The ability of low-temperature plasmas (LTPs) to produce desired chemically reactive environments in gases, on surfaces and in liquids has already made society-wide transformations in our quality of life—from lighting, materials synthesis and water purification, to enabling the information technology revolution through plasma-enabled fabrication of microelectronics devices. Those plasma-enabled societal transformations will continue into the future. A strategic and new opportunity for LTP science and technology is to help enable the electrification of the chemical industry—that is, to drive chemical processing by electrical means facilitated by plasmas. The critical enabling science is controlling the flow of energy through LTPs to produce predictable chemical transformations in gases, on solids, and in liquids. The electrification of the chemical industry is a grand science and engineering challenge that will enable an economically viable and sustainable future based on renewable electricity.
This chapter presents the case that improving our understanding of the fundamental processes in LTPs will lead to translational research benefiting nearly every sector of society.
Unique Features of Low-Temperature Plasma Physics
Low-temperature plasmas are a unique state of matter composed of neutral atoms, molecules, radicals, excited states, ions and electrons. LTPs represents a distinct and, in many ways, unique form of plasma. Most of the plasmas in the universe, including other areas of plasmas discussed in this report, are fully ionized, often magnetized and have average temperatures much higher than ambient. In most LTPs, only a small fraction of the gas is ionized while the mean energy of electrons (a few to 10 eV) is much larger than the temperature of heavy particles (ions and neutrals) that can be as low as room temperature. So even with energetic electrons, the average temperature of an LTP is low enough that these plasmas can be in contact with heat sensitive surfaces, including living tissue, an ability that has enabled the field of plasma agriculture and medicine. Maintaining this nonequilibrium state is possible because energy transfer from electric fields to the electrons is generally much faster and more efficient than the subsequent collisional energy transfer between electrons and heavy particles. A large fraction of the electron energy can then be channeled into the production of electronically excited states, and, in molecular gases, vibrationally excited species and short-lived radicals. The generation of chemically reactive environments at low gas temperature is a defining property of LTPs that enables their use in so many beneficial technologies.
In spite of the unique nature of LTPs, there are common features with other ionized gases. For example, dust in LTPs can behave similarly to dusty plasmas in space. When high energy density plasma comes into contact with cold walls—for example, in fusion devices—the plasma in the boundary layer cools and shares many characteristics with LTPs. The interaction of electromagnetic radiation with LTPs is similar to phenomena in other types of plasma. Some of the instabilities and waves found in higher energy density plasmas also occur in LTPs.
One feature of LTPs, shared with plasmas in fusion devices, is that nearly all LTPs interact with surfaces that bound the plasma, either by the need to confine the plasma or intentionally to change the characteristics of those surfaces. In LTPs, the neutral gas temperature is near ambient while the electron temperature is elevated, resulting in the ability to produce controllable fluxes of reactive species onto surfaces without excessively heating the surface. The relatively high electron temperature and low electron mass leads to the formation of thin space charge regions—sheaths—at boundaries. The electric fields in the sheaths accelerate positive ions to surfaces which then provide activation energy for material changing reactions on surfaces. Low temperature radical generation coupled with normal incidence positive ion bombardment at surfaces has enabled a tremendous range of industrial applications based on LTP processing of material surfaces and thin films; and in particular has enabled industrial scale fabrication of microelectronics.
The field of LTPs is exceedingly broad and so the review of progress since the last decadal, Plasma Science: Advancing Knowledge in the National Interest1 (hereafter the “Plasma 2010 report” is grouped into four representative application areas: aerospace, life sciences, materials interactions, and environment and sustainability.
During the past decade there have been significant advances in the science and technology of Hall thrusters. The Hall thruster (HT) is one of the mainline architectures for plasma propulsion, devices used to accelerate and control the trajectories of spacecraft in earth orbit, geosynchronous orbit and in missions to the planets. In a HT, ions are accelerated to high energies, and emitted as plume of exhaust that pushes the spacecraft in the opposite direction—akin to the exhaust of chemical rockets. This acceleration of spacecraft is very efficient but produces low thrust, thereby requiring long periods of operation for the spacecraft to reach the desired speed. The development of magnetic shielding has greatly reduced the ion-induced sputtering of the HT chamber walls. This has extended the life of HTs from nominally about 1 year to as long as 5 to 10 years, enabling their use in long duration deep space missions. NASA is developing the Power and Propulsion Element (PPE) for the Lunar Gateway human exploration program, which will use 12.5 kW magnetically shielded HTs to move cargo from Earth to lunar orbits (and eventually Mars). HTs (3-4.5 kW) have been flown on commercial communications satellites in the past decade, significantly reducing the propellant needed for orbit raising and station keeping, and similar thrusters are planned for upcoming deep space missions. In the past 5 years low power HTs, compatible with CubeSat architectures have achieved >50 percent efficiencies (conversion of input power to thrust) and long lifetimes both enabled by magnetic shielding.
Other advances have been made in the past decade include nested HTs, devices having multiple annular channels to accelerate ions, which have been demonstrated in powers up to 100 kW with >50 percent efficiency. However, erosion of electrodes and plasma-facing surfaces is increasingly problematic and the topic of continuing research. Investigations of the physics of the cathodes used in HTs have revealed low frequency plasma oscillations and turbulent ion acoustic waves that contribute to transport and energetic ion generation. Electrodeless thruster concepts such as magnetic nozzles and Field Reversed Configuration (FRC) thrusters have been
proposed and investigated, concepts that may reduce erosion to that required for multiyear operation at high powers. However, the efficiencies of these concepts are not currently competitive with state-of-the-art ion and Hall thrusters.
Life Science Applications
There has been impressive progress in applying LTPs to biology and life sciences in the last decade. For example, there have been many demonstrations of LTPs in selectively destroying microbes in diverse environments, ranging from inanimate surfaces to living tissue, from plants to biofilms. Advances have been made in direct therapeutic applications of LTP in both animal models as well as human subjects. There have been investigations in using LTPs in treating cancer (see Figure 5.1); in healing and disinfecting wounds; for dermatological treatments; and in dentistry, among others. The use of LTP to reduce anti-bacterially resistant bacteria strains in wounds has shown promise in many studies.
A major advance came when the first three LTP sources were approved for human testing in the European Union in 2013. Extensive studies on possible dangers to patients were conducted on these devices and treatment protocols were developed before the approvals for human testing were granted. The available clinical studies generally concluded that LTP treatment causes few or no significant side effects or complications. There has been considerable progress in identifying some of the mechanisms through which LTP acts therapeutically. LTP has been shown to stimulate subcutaneous blood flow and blood O2 content; cell-cell communication; and elements of both innate and adaptive immunity. Another important LTP-associated mechanism is the biological effects of pulsed electric fields.
Since most biological systems of interest involve aqueous solutions, studies of LTP interacting with biological targets and liquids have been closely linked. Many studies in the field have addressed the role of reactive chemical species produced by plasma in contact with air adjacent to aqueous solutions. These synergistic studies have led to a broader understanding of the effects of different types of LTP sources on biological targets and how LTPs can chemically activate liquids. For example, LTP treatment of aqueous solutions (including water) produces a liquid that can remain anti-microbial for many days.
In addition to biomedical effects, progress has been made in agricultural and food-related applications of LTPs. LTP have been shown to promote seed germination and disinfect seeds, and to enhance the growth of plants. Agricultural water treatment for both biocidal and nutrition production has been demonstrated. Food and food container disinfection by use of LTP is being investigated and in demonstration trials. Finally, nitrogen fixation (e.g., production of fertilizer) have advanced over the past decade.
Major developments in plasma materials processing have included the continuing progress in applying LTPs to manufacturing of integrated circuits. During the last decade, microelectronics devices have shrunk to the point that atomic layer control of plasma etching and deposition is required—a requirement that will extend to new quantum devices. Since it is extremely challenging to control plasma
to interact with only a single layer of atoms, self-limited processes and ultra-high materials selectivity are needed for both deposition and etching reactions. Plasma-enhanced atomic layer deposition (ALD) and atomic layer etching (ALE) processes have been developed to meet these needs. (See Figure 5.2.) These goals were met by enhanced control of the interaction of electrons, ions, radicals, excited neutrals and photons with surfaces in LTP environments. When conventional continuous (steady state) plasma-materials processes proved not to be adequate, techniques using pulsing were developed—pulsing of the plasma source power, substrate bias, input gases, substrate temperature, and other process parameters. The development of plasma-materials processes consisting of sequences of individual, self-limited surface reaction steps, where the total deposition thickness or etched depth is controlled by the number of process cycles, has been a successful approach to meet the technological requirements. Device structures continue to get taller (larger aspect ratio), more complex (3-dimensional memory) and with new materials. Plasma technology has, to date, met these multiple challenges.
During the last decade there has been wide penetration of plasma surface engineering (PSE) into other industries including etching, smoothing, precision
free-forming, patterning, hardening, and coating of surfaces. There has been increasing use of atmospheric pressure plasmas for materials treatment, with extension from inorganic to organic surfaces (such as polymers) and biological tissue (skin, wound treatment). These advances have utilized new methods of plasma excitation. Although historically PSE has been quite empirical, the science of PSE has greatly advanced, through use of modern of plasma diagnostics techniques. These diagnostics have led to the discovery of plasma instabilities, turbulence and chaos, in what had been considered quiescent devices. Self-organization in process plasmas has also been discovered, most vividly illustrated by instabilities in sputtering magnetrons. The development and application of simulation tools have greatly improved our understanding of PSE systems. Plasmas have also been extensively investigated for synthesis of new materials and structures. Major advancements have been made in the plasma synthesis of nanoparticles and nanostructured materials including 1D and 2D materials.
Thermal plasmas are also intensively used for material processing including welding, cutting and deposition of thermal barrier coatings. In the last decade, a detailed understanding of the influence of metal vapor, particularly in welding arcs, has been developed. 3-dimensional time-dependents models and spatially resolved time-dependent measurements have clarified the mechanisms driving metal vapor transport, and the influence of metal vapor on arc properties, droplet detachment and weld pool depth. This societal benefit of this scientific advance will come with improved welding processes and wire-arc additive manufacturing.
Sustainability and Environmental Applications
In the last decade, impressive progress has been made in understanding plasma assisted ignition and combustion (PAIC). The motivation of PAIC is to improve conventional combustion processes by enabling lean combustion and ignition under conditions that improve efficiency and reduce the environmental impact of combustion processes. Localized heating and radicals produced by nanosecond pulsed plasmas were found to be effective for triggering combustion processes. Nanosecond pulsed plasmas that produce high electric fields efficiently channel energy deposition into electronically excited states of the molecules in air-fuel mixtures, resulting in rapid oxygen dissociation by electron impact and in excitation transfer from other electronically excited atoms and molecules. The majority of these studies have been performed at or below 1 atmosphere, whereas in many engines, the gas is compressed to high pressures. At high pressures, discharge instabilities and self-organization might affect power deposition, induce hydrodynamic effects and alter dominant plasma kinetics—processes that require further investigation.
The treatment of water contaminated with toxic matter by plasmas has been a strong focus of the LTP community and the effectiveness of plasma in mitigating
different pollutants has been investigated. Recently plasma has emerged as a unique technology that is able to decompose carcinogenic species like perfluoroalkyl acids (PFAAs) with energy efficiencies better than competing technologies, as shown in Figure 5.3.
The combination of plasma with catalysts has been extensively studied in the context of environmental remediation during the past decades. Catalysts combined with plasma have shown increased selectivity to remove pollutants from exhaust gases and also increase catalyst lifetime due to in situ plasma-based regeneration of the catalyst. In recent years there has been an increased interest in using the combination of plasmas and catalysts to increase selectivity and yield of chemical conversions, with the main motivation being energy conversion. That is, use plasmas to convert low value materials to high values materials as a form of energy storage. Examples include partial oxidation of methane to make alcohols and ammonia synthesis. The fundamental processes of plasma-catalysis are not well understood. The majority of research has involved performing optimization studies
assessing the consequences of varying input parameters (e.g., voltage, flowrate) on the output while not necessarily addressing fundamental processes.
Societal Benefits of Advances in the Science of LTPs
An improved ability to control chemical transformations through electricity-driven LTP has tremendous potential in a wide range of current and future societal challenges, including enabling the transformation of the chemical industry from being fossil-fuel driven to being electricity-driven. (See Figure 5.4.) This enhanced capability in LTP will benefit multiple sectors of society. Controlling plasma-surface interactions at the atomic level will enable the next generation of materials for quantum computing, combating anti-microbial resistance, improving agriculture efficiencies and food safety, enabling new energy storage technologies and developing plasma-based propulsion capable of taking mankind to Mars and beyond.
Funding sources for LTP research in the United States are as diverse as its scope. Currently, fundamental research in LTP is focused on plasma generation, nonequilibrium kinetics and plasma chemistry; plasma interaction with solid and liquid surfaces, and near surface (sheath) properties; self-organization, magnetized plasmas and plasma-wave interactions. Funding for fundamental research in LTP science is mainly by U.S. government agencies, including NSF, DOE-FES, AFOSR, ONR, DARPA and NASA. As an example, the NSF-DOE Partnership in Basic Plasma Science and Engineering (PBPSE) supports LTP-focused research (including topics such as dusty plasmas) on the order of $3 million per year. The DOE-FES-supported the Center for Predictive Control of Plasma Kinetics for 10 years (2009-2019) with an annual budget of approximately $1.8 million per year and additionally supported DOE laboratory efforts in LTPs at a level of $0.6 million to $1 million per year. In 2019, this program transitioned into support of smaller centers and distributed user facilities for LTP at the level of approximately $4 million per year.
The AFOSR has been a major sponsor for electric propulsion. In addition, many agencies including ARO, DARPA, DOE-BES, NSF (Engineering, Materials), USDA, and NIH support LTP projects that are mainly focused on the utilization of plasmas for specific applications of interest for these agencies. For example, ARO and AFOSR have sponsored several MURI (Multidisciplinary University Research Initiatives) that have funding of about $1 million per year, focused on specific LTP enabled applications such as nanoparticle synthesis and control of electromagnetic radiation. The funding landscape is therefore highly dispersed and with a primary focus on applications and translational research. There is relatively little interagency coordination of LTP research with the exception of the NSF/DOE Partnership in Basic Plasma Science and Engineering.
Funding for applied research in LTP in the United States includes both government and industrial support. The Small Business Innovation Research (SBIR) and the Small Business Technology Transfer (STTR) programs fund a wide variety of projects that use LTPs for a specialized and applied outcome, but typically do not emphasize fundamental research. That is particularly the case for SBIR. STTR requires the small business to collaborate with a research institution, typically a university, and so those programs tend to have a more fundamental component. Each year, federal agencies with extramural research and development (R&D) budgets that exceed $100 million are required to allocate 3.2 percent (e.g., for FY 2017) of their R&D budget to SBIR and 0.45 percent to STTR awards. To estimate the amount of funding that involves LTPs in some manner, even if approximate, the SBIR/STTR grants database over the past 5 years was searched for the term “plasma,” which would include projects that, for example, simply used a plasma
tool to coat a surface but was exclusively focused on other topics, to more plasma focused work. Projects referring to biological plasma were excluded. The results were $20 million to $40 million per year. The majority of these projects involving LTP are likely using the plasma as a tool and not investigating plasma properties.
Industry funding is focused nearly exclusively on applications. Obtaining precise numbers for this effort is difficult since companies are not required to publish these data and LTP applications in industry are invariably coupled with other subfields of science and technology. One sector that utilizes LTP extensively is the semiconductor equipment industry. For example, one leading U.S.-based semiconductor equipment company (Lam Research, Fremont, California) that uses plasma extensively in their equipment reports an R&D expenditure of about $1.2 billion in fiscal year 2018. Other U.S. semiconductor equipment companies such as Applied Materials (Sunnyvale, California) probably spend a comparable amount on their LTP-based equipment development. The chip manufacturing companies also support internal R&D in their plasma-related activities but no data are publicly available. Given the range and scope of using LTPs in industry (i.e., semiconductor, defense, aerospace, automotive, biotechnology, materials, environmental), there are no doubt many other companies that support LTP R&D.
In summary, U.S. funding for fundamental LTP science is no more than about $10 million per year. By contrast, corporate R&D funding for the development of LTP industrial applications probably exceeds $1 billion per year. Industrial estimates of total federal funding investments in LTP related to semiconductor manufacturing, a part of all LTP research, is only 0.01 percent of the plasma processing market—a small fraction for such a strategically important technology. The proper ratio is certainly debatable. However, given that fundamental LTP research underpins several industries that are critically to our national economy and national security, the applied-to-fundamental research ratio appears to be significantly out of balance.
The central role of LTP is often hidden from public view, creating hidden value to the United States and world economy. The hidden value of plasma technology and the unquestionable value of the underlying basic science have led to inconsistent research funding for the field. In the absence of consistent and reliable funding, it is impossible to sustain a sufficient number of world-class research groups in the United States in LTP science to maintain long term leadership in the LTP field. LTP research is carried out at universities and research institutes in multiple departments. The field is truly cross disciplinary. However, this interdisciplinary strength has, to date, been a liability rather than an asset in securing research funding for the field. Most funding agencies and funding mechanisms continue to be strongly compartmentalized and discipline specific. Agency statements in support of cross disciplinary research do not often translate into actual funding programs for
cross disciplinary research, which is a severe problem for LTP in the United States. The impact of the already small amount of government funding for LTP fundamental science is further reduced by being highly dispersed with very little coordination between funding agencies.
Level of Effort
An estimate of the number of faculty and senior researchers active in LTPs in U.S. academic institutions can be made from a community-generated white paper submitted to the APS Community Planning Process in 2019. The white paper had 157 co-signers from 74 institutions (universities, national laboratories, companies). Of these co-signers, 136 had academic appointments from 58 universities and colleges. Many of these colleagues are also involved in other research areas and many focus on applications. This count is known to be an undercount, and so to be optimistic, the committee estimated 175 academic researchers. This is significantly fewer than corresponding numbers in countries such as France. Colleagues in France estimate that between 240 and 350 researchers are involved in LTP within CNRS (the French National Center for Scientific Research) and in French academic institutions. Using the low-high estimates for both countries, on a per capita basis France (population 67 million) has 3.6-5.2 LTP academic researchers/million population. The United States (population 329 million) has 0.41-0.53 LTP academic researchers/million population. On a per capita basis, France has 9-10 more LTP academic researchers than the United States. The committee concedes that these figures are estimates. As the LTP field is exceedingly broad and many researchers use plasmas as part of their research, it is difficult to provide exact figures.
Another estimate of the size of the U.S. LTP community can be made from the number of PhD and MSc theses published, available from the Proquest database for dissertations and thesis. This database lists most, but not all, theses from U.S. educational institutions. Searches in this database over the last 10 years, using the key words “low temperature plasma,” yield an average of about 75 theses/year. However, typically fewer than 10 percent of these theses involve fundamental plasma studies. It is important to note that whereas LTP research involving applications continues to grow, fundamental research in LTP appears to be declining. There is concern that if this trend continues, within the next 10 years, fundamental research in LTP that has historically been the basis of the development of LTP applications will, with a few exceptions, no longer be practiced in the United States.
The demographics in the LTP field will result in a leadership class retiring within the next decade. Plasma research is a multidisciplinary field and recent university faculty hiring has not produced early career faculty with a focus on the fundamentals of LTP science. There are simply too few early career LTP-oriented
faculty for the United States to continue to be an international leader when the current leadership class retires.
The establishment of the Low Temperature Plasma Science Center program at DOE since the 2010 Decadal study has benefited fundamental LTP research. LTP research at U.S. universities remains highly dispersed and it is not uncommon that only one faculty member to be involved in LTP research at an entire university. This underlines the need for a coordinated funding model for LTP and the need to stimulate and support inter-university collaborative efforts. The DOE Plasma Science Center Program has served this role.
In most LTP applications, control of the plasma to achieve the desired effects is the ultimate goal. Designing the plasma device entails making decisions on particulars of the power supply (e.g., radio frequency, pulsed), method of excitation (inductively coupled, dielectric-barrier-discharge, capacitively coupled) geometry, pressure and composition and rates of gas flow. Proper choices depend on the details of the application. In addition to design variables, the process parameters must be selected and optimized for each application. The challenge of this task is, in many cases, difficult to exaggerate. For example, in modern plasma etching applications in the semiconductor industry, it has been estimated that there can be as many as 15 different control parameters, leading to an astonishing 1015 different process recipes that could be used. This huge number results from the multiple film materials are typically etched, with many different gas compositions, coupled with many different possible operating parameters such as gas flows, pressure, multiple power supplies and control of wafer temperatures. Each of these parameters can and often are altered as a function of time, with time variations that range from slow ramps to abrupt pulsing. Controlling such a process is a major challenge. In spite of this huge set of possible parameters, industry is very good at finding a solution that satisfy their needs. That solution is not necessarily the best solution—it is the solution that time and budget allowed.
Other applications may not have quite as large a parameter space, but similar challenges do exist. How does one design and control the plasma to achieve a desired outcome? LTP is a highly nonlinear and complex physical system, including its interactions with its environment, and having more degrees of freedom than any other plasma discussed in this report. The fundamental research questions in LTP are often oriented toward this complex optimization and control problem. To deal with this complexity, an understanding of the dominant physical and chemical processes are necessary to build models that are by necessity less complex but represent system well enough to enable predictive capabilities.
Plasma-Assisted Propulsion in Space Science and Exploration
Propulsion systems based on plasma, typically called electric propulsion thrusters or electric rockets, are an established technology used to keep communications satellites in their desired orbits against gravitational and solar-flux disturbances that perturb their trajectories. Electric propulsion is also emerging as the system of choice for deep space science missions. Gridded ion thrusters were used by NASA on the Deep Space 1 and Dawn missions to visit asteroids and were used by the Japanese Space Agency JAXA to return samples from a near-Earth asteroid. The European Space Agency (ESA) mission BepiColombo is now using ion thrusters to propel the spacecraft to planet Mercury. The Hall thruster was used by the ESA to propel the SMART mission to the moon. Hall thrusters are also planned for the upcoming NASA science mission to the asteroid Psyche.
LTP Enabling Long-Mission Human Space Science and Exploration
One of the unsolved challenges for long mission human space flight is life-support-systems. This challenge extends to long-term habitation on the moon and planets. Life-support requires energy efficient sources of food, water and oxygen, and recycling of waste products. Plasma-liquid interactions and plasma chemical conversion represent a potential solution for all of these areas. Beyond water treatment, plasma-technologies can address a wide range of space habitat challenges. Space agriculture would benefit from the plasma treatment of seeds to reduce microbial load and improve water uptake to enhance yield and the direct application of nutrients to plants via plasma-activated water.
Plasma-based subsystems are potentially components of life support systems. Unlike conventional systems, which require high temperatures for chemical conversion, in LTPs chemical conversion occurs largely at room temperature. LTPs can support atmosphere control through the decomposition of carbon dioxide into by-products such as CO and oxygen. The potential for a plasma-based life support system has yet to be explored, and would be paradigm shift, enabling for essentially complete recycling of waste-water and gases in the spacecraft. LTP is also key to the safety of spacecraft and astronauts enabling the alleviation of charging. (See Figure 5.5.)
LTP in Advanced Microelectronic Devices Enabling Many Other Areas of Science
The recent detection of colliding black holes by LIGO and the imaging of a black hole by the Event Horizon telescope simply would not have occurred in the absence of modern microelectronics devices. Gene sequencing, simulations of weapons,
remote sensing, satellite communications, 3D manufacturing, the display with which the reader is likely viewing this report, are all enabled by microelectronics devices. The essential role played by LTPs in this enormous advance in human capability enabled by microelectronics is in the manufacture of those microelectronics devices.
A report from the U.S. Semiconductor Industry Association entitled Winning the Future. A Blueprint for Sustained U.S. Leadership in Semiconductor Technology2
2 Semiconductor Industry Association, 2019, Winning the Future: A Blueprint for Sustained U.S. Leadership in Semiconductor Technology, https://www.semiconductors.org/wp-content/uploads/2019/04/FINAL-SIA-Blueprint-for-web.pdf.
addresses semiconductor-related research support in the United States. This report notes the importance of semiconductors in emerging areas of advanced scientific exploration such as artificial intelligence, quantum computing, and advanced wireless networks. The fact that LTP is enabling the modern semiconductor industry illustrates that LTP enables most of modern scientific discovery and technological advances that create a major impact.
Physics of lightning has long been considered as prohibitively difficult to understand from first principles, and so lightning research focused either on observations of the macroscopic phenomena (lightning occurrence and properties) or on lightning protection based on engineering models. Lightning protection remains a topic of growing importance, particularly given the trend toward composites in the aerospace industry. However, the discovery of transient luminous events between clouds and ionosphere in 1989, and the discovery of terrestrial gamma-ray flashes and other high energy phenomena from active thunderstorms in 1994 have stimulated new plasma physics research on atmospheric electricity. A new community has developed that is making new observations, undertaking microphysics-based modeling and simulations and analogous lab experiments.
LTP research will play a crucial role in some of the key science challenges in this area including:
- Lightning inception: It is now roughly understood how a lightning discharge can start near one graupel particle (soft hail or snow pellets) due to the local enhancement of the rather low background electric field in the cloud. The manner how lightning grows to tens of meters and more is not known.
- Polarity dependent lightning propagation and stepping: Our understanding of how space charge dominated streamer discharges propagate is improving, and this understanding also applies to their larger relatives, sprite discharges in the thin upper atmosphere. However, when additional effects like plasma heating and plasma chemistry start to play important roles on larger scales of space and time, our understanding is still quite limited. This increased understanding is necessary to explain polarity dependent lightning propagation and stepping.
- Lightning attachment: Where and how does a lightning leader attach to a structure (on land or an aircraft) leading to lightning damage? Recent optical observations show how counter-leaders emerge from tall objects (like apartment buildings) and approach lightning leaders growing downward from the cloud. A better understanding of this process could lead to improved lightning protection schemes.
Plasmas in Hypersonics
Hypersonics is the field of fluid dynamics for speeds greater than Mach 5 (five times the speed of sound). Bodies moving at hypersonic speed through gas will produce an enveloping plasma due to heating by the shock waves at the leading edge of the body. Understanding these plasma dynamics is critical for the design of efficient, reliable and safe hypersonic platforms—for space access and return, planetary entry, defense applications, and high-speed civil transport. Understanding these LTP dynamics is also of importance in predicting meteor penetration through the atmosphere and the associated risks that a meteor impact might have to civilization. The plasma surrounding a body entering into the atmosphere at high velocity produces tremendous heat loads and a communication blackout. These particular phenomena have been long known, but poorly characterized. However, for vehicles that have more lift, the leading edges are sharper and heating is much more severe, exceeding many kW/cm2. Methods to reduce the heating and accurately predict the heat loads are needed to improve the safety of the crew and reduce the weight of thermal protection system. As the plasma passes around the hypersonic body, expansion and recombination processes become important. Research on plasma produced radiation and recombination mechanisms is needed to predict heat loading and to develop methods to minimize it. The conductivity of the plasma is also of interest for the development of advanced control and power extraction methods. For example, a magnetic field may be used to force the leading edge away from the surface, controlling the drag and reducing the heat load. Magnetic fields may also enable MHD type power extraction methods. Injection or ablation of easily ionized species may be used to enhance the conductivity for some applications.
High-Power Microwave Generation
Advances in high power microwave (HPM) generation over frequencies of ~0.1-1,000 GHz are needed for applications such as accelerators, fusion plasma heating, new materials development, advanced radar, remote threat detection, and the transmission of massive volumes of data over long distances. Producing high microwave-energy-density systems requires addressing issues of intense beam generation, dense charged-particle confinement, extreme-environment-compatible materials, intense radiation and particle diagnostic development, and the development of compact pulsed power. HPM research and development requires advances in plasma physics theory, computation, experimental diagnostics, and the integration of advanced electronics (sensors, system control, pulsed power) and signal processing.
Extending conventional magnetron oscillators to GW power and kJ pulse levels requires better understanding of beam-plasma interactions, plasma-induced
pulse shortening, plasma-enhanced mode competition, and scalability. Future opportunities include development of time-domain sources, where HPM radiation is emitted from single-shot (ultrawideband pulses) or periodic-pulse-trains of plasmas generated by ultra-short-pulsed lasers. There is both a need and opportunity for research of higher power, compact (portable) sources of mm-wave and THz-regime radiation.
New materials are both the enabler and a research frontier for advances in HPM science. Advances in predictive computational algorithms, computational materials by design, hardware, and physical models are leading to new electromagnetic materials (especially plasmonic and metamaterials), new extreme-energy-tolerant refractory materials (for low-outgassing anodes) and new cathode materials for thermionic, photo- and field emission. Frontiers in cathode physics include understanding how interfaces, morphology, microstructural heterogeneity, bipolar flows and space charge effects, including nanoscale charge transport, determine emittance, brightness, and cathode lifetime, especially in cold (field emission) cathodes.
Understanding and controlling interactions between localized, dense plasmas and strong EM fields is critically important to HPM sources. Multipactor avalanche (where electrons are scattered and multiply along a microwave window) on conducting and dielectric surfaces by HPM fields can lead to a localized plasma discharge. Controlling surface breakdown of both distributed and spatially periodic discharges would enable longer HPM pulses.
Plasmas for Optics and Wave Manipulation
The field of “plasma metamaterials and plasma photonic crystals” involves LTP science in which plasma elements (individual plasmas, plasma gratings or plasma arrays) serve as, or are integrated into, electromagnetically active artificial materials to produce desired response to electromagnetic (EM) waves. “Plasma metamaterials” act as a filter that will pass or reflect only certain wavelengths due to there being resonances of the EM wave with the plasma array. “Plasma photonic crystals” and gratings relay on EM wave interactions with repeating plasma structures comparable in spacing to the wavelength, leading to Bragg interferences, that will reflect only selected wavelengths.
Plasma integration into metamaterials (MMs) and photonic crystals (PCs) have already proven important in a wide range of applications in microwave, mm-wave, and optical-wave photonics—applications important to communications systems. These devices are typically “static” systems. The potential of plasma-based MMs and PCs is the ability to rapidly reconfigure these structures by changing the properties of the plasma. Plasmas can be used to more effectively focus and shape the radiation field of antennas or reflectors, be used as invisibility cloaks, provide improved impedance matching, and they can spectrally filter, guide, and confine EM waves with high quality factors. The plasma introduces a degree of reconfigurability
at potentially high bandwidths. In communications applications, several of these features relate to the improvement or enhancement of speed and bandwidth (the amount of information that can be communicated). An example of a plasma photonic crystal is illustrated in Figure 5.6.
LTP Benefits to the Environmental and Sustainability
LTP processes help mitigate environmental hazards and processes that contribute to man-made climate change. For example, LTPs as ozone generators have long served as the basis of water purification in municipal water systems.
Water is commonly polluted by pharmaceutical wastes, organic compounds, odor, NOx, SOx, viruses, agricultural runoff and other waste products, including fracking effluent and industrial waste water with persistent pollutants. Cleaning water in an efficient and scalable manner challenges traditional means of water purification. A relatively new approach utilizes advanced oxidation processes (AOP). These methods use the oxidizing potential of the hydroxyl radical (•OH) which is more reactive than ozone (O3) and hydrogen peroxide (H2O2). LTPs are emerging as a strategic technology that can provide the reactants in AOP treatment of water. LTPs are being investigated as in-water sources of oxidizing species such •OH, O•, and H•, and for the emission of UV light for disinfection. Plasma based water treatment is especially attractive since there is no input required other than electricity. The radicals required to remove contamination, pollutants and organic matter are derived from the water itself (or air in contact with the water), requiring no additional chemicals.
Closed Carbon and Sustainable Energy Cycles
Of all the environmental and sustainability issues that must be addressed, CO2 engineering is perhaps the most pressing. There is currently no technology available to economically and permanently remove CO2 from the environment, or to capture and recirculate the carbon in a carbon neutral manner. LTPs represent a science able to address many of these needs. Current research is addressing plasma conversion of CO2 to CO for syngas (a mixture of CO and H2) to recirculate the carbon for carbon-neutral combustion. Similar LTP processes are being investigated for plasma conversion of CH4 to hydrogen (the second component of syngas) and to higher value hydrocarbons. The ultimate closed carbon cycle may involve bio-based carbon raw materials to replace currently manufactured petro-chemicals. Recent research indicates LTPs may play a key role in this effort.
Utilizing plasmas in energy applications already has a record of success. Thin film solar cells are economically viable due to the efficiency and selectivity of plasma-assisted deposition and thin-film etching in industrial scale fabrication processes. LTPs for pollutant mitigation and waste treatment have potential applications across the industrial and municipal landscape. Pilot plants use plasma torches for converting municipal solid waste to syngas and minimizing the need to dispose of solids. The syngas can then be used to sustainably produce electricity. Plasma based systems are used to treat contaminants in industrial gases, to treat SOx/NOx emission from power plants and to remediate medical waste.
Fossil fuel combustion will likely play a significant role in modern society for the next several decades. Using those precious resources more efficiently positively impacts every measure of environmental stewardship. As discussed above, plasma-aided ignition and combustion (PAIC) is a highly promising technology to address some of those needs.
SF6 Replacement Gases in High-Voltage Switchgear in Electricity Distribution Systems
SF6 has several favorable properties that have led to its use in high-voltage circuit breakers—it is nontoxic, stable, is gaseous even under pressures of the several atmospheres that are typically used, it has excellent insulating properties and, since it has lower energy than any of its decomposition products and is thus reformed after an arcing event. However, because of its high global warming potential, it is scheduled to be replaced. CO2 is now being used in some installations but has poorer insulating properties and is also a global warming gas. New gases, in particular C4F7N and C3F8O, show strong promise; however, they have relatively high boiling points and therefore have to be mixed with CO2 or other gases. They are also gradually decomposed after arcing events. Research into the decomposition pathways of these gases under typical operating conditions, their reactions with metals and vapors ablated by the arc, and computational modeling of the arcing process are all required to optimize the design of circuit breakers that use such gases.
LTP in Education and Workforce Development
LTP is intrinsically multidisciplinary, with investigations extending to other research fields from material science to medicine. As a result, students pursuing LTP topics in their graduate studies receive an interdisciplinary education. The research in these allied fields similarly has far-reaching educational impact, helping to train the next-generation of investigators in both the application area and in plasma science. Graduates from LTP programs begin their careers in a wide range of science and engineering disciplines in industry, national laboratories, and academia.
LTP science and technology have long addressed critical societal problems and have created significant economic impact through the interplay between basic science, applied science, and technological challenges. Since plasma technologies are primarily enabling technologies, their contribution to a specific product or method often remains hidden or even unknown and their direct impact, especially their economic impact, is often difficult to assess. Success of LTP in advanced applications has been the result of a significant and sustained investment in basic and applied plasma research in North America, Asia and Europe over many years. This critical support enabled the plasma community to (1) leverage basic plasma science breakthroughs for the development of strategic applications (e.g., integrated circuits in microelectronics) and to (2) use the success of plasma applications to challenge basic plasma science to thoroughly investigate these new phenomena and provide a scientific underpinning.
Materials Processing: Semiconductor and Related Industries
As one of the top 10 industries in the United States, the semiconductor industry is a strategic asset to the U.S. economy and national security. Semiconductor chips are the fundamental technology that enables computers of all types (laptops, mobile phones, data centers, and supercomputers) and their applications, from medicine to national defense. Microelectronic chips enable the rapidly expanding world of artificial intelligence (AI), where state-of-the-art chips are increasingly embedded into products such as autonomous vehicles, web commerce sites, industrial manufacturing plants, and ultimately supporting the growing sophistication of our national security infrastructure.
The chemically reacting environment capable of delivering highly controlled activation energy to wafers enabled by LTPs is absolutely essential to the manufacture of these devices. Of the 400-950 processing steps required to manufacture leading integrated circuit (IC) chips, 40 to 45 percent are plasma based. It is no exaggeration to say that virtually every chip in circulation today has been touched by plasma. If there was a better, cheaper, more reliable method for microelectronics manufacturing than use of LTPs, the committee believes the industry would have gone in that direction. The reality is, that after decades of exponential semiconductor scaling, advances in new technologies depend more than ever on plasma-based processing. Plasma etching, alone (“trimming”), or in conjunction with atomic layer deposition (ALD) has recently been used to create mask features with widths of tens of nm to <10 nm. This “disruptive” technology may obviate the need for extreme UV lithography in many applications.
The economic impact of LTPs is enormous and can be quantified in the microelectronics industry. Currently, the United States leads that industry with close to half of the global IC market worth nearly $500 billion, which serves a roughly $2,000 billion electronics market. Estimates of future growth in the IC industry based on current trends suggest that a 5-year doubling in revenues is likely. To manufacture the microchips that enable this market, the global IC industry purchases $50 billion (2018) of wafer fabrication equipment per year, of which approximately $15 billion to $20 billion is plasma based.
Materials Processing: Polymers
The use of LTPs in materials processing beyond microelectronics fabrication has and continues to have a huge impact. LTPs for functionalization of polymers for wettability and adhesion is the basis of large industries as well as emerging applications. Commodity polymers such as polypropylene, polyethylene and polystyrene are treated with plasmas to produce hydrophilic surfaces that will wet and adhere to other materials. These techniques are now being applied to high
value materials such as metals, carbon fiber laminates, 3D-manufactured parts to remove contaminants and to improve adhesion. The development of plasma processing techniques for biotechnology is also an established industry with emerging applications. LTPs are used for biocompatibility (e.g., cell adhesion, specificity) and sterilization, and is now finding new markets in treating medical polymers to enable more precise and reproducible medical studies.
In 2018, the worldwide production value of polymer films and sheets was $120 billion, growing at 5 to 6 percent annually, of which 70 percent involves forms of polyethylene and polypropylene films, virtually all of which require some form of surface modification. The products using these films and sheets have a market value of over $330 billion. The majority of all products utilizing polymer films require LTP (corona) treatment to enable acceptable final product performance.
Materials Processing: Coatings
LTP sputtering processes are extensively used in industry for coatings and functional films. The cathodic arc deposition has remained a base technology for high-rate coatings on tools and automotive parts and gained new importance in deposition of thick thermal barrier and erosion protection coatings such as on turbine parts. At the same time alternative technologies such as HiPIMS (high power impulse magnetron sputtering) are becoming established in segments of the industry, especially the high-end tooling industry. Another major application includes plasma deposition on glass substrates for reflection and antireflection coatings.
Materials Processing: Additive Manufacturing
Metallic additive manufacturing is an increasingly important process, enabling prototyping, production of custom-designed parts and production of complex structures not possible using traditional methods. Many metallic additive manufacturing processes use metal powder as the precursor (e.g., powder-bed approaches such as selective laser melting and selective electron-beam melting, blown-powder approaches). Thermal plasma processes, including spheroidization using inductively coupled thermal plasma and wire-to-powder using thermal plasma jets, have proven ideal for production of such powders.
Wire-arc additive manufacturing (WAAM) is an emerging additive manufacturing approach. In this process, a wire is fed into a high intensity plasma arc, which rapidly melts the wire to redeposit on an adjacent surface. WAAM is well suited to the production of large components, since it is fast and relatively inexpensive (metal wire is much cheaper than powder). There are several challenges that have to be overcome. These include improved resolution and control, which requires control of the arc and ideally a spatially confined arc, separate control over the
wire feed rate and the arc current, methods to reduce residual stress and distortion (which occurs due to the repeated heating and cooling cycles), and methods to control the microstructure of the deposited metal, which determines the mechanical properties of the component. An example is shown in Figure 5.7.
LTP Applied to Lighting
Plasma lighting sources have dominated commercial, industrial and public lighting needs for 150 years. In spite of this longevity, plasma-based lighting has made progress in the development of compact fluorescent lamps and back-lighting for flat panel displays, both of which have extensive commercial, industrial and
residential use. In the Plasma 2010 report,3 it was reported that plasma light sources—fluorescent bulbs and high-intensity-discharge lamps—produced 80 percent of all the light used in general lighting. While consumers were switching to more efficient plasma (fluorescent) lighting at that time due to improvements in the quality of the light and the life expectancy of the lamp compared to incandescent bulbs, lighting still accounted for 22 percent of all electricity produced in the United States. Plasma-display panels and televisions, also controlled a significant amount of the display market in family households at that time.
That dominance of plasma lighting is beginning to erode with the development of solid-state lighting sources (e.g., light-emitting-diodes and laser-diodes) and flat panel displays. However, the essential role of LTPs in lighting has actually increased in the transition to solid state light sources. The high-volume manufacturing of these devices requires the controllable chemical reactivity produced by LTPs in the same manner as microelectronics fabrication. Essentially all solid-state lighting and flat panel displays use plasma deposition, etching, cleaning and implantation steps in their manufacturing. While plasma may no longer be the main source of the light, it is the source of the higher efficiency and lower cost electronic devices that make and control the light today. The U.S. Energy Information Administration (EIA) estimated that in 2018, only about 8 percent of the total electricity consumed by the combined residential and commercial sectors was used for lighting. Advances in plasma processing in the last decade directly resulted in significant reductions in the amount of electricity required for lighting in the U.S.
LTP Applied to Flow Control
Plasma-based flow control actuators have seen major advancements that improve the operation of aircraft. For example, if efficient arrays of LTP can be applied for on-demand vortex generation during takeoff and landing of aircraft, and be deactivated during the cruise phase, there is the possibility of significant fuel savings during the overall flight. This technology would eliminate the drag penalties associated with conventional vortex generation in the cruise phase. Alternative opportunities may also exist for drag reduction by generating plasma at other positions on the aircraft to reduce drag, reduce noise, or eliminate instabilities.
LTPs, as ozone generators, have for many decades served as the basis of water purification in municipal water systems. The global ozone generation market was valued at $880 million in 2016 and is expected to reach $1.5 billion by 2023.
To be economical, ozone generation systems typically need to be on the scale suitable for municipal water treatment. This discourages their use for rural point-of-use water treatment far from municipal systems. A recent development is using microplasmas for rural point-of-use water treatment. (See Figure 5.8.) (A microplasma is a plasma confined to less than 1 mm dimension.) Using modular arrays of microplasmas powered by solar cells, ozone-based water treatment has been made available to “off grid” villages. While ozone generation processes are well known, technological advances continue to be made in the field and are often highly focused on engineering. At the same time, science challenges do remain. For example, the ozone zero phenomenon (where ozone production ceases) in pure O2 is not well-understood. This is of significant relevance for applications requiring pure ozone as in some semiconductor processing applications.
Kinetics and Collisional Processes in a Highly Nonequilibrium Regime
The key distinctive feature of LTPs is that power transfer to gases, solids and liquids occurs through energizing electrons (and ions in sheath regions) followed
by collisions of those particles with gases, solids and liquids. This leads to highly nonequilibrium plasma kinetics. The LTP field is often driven by applications with plasmas in complex molecular gas mixtures and operating over a wide range of gas pressures. The fundamentals of plasma kinetics have been studied in atomic low-pressure plasmas. The investigation of plasma kinetics in complex molecular plasmas and at higher pressures (often atmospheric pressure and above) and even in liquids has received far less attention, yet applications of these systems are where future opportunities lie. The analysis of molecular plasmas often suffers from a lack of fundamental data, ranging from electron impact cross sections to ion mobilities. At the interface between plasma and liquids, such as water, electron and ion solvation play a dominant role, a topic that is only beginning to be understood.
The driving focus for LTP science is the following: Controlling the nonequilibrium energy deposition and dissipation in collisional LTPs to enable plasma-produced selectivity. This is an extremely challenging topic that has a common science base—that being plasma kinetics and collision physics. However, even fundamental plasma science investigations will have system and application specific solutions. (See Chapter 2.) This specificity is due to the large number of reactions and species, and the wide range of plasma conditions, for example, nearly 9 orders of magnitude difference in pressure between microelectronics processing and plasma-in-water treatment. Bridging this large gap and exploring selectivity for a broad range of applications places great emphasis addressing fundamentals that will scale and the development of predictive modeling.
Plasmas generate infrared, optical, UV, and VUV radiation—this is the basis of plasma lighting sources. The consequences of the production and transport of radiation, and particularly UV/VUV radiation, is perhaps one of the greater unknowns in generation and propagation of LTPs and in plasma material processing.
Scaling, Instabilities, and High-Pressure Regimes
The high rate of electron- and ion-neutral collisions at atmospheric pressure not only leads to increased gas heating but also enhances the tendency to develop spatio-temporal instabilities, and self-organization. This increased sensitivity stems from fundamental scaling laws and plasma kinetics. To first order, maintaining constant E/N (electric field divided by gas number density) in many LTPs typically produces similar electron temperatures. At constant E/N, an important scaling law is pd ≈ constant, where p is the gas pressure and d the plasma scale length. The higher the pressure p, the smaller the plasma scale length d. In a typical glow-discharge plasma, pd ≈ 1 Torr-cm. Spatially dependent plasma phenomena that occur over many cm in a 10 mTorr plasma occur over a few microns at atmospheric pressure. This scaling has enabled an entirely new field of microplasmas, which tends to produce filamentary behavior and self-organization. In addition,
the larger range of length scales down to the micrometer level provides unique challenges for diagnostics and modeling.
Plasma sustained at high neutral gas density experiences correspondingly higher collision frequencies of electrons and ions with neutral species compared to lower pressure plasmas. At constant E/N, another important scaling law is pτ ≈ constant. The higher the pressure, the shorter the time τ over which a collective plasma process (e.g., ionization wave) occurs. Plasma phenomena that occur over μs to ms in a 10 mTorr plasma tend to occur over ps to ns timescales at atmospheric pressure. These shorter time scales significantly increase the complexity of controlling plasma kinetics at atmospheric pressure. Under these conditions, plasma phenomena that need to be time-resolved create unprecedented challenges for both diagnostics and modeling. Several strategies have been developed to minimize gas heating and the associated development of plasma instabilities. These include nanosecond pulsed plasma excitation to produce the plasma on time scales shorter than the time for an instability to grow. Such fast excitation approaches with large amplitude voltages have enabled the production of novel, highly energetic neutral species as well as run-away electrons. Similar phenomena are thought to occur in poorly understood upper atmospheric plasma phenomena such as sprites and elves.
An example of a new plasma regime is transient plasmas in atmospheric pressure plasmas and liquids with high ionization degrees (more than 10 percent) generated by nanosecond voltage pulses. These unique conditions, particularly at such short time scales, enable the gas temperature to remain relatively low. Phenomena associated with strongly coupled plasmas start to become important under these conditions, offering many opportunities to explore novel LTP with analogs to warm dense matter as well as non-neutral and dusty plasmas.
Crossed Electric and Magnetic Fields Transport
Anomalously high transport of plasma across magnetic field lines is pervasive, spanning many branches of plasma physics from fusion to astrophysical plasmas. In LTPs, crossed electric and magnetic fields, E×B, transport is most commonly associated with magnetrons used to sputter in materials fabrication and in electric propulsion (EP) devices. In these E×B systems, instabilities and waves often occur. The lack of understanding of transport in crossed electric and magnetic fields has precluded the development of the types of predictive, numerical models that are highly desirable for both analysis and design. An emerging consensus is that these instabilities can be attributed to nonclassical effects such as self-organized oscillations and micro-scale turbulence.
There are a number of remaining knowledge gaps about transport process in EP systems such as Hall effect thrusters (HTs) which are E×B devices. For example, there are discrepancies between modeling results and experimental measurements
related to the shape, dominant energy modes, direction of propagation, and influence of micro-turbulence on plasma transport. Recent particle-in-cell simulations have shown that fast moving electron waves drive coherent, large amplitude, ion acoustic waves. When coupled with local ionization, these waves can produce the cross-field transport and acceleration potential profiles observed in HTs. (Another form of EP, the magnetic nozzle, have similar unresolved issues.)
In magnetrons, an array of magnets and a high voltage applied to a cathode produces a closed loop of electron motion adjacent to the cathode. The plasma in the closed loop can be highly ionized, producing a large flux back to the cathode to sputter atoms. Magnetrons are geometrically similar to HTs in having an E×B structure that is prone to instabilities and waves, which can lead to reproducibility problems in industry.
In an effort to address open questions in E×B transport, on-going modeling efforts are focusing both on building direct numerical simulations enabled by increased computational capabilities as well as physics-based fluid/hybrid models that can approximate nonclassical transport. Expanded experimental efforts combined with new diagnostics will need to be able to measure energy coupling across several scale lengths, the phase relation between microscale electric field and density, and particle distributions.
Plasma Surface Interactions
In most cases, LTPs are bounded and the bounding interface dominates plasma properties. Interactions of plasma with interfaces are also recognized as a major challenge in other plasma disciplines. Examples include the interaction of fusion plasmas with the diverter wall and interactions of interplanetary dust with space plasmas.
Plasma-interfacial interactions can be highly complex due to strong coupling between the plasma properties and the interfacing material surface properties. This interaction can be due to surface charging, electron generation, local field enhancement, sputtering or evaporation of surface material into the plasma, surface deformation, plasma-induced surface property changes and surface reactions in reactive plasmas. Many of these important interfacial processes are not well understood particularly for complex surfaces like volatile liquids and surfaces with complex nanometer to micrometer scale surface morphology such as catalysts. Understanding plasma interactions with complex surfaces will ultimately require developing a broader range of in situ surface diagnostics and multiphase modeling leveraging knowledge from different communities. For example, understanding and capitalizing on the synergism between catalysts and plasma remains a major challenge and topic for future intensive fundamental and highly interdisciplinary research. (See also Chapter 2.)
As noted above, low pressure LTPs are used to alter surfaces and thin films for semiconductor device fabrication. For example, progress in maintaining accurate control of ion energies and angles at surfaces in plasma etching is an enabling technology in the semiconductor industry. (See Figure 5.9.) The materials challenges in the semiconductor industry have been and will continue to be significant. A new group of materials are becoming more important in this industry, including graphene and other 2D materials, III-V compound semiconductors, ultra-high dielectric constant materials, complex oxides and nanoparticles, among others. Control of plasma-surface interactions will become ever more important, and new experimental and theoretical approaches will be needed.
Self-organization is an often-observed phenomenon in LTPs. Self-organization can occur in the bulk plasma or in the anode or cathode layer at the interface between a plasma and a resistive or dielectric medium. For example, a resistive or dielectric medium can stabilize the plasma into an array of spots. In glow-discharges interacting with liquids, and typically when the water is the anode, self-organization occurs on the surface with visible patterns ranging from circular to star-like shapes. When ionization fronts propagating through the plasma impinge on a dielectric surface the discharge tends to spread as a surface ionization wave that can display self-organization. The self-organization is thought to result from “‘memory”‘ effects associated with surface charge patterns or through nonlinear streamer-streamer interactions. Three dimensional simulations of ionization waves and streamers have been performed in the gas phase whereas surface ionization waves have only been recently modeled and intrinsic 3D phenomena have not yet been addressed. We currently do not have a general understanding of the mechanisms responsible for self-organization in atmospheric pressure plasmas interacting with surfaces. (See also Chapter 2.)
Self-organization can also occur in the bulk plasma. These patterns can arise from nonlinear electron kinetics, memory effects and plasma-wave interactions in magnetized plasmas. Examples including striations in nonmagnetized glow discharges for a wide range of pressures, filament patterns in dielectric barrier discharges, spokes in magnetron E×B discharges (see Figure 5.10) and in Hall effect thrusters. While specific models have been able to reproduce some observations, a general scientific understanding is still emerging for many of these self-organized processes. To gain a better understanding of self-organization the community needs to answer questions related to the role of gradients, electron kinetics, and the coupling between large- and small-scale plasma structures (energy cascade).
Due to the close coupling of fundamental science and motivating applications in LTP, the research is nearly always in a state of translation. (Translational research refers to a smooth continuum that begins with fundamental studies and leads to applications.) The translational nature of LTP science is a tremendous strength, but it also places an implied obligation on LTP science to perform that science in regimes that will produce results that quickly convert to applications. An ever-present theme and challenge to LTP science is how to bridge the gap between science discoveries and technologies developed in the laboratory and products that benefit society. Laboratory developed technologies typically require a combination of advanced testing and predictive modeling to understand how they may perform beyond controlled laboratory environments. Two representative examples that highlight the critical challenges faced in implementing LTP technologies for electric propulsion and energy, material, life and agricultural science are highlighted below.
LTP for electric propulsion (EP):
- Challenges arise in the ground testing of thrusters—“facility effects.” Thrusters in space emit plasma into nearly perfect vacuum, conditions that are difficult to replicate on Earth in large enough volumes and for long enough time to conduct life testing.
- High-power, special purpose facilities can address some of the “facility effects.” However improved understanding of EP physics would also enable alternative, cost-effective tests based on numerical models or accelerated wear tests. High fidelity, experimentally validated models are need to understand EP physics and mitigate life-limiting mechanisms.
LTP for energy, material, life and agricultural applications:
- While there are many different surfaces and structures that are treated with diverse goals and objectives, there is a common challenge. Little is understood about how to couple plasma source design and operation to specific applications for chemical, biological and material systems. Translating fundamental studies of particle distributions to activating a desired surface modality is at the forefront of LTP science.
- There remain significant theoretical and experimental challenges to understanding the correlation between a plasma treatment “dose” (the sum of all reactant species incident on the target) and the subsequent biological effect for biomedical applications.
- There are extreme challenges due to coupling of phenomena at vastly different time scales. For example, plasma-tissue or plasma-cell culture exposures usually last from seconds to minutes whereas the biological responses occur over minutes to days, or even longer in some cases. Similarly, plasma produced radicals in the gas phase evolve over much shorter times (microseconds to milliseconds) compared to surface catalytic reaction time scales in plasma catalysis (seconds to minutes). Plasma can induce liquid phase convection over similar long time scales, all of which impact plasma-liquid treatment.
- The central challenge in many of the proposed applications of LTP activated processes to energy, water, food and agriculture is in scaling. Even if plasma can be shown to be effective on small laboratory scales, the process must be scaled sufficiently to make it useful in an industrial setting, whether in the factory or the corporate farm. One exception might be plasma medicine applications since the focus is on safety rather than scale-up.
- It is highly likely that scale-up will take a modular approach—arrays of highly efficient plasma modules that are combined for higher throughput.
The modular nature of the technology then enables off-grid point-of-use applications. These would be, for example, small farms or shops using locally generated solar or wind power.
- For many anticipated applications in medicine, and agriculture, the intended users (e.g., physicians and farmers) are unlikely to be trained in the complex field of LTP science and technology. This is particularly the case of off-grid point of use, but also true for medical professionals. There may be a need for LTP source autonomy where the plasma source is intelligent enough to adapt to changes in the surface being treated (every patient and plant is different), perhaps borrowing technologies from machine learning and artificial intelligence driven autonomous vehicles. The concept is “‘one doctor, one button’” or “‘one farmer, one button.’”
Recent developments in LTP diagnostics have leveraged laser-based techniques to produce high spatial and temporally resolved measurements. Laser induced fluorescence (LIF) is a widely used diagnostic that can probe species-specific density and velocity distribution functions (VDFs). Since laser diagnostics typically involve the absorption and emission of photons, the collisional nature of most LTPs requires new insights into the consequences of collisions on the measurement. Close collaboration with the atomic, molecular and optics (AMO) community is needed to fully exploit these new laser diagnostics. New diagnostics have been developed based on femtosecond and picosecond pulsed lasers. These short timescale probes, only recently applied to atmospheric pressure collisionally dominated plasmas, can make measurements between collisions. These techniques hold great promise for future investigations. The implementation of Electric Field Induced Second Harmonic (EFISH) diagnostics has enabled, for the first time, measurement of electric fields in a plasma with subnanosecond time resolution. (See Figure 5.11.)
While traditionally reserved for higher temperature and density plasmas, recent advances in laser Thomson scattering (LTS) have opened the door to more extensive use of this diagnostic in low plasma density (e.g., plasma materials processing systems, EP) and atmospheric pressure LTP research. For more compact LTS setups, the use of volume Bragg gratings (in place of triple grating spectrometers) has been demonstrated. Development of laser-based techniques with improved spatial resolution that can be used near surfaces, remains a major challenge.
While the sophistication of experimental techniques continues to progress, there are several critical aspects of low pressure LTPs that to date have not been experimentally accessible. These include noninvasive measurements of high frequency (>1 MHz), mid-wavelength plasma oscillations (e.g., the relationship between density and potential fluctuations). Direct, nonintrusive measurements of
the most fundamental plasma properties (electron densities and temperatures, and electric fields) also continues to be challenging at atmospheric pressure. Extending diagnostics originally developed for low pressure (e.g., laser collisional induced fluorescence and microwave scattering) to higher pressures continue to hold great promise. At the same time, diagnostics that take advantage of high pressure should also be pursued.
The complexity of reactions and range of species in nonequilibrium molecular plasmas provide major challenges for diagnostics. A large variety of diagnostics are available including molecular beam mass spectrometry, laser induced fluorescence, laser scattering techniques, and a range of absorption techniques including broadband absorption, cavity ring down spectroscopy, and tunable diode laser absorption. However, each of these diagnostics, powerful in their own right, have limitations on pressure, species, spatial resolution and timescale. Further developments to increase the capabilities of these techniques, and to broaden the types
of species that can be measured, should be a key priority, together with exploring new approaches such as the recent development of frequency comb spectroscopy.
Diagnostics tools are available for material surface characterization and measuring active species in liquids. However, the majority of such diagnostics are focused on ex situ characterization. An increased understanding of the coupled physico-chemical processes at plasma-solid and plasma-liquid interfaces requires further development of in situ diagnostics amenable to a harsh and complex plasma environment. The development and implementation of new in situ diagnostics to probe changes in surface properties (solid, liquid and soft organic surfaces) and structure during plasma exposure is a critical priority for the LTP field. This could be accomplished by developing new surface diagnostics or adapting, where possible, surface science techniques.
Modeling and Simulation
Predictive computational modeling capabilities are critically important in advancing LTP science and technologies. There are three main physics-based approaches employed to model LTP devices: fluid, particle kinetic and grid-based direct kinetic methods. Hybrid models, where fluid and kinetic methods are combined for different species and conditions, are also widely used. Despite significant advances in the field in the past decade, modeling LTP remains challenging due to the multiphysics and multiscale nature of the discharge phenomena.
While fluid models are inherently computationally less expensive, they have limited capability in representing the detailed aspects of kinetic-based processes. Examples include instabilities found in E×B devices, turbulence in low pressure systems, the dynamics of double-layers and in high electric field regions as in sheaths and near ionization fronts of atmospheric pressure streamers. The challenge for these models is to develop time-dependent, physics-based equations and closures that account for these kinetic effects. For kinetic models, which in principle have the highest fidelity, computational time is a major limitation. For example, current simulations are limited at best to two-dimensional phenomena up to a few tens of microseconds using explicit particle methods. This is too short with insufficient dimensional fidelity to resolve or understand, for example, interplay between collisionless phenomena (beam-bulk instabilities) and three-dimensional collisional phenomena (e.g., plasma wall interactions and intermolecular collisions). Three-dimensional models have been increasingly exploited in the last few years to tackle the intrinsic 3D phenomena of self-organization and inhomogeneous nature of atmospheric pressure plasmas. (See the example in Figure 5.12.)
The ultimate LTP challenge is achieving predictive control of the plasma activated chemical processes. This is particularly complex due to the sensitive two-way coupling between the electron energy distribution (EED) and the gas composition,
including the species produced through collision processes. Given the translational nature of LTP research, models are required that are fully fundamentally physics based, but also have the robustness to be used for design and optimization of devices. For example, complex plasma chemistries may include a hundred individual gas phase species (ions and neutrals), a thousand reactions, and similar complexity in plasma-surface interactions needing resolution from microns to tens of cm. High-performance computing is required for models that contain the needed physics and reaction complexity while also representing the geometries of interest for LTPs on timescales of interest.
Significant progress has been made in plasma-surface interaction modeling for both nonthermal and thermal plasmas. Nonetheless comprehensive models with a two-way coupling of nonequilibrium plasma kinetics in plasmas intersecting with complex interfaces such as liquids, including evaporation, charging, deformation,
liquid interface dynamics and liquid phase convection, continue to be a challenge. While models of thermal plasmas have made major progress in the last few years, such as that gained in the detailed understanding of the influence of metal vapor in welding arcs, for example, no self-consistent physically based models of arc-electrode interaction exist for many situations of practical interest. Similarly, models that resolve nano-scale features at plasma interfaces and evaluate their impact on the plasma properties have yet to be fully developed. In large part, these challenges are due to the enormous range of coupled length scales and timescales that must be included in such comprehensive models.
Developing standards for verification, validation, and benchmarking of new and existing models will continue to be extremely important. Validation of models and reaction sets is increasingly important as a comprehensive understanding of LTP phenomena requires coupling diagnostics with validated models. The development of validated predictive capabilities requires a large team and long-term efforts that are currently not in place for the U.S. LTP community. Such efforts to date have focused mainly on gas phase kinetics. Model validation including interaction of LTPs with interfaces has yet to be systematically addressed. Chemical reaction sets are, in some cases, available for both gas and liquid phase plasma models. However, plasma-surface interaction models generally do not match the rigor associated with models of multiphase phenomena characteristic in fields such as atmospheric aerosol models. Descriptions of plasma-surface interactions developed for low-pressure etching and deposition plasmas will need to be extended and adapted to include important processes relevant for a broader class of material properties and plasma conditions.
The purpose of modeling is to better understand the current state of experiments and to predict well beyond the current state of the art—new systems, new excitations schemes, new configurations, new applications, new physic for which experimental data for validation might not exist. The required precision for computationally analyzing well characterized benchmarking experiments is greater than the precision required for exploratory, first of their kind simulations beyond the current state of the art.
Data Science, Machine Learning, and Artificial Intelligence
Machine learning (ML) is a branch of artificial intelligence (AI) that seeks to find patterns from statistical or probabilistic analysis of large amounts of data. ML is attractive for multiple applications in not LTP but in all systems involving plasma. (See Figure 5.13.) ML involves training a computer algorithm to predict the behavior of a complex system by collecting many examples of input-output behavior. ML methods can be significantly simpler than using exclusively physics-based models. Fundamentals of ML were developed over the last several decades,
but only recently has it become practical to obtain and analyze the enormous quantities of data needed for the schemes to work. Learning-based control approaches can potentially transform LTP control, enhancing plasma reliability, flexibility, and effectiveness. ML has already had a large impact for high volume manufacturing using plasma processing in the semiconductor industry where very large datasets are available. However, in laboratory scale experimental LTP, in most cases, ML will not have true “big data” to work with. The data sets will be relatively small and incomplete. For big data approach to be useful in LTP, it should be well coupled with our prior knowledge of underlying physics and chemistry—that is, physics or algorithmically based ML. This is a new unique challenge for the LTP field.
Even with its challenges, ML holds the promise to transform LTP modeling, diagnostics, and control. ML and AI could lead to the development of self-aware and self-correcting LTP systems, as will likely be needed in LTP applications where the target varies from case to case (e.g., medicine, biotechnology, agriculture). ML is rapidly expanding into many novel applications, typically driven by practical applications.
It is expected that LTP will be not be an exception in the next decade. However, LTP applications are unique challenges to ML methods due in part to the intrinsically strong nonlinear coupling of multiple parameters for most LTP systems and processes.
Given the great diversity of science areas and applications in LTPs, future opportunities that cross the area may best be expressed in terms of high-level Strategic Challenges. Here are four strategic challenges that, while not exhaustive, encompass the breadth of the field.
Challenge 1: Developing plasma-based tools for future health care and food cycle needs.
LTP is a unique state of matter with characteristics that have, until recently, been exploited mainly for nonliving materials processing and chemical applications. Recent advances have shown that LTP can be used to decontaminate both inanimate and living material surfaces through a range of physico-chemical mechanisms. LTP has demonstrated human therapeutic benefits for applications that include promoting wound healing and cancer treatment. The fact that LTP can influence biological systems through multiple pathways and mechanisms suggests LTP can be used for many different biological applications as well.
For example, over the last several decades, antimicrobial resistance (AR) has reached nearly crisis proportions. (AR is where microbes, bacteria and viruses, become immune to drugs, rendering those drugs ineffective or useless for treating infections.) All reports indicate that AR will continue to become more severe for the foreseeable future. It is well known that the development of systemic antimicrobial drugs has slowed down and some knowledgeable observers predict a post-antimicrobial world where drugs are no longer effective against major infections. Plasma disinfection and sterilization tools would certainly not solve all problems associated with AR, but they could be powerful tools in localized, resistance-free, selective disinfection devices. Many bacterial infections in humans and on medical devices occur in the form of biofilms. The highlight in Figure 5.14 illustrates one field of active research in applying LTP to biofilms.
There are many other potential LTP applications in health care. LTP has been shown to stimulate immune systems in animal models, promising for many medical conditions. A relatively unexplored area is use of LTP devices for cosmetics applications and skin treatment. The field of LTP therapeutics is just beginning to indicate its potential.
Promising LTP applications in agriculture could greatly impact the food cycle, from plant growth to food safety. Food, food system, and water disinfection are possible applications of LTP, although costs and scaling issues represent additional
challenges. Other potential applications would use LTP to reduce or minimize the use of pesticides or herbicides.
The field of “‘bioprocessing’” in which biological processes are conducted on a large scale to create or alter some chemical species, could benefit from LTP treatment or enhancement. Recent advances include using LTP to treat organic waste to improve its fertilizer characteristics. This technique uses the plume from air plasma interacting with biologically decomposing waste in bioreactors. This concept could be extended to other bioprocesses.
In each of the potential applications listed above, there are corresponding challenges, opportunities and needs to expand and extend LTP science.
Challenge 2: Controlling plasma-surface interactions at the atomic level to enable the next generation of materials for quantum computing, new communication, sensor, energy storage and harvesting technologies.
Advances in plasma-materials processing are challenged by the need to choose operating conditions and plasma devices from and enormous parameter space. There is a serious need for a more detailed understanding of the fundamental processes underpinning plasma-surface interactions to enable us to develop predictive modeling capabilities. A huge victory would be predictive modeling that can a priori specify the optimum operating conditions and plasma device architectures. From a practical perspective, having modeling reduces the enormous parameter space would enable more productive experiments. The implementation of advanced plasma materials processing may require advanced control schemes, which in turn require better control-oriented mathematical models as well as better in situ, real time plasma diagnostics that are compatible with ultra-clean processing. This offers also opportunities for ML techniques. Major opportunities in plasma materials processing include controlling surface texture at the nanometer scale and controlling interfaces between atomically defined material layers.
Many novel opportunities will no doubt present themselves as the semiconductor integrated circuit device industry continues to grow. Advances in device architectures are now exploiting the 3rd dimension, increasing capability with layers of layers of devices. Memory devices now under development use 256 to 512 layers, each of which need to be deposited followed by etching with aspect ratios (height to width ratio) of the order of 200. These are major plasma processing challenges.
One example in this area of forward-looking research needs involves post-silicon materials. Many post-silicon materials will consist only of a single atomic layer (e.g., 2D materials) or require atomic level precision in their processing. It is clear that plasma etching and deposition of post-silicon materials presents challenges much different than those encountered for silicon. These challenges in turn pose
new plasma science questions, including unprecedented control of ion energies (<10 eV) near the chemical sputtering threshold. Precursor gases for post-silicon materials processing are much more complex than in silicon processing. Properties of atomically thin post-silicon materials will be crucially affected by defects created by the interactions of plasma ions, radicals, and photons. Plasma-surface interaction control will be paramount for defect-free plasma processing of post-silicon materials. Achieving atomic-scale control over surfaces and structures for multiple new materials and devices requires a significant increase in understanding of plasma-surface interaction mechanisms and the development of advanced diagnostics and predictive modeling.
Challenge 3: Electrification of the chemical industry based on renewable electricity to enable a sustainable society.
LTP science could be crucially important to enable the vision of a future based on renewable electricity. In this vision, electrification of the chemical industry utilizing plasma will convert electricity into chemical transformation in an environmentally friendly way. The key enabling science will be controlling the flow of energy through LTPs to produce predictable chemical transformations in gases, on solids, and in liquids. Liquid phase electrochemistry and photochemistry—both utilizing catalysis—are generally thought to be the only ways to directly convert electricity into chemical transformation. However, plasma acts as gas-phase electrochemistry, a capability that has been greatly underutilized. Renewable energy sources such as solar, wind, water and hopefully fusion energy will continue to lead to a more abundant availability of cheap electricity that will gradually lead to the electrification of the chemical industry.
In addition to energy costs, the other major challenge with using plasma for large scale chemical conversion is lack of selectivity. Plasma tends to create a wide range of different products with a limited ability to control which are created. One obvious strategy to improve plasma chemical selectivity is to combine plasma with catalysis, a topic addressed in the example in Figure 5.15.
The trend toward chemical industry electrification will coincide with a gradual replacement of chemical feedstocks that originate from fossil fuels to feedstocks produced from renewably sourced raw materials such as biomaterials. The challenge to developing this potential application of LTP is to better understand and control the flow of energy from the electrical power supply to the catalytic surface through key intermediate chemical species. This challenge will require detailed studies to enhance our current understanding of kinetic and collisional processes, plasma self-organization, transport and plasma-surface interactions under highly nonequilibrium conditions. Only an improved fundamental understanding of plasma-chemical species-surface dynamics will make this possible.
Challenge 4: Enabling space exploration and safeguarding communication infrastructure.
LTP physics research is crucially important to the future of propulsion used for electric rockets, for in-space propulsion on communications satellites, for NASA science missions, and for the development of advanced electric propulsion (EP) systems that are emerging as a critically needed component of human space flight to the moon and Mars. Industry projections suggest that half of all commercial spacecraft in the next 10 years will have EP onboard. Plasma-based thrusters are highly efficient and can reduce propellant fuel requirements by one to two orders of magnitude over chemical propulsion. This is critically important for commercial applications, and will likely be enabling for human exploration missions that require the transport of large amounts of cargo and even people through space. Fundamental plasma physics investigations on electric thrusters are essential to improve the thruster performance and increase the lifetime. An example of key plasma science challenges linked to Hall thrusters is illustrated in Figure 5.16.
Due to the breadth of applications enabled by LTPs, there is a diverse international community of researchers. To assess the state of international research, this committee reached out to research leaders in LTP science around the world to solicit their input on their own programs and to provide their opinions on the state of the U.S. LTP program. Since this is not a fully comprehensive survey, the committee has limited the reporting of specific numerical data. Nonetheless, the committee believe that there is a level of consistency among the responses that allows us to make reasonably informed statements about the state of international LTP research.
Distribution of Worldwide Effort in Low-Temperature Plasmas
Much of worldwide research and technology development in LTP is concentrated in Europe and Asia. From Europe, the committee received input on the status of LTP research in Germany, Belgium, France and the Netherlands. From Asia, the committee received input from South Korea, Japan, and China. Among the white papers submitted to this study, the authorship included researchers from France, Germany, India, Italy, Mexico, Slovakia, Spain, United Kingdom, and Ukraine. Research topics mentioned by these researchers match many of the topics that are being pursued in the United States. In those nations with reasonably mature LTP research activities, governments generally provide support for both translational research activities and to support new initiatives, for example, the recent expansion
into plasma agriculture. A key difference between the United States and several other countries is that LTP research in U.S. institutions is often based on single-PI activities. By contrast, countries such as Germany, the Netherlands, France and China have clusters of faculty or tenured researchers forming research teams and groups.
Self-Assessment of International Research Activities
In Europe, the LTP community is very active, but the level of activity varies among the different countries. Across Europe, the total number of researchers involved in LTP research activities seems to have remained relatively constant in the last 10 years with fluctuations across the different countries: Germany reports relatively stable numbers, while the Netherlands is experiencing a slight increase, but France and Belgium are possibly experiencing a slight decrease. According to our data sources, funding is generally provided via two mechanisms—by national governments and the European Union—but the majority of the funding generally is at the national level. Because of differences in how funding is provided, it is difficult to assess precisely how much research funding is devoted to LTP research. Estimates from European colleagues suggest tens of millions of Euros per year are being provided to support research in LTP active countries. On a per-capita basis, this would appear to be substantially larger than the funding provided by U.S. funding agencies to support LTP research. It should be noted that some countries—notably Germany, France, and the Netherlands—have active programs from government-based Ministries that promote “translational” activities to bring technologies from universities to industry. Many of these are in the form of center-like activities that include European training and networking grants with multiyear programs that advance a particular topic and stimulate multi-institutional collaborations. Regardless of the mechanism, in Europe, it is reported that there has been a substantial conversion of LTP research into new companies.
In Asia, the state of LTP research is more complex. The major programs in this area are based in South Korea and Japan, but China is emerging rapidly as a significant competitor, as are developing programs in Singapore and Taiwan. State support of LTP research varies widely among these locations. In South Korea and in Japan, where the application of LTP technologies is a large and significant industrial driver, both nations provide active support for LTP research, reportedly ~$10 million per year. In both of these countries, state-sponsored funding is available to support translational activities that bring university and national laboratory ideas to the marketplace. Although much more difficult to quantify, China heavily invests in translational LTP research.
In comparison to overseas investments, U.S. industrial leaders specifically cite the Chinese National Guideline for the Development and Promotion of the IC
Industry. The goal of this program is to develop Chinese capabilities in wafer equipment processing to support China’s burgeoning IC device manufacturing industry, and large component of which is plasma equipment. There is nothing analogous in the United States. China is not alone in support of this critical industry. In South Korea, the government funds major university research partnerships with large companies such as Samsung and SK Hynix, both major IC manufacturers. Other governments around the world have clearly recognized the need to fund enabling and breakthrough technology for plasma-based wafer equipment processing. However, the United States significantly lags in this regard.
U.S. Research Activities in LTP in the Context of the World Program
Data shows that over the last decade, the U.S. publication rate (as a percentage of first authorship) has generally remained stable around 25 percent in a selection of journals publishing LTP research, including the Journal of Applied Physics, Plasma Sources Science and Technology, and Journal of Physics D: Applied Physics. Nonetheless, the submission of papers from Asia and particularly China is growing and although no exact data is made available, Chinese contributions dominate the LTP section of Physics of Plasmas.
Europe, Japan, and South Korea are already making significant investments at the national level to advance their LTP industries. China and India are poised to advance their own LTP research and industries. In areas, such as plasma medicine, the United States has fallen behind its competitors in both basic and applied studies due to a lack of substantial and stable funding. This situation is in stark contrast to Germany, The Netherlands, France, Belgium, Italy, Japan, and South Korea, who have had and continue to have major initiatives in these areas. In newly emergent areas, such as plasma agriculture, there are already substantial international investments and the United States is in danger of falling behind in this area as well.
Importance of the United States to Current International Collaborations
Many informal collaborations between U.S.-based researchers and their European and Asian colleagues exist. This is reflected in the significant number of co-authored peer reviewed articles of U.S. researchers with European or Asian colleagues. However, these are often one-on-one collaborations and, in many cases, no structural (i.e., agency-related) funding is available. This is in contrast to the highly collaborative international research facilitated by the EU, as well as several formal 1-on-1 international collaborations (e.g., Russia-France, Japan-Australia). Many leading research groups in the United States often host international scholars from both Asia and Europe as visiting researchers however even those visits are now
being limited by visa restrictions and universities imposing fees to cover federally mandated security investigations. The U.S. LTP community has benefited considerably from the existing, limited, degree of international collaboration. However, the U.S. LTP community could greatly benefit from increased interactions with their European and Asian counterparts.
Given that LTP science is closely linked to applications, many potential international collaborations are restricted by ITAR (International Traffic in Arms Regulations) considerations. This is particularly the case in electric propulsion. The committee respects the need for national security oversight of international collaborations. However, a review and update of ITAR classifications would benefit international collaborations and also benefit U.S. science.
The last decade of research in LTPs has seen significant technological advances in the various ways in which LTP is produced. At the start of the decade, the majority of the research and applications were performed in low-pressure vacuum systems in highly controlled environments and most applications at atmospheric pressure involved high temperatures (e.g., arcs). This has rapidly evolved over the last decade with the development of nonequilibrium atmospheric pressure plasma sources that aim to produce large volumes of plasma without a vacuum chamber. A further development has seen the shift of research in LTP processing from semiconductor materials to “soft” materials such as liquids, biomaterials, and even agricultural products. This evolution in the operating conditions has necessitated the development of a new generation of complex diagnostics that require unprecedented time and spatial resolution to resolve exceptionally large gradients and changes in plasma properties.
The vast majority of laboratory facilities for the study of low pressure to atmospheric pressure LTPs are in the tabletop scale of devices—often surrounded by a suite of advanced diagnostic tools. The major investments for a typical LTP laboratory are often complex power supplies including nanosecond pulsed power capabilities and modulated multifrequency RF plasma sources, coupled with various probe, optical, surface and mass spectrometry-based diagnostics. Within the United States, each laboratory is effectively an independent facility that is generally responsible for its own development of plasma sources and plasma diagnostic tools. While the last decade has seen some sharing of resources among researchers, the operation of individual laboratory facilities remains the dominant research model. This model provides an excellent training ground for students and seamlessly integrates with the LTP practices in industry.
While LTP facilities are focused on small-scale experiments, the facilities needed for LTPs applied to space systems can be very different. LTPs for in-space propulsion
requires facilities on the ground that simulate the space environment in with the electric propulsion (EP) device will actually operate in. These facilities have vacuum vessels capable of reaching low enough pressures to be relevant to space research and large enough to contain the EP device and its plasma plume, while also having the appropriate diagnostics. These facilities are scattered throughout the United States at NASA and Air Force facilities, universities, and commercial companies, and internationally at locations in Europe, Asia and Russia. Unfortunately, these large facilities are a barrier to entry to the EP due to the high capital and operating costs. This has driven many companies and universities to investigate smaller thrusters with smaller facility needs. Investments over many years to install and upgrade test facilities for higher pumping speeds and higher power levels are typically made to achieve the desired capabilities. As EP power levels increase, especially to support human exploration missions, large pumping and power upgrades, and new larger facilities, will need to be brought online.
Needs and Opportunities
Most of the U.S. LTP research community does not utilize the kind of centralized facilities that are more common in other areas of plasma physics—facilities that are few in number, high in cost, and take years to plan and build. LTP facilities are much more commonly associated with individual PIs, typically in universities, but also at a few National Laboratories (e.g., Sandia National Laboratory in Albuquerque (SNLA) and Princeton Plasma Physics Laboratory [PPPL]). There has been some discussion within the U.S. LTP community of establishing a federally funded user facility with plasma-, surface- and materials-related instrumentation. By coupling plasma and plasma-surface interaction investigations, an opportunity for new fundamental knowledge would be created. One challenge is that the widely different applications of LTP require significant differences in facilities, instruments, and associated expertise in personnel. For example, low pressure plasma studies suitable for semiconductor-related studies would be quite different from atmospheric pressure plasma interactions with biological cells in liquid medium. Nevertheless, this potentially attractive option should receive further examination and discussion. At the time of writing this report, the DOE Office of Fusion Energy Sciences established two modest LTP user facilities at SNLA and PPPL to pilot the concept and gauge use response. The results of these pilot operations will be used in future facility funding decisions.
There appear to be no significant, federally funded international collaborations at present involving U.S. LTP researchers.
LTP science and technology has been of significant industrial importance for more than 150 years. In the section, the committee focus on the role of LTPS in the manufacture of integrated circuits (ICs) and semiconductor devices. This focus is an example to highlight the LTP relation to industry, with the lessons and outcomes being relevant to other industries as well.
The IC industry plays a key role in the U.S. economy and national security as described above. Advanced ICs are essential for defense systems, computers and phones. Cloud computing is growing. Applications in artificial intelligence (AI) are growing rapidly, especially for autonomous vehicles, web commerce sites and numerous commercial and industrial applications. These future technologies will depend on ICs and the technology to manufacture them. U.S. plasma technology industry leaders express concern regarding federal U.S. research funding relevant plasma processes for semiconductors. Many federal initiatives include DoD’s quantum computing, DARPA’s electronics resurgence, and DOE’s Exascale computing project, with a budget of $4 billion spread over 5 years, critically depend on ICs without there being commensurate investments in the plasma processing required to produce those ICs.
Future IC device technologies will depend more than ever on plasma-based processing. The IC industry needs advances in LTP science to meet their diverse and extreme challenges, demanding a deeper understanding of the physics, chemistry and materials-modifying characteristics of LTP used in IC manufacturing as described above.
The opportunities and benefits of a closer connection between the LTP research community and industry are clear. By investigating in the complex underlying plasma science and establishing basic scientific principles, these insights and associated experimental and modeling tools can be translated into industrial practice.
In LTP applications to IC manufacture, one possible suggested strategy is to establish technology incubators to fund collaborative ventures comprising academia, start-ups, and an established company to enable translational research and eventual commercialization of innovations. Such a program would focus on applications of LTP to semiconductors with a goal of developing breakthroughs on a 5-10 year timescale. The federal funding would focus on fundamental research and disruptive breakthroughs that can be transferred quickly to industry. The established companies would support the more translational and applied areas of research. In this vision, research in commercial applications of LTP will encourage U.S. leadership in this crucially important LTP enabled technology. Strengthening university-based research in this area by sponsoring graduate students will simultaneously contribute to the development of the highly skilled workforce needed for that industry as well.
The science of plasma induced chemically reactive environments in gases, on surfaces and in liquids has already made society-wide transformations in our quality of life—from lighting, materials synthesis and water purification, to enabling the information technology revolution through plasma-enabled fabrication. The strong track record of LTP research impacting a broad range of applications demonstrates exceptional interdisciplinary and translational success. The research performed in the LTP field overlaps with physics, chemistry, propulsion, energy and materials, with recent extension to biology, agriculture and medicine. U.S. funding agencies have not embraced this trend, which has led to a loss of U.S. leadership in some specific areas of LTP. For example, U.S. researchers led the field of plasma medicine at its birth more than a decade ago. However, in the last few years, the U.S. leading position has been overtaken by researchers in Europe and Asia. A similar situation is now occurring in plasma chemical conversion and plasma catalysis.
The extremely successful Low Temperature Plasma Science Center program supported by the DOE Office of Fusion Energy Science has had a tremendous impact.
Finding: The success of the DOE Low Temperature Plasma Science Center program underlines that there is a need to sustain LTP research directions for a sufficient period of time (5 to 10 years) and size with support on the level of ~$2 million per year to enable scientific impact and translation of research into society benefiting applications.
Finding: The increasing scope of the LTP field into new materials and biotechnology requires full participation of researchers that are traditionally funded by different agencies not focused on plasma science. This is particularly the case for electrification of the chemical industry.
Finding: U.S. funding agencies are often ill prepared to support initiatives that overlap multiple agencies and should actively pursue synergistic opportunities between agencies to maintain U.S. leadership in LTP in line with the recommendation in Chapter 1.
Recommendation: DOE-FES should establish and coordinate a multiagency Low Temperature Plasma Science Center Program to support multidisciplinary research teams and to establish the scientific basis of emerging application areas of low-temperature plasma science.
Finding: Based on the funding level for the LTP science center program, a possible minimum level of support of $20 million over 5 to 10 years for each topical initiative would be appropriate.
An example of a first initiative could be LTPs aimed at the electrification of the chemical industry and associated sustainability initiatives overlapping with research priorities of ONR, ARPA-E, DOE and NSF.
Finding: Advances in plasma-materials processing are challenged by the need to choose operating and plasma device designs from the enormous set of possible operating and design conditions.
Finding: There is a serious need for a more detailed understanding of the fundamental processes underpinning plasma-surface interactions that will enable us to develop predictive capabilities.
Finding: Advances in our understanding of LTP interactions with materials will enable the control of plasma-surface interactions at the atomic level which in turn will enable the next generation of materials for quantum computing, new communication and sensor technologies, and energy storage and harvesting.
Recommendation: DOE-FES and DOE-BES should develop a synergistic collaborative program to focus on the intersection of plasma and materials.
In addition to FES and BES, initiatives could be coordinated and funded between plasma-focused and materials-focused programs in federal agencies that would lead to advances in the science and technology of both fields.
Industry support could also be leveraged to stimulate fundamental research through public-private partnerships—for example, with the semiconductor industry. This public-private partnership could take the form of the federal government supporting more fundamental interdisciplinary research and industry co-funding more translational research.
Finding: The fundamental research performed in LTP is intrinsically interdisciplinary with societal benefits occurring most rapidly when that fundamental research is guided by applications.
Finding: Although the NSF/DOE Partnership in Basic Plasma Science and Engineering is a strong supporter of LTP research, the translational and convergent nature of LTP research often transcends the scope of the NSF/DOE partnership.
Finding: Support for translational and convergent research in LTP by the NSF Engineering directorate has not been consistent, has not been long term and has not kept pace with the opportunities described in this report. Deliberate actions are needed to empower these interdisciplinary opportunities.
An additional level of support of $3 million per year for fundamental research in LTP, similar to the current support from the NSF/DOE partnership, coupled with a similar budget from the application areas, would enable collaborations between, for example, biologists and plasma physicist. These collaborations would produce convergent research and translational research opportunities.
Recommendation: NSF-MPS and NSF-ENG, funded at a level of $6 million per year, should establish interdisciplinary and inter-directorate support for emerging low-temperature plasma science topics that lead to translational research.
The rationale for this funding level is to enable a critical number of projects focusing on a breadth of application areas, while including researchers from the allied sciences who are critical to translational success.
The establishment of the Low Temperature Plasma Science Center program at DOE since the 2010 Decadal study has benefited fundamental LTP research.
Finding: Continuing initiatives like the DOE Low Temperature Plasma Science Center program will help sustain an internationally competitive LTP community in the United States.
Finding: LTP research at U.S. universities remains highly dispersed and it is not uncommon to find only one faculty member involved in LTP research in an entire university. This situation underlines the need for research networks and student training opportunities.
One model is provided by the European Union. The EU has invested heavily in research networks (e.g., COST Action programs) and collaborative research training programs (e.g., Marie-Skłodowska-Curie Innovative Training Network). Enhanced support to enable mobility of promising early-career researchers between different laboratories in the United States and abroad would be extremely valuable for the field. These initiatives initiate and encourage large-scale collaborative efforts and develop and strengthen emerging research areas.
Finding: There are no multi-institutional, networking programs in the United States focused on LTPs. Training of a new generation of scientists in fundamental LTP science, including diagnostics and modeling, is a critical need for the coming 10 years, and would benefit from such programs.
Finding: Many U.S. PhD students working in the LTP field are trained with an exclusively application perspective. To sustain the field, more fundamental LTP science training opportunities for early career researchers are needed.
Taking the EU again as an example, the annual International Low Temperature Plasma Physics school has provided effective training for the next generation of PhD students in LTP science. There is no long-term support for such an initiative in the United States.
Recommendation: NSF should support low-temperature plasma research networks in the United States by providing funding for graduate students and postdoctoral researchers to participate in exchanges between U.S. universities, and for international research experiences for junior scientists.
Finding: Fundamental research in LTP has declined in the United States over the last decade.
There is a concern that if this trend continues foundational research in LTP will, with a few exceptions, disappear from the United States.
Finding: The demographics in the LTP field show that the leadership class will retire within the next decade with an insufficient number of early career faculty available to assume leadership positions.
LTP research is multidisciplinary and the model for hiring university faculty in multidisciplinary fields has produced too few early career faculty with a focus on the fundamentals of LTP science.
Finding: There are currently too few early career LTP-oriented faculty in the United States. The hiring of faculty within universities in the 21st century needs to be viewed from an interdisciplinary perspective that recognizes the intellectual diversity of a field that spans multiple colleges and departments and would benefit from investment by federal agencies.
Recommendation: To strengthen low-temperature plasma research at universities, NSF and DOE should establish specific programs that fund the creation of faculty positions similar to the NSF Faculty Development in Space Sciences program to address the urgency of losing key expertise and leadership in low-temperature plasma science over the next decade.
Finding: The vast majority of laboratories for the study of LTPs consist of tabletop-scale devices—often surrounded by a suite of diagnostic tools.
While the last decade has seen some sharing of resources among researchers and use of national facilities (e.g., synchrotron sources for VUV diagnostics),
the operation of individual laboratory facilities remains the dominant research model. Because of the cost and technical expertise associated with advanced LTP diagnostic development, there have been discussions about the need for a centralized set of diagnostic tools for the LTP community. That discussion is ongoing and is being put to the test with the recent establishment of two LTP user facilities.
Finding: Based on its established track record, it is clear that the LTP community does not generally require nor is there demand for large, single purpose centralized user facilities having a single plasma source to make societal impact.
Finding: Leveraging the flexibility and interdisciplinarity of individual laboratories should be considered a valuable asset of the LTP community, rather than comparing that style of research to large facilities in other areas of plasma science and declaring that mode of operation a weakness.
Finding: There is a need to support the flexibility and interdisciplinarity of individual LTP laboratories perhaps through a mix of user facilities concentrating on diagnostics and improving diagnostic and source capabilities in individual laboratories that could form a distributed user facility.
Recommendation: NSF and DOE should expand opportunities to develop and acquire diagnostics, plasma sources, numerical models, and reaction mechanisms in support of low-temperature plasmas science, perhaps through the NSF/DOE Partnership in Basic Plasma Science.
This effort could be configured similarly to LasernetUS, with support for both equipment and use of the diagnostics, sources, numerical codes and reaction mechanisms.