Plasma Science Enabling Technology, Sustainability, Security, and Exploration (2021) / Chapter Skim
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2 The Foundations of Plasma Science
2 The Foundations of Plasma Science
Pages 54-109
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From page 54... ...
Many of these plasmas are permeated by magnetic fields, which add further richness and complexity to the underlying dynamics. Despite the great diversity of plasmas, there are underlying unifying phenomena.
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From page 55... ...
Computational plasma physics, which 1 D.E. Stokes,1997, Pasteur's Quadrant – Basic Science and Technological Innovation, Brookings Institution Press.
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From page 56... ...
Fundamental plasma processes such as magnetic reconnection, shocks, turbu lence, and the dynamo effect control the conversion of energy from one form of to another -- from magnetic energy to kinetic energy, from flow energy to magnetic
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From page 57... ...
Examples of self-organization in coordinate space (3D) include the dynamo effect, whereby large-scale and slowly varying magnetic fields emerge from magnetic and velocity turbulent fluctuations on much smaller spatial scales and much shorter time scales.
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Understanding the fundamental processing of plasmas sustained in or crossing multiple phases advances fundamental plasma science and is at the heart of translational research leading to technologies. COMPUTATIONAL PLASMA PHYSICS Computations have become as essential to plasma physics as experiments.
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This extension of theory then seamlessly feeds the translational research that produces society-benefiting outcomes, through the use of these same computer models to, for example, optimize magnetic configurations for fusion reactors, chart coronal mass ejections from the Sun to Earth, or design plasma materials processing reactors. Experiments are the ultimate test of theory and computer simulations.
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From page 60... ...
a step in identifying the appropriate, approximate equations that can be represented with fidelity on existing computer hardware and that will result in calculations that can be completed in a reasonable time. A kinetic approach is used when the dynamics involves detailed changes of the particle dis tribution function in a 6-dimensional phase space.
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From page 61... ...
The electric field strength is shown by the color contours, near zero where it is dark blue, then increasing, shown in red, as the plasma sheath is entered near the wafer. The ions are shown as gray particles, with darker gray indicating a greater speed in the downward vertical (impacting direction)
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From page 62... ...
When the magnetic field in the plasma is strong, one can use the gyrokinetic approach, which assumes that the time scale of variations is long compared with the cyclotron period for charged particles orbiting around magnetic field lines, and so only the time average motion of the particle is tracked. In the case of strong magnetic fields and low-frequency dynamics one can use magnetohydrodynamics (MHD)
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Code Availability In plasma physics, state-of-the-art computation is available to a relatively small group of researchers. This is perhaps best illustrated by way of contrast.
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Computation: Training the Workforce Computational plasma education needs to cover the many methods used in plasma physics -- fluid (in particular, MHD or multifluid) approaches and kinetic approaches (both continuum and PIC)
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How can kinetic effects be represented in global simulations of large systems based on extended fluid equations (such as in the vicinity of black holes, neutron stars and planetary magnetospheres, or in fusion plasmas)
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From page 66... ...
. While the ECP has been very successful and has been strongly endorsed by the 2018 National Academies of Sciences, Engineering, and Medicine report Strategic Plan for Burning Plasma Research,2 the coverage of ECP does not include the broad reach of PSE disciplines.
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With quantum computing on the horizon, device-based computing becomes a more important strategic discussion. How can such computing devices be used in computational plasma physics?
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MAGNETIC RECONNECTION: TAPPING THE ENERGY OF MAGNETIC FIELDS Magnetic reconnection is a fundamental process whereby magnetic fields re configure topologically (sometimes viewed as "breaking" and "reconnecting") and in the process release energy.
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Achievements of the Last Decade in Magnetic Reconnection During the last decade, thanks to space missions, laboratory experiments, analyt ical theory and sophisticated computer simulations, our understanding of magnetic connection has advanced greatly. These accomplishments include the following: • An improved theoretical understanding of the plasmoid instability (an insta bility of thin current sheets)
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From page 70... ...
have provided new data for testing the predictions of theory and simulations of magnetic reconnection, often co-existing with shocks and turbulence. These data have been extended to include magne tized plasmas, having self-generated Biermann magnetic fields as well as externally imposed magnetic fields.
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From page 71... ...
Challenge 1: Understand and predict plasma behavior under extreme conditions that challenge our present models. While our theoretical understanding of reconnection in collisional nonrelativistic laminar plasmas is reasonably mature, there are significant gaps in our understanding of reconnection in weakly collisional and collisionless plasmas, both nonrelativistic and relativistic.
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From page 72... ...
(A plasmoid is a structure that looks like a cat's eyes within which magnetic fields lie on nested surfaces, as seen in Figure 2.3.) Key questions include how the number and size of plasmoids scale with the system-size and plasma parameters, and how the reconnection process responds to turbulent fluctuations which span an enormous range of spatial scales.
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From page 73... ...
Turbulence, often driven by unstable waves, is ubiquitous in plasmas and is widely cited as the dominant mechanism for heating and transport of particles, energy and momen tum in many settings. Instabilities, waves and turbulence also play a role in the plasma dynamo, processes by which magnetic fields are generated and amplified both on small and large spatial scales with lifetimes that can vary widely depending on the plasma medium.
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"Small-scale" dynamos amplify magnetic energy but produce negligible magnetic flux because the averaging of fluctuations over space and time leads to near-perfect cancellations. "Large-scale" dynamos produce large-scale magnetic field structures with nonzero magnetic flux.
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The plasma beta (the dimensionless ratio of the plasma pressure to the magnetic energy density) in space and astrophysical plasmas is often close to or greater than unity, which distin guishes these plasmas from plasmas in the Sun's atmosphere or laboratory that are confined by magnetic fields and typically have small beta values (including fusion plasmas)
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From page 76... ...
Salem, and D Sundkvist, Magnetic fluctuation power near proton temperature anisotropy instability thresholds in the solar wind, Physics Review Letters 103:211101, 2009, https://doi.org/10.1103/PhysRevLett.103.211101; copyright 2009 by the American Physical Society.
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From page 77... ...
This is the realm of high energy density plasma physics. This capability has led to the study of astrophysical processes in scaled laboratory experi ments, including magnetic reconnection, collisionless magnetized shocks, Weibel-mediated shocks, the generation of magnetic seeds at shocks by the Biermann effect, and the small-scale turbulent dynamo.
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From page 78... ...
is not the domi nant source of energy, it does influence the transport properties of the plasma by imparting directionality and new degrees of freedom, thereby influencing the large-scale dynamics. The magnetic field-induced anisotropy introduces a fundamental difference between the dynamics of magnetized plasmas and col lisional plasmas or those that are unmagnetized.
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From page 79... ...
Even for a plasma that satisfies the MHD approximation, the large-scale dy namo problem remains unsolved -- what are the mechanisms whereby magnetic fields erupt and decay? Until recently, it was generally accepted that small-scale fluctuations in the plasma can lead to catastrophic quenching of the growth of large-scale magnetic fields.
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How can we control or mitigate disruptions in magnetic fusion plasmas, which are examples of self-organization? DUSTY PLASMAS: FROM COMETS TO FUSION REACTORS In a dusty plasma (sometimes referred to as a "complex plasma")
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From page 81... ...
The large size of the dust particles enables them to be tracked and illuminated with lasers, providing novel tools to study and diagnose strongly coupled plasma dynamics. Dusty plasmas have been used to experimentally and theoretically investigate fundamental properties of soft condensed matter such as the phase transition from the liquid to the solid state, defect formation, and melting produced by waves and instabilities.
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From page 82... ...
Rosenberg, 2015, The magnetized dusty plasma experiment (MDPX) , Journal of Plasma Physics 81:345810206.
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With gravity often a dominant force in laboratory dusty plasmas, moving to microgravity environments is a method to study the smaller scale, inter-particle forces. In another regime, very large magnetic fields (or very small particle sizes)
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Dusty plasmas can be "tuned" via the Coulomb coupling parameter to exhibit self-organized behavior ranging from solid-like to gas-like, while the particles re main suspended in the plasma. In the presence of a magnetic field, this ordering can
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While progress has been made in providing plausible theoretical models for void formation, a comprehensive theoretical framework for predicting self-organized structure formation in dusty plasmas remains elusive.
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Dusty plasmas are the embodiment of a plasma system that defines the chal lenge of controlling plasma surface interactions. Dust particles are charged solid matter that are embedded in a plasma environment.
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cameras. Two areas in which larger collaborative teams have advanced dusty plasma research are the study of dusty plasmas under microgravity conditions and in magnetized plasmas.
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Non-neutral plasmas have useful properties not shared by neutral or quasi neutral plasmas. One important advantage is the existence in non-neutral plasmas of a confined thermal equilibrium state using only static electric and magnetic fields (the "Penning-Malmberg trap" configuration)
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From page 89... ...
The combination of strong magnetic fields (required in Penning-Malmberg trap confinement) and cryogenic temperatures produces novel strongly magnetized and quantum plasma states with nonideal Γ-factors that can be similar to those in the environment of highly magnetized neutron stars and white dwarfs.
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From page 90... ...
These non-neutral plasma crystals are among the most promising technologies for quantum computation and quantum simulation. More generally speaking, the research in this area of fundamental plasma physics has strong interdisciplinary connections, contributing to and borrowing from the wider world of plasma physics, atomic physics, fluid dynamics, astro physics, soft condensed matter physics, and statistical physics.
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From page 91... ...
Recent studies have focused on vortices driven by external time-dependent shear flows, characterizing several novel effects including vortex stripping. • Neoclassical transport in magnetized plasmas is a form of cross-magnetic field transport of particles, momentum, and energy that is enhanced by symmetry-breaking magnetic and electric field "errors." Such errors are inherent in many plasma experiments.
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(This magnetization regime is difficult to achieve in "hot" neutral plas mas, requiring ultra-large magnetic fields exceeding 109-1010 Gauss.) Being able to sustain non-neutral plasmas under these extreme conditions poses science chal lenges: (a)
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For plasmas where this is not the case (i.e., plasmas in strong magnetic fields) , the classical "Braginskii" coefficients (with transport coefficients to account for the effects of magnetic fields)
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Driscoll, 1995, Relaxation of 2D turbulence to vortex crystals, Physical Review Letters 75:3277, copyright 1995 by the American Physical Society. Bottom: Courtesy of the Magnetized Plasma Research Laboratory, Auburn University.
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This is another example of where fundamental studies of non-neutral plasmas have impact well beyond the laboratory and, in this case, to interstellar astronomy. PLASMA INTERACTIONS WITH LIQUIDS, SOLIDS, AND GASES Fundamental plasma research is often focused on the physics of waves and instabilities, which are properties of the bulk plasma far from boundaries.
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From page 96... ...
A significant part of the plasma physics community is concerned with study ing collective phenomena in plasmas generated at very low pressure and in noble gases to reduce or even eliminate collisional effects. Nonetheless in many plasmas of interest, both atomic and molecular gases are present at significant densities or intentionally introduced.
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From page 97... ...
Variations in gas composition and densities can also have a strong effect on plasma dynamics and properties. An example of the use of changes in gas com position to control plasma generation is the so-called cold atmospheric pressure plasma jet, shown in Figure 2.14.
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From page 98... ...
In the last decade an extreme type of multiphase plasmas has emerged, which can be seen as an equivalent of dusty plasmas, though with the dust replaced by liquid droplets or aerosols having sizes of a few microns to a millimeter. The "plasma-aerosol," a dynamic suspension of liquid droplets dispersed in a gas, encompasses a wide range of scenarios that can involve single microscopic droplets up to full sprays and jets while the plasmas themselves vary from nonequilibrium low temperature to thermal plasmas.
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From page 99... ...
• The last 10 years have seen a move to more advanced control of power de livery and plasma kinetics leading to control of plasma-surface interactions at length scales of a single atom, an example being atomic layer etching. This process has been enabled by an increased knowledge of the underpinning plasma processes by a combination of modeling and diagnostics.
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From page 100... ...
While these diverse funding sources are in some respects a strength of the field, it has unfortunately led to a silo effect between studies focusing on plasma physics and on synthesis/material processing from a materials perspective. A key challenge in the next decade is to bring together experts in plasma science, materials research and chemistry to tackle the major science challenges highlighted below.
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From page 101... ...
Another example of such a new scientific frontier are atmospheric pressure mi croplasmas that can lead to relatively high power density LTPs having microscopic dimensions. In some cases, LTP plasma densities can approach values typical of fusion plasmas (1014-1017 cm-3)
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From page 102... ...
Challenge 4: Control and predict the interactions between plasmas and solids, liquids, and neutral gases. While in situ diagnostics for surface characterization during plasma exposure are emerging, a majority of material characterization to date is based on ex situ diagnostics where the material is analyzed after plasma exposure.
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From page 103... ...
Liquids provide additional challenges over solids as they can have a much more pronounced influence on the plasma state. In many models of bounded plasmas, the surface processes are either neglected or treated using phenomenological parameters such as sticking coefficients, or sput tering rates and secondary electron emission coefficients given by simple theories.
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Integrated modeling of the entire plasma solid/liquid interface is needed and is a major challenge for the plasma community. FINDINGS AND RECOMMENDATIONS The science challenges in basic plasma physics are immense and the opportuni ties to translate advances in fundamental plasma science to develop applications are strategic.
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However, basic plasma science investigations are often perceived as being separate from application-inspired research and are often funded separately. As a result, it is increasingly difficult for fundamental studies and applications to lever age each other's efforts.
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From page 106... ...
are needed. Recommendation: NSF, DOE, NASA, and other federal agencies with an in terest and programs in plasma physics should provide regular opportunities for the continued development, upgrading, and operations of experimental facilities for basic plasma science at a spectrum of scales.
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From page 107... ...
Finding: A network of basic plasma user facilities that would provide oppor tunities for access to new and upgraded plasma science facilities needs more coordination and support than currently exists. Recommendation: Federal agencies, particularly DOE-FES and NSF-MPS, should implement a program for one-time, short-term funding for users of basic plasma science facilities.
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From page 108... ...
Finding: With the rapid growth of interdisciplinary research in plasma physics, it is time to consider the establishment of an annual journal that reviews major developments in all areas of plasma physics, much like the Annual Reviews of Astronomy, for example.
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From page 109... ...
T h e F o u n dat i o n s of Plasma Science 109 Recommendation: Computational plasma science and engineering, sup ported by NSF, should include projects for writing textbooks and devel oping courses to train the current and next generation of computational plasma scientists, and to enable noncomputer experts to make optimal use of computations.
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