Plasma Science Enabling Technology, Sustainability, Security, and Exploration (2021) / Chapter Skim
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3 Laser-Plasma Interactions: Compact Particle Accelerators, New Optics, and Brilliant X-Ray Sources
3 Laser-Plasma Interactions: Compact Particle Accelerators, New Optics, and Brilliant X-Ray Sources
Pages 110-160
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This realm corresponds to the physics of the largest electric fields ever produced by humankind. This is the field of laser-plasma interactions (LPIs)
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In the last 10 years, new methods of controlling sequences of FIGURE 3.1 Transferring energy between laser beams. Recent advances in plasma optics have significantly improved laser beam performance.
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However, the interaction of a laser pulse or particle beam with a plasma can generate longitudinal electric fields, so called wakefields, for which the generated electric field is oriented along the direction of propagation of the particle. The accelerated charged particles can move with the accelerating field for long distances, and consequently be accelerated to very high energies.
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Bottom: Courtesy of Remi Lehe, Lawrence Berkeley National Laboratory; available at https://upload.wikimedia.org/wikipedia/ commons/5/5c/Fjordn_surface_wave_boat.jpg, accessed August 6, 2020. beams with plasmas through shaping the laser fields and plasma profiles to efficiently generate ultra-bright, high energy charged particle beams?
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In the last 10 years, high gradient acceleration of ions by the sheath electric fields has been refined and new physics regimes have been developed. Radiation pressure and magnetic vorti ces have been investigated to provide high performance compact ion beams, which may be useful in medical therapy and for electric and magnetic field probes in high energy density (HED)
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,2 Frontiers in High Energy Density Physics: The X-Games of Contemporary Science (2003) ,3 and Workshop on Opportunities, Challenges, and Best Practices for Basic Plasma Science User Facilities (2019)
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In the following sections, we discuss accomplishments, opportunities and challenges in the application areas of plasma optics, plasma acceleration of light particles, plasma acceleration of heavy particles (ions) , bright X-ray generation and
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PLASMA OPTICS Plasma optical techniques have been developed in the last decade that can control and improve LPI performance in regimes beyond those accessible by conventional means. Plasma-based optical components, already consisting of ionized
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Plasma optical components include plasma gratings; plasma waveguides that combat diffraction, extending the interaction length in LWFAs; and plasma mirrors, along with optical pulse shaping to enhance or suppress specific dynamics. These components can increase intensity contrast by orders of magnitude, allowing for impulsive laser-matter interactions nearly free of premature heating; and redirect laser pulses in multistage LWFAs without degrading electron beam emittance.
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Spatiotemporal shaping can produce laser pulses that appear to violate special relativity. The peak intensity of a self-accelerating light beam can follow a curved trajectory in space, while the peak intensity of a "flying focus" pulse can travel at an arbitrary velocity, surpassing even the vacuum speed of light (see Figure 3.4)
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laser pulses can impart angular mo mentum to a plasma. This transfer of momentum can modify the topology and dispersion of the plasma waves produced by the laser and the phase space of the charged particles they accelerate.
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Experiments using high repetition laser pulses aided by machine learning could identify time sequences and profiles of laser pulses that could tame the highly nonlinear kinetic and chaotic responses exhibited by plasmas driven strongly by laser beams over long periods of time. Such techniques could mitigate laser plasma instabilities by modulating the intensity to shrink individual hot spots and disperse hot spot patterns.
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Based on the promise shown by plasma accelerators, the DOE roadmap Advanced Accelerator Development Strategy Report10 was developed in 2016 that provides a plan for realizing plasma-based high-intensity X-ray photon sources and ultimately, a plasma-based high energy physics collider. A TeV-class electron-positron collider based on plasmas would potentially reduce the size of the machine from tens of kilometers to hundreds of meters, and with lower cost.
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Steinke, et al., 2019, Petawatt laser guiding and electron beam acceleration to 8 GeV in a laser-heated capillary discharge waveguide, Physical Review Letters 122:084801, copyright 2019 by the American Physical Society. Progress and Achievements Plasma-based accelerators are now able to regularly provide multi-GeV energy gain to mono-energetic beams of 10 pC to 1 nC in a single plasma accelerator.
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PWFAs used up to ~20 GeV electron drive beams with ~1 nC of charge in plasma sources roughly 1 meter in length and with plasma densities of ~1016 to 1017 cm−3. Several laboratories are now able to regularly accelerate 10-100 pC of charge to energies of 10-100 MeV in multi-millimeter-length, high-density gas jet plasma sources using laser pulses with a more modest peak power (10-100 TW)
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This structure enabled guiding of the wake-driving pulse over a longer distance, thereby increasing the total energy gain of the electron beam. The highest energy gain achieved for a low energy spread PWFA bunch was at the Facility for Advanced Accelerator Experimental Tests (FACET)
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Stable electron beams with mean energy of 120 ± 5 MeV, a 60 percent energy spread, charge of 33 ± 5 pC, divergence of 4 ± 0.3 mrad and pointing stability of 0.3 mrad were achieved over hours of run time, thousands of laser shots and over more than 10 days. To couple this electron beam to the decelerating stage, a short focal length electron lens was developed based on the discharge current in a plasma capillary enabling a compact setup and preventing degradation of the electron beam over long propagation distances.
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Overall, great advances have been made in the past decade in providing high energy gain with low energy spread for a significant level of charge in both laser-driven and particle beam-driven plasma accelerators. Significant strides have also been made in improving the quality and stability of the accelerated beams.
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electron beams. The outsized longitudinal electric fields in plasma accel erators can serve to achieve this goal through the process of controlled electron beam injection.
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This increases access for universities, to facilitate a more widely distributed base of fundamental research on LWFA, PWFA and PWFAs driven by LWFA electron beams. Sophisticated diagnostics have been developed to more directly probe the plasma accelerator itself as discussed in the diagnostics section.
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These would be electron beams with sufficient control and re producibility that they can be used in production-level applications, requiring elimination of instabilities and kilohertz-level repetition rates. Meeting these goals will require the mastery of controlled injection techniques, improved loading of the plasma wake to minimize beam energy spread, beam emittance preservation at the plasma-vacuum interface, and high energy transfer effi ciency.
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In the highly nonlinear blowout regime used in most high energy plasma accelerators, the plasma electrons are completely evacuated in a bubble-like region behind the wake driver. This leaves behind a column of positive ions that can provide transverse focusing to a negative electron beam due to the
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For example, a laser-ionized gas plasma source may be well suited for electron beam-driven PWFA purposes, whereas a helicon plasma source may be better for a proton beam-driven PDWFA. Continuous evolution of gas jet plasma sources for LWFA are likely to lead to improved injection control and beam extraction capability for lower energy beam TABLE 3.1 Primary Accelerator Laser Driver Parameter Ranges of Interest for Plasma Accelerators Ee EL TauL Rep Example/Science Applications ~20 MeV 5-20 mJ 4 fs kHz UED, keV Thomson medical .
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However, improving the repeatability, reliability, and repetition rate of plasma accelerators depends on the development of new laser technologies for increased precision, control and repetition rate. (See Table 3.1.)
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Advanced plasma accelerators and photon sources will enable advances in fundamental science as well as society benefiting applications, from compact TeV class high energy physics colliders to medical imaging technologies.
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have shown that a dramatic increase of proton energies can be achieved by increasing the laser pulse duration. A technology advance that has had a transformational impact on ion acceleration is the use of plasma mirrors (see section on plasma optics)
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This has had a tremendous impact since 2D simulations were unable to capture the ion energy gain by quasi-static plasma electric fields. 3D simulations made it possible to provide quantitative predictions and meaningful comparisons with experimental results.
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This will require a multifaceted approach that involves further developments in both laser technology and targets to fully exploit effects such as relativistic transparency, radiation pressure, and magnetic vortices. At the same time, these topics advance the fundamental plasma science of laserplasma coupling, heating, and acceleration.
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Broadband betatron X-ray emission results from the betatron oscillations of particles in the plasma and/or laser fields and is a diagnostic of the beam properties inside the accelerator. Nearly monoenergetic Compton, or Thomson, scattering of a laser beam from the par ticle beam produces tunable, narrow bandwidth X rays of higher energy and flux than betatron radiation, and can extract detailed beam evolution information for studying beam interactions with plasma waves.
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In these cases, plasma optics is an important ingredient. Control of these mechanisms and photon sources requires precision shaping of the laser pulses.
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Plasma acceleration based sources can enable: (a) precision high-performance X-ray and MeV photon sources in compact packages suitable for applications in nonproliferation, industry, security, medicine and other areas.
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produces keV broadband radiation sources with micron emission spot sizes that have been used to enable sensitive phase contrast imaging and ultrafast diagnostics of high-energy density experiments. These experiments demonstrate that the quality of X rays produced by plasma accelerators can in many cases exceed those obtained in conventional X-ray sources.
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Both of these capabilities reduce undesired radiation generation. The Thomson process can also provide details on electron beam evolution for studying beam interactions with plasma waves.
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Esarey, and W.P. Leemans, 2013, Electromagnetic cascade in high-energy electron, positron, and photon interactions with intense laser pulses, Physi cal Review A 87:062110, copyright 2013 by the American Physical Society.
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11 and Summary of Strong-Field QED Workshop (2019) .12 Electron beam collisions with high intensity laser pulses produce SF-QED ef fects at the lowest laser intensities.
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Scime, 2019, "Workshop Report: Workshop on Opportunities, Challenges, and Best Practices for Basic Plasma Science User Facilities," arXiv:1910.09084.
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electron beams, either from LWFA or conventional ac celerators. In this sense SF-QED has strong connection with laser and accelerator technologies.
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This is true in all subfields of plasma science and particularly challenging in LPI due to the small volumes and short durations of the laser- or particle beam-plasma beam interactions.
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of plasma acceleration, radiation gen eration, and the associated LPIs has been essential to the development of the field of LPI. Computations are synergistically supported by advances in theory, which enables the improved fundamental understanding required to develop the numeri cal algorithms.
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showing narrow electron beam spreads with accelerations to near GeV were accompanied by simulations performed with U.S.-developed codes. However, that dominance has steadily eroded.
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ENABLING TECHNOLOGY AND FACILITIES Progress in LPI, plasma optics, and plasma acceleration will continue to be driven by advances in laser technology. A healthy ecosystem of facilities, programs and capabilities, having diverse and complementary capabilities and instrument size, is required to enable advances in accelerator, photon source, nonlinear optics, high field, and ion sources.
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Chowdhury, A Galvanauskas, et al., 2019, Petawatt and exawatt class lasers worldwide, High Power Laser Science and Engineering 7:E54, reproduced with permission.
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laser intensities exceeding 1021 W/cm2 for high field physics and new wavelength regimes; and (3) kHz-class repetition rates for precision control and machine learning to drive performance, and for accelerator and photon source applications.
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LaserNetUS benefits the entire field of plasma science by providing broad access to state-of-the-art midscale laser facilities. The network also provides a platform for training the next generation workforce on short pulse, high intensity laser science and applications.
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Lasers with repetitions rates of kHz and higher with greater efficiency and control are required for LWFA. These lasers will enable stability and reproducibility of the accelerated electron beams with repetition rates of kHz through feedback control systems.
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The recent heavy investment in Europe and Asia in laser facilities and science advances has shifted the center of high-power laser development to those areas, with followon industrial benefits to areas such as shock peening for metallic surface hardening and laser machining. Future investment should combine opportunities in laser development, applications and plasma science, areas that synergistically feed on each other and that drive progress.
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Next steps in technology development include kHz repetition rates enabling stabilization and active feedback for precision shaping of the plasma state, and new regimes in driver intensity and wavelength. Ion Acceleration New mechanisms for ion acceleration are now accessible, including via radia tion pressure and magnetic vortices.
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Compact plasma accelerators, plasma X-ray sources, and plasma optical methods were in large part invented in the United States, and the United States has held a leadership position in most of the field in past decades. However, as reported in the National Academies study Opportunities in Intense Ultrafast Lasers: Reaching
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Facilities constructed through the strategy above would produce the tech nologically highest intensities to open up new regimes in high field physics and ion acceleration, having repetition rates at and beyond 1 kHz, with shaped pulses enabling precision control, and with active feedback and machine learning for ac celeration and plasma optics. Finding: Plasma acceleration and controlled laser-plasma optics are rapidly advancing, driven by newly available capabilities in short pulse/broad band width lasers.
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Very high repetition rate, precision-controlled lasers and plasma methods will need to be included as part of the stewardship program. Finding: Collaboration between agencies focused on source development (DOE, NSF)
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Recommendation: NSF-MPS, DOE-SC, and DOE-NNSA should strongly support research in the fundamental physics of plasma optics, high field acceleration, and laser sources in collaboration with other agencies. This includes research, centers, and midscale infrastructure.
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