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Plasma Science: From Fundamental Research to Technological Applications 5 Beams, Accelerators, and Coherent Radiation Sources INTRODUCTION AND BACKGROUND Consideration of the state and health of plasma science within the grouped disciplines of intense charged-particle beams, accelerators, and coherent radiation sources presents a picture perhaps representative of trends relevant to plasma science in general. Recent history suggests themes, several of which appear in common with other areas impacted directly or indirectly by plasma science. Basic and applied research have been supported indirectly within large Department of Defense (DOD) weapons-driven and DOE energy-driven application programs, such as the directed-energy weapons, nuclear weapon effects testing, and magnetic/inertial confinement fusion. There have been notable scientific and technical accomplishments in this area, along with visible examples of yet to be achieved or inflated expectations. Finally, and perhaps most important, there is concern about future funding availability and the organizational advocacy necessary to sustain, and advance, the underlying intellectual, facility and equipment infrastructure in light of evolving national defense, economic, and social priorities. RECENT ADVANCES AND SCIENCE AND TECHNOLOGY OPPORTUNITIES Intense Charged-Particle Beams The mission for intense charged-particle beams has changed considerably over the last decade. Research and development sponsored by DOD, the Strate-
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Plasma Science: From Fundamental Research to Technological Applications gic Defense Initiative Organization (SDIO), and DOE resulted in facilities such as the Advanced Test Accelerator (ATA) at Lawrence Livermore National Laboratory for directed-energy weaponry, low-impedance multi-terawatt pulsed power machines for nuclear weapon effects simulation, and intense beams for fusion plasma heating. Kiloamp-MeV electron beams were developed that support high average power operation in excess of 100 kW with repetition rates approaching 1000 pulses per second (pps). Gyrotrons, devices that utilize a spinning electron beam in an axial magnetic field to produce millimeter waves for electron cyclotron resonance heating, successfully generated several hundred kilowatts in long pulses up to 3 s in duration at frequencies up to 140 GHz. Considering that 10 years ago, 100-ms outputs at 28 GHz and comparable power levels were representative, the technical community is justifiably proud of this technological accomplishment. Similarly, klystron technology has been advanced to higher frequencies (11.4 GHz) and powers (up to 100 MW), and the operation of a gyroklystron amplifier at the 20-MW power level at 11 GHz has been demonstrated. Many of the military mission-oriented efforts have been canceled. However, industrial applications of high-energy electron beams, including bulk sterilization of medical products and food, toxic waste destruction via oxidation, and processing of advanced materials, are in the demonstration stage. Technology transfer from the laboratories to industry is being encouraged actively. Having invested several hundred million dollars over the past decade in developing intense charged-particle-beam systems for military use, the emphasis by federal agencies on technology transfer for industrial applications seems prudent. Charged-particle-beam parameters vary greatly, depending on the application. A NASA concept for beaming power to space requires basic plasma science research addressing such physics issues as low emittance growth (< 20π mm-mr), beam breakup modes, and high current beam transport. Similarly, high energy electron-beam systems proposed for toxic waste cleanup, enhanced welding, heat treatment, and material processing generally have less stringent requirements on voltage flatness and emittance, but require reliable generation and maintenance of very high average powers. The interaction of intense charged-particle beams with plasmas, partially ionized gases, and matter offers rich scope for the study of strongly driven collective processes complementary to intense laser-plasma interactions. Electron and ion sources for intense beams have progressed from an empirical art to a developing science. Experiments, simulation, and analytical theory have contributed to this evolution, stimulated by the needs of inertial confinement fusion and other research programs. Intense ion beams also make possible the creation of magnetic field-reversed ion rings in which the self-magnetic field of the circulating ion current exceeds the externally applied magnetic field. Such a ring would be a compact object of high energy density with unique theoretically predicted features, such
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Plasma Science: From Fundamental Research to Technological Applications as low-frequency stability. If these predictions are borne out experimentally, there may be many uses for such rings, including the possibility of ion acceleration to high energy for various applications. A state-of-the-art electron and ion accelerator is pictured in Figure 5.1. Accelerators Several new techniques have been demonstrated that can produce large-amplitude, coherent, high-phase-velocity electron plasma waves. These include the beat wave and wake field concepts. An electron beam accelerated by a beat wave accelerator is pictured in Figure 5.2. Such beat wave accelerators have achieved accelerated electron energies of 9 MeV within distances of 1.5 cm. Attaining 500-MeV energy gains at GeV-per-meter rates is considered a plausible goal within a five-year period in which progress, funding priority, and follow-on application potential may be assessed. FIGURE 5.1 Schematic diagram of Hermes III, a 16-TW ion and electron accelerator that became operational in 1988. It represents a new class of accelerators that combine state-of-the-art pulsed power designs with high-power linear-induction accelerator cells and voltage addition along an extended magnetically self-insulated vacuum transmission line. This technology is particularly suited for applications requiring high output voltages (tens of megavolts), with megampere-level currents and short pulse widths (e.g., as small as tens of nanoseconds). Hermes III is used in its negative polarity configuration to generate an electron beam of ~20 MeV and 700 kA. It can also be configured in positive polarity to drive an ion beam diode. (Courtesy of J. Ramirez, Sandia National Laboratories.)
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Plasma Science: From Fundamental Research to Technological Applications FIGURE 5.2 In a plasma beat-wave accelerator, a pair of laser beams fired into a dense plasma excite a high-phase-velocity plasma wave, and the electric field of this wave accelerates an externally injected electron beam. Shown is an electron beam that has been accelerated to more than 5 MeV in less than 1 cm in such a beat-wave accelerator. This technique holds promise for developing miniaturized particle accelerators for research, medicine, and industry. (Courtesy of C. Joshi, University of California, Los Angeles.) These present and future very-high-energy, low-emittance, short-pulse electron beams should also further enable progress in other accelerator schemes, such as the plasma wake field accelerator. Relatedly, scaling principles for focusing electron and positron beams using thin plasma slabs as plasma lenses have recently been demonstrated, with 600-µm focal spot sizes achieved. In this case, basic plasma science is being exploited to develop an important "component" of an accelerator system rather than the entire system itself. Relativistic 2 1/2-dimensional particle-in-cell codes, developed for inertial confinement fusion research, are now being employed to study the physics of short-pulse, ultrahigh-intensity laser-plasma interactions. Phenomena including severe hydrodynamic distortion by the intense light pressure, heating of electrons and ions to ultrahigh energies, relativistic penetration to supercritical densities, and relativistic self-focusing have been observed. Other novel applications have also been developed, including frequency upshifting of electromagnetic radiation by reflection from ionization fronts and the generation of picosecond pulses of x-rays by irradiation of dense plasmas with ultrashort pulses. The wide-ranging progress may lead to compact particle accelerators, compact sources of tunable radiation, and new diagnostic tools for materials and biological applications. DOE is supporting the development and operation of state-of-the-art "user
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Plasma Science: From Fundamental Research to Technological Applications test facilities" available to researchers in the field. Examples are the Accelerator Test Facility at Brookhaven National Laboratory and the wake field test facility at Argonne National Laboratory. On the other hand, opinions exist within the community that per-grant funding has not kept pace with what is now perceived to be required for conducting experimental plasma physics research, even with access to state-of-the-art facilities. This situation appears to be compounded further by the absence of any clearly identified "organizational champion" within NSF, DOE, or DOD. Coherent Radiation Sources As discussed above in Chapter 2, nonneutral plasmas, such as intense charged-particle beams, exhibit a wide range of collective phenomena. Some collective instabilities limit the performance of accelerators and storage rings and must be minimized; others can be used to convert beam kinetic energy into coherent radiation. New-generation coherent sources, which use electron beams and are based on beam instabilities, operate from the microwave range to the millimeter, infrared, visible and ultraviolet regimes, with previously unattainable intensities. The most prominent of these new systems is the free-electron laser (FEL). Other configurations include gyrotrons and cyclotron masers, and a variety of Cerenkov devices. The basic principle underlying these devices is electron bunching stimulated by an ambient, co-propagating electromagnetic wave. In a properly prepared system, the electrons of the beam, initially distributed at random, can be made to form clusters or bunches. If the bunch dimensions are comparable to or smaller than the wavelength of the desired radiation, coherent emission ensues. Thus, the principle of bunching is somewhat analogous to stimulated emission in conventional lasers. Free-electron lasers have several important and distinctive features. The oscillation wavelength is not constrained to fixed transitions as in a conventional laser, thus allowing broad tunability. The pulse length is determined primarily by the electron beam, so that rf accelerators can be used without much difficulty to make picosecond pulses. Electron beams can transport high peak and high average power, making the FEL, with its reasonable conversion efficiency, a potentially attractive source of high-power radiation. Because there is no medium except the beam, problems associated with absorption and scattering can be avoided. Over the last decade, pioneering studies have been carried out concerning the physics of the relevant nonlinear electron-wave interactions that govern the processes in these free-electron radiation generators. Concurrently, significant SDIO investment was made in free-electron laser R&D as a strategic missile defense system. Experimental, theoretical, and computational studies addressed relevant nonlinear interactions such as trapping and sidebands, mode-locking and phase stability, three-dimensional effects, time-dependent phenomena, and
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Plasma Science: From Fundamental Research to Technological Applications high-efficiency operation. High power (gigawatts) and high efficiency (40%) were demonstrated at the longer wavelengths. Systems have lased using storage rings, linear rf, induction and electrostatic accelerators, microtrons, and low-energy beams. However, the accomplishments within the strategic missile defense arena fell short of what was promised and expected, potentially leaving a blemish on an otherwise promising technology. Most of the basic experiments referred to above were conducted on accelerators built for applications other than the free-electron laser. The SDIO sponsored several small-scale ''user facilities." For the free-electron laser to find its appropriate place among coherent radiation sources, research is required to gain a detailed theoretical and experimental understanding of the temporal and spectral properties, to extend operation to shorter wavelengths in the VUV regime, and to increase the efficiency at the shorter wavelengths. A collaboration among academic institutions, national laboratories, and industrial organizations in the design and construction of a next generation of user facilities and the pursuit of the ensuing research would seem appropriate, given that the advocates and potential users of this coherent radiation source technology are successful in establishing its relative priority. Coherent radiation source research in the x-ray portion of the electromagnetic spectrum includes synchrotrons/undulators, x-ray lasers, and harmonically converted short-pulse optical lasers. Of these three, major support has been given to the synchrotron/undulator effort (i.e., the Advanced Light Source at Lawrence Berkeley Laboratory and the Advanced Photon Source at Argonne National Laboratory). Brookhaven has an active users program dedicated to many areas of research, including biology, materials science, basic atomic physics and chemistry, and semiconductor physics. Much of the x-ray laser research work to date has been carried out with internal research and development funds at national laboratories. This review did not address the x-ray laser research conducted under the auspices of the Strategic Defense Initiative Office. Concepts for soft x-ray lasers were developed successfully and demonstrated in the laboratory during the past decade. The generation of a dense, hot plasma by laser irradiation was a key feature of this success. This progress was the product of close collaboration among plasma theory, atomic physics, and laser science. Since the initial work, x-ray lasers at more than 50 different wavelengths have been demonstrated in about 10 laboratories worldwide. These x-ray lasers have been demonstrated with wavelengths from 400 to 35 Å, output powers to 100 MW, brightnesses eight orders of magnitude greater than those of undulators, bandwidths of 5 × 10-5 at full power, and near-diffraction-limited and partially coherent output beam characteristics. Short-pulse harmonically produced x-rays are currently in the demonstration phase with wavelengths approaching 100 Å having been generated, albeit at low (< 10-9) efficiency. At present, synchrotrons have become undulators offering high coherent average power, but at great cost. The recent development of high-average-power
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Plasma Science: From Fundamental Research to Technological Applications glass lasers promises a high-average-power x-ray laser alternative to synchrotron sources at substantially reduced costs. Sources are envisioned with wavelengths between 100 and 150 Å and bandwidths of 5 × 10-5, producing 1015 photons per pulse at 1 Hz, that will occupy physical footprints approximately 3 m by 6 m. X-ray laser researchers aim at achieving shorter and shorter wavelengths and high coherent output energy. Exploiting ultrashort, subpicosecond laser pulses, photoionization pumping schemes render plausible lasing at or near 14 Å within 5 to 10 years. Harmonically generated x-rays from ultrashort-pulse laser-gas jet interactions are striving to achieve high efficiency at short wavelengths to go along with their inherent tunability and coherence. In turn, these sources can be envisioned to serve as drivers for further wavelengths down-conversion providing improved radiation sources in the spectral regime of the order of tens of keV, which would significantly broaden potential medical and industrial applications. The features of high power, narrow bandwidth, short pulse, and coherence make x-ray lasers attractive for future applications, such as biological microimaging, photoelectron spectroscopy, and probing of dense plasmas. (See Plate 4.) Given these scientific successes and potential applications and societal benefits, it is a serious deficiency that no federal agency has taken on the mandate to support x-ray laser research. Advances in this area are hampered by the presence of large capital-intensive synchrotron and inertial confinement fusion facilities that historically have either siphoned off the majority of potential funding support or programmatically relegated the research to a piggyback status. The multidisciplinary nature of the research complicates the effort to accrue the critical mass of funding that would support a robust program. CONCLUSIONS AND RECOMMENDATIONS Research and development on particle beams, accelerators, and coherent radiation sources offers a wide range of opportunities for technological advances of importance for our society. Examples include food sterilization; waste treatment; welding and materials processing; advanced accelerator development; and the development of new, intense radiation sources for a wide range of applications. In the past, much of the basic science and development in this area was driven by military applications. However, given recent changes in emphasis on military needs, there is a danger that opportunities will be lost unless research and development continue to be pursued in areas in which there are significant technological opportunities. As discussed elsewhere in this report, there is a general need for support for small-scale basic research. This also is true in the areas of beams, accelerators, and radiation sources.
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Plasma Science: From Fundamental Research to Technological Applications Given these considerations, the panel recommends the following: Dual-use opportunities of defense-driven technologies should continue to be pursued. Existing hardware and facilities no longer needed for ICF and SDIO applications should be made available for other scientific and technological applications. Support should be given to small-scale basic research in these areas. Particular attention should be given to the development of advanced concepts of particle accelerators and of new, intense radiation sources, such as x-ray lasers. Where practical, the use of large facilities by outside users to pursue the scientific and technological goals described in this section should be encouraged.
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