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BEAMS, ACCELERATORS, AND COHERENT RADIATION SOURCES 94 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.)
BEAMS, ACCELERATORS, AND COHERENT RADIATION SOURCES 95 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. 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.) 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
