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BEAMS, ACCELERATORS, AND COHERENT RADIATION SOURCES 96 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
BEAMS, ACCELERATORS, AND COHERENT RADIATION SOURCES 97 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 Ã , output powers to 100 MW, brightnesses eight orders of magnitude greater than those of undulators, bandwidths of 5 Ã 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 Ã 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
