EXECUTIVE SUMMARY

The free electron laser uses a beam of relativistic electrons passing through a periodic, transverse magnetic field to produce coherent radiation. These devices have several advantages. A resonance condition that involves the energy of the electron beam, the strength of the magnetic field, and the periodicity of the magnet determines the wavelength of the radiation. Because one medium, the electrons, provides the gain in all spectral regions, adjusting either the beam energy or the field strength tunes the wavelength easily and rapidly over a wide range. Waste energy leaves the medium as kinetic energy of the electrons at nearly the speed of light. Moreover, the lasing medium consists only of electrons in a vacuum, and it does not have the material damage or thermal lensing problems associated with ordinary lasers. Therefore, free electron lasers can achieve very high peak powers.

The main disadvantages of the free electron laser are its size and cost. Because the free electron laser requires an electron accelerator with its associated shielding, it has not been a device that could be placed in an individual investigator's laboratory and be operated and maintained by graduate students whose primary expertise is in other areas of science. Because free electron lasers are currently used only in central facilities, their utilization in scientific research involves both the cost of the device, and the cost and inconvenience of maintaining a user facility. Unlike synchrotrons, free electron lasers serve one user, or at most a few users, at any one time.

The required electron beam energy increases with decreasing wavelength, and the cost and size of the accelerator as well as the cost and size of the magnetic structure increase with decreasing wavelength. In addition to energy requirements, the electron beam must meet other requirements for emittance, energy spread, and peak current that become more stringent at shorter wavelengths. The shortest wavelength reached by existing free electron lasers is 240 nm, but most scientific uses of free electron lasers have been in the infrared. There are alternative methods (the “other advanced sources of light” in the title of this report) proposed for generating radiation, such as plasma sources, high-order harmonic generation, and Compton backscattering sources. Some of these use terawatt laboratory lasers, which are themselves costly devices, and interest in these other sources is confined to the vacuum ultraviolet and x-ray regions for which the cost of free electron lasers is highest.

Although the same physical principles govern the design of free electron lasers in all wavelength regions, the costs and benefits are a strong function of wavelength. Therefore, this report is organized by spectral region, and the committee's findings and recommendations are very different in the different wavelength regions. However, research and development aimed at improving free



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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities EXECUTIVE SUMMARY The free electron laser uses a beam of relativistic electrons passing through a periodic, transverse magnetic field to produce coherent radiation. These devices have several advantages. A resonance condition that involves the energy of the electron beam, the strength of the magnetic field, and the periodicity of the magnet determines the wavelength of the radiation. Because one medium, the electrons, provides the gain in all spectral regions, adjusting either the beam energy or the field strength tunes the wavelength easily and rapidly over a wide range. Waste energy leaves the medium as kinetic energy of the electrons at nearly the speed of light. Moreover, the lasing medium consists only of electrons in a vacuum, and it does not have the material damage or thermal lensing problems associated with ordinary lasers. Therefore, free electron lasers can achieve very high peak powers. The main disadvantages of the free electron laser are its size and cost. Because the free electron laser requires an electron accelerator with its associated shielding, it has not been a device that could be placed in an individual investigator's laboratory and be operated and maintained by graduate students whose primary expertise is in other areas of science. Because free electron lasers are currently used only in central facilities, their utilization in scientific research involves both the cost of the device, and the cost and inconvenience of maintaining a user facility. Unlike synchrotrons, free electron lasers serve one user, or at most a few users, at any one time. The required electron beam energy increases with decreasing wavelength, and the cost and size of the accelerator as well as the cost and size of the magnetic structure increase with decreasing wavelength. In addition to energy requirements, the electron beam must meet other requirements for emittance, energy spread, and peak current that become more stringent at shorter wavelengths. The shortest wavelength reached by existing free electron lasers is 240 nm, but most scientific uses of free electron lasers have been in the infrared. There are alternative methods (the “other advanced sources of light” in the title of this report) proposed for generating radiation, such as plasma sources, high-order harmonic generation, and Compton backscattering sources. Some of these use terawatt laboratory lasers, which are themselves costly devices, and interest in these other sources is confined to the vacuum ultraviolet and x-ray regions for which the cost of free electron lasers is highest. Although the same physical principles govern the design of free electron lasers in all wavelength regions, the costs and benefits are a strong function of wavelength. Therefore, this report is organized by spectral region, and the committee's findings and recommendations are very different in the different wavelength regions. However, research and development aimed at improving free

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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities electron lasers in a specific wavelength region may be important to the improvement of free electron lasers in all wavelength regions. FAR-INFRARED REGION (1000 to 10 µm) The most compelling case for a free electron laser facility is in the far infrared, the region between 1000 and 10 µm. Because of the lack of suitable nonlinear materials, techniques for generating tunable light using commercial laboratory lasers are not effective at wavelengths longer than 10 µm. It is possible to extend this limit to longer wavelengths with some difficulty using noncommercial instrumentation, but it is unlikely that conventional laboratory lasers will ever be effective at wavelengths longer than 20 µm. Findings The scientific case for a tunable, short-pulse (picosecond) source in the far infrared is compelling, but at present there are no picosecond far-infrared FEL user facilities in the United States. This is the spectral region where molecule-surface vibrations, intermolecular cluster vibrations, and transitions in semiconductor quantum wells can be excited. It is also the region for probing transitions between adjacent high-lying Rydberg states of atoms and low-frequency motions in large biomolecules. There is sufficient scientific interest to efficiently use a far-infrared user facility capable of producing picosecond pulses, and the committee believes that the scientific opportunities justify the establishment of such a facility. Operation of such a facility would provide information on whether additional user facilities might be needed subsequently. Two relevant issues that should then be considered are the quality of the proposed science that cannot be accommodated by a single facility and the existence of alternative infrared sources such as laboratory-sized far-infrared FELs. Free electron lasers already exist in the far-infrared region, even as user facilities, and therefore uncertainty about the technical requirements and cost of establishing a far-infrared free electron laser facility is relatively low. The relatively modest accelerator and undulator requirements mean that a user facility might be built for several million dollars using existing technology. There is some promise that further development will lead to a significantly less expensive free electron laser that could be used by a single academic department or perhaps an individual investigator. A photocathode electron source may increase the utility of a user facility both because of the free electron laser's improved performance and because the laser used to excite the photocathode could also serve as a second, synchronous photon source for pump-probe experiments.

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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities Recommendations A far-infrared free electron laser user facility capable of producing picosecond pulses should be established. Such a facility could be a new construction or a modification of an existing facility. The committee believes that the scientific opportunity justifies the establishment of such a facility. Such a facility should have a means of synchronizing a second tunable laser with the free electron laser for use in pump-probe experiments as well as a means for selecting single picosecond pulses from the free electron laser. One method of doing this involves using a photocathode electron gun. The current research and development directed toward the production of a compact free electron laser that could be purchased and operated by a single academic department or individual investigator should be continued. NEAR-INFRARED, VISIBLE, AND ULTRAVIOLET REGION (10 µm to 200 nm) The region of the near infrared, visible, and ultraviolet accessible with commercial lasers covers wavelengths from roughly 10 µm to 200 nm. Noncommercial instrumentation can extend the limits of commercial laboratory lasers with some difficulty to 20 µm to 100 nm. This region is crucial for scientific research and is of great importance for much current and planned research, not only because photon sources and detectors are highly developed in this region, but also because of the fundamental properties of matter. The fingerprint region of molecular vibrations falls in the near infrared, valence transitions of chemical bonds fall in the visible and ultraviolet, and band gaps of solids fall in the visible or near infrared. A wide variety of laboratory laser sources and nonlinear techniques based on laboratory lasers are effective in this region. These lasers are already widely tunable and capable of producing short pulses with high peak and average powers. Research and development will lead to further improvement. There are also existing free electron lasers that operate in this region, but conventional laboratory lasers will probably remain the mainstay of scientific research in this wavelength region. The national expenditure in 1993 on laboratory lasers used in scientific research was approximately $37M. Findings Laboratory lasers have been and will continue to be an important photon source for research in the region from 10 µm to 200 nm. Much of the science in this wavelength region involves simultaneous use of several sophisticated laboratory lasers. When cost and convenience are considered, it is unlikely that free electron lasers will be competitive with laboratory lasers in this spectral region in the foreseeable future.

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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities Recommendations In the near-infrared, visible, and ultraviolet regions, continued support of lasers in individual investigators' laboratories is the highest priority. The development of nonlinear materials for extending the wavelength capabilities of laboratory lasers deserves support in both universities and industry (e.g., through the Small Business Innovation Research program). Support for this activity should receive a high priority, as even a modest investment is likely to have a considerable impact on scientific research. In the near-infrared, visible, and ultraviolet regions, support for additional free electron lasers should receive relatively low priority with respect to support of conventional lasers and with respect to support for free electron lasers in other wavelength regions. VACUUM ULTRAVIOLET AND EXTENDED ULTRAVIOLET REGION (200 to 10 nm) In the vacuum ultraviolet (VUV) and extended ultraviolet, the region from approximately 200 nm to 10 nm, laboratory lasers become increasingly inefficient. There are important scientific needs for radiation in this region, including photodissociation, single-photon ionization detection, photoelectron spectroscopy of dilute species, and pump-probe photoemission. Findings Modern third-generation synchrotrons will cover the vacuum ultraviolet and extended ultraviolet regions well and will provide opportunities to explore much new scientific research in this wavelength region. There are currently no free electron lasers that operate in this region. The construction of a device should be possible with some additional research and development. It may be possible to construct a large VUV free electron laser facility now, since most of the component pieces have been demonstrated, but construction of a VUV free electron laser using current technology would cost on the order of tens of millions of dollars and involve some risk. A user community for a VUV free electron laser is not well developed at this time but could grow with the exploition of third-generation synchrotron sources, demonstrating proof-of-principle experiments.

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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities Recommendations Scientific opportunities in the vacuum ultraviolet and extended ultraviolet wavelength range should be explored by the use of existing synchrotron sources. The development of technology for a vacuum ultraviolet free electron laser should be supported. Among the goals of this development should be lowering the cost of these devices and increasing their reliability. A free electron laser user facility in the vacuum ultraviolet should not be constructed at the present time. The scientific case for a VUV free electron laser facility does not justify the current cost, and the technology is not sufficiently well developed. X-RAY REGION (100 to 1 Å) The x-ray region for which free electron lasers and other sources have been proposed covers the wavelength region of approximately 100 Å to 1 Å. Some of the terawatt laser-based sources have produced x-rays in this region, but there is no existing x-ray free electron laser. There are existing x-ray sources, both laboratory sources and synchrotrons, but there has never been a high-intensity source of coherent x-rays such as would be produced by an x-ray free electron laser. Therefore, much of the interest in this spectral region arises because many of the possibilities involving coherent x-rays are totally unexplored. The scientific case therefore has both great promise and great uncertainty. It may be that a high-intensity source of coherent x-rays would open up unforeseen opportunities, or it may be that the use of such a source would be an extrapolation of present research. Applications involving imaging, such as x-ray microscopy and holography, are potentially very interesting, but one must compare these techniques to recent advances in tunneling, atomic force, and near-field optical microscopy. Time-correlation spectroscopy, a standard technique at longer wavelengths, would be possible with a source of coherent x-rays and could be used to probe dynamical processes at shorter length scales. Light/x-ray pump-probe experiments might be possible with an appropriate source. It appears that the new applications would require a source operating at the short-wavelength end of this region near 1 Å. As the wavelength decreases, the difficulty and cost of building a scientifically useful x-ray free electron laser increase substantially. Findings Third-generation synchrotron sources, while not as bright or as coherent as an x-ray free electron laser, will provide far more intense and coherent radiation than is currently available. While many scientific studies using x-rays produced by an FEL appear to be possible, the feasibility of these studies should first be explored using

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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities third-generation synchrotron sources that are just beginning to operate. This exploration, in conjunction with further research and development of x-ray FEL technology, will provide the basis for an informed decision regarding possible future construction of an x-ray FEL facility. Even if cost were not a factor, construction of an x-ray free electron laser would require significant research and development, particularly at the shorter wavelengths. The costs and risks of building a free electron laser facility would be much higher in the x-ray region than in any other region, and the uncertainties in both the cost and the technology are also much higher. Recommendations Scientific opportunities and the use of coherence in the x-ray spectral region should be explored initially by the use of existing and planned synchrotron sources. The research and development necessary for the possible construction of an x-ray free electron laser should be supported. The goals of this research and development should be improving the technology and lowering the cost. Research and development on other advanced coherent x-ray sources should continue to be supported. One of the goals of this research and development should be the production of devices of appropriate size and cost to be useful for scientific research on a departmental or individual-investigator scale. Construction of an x-ray free electron laser user facility should not be undertaken at the present time. GENERAL ISSUES There were several issues that the committee believed were important but that were general and did not fit into the sections on individual wavelength regions. A variety of communities potentially benefit from the type of research that will be necessary to produce scientifically useful FELs, and because of the huge disparity in resources among these communities, it is unrealistic to expect any single one of them to assume the financial obligations for all, or even any significant fraction, of the total cost. Because each of these communities tends to focus tightly on its own principal interests, there is little impetus for cooperation directed toward the development of FELs. Unless this happens in the future, the technological promise could be unfulfilled. A thorough analysis of this aspect of the problem would require an in-depth study of all of these communities: accelerator physics, high-energy physics, the Defense Department, industry, and the National Laboratories, including the synchrotron laboratories. Such an analysis is clearly

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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities beyond the scope of this report. The committee therefore proposes some general approaches to these problems. There must be a balance between support for individual investigators and support for major research facilities. If the productivity of the scientific enterprise is to be maximized, support for individual investigator research should continue to receive high priority in the competition for resources. Some of the expertise for research leading to improved FELs is located at Department of Energy laboratories and in university research programs outside those funded by the DOE's Office of Basic Energy Sciences. The results of this research will have benefits that extend beyond FEL development to the larger accelerator community. It is therefore important that this research be coordinated with and jointly funded by programs both within and outside the DOE's Office of Basic Energy Sciences. The committee recommends that such coordination be addressed by a task force, along the lines of the task force on accelerator science and technology recently formed by the DOE Director of Energy Research. Since improvements in accelerators benefit a wide range of programs, it may be desirable to invent new processes for funding and management within or among funding agency organizations. In addition to scientific research, there are potential industrial, defense, and medical applications of FELs. The committee recommends that the Department of Energy, other federal agencies, and the private sector explore coordination of funding for FEL development. Existing synchrotron facilities and their host institutions should use some of their discretionary research funds to support the next phase of FEL research. Before a commitment is made for the construction of a short-wavelength FEL facility, existing facilities should be examined to determine if any older facilities should be discontinued. This examination should consider not only the costs of constructing a new facility but also the costs of operating the new facility.

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