Prospects for an x-ray FEL light source and some possible scientific applications

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Abstract
Free electron lasers are now being designed which will operate at wavelengths down to about 1 Å. Due to the physics of the high‐gain, single pass FEL process that these sources will exploit, the radiation produced will have unique properties. In particular: • The FEL peak intensity and peak brightness will be many orders of magnitude higher than can be produced by any other source. • The pulse length will be less than 1 picosecond, orders of magnitude shorter than can be achieved with any other bright source such as a synchrotron. • The FEL radiation will have full transverse coherence and a degeneracy parameter (photons/coherence volume) equal to 109 or more. No other source can produce hard x‐radiation with a degeneracy parameter significantly greater than 1. These properties offer the chance to study chemical, biological, and condensed matter dynamical processes with sub‐picosecond time resolution and angstrom spatial resolution. The high peak power of the FEL radiation (greater than 1014 W/cm2) could be used to create precisely‐controlled chemical and structural modifications inside samples. There is also the possibility that nonlinear x‐ray interactions could be used to give increased resolution for spectroscopic studies, to greatly expand the parameter space for atomic physics studies, and to permit new fundamental tests of quantum mechanics. The exploration of these new x‐ray techniques will require considerable development, not only in technical areas such as optics and detectors, but also in understanding the basic physics of the interaction of very intense x‐radiation with matter. A large collaboration of US institutions is now conducting preliminary research and development in these areas, with the intention of creating an FEL operating at 1.5 Å in about the year 2006. Germany also has a strong short‐wavelength FEL research program, with a soft x‐ray FEL under construction and a proposal for a future large facility which would produce a variety of hard and soft x‐ray laser beams.

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