Storage rings have served well as x-ray sources, achieving continued increases in flux and brightness. While further improvements in performance may still be expected, the performance of these machines is circumscribed by radiation fluctuations and by the Touschek effect. The practical effect of the radiation fluctuations is to limit the minimum 6D emittance of the rings in general, particularly the bunch length. Through limiting the lifetime, the Touschek effect also limits the charge density in the bunches. These effects can be greatly ameliorated through use of a linac driver for the x-ray producing devices such as small gap undulators because the beam passes through only once and in a time short compared to the radiation equilibration time. In this case, the emittance is limited only by the source and the beam is thrown away after each pass so that radiation and collision effects are no longer relevant. By recovering the kinetic energy carried by the beam through use of the energy recovery principle in a superconducting linac, one can in principle support beam currents comparable to those of storage rings but with smaller emitting source area. These advantages permit the achieving of superior brilliance while giving great flexibility in bunch pattern and bunch length. The concept of a 5 - 7 GeV facility using this principle is described and parameter goals are given.
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