Accelerator Physics Challenges of X-Ray FEL SASE Sources

A great deal of international interest has recently focused on the design and construction of free-electron lasers (FEL) operating in the x-ray region (~1 Å). At present, a linac-based machine utilizing the principle of self-amplified spontaneous emission (SASE) appears to be the most promising approach. This new class of FEL achieves lasing in a single pass of a high brightness electron beam through a long undulator. The requirements on electron beam quality become more demanding as the FEL radiation wavelength decreases, with the 1-Å goal still 3-orders of magnitude below the shortest wavelength operational SASE FEL (TTF-FEL at DESY (1)). The sub- picosecond bunch length drives damaging effects such as coherent synchrotron radiation, and undulator vacuum chamber wakefields. Unlike linear colliders, beam brightness needs to be maintained only over a small 'slice' of the bunch length, so the concepts of bunch- integrated emittance and energy spread are less relevant than their high-frequency (or 'time-sliced') counterparts, also adding a challenge to phase space diagnostics. Some of the challenges associated with the generation, preservation, measurement, and stability of high- brightness FEL electron beams are discussed here.