Superconducting linacs as free electron laser drivers

The superconducting RF linac played a central role in the early development of free electron lasers. Now as FEL facilities are being constructed in the far-infrared and the vacuum ultraviolet in order to pursue frontier optical science, and as the feasibility of near-ultraviolet FEL facilities for commercial surface processing of materials is being evaluated, there is renewed interest in the special characteristics that superconducting RF linacs offer. These special characteristics include long pulse or CW operation, feedback regulation of accelerating fields, high efficiency, and availability of high gradient, low impedance structures. There are exciting new opportunities on the horizon for FEL facilities and it is clear that superconducting linacs will play an important role in their development.