Optical Cavity vs. Electron Beam Requirements for the Operation of a 1.5 a Lcls in a Regenerative Amplifier Mode*

In current designs for 100-l 8, Linac Coherent Light Sources (LCLSs), i.e., singlepass gain-saturating FELs [l], a major cost component is the undulator. If the mode of operation could be switched to that of an oscillator (or, more appropriately, regenerative amplifier), substantial size and cost savings could be realized. Unfortunately, simple cavities based on normal-incidence specular or multilayer optics cannot be realized straightforwardly in this spectral regime. In prior work, e.g., the use of diffracting crystal resonators was discussed [2], but the operation of such a cavity is tied to a single frequency, negating one of the main advantages of the FEL. A broad-band multi-facet ring cavity operating at grazing incidence has been proposed, fabricated, and tested at LANL [3] for operation down to 1000 A, but the extension of this scheme down to substantially shorter wavelengths has not as yet been pursued. The basic requirements necessary for progress in this direction can be associated with four areas: 1) specular or interference-based reflectivity, viz., the optical and lattice constants of materials and material systems; 2) optical scattering, viz., the surface quality of materials and interfacial quality of material systems; 3) peak power fundamental and harmonic damage effects; and 4) electron/photon pulse synchronization. Of these, perhaps the most demanding in terms of current technology is synchronization. For example, efficient