Design of High-Average-Power Near-Millimeter Free Electron Laser Oscillators Using Short-Period Wigglers and Sheet Electron Beams

The design and feasibility of a 1-MVV continuous wave (CW) free electron laser (FEL) oscillator are reviewed. The proposed configuration will include a short-period (lw-1 cm) planar wiggler, a sheet electron beam, a 0. 5-1.0 MY thermionic electron gun, a hybrid waveguide/quasi-optical resonator, commercial dc power supplies, and a depressed collector. Cavity ohmic RF losses are estimated to be extremely low (≲ 10-100 W/cm2) at 1-MW output power, while thermal heat transfer studies conservatively indicate that wall cooling up to 1500 W/cm2 should be possible. Experiments have convincingly verified theory and simulations which predict that negligible body currents will be achievable with low-emittance low-space-charge sheet beams. High-voltage sheet beam gun design studies indicate that the required beam quality can be achieved with CW compatible devices. The spent beam energy distribution is consistent with highly efficient spent beam energy recovery, and the proposed resonator cavity should provide mode discrimination and beam/RF separation capability. Finally, recent advances in superconducting wiggler designs suggest that even more compelling improvements in FEL design are possible. Specifically, the substantially increased dc wiggler fields can yield better beam confinement. Furthermore, options for a strong-pump FEL amplifier with a tapered, superconducting, short-period wiggler are now being considered.