e-Guns and depressed collectors for two-stage free electron lasers

An -gun, depressed collector, and guide-field solenoid have been designed and tested for operation at a maximum of 400 kV and 38 A in order to simulate key components of a typical low-voltage free electron laser (FEL). Detailed beam-energy-spread analyses and computer simulations are presented for the -gun and the drift regions. The results of experiments with the depressed collector show beam-current-recovery efficiencies of over 90 percent for -beam voltages from 160 to 400 keV. We have also set up and confirmed computer-code predictions that a space-charge depression in the collector (which is essential for efficient collection) forms at a collector-voltage setting of 1 percent of the beam voltage. These results demonstrate the validity of substantially increasing the efficiency of systems by using depressed collectors to recover the bulk of the energy which is left in the -beam after it leaves the laser resonator. The recovery-efficiency parameter (recovered current/cathode-emission current) is shown to be a function of the relativistic-orbit parameter ( ) and reaches a maximum of 94 percent. These results imply that FEL system efficiencies can be achieved which are an order of magnitude higher per stage than would be possible without the use of depressed collection.