Experimental Investigation of the Free Electron Laser.

An e-gun, depressed collector and guide-field solenoid has been designed for operation at a maximum of 400 kV and 38 A. The results of experiments with this brassboard system include demonstration of depressed collector recovery efficiencies of over 90% for e-beam voltages from 160 to 300 keV and currents up to o25 A. We have also confirmed Hughes computer-code predictions that a space-charge depression in the collector (which is essential to efficient collection) forms at a collector-voltage setting of 1% of the beam voltage. These results demonstrate the validity of substantially increasing the efficiency of FEL systems, by using depressed collectors to recover the bulk of the energy which is left in the e-beam after it leaves the laser resonator. Preliminary indications are that the recovery efficiency (recovered current/cathode-emission current) increases gradually with beam voltage (reaching 94% at 300 keV) as beam spreading due to thermal effects and sensitivity to the focusing-field strength are reduced. We expect to translate these results into FEL-system efficiencies of an order of magnitude higher per stage than would be possible without the use of depressed collection. End-on Thomson scattering was identified as the most feasible technique for this measurement. A Thomson scattering experiment has been designed and hardware implemented into the brassboard system.