A High Current Injector for the Boeing Radiation Laboratory FEL Experiment

A two-stage subharmonic injector has been designed and installed to accelerate 150 ampere micropulses with the Boeing 20 MeV, S-band linac. The accelerator will provide microbunches with interpulse time of 50 nanoseconds throughout the RF macropulse of 20 microseconds. The electron source is a 5 cm2 triode electron gun which is gated to produce 5A, 3ns pulses. Two standing wave prebuncher cavities are used, the first at 119 MHz, the 24th subharmonic of the linac frequency, and the second at 476 MHz, the sixth subharmonic. A short tapered phase velocity traveling wave structure provides the final bunching. The solenoidal containment field is adjustable for optimization using injector profile diagnostics. Computer analysis of bunching and radial trajectories predict beams of 2.2 nanocouloumbs with radius of 0.2 cm and phase width of ?/2 of the fundamental at the entrance to the traveling wave section.