Subharmonic triple buncher for a high-efficiency free-electron laser

A high efficiency free electron laser oscillator experiment was constructed. A buncher system was designed to deliver 30-ps, 5-nC electron bunches to a 20-MeV standing wave linac at the 60th subharmonic of the 1300-MHz accelerator frequency. The first 108.3-MHz buncher cavity accepts a 5-ns, 5-A peak current pulse from a triode gun. Following a 120-cm drift space, a second 108.3-MHz cavity is used, primarily to enhance the bunching of the trailing half of the bunch. A 1300-MHz cavity with 20-cm drift spaces at each end completes the beamline components. The bunching process continues into the linac's first three accelerating cells. Two thin iron shielded lenses and seven large diameter solenoids provide axial magnetic fields for radial focusing.