Versatile H+ Beam Chopper System at LAMPF

The H+ chopper system at LAMPF provides two modes (A and B) of beam chopping. Mode A provides single micropulses separated by 1 ¿s or more. Mode B provides a single pulse 50 ns to 10 ¿s long at the LAMPF repetition rate of 120 Hz. The mode of operation is selected with coaxial relays which can switch during the dead time between LAMPF macropulses. The chopper consists of 1-m-long push-pull helical-wound slow-wave deflecting structures having an axial pulse velocity of 0.04c, which matches the velocity of the proton beam. The helix ground-planes are biased by a pulser providing push-pull 500-V pulses to deflect the H+ beam from the injection channel. In chopping mode A, the helical windings are driven by push-pull, 1000-V, 5-ns pulses delivered by a circuit utilizing avalanche transistors and planar triodes. In mode B, the helical windings are driven by push-pull, 1000-V, 50-ns to 10-¿s long pulses, produced by a circuit utilizing beam-power tetrodes and fast SCRs. In both modes, these pulses cancel the ground-plane bias and permit beam pulses to be delivered to the linac.