Pulsed Beam Chopper for LAMPF

A pulsed beam chopper has been designed and tested for use on the 750 keV proton injector transport l ine of LAMPF. The deflection structure, which is 1 m long, is similar to the traveling wave beam deflector in a high speed oscilloscope. The proton beam travels between two deflection "plates" each consisting of the closely spaced turns of a helical delay line. A 5 ns long positive pulse on one helix with a simultaneous negative pulse on the other helix creates a transverse electric field in the beam region which travels with the beam. The longitudinal velocity of the electric field is matched to the proton beam velocity (~1 cm/ns) by the pitch of the helices. This scheme allows the 5 ns (5 cm) long "micropulses" of LAMPF to be chopped into or out of the linac with 1 ns transition times. This method avoids the constraints of resonant chopping systems and will permit completely arbitrary patterns of LAMPF micropulses to be selected and accelerated. One exotic application is the generation of pseudorandom pulse sequences for neutron time-of-flight measurements.