Conceptual designs of beam choppers for RFQ linacs

A design study at Los Alamos of a linac/accumulator ring facility for a pulsed neutron spallation source calls for an H^- beam with a chopped structure of approximately 200-ns beam-free segments every 600 ns. The required angular impulse can easily be provided with existing pulse power technology and traveling wave structures with a transverse electric field similar to those now available. The deflected beam is then restored by suitable collimation. Chopping is relatively easily done at sufficiently low energies, where the beam is easily deflected, and beam powers are not too large. However, the energy should be high enough so that the space charge blow-up of the beam can be controlled with adequate focusing. LAMPF presently uses a traveling-wave beam chopper at 750 keV, before injection into the drift-tube linac (DTL). In the new linac designs, a radio-frequency quadrupole (RFQ) linac would typically bunch and accelerate the high intensity H^- beam from 100 keV to 7 MeV. In this paper, we present concepts for beam-chopper systems both before and after the RFQ. The beam-optics designs are presented together with numerical simulation results