Development of a Fast Traveling-wave Beam Chopper for the National Spallation Neutron Source*

High current and severe restrictions on beam losses, below 1 nA/m, in the designed linac for the National Spallation Neutron Source (NSNS) require clean and fast — with the rise time from 2% to 98% less than 2.5 ns to accommodate a 402.5-MHz beam structure — beam chopping in its front end, at the beam energy 2.5 MeV. The R&D program includes both modification of the existing LANSCE coax-plate chopper to reduce parasitic coupling between adjacent plates, and development of new traveling-wave deflecting structures, in particular, based on a meander line. Using analytical methods and three-dimensional time-domain computer simulations we study transient effects in such structures to choose an optimal chopper design.