Electromagnetic modeling of beam position and phase monitors for the LANSCE linac

Electromagnetic modeling has been used to compare designs of pickups for the beam position and phase monitors (BPPM) for the Los Alamos neutron science center (LANSCE) linac. This study is a part of the efforts to upgrade LANSCE beam diagnostics [1]. MAFIA 3-D time-domain simulations with an ultra-relativistic beam allow computing the signal amplitudes and phases on the BPPM electrodes for the given processing frequency, 201.25 MHz, as functions of the beam transverse position. An analytical model can be applied to extrapolate the simulation results to lower beam velocities. Based on modeling results, a BPPM design with 4 one-end-shorted electrodes each covering 60-degree subtended angle, similar to the SNS linac BPPM [2], appears to provide the best combination of mechanical and diagnostics properties for the LANSCE side-coupled linac.