An investigation into the behaviour of residual gas ioni-sation profile monitors in the ISIS extracted beamline

Copyright © 2016 CC-BY-3.0 and by the respective authors. Non-destructive beam profile measurements at the ISIS neutron source are performed using Multi-Channel Profile Monitors (MCPMs). These use residual gas ionisation within the beam pipe, with the ions being guided to an array of 40 Channeltron electron multipliers by a high voltage drift field. Non-uniform transverse electric fields within these monitors are caused by the drift field and the beam’s space charge. Longitudinally, a saddle point located between the drift field plate and the opposing compensating field plate introduces extra complexity into the ion motion. To allow for detailed studies of this behaviour, an MCPM has been placed in Extracted Proton Beamline 1 (EPB1) where the beam is well defined. Simulations of the profiles obtained by this monitor are performed using machine measurements, CST EM Studio and a simple C++ particle tracking code. This paper describes the process used to simulate MCPM profiles along with a comparison of simulated and measured results. Trajectories of detected ions from their creation to the Channeltrons are discussed, together with a study of Channeltron detection characteristics carried out in the ISIS diagnostics laboratory vacuum tank.