Design of an electrostatic energy separator for the ISIS RFQ test stand

It is planned to replace the 665 keV Cockroft Walton set of the ISIS spallation neutron source in the UK with a four rod RFQ. Before this replacement is made, the performance of the RFQ will be extensively assessed on a test stand. One of the quantities we hope to measure is the proportion of accelerated (trapped) beam at the exit of the RFQ for various H^- ion source currents and rod voltages. In order to do this it is necessary to separate the untrapped beam which is largely at the ion source extraction energy of 35 keV from the trapped beam which has an energy centred on 665 keV. This paper describes the design process for an instrument to perform this separation using parallel electrostatic plates to bend away the low energy beam while allowing all the higher energy beam to pass through, therefore measuring directly the fraction of trapped beam, and by implication the fraction of untrapped beam