Controlling the shape of the ISIS H− penning ion source beam pulse

The Penning ion source serving the ISIS Pulsed Spallation Neutron and Muon facility routinely delivers 55 mA beam of negative hydrogen ions (H−) in 250 µs pulses at 50 Hz repetition rate. The Front End Test Stand (FETS) specifications require 60 mA, 2 ms, pulses at 50 Hz. Extending the ion source discharge pulse length to 2.2 ms will need overcome the observed beam current droop caused by thermal transients in long pulse operation. Recent experiments at 25 Hz have demonstrated square 60 mA beam pulses up to 1.2 ms with the permanent magnet version of the ISIS source and 100 mA pulses up to 1.65 ms with the same source equipped with a double-width extraction slit. Droop was compensated by ramping up the discharge current during the beam pulse. The physical phenomena underlying the droop and its countermeasures are discussed, and further technical developments that are necessary to reach the FETS specifications are described. In addition, various experimental shapes of the ion source discharge pulses and H− beam pulses were achieved by controlling the ion source discharge current and accelerated through the ISIS 70 MeV linac. The technique allows almost arbitrary shaping of the H− beam pulses for injection studies into the 800 MeV rapid cycling proton synchrotron.