Electromagnetic modeling of the extraction region of the ISIS H− ion source

Abstract

The ISIS Penning surface plasma source, which routinely produces 35 mA of H− ions during a 200 μs pulse at 50 Hz for uninterrupted periods of up to 50 days, is regarded as one of the leading operational sources in the world. The ISIS source should provide an excellent starting point for a development program to produce H− ion sources with performances exceeding those achieved today, which will be a key requirement for the next generation of high power proton accelerators. One goal is to produce 60 mA of H− ions from the source without large departures from the optimum conditions for source lifetime or increased emittance. As the ISIS source operates in the space-charge limited mode it is predicted that an increase in extraction potential from 17 to 25 kV should be sufficient to achieve this, and a suitable pulsed power supply for the ion source extraction electrode has been manufactured. An understanding of how extract geometry changes affect beam transport is essential for operation at higher extraction potential. An examination has been undertaken of the electromagnetic fields in the extract electrode region using MAFIA finite element analysis software. The effects of changing the extraction potential, gap, and electrode geometry are described.