How to Make Extraction Electrode Current Lower than Beam and Corresponding Beam Qualities in J-PARC Cesiated RF-Driven H- Ion Source 66 mA Operation

The beam brightness of the Japan Proton Accelerator Research Complex cesiated RF-driven H- ion source has been increased by using several unique measures, such as a low plasma electrode temperature operation, slight H2Os feeding in hydrogen plasma and so on. The source enables high energy linear accelerators (LINACs) to eject a beam with 60 mA peak intensity. In order to use the source stably for the LINACs operated with a duty factor higher than the present value of 1 msecx25 Hz, the conditions to minimize the extraction electrode current (I-EE), whose main component was the electron current co-extracted with a 66 mA beam, were investigated. The 66 mA H- ion beam with a low I-EE of 39 mA, which was about one-fourth of that in the ordinary operation, were stably operated by optimizing rod-filter-field (RFF), cesium (Cs) density and axial magnetic field correction (AMFC). Especially, the AMFC of only-30 Gauss had the largest I-EE reduction by about one-third. The corresponding 95 % beam transverse normalized rms emittances were degraded by about 24 % due to the Cs density and RFF higher than those in the ordinary operation.