Solving beam intensity bottlenecks and 100 mA operation of J-PARC cesiated RF-driven H− ion source

Abstract

The Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven H− ion source has been stably operated without any serious problems for about four years. The J-PARC LINAC successfully accelerated the 60 mA beam required for the J-PARC, when a 72 mA beam was extracted from the source. The high intensity beam with transverse emittances suitable for the RFQ is produced with several unique measures, such as, slight water molecules addition into hydrogen plasma, low temperature (about 70 °C) operation of 45°-tapered plasma electrode with a 16-mm thickness accompanying with precise cesium density control, impurity (argon and/or nitrogen) elimination in the hydrogen plasma along with filter-field optimization, continuous-wave igniter plasma driven with a 50-W 30-MHz RF, and so on. In order to circumvent the beam intensity bottlenecks of the source, the extraction and acceleration voltages (VE and VA) higher than the design values of 10 kV and 40 kV were examined. A 100 mA beam, whose 93.2 mA has particle distributions in horizontal and vertical phase planes similar to those used for a common RFQ design, was stably operated with a duty factor of 5 % (1 ms x 50 Hz) by using the VE and VA of 12.4 kV and 49.6 kV, respectively. This beam intensity breakthrough will realize the next generation benchmark H− ion source for high intensity and high energy H− LINACs. The important information on the space-charge limited bottlenecks in the extraction and acceleration gaps will derive the optimal electrode shapes for the source operated with a beam intensity higher than 100 mA.