110 mA operation of J-PARC cesiated RF-driven H− ion source

The Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven H− ion source (IS) has been stably operated for about six years. From Sep. 2018, the J-PARC 400 MeV LINAC has been steadily operated with the design H− ion beam intensity (IH−) of 50 mA, when a 60 mA beam is injected from the IS. At each end of about 3 months J-PARC operation, the IS supplies a 52.5 keV 72 mA beam for the LINAC 60 mA acceleration. The IS superior emittances predicted in a test-stand were proven by the high RFQ acceleration efficiency of 94.3 % (IH− = 67.9 mA). The high intensity beam with transverse emittances suitable for the RFQ is produced with several unique measures, such as slight water molecule addition into hydrogen plasma; low temperature (about 70 °C) operation of 45°-tapered 16-mm thick plasma electrode with the precise cesium density control; impurity elimination in the hydrogen plasma along with the filter-field optimization; continuous-wave igniter plasma driven with a 50-W 30-MHz RF, and so on. The 110 mA operation results of the IS in the test-stand are presented in this paper. A 65 keV 110 mA H− ion beam, about 103 mA of which was inside the transverse emittances used for a common RFQ design, was stably operated with a duty factor of 4.5 % (1 ms x 45 Hz). The interesting relationship of the IH− without any emittance blowups upon the required beam energy (WH-) was derived as IH−(WH−) = 71.2 x (WH- / 52.5)^2.03. The IS is the benchmark one for the next generation high energy H− LINACs aiming for 100 mA class intensity.