Development of a Cs-seeded rf-driven H¯ ion source for the J-PARC

In order to satisfy the J-PARC (Japan Proton Accelerator Research Complex) 2nd stage requirements of an H¯ ion beam current of 60mA within normalized emittances of 1.5πs·mrad both horizontally and vertically, a flat top beam duty factor of 1.25% (500μsx25Hz) and a life-time of more than 50 days, the J-PARC rf-driven H¯ ion source is under development. It was designed based upon the operating J-PARC ion source with several modifications; the adoptions of an rf-antenna developed at the SNS (Spallation Neutron Source) instead of a LaB₆-filament, the cooling plate attached to the plasma electrode (PE), the rod filter magnets instead of the external filter magnets, the cesium (Cs) oven, the axial magnetic field correction (AMFC) coil, the enlargement of the plasma chamber inner diameter from 100mm to 120mm and the shorting of extraction and acceleration gaps, and so on. The source plasma is produced by an about 50W CW 30MHz-rf and an about 40kW pulsed 2MHz-rf. The Cs is installed into the plasma chamber by opening the Cs-valve between the plasma chamber and the Cs oven, which is heated to 200∼240degC. The H¯ ion beam current is maximized by optimizing the Cs quantity depending upon the Cs-valve opening time, the PE temperature depending upon the flow rate or heater power around air pipe of the air through the stainless pipe brazed to the PE cooling plate, the AMFC by the AMFC coil current, and so on. The experimental results using the nickel (Ni) plated plasma chamber made of oxygen free copper (OFC), which satisfied the above requirements, were reported last year. The experimental results using the plasma chamber made of stainless steel (SUS) are presented in this paper. The stainless steel was adopted in order to save the weight. It was necessary to install the integrated plasma chamber from the PE to the end-plate, which made it possible to pre-check the vacuum leak and to shorten the time to exchange the ion source as same as the operating J-PARC ion source, by hands. Although the necessary amount of Cs for the SUS chamber was much smaller than that for the Ni-plated OFC chamber, the former H¯ ion beam current was about 20% lower than the later one. (author)