Beam stabilization of the J-PARC ion source by ARC current feedback control

Okoshi, Kiyonori ; Ito, Yuichi; Ueno, Akira  ; Oguri, Hidetomo  

The negative hydrogen ion source driven by an arc discharge with a lanthanum hexaboride (LaB) filament is under operation for the J-PARC (Japan Proton Accelerator Research Complex). In order to minimize the beam loss in the J-PARC, especially in the 50 GeV-MR (Main Ring), the allowable beam fluctuation of the ion source is very low ( 0.3 mA). On the other hand, the arc impedance is always changing due to the accumulation of boron on the low temperature surface of the filament and the exhaustion of the filament itself. The rather rapid increase of the arc impedance is produced by the former reason for a few days after the start-up of the overhauled ion source operation. Since the arc voltage is set to a constant value, basically the beam current is stabilized by keeping the arc current, that is arc impedance. Therefore, a control program simply negative-feedbacking the difference between the measured arc current and the setting one to the setting of the filament voltage was used at first. However, the beam current fluctuation by the feedback was not negligible to minimize the beam loss at the MR due to the insufficient setting resolution of the filament voltage (0.01 V). Therefore, the feedback system was installed into the program, in order to keep the arc current more precisely. The arc current fluctuation was reduced from about 0.6 % to 0.2 % by the improved feedback system. Consequently, the beam current fluctuation was successfully reduced to within about 0.1 mA.