New Journal of Physics (Jan 2017)
Cesiated surface H− ion source: optimization studies
Abstract
The H ^− ion beam intensity required for high-energy and high-intensity proton accelerators is continuously increasing. The required 95%-beam transverse normalized root mean square emittance ( ε _95%rnms _x _/ _y ) of the beam is around 0.25 π mm mrad for all accelerators. The Japan Proton Accelerator Complex (J-PARC) 400 MeV linear accelerator (LINAC) succeeded in accelerating the world’s highest-class H ^− ion beam of 50 mA with a cesiated RF-driven H ^− ion source. This was achieved by increasing the beam brightness through the following measures: (1) 45°-tapered plasma electrode (PE) with a 16 mm thickness to increase beam intensity by 56%, (2) continuous-wave igniter plasma driven by 50 W 30 MHz RF to reduce hydrogen pressure in the plasma chamber (PCH) by 50% and beam loss in low-energy beam transport by 12%, compared with that by the commonly used 300 W 13.56 MHz RF, (3) axial magnetic-field correction around the PE beam aperture to increase beam intensity by a maximum of 15%, (4) operation at a low PE temperature ( T _PE ) of about 70 °C to reduce ε _95%nrms _x _/ _y by 27%, (5) suitable beam apertures of the plasma and the extraction electrodes to increase beam intensity by a maximum of 7% and to reduce ε _95%nrms _x _/ _y by more than 4%, (6) argon/nitrogen elimination and 39% filter-field reduction to reduce ε _95%nrms _x _/ _y by 9% and the required 2 MHz RF power by around 30%, (7) eight-hours conditioning with a 50 kW 2 MHz RF and a 5% (1 ms × 50 Hz) duty factor to reduce ε _95%nrms _x _/ _y by 15%, and (8) slight water molecules (H _2 Os) feeding in hydrogen to avoid ε _95%nrms _x _/ _y increase by 72% and divergence angle expansion by 50%. In the studies, we investigated principally the 66 mA H ^− ion beams extracted from the source in order to achieve a 50 mA beam at the J-PARC LINAC exit regardless of the beam’s brightness. Consequently, the source can produce the required beam for a 60 mA J-PARC LINAC operation, since the world’s brightest-class beam with the ε _95%nrms _x _/ _y of 0.23 π mm mrad and beam intensity of 66 mA is successfully produced through the above measures.
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