Cesiated surface H− ion source: optimization studies

• Published in: New journal of physics 2017-01, Vol.19 (1), p.15004
• Main Author: Ueno, Akira
• Format: Article
• Language: English
• Subjects: Apertures; Argon; Brightness; cesiated surface ion source; Continuous radiation; Divergence; Electrodes; Emittance; Ion beams; Ion sources; Linear accelerators; negative H ion source; Optimization; Physics; Plasma; Proton accelerators; RF-driven ion source; Root mean square emittance; Water chemistry
• ISSN: 1367-2630; 1367-2630
• DOI: 10.1088/1367-2630/aa52e3

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 ( 95%rnmsx/y) of the beam is around 0.25 πmm 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 (TPE) of about 70 °C to reduce 95%nrmsx/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 95%nrmsx/y by more than 4%, (6) argon/nitrogen elimination and 39% filter-field reduction to reduce 95%nrmsx/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 95%nrmsx/y by 15%, and (8) slight water molecules (H2Os) feeding in hydrogen to avoid 95%nrmsx/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 95%nrmsx/y of 0.23 πmm mrad and beam intensity of 66 mA is successfully produced through the above measures.