Field tuning of a radio-frequency quadrupole using full 3D modeling

Morishita, Takatoshi  ; Kondo, Yasuhiro   ; Oguri, Hidetomo  ; Hasegawa, Kazuo; Otani, Masashi*

The radio-frequency quadrupole (RFQ) is operating in the frontend of the J-PARC linac to accelerates 50-mA negative hydrogen beams from 0.05 MeV to 3 MeV. As a backup, the spare RFQ has been fabricated in 2018. The vane-voltage ramping is adopted to improve the acceleration efficiency so that the cross-sectional shape is adjusted longitudinally to produce the designed voltage distribution. Then, the three-dimensional cavity models including modulations and cutbacks were created in CST Micro-Wave Studio. The vane-base widths and cutback depths were optimized to produce the desired vane-voltage distribution. In the final tuning, the insertion depths of the stub turners were also determined based on the tuner responses obtained from the full 3D models.