Cavity voltage calibration for J-PARC Ring RF

Schnase, A.; Ezura, Eiji*; Hara, Keigo*; Hasegawa, Katsushi; Nomura, Masahiro  ; Omori, Chihiro*; Shimada, Taihei ; Suzuki, Hiromitsu; Takagi, Akira*; Tamura, Fumihiko   ; Toda, Makoto*; Yamamoto, Masanobu   ; Yoshii, Masahito*; Takata, Koji*; Anami, Shozo*; Haga, Koichi*; Horino, Koki*

J-PARC RCS (Rapid Cycling Synchrotron) is under commissioning and reached the milestones of acceleration to 3 GeV and extracting the beam to MLF and MR (Main-Ring). We found a discrepancy between expected synchrotron frequency as function of voltage seen by the beam and measured value at injection energy. Also we saw a difference between expected and measured synchronous phase during acceleration. The voltage seen by the beam seemed to be lower than the voltage by the acceleration cavities. We found that the High Voltage probes used at calibration gave a too high value due to unwanted capacitive coupling. We reduced the coupling by probe tip extension and calibrated all 10 RCS cavities again. In RCS beam operation we confirmed that the discrepancy was resolved, so that the beam sees the expected voltage. This knowledge was used in MR calibration. We confirmed that expected and measured synchrotron frequency match very well. Expected and voltage seen by the beam are within 2%.