Pande, S.A. Burt, G. Christou, C. Gu, P. JACoW Geneva, Switzerland 978-3-95450-191-5 10.18429/JACoW-SRF2017-TUPB010 English 405-408 TUPB010 ion cavity pick-up GUI electron Contribution to a conference proceedings 2018 2018-01 https://doi.org/10.18429/JACoW-SRF2017-TUPB010 http://jacow.org/srf2017/papers/tupb010.pdf The Diamond storage ring operates with two CESR-B type Superconducting RF cavities. The cavities suffer from trips with a sudden loss of accelerating field if operated above a certain voltage. Consequently the cavities are operated at voltages up to 1.4 MV for better reliability. These cavities are iris coupled and have fixed Qext. At these lower operating voltages, the optimum condition for beam loading is satisfied at powers around 100 kW. For operation at 300 mA with two cavities, the power needed per system exceeds 200 kW. Therefore 3 stub tuners are used to lower the Qext to move the optimum condition close to 200kW. Additionally, the step due to the difference in the height of the coupling waveguide on the cavity and that of the vacuum side waveguide on the window results in a standing wave between the cavity and the window even at matched operation. The 3 stub tuner further enhances this standing wave. Numerical simulation reveals that the standing wave field from the cavity penetrates into the coupling waveguide increasing the probability of multipactor and breakdown in the coupler region. The results of multipactor simulations in this region with CST Studio are discussed.