New HOM coupler design for ILC superconducting cavity

Four superconducting cavities have been developed at the superconducting RF test facility (STF) at KEK as candidates for the International Linear Collider (ILC) accelerating cavity. These cavities, which are termed TESLA-type STF baseline cavities, are equipped with newly designed higher-order-mode (HOM) couplers that have a short outer cylinder and a single welding point on the antenna that acts as the inner conductor. Each STF baseline cavity is equipped with two types of these new HOM couplers: STF I-type and STF L-type. The HOM couplers were designed and developed, and their RF performance was evaluated by using a Cooper model. Then, niobium models were fabricated and cool down tests were conducted with these cavities. In all, 14 cool down tests were performed for the four cavities using a vertical setup that involved the newly designed HOM couplers. Electrochemical polishing of the inner surface of the cavity and frequency tuning of the accelerating mode were performed before each cool down test in order to maintain the RF performance and obtain a higher gradient. These new HOM couplers were operated up to a gradient of 32 MV/m in the accelerating field of the end cell without any serious problem. Their damping performance is comparable to that of the TESLA cavity except for the TM011 mode, whose performance can be easily corrected by changing the installation angle of the next fabricated cavity. In this paper, the development of the new HOM couplers for STF baseline cavities is described.