Cryogenic RF performance of double-quarter wave cavities equipped with HOM filters

Crab cavities are one of the several components included in the luminosity upgrade of the Large Hadron Collider (HL-LHC). The cavities have to provide a nominal deflecting kick of 3.4 MV per cavity while the cryogenic load per cavity stays below 5 W. Cryogenic RF tests confirmed the required performances in bare cavities, with several cavities exceeding the required voltage by more than 50%. However, the first tests of a Double-Quarter Wave (DQW) cavity with one out of three Higher-Order Mode (HOM) filters did not reach the required voltage. The present paper describes the studies and tests conducted to understand the limiting factor in the operation of a DQW cavity with HOM filters. The recipe to meet the performance specification and exceed the voltage requirement by more than 35% is discussed.