Coaxial HOM Coupler designs tested on a single cell niobium cavity

Coaxial higher order mode (HOM) couplers have been developed for HERA cavities and are used in TESLA, SNS and JLab upgrade cavities. The principle of operation is the rejection of the fundamental mode by the tunable filter of the coupler and the transmission of the HOMs. It has been recognized recently that inappropriate thermal designs of the feedthrough for the pick-up probe of the HOM coupler will not sufficiently carry away the heat generated in the probe tip by the fundamental mode fields, causing a built-up of the heating of the niobium probe tip and subsequently, a deterioration of the cavity quality factor has been observed in CW operation. An improvement of the situation has been realized by a better thermal design of the feedthrough incorporating a sapphire rf window [1]. An alternative is a modification of the coupler loop (?F? ? part) with an extension towards the pick-up probe. This design has been tested on a single cell niobium cavity in comparison to a ''standard TESLA'' configuration by measuring the Eacc behavior at 2 K. The measurements clearly indicate that the modified version of the coupler loop is thermally much more stable than the standard version.