Thermal Tests of HOM Couplers for Superconducting Cavities

In order to restrict the multi-bunch phenomena due to long-range wakefields in the TESLA linac, the higher order modes of the cavity must be damped down to the 104-lo5 level. For this purpose, HOM-couplers, mounted at both ends of the cavity, must couple strongly to the most dangerous modes, while rejecting by means of a filter the accelerating mode. Two versions have been developed [ 11, one is welded to the beam tube and the other one is demountable thanks to an intermediate flange. In this paper, we present the results of high power tests performed on the demountable version with a special test stand, including a single-cell cavity, a 5kW klystron and the coupler itself, as well as thermal simulations, which were initiated previously [2] with the help of a finite elements code. Accelerating field of 21 MV/m in cw mode, limited by cavity quenches and not by the coupler, was achieved, giving a confortable safety margin if one keeps in mind that the duty cycle of the TESLA pulsed mode is lower than 2 %. Moreover, by intentional dehming of the filter, the coupler was capable to transmit a peak power of 1.75 kW for the TESLA beam pulse duration, higher than the maximum HOM power which could be resonantly excited by the beam.