HIGH FIELD TEST RESULTS OF SUPERCONDUCTING 3.9-GHZ ACCELERATING CAVITIES AT FNAL

The XFEL facilities are planning to use a section with several third harmonic cavities (3.9 GHz) to improve beam performance [1]. Fermilab is developing the superconducting third harmonic section for the FLASH (TTF/DESY) upgrade. This section will include four cavities equiped with couplers and blade tuners installed in a cryostat. Currently, two cavities are completed and one of them is under vertical test. The gradient of this cavity was limited by multipacting in the HOM coupler. Visual inspection of the HOM couplers after cold tests showed that both couplers were damaged. In this paper we discuss the results of the vertical tests, an analysis of multipactoring in the HOM coupler, and a new design for the HOM coupler.