New hom coupler design for 3.9 GHZ superconducting cavities at FNAL

Over the last few years Fermilab has developed the superconducting third harmonic section for the FLASH (TTF/DESY) upgrade. Initial vertical tests of 9-cell Nb cavities didn´t reach the designed accelerating gradient. The main factor of gradient limitation was multipacting in the HOM coupler. In this paper we present the results of vertical tests accompanied with 3D analysis simulations of multipacting. Also we discuss the RF design of the new HOM couplers. The goal of the new design is to eliminate multipacting and to increase the second resonance frequency of the HOM coupler. Increasing this frequency reduces electric and magnetic fields, resulting in decreased thermal load on the antenna. Two cavities with modified HOM couplers have achieved operating gradients of 23 MV/m.