HOM damping performance of JLab SL21 cryomodule

The SL21 cryomodule is a first 1.5 GHz, 12.5 MV/m, 7-cell cavity, 8-cavity string superconducting accelerator module produced at JLab. The first two passbands of TE111 and TM110 high order modes (HOM) have to be damped to avoid beam breakup problems at 460 /spl mu/A current. External Q's of the HOM couplers and waveguide were measured on a copper model, on cold niobium cavities in vertical tests and finally in the cryomodule without beam. This paper presents all HOM data and general measurement techniques. A threshold current predicted by MATBBU code based on these data and the 6 GeV machine beam optics is 10 mA.