High Order Modes Survey and Mitigation of the CEBAF C100 Cryomodules

Ten new C100 cryomodules have been fabricated and installed for the CEBAF 12 GeV upgrade project in the past few years. The dipole high order modes (HOM) of these modules need to be controlled to avoid beam breakup (BBU) instability. Over the last few years, we surveyed the HOM for all the 80 cavities of the C100 modules in the Vertical Test Area (VTA), as well as in the JLAB Cryomodule Test Facility (CMTF) and the CEBAF tunnel. Additional measures such as waveguide filters were applied to reduce the quality factor of the out of spec modes. In addition, we also measured the fundamental mode passband (a.k.a. the same passband) of all the cavities. In this paper, we will present the HOM survey methodology and results from CMTF and CEBAF survey, as well as the same passband mode results. We will also discuss the causes and mitigation measures of the high Q modes.