• F. Marhauser, J. Henry, H. Wang
  JLAB, Newport News, Virginia

The 12GeV upgrade of CEBAF is currently in progress. Ten new cryomodules will be installed at completion of the project to increase the energy from 6GeV to 12GeV. Each cryomodule houses eight seven-cell Low Loss type cavities. The damping of HOMs is crucial to prevent from beam break-up (BBU) instabilities at the desired beam currents as experienced with an upgrade demonstration cryomodule which needed to be de-installed recently. Detailed HOM surveys of a complete string of cavities in a cryomodule as well as individual cavities revealed the existence of critical dipole modes below and above beam tube cutoff that needed extensive experimental and numerical analyses. Results and their consequences for the 12 GeV upgrade cryomodules are detailed.