Pursuing the Origin and Remediation of Low Q0 observed in the Original CEBAF Cryomodules

Abstract

We report on results of a new investigation into the Q 0 degradation phenomenon observed in original CEBAF cavities when assembled into cryomodules. As a result, the RF dissipation losses increased by roughly a factor of two. The origin of the degradation, first observed in 1993, has remained unresolved up to current period, despite much effort. Recently, a new investigation has been launched, in parallel with cryomodule refurbishment. Systematic measurements are conducted with respect to the magnetic shielding effects of the double-layer shields and the magnetic properties of various components within the inner shield. This resulted in the new discovery of strongly magnetized strut springs as a major source of remanent magnetic flux near a cavity inside of all magnetic shielding. New springs with superior magnetic properties have been found, evaluated and implemented. Preservation of Q 0 from vertical testing to cryomodule testing in tunnel at 5 MV/m gradient in the range of 79-88% has been measured. Studies for complete Q 0 preservation and possible Q 0 remediation for those 330 cavities already installed in CEBAF are carried out and preliminary results are also presented.