JLab, Newport News, Virginia, USA
PKU, Beijing, People's Republic of China
We report on results of a new investigation into the Q0 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 1994, has remained unresolved up to current period, despite much effort. Recently, a new investigation has been launched, taking advantage of the latest cryomodule to undergo 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 remnant magnetic flux near a cavity inside of all magnetic shielding. New springs with superior magnetic properties have been found, evaluated and implemented into the current cryomodule. In this contribution, we will review the data accumulated so far. Options for complete Q0 preservation of assembled cavities and possible Q0 remediation for those 330 cavities already installed in CEBAF will be presented.
