Improvement of the operational performance of SRF cavities via in situ helium processing and waveguide vacuum processing

The useful performance range of the superconducting RF cavities in the CEBAF accelerator at Jefferson Lab is frequently limited by electron field emission and derived phenomena. Improvements are required to support future operation of the accelerator at higher than 5 GeV. Twelve operational cryomodules have been successfully processed to higher useful operating gradients via RF-helium processing. Progress against field emission was evidenced by improved high-field Q, reduced X-ray production and greatly reduced incidence of arcing at the cold ceramic window. There was no difficulty reestablishing beamline vacuum following the processing. Cavities previously limited to 4-6 MV/m are now operating stably at 6-9 MV/m. By applying a pulsed-RF processing technique, we have also improved the pressure stability of the thermal transition region of the input waveguide for several cavities