Field emission studies from Nb surfaces relevant to SRF cavities

Enhanced field emission (EFE) presents the main impediment to higher acceleration gradients in superconducting rf (SRF) niobium (Nb) cavities for particle accelerators. A scanning field emission microscope was built at Jefferson Lab with the main objective of systematically investigating the sources of EFE from Nb surfaces. Various surface preparation techniques and procedures, including chemical etching, electropolishing, ultrasonic water rinse, high pressure water rinse, air-dry after methanol rinse, air-dry after water rinse in Class 10 cleanroom, were investigated. The capability and process variables for broad-area Nb surfaces to consistently reach field emission free or near field emission free performance at /spl sim/140 MV/m have been experimentally demonstrated using the above techniques/procedures.