Field emission studies of heat treated and chemically treated superconducting cavities

With the newest heat-treatment techniques, average surface electric fields of 50 MV/m with a maximum of 60 MV/m have been reached in 1.5-GHz single cell cavities. Using a high-speed temperature mapping system, extensive studies have been carried out on the nature of emitters in RF cavities under various surface treatments. Results show that, as in the case of emitters studied with DC field, emissive areas are randomly distributed between 10/sup -8/ and 10/sup -13/ cm/sup 2/, and beta values fall between 75 and 500. The density of emitters is about 0.2/cm/sup 2/ at 40 MV/m with standard surface treatment, falling by a factor of 10 with heat treatment, which lowers beta values. Clean air, water, and methanol are shown not to be dominant sources of emitters which limit the performance of chemically treated cavities. Other than debris introduced by improper cleaning or assembly procedures, chemical residues or minute impurity inclusions remain important possibilities to be investigated. Condensed gases are shown to enhance emission from potential sites, and He processing is shown be effective against the associated emission, but its effectiveness decreases when higher initial fields are reached by techniques which provide cleaner surfaces.<>