In-situ plasma cleaning to decrease the field emission effect of half-wave superconducting radio-frequency cavities

The in-situ plasma cleaning technique using the chemical reactive oxygen plasma to remove the hydrocarbon contaminants, the outmost layer of the inner surface of the HWR cavity, has been explored at Institute of Modern Physics, CAS. For the purpose of cleaning the HWR cavity in the cryomodule by plasma, an offline apparatus for the plasma discharge study was set up, which duplicates the assembly of the cavity inside the cryomodule. Plasma ignition procedures were investigated. And the plasma parameters such as free electron number density, electron temperature and dissociated oxygen atom intensity were diagnosed by optical emission spectra to optimize the operation conditions for plasma in-situ cleaning of the HWR cavity. To evaluate the efficiency of the plasma in-situ cleaning of the HWR cavity, the related experiments were carried out on the vertical test stand. The results show that the hydrocarbon contaminants on the inner surface of the cavity were removed, and the maximal available accelerating gradient of the cavity was improved by 29% due to the mitigation of field emission inside the cavity after plasma cleaning. Details of these experimental results and observations are discussed in this article.