Field emission studies in vertical test and during cryomodule operation using precise x-ray mapping system

Field emission is one of the most serious issues in the degradation of superconducting cavity performance. However, surveying field emission sources during the cavity performance test (called the "vertical test") and cryomodule operation is difficult. Therefore, to precisely investigate electron emissions from the field emission source, we developed a diagnostic system for the energy recovery linac 1.3 GHz nine-cell superconducting cavity for both vertical tests and the cryomodule operation. The developed system is comprised of two types of sensors: a carbon sensor and a Si p-i-n diode, that measure the temperature rise and the radiation produced by electron emissions, respectively. Rotating the sensor array around the cavity axis in the vertical test provides detailed information on the entire cavity surface. The precise x-ray mapping profile measured by the system in the vertical test enables identification of the local emission source. This paper describes how the field emission source is identified in the vertical test from the standpoint of not only experimental results obtained with the newly developed rotating mapping system but also the detailed approach based on precise simulations. In addition, field emission studies with the optimized instrumentation have been extended to cryomodule tests, both standalone and in the beam line. These developments allow us to monitor changes in the cavity field emission signatures introduced by cryomodule assembly and beam operation.