Cavity diagnostic system for the performance test of the 1.3 GHz superconducting 9-cell cavity

A new cavity diagnostic system is introduced for a performance test of superconducting cavity at KEK-STF. It is composed of about 300 carbon resistors and several tens of PIN photo diodes. Heating on cavity surface at a thermal quenching is detected by the carbon resistor, which is called “Temperature-mapping”, and X-ray at field emission by PIN photo diode. This system detected successfully the heating location for every performance test, which is carried out totally seven times, and the causes of the field limit were clearly specified. The most apparent phenomenon is the thermal quenching by defect or contamination on the cavity surface and the next is the field emission. It is not clear whether the multipacting actually limits the cavity field or not.