Results from some temperature mapping experiments on Nb3Sn RF cavities

In a previous paper the authors reported about the performances of several high purity niobium cavities which had been coated with Nb3Sn layer by the vapor diffusion process. These cavities performed quite well, showing low residual surface resistance values in the norm range and peak electric surface fields above 25 MV/m with little electron loading. However, a rather strong dependence of the Q-value on the field gradient in the cavity was observed. They have applied a rotating temperature mapping system to some of these cavities during re-testing and acquired temperature maps in subcooled helium at different field levels in the cavities. Three different types of heating distributions were found: a broad area heating along an azimuth caused by trapped magnetic flux generated by thermocurrents, localized heating due to a defect, and homogeneous heating over the whole cavity surface