A new purification technique for improving the thermal conductivity of superconducting Nb microwave cavities

Improvement factors up to 4 in the thermal conductivity of Niobium between 4 and 9K are obtainted by a new purification technique which removes the dominant interstitial impurity (oxygen) found in commercial Nb. In the process, surfaces of the cavity are brought into proximity with Yttrium foil at -10^-5torr for several hours. A vapor deposited film of several μm thickness traps oxygen diffusing rapidly from the bulk to the surface. A single cell, 1500 MHz, elliptically shaped cavity with thermal conductivity improved by this method shows amelioration of thermal breakdown, as predicted by thermal model calculations.