Advances in production of high purity Nb for RF superconductivity

In the last three years, the purity of commercially available Nb has improved a factor of 10 in response to the needs of rf superconductivity for application to electron-positron storage rings for high energy physics as well as to electron linacs for nuclear physics and free electron lasers. Yttrification of this premium quality Nb enhances the purity by yet another factor of 2-3. Superconducting cavities built with high purity Nb provide higher accelerating gradients, increasing roughly as the square root of the purity enhancement. Results are presented on analysis of the interstitial impurity content on the ingot and final sheet product as well as on the sheet properties of the final product that are important to cavity manufacturers, such as residual resistivity ratio, yield strength, grain size, etc. Prospects for further advances are discussed.