Quality requirements and control of high purity niobium for superconducting RF cavities

Abstract

Niobium is a mainly used material for the fabrication of superconducting accelerating RF cavities. The proposed e +e − linear collider TESLA demands about 500 tons of niobium. High purity Nb should reach the TESLA specification. The required high thermal conductivity can be additionally improved by the post-purification. The quality control includes the residual resistivity ratio measurement, microstructure analysis, analysis of interstitial and metallic impurities, hardness measurement, tensile test, examination of the surface roughness and search for clusters. An eddy current scanning system with rotating table is applied for diagnostic of cracks and foreign material inclusions in niobium sheets. 100% of sheets are eddy current tested. Synchrotron fluorescence analysis and neutron activation analysis used for supplemental non-destructive identification and investigation of detected defects. More than 1000 niobium sheets for TESLA test facility were examined. A SQUID based scanning system has potentially higher sensitivity as a conventional pick up coil. First prototype of SQUID apparatus demonstrates sensitivity sufficient to detect inclusions as small as 0.1 mm in diameter.