METALLURGICAL ANALYSIS AND RF LOSSES IN SUPERCONDUCTING NIOBIUM THIN-FILM CAVITIES

Copper cavities with a thin niobium film as used in the large electron positron collider LEP - would be also attractive for future linear co lliders, provided the decrease of the Q-value with the accelerating gr adient could be reduced, We aim at extracting the important parameters that govern this decrease, The dependence on the RF frequency is stud ied by exciting 500 MHz and 1500 MHz cavities in different modes. In a ddition we combined RF measurements for two 1500 MHz cavities of diffe rent RF performance with microscopic tests (AFM, TER I) on samples cut out of the same cavities. Their microstructural characterization ill plan-view allows the extraction of the grain size and the defect densi ties.