Recent Measurements at SLAC on Superconducting Niobium X-Band Cavities

Measurements at 8.6 GHz on TM mode superconducting niobium cavities have been carried out at SLAC in an attempt to establish definitive conditions for reproducibly attaining high peak electric and magnetic fields and high residual Q's. Four cavities, processed by techniques which insure the presence of an oxide layer on the niobium surface before final high temperature outgassing, have given peak magnetic fields exceeding 1000 G and corresponding peak electric fields in excess of 56 MV/m. From this and related experience it is speculated that, in order to achieve high peak fields, carbon present on the niobium surface must be removed through the formation of volatile compounds with oxygen or fluorine during high temperature processing. Data are also presented on the effect on rf properties of exposure at room temperature to various gases. Measurements on anodized cavities are briefly discussed.