Measurements of Superconducting Niobium Cavities at 700 MHz

Niobium TM010 cavities of different accelerating gap, but identical field emission geometries, were studied at 0.7 GHz. The measurement of Q0(Ep)-depressions x-rays and electrons Produced at high Ep-fields show enhanced field emission (FEM). This FEM loading depends on accelerating gap due to electron trajectories and impact generating slow (TSE) and fast (BSE) secondary electrons. In wide gap (dg¿c/2f), oxide coated Nb cavities enhanced FEM produce BSE which in crossing the gap drain energy out of the rf field. This limits the field already at Ep¿107V/m (f/GHz). Hidden in this FEM-BSE loading one side multipactor trajectories of second order at Bp=10mT have been identified, which were fed by FEM electrons and could be overcome.