Properties of a Superconducting Niobium Test Cavity at 350 MHz

Measurements of Q-values as a function of heat treatment and rf field strength for a reentrant niobium test cavity having a gap of one cm indicated that the high field performance was limited by field emission at 25 MV/m peak 5urface field. Q-values at low power of 1.2 x 109 at 4 K and 7 x 109 at 2 K wepe degraded to 2 x 108 at 4 K and 3 x 108 at 2 K after rf processing. Excellent stability was obtained by mechanical control of the resonant frequency of the cavity using a piezoelectric motor made of quartz transducers. The fractional tuning rate of the uncontrolled cavity resonance was observed to be 1.7 x 10-6 per Torr.