Dielectric property measurements on large alumina vacuum seals used on Fermilab accelerator rf cavities

The rf window vacuum seals on the 400-GeV Fermilab accelerating rf cavities are very large circular cylinders of alumina. The cavities were first installed with seals of 94 percent purity Al2O3. Several of these seals failed in high-power operation due to dielectric heating. Higher purity ceramics (99.5 percent Al2O3) were obtained and are now being installed. The complex permittivity of both types of ceramic was measured in a TM010 mode test cavity ten inches high and eight feet in diameter. Frequency shift and Q shift data were analyzed by perturbation analysis and by computer programs. Results confirm that the higher purity ceramics have a loss tangent a factor of five better than the original seals. Some high-power operational test results are also presented. (auth)