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% purity Al2O3. Several of these seals failed in high-power operation due to dielectric heating. Higher purity ceramics (99.5% Al2O3) have been obtained and are now being installed. The complex permitivity of both types of ceramic has been measured in a TM010 mode test cavity ten-inches high and eight feet in diameter. Frequency shift and Q shift data have been 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.