SRS RF cavity windows

Operation of the SRS storage ring was prevented for the last three months of 1983 by a succession of cavity window failures. In order to provide broadband and high coupling factors the SRS uses an aperture type window. Several failures were caused by the growth of high conductivity areas on the windows, resulting in additional local heating and eventual thermal runaway. Although on-line monitoring of the ceramic temperature was introduced it was necessary to reduce the SRS operating level to 1.8 geV to ensure adequate window lifetime. In order to explore the failure mechanism an extensive program of material analysis on damaged windows has been carried out. The merits of different anti-multipactor window coatings have been reassessed, and the advantage of copper black over alternatives, such as titanium, has been confirmed. Improvements to the Daresbury coating system have been introduced. Theoretical and experimental studies of the electromagnetic field distribution in the region of the waveguide-cavity transition have led to a change in window location, and operation in a test cavity confirmed that this produced the expected temperature reduction. All SRS windows have now been modified and full 2 GeV operation restored