Characterisation of a localised broad-band impedance phenomenon on the SRS

A major perturbation to the SRS vacuum envelope was required to install a second superconducting wiggler in 1992. As part of this, all of the new wiggler straight (straight 16) vacuum vessels were re-designed to give a good match to the small elliptical chamber of the wiggler, including the sector vacuum isolation valve. Because of engineering problems this valve was not fitted and a standard circular aperture valve of much larger diameter had to be installed. This caused a localised impedance, which manifested itself by gross overheating due to the energy dissipated in it from the beam. A study of the effect of this valve has been conducted and is summarised. All data from the period of installation in the ring is collated, including beam heating and instability data. On removal from the ring a bench measurement of the effective broad-band impedance has been made, using time-domain measurement techniques.