Source size variation and ion effects in the SRS at Daresbury

Source dimensions in the electron storage ring are routinely monitored on a diagnostic beam line. During recent recommissioning of the SRS, following the installation of a second wiggler magnet, relatively poor vacuum conditions were experienced. Large step changes in source size have been measured and broad tune signals have been seen, together with other effects characteristic of ion influences. Changes of source size and beam stability as a function of ion clearing electrode voltage have also been observed. This paper summarises all such effects observed and comments on their probable causes. It also includes other data on source dimensions, such as measured emittance coupling and the effect of insertion devices in the lattice.< >