Recent Experimnts on Forming Electron Rings at Berkeley

In the initial electron-ring experiments with a new high-current, 2 MeV electron injector an efficient method of trapping of the injected beam was developed, and a suitable compression cycle was found in which betatron amplitude growth on single-particle resonances is negligible. The ring behavior is satisfactory during the compression cycle at low intensity (Ne < 5 × lO11). At higher intensities the lowest mode of the coherent radial instability appeared, but the threshold for its onset was raised considerably by increasing the Landau damping coefficient, so that it is no longer the limiting instability. At intensities Ne > 1012 electrons a longitudinal (negative mass) instability causes particle loss and radial spreading of the electron rings. Increasing the energy spread in the beam has raised the threshold for several phenomena observed in connection with this instability. The effects of changing the electromagnetic environment of the electron rings to suppress this instability are being investigated.