A radio frequency system with a fourth-harmonic "Landau" cavity suppresses coupled-bunch instabilities and increases the beam lifetime of the Aladdin electron storage ring. When the storage ring is operated with a small momentum compaction, instabilities limit the utility of the Landau cavity. Analytical modeling of instability frequencies and growth rates, simulations, and experiments suggest that the observed instabilities result from coupling between dipole and quadrupole Robinson modes.

The longitudinal wake is considered for a bunch of electrons that are suddenly accelerated to an ultrarelativistic velocity. This wake describes the wake of forward transition radiation, and it approximates the edge-radiation wake of a bunch exiting a bending magnet. The wake is large within the radiation formation zone, where it provides resistive impedance. A comparison with the computed wake downstream of a bending magnet yields good agreement, indicating that our wake expressions may be used to approximate the wake without numerical computation. For schemes in which a bunch produced by laserplasma acceleration exits the plasma and then drives a free electron laser (FEL), the transition-radiation wake causes energy losses of many MeV that may affect the FEL process.

Supersonic, counterstreaming plasmas were produced by ablating plasma from the inside surfaces of two parallel disks made of aluminum and magnesium, respectively, with a 0.53 pm laser at an intensity of lOi W/cm2 for 1.3 nsec. Diagnostics included holographic interferometry, a time-integrated x-ray pinhole camera and a gated x-ray crystal spectrograph with imaging slits. The plasmas interpenetrate for the first half of the laser pulse but stagnate once the electron density exceeds 5 X 102' cmF3. Spectroscopic measurements suggest a coronal electron temperature of -800 eV and an ion temperature of -15 keV in the stagnated plasma. The observations are in good agreement with a two ion fluid model of interpenetrating plasmas in which the dominant slowing down process is ion-ion collisions.

JIIIIIPS and hysteresis in discharge current are produced in a low-pressure ( loA4 Torr), magnetic field free, thermionic argon plasma discharge by varying discharge voltage, neutral pressure. or filament temperature. This behavior can be attributed to a region of negative differential resistance in the voltage versus current characteristic of the discharge. A model is presented in order to understsnd the physical processes leading to this negative resistance.

An experimental study of the confinement properties of a low-(average) beta discharge plasma in a spindle cusp magnetic field is described. Electron and ion densities, space potentials, and plasma flow velocities were measured in the ring and point cusps. The leak width of the escaping plasma was measured over a large range of magnetic field strengths and neutral pressures. Leak widths varying between the hybrid gyrodiameter and the ion gyrodiameter were observed. The dependence of the leak width on neutral pressure and magnetic field is accounted for by a simple model in which plasma diffuses across the magnetic field as a result of neutral particle collisions while streaming out of the cusps along the magnetic field lines. At the lowest neutral pressures investigated, the scaling of the leak width with magnetic field strength suggests that noncollisional mechanisms (e.g., Bohm diffusion) determine the leak width.