Status Report of the Experimental and Theoretical Investigations in the Garching Electron Ring Accelerator

In the Pustarex electron ring accelerator low-intensity electron rings were accelerated without ions. Rings with larger electron numbers ran into instabilities with a radial precessional mode, which led to ring widening and particle losses. With resistive walls this instability - probably the resistive wall instability - occurred rather early. After replacement of the resistive walls by capacitive walls the instability happened mostly at the end of compression. Rearrangement of the system is planned to improve the symmetry of the fields and enhance the Landau damping coefficient. To increase the negative mass threshold of the number of trapped electrons in the ring, a beam with instantaneous energy spread has been developed. An arrangement which results in a beam with two energy peaks 5% apart is described. Also connected with the negative mass instability are calculations of the coupling impedance of an electron ring to a realistic squirrel cage. A few of the results, which show a rather large impedance with additional resonances, are presented. The present paper reports three main subjects: 1. the experiments with the Garching Pustarex electron ring accelerator; 2. results of an experiment to produce electron beams with instantaneous energy spread; 3. calculations of the coupling impedance of an electron ring to a squirrel cage structure.