Charge change-induced beam losses under dynamic vacuum conditions in ring accelerators

Abstract

Intense heavy ion beams from the Gesellschaft für Schwerionenforschung (GSI-Darmstadt) accelerator facilities are investigated for their potential to drive inertial fusion targets and are currently used to generate and probe high-energy-density (HED) matter. The existing heavy ion synchrotron facility SIS-18 delivers intense uranium beam pulses for experiments with up to 4 × 109 ions per bunch at a charge state of q = 73+. Higher intensities are potentially available at lower charge states, when related loss mechanisms can be overcome. We have observed intensity-dependent beam losses in SIS-18. The origin of these beam losses is attributed to processes changing the charge state of ions, which is predominantly due to collisions with rest gas atoms. The resulting change in the mass over charge ratio m/q leads to modified trajectories in dispersive beam transport elements, and finally to the loss of the particle at the vacuum tube wall. At the impact position secondary particles are produced by ion-induced desorption and as a result the pressure in the vacuum tube is increased locally. This local rise in pressure in turn leads to increased charge changing processes starting an avalanche process that may eventually lead to a complete loss of the beam during a few turns in the synchrotron. In this paper, we discuss a method to control the problem of desorbed gases with specifically designed beam catchers at critical positions. We have further developed a program package to calculate the most important beam loss mechanisms and to couple them to the dynamic vacuum problem. The basis of this simulation is an ion-optics routine where relevant atomic processes and the effects of dynamic vacuum are implemented. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.