Space Charge Effects in Cyclotron Gas Stopper

Abstract: The cyclotron gas stopper is a newly proposed device to stop energetic rare isotope ions from projectile fragmentation reactions in a helium-filled chamber. The radioactive ions are slowed down by collisions with a buffer gas inside a cyclotron-type magnet and are extracted via interactions with a Radio Frequency (RF) field applied to a sequence of concentric electrodes (RF carpet). The present study focuses on a detailed understanding of space charge effects in the ion extraction region. The space charge is generated by the ionized helium gas created by the stopping of the ions and eventually limits the beam rate. Particle-in-cell simulations of a two-component (electron-helium) plasma interacting via Coulomb forces were performed in the space charge field created by the stopping beam.