Cyclotron gas stopper: simulations and predicted performance

Projectile fragmentation followed by in-flight separation provides fast, chemistry-independent access to a wide range of beta-unstable nuclides. To optimize their use, these exotic beams should be available at energies from rest to several MeV/u. This can be achieved by stopping fast beams in a volume of helium, extracting the stopped ions, and reaccelerating them to the desired energy. A ``cyclotron gas stopper'' has been proposed to overcome the limitations of current and proposed linear gas stoppers. The current design of the NSCL cyclotron-stopper uses a 2 meter diameter superconducting spiral-sector magnet with ion-guiding carpets in the central region. Complete simulations have been performed starting with realistic beam properties for 17 projectile fragments ranging from 6Li to 150I. Details of the NSCL cyclotron-stopper and the simulation package developed to predict its performance will be presented.