  A gas-filled reverse cyclotron for the thermalisation of energetic beams is
under construction at NSCL/MSU. Rare isotopes produced via projectile
fragmentation after in-flight separation will be injected into the device and
converted into low-energy beams through buffer gas interactions as they spiral
towards the centre of the device. The extracted thermal beams will be used for
low energy experiments such as precision mass measurements with traps or laser
spectroscopy, and further transport for reacceleration. Detailed calculations
have been performed to optimize the magnetic field design as well as the
transport and stopping of ions inside the gas. An RF carpet will be used to
transport the thermal ions to the axial extraction point. The calculations
indicate that the cyclotron gas stopper will be much more efficient for the
thermalisation of light and medium mass ions compared to linear gas cells. In
this contribution we will discuss simulations of the overall performance and
acceptance of machine, the beam matching calculations to the fragment separator
emittance, and the construction status.
