The NSCL cyclotron gas stopper – Preparing to go ‘online’

Abstract

The NSCL has successfully used linear gas stopping cells for more than a decade to slow down near-relativistic projectile fragments to energies of tens of keV; first for experiments at low energy and later for reacceleration to Coulomb-barrier energies. In order to stop and rapidly extract light and medium-mass fast ions, which are difficult to efficiently thermalize in linear gas cells, a gas-filled, reverse cyclotron has been constructed. The device uses a cyclotron-type magnet and helium gas in a liquid-nitrogen (LN) cooled stopping chamber to confine and slow down the injected beam. The stopped ions are transported to the center of the magnet by an RF-carpet system, extracted axially through the bore with an ion conveyor and accelerated to <60 keV energy for delivery to the users.For magnet commissioning and low-energy ion tests, the cyclotron gas stopper was constructed in a location that did not allow connection to the NSCL high-energy beamlines. However, all other components have been installed for the device. Efficient ion transport has been demonstrated with ions from a movable alkali source with the magnet energized.An experimental vault is being prepared to allow the connection of the cyclotron gas stopper to the NSCL beamline. The design for a dedicated momentum-compression beam line, similar to the ones feeding the existing linear gas cells [1,2], is complete and the components are under construction. Offline tests have ended and moving the cyclotron stopper to its destination has begun.