Cooling of radioactive ions with the Penning trap REXTRAP

Cooling of radioactive ion beams in a Penning trap is an essential component of the post-accelerator REX-ISOLDE at CERN. Prior to their charge-breeding and acceleration, ions from the on-line mass separator ISOLDE are accumulated, cooled and bunched with REXTRAP. This beam preparation provides short ion pulses with low emittance, key ingredient for a high efficiency of REX-ISOLDE. Two different cooling techniques have been investigated with REXTRAP. Both rely on the use of a buffer gas as the coolant but differ in the way the transversal compression of the stored ion cloud is achieved. Sideband cooling with a light buffer gas as coolant is the standard technique used at REXTRAP so far. With this technique an efficiency of about 45% for the injection, cooling, and extraction process has been obtained for stable and radioactive ions. For about 105 simultaneously stored ions the resulting emittance of the extracted ion pulses is about 10πmm mrad at 30 keV beam energy. For much larger numbers of ions shifts of the resonance frequency of the sideband excitation are observed together with an emittance growth. As an alternative to sideband cooling a “rotating wall” technique was tested. First results with rotating dipole as well as quadrupole fields are presented.