KINETIC-ENERGY MEASUREMENTS OF IONS STORED IN A RF TRAP BY USE OF PHASE-STOPPED EJECTION AND RETARDING POTENTIAL METHODS

The kinetic energy distribution of a cloud of ions stored in a RF trap has been investigated by use of two methods based on phase-stopped ej ection. The oscillation of the RF voltage is stopped at a phase corres ponding to zero field value inside the trap. After their subsequent ej ection from the trap, the ions are detected by an electron multiplier. The instantaneous ion cloud temperature value corresponding to the ph ase-stopping time can be deduced from the recorded profile of the ion signal. An alternative method consists in submitting the ejected ions to a repulsive field, applied before the detector, which allows us to discriminate the velocity classes. In both cases, the experiments are interpreted thanks to a simple theoretical description of a cloud of i ons at thermal equilibrium in a RF trap and to a precise calculation o f the ejection process. Both methods are compared, for the various tra p working points. In all cases, the ion cloud average temperature is o f the order of a tenth of the potential well depth.