Optimization of a charge-state analyzer for ECRIS beams

摘要

A detailed experimental and simulation study of the extraction of a 24 keVHe-ion beam from an ECR ion source and the subsequent beam transport through ananalyzing magnet is presented. We find that such a slow ion beam is verysensitive to space-charge forces, but also that the neutralization of thebeam's space charge by secondary electrons is virtually complete for beamcurrents up to at least 0.5 mA. The beam emittance directly behind theextraction system is 65 pi mm mrad and is determined by the fact that the ionbeam is extracted in the strong magnetic fringe field of the ion source. Therelatively large emittance of the beam and its non-paraxiality lead, incombination with a relatively small magnet gap, to significant beam losses anda five-fold increase of the effective beam emittance during its transportthrough the analyzing magnet. The calculated beam profile and phase-spacedistributions in the image plane of the analyzing magnet agree well withmeasurements. The kinematic and magnet aberrations have been studied using thecalculated second-order transfer map of the analyzing magnet, with which we canreproduce the phase-space distributions of the ion beam behind the analyzingmagnet. Using the transfer map and trajectory calculations we have worked outan aberration compensation scheme based on the addition of compensatinghexapole components to the main dipole field by modifying the shape of thepoles. The simulations predict that by compensating the kinematic and geometricaberrations in this way and enlarging the pole gap the overall beam transportefficiency can be increased from 16 to 45%.