Beamlet Profiles from Multiple-Hole Ion-Extraction Systems

Ion-beamlet structures were studied experimentally to find the conditions for which ion beamlets had noncircular profiles. Seven or less ion beamlets were extracted from a discharge chamber using a multiple-circular-hole ion-extraction system that was composed of a pair of screen and accelerator grids. A beam separator was employed to eliminate all but one ion beamlet. The isolated ion beamlet was observed three dimensionally using arrayed Faraday probes, which showed that the ion-beamlet profiles were distorted hex-symmetrically under certain conditions. To measure the degree of distortion, a distortion factor was introduced. The ion beamlet was generally a superposition of a circular-profiled beamlet consisting of the major part of the ions and a distorted-profiled beamlet consisting of the minor part of the ions. Attempts to demonstrate the hex-symmetrlcal profile computationally using a conventional three-dimensional numerical ion-extraction model that assumed a constant ion density over all of the ion-trajectory origins failed, but the results indicate that inclusion of nonuniform ion production would solve the problem. (Author)