Negative Ion Beam Production in an Ion Source with Chamfered Extraction Opening

The Particle-in-Cell method based numerical model of negative ion (H-) beam production in an ion source with chamfered extraction channel is presented. The model enables calculations of the charge density and electrostatic potential distribution as well as determination of extracted ion current. Influence of the chamfering angle on H- density distributions and the obtained H- current is under investigations. Major (by an order of magnitude) increase of the current is observed in the case of chamfered opening as the most of the extracted H- ions are produced at the extraction channel walls. Changes of the extracted current due to H- ion flux outgoing from the plasma grid are also studied. Current-voltage characteristics of the ion source both with non-chamfered and chamfered extraction channels are presented. Saturation of the C-V curve in the latter case for V-ext larger than 10 kV is observed. A transition of beam profile shape from a single maximum broad beam (V-ext < 0.5 kV) through the ion beam with two maxima to a very intense broad ion beam (V-ext = 10 kV) is presented and discussed.