Suppressing of Co-Extracted Electrons in a Negative Ion Source-Numerical Simulation

Electron co-extraction and suppression by a transverse magnetic fields is studied within a two dimensional particle-in-cell numerical model of surface ionisation ion source with beveled extraction opening. A novel approach of data presentation is proposed, based on the fact that dependences of co-extracted current on the filter strength could be approximated by four parameters only, describing e.g. initial electron current value and cut-off B value. In the paper the influence of extraction system geometry is considered – it is shown that the cut-off B value increases with the size of the opening in the extraction electrode, while the inclination of the extraction opening walls does not play any significant role. It is demostrated that the most of electron is eliminated by hiting the extraction electrode walls, however up to 30 % of electrons were lost by encountering the extraction channel walls due to the modification of their trajectories by the filter field. The influence of the magnetic filter field placement is also investigated – the center of the filter field has to be no further than 2 mm form the extraction channel orifice in order to achieve minimal values of the cut-off field (~20 mT). The possibly low extraction voltages are preferable, as the ammount of co-extracted electrons grows rapidly with Vext resulting in e.g. threefold increase of cut-off parameter value when extraction voltage is changed from 1 kV up to 10 kV. Within the considered model the filter field does not have any significant influecne on extracted H- current.