Effects of the magnetic field topology on the co-extracted electron current in a negative ion source for fusion

The negative hydrogen ion current that can be extracted from ion sources for neutral beam heating in fusion experiments can be strongly restricted by the amount of co-extracted electrons and their increase over time, particularly during long pulses (up to 1 h). Models describing the underlying physics of particle extraction from a low-temperature plasma with a high amount of negative ions are essential for identifying measures for reducing and stabilizing the co-extracted electrons. In this work, the 3D PIC-MCC code ONIX (Orsay Negative Ion eXtraction) for the plasma volume around one extraction aperture in the first grid of the extraction system is used for analyzing the effect of the magnetic field configuration on the co-extracted electrons and the extracted negative ions. The magnetic field topology is the result of superimposing two different fields that are perpendicular to each other, the filter field (dominant in the ion source volume) and the electron deflection field (dominant in the extraction system). A parametric study changing the relative intensity of these two fields is performed. It is demonstrated that on the local scale of the simulation, the strength of the filter field does not affect the amount of co-extracted electrons, while a significant reduction of the co-extracted electron current is observed when strengthening the electron deflection field.