Analysis of the ion flux in the extraction region of the negative ion source based on Mach probes

In neutral beam injection system based on negative ion source (NNBI), negative hydrogen ions are primarily generated via the surface conversion of neutral atoms and positive ions on the plasma grid (PG). The flux of directed positive ion incident on the PG surface is a critical parameter, as it directly governs the surface yield of negative hydrogen ions and the uniformity of the extracted beam. However, for the Comprehensive Research Facility for Fusion Technology (CRAFT) NNBI device, the dependence of the ion flux on discharge parameters and its spatial distribution within the extraction region have remained unclear. To address this, a custom-designed Mach probe diagnostic system was developed to experimentally investigate the positive ion flux and its spatial distribution in the extraction region. The experimental results demonstrate that increasing the radio-frequency (RF) power significantly enhances the positive ion flux perpendicular to the PG surface, which arises from the simultaneous improvement in ionization efficiency and the axial plasma potential gradient. Conversely, elevating the operating pressure increases ion density but attenuates the directed ion flow velocity. Within the investigated parameter range, the ion flux component perpendicular to the PG exhibits a slight overall increase. The introduction of a transverse filter magnetic field flattens the axial potential gradient due to electron magnetization, reducing both the local ion density and the axial flow velocity. Concurrently, the E×B drift of these electrons modifies the local electrostatic field, deflecting the positive ion flux downward and causing pronounced spatial asymmetry. Furthermore, applying a positive bias voltage weakens the axial potential gradient, leading to a decrease in the axial ion flux. These quantitative findings on the ion flux dependence on operational parameters provide essential experimental support for optimizing beam extraction uniformity, suppressing co-extracted electrons, and achieving the stable long-pulse operation of high-power negative ion sources.