Effect of plasma confinement magnets on ROBIN (RF-operated beam source in India for negative ions) performance

The RF-based single-driver negative hydrogen ion source test bed ROBIN is a 100 kW, 1 MHz negative hydrogen ion source test bed at IPR, Gandhinagar. The inductive RF coupling produces plasma in the driver, expanding into an expansion chamber coupled to a three-grid extractor accelerator system. A magnetic filter field, transverse to the plasma flow direction from the RF driver, is produced magnetically using magnet boxes fitted in the diagnostic flange, also called filter field flange. The diagnostic flange is located between the exit of the expansion chamber and the entry to the three-grid system consisting of a plasma grid (PG), extraction grid, and grounded grid. The filter field in conjunction with the bias voltage of a few tens of volts applied to the plasma grid with respect to the source body has a strong influence on the control of plasma electrons, plasma drifts, and plasma confinement, which, in turn, influence plasma uniformity, and so beam profile uniformity, beam divergence, and beam transmission. The present design of ITER sources also envisages magnets lined along the expansion chamber for plasma confinement. Their effectiveness on operations is an important aspect of studies, being performed on fusion-relevant negative ion test beds globally. A systematic study on similar lines has been carried out on the ROBIN test bed and is reported in this paper. The magnets are arranged in boxes and fitted on the walls of the ROBIN expansion chamber extending up to the diagnostic flange. Several magnetic configurations, line and cusp arrangements, have been studied. Significant effects on plasma uniformity across the ion extraction plane in front of the PG are observed. As a consequence, the ratio of co-extracted electron current to negative hydrogen ion (H⁻ ion) current in terms of their current densities (j_e−/j_ion) is also influenced by the magnetic configurations. The findings presented here may be relevant for achieving improved H⁻ uniformity along the extraction plane, thereby helping in improved beam divergence and reduced transport losses from larger sources.