Characterization of ROBIN Ion Source Under Volume Mode Operation Using Langmuir Probe, Cavity Ring-Down Spectroscopy, and Optical Emission Spectroscopy

This article deals with the negative ion characterization of RF operated beam source in India for negative ion (ROBIN), being operated in pure volume mode using three independent diagnostics that are: cavity ring-down spectroscopy (CRDS), optical emission spectroscopy (OES), and Langmuir probe (LP). This will help to understand the ion source performance in the cesiated condition (surface mode operation). Plasma transport and corresponding negative ion transport in the extraction region of the ion source volume, close to the plasma grid (PG), are significantly influenced by the transverse magnetic filter (TMF) field and the positive potential bias applied to the PG. Plasma asymmetry across the extraction plane, orthogonal to the TMF field direction, is registered for both PG-biased and unbiased conditions. The plasma asymmetry is found to increase drastically, when the PG bias voltage goes beyond the plasma potential. On the other hand, the negative ion density profile near the extraction plane follows an opposite trend. The negative ion density registered by both CRDS and OES is found to be within the range of 1015 – 1016 m-3 when the background plasma density is in the order of 1017 m-3. The spatial asymmetry of negative ion density reduces with pressure under constant RF power when the PG is unbiased. However, it is found that the increase in RF incident power has negligible effect. This article contains the first experimental results of CRDS diagnostic employed in ROBIN. Further, such extensive characterization data using three different types of diagnostics are not easily available in the literature. The experimental data substantiate and support the database available from Bavarian test machine for negative ion (BATMAN). The experimental results reported here may be considered important, considering similar test beds operating worldwide after International Thermonuclear Experimental Reactor (ITER) considered RF ion source as baseline design for its neutral beam injectors (NBIs).