Negative ion characterization in a helicon plasma source for fusion neutral beams by cavity ring-down spectroscopy and Langmuir probe laser photodetachment

Negative ions are characterized in the helicon plasma source resonant antenna ion device (RAID) at the Swiss plasma center by means of cavity ring-down spectroscopy (CRDS) and Langmuir probe (LP)-assisted laser photodetachment. A high density and axially homogeneous plasma column is produced via a RF antenna able to sustain the propagation of helicon waves in a steady state regime. An electron density ne~=2.0×1018m-3 in H2plasma at 0.3 Pa and 3 kW of input power is measured in the center of the plasma column by LP and microwave interferometry. The electron temperature profile is peaked on axis reaching Te?5eV and decreasing to 1.5 eV at r=0.05m. Thus, a hot core region forms where H2 molecules are rovibrationally excited (H2(?)), and a cold edge, where low energy electrons can attach to H2(?) and produce H- ions by dissociative attachment. In this work we use LP-assisted laser photodetachment and CRDS diagnostics to measure H- and D- radial density profiles and how they depend on source parameters. We show that negative ions are distributed on a shell of 0.06 m radius with a peak value of ~2.0×1016m-3 in H2 plasma. These results suggest that, although substantial technical development is needed, helicon plasmas could be considered as a possible candidate as sources of negative ions for future NBIs.