| Home > 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 n(e) congruent to 2.0 x 10(18) m(-3) in H-2 plasma 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 T-e approximate to 5 eV and decreasing to 1.5 eV at r = 0.05 m. Thus, a hot core region forms where H2 molecules are rovibrationally excited (H-2(nu)), and a cold edge, where low energy electrons can attach to H-2(nu) 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 similar to 2.0 x 10(16) m(-3) in H-2 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.