Cavity ring-down spectroscopy to measure negative ion density in a helicon plasma source for fusion neutral beams

Cavity Ring Down Spectroscopy (CRDS) is used to measure the D^- absolute density produced in the helicon plasma reactor RAID (Resonant Antenna Ion Device) at the Swiss Plasma Center. The birdcage geometry of the helicon antenna produces a homogeneous, high-density plasma column (n ≅ 1.5 × 10¹⁸ m^-3 in H₂ and D₂ at 0.3 Pa and 3 kW of input power) 1.4 m long. We present the CRDS experimental setup, its positioning on the RAID reactor, and how the mechanical and thermal effects of the plasma affect the measurement. First results in deuterium plasma confirm the production of negative ions (D^-) with a significant density: an average value of 3.0 × 10¹⁶ m^-3 of D^- is obtained at 0.3 Pa and 5 kW of power input in Cs-free plasma. This result is in good agreement with calculations performed with the collisional radiative code YACORA.