A novel diagnostic technique for H⁻(D⁻) densities in negative hydrogen ion sources

A new diagnostic method for the determination of negative hydrogen ion densities H⁻ (D⁻), namely the H_α/H_β line ratio method, is presented and applied to high power (P≈100 kW) negative ion sources operating at low pressure (p < 1 Pa). The basis of the method is the mutual neutralization process which enhances Balmer line radiation selectively. Detailed parameter studies carried out with a collisional radiative model show that the Balmer line ratio H_α/H_β is very well suited to obtain negative ion densities in low temperature hydrogen plasmas with typical plasma parameters of T_e ≈ 1–5 eV and n_e ≈ 10¹⁶–5 × 10¹⁸ m⁻³. The H_α/H_β line ratio method has the great advantage that it can be accomplished easily with the non-invasive and in situ plasma diagnostic method of optical emission spectroscopy. The method is applied to the RF ion sources of IPP Garching, currently developed for the neutral beam injection system of ITER. Line-of-sight averaged negative hydrogen densities in the range of 10¹⁶–10¹⁷ m⁻³ are measured close to the extraction system in hydrogen and deuterium discharge. Absolute values depend on pressure, power and caesium conditioning of the source. Negative ion densities measured with this novel technique in front of the grid correlate very well with the extracted negative ion current densities.