High-speed camera observation of Hα, Hβ and Hγ associated with mutual neutralization and EIR in GAMMA 10/PDX divertor simulated plasma

Abstract

For the purpose of gaining a better understanding of detached plasma formation, the transition of the volumetric plasma recombination processes of the end-loss hydrogen plasma of a tandem-mirror, GAMMA 10/PDX, was investigated. In the Divertor Experimental Module (D-module) on the end cell, where the additional hydrogen gas is injected, the plasma flows into the V-shaped tungsten target plates. A 4-branch high-speed camera system was used to observe the 2D distributions of H_α, H_β and H_γ emission intensities simultaneously. As the neutral gas pressure (p_n) in the D-module increased, two distinct emission phases were observed. The first phase was characterized by an initial increase followed by a decrease in H_α emission, coinciding with a rollover of the electron density (n_e). This behavior is associated with the mutual neutralization of H⁺ and H^-, a process associated with molecular activated recombination (MAR). Further increase in p_n led to the second phase, during which a drastic increase in H_β and H_γ emissions alongside H_α was seen and the interferometer indicated a re-increase in n_e up to at least ∼ 4 × 10¹⁸ m⁻³, while the spectrometer observed an increase in Balmer emission intensities from higher excited states (n ≥ 5). The electron temperature (T_e) was estimated to be less than ∼ 0.1 eV using the Boltzmann plot method. These results suggest a shift in the dominant volumetric recombination mechanisms, with mutual neutralization of H₂⁺ and H^- becoming significant alongside electron–ion recombination (EIR), a process previously thought to be negligible in GAMMA 10/PDX.