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

• Spatio-temporal change of Hα, Hβ, and Hγ in D-module of GAMMA 10/PDX was observed. • High-speed camera with 4-branch optical system tracked detached plasma formation. • Transition from MAR to EIR observed with increasing neutral gas pressure. • Microwave interferometry observed re-increase of electron density during detachment. • Electron temperatures in the volume was estimated using Boltzmann plot. 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 × 10 18 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 H 2 + and H - becoming significant alongside electron–ion recombination (EIR), a process previously thought to be negligible in GAMMA 10/PDX.