About the relevance of molecular processes in divertor plasmas: Which processes determine T vib at Magnum-PSI?

Molecular assisted recombination (MAR) is discussed to be among the key recombination mechanisms for divertor detachment of magnetically confined fusion plasmas. The effective MAR reaction rate depends strongly on the population densities of vibrationally excited levels in molecular hydrogen or deuterium that can typically be characterized by the vibrational temperature T vib. Hydrogen plasma experiments at the linear divertor simulator Magnum-PSI are combined with collisional-radiative (CR) modeling to determine T vib and its influence on the effective MAR rate in ITER-relevant conditions: Optical emission spectroscopy measurements reveal T vib ≈ 2000-10 000 K for the plasmas investigated at T e ≈ 1-3 eV and n e ≈ 2⋅10 19 − 2⋅10 20 m-3. By fitting the measured Balmer lines and Fulcher-alfa band emission with CR models for atomic and molecular hydrogen, the neutral and ionic particle densities are determined. Self-consistent T vib calculations employing the CR model Yacora H2(X1, v) reproduce the measured T vib at the plasma center and give insight on which processes determine T vib and thus the effective MAR rate.