Speaker
Description
Divertor detachment is a promising solution to the heat exhaust problem in future fusion devices by reducing the heat load onto divertor target plates$^1$. During the detachment process a neutral gas layer builds up in-between the plasma and the divertor plates through recombination reactions.
Molecular assisted recombination (MAR) is discussed to be among the dominant volume recombination processes, whereby several multi-step channels based on hydrogenic molecules are distinguished$^2$. Thereby, the effective reaction rate for MAR strongly depends on the vibrationally excited population densities of molecular hydrogen H$_2$ (or its isotopologues) that can typically be characterized by the vibrational temperature $T_{\rm vib}$.
This contribution investigates $T_{\rm vib}$ in divertor-like H$_2$ plasmas by combining optical emission spectroscopy (OES) measurements with interpretation via collisional-radiative (CR) models based on the Yacora code for atomic$^3$ and molecular$^4$ hydrogen applying state-of-the-art reaction probabilities.
OES measurements (including the molecular Fulcher-$\alpha$ band and the atomic Balmer lines) as well as Thomson scattering and two-photon absorption laser induced fluorescence (TALIF) are performed at the linear divertor plasma simulator Magnum-PSI$^5$ for plasma conditions with $T_{\rm e} \leq 3.5$ eV and $n_{\rm e} \approx 10^{19} - 10^{20}$ m$^{-3}$. From the spectroscopic measurements first $T_{\rm vib}$ is deduced and secondly the molecular and ionic particle densities are determined applying a CR model for atomic hydrogen (Yacora H), while considering the results from Thomson scattering and TALIF as constraints. Subsequently, these plasma parameters are used as input for the CR model Yacora H$_2$($X^1$,$v$), which is aimed for self-consistent $T_{\rm vib}$ predictions, in order to benchmark the model and to study, to which extent which processes determine $T_{\rm vib}$ and thus the effective MAR rate.
$^1$ S.I. Krasheninnikov et al., Journal of Nuclear Materials 241-243 (1997) 283-287
$^2$ N. Ohno, Plasma Physics and Controlled Fusion 59 (2017) 034007
$^3$ D. Wünderlich et al., Journal of Quantitative Spectroscopy and Radiative Transfer 240 (2020) 106695
$^4$ R.C. Bergmayr et al., European Physical Journal D 77 (2023) 136
$^5$ M.J. van de Pol et al., Fusion Engineering and Design 136 (2018) 597-601