Formation of a natural X-point multifaceted asymmetric radiation from the edge in numerical simulations of divertor plasmas

Abstract

Multifaceted asymmetric radiation from the edges (MARFEs) are regions of high-density and low-temperature plasma observed under certain conditions at the tokamak edge. Such structures are found to form in a natural way on the closed flux surfaces near the X-point in 2D numerical simulations (B2-EIRENE) of the divertor plasmas at high discharge densities. Stable steady-state solutions can be obtained by the conventional edge code if the hydrogen line-radiation opacity is taken into account in the model, both with and without impurity radiation. In addition to extra power losses from the MARFE the detached divertor solution is affected due to (i) reduction of the separatrix temperature and thus plasma pressure; (ii) additional momentum losses due to friction with atoms produced by the recombining plasma of the MARFE.

Physics of ultimate detachment of a tokamak divertor plasma

“…This served as an 'excuse' to neglect the radiation trapping effects in already very time consuming simulations of the ITER divertor. However, it was demonstrated in Kotov & Reiter (2012) that radiation trapping plays a crucial role in simulating a stable MARFE-like structure for the JET configuration.…”

Multi‐level model of radiation transport in inhomogeneous plasma

“…At the same time, photoexcitation due to transitions between higher levels, for example, Balmer series, virtually do not contribute to excited levels of populations. Therefore, opacity of those lines can be neglected, which justifies assumptions made in EIRENE . The photoexcitation source has a pronounced influence on plasma transport parameters, for example, ionization source.…”

“…Therefore, opacity of those lines can be neglected, which justifies assumptions made in EIRENE. [12] The photoexcitation source has a pronounced influence on plasma transport parameters, for example, ionization source. Non-uniformity in plasma temperature and density causes asymmetry in radiation transport in divertor region, directing the radiation energy flux away from the target.…”

“…However, the module is rarely used because simultaneous Monte Carlo modelling of radiation transport and plasma processes becomes very time consuming. The radiation transport model built in the code has several limitations: (a) photoexcitation is accounted for only for transitions including the ground‐state level; (b) the number of photons sampled in the wings of the line profile is limited due to limitations of Monte Carlo simulations; and (c) the number of computational cells in the region where opacity is important is usually insufficient to resolve plasma parameter profiles with the accuracy required to examine details of the radiation transport. It was also noticed that detailed calculations of the radiation transport role in divertor physics presented in Kotov take into account only Zeeman and Doppler broadening for the line profile.…”

“…The radiation transport model built in the code has several limitations: (a) photoexcitation is accounted for only for transitions including the ground‐state level; (b) the number of photons sampled in the wings of the line profile is limited due to limitations of Monte Carlo simulations; and (c) the number of computational cells in the region where opacity is important is usually insufficient to resolve plasma parameter profiles with the accuracy required to examine details of the radiation transport. It was also noticed that detailed calculations of the radiation transport role in divertor physics presented in Kotov take into account only Zeeman and Doppler broadening for the line profile. The Stark broadening is now also implemented in EIRENE, and a brief discussion of its role is given in Rosato et al See also Rosato et al for a description of the line profile calculation method in divertor conditions.…”