Physics of the geometry-related detachment stability in W7-AS

Abstract

This paper presents a detailed analysis of the transport behaviour of the detached plasmas in W7-AS based on an extended numerical study using the EMC3-EIRENE code, aimed at understanding the underlying physics responsible for the geometry-dependent detachment stability observed in W7-AS island divertor experiments. Here, a stable detachment can only be established when the control coils are switched on to generate sufficiently large islands with relatively short connection lengths. Special attention will be paid to a discussion of the carbon radiation, location and dynamics of the radiation layer, the neutral screening efficiency specific to the island divertor geometry and its impact on the detachment stability. Based on the three-dimensional simulation results, a linear stability model is presented in order to obtain some insight into the mechanisms driving the instability. The radiation behaviour and the location and evolution of the radiation zone in the island divertor will be discussed with respect to those of tokamak-MARFEs.

Comparison between stellarator and tokamak divertor transport

“…Numerically it is shown that the plasma-neutral interaction is not the main reason for the rollover of the recycling flux and downstream density in helical devices. The major features of the partially-detached plasma in W7-AS could be well explained by the EMC3-EIRENE code [26]. First simulation results for stable detachment in LHD have also shown similar tendency in carbon radiation pattern as observed experimentally [55].…”

Non-axisymmetric SOL-transport study for tokamaks and stellarators

“…The detachment becomes stable only if power transport and radiation are in balance at a position inside the island SOL. Both experiment and modeling have shown that this balance can only be established in a divertor geometry having large plasma-tocore distances and small connection lengths [22,23]. In these cases, the modeling predicts the formation, at detachment conditions, of a carbon radiation belt inside the islands at the HFS.…”

Modeling of divertor particle and heat loads during application of resonant magnetic perturbation fields for ELM control in ITER

“…The EMC3 code [12] solves a set of Braginskii fluid equations (Particle, Energy and Momentum) using a Monte-Carlo method iteratively with a 3D kinetic neutral transport code EIRENE [13]. EIRENE supplies the particle, energy and momentum sources from neutrals released by the divertor elements.…”