Bolometer tomography on Wendelstein 7-X for study of radiation asymmetry

Abstract

The algorithm for bolometer tomography at Wendelstein 7-X (W7-X) has been recently improved using a novel regularization functional, based on relative gradient smoothing of the sought radiation profile. It has been validated using radiation patterns provided by 3D modeling under real plasma conditions as phantoms and then applied to bolometer measurements performed during the first divertor operation phase of W7-X. The following results are presented: (1) edge-localized 2D radiation patterns with clearly resolved magnetic island radiation structures, (2) an up-down asymmetry in the impurity radiation that is not captured by the 3D edge plasma transport modeling, (3) reversal of the asymmetry with reversed magnetic field direction. Further analysis reveals a poloidal variation of the emissivity in the outer confined plasma region with a field-direction dependent asymmetry, also supported by the soft x-ray measurements. This asymmetry is considered to be related to asymmetric impurity distributions, driven by the pronounced ion-impurity friction force at the plasma edge where the collisionality of the W7-X plasma is sufficiently high to develop impurity asymmetry as predicted by neoclassical theory of parallel impurity transport.

Plasma radiation behavior approaching high-radiation scenarios in W7-X

“…Here we have reported that poloidal asymmetries in the radiation distribution have been observed for both limiter and divertor plasmas (after boronization). Similar to the description in reference [37], the magnitude of the asymmetry increases toward the high density, high radiation scenarios. This asymmetry reverses under certain plasma conditions, e.g.…”

“…However, the current version of the EMC3-Eirene code (without drifts) for 3D modeling is not yet able to quantitatively reproduce all selected local measurements simultaneously under the current experimental conditions (in particular, the existence of error fields) [52]. Some results using EMC3-Eirene code are reported in [37,51] and com-parisons with the bolometer results (in OP1.2a before boronisation) show qualitative agreements but also differences, especially in the asymmetric feature of the impurity radiation distribution.…”

“…Nevertheless, the impurity radiation in discharges before boronisation behaves quite similarly to that in experiments after boro-nisation, such as edge localization of the radiation zone and visible up-down asymmetry. Yet, they differ in the following respects: (a) In the low-density scenario, the 'double emission zone' in the OP1.2b plasma, as shown in figure 5(e), smears into one [37]. (b) The radiation profiles in OP1.2a (before boronisation) are generally broader around the LCFS than those in the discharges after boronisation.…”

“…The inversion algorithm based on reference [36] uses a new regularization functional based on relative gradient smoothing (RGS) of the sought emissivity profile. Details about the method can be found in reference [37]. The profile evolutions with density for both discharges are as follows:…”

“…In experiments with reversed magnetic field direction, a 'reversal' of the asymmetry is observed, suggesting that particle flux-friction relationship plays a role here. Details can be found in reference [37].…”