Speaker
Description
Snowflake divertor (SFD) experiments show enhanced radial transport near the X-points, more than can be explained by standard 2D edge transport modelling (i.e. with poloidally uniform diffusion coefficients and without drifts). While 3D effects may be relevant here, several 2D transport mechanisms have been proposed in the literature which could explain this phenomenon: 1) a region of enhanced ExB drifts driven by large poloidal gradients in plasma profiles close to the primary X-point [1], and 2) the ‘churning mode’ [2], a toroidally-symmetric plasma vortex in the region of weak poloidal magnetic field near the X-points.
We have investigated these effects through two separate modelling approaches. Firstly, we have carried out interpretive modelling of recent SFD experiments at MAST-U using the code UEDGE with equilibrium drifts and currents included in the model. Secondly, we have developed a model of convective transport in the vicinity of the X-points which includes both the effect of equilibrium drifts and the churning mode. This latter model, implemented in BOUT++, allows us to directly compare the two mechanisms at conditions relevant to current and future SFD experiments.
From UEDGE modelling with drifts, we have found that ExB drifts can drive transport across the X-point region leading to activation of secondary strike points (SPs) and reduced peak heat fluxes at the primary SPs, consistent with the predictions in [1]. Direct heat flux measurements at the SPs were not available on the experiments modelled, but the UEDGE results show good agreement in midplane $T_e$, $n_e$ profiles and radiated power profiles in the divertor region.
From BOUT++ modelling, we find that the equilibrium drifts effect is dominant when the ratio of plasma pressure at the X-points to magnetic pressure at the midplane is low,$\beta_{pm} \lesssim 1\%$. Above this, electromagnetic effects become increasingly important in driving additional transport. Such conditions are attainable in future SFD experiments at MAST-U and NSTX-U, as well as during ELMs in current devices.
This work was carried out under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC5207NA27344.
[1] Canal et al., Nucl. Fusion 55 (2015)
[2] Ryutov et al., Phys. Scr. 89 (2014)