Speaker
Description
A snowflake divertor (SFD) is formed by bringing a second X-point into the vicinity of the first, resulting in four divertor legs instead of two. SFD experiments have provided evidence of enhanced transport across the region close to the X-points, resulting in redistributed exhaust power across divertor legs and reduced peak heat fluxes at the targets [1]. Two leading candidates for this transport are 1) the ‘churning mode’, a loss of toroidal equilibrium close to the X-points leading to plasma convection in the poloidal plane [2], and 2) strong ExB drifts across the X-points driven by large poloidal gradients [3]. Our previous work has focused on the churning mode [4], while here we focus on the effect of drifts.
NBI-heated, H-mode SFD experiments at MAST-U have been modelled with the edge transport code UEDGE, with and without self-consistent electromagnetic drift effects included. The secondary X-point was around 10cm from the first in these experiments, and its relative position was varied to capture the three unique topologies of SFD: in the private flux region, on the high-field side SOL, and on the low-field side SOL. The experiments featured both upper and lower divertors and so four X-points in total, but UEDGE can only simulate up to two X-points. To overcome this, we simulate a lower-divertor-only scenario by stitching together flux surfaces at the midplane. We can then reverse the toroidal field direction to approximate conditions in the upper divertor.
Anomalous transport coefficients $D_n$ and $\chi_{e,i}$ are tuned in UEDGE to match experimental measurements, and we find that simulations with drifts require around 50% lower $D_n$ and $\chi_{e,i}$. In the lower divertor, ExB drifts are responsible for reduced peak heat fluxes at the primary strike points (SPs) by 20-40% and increased activation of the secondary SPs. In the upper divertor we find that the drift-driven transport across the X-points is reduced. By varying the core heating power boundary conditions, we find that the effect of drifts on transport to the secondary SPs decreases at higher heating, in contrast with predictions of the churning mode.
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] Ryutov & Soukhanovskii, PoP 22 (2015)
[2] Ryutov et al., Physica Scripta 89 (2014)
[3] Canal et al., Nuclear Fusion 55 (2013)
[4] Power et al., Physics of Plasmas 32 (2025)