Drift effects on W7-X divertor heat and particle fluxes

Abstract

Classical particle drifts are known to have substantial impacts on fluxes of particles and heat through the edge plasmas in both tokamaks and stellarators. Here we present results from the first dedicated investigation of drift effects in the W7-X stellarator. By comparing similar plasma discharges conducted with a forward- and reverse-directed magnetic field, the impacts of drifts could be isolated through the observation of up-down asymmetries in flux profiles on the divertor targets. In low-density plasmas, the radial locations of the strike lines (i.e. peaks in the target heat flux profiles) exhibited discrepancies of up to 3 cm that reversed upon magnetic field reversal. In addition, asymmetric heat loads were observed in regions of the target that are shadowed by other targets from parallel flux from the core plasma. A comparison of these asymmetric features with the footprints of key topological regions of the edge magnetic field on the divertor suggests that the main driver of the asymmetries at low density is poloidal E × B drift due to radial electric fields in the scrape-off layer and private flux region. In higher-density plasmas, upper and lower targets collected non-ambipolar currents with opposite signs that also inverted upon field reversal. Overall, in these experiments, almost all up-down asymmetry could be attributed to the field reversal and, therefore, field-dependent drifts.

Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

“…Note that the heat flux contains photon contributions. Although a significant local up-down asymmetry, presumably due to edge drifts [29], is observed in the attached state at t < 3 s, it vanishes when transitioning into detachment. As the radiation characteristics of carbon are similar to that of nitrogen, the results obtained here should, to a large extent, be valid for detachment driven by nitrogen seeding [62].…”

“…This feature allowed us to perform detailed studies on the edge drifts. By comparing similar plasma discharges conducted with a forward-and reverse-directed magnetic field, the impacts of drifts could be isolated through the observation of up-down asymmetries in flux profiles on the divertor targets [29]. The measurement for two discharges performed in the low-iota configuration is shown in figure 5.…”

Understanding baffle overloads observed in high-mirror configuration on Wendelstein 7-X

“…The upper baffle tiles generally receive more heat loads flowing towards the low-iota end, while the lower baffle tiles receive more from the low-iota end, when operating in the positive field direction, which has the toroidal field component in the positive φ direction as marked in figure 1. The enhancement or reduction of particles flowing in either direction by particle drift is observed but the detailed understanding of the effects is out of the scope of this paper (for more see [18]).…”

Filament simulations in regions of highly-varying parallel connection length

“…The scrape-off-layer of Wendelstein 7-X exhibits a patchwork L structure, where neighbouring field lines can have completely different connection lengths -by over an order of magnitude (Hammond et al 2019). For the following simulations, FIGURE 4.…”

“…The edge and SOL topology of the Wendelstein 7-X stellarator provides a novel environment for filament physics. Unlike tokamak scrape-off-layers, W7-X exhibits abrupt transitions in connection length (Hammond et al 2019) due to the edge island topology.…”