Current structure in the scrape-off layer of a tokamak in a quiescent state

Abstract

Currents structure in the scrape-off layer (SOL) of a tokamak is analyzed. It is demonstrated that poloidal currents measured in the experiments are a combination of several current types of different physical nature. Besides known Pfirsch-Schlüter (PS) currents and thermoelectric currents, so-called plate closing currents (PCC) flowing to/from the divertor plates are also analyzed. The latter close radial currents in the SOL and below/above the Xpoint in the SOL and private flux region (PFR). In particular, current flowing to the outer plate in the private flux region, opposite to thermoelectric current is predicted for the standard singlenull configuration and favorable direction of drift. In addition, a pair of currents to and away from the outer plate should flow. In the single-null configuration they are often masked by a larger thermoelectric current, however for the connected double null (CDN) case, where thermoelectric current is strongly reduced due to smaller temperature asymmetry, these currents dominate. The suggested physical model is supported by results of simulations performed with SOLPS-ITER transport code. Simulations were done for ASDEX Upgrade (AUG), L and H-modes, single-null configurations, and for GLOBUS-M H-modes, both disconnected and connected double null configurations. Results of the simulations are compared with probe measurements for AUG and Globus-M tokamaks, and reasonable agreement has been found. The role of parallel currents in the formation of the potential maximum/minimum in the vicinity of X-point for strongly detached regimes is also analyzed.

SOLPS-ITER simulations of an X-point radiator in the ASDEX Upgrade tokamak

“…Radial electric fields were generated by the potential gradients, pointing from the inner confined region to the highly ionizing region, resulting in E r × B drifts from the X-point to the high-field-side upstream position. A potential hill was found at the X-point, which corresponds to the poloidal current leaving the X-point to the divertor targets and to the upstream position [26]. Given the temperature and its gradient as well as the electric conductivity near the X-point were already low, the currents were mainly driven by the poloidal electric field.…”

Detached regime with highly radiating X‐point: Physics and modelling

“…To explain the role of drifts and currents in divertor asymmetry, let us start from consideration of currents type flowing in the SOL and private flux region (PFR), following the paper. [ 34 ] Besides the well‐known, vertical currents, Pfirsch‐Schlüter current, and thermoelectric current there exist so‐called plate‐closing currents, which close the divergent part of perpendicular currents through targets, see Figure 1. The requirement for plate‐closing currents is that the net current to all short‐circuited plasma‐facing components (including targets) is zero.…”

“…The relative role of plate‐closing currents and local magnitude of current density along the target depends on parameters and operational regime, which is demonstrated in AUG experiments. [ 34 ] Figure 2 shows how the current density changes in direction and magnitude along the outer target in the H‐mode AUG discharge, and how current flow directions in PFR, near SOL, and far SOL are reproduced by SOLPS‐ITER modelling.…”

Evaluation of ITER divertor shunts as a synthetic diagnostic for detachment control

“…The analysis of drift current contributions in the SOL and PFR shows that, beyond the thermoelectric current contribution, both plate closing currents (PCCs) compensating downward ∇B current [20][21][22] and parallel currents compensating neutral friction currents [22][23][24] contribute to the j ∥ profile along the divertor targets. We note for completeness that the well known Pfirsch-Schlüter parallel currents [23], which arise to short circuit ∇B and curvature drifts in the SOL above the X-point, do not make any significant contribution to the target integral current.…”

“…These currents then run through their respective divertor targets, re-entering the plasma outside the separatrix, completing 'plate-closing' current circuits. These drift-driven PCCs cancel themselves out when integrated over the entire plate, resulting in the total SOL current equalling that through the divertor cassette (SOLPS-ITER simulations are 2D and calculate the total, toroidally symmetric current flowing into all divertor targets) [21,22]. The divertor shunt diagnostic only measures this integrated total SOL current and therefore is blind to the effects of the neutral-friction current and PCCs.…”