SOLPS-ITER analysis of drift effects on plasma profiles in the EAST scrape-off layer

Abstract

Drift effects on the plasma profiles of the scrape-off layer (SOL) in the Experimental Advanced Superconducting Tokamak (EAST) have been numerically investigated using the comprehensive 2D edge modeling package, SOLPS-ITER, based on a generic magnetic equilibrium with lower single null configuration. SOL particle diffusivity (D SOL) has been scanned from high (1.0 m2 s−1) to extremely low (0.02 m2 s−1), to gradually highlight the role of drift-based neoclassical mechanisms in radial particle transport. To address the impact of magnetic field direction on drift-driven transport, plasma profiles, flows and currents in the SOL of EAST plasmas, with the toroidal magnetic field (B T) direction favorable and unfavorable for H-mode access, i.e. with the ion B × ∇B drift pointing towards and away from the active X-point, are simulated and analyzed. Results demonstrated that drift-driven transport, considered as the key process in the formation of SOL plasma profiles, is dependent on magnetic field direction and thus SOL flows and currents, as well as SOL widths, can obviously be affected by the direction of drifts. With B T changed from the favorable direction to the unfavorable one, the flattening of the density radial profile as well as the increase in power decay length, in the SOL, can be achieved and can be further enhanced as the weight of turbulent transport (i.e. D SOL) gets reduced, due to the increased contribution of ion parallel viscosity to the radial ion flow. In particular, with D SOL ⩽ 0.05 m2 s−1 in the simulations, the dominant role of drift-based neoclassical mechanisms in the radial particle transport will lead to the formation of the so-called edge density-shelf in plasmas with unfavorable B T. The power scrape-off width in plasmas with unfavorable B T is very insensitive to the turbulent transport level and can remain relatively high even when D SOL has been decreased to an extremely low level. Due to the compressing/widening effect of the drift-driven inward/outward radial particle flow, the simulated power scrape-off width exhibits an in-out asymmetry, which is also dependent on magnetic field direction . This work represents a step towards a deeper understanding of the physics mechanisms determining SOL widths in EAST.