A model for microinstability destabilization and enhanced transport in the presence of shielded 3-D magnetic perturbations

TL;DR

This paper proposes a mechanism where shielded 3-D magnetic perturbations destabilize microinstabilities and enhance transport in tokamaks, potentially explaining observed pressure gradient reductions during ELM suppression.

Contribution

It introduces a local 3-D equilibrium theory showing how small 3-D deformations perturb the ideal MHD ballooning stability boundary near rational surfaces.

Findings

3-D magnetic perturbations strongly perturb local magnetic shear.

Near-resonant Pfirsch-Schluter currents are driven by 3-D fields.

Pressure gradient may be lowered near rational surfaces with 3-D fields.

Abstract

A mechanism is presented that suggests shielded 3-D magnetic perturbations can destabilize microinstabilities and enhance the associated anomalous transport. Using local 3-D equilibrium theory, shaped tokamak equilibria with small 3-D deformations are constructed. In the vicinity of rational magnetic surfaces, the infinite-n ideal MHD ballooning stability boundary is strongly perturbed by the 3-D modulations of the local magnetic shear associated with the presence of nearresonant Pfirsch-Schluter currents. These currents are driven by 3-D components of the magnetic field spectrum even when there is no resonant radial component. The infinite-n ideal ballooning stability boundary is often used as a proxy for the onset of virulent kinetic ballooning modes (KBM) and associated stiff transport. These results suggest that the achievable pressure gradient may be lowered in the vicinity of low order rational surfaces when 3-D magnetic perturbations are applied. This mechanism may provide an explanation for the observed reduction in the peak pressure gradient at the top of the edge pedestal during experiments where edge localized modes have been completely suppressed by applied 3-D magnetic fields.