Effect of magnetic perturbations on turbulence-flow dynamics at the L-H transition on DIII-D

Abstract

Detailed 2D turbulence measurements from the DIII-D tokamak provide an explanation for how resonant magnetic perturbations (RMPs) raise the L-H power threshold PLH [P. Gohil et al., Nucl. Fusion 51, 103020 (2011)] in ITER-relevant, low rotation, ITER-similar-shape plasmas with favorable ion ∇B direction. RMPs simultaneously raise the turbulence decorrelation rate ΔωD and reduce the flow shear rate ωshear in the stationary L-mode state preceding the L-H transition, thereby disrupting the turbulence shear suppression mechanism. RMPs also reduce the Reynolds stress drive for poloidal flow, contributing to the reduction of ωshear. On the ∼100 μs timescale of the L-H transition, RMPs reduce Reynolds-stress-driven energy transfer from turbulence to flows by an order of magnitude, challenging the energy depletion theory for the L-H trigger mechanism. In contrast, non-resonant magnetic perturbations, which do not significantly affect PLH, do not affect ΔωD and only slightly reduce ωshear and Reynolds-stress-driven energy transfer.