Increase of turbulence and transport with resonant magnetic perturbations in ELM-suppressed plasmas on DIII-D

Abstract

Long-wavelength turbulence increases dramatically in the outer regions of DIII-D plasmas with the application of resonant magnetic field perturbations (RMPs) that suppress edge-localized modes (ELMs). Correspondingly, transport increases and global energy confinement decreases in these low-collisionality RMP-ELM suppressed discharges. The core and pedestal density are sharply reduced, while ion and electron temperatures may change only slightly. Low wavenumber density turbulence (k⊥ρi < 1) in the range of 60–300 kHz, measured with beam emission spectroscopy, is modified and generally increases throughout the outer region (0.6 < ρ < 1.0) of the plasma in response to RMPs over a range of q95 values; ELM suppression, in contrast, occurs for a narrower range in q95. Radial magnetic field modulation experiments indicate that these turbulence modifications occur on a time scale of a few milliseconds or less near ρ = 0.85–0.95, significantly faster than transport time-scales and faster than the local pressure gradients and shearing rates evolve at these locations. As the internal coil current is modulated in a square-wave fashion from 3.2 to 4.2 kA, the turbulence magnitude varies in phase by 30% or more, while local density changes by only a few per cent. This dynamical behaviour suggests that the turbulence is directly affected by the RMP, which may partially or largely explain the resulting increased transport and stabilization of the pedestal against peeling–ballooning instabilities that are thought to drive ELMs.

Direct effects of the resonant magnetic perturbation on turbulent transport

“…In this range, the quasilinear regime is characterized by a stronger influence of the RMPs than the nonlinear transport ( Figure 2). The smooth threshold in the dependence of the diffusion coefficient on the amplitude of the RMPs is in agreement with the experiments [7], [4].…”

Propagation Dynamics Associated with Resonant Magnetic Perturbation Fields in High-Confinement Mode Plasmas inside the KSTAR Tokamak

“…This shows that the density fluctuation radial scale length increases with application of a steady state RMP for both the LFT and the AT. This is consistent with the turbulence causing additional radial transport during the RMP [22]. The increased turbulence and V ϕ braking produced by the RMP result in the reduced height of the pedestal toroidal rotation [ Fig.…”

The density dependence of edge-localized-mode suppression and pump-out by resonant magnetic perturbations in the DIII-D tokamak

“…Also, there is evidence that the nonlinear interaction of RMPs with PBMs 10,11 can account for ELM mitigation, 42,43 and may be key to understanding ELM suppression at high pedestal collisionality. 2,5 Finally, enhanced micro-turbulence 44 is correlated with the penetration of resonant fields 18,45 and these could act synergistically with magnetic islands to suppress ELMs.…”