The role of radial particle pinches in ELM suppression by resonant magnetic perturbations

Abstract

The force balance in the plasma edge in a matched pair of DIII-D (Luxon 2002 Nucl. Fusion 42 6149) tokamak discharges with and without resonant magnetic perturbations (RMPs) is evaluated in order to investigate the effects on particle transport of RMP applied for the purpose of suppressing edge-localized modes (ELMs). Experimental data are used to evaluate the radial and toroidal force balances, which may be written as a pinch-diffusion relation for the radial ion flux to facilitate investigation of transport effects. The radial electric field in the H-mode plasma had a sharp negative dip in the steep gradient region of the edge pedestal, associated with which was a large inward pinch velocity. The main effect of RMP was to make the edge electric field less negative or more positive, reducing this strong negative dip in the radial electric field (even reversing it from negative to positive over some regions), thereby reducing the strong inward particle pinch in the edge of an H-mode discharge, thus causing a reduction in edge density below the ELM threshold.

Intrinsic rotation produced by ion orbit loss and X-loss

“…Based on magnetic field contours for the H-mode DIII-D shot [36][37][38] discussed next, a value of the angular width of the X-loss region Dh x ¼ 0.15 radians was estimated. Using this value of Dh x ¼ 0.15 and a larger value Dh x ¼ 0.25, intrinsic rotation due to X-loss was calculated to be 12 and 17 km/s, respectively, at q ¼ 0:994 and of 2.6 and 8.4 km/s at q ¼ 0:988.…”

The effect of ion orbit loss and X-loss on the interpretation of ion energy and particle transport in the DIII-D edge plasma

“…A relationship between changes in the radial electric field E r and in the poloidal rotation velocity V h in the plasma edge and changes in the edge pressure, temperature, and density gradients in the plasma edge has long been observed experimentally, 3 suggesting that an understanding of the causes of the rotation velocities and the radial electric field may provide insight to an understanding of edge pedestal physics, and recently it has been demonstrated that changes in these experimentally observed quantities are correlated by momentum balance requirements. [4][5][6] A second, and more widely held, school of thought postulates that the stabilization or destabilization of electromagnetic microinstabilities [7][8][9] and the corresponding changes in fluctuation-driven transport cause the observed changes in temperature and density gradients in order for diffusive heat and particle fluxes to remove the input heat and particles. A third school of thought is that the physics of the edge plasma is determined, at least in part, by the loss of energetic ions and their consequences.…”

X-transport of ions in diverted tokamaks, with application to DIII-D

“…Implicit in much of this fluid code analysis is the assumption that the particle and heat fluxes in the edge plasma are diffusive in nature (i.e., that the particle and heat fluxes are proportional to gradients of the measured density and temperature profiles), although many of these codes have the capability to represent convective transport and some recent analyses have explicitly taken into account the particle pinch required to satisfy momentum balance. [10][11][12] However, direct ion-orbit-loss (i.e., the loss of particles and energy due to particles following orbits which leave the confined plasma and do not return) is not usually taken into account in interpreting edge pedestal physics. In the "standard" type of ion-orbit-loss (e.g., as described in Ref.…”