DIII-D Research Advancing the Scientific Basis for Burning Plasmas and Fusion Energy

W.M. Solomon, (E. Schuster), et al. (Collaboration Paper)

26th IAEA Fusion Energy Conference

Kyoto, Japan, October 17-22, 2016

Abstract

The DIII-D tokamak has addressed key issues to advance the physics basis for ITER and future steady-state fusion devices. The ITER baseline scenario is challenged at low rotation, where magnetic probing identifies a decrease in ideal stability correlated with changes in the current profile and increased tearing instability, providing a basis for active instability sensing. Improved understanding of 3D interactions is emerging, with RMP-ELM suppression correlated with exciting an edge current driven mode, while core tearing mode drives are mediated through a global kink response. n = 2 error fields are found to drive locked mode instabilities at levels similar to n = 1. Should plasma termination be necessary, shattered neon pellet injection has been shown to be tunable to adjust radiation and current quench rate; the technique also proves effective in dissipating runaway electron beams. Reduced transport models such as TGLF reproduce the reduction in confinement associated with additional electron heating in ITER baseline plasmas. Raising the pedestal density can recover the performance in the ITER baseline through an increase in the pedestal pressure, and can even give access to Super H-mode for ITER. A new wide-pedestal variant of QH-mode has been discovered where increased edge transport is found to allow higher pedestal pressure, consistent with peeling-ballooning theory. New dimensionless scaling experiments suggest an intrinsic torque comparable to the beam-driven torque on ITER. Complete ELM suppression has been achieved in steady-state "hybrid" plasmas that is relatively insensitive to q95, having a weak effect on the pedestal. Both high-qmin and hybrid steady-state plasmas have avoided fast ion instabilities and achieved increased performance by control of the fast ion pressure gradient and magnetic shear, and use of external control tools such as ECH. In the boundary, ExB drifts are required in simulations to match observed asymmetries in divertor detachment, and the erosion rate of high-Z materials is found to be reduced through control of the electric field in the pre-sheath. Between-ELM heat flux asymmetries in the presence of RMP fields are determined to be eliminated in detached divertor conditions.