Suppression and Mitigation of Edge Localized Modes in the DIII-D Tokamak with 3D Magnetic Perturbations

Abstract

Uncontrolled Type-I Edge Localize Modes (ELMs) are expected to cause melting of the tungsten divertors in ITER. Methods for controlling ELMs in ITER include pellet pacing and Resonant Magnetic Perturbation (RMP) fields produced by in-vessel, non-axisymmetric, coils. Type-I ELMs have been reproducibly suppressed and mitigated in DIII-D H-mode plasmas with a variety of shapes and pedestal collisionalities using RMP fields of order 10 −3 B T. In these experiments the response of Type-I ELMs to applied RMP fields, with a principal toroidal mode number n = 3, varies dramatically. In some cases there is an evolution in the ELM dynamics involving combinations of small high frequency D α spikes mixed with mitigated Type-I ELMs prior to reaching an ELM suppressed state. In other cases, there is continuous change in the frequency and amplitude of the Type-I ELMs. A reduced set of plasma parameters, that significantly affect the dynamics of the ELMs immediately following the application of the RMP field, have been identified. The dynamics of mitigated ELMs are generally consistent with those seen during Type-I through Type-V ELMs although several new types of ELM dynamics have also been observed in plasmas with relatively low toroidal fields as well as during q 95 ramps.

Suppression of type-I ELMs with reduced RMP coil set on DIII-D

“…The threshold amplitudes for RMP ELM suppression with a reduced I-coil set under other conditions may in fact be lower than those obtained in these experiments. RMP ELM suppression is a dynamic process [21], and often the DIII-D ISS discharges (with higher triangularity) undergo a few hundred milliseconds of ELM mitigation phase before complete ELM suppression is achieved.…”

Evidence of Toroidally Localized Turbulence with Applied 3D Fields in the DIII-D Tokamak

“…While complete ELM suppression only occurs in certain ranges of edge safety factor q [3], density pumpout is a more consistent feature of DIII-D H-mode plasmas with applied 3D fields [4]. Recent experiments have linked both ELM suppression and density pumpout to resonant excitation of modes on magnetic surfaces with rational q values [5][6][7].…”

DIII-D research to address key challenges for ITER and fusion energy

“…It is also strongly supported by two fluid MHD calculations with the M3D-C1 code, which predict a significant enhancement of the tearing drive at the top of the pedestal in the ELM-suppressed phase. Here, stochastization due to island formation at multiple closely spaced rational surfaces is predicted to increase cross-field transport in this region [4]. This leads to a predominantly n = 0 predicted profile response, also explaining the lack of a clear observed helical island structure when suppression occurs.…”