DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
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Title	204: Suppression of Density Limit Disruptions with RMP Fields
Name:	Todd Evans evans@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Torkil Jensen Award	Presentation time:	Not requested
Co-Author(s):	Contributions from collaborators at DIII-D, ITER, AUG, KSTAR, LHD and TEXTOR	ITPA Joint Experiment :	No
Description:	ITER will operate at high pedestal density and will attempt to control the energy released by ELMs using pellet pacing. If the neutral particle load from the pellet pacing exceeds the ITER pumping throughput capability at high pellet frequencies the pedestal density may build up and exceed the edge radiation limit triggering a radiative collapse of the current profile. This will induce a voltage spike that will generate a massive runaway electron current. Very little work has been done in tokamaks to prevent or control the initial edge radiative instability that is responsible for this process. The goal of this experiment is to use a technique that was developed in Tore Supra using RMP fields to control radiative instabilities (T. E. Evans, et al., J, Nucl. Mater., 196-198 (1992) 421). The Tore Supra results suggest that the DIII-D I-coil will be able to prevent the radiative collapse and the subsequent disruption. A successful demonstration of this idea in ISS plasmas may allow ITER to operate above the Greenwald density limit, using their in-vessel ELM coil, without disruptions thus enhancing Q in various operating scenarios.	ITER IO Urgent Research Task :	Yes
Experimental Approach/Plan:	Two sets of discharges will be used to test this concept. Using inner wall circular limited discharges (ref. 146121) with q_a = 3, the density will be ramped up to the density limit with a continuous deuterium gas feed. Once conditions have been established for a reproducible density limit radiative collapse and disruption, the I-coil will be applied to hold the radiating zone between the edge plasma and the high field side wall as was done in the Tore Supra experiment. If the radiating zone can be robustly stabilized by the RMP field at the Greenwald limit, the density will be increased to see how far above the Greenwald limit the plasma can be pushed without triggering a disruption. After establishing the new upper bound on the density with the stabilized radiating zone, the NBI power will be dropped in order to quench the the radiation asymmetry. The next step is to repeat this procedure in a lower single null plasma and test its applicability to ITER.
Background:	In Tore Supra the ergodic divertor coil produced a dominate m,n = 18,6 mode spectrum on the q = 3 surface. The RMP coil was capable of operating with up 45 kA of current but it was found that the radiative instability could be stabilized and controlled with a coil current of only 18 kA and 3 MW of LH power. LHH was used to heat the plasma and produce a highly localized radiating zone between the edge of the plasma and the high-field side graphite wall. The DIII-D I-coil, when configured to produce an m,n = 9,3 resonant field on the q = 3 surface with ~ 7 kA, provides a delta_br approximately equivalent to that of the 30 kA Tore Supra RMP field implying that it will be possible to reproduce the Tore Supra results in DIII-D. In order to produce a significant radiation source needed to enhance the radiative instability and test the limits of the stabilizing effect of the RMP field, it may be necessary to add an impurity puff such as methane or neon.
Resource Requirements:	This experiment can be completed in one day.
Diagnostic Requirements:	IR camera, bolometer array, fast UCSD camera, SXRI and the full array of magnetic, fluctuation and pedestal profile diagnostics.
Analysis Requirements:	--
Other Requirements:	--