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Title	189: Measurement of optimal n=2 phasing for RMP & ELM suppression without q95 windows
Name:	Carlos Paz-Soldan paz-soldan@fusion.gat.com	Affiliation:	Columbia University
Research Area:	ELM Control	Presentation time:	Not requested
Co-Author(s):	M. Lanctot	ITPA Joint Experiment :	No
Description:	This experiment will use n=2 I-coil phasing as the dynamic variable during the shot to measure the optimum phasing for a given set of plasma parameters. Recent results suggest that the optimum phasing may not be where the vacuum-field pitch-resonant harmonics (m = nq) are maximized, but instead at the kink-resonant maximum (m ~ 2nq). We propose a direct technique to test this hypothesis by dynamically scanning the phasing during the shot (at constant plasma parameters) and looking for ELM suppression windows --in phasing--. Due to the symmetry of the 3-D field spectrum with phasing, the optimum phasing should be in the center of the phasing suppression window. Furthermore, it is possible and proposed to extend this technique to discover the optimum phasing as a function of relevant plasma parameters, likely the most important being q95 (but possibly also beta or rotation). Other key metrics, such as density pump-out, can be measured as a function of phasing in the same discharges. A demonstration of improved understanding is also proposed. By measuring the optimal phasing at a 2-3 values of q95, an algorithm can be developed which tracks q95 and maintains the optimal I-coil n=2 phasing. This algorithm can then be applied to a discharge with significant variations in q95, with suppression expected throughout - in contrast to the usual narrow q95 windows observed.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	To develop an algorithm for q95 following, the required phasings at a few q95's must first be determined. To do this, the q95 will be held constant while the I-coil phasing is varied. The phasing could be either stepped or ramped. The sweep will center on the optimum value found in 2011 n=2 experiments (shot 145592). An ELM suppression window in phasing is expected, with the center point taken to be the optimum for that q95. This will be repeated at 1-2 more well-spaced values of q95 in order to build up an algorithm between q95 and phasing, which is expected to be linear. Greater diagnostic sensitivity can be achieved by modulating the outer gap during the experiment. Once this mapping is determined, discharges for which q95 is ramped will be run. The q95 evolution will be known a-priori by using a well-known target discharge. As q95 is evolving, the n=2 I-coil phasing will be fed-forward based on the mapping. A successful outcome would be no ELMs found within the q95 range of the algorithm, and possibly beyond. This experiment will also give useful piggy-back opportunities to the new 3-D magnetics diagnostic. The optimal (and sub-optimal) n=2 field can also be rotated to enable synchronous detection and thus better noise rejection. This experiment could be accomplished in 9 good shots (4 per q95, 1 demonstration discharge).
Background:	n=2 RMP fields provide the capability (unlike n=3) of tailoring the poloidal structure of the 3-D field used for ELM suppression. This capability will be used to determine the optimum phasing at several q95s. In 2011 it was found that the q95 suppression window was different for different I-coil phasings, a favorable result for this technique to yield a successful measurement.
Resource Requirements:	This experiment will require 6 power supplies. Thus, C1, C2 and all SPAs must be operational. 1 SPA is dedicated to n=1 error field control with the C-coils.
Diagnostic Requirements:	Thomson, SXR, ECE, 3D magnetics, MSE, CER
Analysis Requirements:	M3D-C1 of n=2 plasma response and ELITE of the resultant pedestal profiles.
Other Requirements: