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Title	186: Optimal Mixing of I & C coils for n=1 EFC
Name:	Carlos Paz-Soldan paz-soldan@fusion.gat.com	Affiliation:	Columbia University
Research Area:	Stability & Disruption Avoidance	Presentation time:	Requested
Co-Author(s):	E. Strait, M. Lanctot, R. La Haye, J. Hanson	ITPA Joint Experiment :	No
Description:	This experiment proposes to empirically obtain the optimal poloidal spectrum for n=1 error field control (EFC) via ??optimal mixing?? of I and C coil currents. Optimal mixing means proportionally trading-off currents in each coilset based on the optimum current of each coilset alone (see background). In the language of kink-resonant (IPEC single-dominant mode) vs. non-resonant (NTV producing) fields, optimal mixing means nulling the single-dominant mode field while systematically varying the other non-resonant harmonics. This proposal thus allows the role of second order effects (NTV, second-dominant mode) on the low-density limit to be directly measured (by holding the first-order effect constant), and also holds the promise of developing an improved n=1 error field control algorithms for general DIII-D use. The optimal mixing levels of I & C coils can also be compared to NTV theory from various codes. It is also likely that this experiment will produce measurable amounts of non-thermal electrons due to foreseen low-density operation. Piggyback experiments with the runaway electron group can be proposed to diagnose or control this population on the back-end of the discharge. Finally, this experiment provides an opportunity for the new 3-D magnetics diagnostic to measure the detailed structure of the n=1 locked mode found at low density.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Density rampdown discharges will be performed in the low-q, Ohmic plasma commonly used in EFC experiments. The currents in the I and C coil will be proportionally varied, such that a reduction in I-coil current is compensated by an increase in C-coil current, while the phase of both is held constant. The ??over-driven?? case will also be tried, where one coilset is compensated by the other coilset with a 180-degree phase difference from optimum levels. The minimum density prior to locked mode detection will be measured in each discharge and an optimum found along this scan. Additionally, a scan of increasing I & C coil currents together can further constrain the optimum field structure.
Background:	Previous compass scans have yielded an optimum current and phase for n=1 EFC for the I & C coils individually. Strikingly, it is found that the I & C coils are each correcting the -same- single-dominant mode of the intrinsic error field as calculated by IPEC, and thus both point in the same toroidal phase. For this reason, proportionally trading off the amplitude of I & C coil currents will maintain the same single-dominant mode field. Adding multiple coilsets thus allows the single dominant mode field to be held at zero while other parts of the spectrum can be optimized.
Resource Requirements:	Ohmic plasmas only. No beams, no ECH, etc. 4 SPAs required.
Diagnostic Requirements:	Thomson, 3-D magnetics, SXR, CO2 and 288 GHz interferometers
Analysis Requirements:	All presently in existence or underway.
Other Requirements:	--