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Title	338: Local correction of the TBM error field
Name:	Carlos Paz-Soldan paz-soldan@fusion.gat.com	Affiliation:	Columbia University
Research Area:	Stability & Disruption Avoidance	Presentation time:	Not requested
Co-Author(s):	R. La Haye, M. Lanctot, E. Strait, and the TBM group	ITPA Joint Experiment :	No
Description:	A brief experiment is proposed to study the efficacy of local correction of the TBM error field. Local correction may prove superior to prior schemes due to the ability of the local coilset to correct several different toroidal harmonics (n) of the error field simultaneously. Thus, this experiment will shed light on the role of higher n fields on important parameters such as rotation degradation and low-density operation limits. Local correction is achieved by energizing the two C-coil elements adjacent to the TBM (C319 & C259). It is proposed to measure the standard metrics of error field control in both low-beta (Ohmic) and high-beta plasmas as these fields are applied.	ITER IO Urgent Research Task :	Yes
Experimental Approach/Plan:	In low beta (Ohmic) plasmas, the current in the local coil pair will be ramped in each polarity until a locked mode is induced. An offset in the required current will likely be found, and this offset will define the optimum. A low-density rampdown discharge will then be performed at the optimum level and compared to the prior low-density limit results from 2009. The reference shot is 149499. In the high beta target plasma, the local coil currents will again be ramped at each polarity, beginning from the offset identified in the low-beta case. Rotation, angular momentum, plasma response, and the plasma-wall Maxwell stress will be measured throughout the ramps. It is again expected that these metrics will be optimized at a non-zero value of local coil current. The degree of metric optimization can then be compared to rotation degradation results from 2011, with the reference shot being 147131. Throughout the experiment, n=1 error field control will be maintained via the I-coils. It will likely be necessary to de-rate the n=1 I-coil currents in order to compensate for the n=1 pollution by the local correction coils, thus isolating the experimental effects to higher n as much as possible. This experiment could be accomplished in 3 (Ohmic) + 3 (H-mode) good shots.
Background:	TBM experiments in 2011 have found optimum levels of I-coil currents needed for correction of the n=1 TBM error field. However, applying this optimal correction only recovered 25% of the rotation degradation introduced by the TBM. This modest result clearly demonstrates that there is room for improvement in the development of the best 3D field for correction of the TBM field. One such improvement may be the use of a local field (in addition to the optimal n=1) correction, as discussed in this proposal.
Resource Requirements:	TBM installed on DIII-D All 4 SPAs functional
Diagnostic Requirements:	Ohmic: Thomson, 3D magnetics, CO2 and 288 GHz interferometers H-mode: Thomson, ECE, CER, MSE, 3D magnetics
Analysis Requirements:	L-mode: Standard locked mode analysis tools already in existence. H-mode: TRANSP runs for all discharges to keep track of total angular momentum.
Other Requirements: