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	183: Measurement of multiple mechanisms in the low-density limit via ellipticity in the compass scan
Name:	Carlos Paz-Soldan paz-soldan@fusion.gat.com	Affiliation:	Columbia University
Research Area:	Stability & Disruption Avoidance	Presentation time:	Requested
Co-Author(s):		ITPA Joint Experiment :	No
Description:	This experiment proposes to perform a high-resolution low-beta compass scan, consisting of 6-8 points, in order to fully constrain an elliptical curve to the locked mode onset data. This elliptical curve will provide a unique test of theoretical models, requiring at least two physical mechanisms to fully define the locked mode onset. Fitting the elliptical curve will thus yield data to weight the relative importance of the two mechanisms.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	The experiment will be conducted on the standard low-q, low-beta target plasma used for DIII-D error field studies (for example 149499). An n=1 compass scan will be performed by ramping the applied field at a fixed toroidal phase until a locked mode is found. However, instead of the standard 3-4 phases, somewhere between 6-8 phases will be tried. This will fully constrain the elliptical model, which has 5 free parameters. The largest apparent ellipticity in existing scans occurred with the C-coil, thus it is proposed to use the C-coil for this experiment. If ellipticity is verified, a modeling effort will be undertaken to understand this new result. A successful model of the dataset will yield an empirical weighting between competing physical mechanisms setting the low-density locked mode limit. This experiment could be accomplished in 6-8 good shots.
Background:	The standard technique for low-beta empirical error field correction on DIII-D is the 'compass scan', in which a correcting field is applied at various toroidal phases in order to induce a locked mode. The premise behind this approach is that locked mode onset is proportional to the magnitude of the applied field vector summed with the intrinsic machine error. Thus, the coil currents at which a mode appears can be fit (to zeroth order) to an offset circle in the compass scan phasor space (current amplitude and toroidal phase). DIII-D has performed 22 such 'compass scans' since 2004. Despite the common interpretation of these scans, recent analysis indicates that nearly all compass scans systematically deviate from this picture. That is, that a measurable amount of -ellipticity- is present in fits to the locked mode onset. This ellipticity is not easily explained; as all known factors for locked mode onset are circular on the compass scan space, such as resonant and non-resonant (NTV) breaking. Thus, the ellipticity of the compass scan poses a unique challenge for modeling.
Resource Requirements:	No beams will be required for this experiment. A standard patch panel (3 SPAs on C-coil pairs) will be used. This experiment lends itself well to a short time slot (evening or startup, for example).
Diagnostic Requirements:	Thomson, magnetics, SXR, CO2 and 288 GHz interferometers.
Analysis Requirements:	All data analysis tools are already in existence. Modeling of the NTV for the equilibrium is already in progress in the context of the 2011 proxy field experiment (which shares the same target plasma)
Other Requirements: