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	190: Measurement of n=3 error field effects in RMP phase-flip experiments
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 proposes to apply the techniques of error field control (EFC) to understand and verify the offsets used in n=3 'phase flip' experiments commonly used to diagnose RMP ELM-suppression discharges. This will provide a physics basis for the continued use of this offset benchmarked against global EFC metrics, as well as providing useful data for the new 3-D magnetic sensors.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	First a high-beta 'compass scan 1-D analog' will be performed. Using the same target plasma as used for previous n=3 phase flip measurements, the n=3 field will be slowly ramped (or stepped) at +ve polarity, and in a subsequent shot at -ve polarity. Throughout, the NBI torque and beta will be held constant, and the global momentum of the plasma calculated. Where the momentum is maximized will be compared with the offset used in previous phase flip experiments. The n=3 plasma response minimum can also be compared, or in fact any number of parameters along this scan. Second, a low-beta scan will be performed in which the key metric will be the locked mode onset. The plasma target would likely be the Ohmic test plasma commonly used in error field experiments. The n=3 field will be ramped until a mode appears, first at +ve polarity then at -ve polarity. The mean value will be compared with the offset used in RMP-ELM phase flips. (Standard C-coil n=1 EFC will be enabled throughout these discharges.) As the n=3 even parity field has only 1 free parameter (amplitude), the number of shots required for this study is very modest (~4). Considering the other n=3 parity would add another ~4 shots.
Background:	A very useful diagnostic technique for RMP-ELM experiments is currently the 'phase-flip' experiments, in which the n=3 field is quickly flipped from +ve to -ve polarity. However, it has been empirically found that a ~ 0.5kA offset is required to symmetrize the core Thomson response at each polarity, presumably due to an intrinsic machine n=3 error. As the value of this offset materially affects the measurements extracted during phase flips, it is important to understand its origin and confidently apply it in future experiments. This experiment will seek to gain this understanding by applying the tools developed in n=1 error field correction to the special case of the n=3 phase flip experiment. This experiment is also complementary to proposed studies to further vary the offset for phase flip RMP experiments.
Resource Requirements:	Beta_N and injected torque feedback will be critical to the high-beta phase of the experiment. Bipolar current requirements necessitate the use of SPAs.
Diagnostic Requirements:	Thomson, CER, ECE, MSE, SXR, new 3-D magnetics
Analysis Requirements:	EFC Analysis tools may need slight modifications to treat n=3 fields.
Other Requirements: