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Title	163: Exploration of DEFC in the feedback stabilized plasma in low-q discharges
Name:	Michio Okabayashi mokabaya@pppl.gov	Affiliation:	Princeton Plasma Physics Laboratory
Research Area:	Stability & Disruption Avoidance	Presentation time:	Requested
Co-Author(s):	Ted Strait, Yongkyoon In	ITPA Joint Experiment :	No
Description:	DEFC is an attractive tool to evaluate uncorrected error field in stable plasma condition. The necessary condition is simply that the resonant field amplification can take place to the applied non-axi-symmetric field. DEFC should function also even in feedback stabilized plasmas according to a simple model(YongKyoon In 2010 IAEA). However, the operation will be very sensitive to the gain setting. In addition, the magnitude of coil current of DEFC changes, confusingly, from marginally-stable condition to marginally-unstable since the coil current in unstable condition does overestimate the error field correction in contrast to underestimate in the stable condition. The improved approach near the marginal condition was proposed by Yongkyoon In andTed Strait. Recent achievement of q~2 condition approaches asymptotically toward marginal condition. This condition provides an excellent opportunity to verify the model prediction and improved approach. This exercise is useful not only for the DEFC in marginal condition of low-q, but also for direct feedback once we enter deeply in the unstable regimes. This study can be assisted by fast active MHD spectroscopy to determine the instantaneous stability condition.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	When the plasma condition approaches towards marginal condition, we apply AMS of 250-500 Hz (Go Matsunaga measured reasonable response) to observe the plasma response. This ASM would be useful to assess EFC in low-q ~2 operation and high beta operation near ??real stability limit? including rotation and kinetic stabilization effect. We need careful gain setting before and after the estimated marginal time period considering the model prediction. The resetting of error field correction based on the actual feedback current are also made based on the model prediction
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