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Title	122: Bid farewell to locked modes - NTM-locking avoidance by feedback synchronization assisted with ECCD
Name:	Michio Okabayashi mokabaya@pppl.gov	Affiliation:	Princeton Plasma Physics Laboratory
Research Area:	Stability & Disruption Avoidance	Presentation time:	Requested
Co-Author(s):	Daisuke Shiraki, Francesco Volpe, Andrea Garofalo, Rob Lahaye, Ted Strait	ITPA Joint Experiment :	No
Description:	Comprehensive utilization of Internal non-axisymmetric coils, ECCD system and a PCS with an appropriate choice of feedback algorithm would reduce or even eliminate the risk of NTM-locking disruptions in a tokamak reactor. The feedback experiments in piggyback a few years ago had revealed very interesting results on the tearing mode behavior. The feedback operated with toroidal phase shift can synchronize the I-coil currents to the tearing mode, and not only prevent the mode from locking, but also modify the mode characteristics. The successful maneuverability of tearing mode with Internal non-axisymmetric coils is very crucial for the Error Field Correction, but also carry very significant impact on determining the use of internal coils in the ITER.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Observations in piggyback experiment are: (1) The Feedback can shift freely the tearing mode direction and frequency without losing the stored energy. The feedback fields accelerate/decelerate toroidally the mode and, interestingly, the mode frequency is lowered by ten-hundred times within short period with minimum stored energy loss. (2) The synchronization-transition from 0.5-1 kHz frequency to the feedback-controlled frequency (20-60 Hz) is smooth when the phasing is properly adjusted. The choice of phase shift seems less sensitive to plasma condition. These results suggest that the tearing mode is quite maneuverable so that the mode can be slowed down to low frequency range, and then can be controlled. Although it is to be examined further whether the tearing mode can be quenched in time, it seems, at least, we can sustain as a feedback-forced slowly-rotating-tearing mode. However, caution is to be made that this can occur only when the error field correction is extremely well compensated and of course only with high bandwidth amplifier-coil system. However, the feedback itself should perform simultaneously as a dynamic error field correction process, if resonant field amplification occurs during NTM-locking period. Experimental Approach We apply feedback by Dud detector onset: (1) To survey systematically the impact of feedback toroidal phase on tearing modes (2) To explore possibilities of quenching by varying the phase/gain in time (3) To explore the implication to ITER internal coil option. (4) this approach can be combined with ECCD, - no ECCD, or minim ECCD power - increase overall Q in ITER, - slower frequency with o- or x- points sychronization easier (5) while rotating, the electromagnetic force can be added (6) same time, dynamic error field correction takes place, if resonant field amplification occurs due to equilibrium shift during NTM This technique should be tested in any plasma conditions to observe the versatility of this approach.
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