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Title	377: NTM locking disruption avoidance by the EM torque with toroidal-phase forward magnetic feed
Name:	Michio Okabayashi mokabaya@pppl.gov	Affiliation:	Princeton Plasma Physics Laboratory
Research Area:	Disruption Mitigation	Presentation time:	Not requested
Co-Author(s):	A. Garofalo, R. LaHaye, D. Shiraki, Ted Strait and Francesco Volpe.	ITPA Joint Experiment :	No
Description:	For successful operation of reactor-oriented devices like ITER, the NTM mode-locking is one of potentially-serious MHD events leading to major disruptions. Here, it has been proposed to apply accelerating electromagnetic (EM) torque and to overcome the mode-locking torque due to imperfect 2D magnetic fields (error fields). This overcoming against mode-locking is produced by magnetic feedback system utilizing internal 3D coils. A key element is to introduce the feed-forward toroidal-phase shift between the observed NTM mode and the applied feedback magnetic field. An advantage of feedback approach is that the applied total torque increases quadratically when NTM amplitude increases. Secondly, the presetting feed-forward phase shift is convenient to control the amount of applied toque input. Thirdly, the dynamic error field correction (DEFC) process takes place simultaneously since the feedback parameters are typical DEFC settings. This feedback scheme is expected to find a quasi-steady state NTM rotation condition at very low rotation during the slowing down period toward mode-locking. A simple toroidal-phase control stability model predicts that the direction of mode propagation depends on the direction of toroidal-phase shift setting and that the NTM frequency at a very low rotation steady state is the order of the inverse of the filtering time constant preset in the feedback system.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Preliminary results of its application in DIII-D high beta plasmas are promising. By proper presetting of the feed-forward toroidal phase shift, the NTM propagating initially with ~ 5-6 kHz was slowed down, but was sustained around ~50 Hz as a new low frequency steady-state equilibrium point without leading to locking. The stored plasma energy was kept within 70% range of initial level. The C-VI rotation was initially around 5kHz at q=2 surface and reduced to well below 500 Hz. It is hard to estimate accurately the bulk D-plasma rotation frequency. The final NTM frequency of ~50 Hz corresponds to the inverse of the filtering time constant (~ 1/40 ms). The mode propagation was found to flip its direction depending upon the preset of toroidal-shift direction. During the feedback in the low frequency quasi-steady state, the toroidal-phase difference between the NTM and the feedback field was fluctuated but remained stable, consistent with a simple model prediction. However, non-linearity of this toroidal phase difference in time implies the possibility of the influence due to uncorrected error field. This approach can be applied to any phase when the NTM or TM is ecited in the middle of disruption mitigation.
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