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Title	175: Beta collapse avoidance in SSI discharges by de-synchronizing toroidal harmonics by using feedback
Name:	Michio Okabayashi mokabaya@pppl.gov	Affiliation:	Princeton Plasma Physics Laboratory
Research Area:	Steady State Heating and Current Drive	Presentation time:	Not requested
Co-Author(s):	Go Matsunaga, Ted strait	ITPA Joint Experiment :	No
Description:	The OFM-ELM pacing study (2012IAEA) revealed how the coupling between toroidal harmonics in low n mode takes place. (1) the poloidal wavelength of n=1,2,(3) is comparable at outboard side. This is why they couple well. (2) The toroidal phase of each harmonics are synchronized producing the total amplitude maximum at mid-plane and causes ELM crash, creating massive impurity inflow and betan collapse. Toroidal harmonics coupling has been said many years. However, until now, we have not visualized how the toroidal harmonics couple in two-dimensional mode structure. It is to be said that the fast growth of higher harmonic component is the major part leading to disruption, not the fundamental n=1. Although this was studied with OFM-ELM pacing, but, relation of fundamental /harmonics eigenmode patterns on surface (real part) should remain similar in any high-beta modes (because of ballooning effect), imaginary part varies depending upon the growth term. This behavior may be universal	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Now, we would like to utilize this knowledge in disruption / beta-collapse avoidance. It is better to control toroidal harmonics n=1, 2, 3 together. However, the present DIII-D RWM control system is only capable to inlcude n=1 and n=3. Thus, the initial experiment is a proof of principle trial with n=1 and n=3. First, in order to test the phase relationsignificance, we need to slow down the mode frequency by reducing the plasma rotation and handling the system within reasonable time resolution. The SSI target plasma with additional 210 beams, we expect the mode frequency can be lowered considerably. We will compare the n=1 and 3 feedback in-phase and out-of-phase between n=1 and n=3 to observe the ratio evolution in toroidal harmonics n=1 and 3. Phase of n=3 harmonics should be desynchronized relative to n=1 toroidal phase When new magnetic sensors are available, it should be easier to look at n=1,2,3.
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