Root Cause of Disruptive NTMs in DIII-D ITER Baseline Scenario Plasma*

DIII-D experiments identify the rotation profile flattening as the direct cause of the majority of disruptive m,n=2,1 tearing modes in low-torque ITER baseline scenario plasmas, while long pulse stable operation is achieved when the differential rotation is preserved. 

The observed exponential onset time distribution and constant onset rate of these islands is consistent with the wait time of Poisson point processes [1]. This behavior indicates the existence of a threshold for instability growth that is crossed with uniform temporal probability occurring from a transient imbalance of stabilizing and destabilizing mechanisms, such as differential rotation and MHD triggering. The probabilistic onset with constant rate clarifies that these modes do not favor any phase of the current relaxation in the pressure flattop. Moreover, the magnetic perturbation amplitude of these islands grows linearly in time, in accordance with the bootstrap current dominated asymptotic limit of the modified Rutherford equation, regardless of how far the current relaxation has evolved by the onset time. This shows that changes in the classical tearing stability due to the current relaxation do not affect the 2,1 tearing mode evolution either. The majority of the mode triggers are consistent with non-linear 3-wave coupling between the sawtooth precursor, the 3,2 island and the equilibrium when the differential rotation between the q=1 and q=2 rational surfaces approaches zero [2]. This flattening is caused by n>1 islands whose amplitude shows chaotic behavior possibly due to non-linear couplings between these core islands and the equilibrium current, pressure and rotation profiles through phase-space resonances of energetic ions. We present shot programs where long pulse stable operation was achieved by avoiding large core n>1 modes and preserving the favorable differential rotation. Machine learning analysis of multi-scenario database of over 13,000 discharges shows the importance of differential rotation in the onset and avoidance of disruptive 2,1 tearing instabilities is general in low edge safety factor H-mode plasmas [3].