Flow, Mode Effects That Seed Neoclassical Tearing Modes

This poster discusses how perturbations seed a robustly unstable neoclassical tearing mode NTM) by exceeding a "gate." In the NTM instability equation the destabilizing bootstrap current drive d_NTM is approximately 3 (j_boot / j_parallel). It is balanced by an ion polarization current that produces a stabilizing flow dependent gate function F (f_m). The polarization current gate function in the plasma is proportional to the difference between the frequencies of the NTM instability mode and the radial electric field toroidal reference frame. In the DIII-D ITER Baseline Scenario (IBS) discharge 174446, the NTM drive d_NTM is about 0.5 and the ion polarization width w_pol of 2.7 cm is about twice the ion banana width. The NTM is unstable when the NTM drive exceeds the stabilizing gate in the island evolution equation: d_NTM > (w_pol^2 / w^2) F (f_m). Here, the island width w (cm) equals 3 B_rms(G)^1/2, where B_rms(G) is a transient external Mirnov signal (e.g. ELM). When the gate function is closed [F(f_m) equals 1], growing NTMs are seeded in 174446 if B_rms > 1.8 G (w > w_pol / d_NTM^1/2 simeq 4 cm). However, if the gate function F (f_m) is reduced because flow differences are reduced, NTM growth can be seeded by a slightly smaller B_rms, w.