Tearing Mode Issues in Burning Plasmas

This poster will highlight the key physics involved in seeding the robust growth of neoclassical tearing modes (NTMs) in burning plasmas. In seeking maximum plasma performance, physics parameters are pushed to conditions where tearing modes can grow and precipitate disruptions. In ITER baseline scenario (IBS) discharges q95 is reduced to near 3 and beta_N is increased to about 2 or more. The most problematic tearing instabilities occur at the q = 2/1 surface because they are the furthest out radially and lowest m/n modes. Their growing magnetic island widths induce resistive wall drag, mode locking to the wall and then plasma disruption. The nonlinear NTMs are seeded by MHD transients (e.g., ELMs, sawtooth crashes, or three tearing mode resonances). Recent studies of IBS-type discharges in DIII-D show the seeded, robustly growing 2/1 tearing modes that evolve into locked modes and then disruptions are pressure-gradient-driven NTMs. The classical tearing drive is negligible during seeding and as the seeded 2/1 magnetic perturbations grow algebraically (~t) in time. The 2/1 NTMs grow in accord with a novel toroidal-based modified Rutherford equation. Major experimental studies of various predicted means for preventing or controlling 2/1 NTMs at small amplitudes are urgently needed.