Tearing Modes in DIII-D IBS Discharges, CTMs or NTMs?

This poster explores whether classical tearing modes (CTMs) or neoclassical tearing modes (NTMs) are dominant in DIII-D ITER Baseline Scenario (IBS) discharges. Growing tearing modes are problematic in IBS discharges because they can lead to plasma disruptions. Tearing mode evolution is governed by the modified Rutherford equation (MRE). CTMs are driven by Δ' ∼ d j_∥ / dr; NTMs are driven by d_NTM ∼ 3 j_boot / j_‖ ∼ dP / dr. When 2/1 tearing modes are growing robustly in DIII-D IBS discharges, the MRE predicts CTMs should grow as t ² and NTMs should grow as t. As an example, in DIII-D IBS discharge 174446, assuming a reasonable positive r₀'Δ=1 for CTMs, the Mirnov magnetic signal δB should grow by about 1 Gauss (G) in 160 ms. In contrast NTMs with d_NTM = 0.5 should increase δB by about 1 G in 9 ms. The 174446 data for δB > 5G exhibits linear growth in t and δB increases by 1 G in 8.5 ms, at a rate of 120 G/s. NTM predictions agree with this experimental data. In contrast, the MRE-predicted CTM t ² scaling disagrees with it and its growth time is too long. Other DIII-D IBS data provide similar results. Thus, the experimental data strongly indicate NTMs are the dominant tearing modes in DIII-D IBS discharges.