The causes of the disruptive tearing instabilities of the ITER Baseline Scenario in DIII-D

Analysis of the evolving current density (J), pedestal and rotation profiles in a database of 200 ITER Baseline Scenario (IBS) demonstration discharges in the DIII-D tokamak identifies the current profile as a key determinant of the disruptive instability limiting both high and low torque operation of these q(95) = 3 plasmas. The m = 2/n = 1 tearing modes, occurring after several pressure-relaxation times, are linked to the shape of the current profile in the outer region of the plasma. The q = 2 surface is located at rho similar to 0.78, near a minimum in J. A steeper 'well' around the q = 2 surface is correlated with the instabilities, and this finding is consistent for modes that occur both early (<1 tau R) and late (>2 tau R) on the beta(N) flattop, at high and low input torque, for discharges with and without direct electron heating. Low rotation or differential rotation in the core plasmas is shown to not be the direct cause of the onset of these instabilities. The current profile trends identified in this work indicate that the modes limiting the Q = 10 operation in the ITER demonstration plasmas are likely due to the slow evolution of the current profile, and reveal which shape of the current profile is more prone to the 2/1 tearing instability.