Fast and pervasive heat transport induced by multiple locked modes in DIII-D

The nonlinear MHD code TM1 [1] is used to understand the transition process from multiple locked modes (LMs) to thermal quench (TQ) in DIII-D. It is found that the co-existence of 2/1, 3/1 and 4/1 locked islands flattens the temperature at the corresponding rational surfaces (RS) and produces a large (~50%) reduction in the central temperature T_e. This modeling reproduces the DIII-D experimental results well. The modeled T_e profile from 2/1 to 4/1 RS is nearly flattened even in cases with no island overlap. The observed reduction in the edge T_e, however, requires island overlap within the TM1 model. The modeled T_e profile is reduced further when applying larger EFs that drive larger island widths, wider edge stochastic regions and secondary island structures. These results indicate that the co-existence of multiple LMs deteriorate plasma thermal confinement more than the sum of their isolated impacts would and that this may be responsible for the fast TQ observed prior to major disruptions.

[1] Yu Q., Phys. Plasmas 10 (2003) 797