Direct Measurements of Internal Structures of Born-locked Modes and the Key Role in Triggering Tokamak Disruption

The internal structure of a born-locked mode in an Ohmic-heated, low-density plasma is directly measured for the first time. A novel dual soft X-ray imaging system has recently been developed in the DIII-D tokamak to measure the radiation symmetry breaking by the presence of static, three-dimensional helical structures. Small locked island chains with helicities of m=3/n=1 and m=4/n=1 are observed simultaneously with the m=2/n=1 island (m, n: poloidal and toroidal number) and the locked phase of each island chain is uniquely determined. It is found that the 3/1 and 4/1 island, initially well separated from the major 2/1 island, govern the cooling process in a long quasi-stationary phase, but later trigger thermal quench, when starting to overlap. A simulation of intrinsic error field penetration at multiple rational surfaces and its impact on electron thermal transport by a non-linear single fluid MHD code (TM1) is qualitatively consistent with the observations. The result shows the essential role of co-existing multiple locked islands in the deterioration of global thermal confinement preceding thermal quench.It should be emphasized that this study is important for the understanding of potential major disruption in the current ramp-up phase of ITER discharge, since the amplitude of fully penetrated resonant n=1 error field locked modes was demonstrated to be independent of resonant intrinsic error field correction in DIII-D tokamak [1].

[1] R.J. La Haye, C. Paz-Soldan and E.J. Strait, Nucl. Fusion 55 023011 (2015).