Integrated Modeling of Advanced Tokamaks for Tearing Mode Avoidance on DIII-D

A newly developed tearing mode (TM) onset model has been integrated to the theory-based IPS-FASTRAN scenario modeling, allowing a “predict first” TM stable solution for non-inductive, high β_N>4 operation with the planned DIII-D heating and current drive upgrades. Understanding the physics of the onset and evolution of TM in tokamak plasmas is crucial for high-performance steady-state operations. For TM stability analysis, a classical tearing stability index Δ' is calculated with experimental equilibria using PEST3 and resistive DCON. The onset condition of tearing stability, Δ'>Δ'_c>0, has been studied with an analytic model of the tearing stability threshold Δ'_c based on the nonlinear neoclassical tearing mode theory, showing qualitatively reasonable agreement against various DIII-D advanced tokamak (AT) discharges including steady-state hybrid, elevated q_min, and high l_i. A database of DIII-D AT discharges indicates that the onset of (m,n)=(2,1) TM is more sensitive to the global MHD equilibrium than to the local profile gradients at the q=2 rational surface. A stability diagram for the TM onset is suggested by the difference between Δ' and Δ'_c with a parametric variation of the current density and pressure profiles around a given MHD equilibrium to evaluate its proximity to the TM onset. A reduced model of Δ'_c is constructed from nonlinear 3-D MHD simulations using NIMROD for more accurate prediction of the TM onset in the IPS-FASTRAN scenario modeling. A systematic optimization of the current and pressure profiles is done with a massive ensemble of the IPS-FASTRAN simulations enabled by High Performance Computing at scale, identifying a trade-off between tearing onset and ideal MHD stability limit to achieve the stable β_N>4 in a fully non-inductive condition.