Modeling Steady-State DIII-D Plasmas for Tearing Stability Studies

In DIII-D, steady-state high-$\beta$ discharges are limited by a $n=1$ tearing mode, causing a radial redistribution of the current density not recoverable with the available noninductive current drive sources. The use of electron cyclotron (EC) current with a broad deposition can prevent the mode onset. The current density profile from an experimental DIII-D equilibrium has been perturbed numerically, mimicking the injection of EC current. The tearing stability index $\Delta^\prime$ is evaluated by the PEST3 code as a function of the perturbation amplitude, shape and radial position. The results are compared to the evolution of the experimental current density found previously to characterize discharges unstable to the $n=1$ tearing instability, and to a previous analytical study performed in cylindrical geometry for similar conditions.