Effect of on-axis safety factor on infernal instability in tokamak plasmas with negative-triangularity and negative central shear

The effect of the on-axis safety factor (q0) on infernal-kink instabilities is investigated in negative-triangularity (neg-D) plasmas with negative central shear. Linear stability analyses are performed using the MARS-F code for a family of equilibria with varying q0 while keeping the minimum safety factor (qmin), its radial location (sqmin), and normalized pressure (βN) fixed. The results indicate that q0 has a rather limited impact on stability, with the magnitude of this effect strongly dependent on sqmin. For the pure infernal mode, which occurs when qmin is located sufficiently far from the magnetic axis, increasing q0 only slightly widens the stability window in triangularity, and mainly for weakly positive triangularity plasmas. In neg-D configurations, the effect is negligible since the instability peaks far from the magnetic axis, where q0 has little influence. When qmin moves close to the axis, the mode develops an infernal-kink mode. In this case, q0 again has little influence on stability in neg-D plasmas because the global kink component dominates the mode structure, which is insensitive to q0. However, scans of qmin near a rational value reveal that increasing q0 stabilizes the infernal mode while destabilizing the infernal-kink mode.