This study employs the magnetohydrodynamic code CLT to simulate the excitation of a ballooning mode instability by a nonlinear internal kink mode. The nonlinear saturation of the 1/1 internal kink mode leads to a collapse of the core plasma pressure while simultaneously increasing the pressure in the outer region. This redistribution results in a steepened pressure gradient at the edge, which in turn destabilizes a previously stable ballooning mode. We propose that this mechanism underlies the limit-cycle oscillations observed during the sawtooth-triggered I-phase. Furthermore, the triggered ballooning mode exhibits several key characteristics: (i) its toroidal mode amplitude displays an n = 1 periodicity; (ii) it is resistive in nature, with its saturation amplitude increasing as the resistivity rises; and (iii) it can be stabilized by ion and electron diamagnetic drift effects within a two-fluid framework. Finally, the combined effects of plasma heating and sawtooth oscillations lead to a significant outward shift of the plasma pressure profile, which may offer an explanation for the experimentally observed sawtooth-triggered L–H transition.
Pressure-driven relaxation instability in a current-free high-shear helical system