Observation of Nonlinear Neoclassical Pressure-Gradient–Driven Tearing Modes in TFTR

A detailed comparison is made between the tearing-type modes observed in TFTR supershot plasmas and the nonlinear, neoclassical pressure-gradient--driven tearing mode theory. Good agreement is found on the nonlinear evolution of single helicity magnetic islands $(m/n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3/2$, $4/3$, or $5/4$, where $m$ and $n$ are the poloidal and toroidal mode numbers, respectively). The saturation of these neoclassical tearing-type modes requires ${\ensuremath{\Delta}}^{\ensuremath{'}}<0$ (where ${\ensuremath{\Delta}}^{\ensuremath{'}}$ is the well-known parameter for classical current-driven tearing instability), which is also consistent with the numerical calculation using the experimental data.