Temperature-Gradient-Driven Tearing Modes

The observation of spontaneous islands growth in tokamak experiments indicates that the tearing mode could also be linearly unstable. However, in the framework of resistive magnetohydrodynamics, previous works have demonstrated that drift effects, electron heat transport along magnetic field lines and ion Larmor radius effects have stabilizing effects. Recently, numerical calculations and analytical theory in the collisional regime have demonstrated that inclusion of perpendicular electron heat transport carries out a new type of linear tearing mode driven by a large electron temperature gradient. In this paper, we review the linear tearing mode equations in order to identify the relevant instability regimes. The relevance of some regimes to experiments is discussed and analytical results in different regimes are presented. In all cases, realistic value of the perpendicular electron heat transport produces a strong destabilizing term proportional to the electron temperature gradient in the expression of the island growth rate.