Abstract
The linear stability theory of an electron temperature gradient driven microtearing mode, an instability recently proposed as a possible cause for anomalous electron thermal transport in tokamaks, is considered. The theory is electromagnetic and is carried out within the context of a slab model with a sheared magnetic field. In contrast to the linear theory of drift waves, where any magnetic shear is completely stabilizing, shear may actually increase the growth rate of microtearing modes. The crucial feature required for instability is the energy dependence of electron-ion collisions. The mode is shown to be unstable for electron temperature gradients and degrees of collisionality typical of present day tokamaks. It is found that previous theories of these modes were based on assumptions which are not, in general, justified; a case in point being the fact that the usually neglected electrostatic effects are actually quite important in producing instability.
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