Microtearing Modes and Anomalous Transport in Tokamaks

Microtearing (high-$m$) modes driven by the electron temperature gradient are found to be unstable for present tokamak parameters. A self-consistent calculation of the nonlinear saturation of this instability yields magnetic fluctuations $\frac{\left|\tilde{B}\right|}{B}\simeq \frac{{\rho }_e}{L_T}$. The associated crossfield electron thermal conductivity is shown to be inversely proportional to density, consistent with Alcator scaling, and comparable in magnitude with that inferred from experiments.