Microtearing Modes and Anomalous Transport in Tokamaks

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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 $\frac{|\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}|}{B}\ensuremath{\simeq}\frac{{\ensuremath{\rho}}_{e}}{{L}_{T}}$. 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.