Stabilization of the Tearing Mode in Low-Density Tokamak Plasmas by Turbulent Electron Diffusion

The effect of turbulent electron diffusion from stochastic electron orbits on the stability of low-beta fluctuations is considered. A set of coupled self-adjoint equations is derived for the fluctuation potentials $\tilde{\varphi }$ and ${\tilde{A}}_{\parallel }$. For the tearing mode, it is shown that stability is obtained for sufficiently large values of the diffusion coefficient. Provided $D_e\sim \frac{1}{n}$, this implies that a density threshold must be surpassed before the tearing mode is observed. Numerical calculations also support this conclusion.