Unstable dissipative drift modes in a sheared magnetic field

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Abstract
Collisional, electrostatic drift waves are shown, within the context of a slab geometry with magnetic shear, to be destabilized by a positive electron temperature gradient ηe=dlnTe/dlnn. The temperature gradient produces a well which localizes the drift eigenmode near the mode rational surfaces and for reasonable magnetic shear, e.g., Ln/Ls=0.05, these dissipative drift waves are linearly unstable when the temperature gradient exceeds a threshold of νe≃3. The crucial feature of this analysis is the inclusion of an energy dependent collision model (Lorentz model). Generally, the mode is most unstable for νc/ω*≃20 and is stable for νc/ω* ≳≳<<20. We believe this is the first drift mode, driven solely by diamagnetic currents, which is absolutely unstable within the context of a slab model with magnetic shear. Other collisionless and collisional drift modes are found to be stable when the full electron dynamics are included. Finite β effects are found to have a strong stabilizing influence on this instability.