On the instability mechanism of a slab microtearing mode

The electron temperature gradient destabilizes short-wavelength magnetic field perturbations and enhances transport to degrade performance of fusion devices[1]. The theoretical works predicted that the energy dependent electron-ion collisions leading to the thermal force are indispensable for such an electromagnetic micro-instability, called the microtearing mode [1][2]. More recent work have reported there exists a collisionless electromagnetic ETG mode causing magnetic reconnection in a slab [3], while in more realistic magnetic geometry for fusion devices, the collisionless MTM has been demonstrated [4].

The fundamental physics of MTM or electromagnetic ETG has not been fully understood. In this work, we study the destabilization mechanism of those modes in a slab using the gyrokinetic simulation code AstroGK [5]. Specifically, we focus on the detail of collision models, and discuss how the energy dependence of the collision frequency affects the thermal force destabilizing the collisional MTM.

[1] J. F. Drake et al., Phys. Fluids 20, 1341 (1977).

[2] A. B. Hassam et al., Phys. Fluids 23, 38 (1980).

[3] Geng et al., Plasma Phys. Contl, Fusion 62, 085009 (2020).

[4] Dickinson et al., Plasma Phys. Contl, Fusion 55, 074006 (2013).

[5] R. Numata et al., J. Comput. Phys. 229, 9347 (2010).