The physics of a collisionless micro-scale tearing mode

Microtearing modes and electron temperature gradient modes are two types of micro-instability that are driven by the electron temperature gradient in magnetised plasmas. Both have been widely studied and are well known respectively as an electromagnetic tearing parity mode and an electrostatic twisting parity mode. Microtearing modes, as the tearing parity one, can cause fine scale reconnection of the magnetic field lines in the vicinity of rational flux surfaces. This leads to formation of magnetic islands, which increases the heat and particle flux across the magnetic confinement devices. Microtearing modes therefore are considered as a candidate for anomalous electron heat transport in tokamak plasmas.
Gyrokinetic theories are used in modelling the physics drive mechanism and the stability of micro-scale modes. Early theories for microtearing modes in slab geometry concludes that the drive mechanism of this mode requires a finite collision frequency; thus it is stabilised at low collision frequencies. However, we find in linear gyrokinetic simulations that a fine scale tearing parity instability, driven also by the electron temperature gradients, persists even in the collisionless or electrostatic limit. We demonstrate that this mode has a much larger radial wavenumber than the binormal one and poses numerical challenges to resolve in simulations. The mode growth rate is sensitive to electron finite Larmor radius effects, which are often neglected in previous studies. We develop two linear analytic gyrokinetic models to identify that this collisionless tearing parity mode is consistent with a higher harmonic of the electron temperature gradient mode, which becomes more unstable than the conventional twisting eigenmode under the parameter range in this thesis. When including electromagnetic fields, this mode is capable of forming magnetic islands even in the absence of collisions.
Our study provides an example that tearing parity micro-instabilities can arise from various physics drives. This result brings up thoughts for further studies on turbulent transport in magnetised plasmas.