Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

Electromagnetic (EM) instabilities and turbulence driven by the electron-temperature gradient are considered in a local slab model of a tokamak-like plasma, with constant equilibrium gradients (including magnetic drifts, but no magnetic shear). Derived in a low-beta asymptotic limit of gyrokinetics, the model describes perturbations at scales both larger and smaller than the electron inertial length d_e, but below the ion Larmor scale ρ_i, capturing both electrostatic and EM regimes of turbulence. The well-known electrostatic instabilities --- slab and curvature-mediated ETG --- are recovered, and a new instability is found in the EM regime, called the Thermo-Alfvenic instability (TAI). It exists in both a slab version (destabilising kinetic Alfven waves) and a curvature-mediated version, which is a cousin of the (electron-scale) kinetic ballooning mode (KBM). The curvature-mediated TAI is shown to be dominant at the largest scales covered by the model (greater than d_e but smaller than ρ_i), its physical mechanism hinging on the fast equalisation of the total temperature along perturbed magnetic field lines (in contrast to KBM, which is pressure balanced). A priori critical-balance estimates suggest that the TAI-driven heat-flux scales more steeply with the temperature gradient than that due to electrostatic ETG turbulence, giving rise to stiffer transport. Numerical results on the saturation of the resultant turbulence and the role of zonal flows and zonal fields are presented.