Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

Abstract

Electromagnetic (EM) instabilities and turbulence driven by the electron-temperature gradient are considered in a local slab model of a tokamak-like plasma. The model describes perturbations at scales both larger and smaller than the flux-freezing scale $d_e$, and so captures 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-Alfvénic instability (TAI). It exists in both a slab version (sTAI, destabilising kinetic Alfvén waves) and a curvature-mediated version (cTAI), which is a cousin of the (electron-scale) kinetic ballooning mode (KBM). The cTAI turns out to be dominant at the largest scales covered by the model (greater than $d_e$ but smaller than ${\rho }_i$), 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 turbulent cascade theory is then constructed, with two energy-injection scales: $d_e$, where the drivers are slab ETG and sTAI, and a larger (parallel system size dependent) scale, where the driver is cTAI. The latter dominates the turbulent transport if the temperature gradient is greater than a certain critical value, which scales inversely with the electron beta. The resulting heat flux scales more steeply with the temperature gradient than that due to electrostatic ETG turbulence, giving rise to stiffer transport. This can be viewed as a physical argument in favour of near-marginal steady-state in electron-transport-controlled plasmas (e.g., the pedestal). While the model is simplistic, the new physics that is revealed by it should be of interest to those attempting to model the effect of EM turbulence in tokamak-relevant configurations with high beta and large electron temperature gradients.

104 pages, 17 figures

References in corpus (13)

• Linearized model Fokker-Planck collision operators for gyrokinetic simulations. I. Theory
• Simulations of Electron Magnetohydrodynamic Turbulence
• Multiscale nature of the dissipation range in gyrokinetic simulations of Alfvénic turbulence
• Gyrokinetic turbulence: a nonlinear route to dissipation through phase space
• Microtearing modes as the source of magnetic fluctuations in the JET pedestal
• Collisionality scaling of the electron heat flux in ETG turbulence
• Kinetic microtearing modes and reconnecting modes in strongly magnetised slab plasmas
• A solvable model of Vlasov-kinetic plasma turbulence in Fourier-Hermite phase space
• Linear gyrokinetic stability of a high $\beta$ non-inductive spherical tokamak
• Extended electron tails in electrostatic microinstabilities and the nonadiabatic response of passing electrons
• Multiscale Gyrokinetics for Rotating Tokamak Plasmas II: Reduced Models for Electron Dynamics
• Dimits transition in three-dimensional ion-temperature-gradient turbulence
• Linear Vlasov theory of a magnetised, thermally stratified atmosphere

Cited by in corpus (8)

• Three-Dimensional Inhomogeneity of Electron-Temperature-Gradient Turbulence in the Edge of Tokamak Plasmas
• Kinetic-Ballooning-Limited Pedestals in Spherical Tokamak Plasmas
• An analytical form of the dispersion function for local linear gyrokinetics in a curved magnetic field
• The effects of finite electron inertia on helicity-barrier-mediated turbulence
• Suppression of temperature-gradient-driven turbulence by sheared flows in fusion plasmas
• Scale invariance and critical balance in electrostatic drift-kinetic turbulence
• Turbulent heating in collisionless low-beta plasmas: imbalance, Landau damping, and electron-ion energy partition
• The gyrokinetic field invariant and electromagnetic temperature-gradient instabilities in `good-curvature' plasmas