Extended electron tails in electrostatic microinstabilities and the nonadiabatic response of passing electrons

Abstract

Ion-gyroradius-scale microinstabilities typically have a frequency comparable to the ion transit frequency. Hence, it is conventionally assumed that passing electrons respond adiabatically in ion-gyroradius-scale modes, due to the small electron-to-ion mass ratio and the large electron transit frequency. However, in gyrokinetic simulations of ion-gyroradius-scale modes, the nonadiabatic response of passing electrons can drive the mode, and generate fluctuations with narrow radial layers, which may have consequences for turbulent transport in a variety of circumstances. In flux tube simulations, in the ballooning representation, these instabilities reveal themselves as modes with extended tails. The small electron-to-ion mass ratio limit of linear gyrokinetics for electrostatic instabilities is presented, including the nonadiabatic response of passing electrons and associated narrow radial layers. This theory reveals the existence of ion-gyroradius-scale modes driven solely by the nonadiabatic passing electron response, and recovers the usual ion-gyroradius-scale modes driven by the response of ions and trapped electrons, where the nonadiabatic response of passing electrons is small. The collisionless and collisional limits of the theory are considered, demonstrating interesting parallels to neoclassical transport theory. The predictions for mass-ratio scaling are tested and verified numerically for a range of collision frequencies. Insights from the small electron-to-ion mass ratio theory may lead to a computationally efficient treatment of extended modes.

Linear gyrokinetic stability of a high β non-inductive spherical tokamak

“…Figure 5(c) overlays the eigenfunctions at different θ 0 around θ = 0 which shows that the central peak moves across θ as θ 0 in increased. Figure 5(d) shows the full eigenfunction, and the extended tail region all have a similar decaying envelope, indicating that the physics along these tails is not impacted by the central region, which is qualitatively similar to the extended electron tails seen recently in [47]. It is noted that compared to the θ 0 = 0.0 case the relative amplitude of the tails is smaller than for the cases with finite θ 0 .…”

Linear gyrokinetic stability of a high $β$ non-inductive spherical tokamak

“…Figure 5c overlays the eigenfunctions at different θ 0 around θ = 0 which shows that the central peak moves across θ as θ 0 in increased. Figure 5d shows the full eigenfunction, and the extended tail region all have a similar decaying envelope, indicating that the physics along these tails is not impacted by the central region, which is qualitatively similar to the extended electron tails seen recently in [47]. It is noted that compared to the θ 0 = 0.0 case the relative amplitude of the tails is smaller than for the cases with finite θ 0 .…”