Stabilisation of short-wavelength instabilities by parallel-to-the-field shear in long-wavelength E × B flows

Abstract

Magnetised plasma turbulence can have a multiscale character: instabilities driven by mean temperature gradients drive turbulence at the disparate scales of the ion and the electron gyroradii. Simulations of multiscale turbulence, using equations valid in the limit of infinite scale separation, reveal novel cross-scale interaction mechanisms in these plasmas. In the case that both long-wavelength (ion-gyroradius-scale) and short-wavelength (electron-gyroradius-scale) linear instabilities are driven far from marginal stability, we show that the short-wavelength instabilities are suppressed by interactions with long-wavelength turbulence. Two novel effects contributed to the suppression: parallel-to-the-field-line shearing by the long-wavelength ${{\boldsymbol {E}} \times \boldsymbol {B}}$ flows, and the modification of the background density gradient by the piece of the long-wavelength electron adiabatic response with parallel-to-the-field-line variation. In contrast, simulations of multiscale turbulence where instabilities at both scales are driven near marginal stability demonstrate that when the long-wavelength turbulence is sufficiently collisional and zonally dominated the effect of cross-scale interaction can be parameterised solely in terms of the local modifications to the mean density and temperature gradients. We discuss physical arguments that qualitatively explain how a change in equilibrium drive leads to the observed transition in the impact of the cross-scale interactions.

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

“…[34]) and theories of cross-scale interactions between k y ρ th,i ∼ 1 and k y ρ th,e ∼ 1 scales (cf. [33,46]).…”

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

“…where we have used the definition of the drift-kinetic electron collision operator C [•], equation (46). To expand the collision operator (34) for electrons, we have used the definition (13), equations ( 95) and (96), the estimate (100), and the identities ik…”

Three-Dimensional Inhomogeneity of Electron-Temperature-Gradient Turbulence in the Edge of Tokamak Plasmas

“…This is relevant for tokamak turbulence modeling [52,[106][107][108][109][110][111] that aims to predict experimental fluxes accurately. While this result is not the first to show k y ρ i ∼ 1 turbulence suppressing k y ρ e ∼ 1 turbulence [21,23,24,112], it is the first to show ETG turbulence at k y ρ i ∼ 1 suppressing ETG turbulence and transport at k y ρ e ∼ 1. It is important to re-emphasize that the ETG turbulence at k y ρ i ∼ 1 is strongly-driven not because of kinetic-ion physics, but because the pedestal temperature gradients are so steep [see discussion surrounding Equation (12)].…”

“…The multiscale mechanism for the suppression of electron-scale transport in the pedestal remains to be investigated in future work. In the core, cross-scale interactions between electron-scale turbulence (driven by ETG instability) and ion-scale turbulence (driven by ITG and other instabilities) can suppress electron-scale and enhance ion-scale transport [22][23][24]112]. In contrast, because steep temperature gradients in the pedestal break electron-ion scale separation, interactions between electron-scale turbulence and ion-scale turbulence, where turbulence at both scales is driven by ETG instability, is possible.…”