Cross-Scale Interactions between Electron and Ion Scale Turbulence in a Tokamak Plasma

Abstract

Multiscale gyrokinetic turbulence simulations with the real ion-to-electron mass ratio and β value are realized for the first time, where the β value is given by the ratio of plasma pressure to magnetic pressure and characterizes electromagnetic effects on microinstabilities. Numerical analysis at both the electron scale and the ion scale is used to reveal the mechanism of their cross-scale interactions. Even with the real-mass scale separation, ion-scale turbulence eliminates electron-scale streamers and dominates heat transport, not only of ions but also of electrons. Suppression of electron-scale turbulence by ion-scale eddies, rather than by long-wavelength zonal flows, is also demonstrated by means of direct measurement of nonlinear mode-to-mode coupling. When the ion-scale modes are stabilized by finite-β effects, the contribution of the electron-scale dynamics to the turbulent transport becomes non-negligible and turns out to enhance ion-scale turbulent transport. Damping of the ion-scale zonal flows by electron-scale turbulence is responsible for the enhancement of ion-scale transport.

Impact of plasma parameter on self-organization of electron temperature gradient driven turbulence

“…Recent multi-scale gyrokinetic (GK) simulations involving both ion and electron scale fluctuations showed that the ETG turbulence might be suppressed when ion scale turbulence is strongly excited. 2,3 On the contrary, when ion scale turbulence is suppressed by strong E Â B shear flows, electron scale turbulence is dominated by the streamer and thus becomes the main channel of the electron heat losses. 4 In fact, transport barriers 5,6 and spherical tokamaks 7 are characterized by strong E Â B shear flows and ion heat transport at neoclassical levels, and the remaining anomalous electron heat transport is often discussed based on the ETG turbulence.…”

Validation metrics for turbulent plasma transport

“…Since these nonlinear gyrokinetic simulations can require 10 3 processor-hours or more (even 10 7 and beyond in some cases [65][66][67][68] ) to calculate the statistics at a single location in the plasma, reduced models of the turbulence have been developed which can make predictions on the processorminute or less timescale, to facilitate practical transport modeling with feasible resource requirements. The general approach of most such models is to decompose the turbulent fluxes into two components along the lines of (using the ion energy flux Q i as an example)…”

“…A third, even more computationally expensive avenue of approach would be to investigate the impact of multiscale simulations which self-consistently incorporate q e -scale ETG fluctuations into these q i -scale simulations. [65][66][67][68][140][141][142][143][144][145][146] Other research avenues are possible as well. However, for the goals of this paper, it should be clear how utilizing a variety of local validation metrics for multiple predicted quantities can be employed to test model fidelity and our physical understanding at a level not possible by earlier global metrics.…”

“…To address this question, one would extend the validation methodology discussed so far to calculation of a local sensitivity map in which both R=L Ti and R=L Te are varied, from which a fluxmatching fractional gradient error vectorẼ z ¼ ðE LT i ; E LT e Þ could be calculated by determining the simultaneous values of R=L Ti and R=L Te which when input into the microturbulence model yield predictions of Q i and Q e that simultaneously match the power balance Q i and Q e results. While the computational resources needed to perform such an analysis using long-wavelength gyrokinetic simulations (to say, nothing of multiscale simulations which incorporate electron-scale ETG dynamics that likely contribute to Q e in many cases [65][66][67][68][140][141][142][143][144][145][146] ) over many conditions or discharges remain prohibitive for current-day computing platforms, such approaches will likely be feasible on next-generation exascale platforms. Moreover, such an approach, or even further generalizations to include matching of particle and momentum fluxes via additional variations of density and rotation gradients, is readily feasible now for most reduced turbulent transport models with fairly modest computing resources, and should be pursued further.…”

Validation of nonlinear gyrokinetic simulations of L- and I-mode plasmas on Alcator C-Mod

“…[15][16][17][18] Proper characterization of these effects requires "multi-scale" simulations (with realistic electron mass), which simultaneously capture the electron and ion scales. It has been shown that in an Alcator C-Mod L-mode plasma, the inclusion of multiscale effects can resolve discrepancies between values of ion heat flux, electron heat flux, and perturbative thermal diffusivity from experimental measurements and those calculated from ion-scale GYRO simulations.…”