The effect of ion-scale dynamics on electron-temperature-gradient turbulence

Abstract

This work reports on numerical studies of small-scale electrontemperature-gradient (ETG) turbulence embedded in large-scale turbulence driven by both ion-temperature-gradient (ITG) modes and trapped-electron modes (TEM). To begin with, we find that the simplified adiabatic-ion model of ETG turbulence does not always saturate nonlinearly, suggesting that corrections to the purely adiabatic ion response are required for robust saturation. Our results also qualitatively confirm a prediction of Holland and Diamond that the back-reaction of ETG on ITG turbulence is insignificant. For the parameters studied, we find that ETG turbulence levels are reduced as ion driving gradients are increased. This result is at least partially explained by linear physics. An important practical result of this work is the finding that most of the electron energy transport arises from ion scales (k θ ρ i < 1) in cases for which ionscale instabilities are not suppressed. Specifically, for the Cyclone base case parameters, only 16% of the energy diffusivity, χ e , arises from the spectral region k θ ρ i > 1.0.

Gyrokinetic simulations of spherical tokamaks

“…Figure 3(a) also shows that: including electron collisions (at the experimental level) in the AI model cures the problem and considerably improves the convergence of the spectrum (the same is true for the KI model); and that with a small level of equilibrium flow shear (γ E = 0.005 v te /a) the collisionless AI simulation recovers from what appeared a catastrophically unresolved situation. The healing of the simulation, with the addition either of electron collisions or flow shear, is probably due to the stabilisation of weakly unstable trapped electron driven modes in the region k y ρ i ∼ 1, as discussed at the end of Appendix A. GYRO simulations have also found that flow shear helps in resolving ETG turbulent saturation with the AI model [32].…”

Validation metrics for turbulent plasma transport

“…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.…”

“…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.…”

Multispecies density peaking in gyrokinetic turbulence simulations of low collisionality Alcator C-Mod plasmas

“…For some C-Mod plasmas the inclusion of ETG modes can raise the electron heat transport and allow matching of both the ion and electron heat fluxes 62 . In some multi-scale simulations the presence of short wavelength modes does not qualitatively change the results for ion transport because that is dominated by strongly-driven long-wavelength ITG turbulence [74][75][76] . However, when the ion drive is sufficiently weak the inclusion of ETG modes may significantly alter the long wavelength ion heat flux 62 (and, presumably, the particle flux) so a significant role for ETG modes cannot be ruled out for the conditions examined here without further investigation.…”