Criteria for the importance of multi-scale interactions in turbulent transport simulations

Abstract

Turbulent transport simulations have been used to develop criteria that indicate when multi-scale turbulent phenomena are important in tokamak plasmas. Twelve experimental plasma discharges from the Alcator C-Mod and ASDEX Upgrade tokamaks are compared to ion-and multi-scale simulations with the Trapped Gyro-Landau Fluid (TGLF) turbulence code. Multi-scale TGLF agrees with all available validation constraints (ion heat flux, electron heat flux, electron temperature fluctuations, and electron perturbative thermal diffusivity) within uncertainty for all cases analyzed. Ion-scale TGLF agrees in only some cases. Two criteria based on the ratios of normalized linear growth rates are able to distinguish cases in which ion-scale simulations are sufficient from cases for which multi-scale simulations are necessary. The form of these criteria reveal the key role of zonal flow mixing in moderating multi-scale effects.

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

“…2016, 2017; Creely et al. 2019), though their interpretation differs from that given here: Staebler et al. (2016, 2017) and Creely et al.…”

“…Near-marginal ITG turbulence is dominated by zonal modes, see, e.g. , Dimits et al (2000) and Rogers et al (2000). Zonal modes have a structure that depends only on radial position, and hence the long-wavelength E × B driftv E only has the effect of shifting the ETG fluctuation frequency by a Doppler shift: for zonal modes whereφ =φ(x), the termv E • ∇g in (2.2) can be removed by boosting to a toroidally rotating frame (Hardman 2019).…”

“…(2016, 2017) and Creely et al. (2019) neglect the variation of drifts along field lines, and so neglect the key physical mechanism that is critical to the quasilinear argument: the effect of parallel-to-the-field shear. We note that parallel-to-the-field flow shear has previously been considered as a part of a model of hyperviscous dissipation of ITG-driven turbulence (Smith 1997).…”

“…Weakly driven IS turbulence can reach a Dimits-shift regime where the fluxes are low, and a long-wavelength, long-lived zonal flow regulates the IS dynamics; see, e.g. , Dimits et al (2000) and . This represents a very different state from strongly driven, critically balanced, IS turbulence, cf.…”

Predictions of core plasma performance for the SPARC tokamak

“…ion temperature gradient (ITG) modes, with some electron-scale activity in the outer part of the plasma core. Recent work (Howard et al 2016;Creely et al 2019) has suggested that multi-scale effects may play a strong role in the prediction of turbulent transport only if (γ /k) high-k > (γ /k) low-k , which is a condition not met at most radii in SPARC under the modelling assumptions used in this analysis.…”