Electron temperature gradient driven transport in a MAST H-mode plasma

Abstract

Gyrokinetic simulation of a MAST-like equilibrium is used to establish the turbulent transport resulting from the electron temperature gradient driven mode for core and edge parameters. The thermal diffusion coefficients calculated in these simulations are found to be experimentally significant for core parameters but underestimate the observed transport on outer flux-surfaces.

Gyrokinetic simulations of spherical tokamaks

“…In numerical experiments with artificially raised levels of γ E , it is found that the ETG turbulence only starts to be diminished as γ E approaches the value of the maximum linear ETG growth rate. These results add weight to the main conclusion of [7] that ETG induced transport can be significant at mid-radius in MAST.…”

“…Nonlinear calculations for STs have therefore generally focussed on modelling ETG turbulence. GS2 simulations (without flow shear) for a typical H-mode plasma, suggest that ETG can carry significant electron heat flux at mid-radius in MAST [7], consistent with findings from GYRO simulations for NSTX [2]. In this paper flow shear will be included self-consistently in local gyrokinetic simulations, to assess more thoroughly the impact of flow shear on ITG and ETG turbulence in MAST.…”

“…Here we probe further the robustness of this very important conclusion with longer GS2 simulations that include kinetic ions, collisions and equilibrium flow shear. Despite the large impact of magnetic perturbations (especially δB ) on linear ETG growth rates [17], the saturated electron heat fluxes q e in electrostatic and electromagnetic simulations were similar and predominantly electrostatic [7,30]. For reasons of economy with computing resources, most of the GS2 simulations reported here are electrostatic, although a subset of key results were checked and found to be consistent with fully electromagnetic simulations.…”

“…For reasons of economy with computing resources, most of the GS2 simulations reported here are electrostatic, although a subset of key results were checked and found to be consistent with fully electromagnetic simulations. Recent longer ETG simulations, to t ∼ 1500 a/v te , with an electrostatic version of the AI model of [7], lead at long times to the runaway growth of q e that is illustrated in Figure 3(a). At late times the Φ spectrum becomes dominated by a single k x = 0 mode at low k y , and this may be associated with a slowly growing mode close to the edge of the spectral domain.…”

“…In this section we have presented simulations, which run to long times and include the physics of collisions, kinetic ions and sheared toroidal equilibrum flow, that add weight to the conclusion from [7] that ETG turbulence can carry significant electron heat transport at mid-radius in MAST. ¶ The linear drive term had a negligible impact on ETG turbulence in GYRO simulations for MAST [34], and was excluded from these GS2 studies of the effect of flow shear on ETG turbulence.…”