Global ‘zero particle flux-driven’ gyrokinetic analysis of the density profile for a TCV plasma

Abstract

TCV is a small-sized tokamak, where finite size effects (often called ‘rho-star’ or ‘global’ effects) could significantly impact the heat and particle fluxes, leading to discrepancies between gyrokinetic flux-tube results and global ones [McMillan et al., 2010 Phys. Rev. Lett.]. The impact of global effects on the radial profile of the plasma density has been investigated in a previous study for a particular TCV discharge with negligible particle source, satisfying the ‘zero particle flux’ condition. A radially local flux-tube analysis, reconstructing the dependence of the peaking of the density profile on the main physical parameters, invoking the zero particle flux constraint, was pursued close to mid-radius in [Mariani et al., 2018 Phys. Plasmas]. This analysis was followed by a global one [Mariani et al., 2019 Plasma Phys. Control. Fusion], where local quasi-linear and nonlinear results were compared with global simulations, showing small global effects on the density peaking. However, these gradient-driven global runs considered Krook-type heat and particle sources to keep temperature and density profiles fixed on average, which differ from the experimental radially localized sources. To remove this possible bias on the results, a different evaluation of the density peaking for the same case is performed here, based on global nonlinear hybrid simulations where the temperature profiles are [still] kept fixed with the Krook-type sources, however the density profile relaxes in a flux-driven way (with zero particle source). The new hybrid simulations show a good agreement with the old gradient-driven runs. A global quasi-linear model is also developed and applied using the output from linear global runs, to estimate ratios of fluxes, showing a good agreement with the results of global nonlinear gradient-driven simulations. The effect of collisions on the flux-tube results is also investigated, in order to evaluate their impact on the radial variation of the density peaking.

Study of fast-ion-driven toroidal Alfvén eigenmodes impacting on the global confinement in TCV L-mode plasmas

“…To conclude this section on the modelling, it has been observed that the local gyrokinetic approximation employed in these analyses with the GENE code is not sufficient to grasp the accurate dynamics of the unstable fast-ion-driven modes. Global simulations are therefore required, since firstly the turbulence structures is way much larger than the few lowest mode rational surfaces and, moreover, the non-local effects may play an important role when evaluating the global level of turbulent transport in TCV [76][77][78]. For these reasons, future efforts will be dedicated to global modelling of the here discussed TCV scenario including bulk and fast-ion transport in flux-driven gyrokinetic simulations.…”