Impact of collisionality on turbulence in the edge of tokamak plasma using 3D global simulations

Abstract

Collisionality is one of the key parameters in determining turbulent transport in the plasma edge, regulating phenomena such as "shoulder formation", separation of scale lengths in the scrape-off layer, turbulence damping and zonal flow dynamics. Understanding its role is therefore of primary importance for future reactors like ITER. Obtaining reliable predictions and a better characterization of plasma flow properties when varying collisionality remains, however, a critical challenge for the simulations. This paper focuses on the impact of varying collisionality in a nonisothermal three-dimensional fluid model of the plasma edge. A high field side limited configuration encompassing open and closed magnetic field lines with parameters typical of a medium-sized tokamak is considered. The present model can consistently account for the variations of collisionality and its impact on both the parallel resistivity η and the ion and electron parallel thermal conductivities χ e,i . Details on mean flow and turbulence properties are given. Changing collisionality leads to significant changes in the flow properties both on the mean and fluctuating quantities. In particular, lowering collisionality decreases the size of coherent structures, the fluctuation levels of turbulence, and steepens the density and temperature equilibrium profiles around the separatrix leading to a global reduction of the turbulent transport. The scrape-off layer (SOL) width is observed to increase with collisionality, eventually resulting in the disappearance of the scale lengths separation between near and far SOL, consistently with previous experimental observations. At low collisionality, where the presence of narrow feature is well-established, a contribution of heat conduction increases up to compete with heat convection.

Validation of edge turbulence codes against the TCV-X21 diverted L-mode reference case

“…Whereas GRILLIX used the Braginskii values for resistivity and heat conductivities directly, relaxed parameters were used in GBS (η increased by factor 3 and χ ,e,i reduced by factor between 4.8 (at the targets) and 20 (at the OMP)) and TOKAM3X (η increased by factor 1.8 and χ ,e,i reduced by factor 1.8) in order to reduce computational costs and improve the numerical stability. Previous studies have indicated that artificially increased collisionality leads to broadened SOL profiles [13], [15], larger blobs and higher fluctuation levels [13]. Increased resistivity can directly affect the plasma potential (affecting the scale of potential fluctuations and the magnitude of the E × B transport) [13], extend the inertial regime of SOL blobs [69] and change the position where blobs are generated [70].…”

“…In this section we first provide a brief introduction of the models in each of the codes, and then discuss the choice of sources and parameters to model the TCV-X21 scenario. For the complete physical models (excluding boundary conditions) of the respective codes, the reader is directed to Giacomin and Ricci, 2020 [51] for GBS, appendix A of Zholobenko et al, 2021 [20] for GRILLIX, and Tatali et al, 2021 [13] for TOKAM3X. For a discussion of the sheath boundary conditions used in this study, see Sec.4.1 and appendix A.…”

“…[55] for GBS and Ref. [13] for TOKAM3X) and treat the electrostatic limit of the dynamics, while GRILLIX does not apply the Boussinesq approximation and includes the effects of electromagnetic induction. Additionally, GRILLIX and TOKAM3X include terms for electron-ion heat exchange, in contrast to GBS.…”

“…The codes use different methods to discretise their model equations -from a fourth-order non-fieldaligned scheme in GBS [57], to a domain-decomposed flux-aligned scheme in TOKAM3X [13], to a locallyfield-aligned scheme in GRILLIX [12]. In GBS and TOKAM3X, the boundary conditions are enforced at boundary grid points, while in GRILLIX an immersed boundary method is used [12].…”

“…Previous studies have indicated that artificially increased collisionality leads to broadened SOL profiles [13], [15], larger blobs and higher fluctuation levels [13]. Increased resistivity can directly affect the plasma potential (affecting the scale of potential fluctuations and the magnitude of the E × B transport) [13], extend the inertial regime of SOL blobs [69] and change the position where blobs are generated [70]. Reduced heat conductivity leads to a higher fraction of the heat flux being convected by parallel flows [71], broader profiles [72] and allows larger parallel temperature gradients to develop.…”