Simulations of tokamak boundary plasma turbulence transport in setting the divertor heat flux width

Abstract

The BOUT + + code has been used to simulate edge plasma electromagnetic (EM) turbulence and transport, and to study the role of EM turbulence in setting the scrape-off layer (SOL) heat flux width λq. More than a dozen tokamak discharges from C-Mod, DIII-D, EAST, ITER and CFETR have been simulated with encouraging success. The parallel electron heat fluxes onto the target from the BOUT + + simulations of C-Mod, DIII-D, and EAST follow the experimental heat flux width scaling of the inverse dependence on the poloidal magnetic field. Further turbulence statistics analysis shows that the blobs are generated near the pedestal pressure peak gradient region inside the separatrix and contribute to the transport of the particle and heat in the SOL region. Transport simulations indicate two distinct regimes: drift dominant regime and turbulence dominant regime. Goldston's heuristic drift-based (HD) model yields a consistent divertor heat flux width in the drift dominant regime. For C-Mod enhanced Dα H-mode discharges, drifts and turbulence are competing in setting the divertor heat flux width, possibly due to its compact machine size and good pedestal confinement. The simulations for ITER and CFETR indicate that divertor heat flux width λq of the future machines may no longer follows the 1/Bpol,OMP HD-based empirical (Eich) scalings and the HD model gives a pessimistic limit of divertor heat flux width. The simulation results show a transition from a drift dominant regime to a turbulence dominant regime from current machines to future machines such as ITER and CFETR for two reasons. (1) The magnetic drift-based radial transport decreases due to large CFETR and ITER machine sizes. (2) The SOL turbulence thermal diffusivity increases due to larger turbulent fluxes ejected from the pedestal into the SOL when operating in a different pedestal structure, from an ELM-free H-mode pedestal regime to a small and grassy ELM regime.

Overview of the SPARC tokamak

“…2017; Xu et al. 2019) predicts considerably wider heat flux widths in low- plasmas. For this reason, data from SPARC on the divertor heat flux width scaling will be an important physics result and may help resolve the differences between empirical scaling and modelling predictions.…”

“…Since the ability to balance a perfect double-null is uncertain, the SPARC divertors are being conservatively designed to operate in single-null. SPARC operation will represent an extrapolation in the empirical parallel heat flux width scaling (λ q ≈ 0.2 mm, with peak unmitigated parallel heat fluxes estimated to be of the order of 10 GW m −2 ) (Eich et al 2013;Brunner et al 2018) and some recent modelling (Chang et al 2017;Xu et al 2019) predicts considerably wider heat flux widths in low-ρ * plasmas. For this reason, data from SPARC on the divertor heat flux width scaling will be an important physics result and may help resolve the differences between empirical scaling and modelling predictions.…”

Constructing a new predictive scaling formula for ITER's divertor heat-load width informed by a simulation-anchored machine learning

“…[12]. There is also a BOUT++ fluid turbulence simulation result [15] that shows broadening of λq in the 15MA Q = 10 ITER plasma. Since a fluid modeling does not contain the kinetic physics that are essential in the present work, such as the finite ion orbit width and trapped electron modes, we do not attempt to compare the present work with Ref.…”

“…Since a fluid modeling does not contain the kinetic physics that are essential in the present work, such as the finite ion orbit width and trapped electron modes, we do not attempt to compare the present work with Ref. [15]. SOLPS-ITER modeling of the 15MA ITER discharge has shown that an anomalous electron thermal diffusivity of 1m 2 /s in the SOL can lead to a λq enhancement from Eich/Goldston formula prediction (≲ 1mm) to 3-4mm [16].…”

Drift reduced Landau fluid model for magnetized plasma turbulence simulations in BOUT++ framework

“…Our initial analysis suggests that Landau fluid closure results in a nearly one order of magnitude weaker heat conduction comparing to the value given by the flux-limited model both in the closed flux region and in the SOL, as attested by the substantial diminishing amplitudes of ∇ q (red dotted lines) in Figure 7. However, the impact of the improved Landau fluid closure on edge instability, turbulence, transport and most importantly, divertor heat flux width [42] doesn't fit in the scope of this paper and will be left for a future study.…”