Investigation of turbulent transport regimes in the tokamak edge by using two-fluid simulations

Abstract

The results of flux-driven, two-fluid simulations in single-null configurations are used to investigate the processes determining the turbulent transport in the tokamak edge. Three turbulent transport regimes are identified: (i) a developed transport regime with turbulence driven by an interchange instability, which shares a number of features with the standard L-mode of tokamak operation; (ii) a suppressed transport regime, characterized by a higher value of the energy confinement time, low-amplitude relative fluctuations driven by a Kelvin–Helmholtz instability, a strong $\boldsymbol {E}\times \boldsymbol {B}$ sheared flow and the formation of a transport barrier, which recalls the H-mode; and (iii) a degraded confinement regime, characterized by a catastrophically large interchange-driven turbulent transport, which recalls the crossing of the Greenwald density limit. We derive an analytical expression of the pressure gradient length in the three regimes. The transition from the developed transport regime to the suppressed transport regime is obtained by increasing the heat source or decreasing the collisionality and vice versa for the transition from the developed transport regime to the degraded confinement regime. An analytical expression of the power threshold to access the suppressed transport regime, linked to the power threshold for H-mode access, as well as the maximum density achievable before entering the degraded confinement regime, related to the Greenwald density, are also derived. The experimental dependencies of the power threshold for H-mode access on density, tokamak major radius and isotope mass are retrieved. The analytical estimate of the density limit contains the correct dependence on the plasma current and on the tokamak minor radius.

Numerical turbulence simulations of intermittent fluctuations in the scrape-off layer of magnetized plasmas

“…Numerous SOL turbulence models and codes are now being extended to include these features. [92][93][94][95][96][97][98][99][100] The statistical framework with super-position of filaments can be used for analysis and interpretation of simulation results in these more advanced models, similar to what has been done here and previously for experimental measurements. As such, this work sets a new standard for validation of turbulence simulation codes.…”

The GBS code for the self-consistent simulation of plasma turbulence and kinetic neutral dynamics in the tokamak boundary

“…The E × B shear can play an important role in suppressing turbulence at the tokamak edge, as discussed in detail in Ref. [38], where GBS simulations are used to investigate the turbulent transport regimes in this region. Here we show that the E × B shear persists also when the neutral dynamics is included.…”

“…A new coordinate system is chosen adapted to the new geometry. GBS simulations that leverage this new domain and coordinate system have been recently used to investigate the turbulent transport regimes of the tokamak boundary [38] and to develop a theory-based scaling law of the pressure and density decay lengths in both the near and the far SOL [39]. Ultimately, the domain implemented in the present version allows for more flexibility on the choice of the magnetic configuration, which can also be loaded from an equilibrium reconstruction or a Grad-Shafranov solver.…”

Theory-based scaling laws of near and far scrape-off layer widths in single-null L-mode discharges

“…A detailed description of the model can be found in Ref. [22]. The use of a drift-reduced fluid model is justified by the high plasma collisionality in the SOL, λ e L ∼ 2πqR, with λ e the electron mean-free path, and by the large scale fluctuations, k ⊥ ρ i 1, with ρ i the ion Larmor radius, that dominate transport in this regime.…”

“…In parallel to the experimental effort, recent theoretical and numerical studies based on two-fluid models, justified by the high plasma collisionality of this region, have investigated the mechanisms that regulate the near SOL width in L-mode, leading to analytical and numerical scaling laws of the SOL density and pressure gradient lengths in both limited [16][17][18][19] and diverted [20][21][22][23] geometries. A direct comparison of theoretical scaling laws to experimental data has been carried out in limited geometry [16,18,24,25], showing a good agreement with experimental measurements.…”