Turbulent transport regimes and the scrape-off layer heat flux width

Abstract

Understanding the responsible mechanisms and resulting scaling of the scrape-off layer (SOL) heat flux width is important for predicting viable operating regimes in future tokamaks and for seeking possible mitigation schemes. In this paper, we present a qualitative and conceptual framework for understanding various regimes of edge/SOL turbulence and the role of turbulent transport as the mechanism for establishing the SOL heat flux width. Relevant considerations include the type and spectral characteristics of underlying instabilities, the location of the gradient drive relative to the SOL, the nonlinear saturation mechanism, and the parallel heat transport regime. We find a heat flux width scaling with major radius R that is generally positive, consistent with the previous findings [Connor et al., Nucl. Fusion 39, 169 (1999)]. The possible relationship of turbulence mechanisms to the neoclassical orbit width or heuristic drift mechanism in core energy confinement regimes known as low (L) mode and high (H) mode is considered, together with implications for the future experiments. V C 2015 AIP Publishing LLC. [http://dx.

Physics of ultimate detachment of a tokamak divertor plasma

“…As a result, such a breakdown caused by a variation of the parameters of both the plasma and the plasma-facing components may cause a massive gas release from the wall and a bifurcation-like jump to detachment. Such a jump was indeed observed experimentally in long pulse discharges (Nakano et al 2006).…”

“…However, some turbulence-based theoretical models (e.g. Halpern et al 2013;Myra, D'Ippolito & Russel 2015) do also show reasonable agreement with the experimental database. Note, however, that applicability of the fluid models based on the Braginskii-like equations, often used in both analytic and numerical turbulence studies, may be questionable in the weakly collisional, H-mode SOL plasma.…”

“…However, more work is needed to make definite conclusions on the physics of the plasma-wall interactions during large ELMs, on the impact of the wall material and on the hydrogen outgassing processes after the ELM bursts. Another reason, for which the quasi-equilibrium between the absorption and desorption processes breaks, is the increase of the wall temperature in the long pulse discharges, which stimulates the desorption process and results in an influx of a large amount of hydrogen into the discharge and, finally, leads to a disruption (Nakano et al 2006). …”

Mean flows and blob velocities in scrape-off layer (SOLT) simulations of an L-mode discharge on Alcator C-Mod

“…12,13 In addition to contributing to L-H mode transition physics, some models predict that turbulence in the vicinity of the separatrix could be sufficiently large so as to impact the width of the heat flux channel. [14][15][16][17][18][19][20][21] Binormal (approximately poloidal) flows that vary in the radial direction are routinely observed in the outer midplane region of tokamaks as for example in Refs. [22][23][24].…”

“…Profile-modification is active as a saturation mechanism where the radial gradient of the pressure fluctuation is comparable to the gradient of the profile, which drives the interchange instability, and so acts locally to lower the overall growth rate., 21 viz.,…”