Publication

Dissipative processes in interchange driven scrape-off layer turbulence

Richard Pitts, Jonathan Graves, Jan Horacek
2007
Journal Articles

Abstract

First principles expressions are given for the parameters governing collisional diffusion and parallel losses of mass, momentum and energy in tokamak scrape-off layer (SOL) plasmas. These dissipative, or damping, coefficients are based on neoclassical perpendicular transport (Pfirsch-Schluter diffusion) and classical parallel transport (sub-sonic advection and Spitzer-Harm diffusion). When numerical values derived from these expressions are used to compute damping coefficients for the edge-SOL electrostatic (ESEL) turbulence code, simulations correctly reproduce the radial profiles of particle density, n, and electron temperature, T-e, as well as statistical distributions and temporal correlations of particle density and flux density measured in Ohmic and L-mode plasmas on the TCV tokamak. Similarly, preliminary calculations agree reasonably well with radial profiles of T-e measured in Ohmic and L-mode plasmas on JET, although the particle density e-folding length is over-estimated by a factor of 3; this discrepancy is largely removed by reducing the parallel density gradient length by a factor measuring the poloidal asymmetry (ballooning) of filament displacements. These encouraging results suggest that turbulent SOL transport is driven by interchange motions, caused by unfavourable curvature and strong pressure gradients in the edge region, with the level of turbulence being influenced by neoclassical diffusion and parallel losses in the SOL region. Moreover, the curvature drive offers a viable mechanism for the origin of the B x del B-independent part of the parallel SOL flow measured on many tokamaks, including JET and TCV, with ESEL simulation predicting a parallel Mach number of approximate to 0.2 in JET Ohmic and L-mode plasmas, in fair agreement with Mach probe measurements.

Official source

https://infoscience.epfl.ch/entities/publication/410bdc51-125e-4314-b6f9-e76524f76df4

Richard Pitts, Jonathan Graves, Jan Horacek
2007
Journal Articles

Abstract

Official source

https://infoscience.epfl.ch/entities/publication/410bdc51-125e-4314-b6f9-e76524f76df4

Related concepts (30)

Tokamak

A tokamak (ˈtoʊkəmæk; токамáк) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. , it was the leading candidate for a practical fusion reactor. Tokamaks were initially conceptualized in the 1950s by Soviet physicists Igor Tamm and Andrei Sakharov, inspired by a letter by Oleg Lavrentiev. The first working tokamak was attributed to the work of Natan Yavlinsky on the T-1 in 1958.

Classical diffusion

Classical diffusion is a key concept in fusion power and other fields where a plasma is confined by a magnetic field within a vessel. It considers collisions between ions in the plasma that causes the particles to move to different paths and eventually leave the confinement volume and strike the sides of the vessel. The rate of diffusion scales with 1/B2, where B is the magnetic field strength, implies that confinement times can be greatly improved with small increases in field strength.

Bohm diffusion

The diffusion of plasma across a magnetic field was conjectured to follow the Bohm diffusion scaling as indicated from the early plasma experiments of very lossy machines. This predicted that the rate of diffusion was linear with temperature and inversely linear with the strength of the confining magnetic field. The rate predicted by Bohm diffusion is much higher than the rate predicted by classical diffusion, which develops from a random walk within the plasma. The classical model scaled inversely with the square of the magnetic field.

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