Radial convection of isolated filamentary structures due to interchange motions in magnetized plasmas is investigated. Following a basic discussion of vorticity generation, ballooning, and the role of sheaths, a two-field interchange model is studied by means of numerical simulations on a biperiodic domain perpendicular to the magnetic field. It is demonstrated that a blob-like plasma structure develops dipolar vorticity and electrostatic potential fields, resulting in rapid radial acceleration and formation of a steep front and a trailing wake. While the dynamical evolution strongly depends on the amount of collisional diffusion and viscosity, the structure travels a radial distance many times its initial size in all parameter regimes in the absence of sheath dissipation. In the ideal limit, there is an inertial scaling for the maximum radial velocity of isolated filaments. This velocity scales as the acoustic speed times the square root of the structure size relative to the length scale of the magnetic field. The plasma filament eventually decelerates due to mixing and collisional dissipation. Finally, the role of sheath dissipation is investigated. When included in the simulations, it significantly reduces the radial velocity of isolated filaments. The results are discussed in the context of convective transport in scrape-off layer plasmas, comprising both blob-like structures in low confinement modes and edge localized mode filaments in unstable high confinement regimes.

Fluctuations and particle transport in the scrape-off layer of TCV plasmas have been investigated by probe measurements and direct comparison with two-dimensional interchange turbulence simulations at the outer midplane. The experiments demonstrate that with increasing line-averaged core plasma density, the radial particle density profile scale length becomes broader. The particle and radial flux density statistics in the far scrape-off layer exhibit a high degree of statistical similarity with respect to changes in the line-averaged density. The plasma flux onto the main chamber wall at the outer midplane scales linearly with the local particle density, suggesting that the particle flux here can be parameterized in terms of an effective convection velocity. Experimental probe measurements also provide evidence for significant parallel flows in the scrape-off layer caused by ballooning in the transport of particles and heat into the scrape-off layer. The magnitude of this flow estimated from pressure fluctuation statistics is found to compare favourably with the measured flow offset derived by averaging data obtained from flow profiles observed in matched forward and reversed field discharges. An interchange turbulence simulation has been performed for a single, relatively high density case, where comparison between code and experiment has been possible. Good agreement is found for almost all aspects of the experimental measurements, indicating that plasma fluctuations and transport in TCV scrape-off layer plasmas are dominated by radial motion of filamentary structures.

Probe measurements of electrostatic plasma fluctuations in the scrape-off layer (SOL) of the TCV tokamak are compared with the results from twodimensional interchange turbulence simulations. Excellent agreement is found for both the radial variation of statistical moments and temporal correlations, clearly indicating that turbulent transport in the tokamak SOL is due to radial advection of blob-like filamentary structures. This offers an explanation both for the basic mechanism driving the anomalous SOL particle transport and the now commonly observed broad particle density profiles, extending deep into the SOL and thought to be the cause of high levels of main chamber plasma-wall interactions.

Large-scale radial advection of isolated structures in nonuniformly magnetized plasmas is investigated. The underlying mechanism considered is due to the nonlinear evolution of interchange motions, without any presumption of plasma sheaths. Theoretical arguments supported by numerical simulations reveal an inertial scaling for the radial velocity of isolated structures in the ideal limit. This velocity increases as the square root of the structure size relative to the length scale of the magnetic field. The magnitude of the radial advection velocity, as well as the dynamical evolution of the structures, compares favorably with recent experimental measurements of radially propagating blob structures in the scrape-off layer of magnetically confined plasmas.

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-Schlüter diffusion) and classical parallel transport (sub-sonic advection and Spitzer-Härm 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 overestimated 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 × ∇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 ≈0.2 in JET Ohmic and L-mode plasmas, in fair agreement with Mach probe measurements.