Garcia, O.E.

Horacek, J.

Larsen, J.S.

Madsen, S.

Naulin, V.

Nielsen, A.H.

Pitts, R.A.

Rasmussen, J.J.

Infoscience - École polytechnique fédérale de Lausanne

Convective transport byfilamentary structures in scrape-off layer plasmasO.E. Garcia1, J. Horacek2, J.S. Larsen1, J. Madsen1,V. Naulin1, A.H. Nielsen1, R.A. Pitts3, J. Juul Rasmussen1, TCV Team31 Association EURATOM–Risø National Laboratory, Technical University of Denmark, Denmark2 Association EURATOM–Institute of Plasma Physics, Prague, Czech Republic3 Ecole Polytechnique Fe´de´rale de Lausanne, Centre de Recherches en Physique des Plasmas,Association EURATOM–Confe´de´ration Suisse, Lausanne, SwitzerlandHigh levels of turbulence and anomalous transport are ubiquitous in the boundary re-gion of magnetically confined plasmas, experimentally observed in virtually all magneticgeometries and confinement regimes. Recent measurements indicate that the radial trans-port is caused by convective motions of filamentary structures which are elongated alongthe magnetic field lines and appear as blobs when viewed across the field lines. Such fasttransient transport events seem to be what underlies the commonly observed large relativefluctuation levels, broad particle density profiles, and strong plasma–main chamber wallinteractions. In this contribution we present new theoretical and experimental results re-vealing the important role of convective transport by localized structures, and explore itsdependence with plasma parameters. We will conclude that turbulent interchange motionsare the origin of the anomalous cross-field transport.Using a two-field fluid model for interchange motions driven by the non-uniform mag-netic field, it is demonstrated that an isolated blob structure can propagate a radial distancemany times its initial size. In the collisionless limit, electric currents through sheaths atthe divertor targets lead to strong dissipation of the blob structures. However, for colli-sional scrape-off layer plasmas, the parallel motions are impeded and the role of sheathcurrents is diminished. As a result, the perpendicular transport can be strongly enhanced.In the absence of sheath dissipation, the model predicts a maximum radial velocity Vscaling as V/Cs ∼ (2ℓ/R)1/2, where Cs is the acoustic speed, ℓ is the structure size and Ris the magnetic field radius of curvature [1]. Numerical simulations show that the sheathdissipation strongly reduces the radial transport due to localized filaments.The collisionality dependence of convective transport is supported by electric probemeasurements from experiments in Ohmic TCV plasmas. These comprise scans in bothline-averaged plasma density and plasma current. Here it will be demonstrated that in-creasing the plasma collisionality by altering either of these control parameters resultsin a broadening of the radial plasma particle density profile, increasing the fluctuationlevel and the radial convective flux to the main chamber walls. Yet the experimental datadisplay a universal radial variation of the statistical moments of the particle and turbu-lent flux density fluctuations across a broad variation in these parameters, suggesting thatradial interchange motions of filamentary structures prevail in all parameter regimes.We finally present new results from interchange turbulence simulations, which havepreviously been shown to yield excellent agreement with the radial variation of statisticalmoments and temporal correlations obtained from probe measurements in high-densityTCV plasmas [2].References[1] O. E. Garcia, N. H. Bian and W. Fundamenski, Phys. Plasmas 13, 082309 (2006)[2] O. E. Garcia, J. Horacek, R. A. Pitts et al. Plasma Phys. Control. Fusion 48, L1 (2006)

Convective transport by filamentary structures in scrape-off layer plasmas

http://infoscience.epfl.ch/record/124606/files/075001202.pdf

Convective transport by filamentary structures in scrape-off layer plasmas

Abstract

Similar works

Full text

Available Versions

Infoscience - École polytechnique fédérale de Lausanne