The rôle of clustering effects in interpreting non-diffusive transport measurements in tokamaks
The transport of particles and energy in magnetised plasmas is a fundamental area of controlled fusion research. The present contribution provides a quantitative account of the close parallels that exist between the measurements of edge density fluctuations in the DIII-D tokamak and the height fluctuations of the sandpile cellular automaton. An important and distinctive feature of this cellular automaton is that it enables the microphysics of the avalanches to be resolved at the level of tracing the motion of individual grains. The present work explores the single interval statistical properties of physically measurable quantities associated with these motions, together with their spatial and temporal correlation properties that are revealed by the application of �multi-channel� measurements. Central to this study is using the concept of clustering as a basis for the statistical characterisations of non-diffusive transport behaviour in plasmas. In broad terms, clustering is manifested by correlations between observables measured at different times, different locations, or both. Clustering is both a statistical observable and a physical concept: it originates in the underlying physical processes driving the transport, and can be quantified and modelled mathematically. A fundamental message will be that, once a critical gradient has been exceeded, clustering of transport events is sufficient to generate much of the avalanching phenomenology and non-Gaussian statistical manifestations outlined above.
Bibliographic Reference: A paper presented at the Joint Varenna-Lausanne International Workshop on "Theory of Fusion Plasmas", Varenna, Italy, August 27-30 2002, and published in EPFL Internal report 737/02, October 2002, pp.45-50
Availability: Available free of charge from: École Polytechnique Fédérale de Lausanne (EPFL), Ecublens, CH-1015 Lausanne Fax +41-21-6934747 Available online on the workshop website: http://varenna-lausanne.epfl.ch/
Record Number: 200215545 / Last updated on: 2002-11-20
Original language: en
Available languages: en