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Statistical description and modelling of fusion plasma edge turbulence

By Joseph Michael Dewhurst

Abstract

In tokamaks, heat and particle fluxes reaching the wall are often bursty and\ud intermittent and understanding this behaviour is vital for the design of future reactors.\ud Plasma edge turbulence plays an important role, its quantitative characterisation and\ud modelling under different operating regimes is therefore an important area of research.\ud Ion saturation current (Isat) measurements made in the edge region of the Large\ud Helical Device (LHD) and Mega-Amp Spherical Tokamak (MAST) are analysed. Absolute\ud moment analysis is used to quantify properties on different temporal scales of\ud the measured signals, which are bursty and intermittent. In all data sets, two regions\ud of power-law scaling are found, with the temporal scale τ≈40μs separating the two\ud regimes. A monotonic relationship between connection length and skewness of the\ud probability density function is found for LHD.\ud A new numerical code, ‘HAWK,’ which solves the Hasegawa-Wakatani (HW)\ud equations is presented. The HAWK code is successfully tested and used to study the\ud HW model and modifications. The curvature-Hasegawa-Wakatani (CHW) equations include\ud a magnetic field strength inhomogeneity, C = −∂lnB/∂x. The zonal-Hasegawa-\ud Wakatani (ZHW) equations allow the self-generation of zonal flows. The statistical\ud properties of the turbulent fluctuations produced by the HW model and variations\ud thereof are studied. In particular, the probability density function of E × B density\ud flux Γn = −n∂φ/∂y, structure functions, the bispectrum and transfer functions are\ud investigated.\ud Test particle transport is studied. For the CHW model, the conservation of\ud potential vorticity Π = ∇2φ − n + (κ − C)x accounts for much of the phenomenology.\ud Simple analytical arguments yield a Fickian relation Γn = (κ − C)Dx between the\ud radial density flux Γn and the radial tracer diffusivity Dx. For the ZHW model, a\ud subtle interplay between trapping in small scale vortices and entrainment in larger scale\ud zonal flows determines the rate, character and Larmor radius dependence of the test\ud particle transport. When zonal flows are allowed non-Gaussian statistics are observed.\ud Radial transport (across the zones) is subdiffusive and decreases with the Larmor radius.\ud Poloidal transport (along the zones), however, is superdiffusive and increases with small\ud values of the Larmor radius

Topics: QC
OAI identifier: oai:wrap.warwick.ac.uk:3903

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