Dewhurst, Joseph Michael (2010) Statistical description and modelling of fusion plasma edge turbulence. PhD thesis, University of Warwick.

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Abstract

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

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Tokamaks, Plasma (Ionized gases) -- Mathematical models
Official Date:	April 2010
Dates:	Date Event April 2010 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Hnat, Bogdan A.
Extent:	xv, 132 leaves : ill., charts
Language:	eng

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