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Title: Properties of intermittent transport in the Mega Ampere Spherical Tokamak
Author: Walkden, Nicholas Ross
ISNI:       0000 0004 5364 7502
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2014
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This thesis investigates the properties of intermittent transport on the Mega Amp Spherical Tokamak (MAST) both through dedicated experiment and numerical modelling. A ball pen probe (BPP) diagnostic has been operated on MAST to make a direct mea- surement of the plasma potential at the plasma boundary. By combining the BPP measurement with the floating potential from a Langmuir probe (LP) a measurement of the electron temperature is obtained, which is compared to the Thomson scattering diagnostic allowing for the accuracy of the BPP measurement of plasma potential to be assessed. From the measurement of the plasma potential the profile of the radial elec- tric field is extracted. Using a third LP on the probe face fluctuation statistics of the ion-saturation current are determined. The skewness of the Isat fluctuations is shown cross zero at the point where the radial electric field gradient maximizes. This suggests that intermittency in the plasma edge of MAST is produced at the centre of the radial shear layer, with negative skewness inside and positive skewness outside the centre of the shear layer. The intermittent transport in the scrape-off layer (SOL) can be approximated as the result of successive discrete transport events arising from the motion of individual fil- aments. The BOUT++ framework has been used to model the motion of individual filaments in a geometry representative of the SOL of MAST. The SOL geometry gives rise to gradients in the filament parallel to the magnetic field through two mechanisms; enhanced diffusion in the divertor region and resistive ballooning. Parallel gradients cause a departure of the filament dynamics from 2D predictions due to the presence of the Boltzmann response in 3D. This provides a mechanism for poloidal motion of the filament alongside a breakdown into resistive drift-wave turbulence with the turbulent scale-length determined by the level of diffusion present in the system. The motion of the filament in the direction normal and bi-normal to the magnetic field line is shown to be approximately decoupled. The velocity of the filament in the normal direction displays a scaling with temperature and filament size congruent with the resistive bal- looning or ideal interchange regimes of 2D blob dynamics whilst the bi-normal velocity of the filament adheres to a scaling based on the Boltzmann response. An increase in temperature is shown to provide a slight increase in the net radial transport of particles due to the motion of the filament at the outboard midplane, however an increase in filament width provides a far stronger increase in particle transport. This suggests that for MAST the filament with is the main parameter determining the level of intermittent particle transport in the SOL.
Supervisor: Dudson, Benjamin ; Fishpool, Geoff Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available