Analysis of Plasma Filaments with Fast Visible Imaging in the Mega Ampère Spherical Tokamak



Farley, Thomas
(2019) Analysis of Plasma Filaments with Fast Visible Imaging in the Mega Ampère Spherical Tokamak. PhD thesis, University of Liverpool.

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Abstract

The cross-field transport of particles in the scrape-off layer (SOL) of magnetic fusion devices is dominated by the convection of coherent filamentary plasma structures. In this thesis, we present a new technique for the analysis of filaments in fast visible camera data. The new technique operates by inverting the background subtracted emission in the camera images onto a basis set of uniformly emitting field line images, constructed using information from magnetic equilibrium reconstructions. The output of the inversion is a 2D mapping of emission parametrising the average intensity of field lines in the SOL by the coordinates of their intersection with the mid-plane. Filaments manifest in the inverted emission profile as blobs of raised emission. A filament detection technique has been developed to identify these regions of increased emission and fit them with 2D Gaussians. This yields the positions, widths and amplitudes of the filaments. A tracking algorithm is then applied to calculate the filaments velocities and lifetimes. Data from a synthetic camera diagnostic is used to assess the capabilities and limitations of the new technique and quantify its errors. This exercise shows it can detect ~36% of all filaments in the analysis region, corresponding to ~74% of filaments above the targeted amplitude threshold. This sensitivity is achieved with a true positive detection rate of 98.8%. Standard errors in the radial and toroidal positions of the filaments are estimated to be ~2 mm, while errors in the toroidal and radial widths are around ~3 mm and ~7 mm respectively. The shapes of the probability density functions (PDFs) of the filament parameters are also qualitatively recovered and the effect of filament overlap on filament amplitude measurements is investigated. Valuable insight is gained into effects from the non-orthogonality of the field line basis functions and the resulting spatial dependence of the measurement errors. Finally, the technique is applied to MAST data and compared to Langmuir probe measurements. Good agreement is found between the two diagnostics, including exponential waiting times and symmetrical conditionally averaged waveforms. Measurements of the PDFs of filament properties provide valuable inputs for analytic models of SOL transport and show filament lifetimes to be exponentially distributed. The depth of field of the technique enables measurement of the toroidal filament spacing, with results supporting the assertion that filaments are generated uniformly and independently, and are thus described by Poisson statistics underpinning several analytic models.

Item Type: Thesis (PhD)
Uncontrolled Keywords: plasma, filaments, scrape-off layer, tokamak, visible imaging, tomography
Divisions: Faculty of Science and Engineering > School of Electrical Engineering, Electronics and Computer Science
Depositing User: Symplectic Admin
Date Deposited: 10 Jan 2020 08:29
Last Modified: 01 Oct 2021 07:48
DOI: 10.17638/03066785
Supervisors:
  • Bradley, james
  • Militello, Fulvio
URI: https://livrepository.liverpool.ac.uk/id/eprint/3066785