Bradshaw, Jack Callum (2020) Modelling of compressional alfvén eigencmodes in axisymmetric toroidal fusion plasmas. PhD thesis, University of Warwick.

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Abstract

Compressional Alfven eigenmodes (CAEs) have been linked to electromagnetic emission detected in tokamaks in the frequency range around harmonics of the ion cyclotron frequency, and more recently to sub-ion cyclotron frequency emission in spherical tokamak experiments. This has sparked interest in using CAEs as a diagnostic for fast-ion distributions in fusion devices. This thesis presents a linear stability code, Whales2, that solves for CAEs using the linearised cold ideal Hall- MHD equations. Whales2 is a mixed finite elements spectral code that operates in a 2-dimensional axisymmetric toroidal geometry to solve for the frequency and spatial structure of CAEs in a given ideal-MHD equilibrium.

In this thesis we present how the Whales2 code is designed to calculate CAEs whilst avoiding the physical coupling of CAEs to the slow-magnetoacoustic and shear Alfven eigenmodes. We demonstrate that Whales2 is free from spectral pollution and that the self-adjointness of the ideal-MHD equations is preserved in the numerical methods employed by Whales2. We show that Whales2 well reproduces analytical and qualitative predictions of CAE theory in a range of test cases in cylindrical and toroidal geometries. We also use output from Whales2 to establish the behaviour of CAEs with respect to including the Hall term in the MHD equations, particularly in the lifting of the positive/negative frequency degeneracy that is present in ideal-MHD. Results from Whales2 show the impact that the Hall term can have on the spatial localisation of CAEs. Whales2 manipulates the MHD equations based on the method detailed in [3] to avoid CAE coupling to the shear Alfven continuum - the effectiveness of this method is demonstrated clearly for the first time, to the author's knowledge. xi

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Magnetohydrodynamic waves, Tokamaks, Cyclotron waves
Official Date:	December 2020
Dates:	Date Event December 2020 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Verwichte, E. (Erwin)
Format of File:	pdf
Extent:	xi, 130 leaves : illustrations
Language:	eng

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