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Title: Investigating strong flow-turbulence dynamics via numerical simulations
Author: Collier, James D.
ISNI:       0000 0004 5916 0324
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2016
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In a magnetically confined fusion plasma, small scale instabilities drive the anomalous transport which determines the confinement. It is possible for the plasma in a tokamak to have a toroidal rotation, either formed spontaneously or by being given some external drive. This rotation may help improve confinement. Therefore, this thesis aims to investigate the effects of strongly rotating plasmas, with toroidal Mach number approaching unity, on the turbulence dynamics in numerical simulations. For this purpose, the global gyrokinetic PIC code ORB5 has been extended to include these strong- flow terms; retaining the background E x B drift terms typically neglected. Investigations into GAMs appearing as eigenfunctions with forms similar to the Airy function found that although the behaviour of GAMs with increasing radial wavenumber were poorly predicted, eigenfunctions of the predicted form were still found. As radial wavenumber increased the eigenfunctions became less well defined. Linear simulations with a solid-body rotation found that the frequency of the GAMs and toroidal modes with n n = 0 exhibited an increase for larger magnitude of toroidal velocity and were largely independent of direction of rotation. Further studies found that an increasing toroidal rotation initially showed a destabilization effect on the linear modes, acting against the stabilizing effects of already present gradient profile flows, before beginning to reduce the mode growth at large toroidal velocities. This behaviour was found to be largely reflected in the tilting of the mode structures caused by rotation. A stabilizing effect was also observed in collisionless non-linear simulations. The presence of a positively rotating plasma gave reductions to turbulence, but a much stronger turbulence suppression was found when the plasma was rotated in the opposite direction. It is suspected that the large ows caused by the equilibrium profile gradients give rise to some of this observed asymmetry.
Supervisor: Not available Sponsor: University of Warwick
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics