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Title: The effect of toroidal flows on the stability of ITGs in MAST
Author: Hill, Peter
ISNI:       0000 0004 2748 800X
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2012
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The free energy in the large temperature and density gradients in tokamaks can drive microinstabilities, which in turn drive turbulence. This turbulence is responsible for the transport of energy and particles over and above that predicted by neoclassical theory. Sheared toroidal rotation can suppress the turbulence and stabilise the underlying microinstabilities, thereby reducing the transport. This thesis investigates how variation of the equilibrium temperature and density profiles, over the same scales associated with the microinstabilities, affects how the ow shear stabilises the linear modes and suppresses the turbulence. A global gyrokinetic code is employed in this investigation, which retains the profile variation and simulates the full 3D domain of a tokamak plasma. How much ow shear is needed to stabilise the linear ion temperature gradient (ITG) mode is found to be dependent on its poloidal wavenumber, with longer wavelength modes needing more ow shear than the fastest growing mode. This dependence is present whether the ow shear is constant across the radius or if it has the variation typical in an experimental rotation profile. There is an asymmetry with respect to the sign of the ow shear in the effectiveness of the stabilisation, with the maximum linear growth rate occurring at finite negative shearing rates for the plasma studied here. This asymmetry arises from the profile variation, and is found to be significant in simulations of MAST L-mode plasmas, especially when the effects of kinetic trapped electrons are included in the simulations. Flow shear asymmetry is still present in nonlinear simulations, and the suppression of fully-developed turbulence depends on the sign of the shearing rate. With the experimental rotation profile, the heat ux arising from ITG turbulence is reduced by an amount comparable to the reduction in the linear growth rates. When the direction of the rotation profile is reversed, such that the sign of the ow shear is ipped while the magnitude remains the same, the turbulence is almost completely suppressed. A new diagnostic on MAST, beam emission spectroscopy (BES), is used to make a direct comparison between density fluctuations from simulation, and from experiment. Collisionless, electrostatic simulations with rotation are found to disagree significantly with experiment in the level of ITG turbulence activity and the correlation times and lengths of the turbulence. The inclusion of electron-electron and electron-ion collisions into static simulations is enough to bring the level of turbulent density uctuations down to within a factor two of the experimental levels, with the correlation lengths becoming comparable, while the correlation times remain an order of magnitude too large.
Supervisor: Not available Sponsor: Research Councils UK (EP/I501045) ; European Commission (EC) ; Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics