Turbulent transport in rotating tokamak plasmas

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Abstract

Small scale turbulence in a magnetically confined fusion plasma drives energy
and particle transport which determine the confinement. The plasma in a tokamak
experiment has a toroidal rotation which may be driven externally, but can also
arise spontaneously from turbulent momentum transport. This thesis investigates
the interaction between turbulence and rotation via nonlinear numerical simulations,
which use the gyrokinetic description in the frame that corotates with the plasma.
A local gyrokinetic code is extended to include both the centrifugal force, and the
stabilising effect of sheared equilibrium flow.
Sheared flow perpendicular to the magnetic field suppresses the turbulence,
and also breaks a symmetry of the local model. The resulting asymmetry creates
a turbulent residual stress which can counteract diffusive momentum transport and
contribute to spontaneous rotation. The competition between symmetry breaking
and turbulence suppression results in a maximum in the nondiffusive momentum flux
at intermediate shearing rates. Whilst this component of the momentum transport
is driven by the sheared flow, it is also found to be suppressed by the shearing more
strongly than the thermal transport. The direction of the residual stress reverses
for negative magnetic shear, but also persists at zero magnetic shear.
The parallel component of the centrifugal force traps particles on the outboard
side of the plasma, which destabilises trapped particle driven modes. The
perpendicular component of the centrifugal force appears as a centrifugal drift which
modifies the phase relation between density and electric field perturbations, and is
stabilising for both electron and ion driven instabilities. For ion temperature gradient
dominated turbulence, an increased fraction of slow trapped electrons enhances
the convective particle pinch, suggesting increased density peaking for strongly rotating
plasmas. Heavy impurities feel the centrifugal force more strongly, therefore
the effects of rotation are significant for impurities even when the bulk ion Mach
number is low. For ion driven modes, rotation results in a strong impurity convection
inward, whilst a more moderate convection outward is found for electron driven
modes.

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