Bistable turbulent transport in fusion plasmas with rotational shear

Thesis

Turbulent transport is a major limiting factor for the energy yield of magnetic- confinement fusion experiments. Turbulence is driven by gradients in the profiles of equilibrium plasma parameters, and it can be regulated by the presence of sheared flows. Direct numerical simulations of turbulent transport are challenging because of the broad range of temporal and spatial scales characterising fusion plasmas. In this thesis, a novel algorithm is developed to include an externally imposed, mean rotational flow shear in linearly implicit, local δf gyrokinetic codes. Unlike the discrete-in-time approach currently followed by a number of codes, it treats flow shear continuously in time, allowing for sparser numerical grids to be used. In some cases, the new algorithm leads to a difference in the level of turbulent transport compared to the discrete-in-time approach. In the prevailing paradigm for plasma turbulence, a unique, stationary turbulent state is associated to given equilibrium plasma parameters. This thesis reports the first instance of bistable turbulence in a fusion plasma with rotational shear. Two distinct states, obtained with identical equilibrium parameters in first- principle gyrokinetic simulations, have turbulent fluxes of particles, momentum and energy that differ by an order of magnitude – with the low-transport state agreeing with experimental observations. The occurrence of the two states is regulated by the competition between the externally imposed, mean rotational flow shear and “zonal” flows generated by the plasma. With small turbulent amplitudes, zonal flows have little impact on transport, and the mean shear causes turbulence to saturate in a low-transport state. With larger amplitudes, the zonal shear can substantially oppose the effect of the mean shear, allowing the system to sustain a high-transport state. This poses a new challenge for research that, so far, has assumed a uniquely defined turbulent state.

Authors: Christen, N

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: physics; plasma; fusion; gyrokinetics; turbulence
• Subjects: Plasma turbulence