Gyrokinetic full-f transport simulations of ohmic FT-2 tokamak discharges

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Susan Leerink Name of the doctoral dissertation Gyrokinetic full-f transport simulations of ohmic FT-2 tokamak discharges Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 176/2012 Field of research Fusion and Plasma Physics Manuscript submitted 12 June 2012 Date of the defence 21 December 2012 Permission to publish granted (date) 14 November 2012 Language English Monograph Article dissertation (summary + original articles) Abstract The work described in the thesis focuses on understanding large-scale mean ExB flows, finescale zonal flows and turbulent transport in ohmic plasma discharges in the small Russian FT2 tokamak from a first principal basis. The investigations are performed with the full-f particle-in-cell (PIC) code Elmfire which calculates the time evolution of the full distribution function of the electrons and a selection of ions while self-consistently calculating the electric field making the code suitable to study neoclassical and turbulent transport processes simultaneously. The equations solved in the Elmfire code are based on a set of gyrokinetic Vlasov equations which explicitly includes the polarization drift in the equations of motion by using an alternative definition for the gyrocenter. The differences to the standard gyrokinetic model and the advantages of this treatment for particle-in-cell codes are addressed. The coefficient matrix for the Poisson equation is constructed implicitly from the ion polarization and the electron parallel nonlinearity. Two types of interpolation schemes to obtain the particle electric field from the grid are implemented in the Elmfire code and their influence on the particle transport and momentum conservation are discussed. Collisions are treated by a binary collision model. The dependence of the transport quantities on the spatial resolution and the influence of particle noise are addressed. Steady state simulation profiles are obtained by the balance of ohmic heating and particle/energy transport, radiation losses and energy transported to a limiter surface. Direct measurements of transport phenomena in ohmic FT-2 discharges are shown to be quantitatively reproduced by the Elmfire simulation predictions. A detailed agreement with mean equilibrium ExB flows, oscillating fine-scale zonal flows and turbulence spectra observed by a set of sophisticated microwave back-scattering techniques as well as a good fit of the thermal diffusivity data are demonstrated. The measured and simulated mean equilibrium ExB flows are validated against various analytical models where the effect of the Te/Ti ratio, ion orbit losses and impurities are investigated in more detail. Statistical analysis of the oscillating fine-scale zonal flow are presented and a comparison to geodesic acoustic mode statistics while including the impurity contribution is performed