Modelling and measuring transport in fusion plasmas

Licentiate thesis, 2009

In the present thesis we consider theoretical and experimental aspects of the turbulent transport which is a crucial issue in fusion plasma physics. Experimental observations and gyrokinetic simulations show that collisions strongly influence the turbulent flux of particles. In this thesis we investigate the collisionality dependence of the quasilinear particle fluxes due to ion temperature gradient (ITG) and trapped electron (TE) modes. A semi-analytical, collisional model of electrostatic turbulence (COMET) has been developed, where collisions are modeled by the Lorentz operator. We point out that the form of the collision operator affects the collisionality scaling of particle flux. COMET has been benchmarked with the gyrokinetic code GYRO, and it is used to calculate quasilinear particle and energy fluxes and ITG mode stability thresholds. Closed analytical expressions are provided for the density and temperature responses without expansion in the smallness of magnetic drift frequency. We find that the temperature gradient threshold for stability is significantly affected by the electron-ion collisions for high enough logarithmic density gradients. Alkali beam emission spectroscopy (BES) is widely used for the measurement of electron density and its fluctuations, contributing to the understanding of transport processes in the outer plasma regions. In the evaluation of density profile measurements the width of the diagnostic beam is often neglected, which might cause a non-negligible underestimation of the pedestal density. A de-convolution based correction algorithm has been introduced which estimates the emission density on the beam axis from a measured light profile allowing the use of the conventional one-dimensional density calculation methods.