Performance of tokamak fusion reactors is limited by heat and particle losses. Fusion reactions require extreme temperatures that ionise the fuel and transform it into a plasma, which is confined by strong magnetic fields in tokamaks. Designing future reactors requires the capability to predict the efficiency of the confinement based on known parameters. Generally the design process utilises simple scaling laws based on databases of previous experiments. These scaling laws, however, give little insight into the physical mechanisms that determine the confinement properties. The heat and particle losses are understood to be governed by the non-linear interplay of turbulence and plasma flows, but uncertainty remains on e.g. sudden transitions between confinement regimes and isotopic scaling of confinement. Experimental studies of these issues are aided by advanced computer models. This thesis investigates the interplay of flows and turbulence in ohmically heated tokamak plasmas via gyrokinetic simulations with the ELMFIRE and GENE codes. The first part of the thesis presents the verification of ELMFIRE predictions against theoretical estimates of fundamental physics properties with ad-hoc plasma parameters. The simulation predictions agree quantitatively with the analytical estimates for the neoclassical mean plasma flow and electrical conductivity in different collisionality regimes. ELMFIRE predictions for the frequency of oscillating flows are also within a few percent of the analytical estimate. The second part presents studies of isotope effect on transport and plasma flows in Ohmic tokamak plasmas. Gyrokinetic simulations predict decreased particle transport, when the fuel is switched from hydrogen to deuterium and other parameters remain comparable in the FT-2 tokamak. Experimental measurements of the corresponding plasmas validate the prediction qualitatively. Simulations indicate that the reduction of particle flux results from less intense fluctuations at small spatial scales for the heavier isotope. Linear analysis of turbulence identifies the dominant instability as the trapped electron mode driven by the density gradient. Experiments and simulations show clear evidence of geodesic acoustic mode (GAM) activity; they follow an isotopic scaling of GAM frequency, wavelength, and amplitude. The interplay of GAM and turbulence results in modulation of particle flux, which is more distinct for the deuterium plasma. The deuterium parameters also have a larger GAM amplitude. The simulations predict that both neoclassical effects and turbulence determine the mean flow profile in the plasmas, and that the safety factor profile is important for the organisation of mean flows and turbulence.
|Translated title of the contribution||Turbulenssin ja virtausten vuorovaikutus simuloiduissa tokamak-plasmoissa|
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- nuclear fusion
- computer simulation
- zonal flows