A study on the ion temperature gradient driven turbulence in tokamak plasmas

The continuous growth of the energy worldwide consumption is one of the most important challenges to our civilization. New kinds of energy resources are without doubt needed. Nuclear fusion promises to supply large amounts of energy, with minimal environmental impact. This has motivated at least sixty years of research in which substantial progress has been achieved, but without breakthrough result. Nuclear fusion can occur at temperatures of the order of 150 million degrees Celsius (thermonuclear fusion). At these temperatures atoms are completely ionized, the fuel is then in the state of matter called a plasma, a gas of ions and electrons. The most promising approach towards the goal of using thermonuclear fusion for large scale energy production is to confine the plasma using magnetic fields. The tokamak is the device that produces the best results concerning plasma magnetic confinement to date. One of the main tasks of fusion research is the understanding of plasma confinement. The energy confinement must be sufficiently good such that a large amount of reactions take place, this in order to make the process economically convinient. This translates in the necessity of minimizing the heat fluxes out of the plasma. The heat fluxes observed experimentally in tokamak plasmas are much higher than those that can be ascribed to collisions. This so called anomalous transport is largely controlled by the destabilization of low frequency drift wave fluctuations, resulting in turbulence in the plasma on small scales compared to the tokamak size. The drift waves are collective modes of plasma oscillations that propagate through the plasma, arising as a result of the independent dynamics of ions and electrons in the presence of gradients of quantities describing the plasma (temperature, density, etc.). In this thesis, physical phenomena connected with the global description of turbulence in tokamak plasma have been analysed. Quasi-local simulations of electrostatic Ion Temperature Gradient (ITG) modes instabilities, i.e. electrostatic microinstabilities driven in the plasma by the presence of an ion temperature gradient, have been performed. Quasi-local refers to the case in which background quantities are assumed constant throughout the simulation domain, but inhomogeneities in the profiles of the turbulent quantities are taken into account. The work consists of two main parts. In the first part of the thesis, the electrostatic linear ITG modes growth rate (γ) spectrum is numerically calculated. It is observed that γ as a function of the poloidal wave vector (kΘ) is given by a double-humped curve. In particular, it is observed that modes with high value of kΘ have a maximum amplitude at a position that is shifted away from the low field side. The physical mechanism responsible for this behaviour is clarified through the use of a fluid model. It is shown that the shift of the mode away from the low field side reduces the effective drift frequency which allows…