Study of the influence of zonal flows on plasma turbulence, L-H transition and entropy production

One of the main obstacle to the success of thermonuclear magnetic confinement fusion is turbulence, which is mainly responsible for radial transport towards the walls. A shell model is used to study the mechanism of turbulence suppression by zonal flows, which are considered to play an important role in the L-H transition. Based on generalized Hasegawa-Mima equations, it gives an insight of predator-prey interactions between zonal flows and drift wave turbulence. The collisional drag of zonal flows is shown to play a major role in the behaviours of the system that is considered. With a larger number of shells, this model agrees well with the suppression rule of turbulence by E x B shear. According to the model, potential enstrophy (enstrophy is defined as the variance of vorticity) transfer occurs from injection scales toward dissipation scales, resulting from the action of the shear flow. On the other hand, a simple radial transport model is proposed. With 3 equations for the evolution of pressure, density, and intensity of turbulence, it can reproduce a basic mechanism of L-H transition. The influence of several parameters of the system on the nature of L-H transition is highlighted. Then, a new multi-scales model is built by coupling both transport and shell models, in order to reproduce the L-H transition in real space and k-space simultaneously. The behaviour of the system is somewhat similar to several experimental observations, and the crucial role of zonal flows as a trigger of L-H transition is underlined. Due to its spectral nature, the coupled model allows us to predict the self-consistent flattening of the spectrum when the H mode is reached. The influence of turbulence spreading on the dynamic of L-H transition is also discussed. Finally, an interpretation of the L-H transition through entropy production is proposed. (author)