Waves and turbulence in plasmas: from MHD to vlasov and beyond

Abstract

This thesis fo uses on the physi al onne tion of large, uid s ales with small, kineti wavelenghts and on the introdu tion of ollisional e e ts in weakly- ollisional plasmas. In the rst part, the Mo att & Parker problem, namely the ollision of two ounter-propagating Alfvéni wave pa kets, has been revisited by means of magnetohydrodynami s (MHD), Hall MHD and hybrid kineti simula- tions. The goal of this study was to extend the Mo att & Parker problem to the realm of kineti physi s and show that, when introdu ing more om- plex physi al ingredients, the dynami s is quite di erent with respe t to the pure ideal MHD ase. When the energy is transferred towards kineti s ales through nonlinear oupling me hanisms, the distribution fun tion is strongly perturbed and departs from lo al thermodynami al equilibrium. The wave pa kets intera tion has been also hara terized in terms of strong and weak turbulen e, showing that features explained in terms of both kinds of turbu- len e theories oexist. In the se ond part, a spe ial attention has been devoted to weakly olli- sional plasma systems, in whi h kineti e e ts and parti le ollisions oexist and ompete in shaping the parti le velo ity distribution. By means of nu- meri al simulations of relaxation towards equilibrium in presen e of the full Landau ollisional integral, it has been pointed out that ollisionality an be e e tively enhan ed by the presen e of ne velo ity stru tures in the parti le distribution fun tion. However, due to the high omputational ost of the Landau integral, sim- pli ed ollisional operators have been employed to simulate self- onsistently the dynami s of weakly- ollisional plasmas. In parti ular, the Dougherty operator has been employed in 1D 3V phase spa e on guration (1D in physi al spa e, 3D in velo ity spa e) to address the role of ele tron-ele tron ollisions in the nonlinear regime of ele trostati waves propagation. Finally, with the aim of simulating realisti physi al onditions in experiments with plasmas trapped in longitudinal ma hines, numeri al simulations in redu ed 1D 1V phase spa e have been run to reprodu e the pro ess of wave laun hing in real plasma devi es. The ultimate goal of this work was to support the idea that the om- petition between kineti e e ts, whi h tend to drive the system away from equilibrium, and ollisions, whi h work to thermalize the plasma, ould be the physi al ingredient underlying the me hanism of parti le heating in weakly ollisional systems, su h as the solar wind.