Studies on the parametric decay of waves in fusion plasmas: Dissertation

Parametric instabilities of large-amplitude electromagnetic waves are investigated in fusion applications. In laser fusion, the electromagnetic wave reflected from the overdense plasma can act as a secondary pump wave and excite parametric instabilities. In double stimulated Brillouin scattering (DSBS), both the incoming and the reflected pump wave scatter from a common ion sound wave. The stationary states and the dynamics of DSBS are investigated by using a simple envelope model. The ion sound wave that is excited in DSBS is shown to have soliton-like properties. The stimulated Raman scattering (SRS) of free-electron-laser radiation can be applied to current drive in tokamaks. SRS generates fast longitudinal electron plasma waves which accelerate electrons to relativistic energies. Since the energetic current-carrying electrons are almost collisionless, the current decays very slowly. The feasibility of the Raman current drive in tokamaks is investigated theoretically. The current drive efficiency and the optimum free-electron-laser parameters are determined. The energy transfer to the fast electrons from the electrostatic wave is studied with relativistic Vlasov-Maxwell simulations. The parametric decay of a wave to half-harmonics is investigated. It is shown that the growth rate of the decay vanishes in the limit of a long wavelength of the pump wave even for general elecromagnetic or electrostatic decay modes. The results are applied to the decay of a fast magnetosonic wave to two slow waves in tokamak plasmas.