SIMULATIONS OF WAVE PARTICLE INTERACTIONS IN STIMULATED RAMAN FORWARD SCATTERING IN A MAGNETIZED PLASMA

Stimulated Raman forward scattering in a high-temperature, magnetized plasma is investigated with relativistic Vlasov-Maxwell simulations and with envelope and test particle calculations. The parameters correspond to Raman current drive by free-electron lasers in reactor grade tokamak plasmas. The phase velocity of the Raman excited plasma wave is large, and therefore the Landau damping is initially weak. The electron plasma wave grows to a large amplitude and accelerates electrons to high energies. Simultaneous pump depletion weakens the driving ponderomotive force, which leads to a collapse of the plasma wave if the number of the interacting electrons is large enough. Spatially the wave-particle interaction takes place in a distance of a few wavelengths of the plasma wave. The electron energies can largely exceed the kinetic energy at the phase velocity of the electron plasma wave. Short-wavelength amplitude modulations of the plasma wave appear at high amplitudes. Efficient generation of the nonresonant anti-Stokes wave and of the second Stokes wave are also observed. Analytical growth rates and envelope calculations explain well the early evolution of the Raman process. Later on, nonlinear wave-particle interactions and relativistic effects start to dominate, and the system is not satisfactorily described by simple envelope equations.