ABSTRACT
The long-time dynamics of particles interacting resonantly with large-amplitude coherent plasma wave is investigated in the kinetic regime of stimulated Raman scattering in which particle trapping plays a major role (and which corresponds to a high value of the parameter k_{EPW}lambda_{D}, where k_{EPW} is the plasma wave vector and lambda_{D} is the electron Debye length). Using Vlasov simulations, the dynamics of such particles become stochastic when repeated wave-particle interactions take place. For small values of the ratio tau_{auto}/tau_{b} of the autocorrelation time to the bounce time of particle (condition usually met in backward propagation of the scattered wave) the turbulent regime results in the merging of phase-space trapping vortices according to a weak turbulencelike scenario. For high values of tau_{auto}/tau_{b} (or narrow spectrum of longitudinal electric field as met when only one plasma wave is present), the stochasticity is now induced by particle trapping, detrapping, and retrapping in the adiabatically fluctuating field. The stochastic transitions performed by resonant particles above (or below) the separatrix limit in phase space determine now the long-time plasma evolution.
ABSTRACT
Particle simulations on a flat-topped somewhat underdense (typically n0/nc = 0.6) plasma slab by Nikolic [Phys. Rev. E 66, 036404 (2002)] were seen to give transient stimulated scattering behavior with frequency shift [omega0 - omegas(approximately omegap)] considerably less than the plasma frequency omegap. This has been linked to the electron acoustic wave (EAW) and the scattering was thus seen as another example of stimulated electron acoustic scattering inferred by Montgomery [Phys. Rev. Lett. 87, 155001 (2001)] from experiments on low-density plasmas. Montgomery had noted the difficulty of how one could have a very narrow observed scattering from a wave whose damping was at least initially very high. Our Vlasov-Maxwell simulations for such somewhat underdense (n0/nc > or = 0.25) plasmas show that the simulation resonance was in fact determined by the beating of the pump with a new "radiating pseudocavity" electromagnetic mode for the slab at a frequency close to omegap with relatively low loss. This allows the initial narrow-band excitation of the kinetic electrostatic electron nonlinear (KEEN) waves (the nonlinear "cousins" of EAWs) at a well-defined frequency (omegaK approximately omega0 - omegap < omegap) which is not necessarily the value given by the EAW dispersion relation. (The KEEN wave characteristics have been discussed by Afeyan [33rd AAAC (2003), #238, IFSA 2003].) The consideration of such a mechanism is relevant to moderately underdense hot plasmas.
