Parametric instabilities in underdense plasma and stochastic heating of electrons

Several aspects of ion-acoustic decay (IAD) and oscillating two-stream (OTS) instabilities are studied by theory and particle simulations in homogeneous plasma for densities 0.5≤ne/ nc ≤0.8. Linear theory for IAD and OTS is reviewed and compared with particle-in-cell simulations. Nonlinear aspects of IAD such as the appearance of ion-acoustic second harmonic are observed and theoretically investigated. A saturation mechanism for IAD in which the second harmonic plays an important role is proposed. A simple model in which particles move in the fields of coherent waves related to IAD (OTS) is proposed in order to explain heating of the electron distribution function. If the wave amplitudes exceed a certain threshold, large scale stochasticity (LSS) develops allowing appreciable momentum transfer to particles. The appearance of a characteristic Maxwellian tail in the electron distribution function is related to LSS in particle motion. The method presented here should also be useful for analyzing hot particle production in other parametric instabilities.