Novel features of non-linear Raman instability in a laser plasmaM. Mašek and K. Rohlena
Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
Received 17 March 2009 / Received in final form 17 September 2009 / Published online 6 November 2009
The electron phase space evolution in a non-relativistic and homogeneous laser plasma generated by a nanosecond laser in a near infrared region in the presence of stimulated Raman scattering is investigated by a numerical simulation. The mechanism of electron acceleration in the potential wells of the plasma wave accompanying the Raman back-scattering is analyzed in a 1D Vlasov-Maxwell model. The dominant wave modes are both the backward and the forward propagating Raman waves, each accompanied by a daughter electrostatic wave. In addition to a strong interaction of plasma electrons with the primary electrostatic wave in the case of back-scattering, a cascading is observed consisting in a secondary scattering of the primary Raman back-scattered wave. This phenomenon reduces the Raman reflectivity and causes an acceleration of electrons against the direction of the heating laser beam. Moreover, the strong trapping in the primary electrostatic wave generated by the Raman back-scattering leads due to the trapped particle instability to a significant spectral broadening of the original plasma wave and a subsequent intermittent behaviour of the scattering process. The high phase velocity electrostatic daughter wave of the forward Raman scattering cannot trap the electrons directly, but there is an indication of non-resonant quasi-modes combined of this wave and of the simultaneously existing electrostatic daughter wave accompanying the Raman back-scattering. The transform method is used for a solution of the set of partial differential equations, which consists of the Vlasov equation and of the full set of Maxwell equations in a 1D approximation. A simplified Fokker-Planck collision term is added to overcome the numerical instabilities during the simulation. The model has relevance to a long scale plasma geometry, such as occurring in the indirect drive experiments near the light entrance holes of target hohlraum.
52.35.Mw - Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.).
52.38.Bv - Rayleigh scattering; stimulated Brillouin and Raman scattering.
52.38.Kd - Laser-plasma acceleration of electrons and ions.
52.65.Ff - Fokker-Planck and Vlasov equation.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2009