DOI: 10.1140/epjd/e2009-00271-9
Novel features of non-linear Raman instability in a laser plasma
M. Mašek and K. RohlenaInstitute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
masekm@fzu.cz
Received 17 March 2009 / Received in final form 17 September 2009 / Published online 6 November 2009
Abstract
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