Vlasov simulations of kinetic enhancement of Raman backscatter in laser fusion plasmas

Stimulated Raman scattering (SRS) is studied in plasmas relevant to inertial confinement fusion (ICF). The Eulerian Vlasov-Maxwell code ELVIS was developed and run for this purpose. Plasma waves are heavily Landau damped in the regimes of interest, and coupled-mode theory predicts back-scattered SRS is a convective instability. Simulations in a finite length, homogeneous plasma show electron trapping drastically elevates the reflected light over convective gain values ("kinetic enhancement"). Average reflectivities are [approx.] 10%, while the instantaneous reflectivity is chaotic and does not reach a steady state. Trapping reduces the plasma-wave Landau damping and downshifts the observed frequencies from their linear values. Two longitudinal acoustic (? ? k) features and light from possible stimulated electron acoustic scattering (SEAS) are present. The phase-matched SEAS plasmon lies on the observed acoustic mode with phase velocity 1.3(Te/me)1/2. As the pump laser intensity is increased or the electron temperature is decreased, SRS transitions sharply from the coupled-mode steady state to kinetically enhanced levels. Enhancement happens for different back SRS seed levels and monochromatic or broadband seeds. Simulations with a Krook relaxation operator to mimic speckle sideloss display enhancement when resonant electrons complete a bounce orbit before escaping, with a sharp onset as the relaxation rate varies. The sudden development of kinetic enhancement as parameters change suggests trapping makes SRS absolutely unstable.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Physics.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2006

Author dc:creator

• Strozzi, D. J. (David J.)

Advisor dc:contributor.advisor

• Abraham Bers.

• Physics.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/34974

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/34974