B.W. Li

Lj. Nikoli?

M.M. ?kori?

S. Ishiguro

T. Sato

National Institute for Fusion Science (NIFS-Repository)

§ 5. Stimulated Scattering of RelativisticLaser Light in Subcritical Plasmasto future fast ignition and hohlraum target experi-ments. We plan to further address this problem bytwo-dimensional (2D) and 3D simulations.0.90.712 2E0.0 to 1024.0.. kit0.013 ......--r---.---,"""T"'""....-........-.S 0.010or¥ 0.007ei 0.0031480 2220 2960ii' 0.40.2 '\ If.\-0.00 740Ishiguro, S.,Skoric, M.M. (Vinca Inst., Yugoslavia)Li, B.W., Nikolic, Lj. (The Grad. Univ. for AdvancedStudies)Sato, T. (Earth Simulator Center)0.8 0.8Fig. 3. x - Px phase space plot of electrons.1006040204u 208...........0-2-40x(c/coo)= 0.80 (Oot = 247.512,..,""to>dl,..,+172.51,..,+C1\)t= 222.51-1 -10.8 0.8100 200 100 200X(c/ffi()) X(C/ffi()),..,""to>dl,..,+>-,297.51,..,+>-,C1\)t= 347.51-1 -10.8 0.80 100 200 100 200X(c/ffi()) X(c/ffi())Fig. 2. Time evolution of electromagnetic field.Fig. 1. Time evolution of reflectivity (left) and spec-trum of scatterd electromagnetic field (right).Relativistic laser-plasma interaction is a source ofvarious electronic instabilities. In particular, a largeeffort has been put into studies of backward and for-ward stimulated Raman scattering (SRS) and relativis-tic modulational instability (RMI) for varying laser-plasma parameters. The theory of electron paramet-ric instabilities predicts the relativistic broadening andmerging of unstable regions for SRS and RMI at highlaser intensities.We study by one-dimensional relativistic particle-in-cell simulations, a linearly polarized relativistic laser(10 17 W/cm2 < I < 1019 W /cm2 ) interacting with aplasma layer (Te rv IkeV) at a subcritical density range(n c /4 < n/I < nc ); where I is the relativistic fac-tor. In regions which are overdense for standard SRS,intense coherent reflectivity pulsations at frequenciesbelow the electron plasma frequency (wp ) are typi-cally observed in our simulations. In early stage, thespectrum is well explained by a resonant 3-wave para-metric decay of the relativistic laser pump into theslowed (rv "critical") Stokes light sideband (w s rv wp )and the trapped electron-acoustic wave (TEAW, withW a < wp ). This appears a relativistic version of stim-ulated electron-acoustic scattering (R-SEAS), recentlystudied by a number of authors. In nonlinear satura-tion, there is a rapid growth and strong localization ofthe Stokes wave by forming narrow intense EM soliton-like structures with (downshifted) laser light trappedinside. The train of relativistic EM solitons gets ir-radiated through the front vacuum-plasma boundaryin a form of intense coherent reflection of the down-shifted laser light (Fig. 1, 2). Large TEAW excitedin the plasma quickly heats up electrons to relativisticenergies (Fig. 3) which eventually suppresses the insta-bility in our simulations. For example, 1 psec pulse ofthe fundamental light at 1018 W/cm2 transmits about70 % of laser energy while propagating through the 20microns thick uniform subcritical plasma at n = 0.7nc ,placed in vacuum. However, in a similar case, a shorter0.5 psec pulse experiences just 40 % transmission dueto large transient backscatter and absorption. Thisnovel behavior which alters the distribution of rela-tivistic laser energy between transmission, scatteringlosses and generation of energetic electrons has es-caped earlier attention while being possibly relevant326

Stimulated Scattering of Relativistic Laser Light in Subcritical Plasmas

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)