Development of a nanosecond-laser-pumped Raman amplifier for short laser pulses in plasma

Multi-beam effects on backscatter and its saturation in experiments with conditions relevant to ignition

“…The most complete studies to date were carried out at Jupiter, where the amplification of both 0.5 and 3.5 ps duration seeds of broad spectrum light by interaction with a single crossing pump beam in a strongly damped plasma (kk D ¼ 0.38) with a small amplitude gain exponent ($2) was demonstrated on a time scale that does not allow ion wave instabilities. This demonstrated, for the first time under such conditions, 25,26 a non-linear, electron kinetic saturation of the amplification that has now been described by 2D VPIC simulations. [26][27][28] The shortest pulse used would allow the ion waves involved in secondary decays to undergo no more than 1 radian of oscillation (wt pulse $1) so they are unable to saturate the SRS Langmuir waves, which can then grow and be saturated by the electron kinetic effects relevant to the higher kk D of ignition targets.…”

“…The optimum intensity determined the results from a series of earlier experiments that observed a sharp onset of angular beam spray and scattering of the transmitted light outside the f=cone of the incident beam when intensity was in excess of the value 1.2 Â 10 14 W=cm 2 that is studied here. 25 Figures 3 and 4 show the measured transmitted seed spectrum from experiments in which a 0.5 ps seed with a spectral energy density of $1.5 lJ=nm near 1200 nm was intersected with the pump beam, and compares it with measurements with the seed beam only (no plasma) and the pump beam only (with plasma), at the same intensities and conditions. The data in Figure 3 clearly show the plasma amplifier both enhances and narrows the seed spectrum relative to the "seed only" case, which is consistent with Raman amplification in the uniform plasma region.…”

“…[24][25][26] Those experiments are then modeled with both linear wave analysis and our massively parallel VPIC simulations 27,28 that include the electron kinetic non-linearities such as particle trapping, wave front bowing, wave mode frequency detuning, and other trapped electron effects. In Raman amplification experiments carried out at laser facilities that have much less power and energy than is needed for ignition, it has been challenging to emulate the plasma conditions where reamplification occurs in ignition targets because the electron temperatures are very high in ignition targets (2-4 keV) so that the kk D products tend to be in the range 0.4 to 0.9 which causes strong linear damping of plasma waves.…”

Generation of multi-hundred petawatt pulses by resonant Raman amplification in plasma

“…BRA has been theoretically and experimentally demonstrated as an effective way for the fast compression of ultrashort pulses [2,[4][5][6][7][8][9][10]. Theoretical studies have indicated that centimeter-aperture beams are required to produce exawatt laser pulses given that the unfocused output seed intensity can reach 10 17 W/cm 2 [2,3,6].…”

Short-pulse amplification by strongly coupled stimulated Brillouin scattering

“…Amplification by stimulated Raman scattering (SRS) offers higher growth rates and shorter compressed pulses, and a large volume of work has been devoted to understanding its limiting factors, including wavebreaking [1,[4][5][6], Landau damping [7][8][9][10][11][12][13][14], spontaneous Raman scattering [1,4,[15][16][17][18], plasma inhomegeneities [19], and relativistic non-linearities [1,4,8,[20][21][22][23]. A series of experiments have demonstrated the viability of the mechanism [14,[24][25][26][27][28][29][30][31][32][33][34][35], though maximum achieved intensities are somewhat below the theoretical limits.…”