Backward Raman amplification in the Langmuir wavebreaking regime

Abstract

In plasma-based backward Raman amplifiers, the output pulse intensity increases with the input pump pulse intensity, as long as the Langmuir wave mediating energy transfer from the pump to the seed pulse remains intact. However, at high pump intensity, the Langmuir wave breaks, at which point the amplification efficiency may no longer increase with the pump intensity. Numerical simulations presented here, employing a 1D Vlasov-Maxwell code, show that, although the amplification efficiency remains high when the pump only mildly exceeds the wavebreaking threshold, the efficiency drops precipitously at larger pump intensities

Raman amplification in the coherent wave-breaking regime

“…It is interesting to note, however, that simulations carried out in this regime have yielded significantly different efficiencies, even for similar parameters, ranging from 35% in the work of Trines et al [2], to less than 10% in that of Toroker et al [11]. These apparently contradictory results have been the subject of significant discussion [11,12], although the main cause has not previously been identified.…”

“…This corresponds to ω a /ω p = 20, ω b /ω p = 19, and a pump intensity 30 times above the threshold for wavebreaking to occur [3]. These are the same parameters as used by Toroker et al [11] in their investigation of the strongwavebreaking regime, with the longest pulse used here equivalent to the probe used in that work. Although the longer pump length used here (30 ps compared to ∼1 ps) would result in higher plasma temperatures, we retain the 10 eV temperature to allow direct comparison to those results.…”

Advantages to a diverging Raman amplifier

“…As predicted, there are no secondary trailing peaks. Although, suppression of these peaks is also possible when the pump exceeds the threshold wavebreaking intensity [35][36][37][38][39] , here this was only true for the first 20% of the propagation. We therefore attribute the suppression to the divergence of the pulse, rather than premature breaking of the plasma wave.…”

Short-pulse amplification by strongly coupled stimulated Brillouin scattering

“…Studies of plasma amplification have primarily considered stimulated Raman [1] and Brillouin [3] backscattering, which are based on three-wave coupling of pump and probe laser beams with Langmuir or ion-acoustic plasma waves, respectively. 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.…”