Experimental investigation of stimulated Raman and Brillouin scattering instabilities driven by two successive collinear picosecond laser pulses

Backward stimulated Raman and Brillouin scattering (SRS and SBS) are experimentally investigated by using two successive 1-μm, 1.5-ps FWHM laser pulses. The collinear pulses, separated by 3 or 6 ps and of moderate laser intensities $\left(\sim 2\times {10}^{16}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}\right)$, are fired into a preionized He plasma of density $\sim 2.5-6\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$. The electron plasma waves and ion acoustic waves, respectively driven by SRS and SBS, are analyzed through space- and time-resolved Thomson scattering. Depending on the laser and plasma parameters, we observe the effect of the first pulse on the time-resolved SRS and SBS signals of the second pulse. The measurements are found to qualitatively agree with the results of a large-scale particle-in-cell simulation.

Figure 1

(a) Temporal evolution of the SRS-EPW (red circles) and SBS-IAW (blue squares) driven by two 1.5-ps laser pulses with $\mathrm{\Delta }t=2.9\mathrm{ps}$ (dashed gray line, labeled “laser bispeckle”), $I_0=2\times {10}^{16}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}$ and $n_e=6\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$. The upper-red (resp. lower-blue) dashed lines are the EPW (resp. IAW) responses measured using a single pulse centered at $t=5.8\mathrm{ps}$ (dashed gray line, labeled “laser monospeckle”). (b) Same conditions except for $\mathrm{\Delta }t=5.8\mathrm{ps}$. In both (a) and (b), the signals result from an integration over an $\sim 600\mu \mathrm{m}$ length along the lasers’ direction of propagation.

Figure 2

Temporal evolution of the SRS-EPW (red circles) and SBS-IAW (blue squares) driven by two 1.5-ps laser pulses (dashed gray line) with a 5.8-ps time delay. $I_0=2\times {10}^{16}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}$ and $n_e=2.5\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$. Same scales as in Fig. 1.

Figure 3

Time evolution of the synthetic EPW ($I_{\mathrm{EPW}}$, solid red line) and IAW ($I_{\mathrm{IAW}}$, dashed blue line) signals as extracted from the 2D PIC simulation. $t_{\max ,1}$ and $t_{\max ,2}$ denote the times of peak intensity of the two successive pulses at the center of the diagnosed region.

Figure 4

Spatial spectra (resolved in the transverse direction) of the electron density at times $t=t_{\max ,1}-0.9\mathrm{ps},t_{\max ,1}-0.2\mathrm{ps}$, and $t_{\max ,2}-0.2\mathrm{ps}$. The longitudinal wave number is normalized to $k_0={\omega }_0/c$, the laser wave number in vacuum. The spectral intensities are plotted in log scale.