Experimental Investigation of the Collective Raman Scattering of Multiple Laser Beams in Inhomogeneous Plasmas

Experiments have been performed evidencing significant stimulated Raman sidescattering (SRS) at large angles from the density gradient. This was achieved in long scale-length high-temperature plasmas in which two beams couple to the same scattered electromagnetic wave further demonstrating for the first time this multiple-beam collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. These findings obtained in plasmas conditions relevant of inertial confinement fusion experiments similarly apply to the more complex geometry of these experiments where anomalously large levels of SRS were measured.

Figure 1

(a) Schematic representation of the two cones of beams in the foam target; (b) density profile along the $z$ axis at $t=0.5$ and 1 ns; (c) 3D representation of the sidescattering geometry in the bisecting plane of beams 45 and 50. The conjugate EPWs in this collective excitation are represented by the green arrows for beams 45 and 50.

Figure 2

(a) Angular distribution of the 6 beams incident on the H18 side and scheme of the NBI diffuser plate; (b) typical image recorded with the NBI diagnostic for a $7\mathrm{mg}/\mathrm{cc}$ foam; (c) time-resolved spectrum of the SRS light collected in the direction of FABS25 in the same shot as for (b).

Figure 3

(a) calculated refraction of the 351 nm laser light and of 600 nm SRS light for ${\theta }_{\mathrm{SRS},p}=55°$, 71°, and 85° in the density profile for $t=1\mathrm{ns}$; (b) Rosenbluth gain ($G_{\mathrm{SRS}}$) for $n_e/n_c=0.17$ as a function of ${\theta }_{\mathrm{SRS},p}$ and transmission (T) of the SRS light for ${\lambda }_{\mathrm{SRS}}=600\mathrm{nm}$ through the density profile at $t=1\mathrm{ns}$. $G_{\mathrm{SRS}}$ is calculated for 2 beams with polarizations aligned with the scattered light; (c) exit angle (${\theta }_{\mathrm{SRS},v}$) of the SRS light produced at $n_e/n_c=0.17$ as a function of ${\theta }_{\mathrm{SRS},p}$.

Figure 4

(a) SRS spectra measured in FABS25 for different conditions: in $4\mathrm{mg}/\mathrm{cc}$ and $7\mathrm{mg}/\mathrm{cc}$ foams at 400 J and in a $5\mathrm{mg}/\mathrm{cc}$ foam at 350 J; (b) exit angle of the SRS light for the various densities in the crossing beams region shown as a function of its wavelength; the gray dashed lines represent the aperture of FABS25; representation of the two beams overlay in the planes (c) $z=-300\mu \mathrm{m}$ and (d) $z=-350\mu \mathrm{m}$. The laser beam spots are shown for two beams incident at 71° of the $z$ axis and overlapping in $z=-300\mu \mathrm{m}$. The contours are shown at 50% of the maximum intensity.