Laser pulse bandwidth broadening via stimulated forward Brillouin scattering

One of the major limitations for implementing direct drive on the National Ignition Facility is the reduced bandwidth (220 GHz) now available for beam smoothing purposes. A bandwidth of around 1 THz is needed to reduce the Brillouin backscattering growth rate by 30\% for typical NIF parameters [1]. Here we present a novel method to increase the bandwidth of the incoming laser pulse via explicit stimulation of the Brillouin forward scattering process. A similar process has already been demonstrated for stimulated Raman forward scattering [2]. The incoming laser beam with frequency $\omega_0$ is accompanied by a low-intensity beam (1-2 \% of the main beam intensity) at the frequency of the first Stokes sideband for Brillouin forward scattering: $\omega_1 = \omega_0 - \omega_B$ with $\omega_B \sim 0.001\omega_0$. The beating between these beams drives the Brillouin forward scattering instability and causes several orders of (anti-)Stokes side bands to emerge, leading to an effective spectral broadening. In addition, the long wavelength ion-acoustic wave corresponding to Brillouin forward scattering will reduce the growth of the short wavelength ion-acoustic wave corresponding to Brillouin backscattering. [1] B. Brand\~ao, L.O. Silva, J.E. Santos, R. Bingham, Phys. Plasmas, submitted (2010). [2] R. Trines et al., Europhys. Lett. 66, 492 (2004).