Brillouin amplification: towards next generation laser energy densities
High-intensity lasers beams allow to create high-energy-density (HED) conditions, that mimic extreme astrophysical scenarios, in controlled laboratory settings. These exciting experiments give us a closer look at complex astrophysical processes that would otherwise have to be inferred indirectly by their radiation collected by our telescopes. Advances in the generation of higher laser powers and intensities is highly desirable, since these will give access to novel astrophysical conditions and exotic physical regimes of HED science, like “boiling the vacuum”.
Further increasing current state-of-the are laser energy densities is highly expensive using solid state optics. Unfeasibly massive gratings, mirrors and lenses are required to manage impinging laser intensities below their damage threshold. Plasma-based laser amplifiers, leveraging on parametric processes like stimulated Brillouin backscattering, overcome the optical damage limitations of solid state optics by several orders of magnitude, providing a promising alternative to the production of next generation laser energy densities.
Stimulated Brillouin backscattering consists in the coupling between two counter propagating electromagnetic waves, that are tuned at slightly different frequencies, via the interaction with an ion-acoustic wave. In the case where the two electromagnetic waves are composed of a short probe and a long pump laser pulse, this process can be used to efficiently transfer the energy from the pump to the probe pulse, thus greatly amplifying the latter to high powers and intensities.
This animation is of a large-scale two-dimensional particle-in-cell simulation of laser amplification via stimulated Brillouin backscattering. This simulation is performed using a moving window that follows the evolution of the amplifying probe pulse. Here, the the contour surfaces represent the intensity envelopes of the counter propagating laser pulses, and the spheres represent the plasma ions that oscillate in the ion-acoustic wave. The initial pump and probe intensities are both 10^16 W/cm^2 and the plasma density is 30% critical. It is shown that the probe pulse efficiently picks up the pump laser energy, reaching a peak intensity of 1.5×10^17 W/cm^2 (15x the pump intensity) while preserving a smooth envelope. However, as the probe is amplified it begins to break up due to ponderomotive filamentation. This instability is the main limiting effect to the amplification process in this parameter regime. Therefore, the interaction length has to be carefully adjusted to achieve maximum probe amplification while keeping filamentation at a tolerable level.