First laser–plasma interaction experiments on Shenguang Octopus laser facility

On laser facilities for achieving inertial confinement fusion ignition, several beams are usually bundled together and injected into the target chamber through a common port. This specific configuration could lead to laser–plasma interaction (LPI) processes that are quite different from those of a single laser beam, as well as to more options for suppressing LPI via combination of different laser smoothing techniques on the beams in a bundle. For studying LPI of a bundle of beams, a novel facility named Shenguang Octopus has been developed. Recently, the first LPI experiments have been performed by axially irradiating a gas-filled glass pipe or a gold hohlraum target with the bundled eight laser beams at an overlapped intensity of ∼1×1015W/cm2, producing ∼5-mm-long homogeneous plasmas with densities of ∼3×1020cm−3 and temperatures of 1.2–1.5 keV. Efficient laser propagation without plasma filamentation and beam spraying has been observed via side-on X-ray images of glass pipe targets. Results from the gold hohlraum targets indicate that single-beam backward stimulated Brillouin scattering (SBS) and multibeam seeded side-SBS play major roles in the interaction, while other multibeam LPI processes expected before the experiments are not observed. Several beam smoothing techniques, such as mixed polarizations, smoothing by spectral dispersion, and multicolor mode, are demonstrated to be effective in suppressing LPI under current conditions.