The direct-drive inertial confinement fusion scheme with a gradually increasing intensity nanosecond pulse needs to avoid fuel preheating during the low-temperature implosion stage, in which the occurrence and characteristics of two-plasmon decay with low laser intensity is worthy of attention as it produces hot electrons that preheat the fuel. In this paper, we present experimental results regarding the occurrence and characteristics of two-plasmon decay inferred from the observation of three-halves harmonic light under an irradiance of ∼1 × 1013 W cm−2, with which three plasma density scale-lengths were obtained using different laser pulses. Spatial and temporal discontinuities of the laterally emitting three-halves harmonic light in both the parallel and the perpendicular directions with respect to the target surface were observed, which are interpreted by laser filamentation analyses based on plasma parameters calculated from the measured spectra and interferograms, and supported by radiation hydrodynamics and particle-in-cell simulations. From the perspective of filamentation, suppression methods of the observed phenomena under such conditions are discussed.
Focus optimization of relativistic self-focusing for anomalous laser penetration into overdense plasmas (super-penetration)