Confirmation of hot electron preheat with a Cu foam sphere on GEKKO-LFEX laser facility

Experiments with a solid Cu foam (∼1.3 g/cm³) sphere coated by a 20 μm CH ablator are performed on the GEKKO-LFEX laser facility to study the effect of hot electron preheat on the implosion performance. When the target is imploded by the GEKKO lasers (∼1.2 × 10¹⁵ W/cm² in peak intensity), plenty of hot electrons are measured through the induced Cu Kα emission, indicating that the target could suffer strong preheat. This suffering of preheat is confirmed by the temporal evolution of the target self-emission, which is well reproduced by a 2D cylindrically symmetric radiative hydrodynamic code (FLASH) when a module handling the hot electron preheat is coupled. The results given by this benchmarked code indicate that, in the typical experiments with a small (∼200 μm in diameter) solid sphere target conducted on the GEKKO-LFEX laser facility, the hot electron preheat greatly degrades the implosion performance, reducing the peak areal densities of a Cu foam sphere and a CD sphere by ∼20% and ∼35%, respectively.