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

Abstract

Experiments with a solid Cu foam ($1.3 g/cm 3) sphere coated by a 20 lm 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 Â 10 15 W/cm 2 in peak intensity), plenty of hot electrons are measured through the induced Cu Ka 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 lm 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%

Direct observation of imploded core heating via fast electrons with super-penetration scheme

“…We figured out that the laser-to-core energy coupling efficiency could be improved by optimizing the implosion and super-penetration performances, because it was limited by two aspects in experiments reported here. One aspect was the low areal density ( ρR ~ 0.06 g cm −2 ) of the core, which resulted from the low driver energy (~200 J beam −1 ) and the target preheat caused by the driver-produced suprathermal electrons 38 . The kinetic energy of fast electrons optimal for heating a core of 0.06 g cm −2 ranges from 200 keV to 500 keV.…”

“…The hot-electron-preheat module was coupled to take into account the effect on the target compression caused by the energy deposition of GXII-produced suprathermal electrons. The necessity of considering this preheat effect was demonstrated by our previous experiments under similar configurations 38 . This simulation code was benchmarked by comparing the simulated temporal and spatial evolutions of the target self-emission (i.e., the XSC result) with the experimental data, as shown in Fig.…”

“…The densities in both parts were ~1 g cm −3 . Our previous experiments in similar configurations indicated that, when a ~20 μm thick coating layer was used, plenty of suprathermal electrons produced by the drive (GXII) lasers were able to penetrate into the inner region, resulting in strong background Cu Kα emission 38 . Therefore, a thicker (30 μm) coating layer was used in the experiments reported here to reduce the background Cu Kα emission, thereby facilitating the detection of fast electron transport.…”

“…The compression of the target was simulated by the FLASH code 32 in 2D-cylindrical geometry coupled with a hot-electron-preheat module 38 . FLASH is a radiative hydrodynamic code using 3D-in-2D ray trace algorithm for the laser energy deposition.…”