Inferences of hot electron preheat and its spatial distribution in OMEGA direct drive implosions

Hot electrons generated from laser plasma instabilities degrade performance of direct drive implosions by preheating the deuterium and tritium (DT) fuel resulting in early decompression and lower areal densities at stagnation. A technique to quantify the hot electron preheat of the dense DT fuel and connect it to the degradation in areal density is described in detail. Hot electrons are measured primarily from the hard x-rays they emit as they slow down in the target. The DT preheat is inferred from a comparison of the hard x-ray signals between a DT-layered implosion and its mass equivalent ablator only implosion. The preheat energy spatial distribution within the imploding shell is inferred from experiments using high Z payloads of varying thicknesses. It is found that the electrons deposit their energy uniformly throughout the shell material. For typical direct-drive OMEGA implosions driven with an overlapped intensity of ∼9·10^14 W/cm2, approximately ∼0.02%–0.03% of the laser energy is converted into preheat of the stagnated fuel which corresponds to areal density degradations of 10%–20%. The degradations in areal density explain some of the observed discrepancies between the simulated and measured areal densities.由激光等离子体不稳定性产生的热电子，会通过预加热氘氚（deuterium and tritium，DT）燃料引发早期降压，并在内爆停滞阶段降低面密度，进而劣化直接驱动内爆的整体性能。本文详细阐述了一种可量化致密DT燃料热电子预加热程度，并将其与面密度劣化建立关联的技术方法。热电子主要通过其在靶材内减速过程中辐射的硬X射线进行探测。通过对比DT层状内爆与等效质量纯烧蚀层内爆的硬X射线信号，可推断出DT燃料的预加热状况。借助不同厚度的高Z有效载荷开展实验，可进一步推断内爆壳层内的预加热能量空间分布。研究表明，热电子会将自身能量均匀沉积于整个壳层材料之中。对于典型的直接驱动OMEGA激光装置内爆，当其重叠辐照强度约为9×10^14 W/cm²时，约有0.02%~0.03%的激光能量会转化为停滞燃料的预加热能量，这对应着10%~20%的面密度劣化幅度。该类面密度劣化现象可解释部分模拟结果与实测值之间的观测偏差。