A knock-on deuteron imager for measurements of fuel and hotspot asymmetry in direct-drive inertial confinement fusion implosions (invited)

A knock-on deuteron imager (KoDI) has been implemented to measure the fuel and hotspot asymmetry of cryogenic inertial confinement fusion implosions on OMEGA. Energetic neutrons produced by D–T fusion elastically scatter (“knock on”) deuterons from the fuel layer with a probability that depends on ρR. Deuterons above 10 MeV are produced by near-forward scattering, and imaging them is equivalent to time-integrated neutron imaging of the hotspot. Deuterons below 6 MeV are produced by a combination of side scattering and ranging in the fuel, and encode information about the spatial distribution of the dense fuel. The KoDI instrument consists of a multi-penumbral aperture positioned 10–20 cm from the implosion using a ten-inch manipulator and a detector pack at 350 cm from the implosion to record penumbral images with magnification of up to 35×. Range filters and the intrinsic properties of CR-39 are used to distinguish different charged-particle images by energy along the same line of sight. Image plates fielded behind the CR-39 record a 10 keV x-ray image using the same aperture. A maximum-likelihood reconstruction algorithm has been implemented to infer the source from the projected penumbral images. The effects of scattering and aperture charging on the instrument point-spread function are assessed. Synthetic data are used to validate the reconstruction algorithm and assess an appropriate termination criterion. Significant aperture charging has been observed in the initial experimental dataset, and increases with aperture distance from the implosion, consistent with a simple model of charging by laser-driven EMP.

敲上氘核成像仪（Knock-on Deuteron Imager，KoDI）已搭建并投入使用，用于测量OMEGA装置上低温惯性约束聚变内爆的燃料与热斑不对称性。由氘氚（D-T）聚变产生的高能中子会与燃料层中的氘核发生弹性散射（即“敲上”散射，knock-on），其散射概率依赖于燃料面密度（ρR）。能量高于10 MeV的氘核由近前向散射产生，对其成像等效于对热斑开展时间积分中子成像；能量低于6 MeV的氘核则由侧向散射与燃料内射程效应共同产生，其所携带的信息可反映稠密燃料的空间分布。

KoDI成像仪由两部分组成：通过十英寸操作臂设置在距内爆点10~20 cm处的多半影孔径，以及距内爆点350 cm的探测器组件，用于记录最大放大倍率达35倍的半影成像。射程过滤器与CR-39径迹探测器的固有特性可用于在同一视线上按能量区分不同带电粒子的成像结果。置于CR-39后方的成像板可利用同一孔径记录10 keV X射线成像图。

研究人员已开发最大似然重建算法，可从投影半影图像中反推源项分布。研究团队评估了散射与孔径充电效应对成像仪点扩散函数的影响，并利用合成数据验证了重建算法，同时确定了合适的迭代终止准则。初始实验数据集观测到显著的孔径充电现象，且充电程度随孔径与内爆点的距离增大而增强，这与激光驱动电磁脉冲（EMP）诱导充电的简单模型相符。