The conceptual design of 1-ps time resolution neutron detector for fusion reaction history measurement at OMEGA and the National Ignition Facility

The nuclear burn history provides critical information about the dynamics of the hot-spot formation and high-density fuel-shell assembly of an Inertial Confinement Fusion (ICF) implosion, as well as information on the impact of alpha heating, and a multitude of implosion failure mechanisms. Having this information is critical for assessing the energy-confinement time τE and performance of an implosion. As the confinement time of an ICF implosion is a few tens of picoseconds, less than 10-ps time resolution is required for an accurate measurement of the nuclear burn history. In this study, we propose a novel 1-ps time-resolution detection scheme based on the Pockels effect. In particular, a conceptual design for the experiment on the National Ignition Facility and OMEGA are elaborated upon herein. A small organic Pockels crystal “DAST” is designed to be positioned ∼5 mm from the ICF implosion, which is scanned by a chirped pulse generated by a femtosecond laser transmitted through a polarization-maintained optical fiber. The originally linearly polarized laser is changed to an elliptically polarized laser by the Pockels crystal when exposed to neutrons, and the modulation of the polarization will be analyzed. Our study using 35-MeV electrons showed that the system impulse response is 0.6 ps. The response time is orders of magnitude shorter than current systems. Through measurements of the nuclear burn history with unprecedented time resolution, this system will help for a better understanding of the dynamics of the hot-spot formation, high-density fuel-shell assembly, and the physics of thermonuclear burn wave propagation.

核燃烧历史可为惯性约束聚变（Inertial Confinement Fusion, ICF）内爆的热点形成动力学与高密度燃料壳层组装过程提供关键信息，同时可提供阿尔法加热效应的影响以及多种内爆失效机制的相关信息。获取此类信息对于评估内爆的能量约束时间τ_E与整体性能至关重要。鉴于ICF内爆的约束时间仅为数十皮秒，若要精准测量核燃烧历史，需实现优于10皮秒的时间分辨率。本研究提出了一种基于普克尔斯效应（Pockels effect）的新型1皮秒时间分辨率探测方案。本文详细阐述了在国家点火装置（National Ignition Facility）与OMEGA激光装置上开展相关实验的概念设计。将小型有机普克尔斯晶体"DAST"置于距ICF内爆源约5毫米处，利用经保偏光纤传输的飞秒激光产生的啁啾脉冲对其进行扫描。当受到中子辐照时，原本为线偏振的激光会通过普克尔斯晶体转变为椭圆偏振激光，其偏振调制信号将被分析。本研究通过35 MeV电子束测试表明，该系统的冲激响应为0.6皮秒，其响应时间较现有系统短数个数量级。通过以空前的时间分辨率测量核燃烧历史，该系统将助力更深入地理解热点形成、高密度燃料壳层组装的动力学过程，以及热核燃烧波传播的物理机制。