First large capsule implosions in a frustum-shaped hohlraum

We report on the first indirect-drive implosions driven by a dual conical frustum-shaped hohlraum denoted “frustraum” and the experimental tuning campaigns leading up to two layered implosions. The campaign utilized 1.2 mm and 1.4 mm inner radius HDC capsules and represented the largest HDC capsules to be imploded on the National Ignition Facility via indirect drive. Several techniques were successfully implemented to control the mode 2 symmetry of the implosions including changing the wall angle of the frustraum which is not possible with cylindrical hohlraums. A mode 4 feature was observed and its implications for hot spot mix discussed. Two layered implosions were conducted with 1.2 mm inner radius capsules, the latter of which achieved the highest layered capsule absorbed energy on the NIF using only 1.74 MJ of laser energy. The layered implosion results suggest that increasing capsule absorbed energy by itself is insufficient, and that further reducing coast time (time between end of laser pulse and bang time) to the 1 ns level is warranted to improve areal density, hot spot temperature and alpha heating and yield amplification.

本研究报道了首个由被命名为"frustraum"的双截锥体形金腔（hohlraum）驱动的间接驱动内爆实验，以及为实现两次分层内爆所开展的实验调试工作。本次实验计划采用内径为1.2 mm与1.4 mm的HDC靶丸，其所使用的靶丸为目前国家点火装置（National Ignition Facility，NIF）上通过间接驱动方式开展内爆实验的尺寸最大的HDC靶丸。研究团队成功应用多项技术调控内爆的二阶（mode 2）对称性，其中包括调整frustraum的壁面倾角——这一操作在圆柱形金腔中无法实现。实验中观测到四阶（mode 4）结构，并讨论了其对热点混合的影响。研究团队使用内径1.2 mm的靶丸完成两次分层内爆实验，其中第二次实验仅使用1.74 MJ的激光能量，便达成了NIF上分层靶丸的最高吸收能量纪录。分层内爆实验结果表明，仅提升靶丸吸收能量并不足以优化实验性能，需将停滞时间（激光脉冲结束至点火时刻的间隔）进一步缩短至1纳秒级别，以提升面密度、热点温度、α加热效应与产额放大倍数。