Performance of indirectly driven capsule implosions on the National Ignition Facility using adiabat-shaping

A series of indirectly driven capsule implosions has been performed on the National Ignition Facility to assess the relative contributions of ablation-front instability growth vs. fuel compression on implosion performance. Laser pulse shapes for both low and high-foot pulses were modified to vary ablation-front growth and fuel adiabat, separately and controllably. Three principal conclusions are drawn from this study: (1) It is shown that reducing ablation-front instability growth in low-foot implosions results in a substantial (3-10X) increase in neutron yield with no loss of fuel compression. (2) It is shown that reducing the fuel adiabat in high-foot implosions results in a significant (36%) increase in fuel compression together with a small (10%) increase in neutron yield. (3) Increased electron preheat at higher laser power in high-foot implosions, however, appears to offset the gain in compression achieved by adiabat-shaping at lower power. These results taken collectively bridge the space between the higher compression low-foot results and the higher yield high-foot results.本研究在美国国家点火装置（National Ignition Facility）上开展了一系列间接驱动靶丸内爆实验，旨在评估烧蚀面不稳定性增长（ablation-front instability growth）与燃料压缩（fuel compression）对内爆性能的相对贡献。研究中分别可控地对低足型脉冲（low-foot pulses）与高足型脉冲（high-foot pulses）的激光脉冲波形（laser pulse shapes）进行修改，以独立且可控地调节烧蚀面不稳定性增长与燃料绝热参数（fuel adiabat）。本研究得到三项主要结论：(1) 研究证实，在低足型脉冲内爆实验中抑制烧蚀面不稳定性增长，可在不损失燃料压缩性能的前提下，使中子产额（neutron yield）实现3至10倍的显著提升；(2) 研究证实，在高足型脉冲内爆实验中降低燃料绝热参数，可使燃料压缩性能提升36%，同时伴随中子产额10%的小幅增长；(3) 不过，高足型脉冲内爆实验中更高激光功率对应的电子预加热（electron preheat）效应，会抵消低功率下通过绝热整形（adiabat-shaping）获得的压缩增益。综合上述结果，本研究架起了高压缩低足型实验结果与高产额高足型实验结果之间的研究桥梁。