National direct-drive program on OMEGA and the National Ignition Facility

A major advantage of the laser direct-drive (DD) approach to ignition is the increased fraction of laser drive energy coupled to the hot spot and relaxed hot-spot requirements for the peak pressure and convergence ratios relative to the indirect-drive approach at equivalent laser energy. With the goal of a successful ignition demonstration using DD, the recently established national strategy has several elements and involves multiple national and international institutions. These elements include the experimental demonstration on OMEGA cryogenic implosions of hot-spot conditions relevant for ignition at MJ-scale energies available at the National Ignition Facility (NIF) and developing an understanding of laser-plasma interactions and laser coupling using DD experiments on the NIF. DD designs require reaching central stagnation pressures in excess of 100 Gbar. The current experiments on OMEGA have achieved inferred peak pressures of 56 Gbar (Regan et al 2016 Phys. Rev. Lett. 117 025001). Extensive analysis of the cryogenic target experiments and two- and three-dimensional simulations suggest that power balance, target offset, and target quality are the main limiting factors in target performance. In addition, cross-beam energy transfer (CBET) has been identified as the main mechanism reducing laser coupling. Reaching the goal of demonstrating hydrodynamic equivalence on OMEGA includes improving laser power balance, target position, and target quality at shot time. CBET must also be significantly reduced and several strategies have been identified to address this issue.激光直接驱动（DD）点火方案的一大核心优势在于，相较于等效激光能量下的间接驱动方案，其耦合至热斑的激光驱动能量占比更高，且对热斑的峰值压强与收敛比要求更为宽松。为实现基于DD方案的点火演示成功达成，近期确立的国家战略包含多项核心内容，涉及多家国内外科研机构的协同参与。这些内容包括：在国家点火装置（NIF）可提供的兆焦级激光能量条件下，于OMEGA装置上开展与点火相关的热斑条件低温内爆实验演示；并依托NIF上的DD实验深化对激光等离子体相互作用与激光耦合效率的认知。DD方案的点火设计要求靶芯中心停滞压强需突破100吉巴。当前OMEGA装置上的实验已实现56吉巴的推断峰值压强（Regan等人，2016年，《物理评论快报》117卷，025001号）。针对低温靶实验的详尽分析以及二维、三维模拟结果显示，激光功率平衡、靶偏移与靶品质是制约靶性能的主要因素。此外，交叉束能量转移（CBET）被认定为降低激光耦合效率的核心机制。要实现OMEGA装置上流体动力学等效性验证的目标，需在打靶时刻优化激光功率平衡、靶位置与靶品质。同时必须大幅抑制CBET效应，目前已提出多项可解决该问题的策略。