In contrast to ion beams produced by conventional accelerators, ion beams accelerated by ultrashort intense laser pulses have advantages of ultrashort bunch duration and ultrahigh density, which are achieved in compact size. However, it is still challenging to simultaneously enhance their quality and yield for practical applications such as fast ion ignition of inertial confinement fusion. Compared with other mechanisms of laser-driven ion acceleration, the hole-boring radiation pressure acceleration has a special advantage in generating high-fluence ion beams suitable for the creation of high energy density state of matters. In this paper, we present a review on some theoretical and numerical studies of the hole-boring radiation pressure acceleration. First we discuss the typical field structure associated with this mechanism, its intrinsic feature of oscillations, and the underling physics. Then we will review some recently proposed schemes to enhance the beam quality and the efficiency in the hole-boring radiation pressure acceleration, such as matching laser intensity profile with target density profile, and using two-ion-species targets. Based on this, we propose an integrated scheme for efficient high-quality hole-boring radiation pressure acceleration, in which the longitudinal density profile of a composite target as well as the laser transverse intensity profile are tailored according to the matching condition.

Proton generation, transport and interaction with hollow cone targets are investigated by means of two-dimensional PIC simulations. A scaled-down hollow cone with gold walls, a carbon tip and a curved hydrogen foil inside the cone has been considered. Proton acceleration is driven by a 10²⁰ W?cm² and 1 ps laser pulse focused on the hydrogen foil. Simulations show an important surface current at the cone walls which generates a magnetic field. This magnetic field is dragged by the quasi-neutral plasma formed by fast protons and co-moving electrons when they propagate towards the cone tip. As a result, a tens of kT B_z field is set up at the cone tip, which is strong enough to deflect the protons and increase the beam divergence substantially. We propose using heavy materials at the cone tip and increasing the laser intensity in order to mitigate magnetic field generation and proton beam divergence.

在神光Ⅱ升级装置上完成了国际上首次间接驱动快点火集成实验。实验采用双台阶脉冲整形激光注入黑腔产生X射线准等熵压缩锥壳靶,实现了高密度压缩,然后采用皮秒超短脉冲激光注入加热燃料。实验中观测到中子产额由皮秒激光注入前的5×103增加到2.2×105,中子产额增益达到44倍,实验证实了皮秒激光具有明显燃料加热效果。该实验为进一步开展快点火热斑形成效率和相关物理研究奠定了基础。

研究了基于间接驱动设计的神光Ⅲ原型装置在直接驱动方式下提供的辐照均匀程度,在考虑激光瞄准偏差以及激光束间能量偏差的情况下,通过优化激光光斑内部光强分布实现了球靶表面较高程度的均匀辐照。研究表明光强均方根随激光偏差的减小而降低。在优化结果基础上,分析了优化光强与实际光强的联系,指出可通过同步放缩具有长度量纲的量调整靶球表面平均光强,为后续靶设计提供了依据。最后,研究了优化结果在直接驱动快点火技术途径下的应用,分析了导引锥对激光传输的影响,表明当导引锥位于靶球两极方向,并且半锥角小于30°时,导引锥对锥外部靶球表面的光强分布影响较小,优化结果亦可应用于直接驱动快点火。

为解决快点火分解物理研究中锥丝靶的整体成型问题，基于超精密车削技术，提出了制备无缝、无胶锥丝靶的方法。通过建立的Au丝超精密切削成型过程中的微切削力模型，研究了相关工艺条件下微切削力对锥丝靶成型的影响。采用有限元分析方法和实验验证手段研究多种参数Au丝的切削变形问题，获得了微切削力条件下Au丝的变形规律。研究结果表明:微切削力对丝径 10 μm附近锥丝靶的成型影响较大，对丝径更大的锥丝靶则无明显影响。

提出了一个快点火靶新型概念和结构分析, 在锥的尖端制作一个热斑大小的平台, 以避免燃料从锥方向逃逸; 在平台中间制作一个微纳米点, 以约束热电子发射。或者在平台上制作微纳米点阵列, 以抬高阿尔文效应限制的条件。考虑超热电子发射的静电势、材料的逸出功, 给出了热电子发射能量公式, 根据公式提出了关于快点火靶在材料和结构方面的优化条件和研究内容。比如, 提高超热电子发射的静电势, 和发射材料的选择等, 以优化超热电子的出射能量和方向。

与中心点火相比, 快点火将压缩和点火过程分开, 大大放宽了对压缩对称性和驱动能量的要求。通过在神光Ⅱ激光装置上开展了快点火锥壳靶预压缩实验研究, 利用X射线背光分幅照相方法观察到了清晰完整的快点火锥壳靶内爆压缩过程, 并利用阿贝反演结合剩余烧蚀质量的方法得到了不同时刻燃料密度、面密度分布数据, 当前实验条件下获得的最大压缩密度和面密度分别为30 g/cm3和50 mg/cm2; 为解决金柱腔M带对导引锥的预热以及由此导致的燃料-锥体材料混合问题, 提出了一种在锥体表面镀低Z材料的方法, 实验和辐射流体数值模拟结果验证了该方法的有效性, 该方法的成功解决了间接驱动快点火激光聚变的重要关键技术问题。

针对相对论快电子束在高密度压缩芯区等离子体中的能量沉积过程开展物理建模、程序研制和数值模拟研究。从等离子体粒子碰撞的基本物理出发, 综合考虑了高能电子与背景等离子体之间的短程两体碰撞过程和长程集体效应, 建立了相对论Fokker-Planck动理学模型, 通过采用球谐展开的方法, 推导得到了适于数值求解的方程形式并根据方程特点开展相应的数值算法研究及程序研制并完成了物理考核, 对快点火能量沉积的典型物理算例进行了模拟研究, 并针对即将在神光Ⅱ升级装置上开展的快点火物理实验进行了初步的物理分析。