惯性约束核聚变的离子驱动快点火方案理论与数值模拟研究

Ion-driven fast ignition of inertial confinement fusion is considered to be a promising variant of fast ignition that could achieve a high energy gain and simplify the target fabrication for the future fusion reactor. However, there are two crucial challenges in the ion-driven fast ignition scheme: (a) how to deliver laser pulse closer to the core of pre-compressed DT target, and (b) how to efficiently generate a high-quality large-fluence fast ion beam. In this project, we propose to create a channel into the overdense target by the hole-boring effect of an intense laser pulse and then generate fast ions for ignition by the hole-boring radiation pressure acceleration using another more intense and circularly polarized laser pulse. Firstly, we intend to formulate the laser hole-boring speed in the multidimensional case and understand the evolution of accompanying instabilities by theoretical analysis and Particle-in-Cell simulations, and hence we can design a more stable and efficient hole-boring channeling scheme by optimizing the laser polarization, shaping the pulse temporal and transverse intensity profile, or employing a train of shorter pulses. Secondly, we intend to extend the technique of using temporally shaped pulses for the hole-boring radiation pressure acceleration in one-dimensional inhomogeneous plasmas to the multi-dimensional cases by appending the transverse profile shaping of the pulse, which can generate quasi-monoenergetic ion beams with the higher conversion efficiency, narrower energy spread, and smaller spatial divergence. Our research is expected to finally offer a novel scheme for realizing ion-driven fast ignition of inertial confinement fusion in the future experiments.

因为有望实现高能量增益且简化燃料靶的制造工艺，离子驱动快点火成为非常有竞争力的惯性约束核聚变解决方案之一。然而此方案存在两个关键性的挑战：一，如何使激光传输到更接近燃料靶心的高密度区域；二，如何利用激光高效地产生高质量的高能离子束。为此我们建议利用第一束强激光脉冲的钻孔效应来产生通向高密度区域的通道；接着利用第二束更强的圆偏振激光通过钻孔模式辐射压加速产生高能离子束实现点火。首先，我们将构建描述激光钻孔的多维理论模型和掌握其中各不稳定性的演化规律，并通过对激光脉冲的时间包络和横向强度分布进行整形等技术来实现产生更稳定的通道。其次，我们将把利用时域整形激光脉冲在一维非均匀等离子体中通过钻孔模式辐射压加速产生准单能离子束的方案扩展到多维情况，此方案能以较高的能量转换效率产生高质量的高能离子束。我们的研究有望为实现惯性约束核聚变提供一套新颖的离子驱动的快点火方案。

与传统加速器产生的离子束相比，超短超强激光驱动产生的离子束具有持续时间短、密度高等优点。尤其重要的是，激光钻孔模式辐射压加速还可产生大注量的离子束。然而，目前依然很难同时增强激光驱动的离子束的品质及产额，以便适用于惯性约束核聚变快点火方案等应用中。通过本项目，我们系统研究了强激光脉冲在过临界密度靶中的钻孔过程以及这过程中激光辐射压驱动的离子加速。.首先，我们研究了钻孔模式辐射压加速机制中的加速电场结构，特别是引起加速电场震荡的内在物理机制。然后我们发现通过采用含两种离子的复合靶或者双色激光脉冲等手段可以有效地抑制加速电场的震荡，从而减低所产生离子束的能散。.第二，我们推导了实现高效的钻孔模式辐射压加速的激光强度和等离子体密度匹配条件。依据此条件，我们通过激光脉冲的时域整形可在非均匀靶中实现匀速钻孔，或者通过靶的密度调制在高斯激光脉冲情况下实现匀速钻孔。.第三，我们发现了极端法拉第效应即线偏振激光脉冲在强磁化等离子体中传播时分为两个旋向性相反的圆偏振脉冲。以此可设计一些新型光学仪器，譬如磁化等离子体偏振器，它可产生数十拍瓦的超高功率圆偏振激光脉冲。这种高功率圆偏振激光脉冲在激光钻孔模式辐射压加速中尤其重要。.最后，我们对惯性约束核聚变的离子驱动快点火方案的可行性及其所需要的条件进行了分析。