超强激光作用下纳米丝靶中自生磁场的产生机制研究

相对论电子束由超强激光辐照纳米微结构靶驱动,其准直性好、转化效率高,在诸多领域均有广泛的应用前景,例如超强激光驱动粒子源、高亮度和超快X 射线源、癌症治疗、惯性约束聚变快点火等。

目前实验室所用的纳米微结构靶包括纳米丝靶、纳米结构的天鹅绒靶、多孔靶、纳米管靶等。而在众多微结构靶中,纳米丝靶的表现尤为突出,超强激光与纳米丝靶相互作用可以有效地提高激光能量吸收率,纳米丝靶内部可产生MA(106 A)量级的相对论电子束流,在纳米丝靶内部输运时还会产生极强的自生磁场(100 MG),该自生磁场的箍缩作用非常有利于超热电子的加速和准直。但令人遗憾的是,一直缺乏自洽的理论来细致描述激光纳米丝靶相互作用过程中自生磁场的产生机理。

北京应用物理与计算数学研究所的田建民博士等在High Power Laser Science and Engineering 2020年第2期发表的文章(J. M. Tian, H. B. Cai, W. S. Zhang, et al. Generation mechanism of 100 MG magnetic fields in the interaction of ultra-intense laser pulse with nanostructured target[J]. High Power Laser Science and Engineering, 2020, 8(2): 02000e16)中基于电子磁流体方程组建立了超强激光与纳米丝靶相互作用过程中自生磁场强度和分布的理论模型。

理论模型表明,自生磁场的分布和强度主要由纳米丝靶中超热电子束流的强度和等离子体密度梯度决定,而靶材料对纳米丝靶中自生磁场的影响较小。基于理论模型给出的自生磁场与粒子模拟结果相吻合,模型的准确性得到了验证。该研究对认识纳米丝靶结构在改善强激光与靶相互作用产生高能电子束和离子束品质中的作用有重要意义。

纳米丝靶结构示意图

Generation mechanism of 100 MG magnetic fields in the interaction of ultra-intense laser pulse with nanostructured target

The interaction of relativistic laser pulses with nanostructured targets has stimulated considerable interest because of its practical applications in laser-driven particle acceleration, high-brightness ultra-fast hard X-ray, cancer treatment, fast ignition in inertial confinement fusion, etc. Some experimental and simulation results indicate that the interaction of the intense laser pulse with a nanostructured target can significantly increase the production of the high-quality fast electrons and improve the laser energy absorption. The structured targets include nanowire targets, nanostructured "velvet" targets, multihole targets, and nanotubes. Among them, the performance of the nanowire target is very prominent. The nanowire arrays can greatly improve the laser energy absorption and the generation of mega-ampere relativistic electron beams. Experiments and simulations have shown that very strong magnetic fields (about 100 MG) are produced within the nanowire arrays. It is worth noting that the self-generated magnetic field plays an important role in both the production and the transport of the fast electrons. Therefore, it is of great significance to study the generation mechanism of the self-generated magnetic field produced in the nanowire target when a ultra-intense laser pulse interacts with a nanowire target.

The research group led by Prof. Hongbo Cai from the Institute of Applied Physics and Computational Mathematics has established a fully relativistic analytical model of the intensity and distribution of the self-generated magnetic field in the nanolayered target (two dimensional) based on the electron magneto-hydrodynamic description. The research results are published in High Power Laser Science and Engineering, Vol. 8, Issue 2, 2020 (J. M. Tian, H. B. Cai, W. S. Zhang, et al. Generation mechanism of 100 MG magnetic fields in the interaction of ultra-intense laser pulse with nanostructured target[J]. High Power Laser Science and Engineering, 2020, 8(2): 02000e16). The analytical model shows that the self-generated magnetic field originates in the nonparallel density gradient and fast electron current at the interfaces of nanolayered target. And the material has a small effect on the generation of the self-generated magnetic field in the nanolayered target. The 2D-PIC simulation results are in good agreement with the theoretical analysis. This research is of great significance for understanding the role of the structure of the nanolayered target in improving the quality of the energetic electrons and ions accelerated by the ultra-intense laser pulse in the nanolayered target.

Schematic diagram of the nanolayered target