超强激光驱动的相对论电子束在绝缘材料中的传输研究

超强激光与等离子体相互作用中驱动的强流相对论电子束在很多高能量密度物理领域均有应用,特别是在惯性约束聚变“快点火”、激光驱动离子加速和超短脉冲源等方面。相对论电子束在靶中传输时通过场电离和碰撞电离将靶电离,同时会诱发非线性和集体效应而影响到电子束的传输。深入认识相对论电子束在靶中的传输行为,尤其是在绝缘体中的传输特性,对上述应用尤为重要。

相对论电子束穿入靶中时会被电荷分离场抑制直至回流冷电子或自由电子的出现中和电荷分离场。一维理论分析表明,强流相对论电子束在绝缘体中传输时,电离包括四个阶段:电荷积累、场电离、电流和电荷中和、碰撞电离。实验中观察到相对论电子束在绝缘体传输时,会在靶中形成速度小于光速的电离波传输。然而,之前的工作主要局限于一维的理论分析和数值模拟,为了能够更全面认识相对论电子束传输引起的电离过程,需要开展细致的二维研究。

国防科技大学的杨晓虎博士课题组在High Power Laser Science and Engineering 2020年第8卷第1期 (X. H. Yang, C. Ren, H. Xu, Y. Y. Ma, F. Q. Shao. Transport of ultraintense laser-driven relativistic electrons in dielectric targets[J]. High Power Laser Science and Engineering, 2020, 8(1): 010000e2)报道的工作中,利用二维粒子模拟方法揭示了超强激光驱动的相对论电子束在硅靶中的传输特征,在研究中同时考虑了场致电离和碰撞电离两种机制。研究表明,在硅靶中除了存在很强的鞘层场(强度接近场致电离的阈值)传输外,由于自由电子和离子的集体效应,在鞘层场前面还出现了分布较为广泛的“喷泉”场,该场远小于电离阈值,不会引起电离。电离波的传输速度随着激光强度的增加而增加,但小于光速和一维理论分析的结果,表明多维模拟在更好描述相对论电子束在绝缘体的传输行为的必要性。此外,随着相对论电子束电流强度的增加,在电离波前沿的后面出现了双流不稳定性,将进一步影响电子束的传输。

国防科技大学的马燕云教授相信,研究结果将对激光驱动的相对论电子束在绝缘材料中传输的相关应用如“快点火”和离子加速具有重要借鉴意义。

(a)靶的平均电离度、(b)电子密度(log10)、(c)纵向静电场(d)和横向静电场 。电子密度、磁场和电场的单位分别 是nc、T和 V/m。

Transport of ultraintense laser-driven relativistic electrons in dielectric targets

The transport of high-current relativistic electron beams driven by ultraintense laser interactions with plasmas is relevant to many applications of high energy density physics, particularly in areas of the fast ignition scheme for inertial confinement fusion, laser-driven ion acceleration and production of ultrashort radiation sources. A target can be ionized by relativistic electrons both through field ionization and collisional ionization, inducing nonlinear and collective effects that can feed back to the transport of relativistic electrons. It is important to comprehensively investigate the transport process of relativistic electrons in such a target, especially in insulators that are without free electrons initially.

In such targets, the transport of relativistic electrons can be inhibited by charge separation fields until a cold return current is generated or the electrons from ionization neutralize space charge fields. One-dimensional theoretical analysis of high-current relativistic electron beam interactions with the insulator showed that the ionization process can be separated into four regions, i.e., a charge accumulation region, a field ionization region, a current and charge neutralization region, and a collisional ionization region. It is observed that an ionization wave propagates with a velocity much slower than the speed of the light during ultraintense laser interactions with dielectric targets. However, the previous studies were mainly limited to 1D analysis or numerical simulations. To comprehensively understand the ionization process in the targets, detailed 2D simulations are required.

The work presented by Dr. Xiaohu Yang from National University of Defense Technology in High Power Laser Science and Engineering, Volume 8, Issue 1, 2020 (X. H. Yang, C. Ren, H. Xu, Y. Y. Ma, F. Q. Shao. Transport of ultraintense laser-driven relativistic electrons in dielectric targets[J]. High Power Laser Science and Engineering, 2020, 8(1): 010000e2) shows ultraintense laser-driven relativistic electron transport in a dielectric silicon target using particle-in-cell simulations including the field and collisional ionization processes. In addition to the intense sheath fields (close to the threshold electric field of field ionization), a widely spread ‘fountain’ electric field occurs ahead due to the collective effect of free electrons and ions, whose magnitude is much less than that of the former and cannot induce ionization. The velocity of the ionization wave increases with laser intensity but is much less than the speed of light and also that from the 1D theoretical analysis, indicating that 2D3V numerical simulations are better to describe the relativistic electron transport in dielectric targets. Two-stream instability (TSI) behind the ionization front arises when the relativistic electron current is sufficiently high.

Prof. Yanyun Ma from National University of Defense Technology believes that the results are helpful for applications related to laser-driven relativistic electron transport in dielectric targets such as fast ignition and laser-driven ion acceleration.

(a) Distributions of average ionization degree, (b) log10 of electron density, (c) the longitudinal electrostatic field, and (d) the transverse electrostatic field, respectively. The electron density is in units of nc. The magnetic field and electric field are in units of T and V/m, respectively.