温稠密物质的实验研究方法

随着新型高功率激光器和加速器的出现,近代物理学已经能对物质的极端状态进行研究,而这种状态通常仅存在于宇宙或地核深处。其中一种极端状态被称为温稠密物质(WDM),这是一种达到0.1~100 eV的中高温状态,而它的密度相当于具有完全或部分简并电子的强耦合等离子体。这也是WDM在理论上令人难以理解的一个主要原因。

WDM通常仅仅在实验室或星际中大于1 Mbar的压力下才会存在,例如棕矮星、古老恒星的外壳、白矮星等天体中;或在能产生高压的天文现象中也会出现,如超新星爆炸、天体碰撞和天体喷射等。研究WDM的结构、热力学状态、状态方程(EOS)和传输特性已成为在实验室中研究天体物理和惯性约束聚变(ICF)的重要课题之一,而惯性约束聚变中内爆靶丸的点火过程将经历WDM状态。

发表在High Power Laser Science and Engineering 2018年第6卷第4期的一篇综述(Katerina Falk, Experimental methods for warm dense matter research)介绍了近期令人振奋的实验结果,包括行星幔内不同物质的相位分离,行星内核中或由于小行星撞击产生的极高压力下导致的元素相变等。

该综述还简要概述了WDM结构的主要理论研究工作;全面介绍了研究温稠密物质的实验方法,包括如何利用不同实验设备产生这些状态的各种方法以及所使用的诊断方法;重点介绍了近二十年来利用高功率激光器和自由电子X射线激光器实现高压缩态的新方法,并讨论了使用金刚石压砧实现高压缩态的传统方法。

需要特别指出的是,短脉冲光学和X射线激光脉冲的进步为实验室天体物理学带来了的全新革命。基于这些激光光源,近期已发展出许多研究WDM的新诊断方法。研究人员使用新装置来生成亚皮秒激光脉冲,并且首次进行了超快非平衡动力学的研究。

近年来涌现出许多对天体物理学有直接影响的重大发现,例如在冰巨行星内部才能达到的压力下形成金刚石,在木星发电机类似的条件下形成金属氢,在小行星撞击时产生的极高压力下形成六方金刚石等。这是一篇相对简要且通俗易懂的综述,也是第一篇全面介绍WDM研究相关实验技术的综述,希望能够为学生和同行提供一些有益的借鉴。

在12 eV温度下,等温加热的温稠密物质的分子动力学模拟图。图中显示了原子核(绿色球体)的位置以及电子密度从核心电子到价电子的若干等值面。

Experimental Methods for Warm Dense Matter Research

With the dawn of new high-power laser and accelerator facilities, modern physics was able to reach extreme states of matter normally found only in the universe or deep inside the core of our planet. One of those extreme regimes is referred to as warm dense matter (WDM), which in fact is a type of state reaching moderately high temperatures ranging from 0.1 to 100 eV, and solid densities, which mostly corresponds to strongly coupled plasmas with fully or partially degenerate electron species. This is also a primary reason why WDM is poorly understood by theory.

Often, WDM exists at high pressures reaching above 1 M bar both in a laboratory as well as in astrophysical objects. WDM is common in astrophysical bodies such as brown dwarfs, crusts of old stars, white dwarf stars and high-pressure phenomena such as supernova explosions, collisions of celestial bodies and astrophysical jets. The study of structure, thermodynamic state, equation of state (EOS) and transport properties of WDM has become one of the key aspects of laboratory astrophysicsh as well as inertial confinement fusion (ICF), where the imploding capsule goes through the WDM regime on its way to ignition.

A review article published in High Power Laser Science and Engineering, Volume 6, Issue 4, 2018(Katerina Falk, Experimental methods for warm dense matter research) introduced some of the key research topics including phase separation of species within planetary mantles and phase transitions in elements under extreme pressures inside planetary cores or during asteroid impacts with examples of the most exciting recent experimental results.

The review article makes brief overview of major theoretical efforts to study the structure of WDM. A comprehensive introduction to the experimental methods in WDM research, including various types of generation of these states at different laboratory facilities as well as the diagnostic methods used, was provided. The primarily emphasized the novel methods to reach highly compressed states using high power lasers and free electron X-ray lasers that have generated a rapid development in this field over the past two decades, and also discussed for completeness.

Especially, the development of short-pulse optical and X-ray laser pulses meant a true revolution for laboratory astrophysics. Many new diagnostic methods based on these light sources have recently been developed to study WDM in its full complexity. Ultrafast nonequilibrium dynamics has been accessed for the first time thanks to sub-picosecond laser pulses achieved at new facilities.

Recent years saw a number of major discoveries with direct implications to astrophysics such as the formation of diamond at pressures relevant to interiors of frozen giant planets, metallic hydrogen under conditions such as those found inside Jupiter’s dynamo or formation of lonsdaleite crystals under extreme pressures during asteroid impacts. This article is the first and yet still relatively brief and approachable review that tackles all of the experimental techniques developed for experimental study of WDM and should serve as a good introduction to the field for students or experienced researchers interested to broaden their scope.

A snapshot of a DFT-MD simulation of isochoricaly heated warm dense beryllium at a temperature of 12 eV. Shown are the position of the nuclei (green spheres) and several isosurfaces of the electronic density ranging from core electrons to valence electrons.