Investigating electron dynamics and radiation transport in solid-density plasmas using X-ray FELs

第四代光源--X射线自由电子激光(FEL)的出现正在彻底改变我们在极端条件下研究物质的方式，它提供了从XUV到硬X射线光谱区域可调光子能量的超亮、飞秒、几乎单色的X射线。当聚焦到微米尺寸的光斑时，在X射线波长上可以第一次产生超过10^17 W/cm^3的强度。我们最近发现，这样的高强度足以在几十飞秒内将固体系统加热到几百万开尔文的温度，即类似于太阳中心中途发现的温度和密度条件，从而为天体物理、行星科学、惯性约束聚变研究和国家安全应用中广泛关注的物质的极端状态的新颖研究铺平了道路。除了产生热密度等离子体外，强烈的X射线与物质的相互作用产生了受控良好的非热(热)电子来源，这些电子要么直接由光致电离产生，要么通过内壳原子复合过程产生，如俄歇衰变。由于在X射线波长下有质动能可以忽略不计，这些是唯一在辐照过程中产生的热电子，导致非热电子分布可以通过调制X射线波长和强度来直接控制，并且在理论上也比在强光学激光-等离子体相互作用中产生的热电子简单得多。在这个项目中，我们的目标是利用X射线自由电子激光脉冲的这些独特特性来实验地在热密度等离子体中创建定制的非热电子分布，并在超快时间尺度上跟踪其演化和平衡动力学。这些测量不仅将提供一些强耦合系统中电子-电子碰撞的第一次测量，而且还将更广泛地评估库仑对数框架的有效性，该框架通常用于模拟广泛的电子相互作用过程，包括韧致辐射、电导率、热输运和阻止力。重要的是，我们注意到，用强X射线照射固体样品使我们能够达到与太阳辐射/对流区边界相对应的温度-密度条件。通过使用我们最近开发的光谱技术，我们的目标是研究这些极端条件下低Z元素和中Z元素的辐射传输和不透明度，并确定不透明度是否有助于解决太阳模型和太阳内部结构之间的突出分歧，这些不一致是由太阳地震观测确定的。考虑到Bailey等人最近的实验结果，对这一区域中Fe不透明度的准确独立测量尤其令人感兴趣。(《自然》杂志，517，56,2015)，表明实验中的不透明度与低密度等离子体的等离子体不透明度模型预测的结果有很大偏差。

The advent of 4th generation light sources - X-ray free-electron lasers (FELs) - is revolutionising the way we investigate matter in extreme conditions by providing ultra-bright, femtosecond, nearly monochromatic X-rays at tuneable photon energies from the XUV to the hard X-ray spectral region. When focused to micron-sized spots, intensities exceeding 10^17 W/cm^3 can be generated at X-ray wavelengths for the first time. We showed recently that such high intensities are sufficient to heat solid systems to temperatures of several million Kelvin within a few tens of femtoseconds, i.e., to temperature and density conditions similar to those found half way into the centre of the Sun, thus paving the way to novel investigations of extreme states of matter of broad interest to astrophysics, planetary science, inertial confinement fusion research, and national security applications.Alongside generating hot-dense plasmas, intense X-ray interactions with matter give rise to a well-controlled source of non-thermal (hot) electrons which are generated either directly by photoionization, or via inner-shell atomic recombination processes such as Auger decay. Because ponderomotive energies are negligible at X-ray wavelengths, these are the only 'hot' electrons generated during the irradiation, leading to a non-thermal electron distribution that can be controlled directly by modulating the X-ray wavelength and intensity, and is also far simpler to model theoretically than hot electrons produced in intense optical laser-plasma interactions.In this project we aim to use these unique characteristics of X-ray FEL pulses to experimentally create a tailored non-thermal electron distribution within a hot-dense plasma, and track its evolution and equilibration dynamics on ultra-fast timescales. These measurements will not only provide some of the first measurements of electron-electron collisionality in strongly-coupled systems, but will also more broadly assess the validity of the Coulomb Logarithm framework commonly used to model a wide range of electron interaction processes, including bremsstrahlung emission, conductivity, thermal transport and stopping power.Importantly, we note that the irradiation of solid samples with intense X-ray light allows us to reach the temperature-density conditions corresponding to the radiation/convection zone boundary of the Sun. By using our recently developed spectroscopic techniques we aim to investigate radiation transport and the opacity of low and mid-Z elements in these extreme conditions, and determine whether the opacity can help address the outstanding disagreement between solar models and the internal structure of the Sun determined by helio-seismic observations. Accurate independent measurements of the Fe opacity in this regime are particularly of interest given the recent experimental results from Bailey et al. (Nature 517, 56, 2015), showing a significant deviation in the experimental opacity from that predicted by plasma opacity models for lower density plasmas.