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

第四代光源-X射线自由电子激光器（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，通过提供从XUV到硬X射线光谱区域的可调光子能量的超亮，飞秒，近单色X射线。当聚焦到微米大小的斑点时，首次可以在X射线波长处产生超过10^17 W/cm^3的强度。我们最近表明，如此高的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，温度和密度条件类似于太阳中心的一半，从而为天体物理学，行星科学，惯性约束聚变研究和国家安全应用广泛感兴趣的极端物质状态的新研究铺平了道路。除了产生热密度等离子体，与物质的强烈X射线相互作用产生了良好控制的非热（热）电子源，或者通过内壳层原子复合过程如俄歇衰变。由于有质动力能量在X射线波长下可以忽略不计，因此这些是照射期间产生的唯一“热”电子，导致可以通过调制X射线波长和强度直接控制的非热电子分布，在这个项目中，我们的目标是利用X射线的这些独特特性，射线自由电子激光脉冲，实验上创建一个定制的非热电子分布内的热密度等离子体，并跟踪其演变和超快时间尺度上的平衡动态。这些测量不仅将提供强耦合系统中电子-电子碰撞的一些第一次测量，而且还将更广泛地评估通常用于模拟各种电子相互作用过程的库仑对数框架的有效性，包括韧致辐射发射，电导率，热输运和阻止本领。我们注意到，用强X射线光照射固体样品使我们能够达到与太阳的辐射/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，我们的目标是研究辐射传输和低和中Z元素在这些极端条件下的不透明度，并确定不透明度是否可以帮助解决太阳模型和太阳内部结构之间的突出分歧确定的太阳地震观测。考虑到Bailey等人（Nature 517，56，2015）的最新实验结果，在该区域中对Fe不透明度的精确独立测量特别令人感兴趣，该结果显示实验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不透明度存在显著偏差。

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.