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

点击翻译按钮获取中文摘要

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.