Direct measurements of electron collisional dynamics in a solid density plasma

Ultrafast collisional dynamics is the dominant driver of energy transport, conductivity, diffusion, and relaxation in dense collisional plasmas. Understanding such processes at the microscopic level remains a key challenge for strongly-coupled Coulomb systems, and is motivated by the ubiquitous nature of dense plasmas in astrophysical environments, by their growing importance to industrial laser-plasma applications such as additive manufacturing and extreme ultraviolet lithography, and because of their fundamental role in inertial fusion energy. Although significant theoretical and computational advances have been made over past decades, experimental measurements remain scarce. The limited experimental data available primarily relies on extracting and interpreting macroscopic properties, such as conductivity and heat flux, rather than investigating electron collisional interactions directly. In this talk I report direct measurements of ultrafast collisional relaxation of non-thermal electrons in Mn plasmas at electron densities exceeding $10^{23}$~cm$^{-3}$ and temperatures reaching 2$\times 10^5$~K. Our method bypasses transport-based assumptions in probing ultrafast relaxation and allows us to extract both electron impact ionization and electron-electron collision rates. The measurements are able to discriminate between the most commonly used impact ionization models, and set an upper limit on the electron-electron relaxation rate of $\sim$40~fs$^{-1}$. Our approach is readily extendable to a broader range of plasma conditions, and provides a novel opportunity to explore ultrafast collisional dynamics in matter far from equilibrium {\it in situ} via x-ray emission spectroscopy.