Ionization dynamics of dense matter generated by intense ultrashort X‐ray pulses

Abstract

We investigate a pump‐probe X‐ray Thomson scattering (XRTS) experiment that might be carried out at a free electron laser facility to study warm‐to‐hot states of dense matter. Ultrashort and intense X‐ray pulses with different energies (1,560–1,830 eV) heat a 1 µm thick Al target isochorically and create homogeneous and uncompressed warm‐to‐hot states of dense matter. A second pulse with variable delay probes this heated state via XRTS. The X‐ray laser–target interaction is modelled within radiation‐hydrodynamic simulations applying the HELIOS‐CR code. The HELIOS‐CR results qualitatively agree with Monte‐Carlo simulations, where the laser pulse absorption is simulated based on a uniform random sequence of events. The electron feature in the simultaneously observed X‐ray scattering spectrum is a function of the degree of ionization and the target temperature. Therefore, the temporal evolution of the plasmon peak measures the ionization dynamics on ultra‐short time scales. The XRTS spectrum is calculated based on the Chihara formula utilizing the Born‐Mermin approximation for the free electron dynamic structure factor. The proposed experiment will reveal important details of the ionization dynamics on ultra‐short time scales as well as of the relaxation on ps time scales.

Ab initio simulation of warm dense matter

“…Theoretical methods for WDM out of equilibrium are even more challenging than equilibrium applications that were discussed above. They include time-dependent DFT [69,219], semiclassical kinetics [78], quantum kinetic theory and nonequilibrium Green functions [44,83,220,221], hydrodynamics [80,88,222] or rate equations [223]. Among the problems that were studied are the equilibration of the electron distribution by electron-electron collisions [83,220], non-thermal melting induced by fs xray pulses [219], the density response for nonequilibrium momentum distributions [79,221], density evolution following short-pulse laser excitation [78], collisional heating of quantum plasmas by a laser pulse [224,225], or ionization dynamics in a short laser pulse [80].…”

“…Time-dependent x-ray Thomson scattering was modelled in Refs. [79,80]. Here, a powerful method are quantum kinetic equations [81,82] and nonequilibrium Green functions, e.g.…”