Probing ultrafast heating and ionization dynamics in solid density plasmas with time-resolved resonant X-ray absorption and emission

Abstract

Heating and ionization are among the most fundamental processes in relativistic laser--solid interactions; however, their spatiotemporal evolution remains challenging to capture experimentally. Here we present detailed diagnosis of high-intensity laser interactions with wire targets, leveraging the extreme spectral brightness of an X-ray free-electron laser in sub-picosecond time-resolved resonant X-ray emission spectroscopy and absorption imaging. Experimental results are compared with comprehensive simulations using atomic collisional--radiative models, particle-in-cell, and magnetohydrodynamics codes to elucidate the underlying physics. These multi-scale simulations reveal extreme sensitivity of basic plasma parameters with widely used models, such as temperature and ionization depth, which are able to be constrained by incorporating a detailed accounting of laser spatial profiles, pre-plasma conditions, and collisional processes. These results provide new insights into heating and ionization dynamics in the high-energy-density regime relevant to inertial fusion energy research, both as an experimental platform for accessing theoretically challenging conditions and as a benchmark for improving models of high-power laser--plasma interactions.

Figures (9)

• Figure 1: Atomic simulation of plasma opacity and X-ray emission spectra.(a) Optical depth of a solid Cu plasma with 10 $\textrm{\,$\mathrm{\umu\mathrm{m}}$}$ thickness at different temperatures calculated by the atomic code SCFLY over a broad photon energy range covering bound-bound and bound-free transitions for the hot dense plasma. Each peak corresponds to a specific K-L bound-bound transition of the Cu ions. (b) Temperature dependence of Cu's optical depth at the resonant photon energy of $E_{photon} = 8.2$ keV, along with its corresponding averaged charge state. (c) Comparison of simulated X-ray emission spectra with and without XFEL irradiation at a plasma temperature of 400 eV. The light blue shaded region denotes the full spectral bandwidth of the XFEL beam centered at 8.2 keV.
• Figure 2: Experimental measurements of resonant X-ray emission yield and XFEL transmission.(a) Experimentally measured time-resolved X-ray emission spectra covering the pump-probe delay ranging from -1 ps to 10 ps for a few selected shots. The light blue shaded region denotes the full spectral bandwidth of the XFEL beam centered at 8.2 keV. (b) Temporal evolution of resonant X-ray emission yield in the region of XFEL photon energy and normalized XFEL transmission. The stochastic fluctuation in XFEL transmission is primarily caused by X-ray spatial jitter, particularly evident at negative delays. (c) Correlation of the resonant X-ray emission yield and the XFEL transmission. The error bar of the pump-probe time delays assumes the relative timing uncertainty to be 200 fs for all the hot shots. The error of the spectrum is multiplied with a factor of 5 to make it visible on the figure. The background subtraction and selection of good shots are detailed in the Methods section.
• Figure 3: Experimental measurements of off-resonance emissions with comparable yields on both sides of the XFEL photon energy.(a) Fitted spectrum in the case of strong resonance at 2.5 ps and (b) time resolved normalized line intensity of the fitted spectra at different delays. The central energy of the fitting gaussian functions except the resonant photon energy of XFEL are fixed to be 8.027 keV, 8.047 keV, 8.055 keV, 8.08 keV, 8.12 keV, 8.16 keV, $\sim$8.2 keV, 8.255 keV, 8.3 keV, 8.322 keV, 8.335 keV, 8.352 keV, 8.372 keV and 8.395 keV, labeled with line indices from 0 to 13, respectively. Each index corresponds to a specific bound-bound transition that emits at a characteristic energy.
• Figure 4: Hybrid kinetic and MHD simulations of plasma heating and ionization dynamics. Spatial distribution of electron temperature and ion charge state obtained from 2D PIC simulations in the case of top view of a Cu wire with 10 $\textrm{\,$\mathrm{\umu\mathrm{m}}$}$ thickness, using (a) the LTE based Thomas-Fermi and (b) the NLTE based direct impact ionization models, both at 100 fs after peak laser intensity irradiation. Corresponding 2D PIC and MHD simulations for the side view of the Cu wire using the NLTE ionization model are shown at (c) 100 fs and (d) 2.5 ps, respectively. Longitudinal lineouts of the plasma temperature (e) and average ion charge state (f) of the corresponding 2D simulation maps, extracted from the initial target region indicated by the light gree area. The light violet regions indicate the probed resonant charge sate and its corresponding evaluated plasma temperature. The parameters of the PIC simulation are described in the Methods section.
• Figure 5: Experimental measurements of ReLaX laser spatial profile.(a) A typical example of the spatial intensity profile of ReLaX laser beam. Panels (b) and (c) show the shot-to-shot statistical fluctuations of the FWHM size and the energy fraction within the central focus, respectively. The standard deviation is denoted by “std”.