Demonstration of full-scale spatio-temporal diagnostics of solid-density plasmas driven by an ultra-short relativistic laser pulse using an X-ray free-electron laser

TL;DR

This work addresses the challenge of diagnosing solid-density plasmas formed by ultra-short, relativistic laser pulses. It employs a novel pump–probe platform at European XFEL-HED to perform simultaneous SAXS, RXES, and XPCI diagnostics, capturing preplasma formation, ultrafast heating and ionization, and hydrodynamic shock/compression across nm to μm scales and fs to ps times. The key contributions are the first full-scale, time-resolved measurements of these coupled processes in a single experiment, providing data to benchmark PIC and MHD simulations and informing applications in laser-plasma accelerators and high energy-density matter. The findings reveal a preplasma formed and expanded before the main pulse, resonant X-ray emission indicating sustained high temperatures, and evolving density gradients consistent with blast shocks and return-current–driven compression, illustrating the power of integrated XFEL diagnostics for high-fidelity modeling of laser-mplasma interactions.

Abstract

Understanding the complex plasma dynamics in ultra-intense relativistic laser-solid interactions is of fundamental importance to the applications of laser plasma-based particle accelerators, creation of high energy-density matter, understanding of planetary science and laser-driven fusion energy. However, experimental efforts in this regime have been limited by the accessibility of over-critical density and spatio-temporal resolution of conventional diagnostics. Over the last decade, the advent of femtosecond brilliant hard X-ray free electron lasers (XFELs) is opening new horizons to break these limitations. Here, for the first time we present full-scale spatio-temporal measurements of solid-density plasma dynamics, including preplasma generation with tens of nanometer-scale length driven by the leading edge of a relativistic laser pulse, ultrafast heating and ionization at the main pulse arrival, laser-driven blast shock waves and transient surface return current-induced compression dynamics up to hundreds of picoseconds after interaction. These observations are enabled by utilizing a novel combination of advanced X-ray diagnostics such as small-angle X-ray scattering (SAXS), resonant X-ray emission spectroscopy (RXES), and propagation-based X-ray phase-contrast imaging (XPCI) simultaneously at the European XFEL-HED beamline station.

Figures (7)

• Figure 1: A schematic of the multi-scale spatio-temporal plasma processes induced by electron transport in solid-density plasmas irradiated by ultra-short, relativistic laser pulses, along with the corresponding X-ray diagnostics used in the pump-probe experiment. The black curve shows the realistic temporal profile of the ReLaX laser pulse ranging from -100 ps to 100 ps. The illustration includes the preplasma formation, electron filamentation, blast wave and compression, as predicted by our previous simulationsHuang2017Yang2024, is depicted along with the curve and text.
• Figure 2: A schematic illustration of the experimental setup. Both optical relativistic ReLaX laser and XFEL irradiate the Cu wire of 10 $\textrm{\,$\mathrm{\umu\mathrm{m}}$}$ diameter at normal incidence, polarized in the horizontal direction. The beams intersect at an angle of the ReLaX and X-ray beam is 45. The SAXS patterns and emission spectroscopy are recorded by the Jungfrau X-ray detectors. Two HAPG chromatic mirror with a small gap are used to reflect the SAXS signal, allowing the propagation of primary XFEL beam to an Andor Zyla CMOS camera via a 7.5$\times$ objective for simultaneous XPCI imaging. The X-ray propagation distances from the sample to the SAXS and XPCI detectors are 1.31 m and 6.31 m respectively. The entire setup except the SAXS Jungfrau detector and the last 50 cm before the Zyla detector is placed in vacuum conditions to minimize the air scattering. The SAXS/RXES and XPCI data presented in this study correspond to the small FoV and large FoV on the wires of $\sim$10 $\textrm{\,$\mathrm{\umu\mathrm{m}}$}$ and $\sim$30 $\textrm{\,$\mathrm{\umu\mathrm{m}}$}$ respectively, which are achieved by switching two sets of compound refractive lens (CRL) configurations as described in the main text.
• Figure 3: A summary of time resolved SAXS patterns (a-l) and the horizontal outlines taken through $q_y = 0$ averaging over 10 pixels (m-o) during the rising slope of the ReLaX laser from 40 ps to 1.5 ps prior to the arrival of the peak pulse intensity. The corresponding cold SAXS patterns are plotted as a reference of initial wire edge sharpness (a-f). The colorbar denotes the scattered X-ray energy normalized to the incoming XFEL energy.
• Figure 4: (a) Schematic of resonant X-ray emission stimulated by an external XFEL. (b) Optical depth of an optical transparent solid Cu plasma at different temperatures calculated by the atomic code FLYCHK over a broad photon energy and (c) over the energy region of interest in the solid density plasmaHuangThesis . Each satellite peak corresponds to a specific Cu ionic K-L bound-bound transition.
• Figure 5: Experimentally measured time-resolved X-ray emission spectra in the case of no resonant yield at $\sim$ 0 ps (a) and the case of with strong resonant yield at $\sim$ 2.5 ps (b). The shaded gray region indicates the photon energy range of the SASE XFEL beam, centered at 8.2 keV with a narrow bandwidth of $\sim$40 eV, that is in resonance with the bound-bound transition of interest $1s^22s^22p^3$ to $1s^12s^22p^4$. The white vertical lines represent the gap between two adjacent chips of the Jungfrau detector .