Femtosecond dynamics of relativistic electron heating in a high-intensity laser-produced solid-density plasma

Relativistic electrons generated by a high-intensity short-pulse laser have been studied as a source for creating high-energy-density matter.[1] The transport of the electrons rapidly heats and ionizes a thin metal foil to warm dense matter (WDM) before it hydrodynamically expands. The underlying physics of the relativistic electron isochoric heating has been experimentally confirmed with time-integrated monochromatic x-ray imaging.[2] However, diagnosing transient material conditions has been limited in spatiotemporal resolutions. The recent advent of a high-intensity laser combined with an x-ray free electron laser (XFEL) has enabled ultrafast pump-probe experiments to investigate the interior conditions of solid and high-density matter. Here, we capture femtosecond dynamics of plasma formation driven by relativistic electrons in solid metal for the first time. We demonstrate a diagnostic with femtoseconds and micron-scale resolution using SACLA XFEL pulses and visualize the propagation of the ionization front in a solid copper as a signature of the plasma creation.[3] The novel x-ray transmission imaging with x-ray wavelengths tuned to near the Cu K-edge provides information on the target’s temperatures and ionization states from a smeared K-edge profile and the evolution of the electron-impacted area. Our result reveals that the electron-driven ionization wave produces strongly coupled Fermi degenerate matter. Information on the non-equilibrium WDM could be used to validate quantum molecular dynamics and plasma atomic physics calculations, such as ionization potential depression.

[1] Opportunities in Intense Ultrafast Lasers: Reaching for the Brightest Light. Washington, DC: The National Academies Press (2018)

[2] H. Sawada et al., Phys. Rev. Lett. 122, 155002 (2019).

[3] H. Sawada et al., manuscript is in preparation (2022).