Fig. 1: Fast electron isochoric heating mechanisms in a solid foil and experimental layout.

a Schematic illustration of a high-intensity, short-pulse laser interacting with a thin, solid-density foil. Energetic electrons generated in a preplasma traverse the foil due to sheath potentials, leading to the creation of two distinct regions attributed to different heating and ionization mechanisms: a high-temperature, solid-density area in Region 1 by Joule and diffusive heating and thermal and impact ionization, and Fermi degenerate warm dense matter in Region 2 by Joule heating and electron impact ionization. b Layout of the experimental setup featuring a high-intensity femtosecond (fs) laser and an XFEL pulse. The imaging detector positioned outside the vacuum chamber, ~4.1 m away from the target. Examples of X-ray transmission images recorded (c) before and (d) after laser irradiation with a delay of 0.35 ps. e A ratio of these images provides a raw transmission ratio. An averaged horizontal lineout of the enhanced transmission area is superposed in (e).