Abstract <p>Results of the particle-in-cell simulation that illustrate evolution of parameters of laser-produced plasma formed upon irradiation of steel foils with a thickness of 1–5 µm by femtosecond laser pulses with intensity of ≥5&#xa0;× 10²¹ W/cm² are presented. Analytical estimates for analysis of energy dissipation in a foil of&#xa0;finite thickness are obtained. Numerical simulations are compared with the results of the recent experiment in which diagnostics of plasma parameters was carried out by methods of X-ray spectroscopy. The results of simulation agree with the experimental results and confirm that a microscopic-sized plasma source with energy density exceeding 1 GJ/cm³ and lifetime of about 500 fs can be formed as a result of action of high-contrast femtosecond laser pulses of ultrarelativistic intensity. In addition, simulations demonstrate that a plasma source with a volume of ~1 µm³ and lifetime of ~5 light periods that has parameters close to those existing inside the Sun, i.e., the temperature of ~1–3 keV and energy density of ≥10 GJ/cm³ (pressure of ≥100&#xa0;Gbar), can be formed under the same parameters of the experiment.</p>