Thermodynamic properties of a nonideal helium plasma at quasi-isentropic compression by a factor of 575 at a pressure of 3000 GPa

The quasi-isentropic compressibility of a nonideal helium plasma has been measured in a two-cascade spherical chamber with separated cavities with the use of an explosive charge with the mass of $\approx55$ kg. The experiment has been performed on an X-ray diffraction complex consisting of three betatrons and a multichannel optoelectronic system for recording X-ray images. The density of the compressed helium plasma measured at the initial pressures of the gas in the outer and inner cavities $P_1= 36.4$ MPa and $P_2= 5.1$ MPa, respectively, is $\rho= 4.6$ g/cm$^3$, which corresponds to the degree of compression $\sigma=575$ at pressure $P\approx 3000$ GPa under the condition of strong nonideality ($\gamma\sim7$) and noticeable degeneracy of electrons ($n\lambda^3\sim5$). The pressure of compressed helium has been determined from the results of the gas-dynamics calculation.