Experimental study of the compressibility of a helium plasma at a pressure up to 20 TPa

A nonideal helium plasma has been compressed to a density of $\rho\approx14$ g/cm$^3$ at a pressure of $P \approx 20$ TPa ($200$ Mbar) in a spherical two-cascade device. The design of the device has been presented and the thermodynamic parameters of the helium plasma reached in it have been estimated. Data on the dynamics of the external and internal cascades in the device used to choose the calculation method have been obtained in a preliminary experiment with a hemispherical prototype. The experiment has been performed on the X-ray complex at the Russian Federal Nuclear Center All-Russian Research Institute of Experimental Physics, which includes BIM $234.3000$ betatrons with a boundary energy of $60$ MeV used in the multipulse bremsstrahlung generation regime with a multichannel optoelectronic system of recording X-ray images. A high-current linear accelerator LIU-R-T and an image detector assembly consisting of photochromic ADC screens have been used in addition to betatrons to detect the compressed shell cavity.