The results of four experiments on studying preliminarily statically compressed gaseous helium and deuterium during their subsequent compression in explosive spherical cascade structures providing quasi-isentropic gas compression are presented. For helium, the following parameters were achieved: in one experiment, the compression pressure is Pmean ≈ 4.9 TPa at a density ρmax ≈ 6.4 g/cm3 and the compression ratio is δ = ρ/ρ0 ≈ 320; in another experiment, Pmean ≈ 10.9 TPa, ρmax ≈ 10.3 g/cm3, and δ ≈ 470. For deuterium, these parameters are Pmean ≈ 3.4 TPa, ρmax ≈ 6.0 g/cm3, and δ ≈ 162 in one experiment and Pmean ≈ 13.3 TPa, ρmax ≈ 11.4 g/cm3, and δ ≈ 520 in another experiment. The gas density was determined by an X-ray method using the position of the boundaries of the steel shells compressing a gas. The experiments are simulated with a one-dimensional gasdynamic software package, in which the Kopyshev–Khrustalev equations of state are used for the gases under study. The pressures are determined using calculations, in which the dynamics of gas compression is satisfactorily simulated for the entire set of experiments.

The results of four experiments on studying preliminarily statically compressed gaseous helium and deuterium during their subsequent compression in explosive spherical cascade structures providing quasi-isentropic gas compression are presented. For helium, the following parameters were achieved: in one experiment, the compression pressure is P mean ≈ 4.9 TPa at a density ρ max ≈ 6.4 g/cm 3 and the compression ratio is δ = ρ/ρ 0 ≈ 320; in another experiment, P mean ≈ 10.9 TPa, ρ max ≈ 10.3 g/cm 3 , and δ ≈ 470. For deuterium, these parameters are P mean ≈ 3.4 TPa, ρ max ≈ 6.0 g/cm 3 , and δ ≈ 162 in one experiment and P mean ≈ 13.3 TPa, ρ max ≈ 11.4 g/cm 3 , and δ ≈ 520 in another experiment. The gas density was determined by an X-ray method using the position of the boundaries of the steel shells compressing a gas. The experiments are simulated with a one-dimensional gasdynamic software package, in which the Kopyshev–Khrustalev equations of state are used for the gases under study. The pressures are determined using calculations, in which the dynamics of gas compression is satisfactorily simulated for the entire set of experiments.

The quasi-isentropic compressibility of a strongly nonideal helium plasma in the pressure range 250–600 GPa is experimentally studied in devices with cylindrical geometry. The temperature at the front of a cylindrical shock wave in helium ( T ≈ 10 000 K) and the flight speed of the inner cascade ( W ≈ 3.5 km/s), in the cavity of which the maximum compressed plasma density is achieved, are measured. Data on the compression of a nonideal helium plasma to a density ρ ≈ 3 g/cm 3 at an approximately constant final temperature of 21000 K are obtained. The trajectories of the metallic shells compressing the plasma are detected using high-power pulsed X-ray sources with a boundary electron energy of up to 60 MeV. The helium plasma density is determined using the radii of the shells measured at the time of their “stop.” The compressed plasma pressure is obtained using gasdynamic calculations. Comparative theoretical calculations of the quasi-isentropic compression parameters have been carried out using the following two theoretical models: the traditional chemical plasma model (SAHA code) and an ab initio quantum molecular dynamics (QMD) approach. No anomaly of the experimental data in the pressure range of the plasma phase transition theoretically assumed in helium is detected.

The quasi-isentropic compressibility of a strongly nonideal helium plasma in the pressure range 250–600 GPa is experimentally studied in devices with cylindrical geometry. The temperature at the front of a cylindrical shock wave in helium (T ≈ 10 000 K) and the flight speed of the inner cascade (W ≈ 3.5 km/s), in the cavity of which the maximum compressed plasma density is achieved, are measured. Data on the compression of a nonideal helium plasma to a density ρ ≈ 3 g/cm3 at an approximately constant final temperature of 21000 K are obtained. The trajectories of the metallic shells compressing the plasma are detected using high-power pulsed X-ray sources with a boundary electron energy of up to 60 MeV. The helium plasma density is determined using the radii of the shells measured at the time of their “stop.” The compressed plasma pressure is obtained using gasdynamic calculations. Comparative theoretical calculations of the quasi-isentropic compression parameters have been carried out using the following two theoretical models: the traditional chemical plasma model (SAHA code) and an ab initio quantum molecular dynamics (QMD) approach. No anomaly of the experimental data in the pressure range of the plasma phase transition theoretically assumed in helium is detected.

We present our experimental results on the compressibility of strongly coupled (nonideal) degenerate deuterium and helium plasmas quasi-isentropically compressed to pressures P ~ 20 TPa in devices with a spherical geometry. The trajectories of the plasma-compressing metallic shells were recorded with the help of powerful pulsed X-ray sources (betatrons) with a boundary electron energy of 60 MeV. A high-current accelerator with a penetrability of objects with an equivalent thickness of 250-mm of lead has been used for the first time as an X-ray source in our experiments. Plasma densities up to ρ ≈ 14 g cm–3 were determined from the measured radius of the shell at the instant of its “stopping.” We derived the compressed-plasma pressure based on our gasdynamic computations including the real characteristics of the experimental devices.

We present data on the quasi-isentropic compressibility of a strongly coupled (nonideal) plasma of a deuterium–helium mixture in the range of pressures 150–250 GPa in devices with a cylindrical geometry and a new result obtained in an experiment with “pure” deuterium at a pressure ~200 GPa. The trajectory of the plasma-compressing metallic shells was recorded using powerful pulsed X-ray sources with a boundary electron energy up to 60 MeV. The densities of the plasma of deuterium and its mixture with helium were determined from the measured radii of the shells at the instant of their “stopping.” We derived the pressure of the compressed plasma based on gasdynamic computations including the real characteristics of the experimental devices. The data obtained confirm the conclusion previously reached at VNIIEF about a phase transition in the plasma of compressed deuterium in the range of pressures 150–160 GPa and suggest that this phase transition is retained in the mixture of deuterium with helium.

An experimental result for the quasi-isentropic compressibility of a strongly nonideal deuterium plasma compressed in a spherical device by the pressure P = 11400 GPa (114 Mbar) to the density ρ ≈ 10g/cm 3 has been reported. The characteristics of the experimental device, diagnostic methods, and experimental results have been described. The trajectory of motion of metallic shells compressing a deuterium plasma has been recorded using intense pulsed sources of X rays with the boundary energy of electrons up to 60 MeV. The deuterium plasma density ρ ≈ 10g/cm 3 has been determined from the measured radius of the shell at the time of its “stop.” The pressure of the compressed plasma has been determined from gas-dynamic calculations taking into account the real characteristics of the experimental device.

A nonideal helium plasma has been compressed to a density of ~9 g/cm 3 by a pressure of P ~ 10000 GPa produced by an explosive charge of 85 kg of TNT in a spherical two-cascade device. The experiment has been performed on an X ray setup using simultaneously three betatrons with a boundary energy of about 60 MeV and a multichannel optoelectronic system of recording X-ray images. The pressure of the compressed helium has been determined from the gas-dynamic calculation.

Experiments were performed to study the spherical compression of deuterium and helium to pressures of ≈3000 GPa in a quasi-isentropic regime. The process was recorded by a multiframe radiographic system which produces up to nine x-ray images of a cavity with gas at different times in one experiment. X-ray images show that explosive devices provide a nearly spherically symmetric shape of the cavity with gas up to the maximum compression of the gas. The experimental data are in good agreement with the results of calculations using the equations of state of the gases studied. From the results of these calculations, the parameters of the region of the compressed gas states obtained in the experiments were determined: for deuterium, a density of 5.5 g/cm3 and a pressure of 3.6 TPa; for helium, a density of 4.7 g/cm3 and a pressure of 2.4 TPa.

The thermodynamic parameters of a strongly nonideal helium plasma obtained in experimental devices of hemispherical and spherical geometries are presented. Under shock-wave loading in the hemispherical device, the helium plasma was compressed to a density ρ ≈ 0.76 g cm–3 by a pressure P ≈ 83 GPa at a temperature T ≈ 51000 K. Two-cascade spherical experimental devices of two types were used under quasi-isentropic helium plasma compression. In the devices of the first type at the same initial gas pressure in both cavities of the shells, the helium plasma was compressed approximately by a factor of 200 to a density ρ ≈ 8 g cm–3 by a pressure P ≈ 4800 GPa. In the devices of the second type at a ratio of the initial gas pressures in the cavities of about 9: 1, the thermodynamic parameters of a nonideal helium plasma compressed by a factor of 900 to a density ρ ≈ 5 g cm–3 by a pressure P ≈ 3700 GPa were determined. The compressed-plasma pressure was determined from the results of gasdynamic computations. An X-ray radiograph consisting of three betatrons and a multichannel optoelectronic X-ray imaging system was used to determine the positions of the boundaries of the gaseous-helium-compressing steel shell.

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 ≈55 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 ρ = 4.6 g/cm3, which corresponds to the degree of compression σ = 575 at pressure P≈3000 GPa under the condition of strong nonideality (γ ∼ 7) and noticeable degeneracy of electrons (nλ3 ∼ 5). The pressure of compressed helium has been determined from the results of the gas-dynamics calculation.

The quasi-isentropic compressibilities of deuterium and helium plasmas are measured in the pressure range 1500–5000 GPa at densities up to 8 g/cm 3 using spherical experimental devices and an X-ray complex consisting of three betatrons and a multichannel optoelectronic system for taking X-ray images. The experimental results demonstrate the possibilities of high-energy-density experimental physics to reproduce the extreme states of substance typical of the Universe under laboratory conditions using the energy of traditional condensed explosives.

The quasi-isentropic compressibility of helium and deuterium plasmas at pressures of up to 1500–2000 GPa has been measured using devices with spherical geometry and an X-ray diagnostic complex comprising three betatrons and a multichannel imaging system with electro-optic gamma detectors. A deuterium density of 4.5 g/cm 3 and a helium density of 3.8 g/cm 3 have been obtained at pressures of 2210 and 1580 GPa, respectively. The internal energy of a deuterium plasma at the indicated pressure is about 1 MJ/cm 3 , which is about 100 times greater than the specific energy of condensed chemical explosives. Analysis of the obtained data shows that the degree of helium ionization under the achieved plasma compression parameters is about 0.9.