Quasi-isentropic compressibility of a strongly nonideal deuterium plasma at pressures of up to 5500 GPa: Nonideality and degeneracy effects

We report on the experimental results on the quasi-isentropic compressibility of a strongly nonideal deuterium plasma that have been obtained on setups of cylindrical and spherical geometries in the pressure range of up to P ≈ 5500 GPa. We describe the characteristics of experimental setups, as well as the methods for the diagnostics and interpretation of the experimental results. The trajectory of metal shells that compress the deuterium plasma was detected using powerful pulsed X-ray sources with a maximal electron energy of up to 60 MeV. The values of the plasma density, which varied from ρ ≈ 0.8 g/cm³ to ρ ≈ 6 g/cm³, which corresponds to pressure P ≈ 5500 GPa (55 Mbar), were determined from the measured value of the shell radius at the instant that it was stopped. The pressure of the compressed plasma was determined using gasdynamic calculations taking into account the actual characteristics of the experimental setups. We have obtained a strongly compressed deuterium plasma in which electron degeneracy effects under the conditions of strong interparticle interaction are significant. The experimental results have been compared with the theoretical models of a strongly nonideal partly degenerate plasma. We have obtained experimental confirmation of the plasma phase transition in the pressure range near 150 GPa (1.5 Mbar), which is in keeping with the conclusion concerning anomaly in the compressibility of the deuterium plasma drawn in [1].     

Quasi-Isentropic Compressibility of Deuterium at a Pressure of ~12 TPa

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/cm³ 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/cm³ 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.     

Measurement of Quasi-Isentropic Compressibility of Gaseous Helium at a Pressure of ~10 TPa

A nonideal helium plasma has been compressed to a density of ~9 g/cm³ 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.

     

Direct observation of population inversion between Al+11 levels in a laser-produced plasma

Measured X-ray intensities of the resonance line series of Al⁺¹¹ in a laser-produced plasma shows population inversion between the n = 4,5 levels and the n =3 level at a plasma density N_e～10²⁰ cm^-3. The cooling of the expanding plasma leading to inversion is enhanced by a special target configuration. The gain coefficient in the 4 → 3 transition at 129.7 Å is estimated to be ～ 10 cm^-1, using both measured line intensities and numerical simulation.     

Measurement of quasi-isentropic compressibility of helium and deuterium at pressures of 1500–2000 GPa

The quasi-isentropic compressibility of helium and deuterium plasmas at pressures of up to 1500?C2000 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/cm³ and a helium density of 3.8 g/cm³ 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/cm³, 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.     

Spectroscopic study of a brush cathode plasma in a magnetic field

The brush cathode helium discharge in the magnetic field has been operated stably at discharge currents larger than those without magnetic field. The diameter of the plasma column has been determined by the configuration of the magnetic field. The measurements of the spectral intensities of the recombination continuum followed by the 2³S-n³P series reveals that the electron density is 1·8 × 10¹³ cm^-3 and the electron temperature is 0·17 eV at a discharge current of 500 mA and a pressure of 0·9 torr for a magnetic flux density of 1·3 kG. The principal quantum number for line merging is 20.     

Initiation of ecton processes by interaction of a plasma with a microprotrusion on a metal surface

Evolution of rapid (～10 ns) Ohmic overheating of a microprotrusion on a surface in contact with a plasma by emission current is studied taking into account the energy carried by plasma ions and electrons, as well as Ohmic heating, emissive source of energy release (Nottingham effect), and heat removal due to heat conduction. Plasma parameters were considered in the range of n = 10¹⁴?10²⁰ cm^?3 and T _e = 0.1 eV?10 keV. The threshold value of energy transferred to the surface from the plasma is found to be 200 MW/cm²; above this value, heating becomes explosive (namely, an increase in the temperature growth rate (δ² T/δt ² > 0) and in passing current (δJ/δt > 0) is observed in the final stage at T ～ 10⁴ K and j ～ 10⁸ A/cm²). In spite of the fact that Ohmic heating does not play any significant role for plasmas with a density lower than 10 18 cm^?3 because the current is limited by the space charge of electrons, rapid overheating of top of microprotrusion is observed much sooner (over a time period of ～1 ns) when the threshold is exceeded. In this case, intense ionization of vapor of the wall material leads to an increase in the plasma density at the surface, and the heating becomes of the Ohmic explosion type. Such conditions for the formation of a micr?xplosion on the surface and of an ecton accompanying it can be created during the interaction of a plasma with the cathode, anode, or an insulated wall and may lead to the formation of cathode and anode spots, as well as unipolar arcs.     

Acoustic perturbations in a pulsedRF-plasma

The response of a stationary weakly ionized plasma to a density perturbation in the neutral gas component was studied in a neon plasma with the following typical properties: electron density ¯N _e≈8×10¹² cm^?3, electron temperature on the axis of the vesselT _e0≈3.0 eV; neutral gas densityN _n≈1×10¹⁷cm^?3 and neutral gas temperatureT _n0≈600 °K. A neutral density perturbation, generated 50 cm apart from the plasma, produces a fluctuation in the ion density and a sharp spike in the differential voltage of a floating double probe. The experimental observations demonstrate the propagation of an ion sheath and of an electric field perturbation together with the neutral density perturbation. An interpretation of the plasma response to acoustic wave pulses has been proposed by Ingard and Schulz in a theory on acoustic wave modes in a weakly ionized gas. The experimental results are in good agreement with the theoretical expectations.     

Ne(I) spectral lines from a Ne-O2 d.c. glow discharge

Measurements have been made of intensities of the spectral lines emitted from an Ne-O₂ d.c. discharge with small discharge current (1–4 mA) under the following conditions: gas pressures of 2 and 3 torr and oxygen partial pressures (P₀₂) up to 0.1 torr. All of the Ne(I) line intensities observed decrease when O₂ is added. The Ne(I) λ5852 line (1s₂-2p₁) has been studied in detail as a representative example. The population density of the 2p₁ level of neon has been obtained from the intensity measurements as a function of P₀₂. The energy-distribution function of electrons has been determined using Druyvesteyn's method in order to calculate the population density for a corona model. The high-energy tail of the measured distribution function is markedly reduced when O₂ is added. It is shown that inelastic collisions of electrons with O₂ produce large energy losses for the electrons. These cause a decrease in population density of the 2p₁ level when O₂ is added. The population density of the 2p₁ level at a gas pressure of 2 torr is 1.2×10⁴ cm^-3 in pure neon and 5.2×10² cm^-3 in an Ne-O₂ mixture (P₀₂ = 0.01 torr). The electron densities and average electron energies are 3.5×10⁸ cm^-3 and 8.7 eV and 1.7×10⁸ cm^-3 and 5.3 eV, respectively, for the specified two cases.     

Measurement of total decay rates of excited states in a deuterium plasma by laser-resonance fluorescence

In a deuterium plasma with an electron density of 2.8 × 10¹³ cm^-3 and an electron temperature of 1850 K, the n = 2 level is coupled to the n = 6 level by absorption of short-pulsed laser light. Due to inelastic and superelastic electron collisions, the enhanced n = 6 population density is distributed to its neighbouring levels, i.e., n = 5 and n = 7. The laser-resonance fluorescences of the Balmer lines D₅, D₆, and D₇ were measured with time resolution. The transient behaviour of the fluorescence is compared with numerical calculations, which are based on the rate equations for population densities of a system with 5 states. It is observed that the rate coefficients of Johnson cannot explain our experimental data. The coefficients of Vriens and Smeets compare favourably with the measurements within experimental errors.     

Etude spectroscopique de l'effet d'anode

Spectral lines of LiI emitted by an anodic plasma, produced during electrolysis of a KCl-LiCl mixture, have been observed. This paper deals with diagnostic studies of this plasma and is based on a comparison of observed line profiles for LiI(2P-3D), (2P-4S) and (2P-4D, 4F) with calculated values derived from the Griem-Baranger impact theory. We have found, from the ensemble of the results, that the electron density in the plasma center is N_e(0) ? 1·8×10¹⁷ cm^-3, the electron temperature is T_e(0) ? 9×10³ °K, and the plasma thickness is l ? 200 μ.     

Generation of a Low Flow Atmospheric Pressure Neon ICP

Abstract

A 27.12 MHz low flow (3 1/min), laminar flow, atmospheric pressure neon ICP has been generated. The forward power used is 500 W with a reflected power of less than 5 W. Using higher powers caused the plasma to either extinguish or form numerous filaments. The Hß line is used to determine an electron number density of 8 × 10¹³cm^?3. The N₂ ⁺(0, 0) and OH(0, 0) transitions did not readily emit. This fact, coupled with the low electron density and low input power, indicates a relatively cool plasma.     

A measurement of deuterium neutrals by the balmer-series in the STP-2 high beta screw pinch tokamak

The neutral particle density of a deuterium plasma in the STP-2 screw pinch tokamak has been determined spectroscopically. The particle density for the ground state of deuterium is calculated from the rate equations for population density of the excited levels with principal quantum numbers 3 and 4; both of the population densities were obtained from intensities of the Balmer alpha and beta lines, and the rate coefficients for electronic collision were estimated from the electron density (10¹⁴/cm³) and temperature (10 eV) measured by laser scattering. We find that the neutral density is approx. 2 × 10¹²/cm³ at the center of the plasma and 2 × 10¹⁴/cm³ on the periphery. The time history of the neutral deuterium density is consistent with the observed increase in plasma density.     

Influence of ultrashort laser pulse duration on the X-ray emission spectrum of plasma produced in cluster target

Line emission spectrum of a laser plasma produced in an argon cluster jet target was measured on the n ¹ P1?1¹ S ₀ (n=5–9) transitions of the helium-like Ar XVII ion for a pulse duration varying from 45 fs to 1.1 ps and a constant fluence of ～10⁵ J/cm². The independent modeling of the relative intensities of the transitions from the n=5,..., 10 levels, as well as of the 2¹ P ₁? 1² S ₀ and 2³ P ₁?1² S ₀ lines and dielectronic satellites indicates that the electron temperature is anomalously low and that the electron density in emitting plasma increases with shortening the laser pulse. The excitation from the ground state by a small fraction of hot electrons is expected to be the main channel of populating the Ar XVII levels.     

Intensity distribution of a capillary-discharge 46.9 nm soft X-ray laser

We realize a Ne-like Ar 46.9 nm soft X-ray laser pumped by a capillary discharge. The study of the laserpulse-intensity distribution is important for applications of soft X-ray lasers. The intensity distribution demonstrates the gain distribution, plasma radius, and axial plasma density that contribute to the study of the laser-pulse formation. To measure the intensity in different positions of the X-ray laser spot, we moved transversally an X-ray diode (XRD) assembled with a slit. We obtain the onedimensional intensity distribution. We find a laser divergence (FWHM) of 4.0 mrad. According to the gain-guided model, we calculate the intensity distribution. The measured divergence of 4.0 mrad roughly corresponds to a plasma radius a approximately equal to 230–250 μm and on-axis electron density n _e ≈ 8.0?10¹⁸ ?9.0?10¹⁸ cm ^?3. The results of calculations indicate that the divergence of the intensity distribution increases when the plasma radius decreases and the on-axis electron density increases.     

Reflection measurements in CO2 laser solid deuterium target interaction

Using a 60 nsec, 300 MW CO₂ TEA laser reflection measurements from solid deuterium targets have been investigated. Energy, reflected and scattered pulse shape are recorded at various angles: 0°, 45°, 90°, 135°. Reflection, X-ray measurements and ion mean kinetic energy are correlated at the focussing lens position with respect to the target position. The maximum plasma temperature varies from 20 to 35 eV for incident laser fluxes ranging from 5 × 10¹⁰ up to 5 × 10¹¹ W/cm². The cut-off density inside the deuterium ice has been observed and located. In each case reflection has been found to be weak less than 5% for each direction. For the maximum fluxes X-ray energies greater than 0.5 keV have been observed.     

Isomer shift in BaSnO3 under a pressure up to 15 GPa

The dependence of the unit cell volume of BaSnO₃ on the pressure up to 15 GPa has been investigated and the constants of the Murnaghan equation of state B ₀ = 178.39 ± 4.09 GPa and B′₀ = 4.68 ± 0.56 have been obtained using the X-ray diffraction method. The change of the isomer shift (IS) in BaSnO₃ with a variation in the pressure P has been examined using the gamma resonance method. This quantity is ?IS(P)/?P = ?(0.00474 ± 0.0002) mm s^?1 GPa^?1 or, taking into account the measurements of the unit cell parameter under pressure, ?IS/?L = 1.42 mm s^?1Å^?1, where L is the tin-oxygen distance.     

Generation of H− ions in a low-pressure xenon-hydrogen discharge. I

A theory of a low-pressure discharge in a xenon-molecular hydrogen mixture is developed. It is shown that, in such a discharge, at an interelectrode distance of L = 1 cm and a total plasma pressure of p ₀ ～ 1 Torr, the density of negative hydrogen ions produced via the dissociative attachment of thermal electrons to vibrationally excited molecules H₂ can reach a value as high as N_H ^? ≥ 10¹² cm^?3. According to calculations, the electron temperature in discharge operating regimes under study attains T _e ≈ 1?2 eV, which corresponds to the maximum of the e-v exchange rate constant of H₂ molecules. This ensures a relatively high rate of vibrational pumping of H₂ molecules in the discharge.     

Spectrum of X-ray emission from nsec CO2-laser produced (CH2)n and A1 plasmas

Studies of the X-ray emission from nsec CO₂ laser produced plasmas indicate a stronger deviation of the electron distribution from equilibrium for a (CH₂)_n plasma than for an A1 plasma. The lowest spectral temperature measured is ～ 300 eV at the maximum flux of 5 × 10¹²W·cm^-2.     

Die Erzeugung von Seriengrenzkontinua mit Hilfe von Gleitfunken zur Absolutmessung im Vakuum-Ultraviolett

A lithium plasma is produced by discharging a 40 kV, 0,3 μF capacitor through a lithium-hydride-capillary (diameter 2 mm, length 20 mm) in vacuum (p～10^?4 Torr). During the first half-cycle (0,6 μs) Bremsstrahlung of Li III is observed in the visible and infrared, and the Lyman series of Li III together with the recombination continuum in the vacuum UV (100 Å). The high members of the Lyman series are broadened by Stark effect giving an electron density of about 6 · 10¹⁸ cm^?3. In the infrared the radiation is emitted from an optically thick plasma at a time when the free-free continuum in the visible is emitted from an optically thin plasma. Temporal development of electron temperatureT _e and electron density N_e has been measured from the absolute intensity in these spectral regions. Typical values ofT _e=230 000° K andN _e 5 · 10¹⁸ cm^?3 e.g. have been obtained. For these values the relaxation time for an ionisation equilibrium is short compared to the observation time. The complete ionisation of Li III has been checked by absorption measurements near 100 A. Thus the absolute intensity of the recombination continuum could be calculated. An experimental arrangement was built to measure simultaneously the time history of the intensity in the vacuum ultraviolet, in the visible, and infrared spectral regions.