Study of metal conductivity near the critical point using a microwire electrical explosion in water

Electrical explosion of aluminum and tungsten microwires in water was studied both experimentally and numerically. The experimental range of currents through the wire was 0.1–1 kA for explosion times of 40–300 ns and current densities up to 1.5×10⁸ A/cm². The experimental results were interpreted on the basis of magnetohydrodynamical simulation with various metal conductivity models. A comparison of the experimental and numerical results allows the conclusion to be drawn that the metal conductivity models used in this work are adequate.     

Three-body reaction rate in spin-polarized atomic hydrogen

An order-of-magnitude estimate of the three-body reaction of spin-polarized atomic hydrogen is obtained by expressing this rate as a product of the gas-kinetic reaction rate and the probability of a spin-flip transition. The former is estimated from experimental results on atomic hydrogen and the latter by using a Landau-Zener type approach. We find k₃ (H↑) ≈ 5 × 10^?40 cm⁶ atom^?2 s^?1, which would lead to a density of 10¹⁸ atoms cm^?3 for times of the order of 10³ s.     

Channels of radiative recombination and phase transitions in a system of nonequilibrium carriers in a Si0.93Ge0.07/Si thin quantum well

The “exciton gas-plasma” transition (the Mott transition) in a Si_0.93Ge_0.07/Si thin quantum well is investigated using low-temperature photoluminescence. It is demonstrated that this transition is smooth and occurs in the concentration range from approximately 6 × 10¹⁰ to 1.2 × 10¹² cm^?2. At a temperature of 23 K and excitation densities of higher than 10 W/cm², the shape and location of the luminescence line associated with the electron-hole plasma remain unchanged with an increase in the pump density. This can indicate the occurrence of an “electron-hole gas-liquid” transition. It is shown that, in the spectrum of the quantum well, the luminescence of boron-bound excitons dominates at liquid-helium temperatures and low excitation densities, whereas the free-exciton luminescence dominates at temperatures above 10 K. The influence of the homogeneous and inhomogeneous broadening on the electron-hole plasma and exciton luminescence is discussed.     

Shunting effect in explosive electron emission

An explanation is given to the results of an experiment on studying the explosive electron emission in a wire-cathode diode where a strongly nonuniform energy deposition into the wire material was observed using an X pinch as a radiation source for projection x-ray imaging. The specific input energy, contrary to the well-known observations, was not a maximum at the wire end, i.e., in the region of the strongest electric field, and the wire explosion occurred in the bulk, distant from the end. This is accounted for by the contribution of the wire side surface to explosive electron emission and by the gas desorption from the wire intensely heated by a current of density 10⁸ A/cm². Thus, the space between anode and cathode (wire end) is bridged by two plasmas: one generated due to the explosive electron emission from the wire side surface and the other produced from the desorbed gas.     

Hydrodynamics of a dense plasma created during laser annealing pulses

We show that an electron-hole plasma with density well above the droplet one (≈ 10¹⁸ cm^?3 in Si) expands very fast. Adding Auger recombination and phonon collisions, the maximum density for typical laser annealing pulses is found to be at most a few times 10²⁰ cm^?3. As a plasma density of a few times 10²¹ cm^?3 is necessary to modify the stability of the solid, we conclude that the laser pulse has essentially a thermal effect.     

Homogeneous electrical explosion of tungsten wire in vacuum

Experimental results on Joule energy deposition upon initiation of a fast electrical explosion of 16-μm tungsten wire in vacuum at current densities of more than 10⁸ A/cm² are reported. We have found that explosion with a fast current rise time (～170 A/ns into a short) results in homogeneous and enhanced deposition of electrical energy into the tungsten before surface flashover. The maximum tungsten wire resistivity reaches a value of up to ～185 μΩ cm before surface flashover that significantly exceeds the melting boundary and corresponds to a temperature of ～1 eV. The highest values for light radiation and expansion velocity of wire ～1 km/s were observed for the fast explosion. For the explosion mode with a slower current rise time (～22 A/ns into a short), we observed the existence of an “energy deposition barrier” for tungsten wire. In the slow explosion mode, the current is reconnected to the surface shunting discharge before melting. The maximum tungsten wire resistivity in this case reaches the value of ～120 μΩ cm, which is less than indicative of melting. Also, the energy deposition along the wire is strongly inhomogeneous, and wire is disintegrated into parts. We attribute the early reconnection of the current to the surface discharge for the slow explosion to high electron emission from the wire surface, which starts before melting.     

Fabrication of a three-dimensional complex carbon nanoneedle from carbon nanowalls

A three-dimensional complex carbon nanoneedle has been fabricated from carbon nanowalls by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibits novel three-dimensional complex nanostructure, the center of which is a carbon nanoneedle, and many carbon nanowalls growing from the needle. The density of unique nanostructure emitters was about 5 × 10⁷/cm². The I-V characteristic addressed an emission current density of 314 mA/cm² at the electric field of 2.5 V/μm.     

Parameters of alloy p-n junctions in Cu2O

The p-n junctions are made by melting copper on single-crystal Cu₂O plates. A change in specific resistance of the Cu₂O from 2.8 × 10² to 1.15 × 10⁵ ohm-cm causes the reverse voltage at a reverse-current density of 2 m A/cm² to increase from 15 to 500 V, while the forward current density at 2 V decreases from 250 to 2 mA/cm².     

Spatial-temporal measurements of magnetic fields in laser-produced plasmas

The results of an investigation of the electromagnetic wave polarization, probing high-temperature laser plasma, as well as spatial-temporal structure of the magnetic fields, electron density, current density, and electron drift velocity are presented. To create the plasma, plane massive Al targets were irradiated with the second harmonic of a phoenix Nd laser at intensities up to 5·10¹⁴ W/cm². It was shown that the magnetooptical Faraday effect is the main mechanism responsible for the changing polarization of the probing wave. Magnetic fields up to 0.4 MG with electron densities ∼10²⁰ cm⁻³ were measured. Analysis of the magnetic field spatial distribution showed that the current density achieved the value ∼90 MA/cm² on the laser axis. The radial structure of the magnetic field testified to the availability of the reversed current in the laser plasma. The spatial and temporal resolutions in these experiments were equaled to ∼5 μsec and ∼50 psec, respectively. Translated from Preprint No. 35 of the Lebedev Physics Institute, Moscow, 1993.     

Electron density diagnostics by use of a collisional excitation model of the oxygen VI doublet in the solar transition region

ABSTRACT

For spectral diagnostics in the solar transition region under the collision–equilibrium condition, the method for electron density diagnostics by the collisional excitation is discussed in this paper. By using observed signal ratio of spectral lines produced by the oxygen VI ion, we discuss the electron density by this method. We find that this method is valid for a large region of signal ratio (from 2.001 to 4.162). We obtain that the electron density increases with increasing signal ratio of the oxygen VI doublet, and the plasma has an electron density on the order of magnitude of 10⁶–10⁷?cm^?3. This discussion will be significant for study on spectral diagnostics of the solar transition region.     

Low beam current density Auger spectroscopy and surface analysis

Auger and secondary electron spectroscopy become a more and more routine technique in surface characterization. Even with primary electron beam current density as low as 10^?2 or 10^?3 A cm^?2 beam damage were reported in both Auger and LEED experiments. So we developed and compared counting method, brightness modulation and Harris' modulation techniques in terms of signal to noise ratio. The two first methods offer the advantage of a primary beam current density decreasing about 10⁴ times. So various mechanisms of beam damage were identified as thermal, chemical and electrical. The advantage of the method is shown with hydrocarbons adsorption layer; the beam cracking of the organic chain produces a chemical shift of the C_KLL maximum Auger line about 5 eV. This progressive shift is observed with current densities of 10^?5 A cm^?2 order of magnitude. The reproducibility of this low current density Auger spectroscopy allowed the study of the background and the true secondary electron yield modifications when adsorbed layers are built up.     

Dynamics of hydrogen accumulation and radiation-stimulated release from steels

Hydrogen accumulation during electrolytic saturation of 12Kh18N10T and 12Kh12M1BFR steels, as well as during thermally and radiation-stimulated hydrogen release from the same materials, was studied. It was shown that there is a critical hydrogen concentration in the sample, which is reached in 50 h for this saturation method (1 M H₂SO₄ electrolyte, current density is 0.5 A/cm²). Initially, hydrogen is trapped at low-temperature (400–500°C) traps of several types in surface layers. At saturation times of 50 h and longer, hydrogen penetrates to high-temperature (800–900°C) traps in the sample bulk. Under electron irradiation of saturated samples, the hydrogen yield nonlinearly increases with electron current density and energy above 40 keV. It was concluded that electronic processes (Auger process and plasmon excitation) play a dominant role in hydrogen diffusion and desorption activation.     

Raman scattering of high-temperature phase transition in KH2PO4

Raman scattering was applied to study the high-temperature phase transition (near 175°C) in KH₂PO₄. Drastic temperature-dependent changes were observed to take place in the normal modes of B₁ symmetry between 1000–3400 cm^?1. The disintegration of the dominant broad feature near 2500 cm^?1 when temperature rises beyond 150°C suggests that the alteration of the hydrogen-bond network is closely connected with this high-temperature phase transition.     

Field emission investigations of RuO2-doped SnO2 wires

Field emission studies of a bunch and a single isolated RuO₂:SnO₂ wire have been performed. A current density of 5.73 × 10⁴ A/cm² is drawn from the single wire emitter at an applied field of 8.46 × 10⁴ V/μm. Nonlinearity in the Fowler-Nordheim (F-N) plot has been observed and explained on the basis of electron emission from both the conduction and the valence bands of the semiconductor. The current stability recorded at the preset value of 1.5 μA is observed to be good. Overall the high emission current density, good stability and mechanically robust nature of the RuO₂:SnO₂ wires offer advantages as field emitters for many potential applications.     

Effects of the AlN nucleation layer thickness on the crystal structures of an AlN epilayer grown on the 6H-SiC substrate

The influence of the LT-AlN(NL) growth times on the mosaic structure parameters of the AlN layer grown on the LT-AlN(NL)/6H-SiC structures as well as the dislocation densities and the strain behaviours in the AlN epilayers has been investigated using XRD measurements. The growth times of the LT-AlN(NL) were changed to 0, 60, 120, 180, and 240?s. We observed that the mosaic structure parameters of the AlN epilayers were slightly affected by the LT-AlN(NL) growth times. However, the dislocation densities in the AlN layer are affected by the growth times of the LT-AlN(NL) layer. The highest edge dislocation density (5.48?×?10¹⁰?±?2.3?×?10⁹?cm^?2) was measured for the sample in which 120?s grown LT-AlN(NL) was used. On the other hand, highest screw type dislocation density (1.21?×?10¹⁰?±?1.7?×?10⁹?cm^?2) measured in the sample E that contains 240?s growth LT-AlN(NL). The strain calculation results show that the samples without LT-AlN(NL) suffered maximum compressive in-plane strain (?10.9?×?10^?3?±?1.8?×?10^?4), which can be suppressed by increasing the LT-AlN(NL) growth times. The out-of-plane strain also has a compressive character and its values increase with LT-AlN(NL) growth times between 60 and 180?s. Same out-of-plane strain values were measured for the LT-AlN(NL) growth times of 180 and 240?s. Furthermore, the form of the biaxial stress in the AlN epilayer changed from compressive to tensile when the LT-AlN(NL) growth times were greater than 120?s.     

The problem of “thermal explosion” in spin-polarized atomic hydrogen gas

The exponential character of the depolarization-rate temperature dependence in spin-polarized atomic hydrogen gas results in a “thermal explosion” phenomenon with increasing density. The inhomogeneous heating is stimulated by the three-body recombination, which in allowance for dipole-dipole interaction takes place at arbitrary spin states of colliding particles. The critical gas density is found to be in the range of 10¹⁸–10¹⁹ cm^?3 for the real parameters of the system.     

High-current plasma opening switch fed by a magnetic explosion generator

A plasma opening switch fed by a helical magnetic explosion generator is developed. A plasma sheath with an axial length of ≈4 cm and an electron density of ～10¹⁷ cm^?3 is produced in the electrode gap of the switch by six coaxial gaseous-plasma injectors. A series of explosion experiments shows that the system developed allows one to study the switch at currents of about 2 MA.     

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.     

Observations on the electronic spectrum of gaseous FeO

About 400 lines are assigned to FeO emission bands in the region 5580 to 6265 Å. The lower state of all the bands analyzed is identified as the ground state of the molecule, for the value of the lower-state vibration frequency (ω_e = 880.61 ± 0.02 cm^?1) is in excellent agreement with that observed in low-temperature matrix isolation, itself confirmed by isotopic substitution. This state is also observed as the lower state in laser-induced fluorescence. However, 880 cm^?1 is significantly smaller than the value found in laser photodetachment studies of FeO^? (970 ± 60 cm^?1). The rotational analysis is consistent with a parallel transition, ΔΛ = 0, but the value of Λ is not determined. According to theoretical calculations, the orange bands most probably arise from a ⁵Σ-⁵Σ transition. There is at least one nearby excited state, for all analyzed upper levels are perturbed.     

Spatially-resolved spectroscopic diagnosing of aluminum wire array Z-pinch plasmas on QiangGuang-I facility

Spatially-resolved crystal spectrometers with a high spectral resolution are developed to diagnose K-shell x-ray radiation from Z-pinch plasmas. These diagnostic apparatuses are successfully applied to aluminum wire array Z-pinch experiments on QiangGuang-I facility, a driver with a pulsed current up to about 1.5 MA in 80 ns. Time-integrated experimental results show that the K-shell x-ray emission lines of aluminum Z-pinch plasmas are dominated by line emissions from helium-like ionisation state. Bright spots that might have higher electron temperature or density are produced randomly in location and size along the z-axis during implosions. According to the experimental data, the electron temperature and the ion density are estimated to be between 250 eV and 310 eV, and between 7.0×10¹⁹cm^-3 and 4.0×10¹⁹ cm^-3 respectively, while the ion temperature is inferred to be about 10.2 keV, which is much higher than the electron temperature.