Specific Features of the Longitudinal Distribution of Plasma Parameters in the Initial Segment of a Supersonic Jet Formed by a Pulsed Capillary Discharge

Plasma Physics Reports - The paper presents the results of spectroscopic studies of the initial segment of a supersonic plasma jet formed by a pulsed discharge in a capillary made of...     

Formation of a micropinch and generation of multiply charged ions at the front of a current-carrying plasma jet

The formation of a neck in the cathode plasma jet in the initial stage of a low-voltage vacuum spark is investigated experimentally and theoretically. X-ray bursts corresponding to an electron temperature of 150–300 eV are detected. With the use of a pinhole camera, it is found that an emitting region less than 1 mm in size is located near the cathode. The free expansion of a current-carrying cathode plasma jet with a current growing in accordance with the experimentally observed time dependence is simulated using a hydrodynamic model. It is shown that the neck forms at the front of the plasma jet due to the plasma compression by the magnetic self-field. In the constriction region, the plasma is rapidly heated and multiply charged ions are generated. The calculated spatial and temporal variations in the electron temperature and average ion charge are close to the measured dependences over a wide range of the discharge parameters.     

Study of the current-sheath formation during the implosion of multishell gas puffs

Results are presented from experimental studies of the dynamics of large-diameter multishell gas puffs imploded by microsecond megampere current pulses. The experiments were conducted on the GIT-12 generator in the regime of microsecond implosion (t _imp = 1.1–1.2 μs, I ₀ = 3.4–3.7 MA). The influence of the load configuration on the dynamics of current losses and gas-puff radiative characteristics was studied. The correlation between the radial compression ratio (the ratio between the initial and final Z-pinch radii) and the magnitude of the current flowing at the plasma periphery was investigated. The experiments show that, in a multishell gas puff, large-scale instabilities insignificantly affect the gas-puff implosion even over microsecond time intervals and that a compact dense pinch with a relatively high average electron temperature (400–600 eV) forms at the Z-pinch axis. The diameter of the plasma column radiating in the K-shell lines of neon is about 3–4 mm, the K-shell radiation yield being 5–11 kJ/cm. In the final stage of implosion, only a small portion of the current flows through the high-temperature central region of the pinch plasma, whereas the major part of the generator current flows through the residual peripheral plasma.     

Modeling of electron heating and the spatiotemporal evolution of emission in a current-sheet plasma

Results are presented from a detailed study of the behavior of the electron temperature during the evolution of a current sheet by comparing the data from spectral measurements with the spatiotemporal evolution of the emission intensities of the atomic and ionic lines of the working gas (He) and impurities (C, O) calculated in the collisional-radiative model. It is shown that the electron temperature in the center of the sheet attains a value of T _e =110±40 eV; under these conditions, taking into account metastable states affects the calculated results only slightly. The spatial profiles of the electron temperature and the plasma emission in the spectral lines of various atoms and ions across the plasma sheet are calculated as functions of time. It is shown that as the electron temperature grows most of the spectral lines of atoms and ions of the working gas and impurities are depleted in the center of the sheet and the emission region shifts toward the periphery of the sheet. The results obtained confirm the previous conclusion that, in this regime, a hot plasma is formed in the center of the sheet.     

Optical characteristics of the plasma of a glow discharge in a He/H2O mixture

The emission from the plasma of a steady-state electric discharge in a He/H₂O mixture in the wavelength range 130–670 nm is investigated. It is shown that, at a water vapor partial pressure of P=2.0–2.5 kPa, the discharge mainly emits within the range 306–315 nm. The emission consists of an OH (A-X; 0-0) 307.4-nm narrow peak and a broad band with a maximum at λ_max=309.1 nm. As the partial pressure of water vapor decreases to 50–150 Pa, VUV emission at wavelengths of λ=186, 180, and 157 nm becomes dominant. In the visible region, H_α 656.3-nm and H_β 486.1-nm spectral lines and HeI lines in the range 447.1–667.8 nm, which are of interest for diagnosing the plasma, prevail. The intensities of the main bands and spectral lines are determined as functions of the helium partial pressure and discharge current.     

Spectroscopic measurements of the electron and ion temperatures and effective ion charge in current sheets formed in two- and three-dimensional magnetic configurations

The spatial distributions of the electron temperature and density, the effective and average ion charges, and the thermal and directed ion velocities in current sheets formed in two-dimensional magnetic fields and three-dimensional magnetic configurations with an X line were studied using spectroscopic and interference holographic methods. The main attention was paid to studying the time evolution of the intensities of spectral lines of the working-gas (argon) and impurity ions under different conditions. Using these data, the electron temperature was calculated with the help of an original mathematical code based on a collisional-radiative plasma model incorporating the processes of ionization and excitation, as well as MHD plasma flows generated in the stage of the current-sheet formation. It is shown that the electron temperature depends on the longitudinal magnetic field, whereas the ion temperature is independent of it. The effective ion charge of the current-sheet plasma was determined for the first time.     

High-Power X-Ray Line Radiation of the Plasma Produced in a Collision of High-Energy Plasma Flows

Results are presented from experimental studies of a pulsed source of soft X-ray (SXR) emission with photon energies in the range of 0.4–1 keV and an output energy of 2–10 kJ. SXR pulses with a duration of 10–15 μs were generated in collisions of two plasma flows propagating toward one another in a longitudinal magnetic field. The plasma flows with velocities of (2–4) × 10⁷ cm/s and energy contents of 70–100 kJ were produced by two electrodynamic coaxial accelerators with pulsed gas injection. Nitrogen and neon, as well as their mixtures with deuterium, were used as working gases. The diagnostic equipment is described, and the experimental results obtained under different operating conditions are discussed. In particular, X-ray spectroscopy was used to study the high-temperature plasma produced in a collision of two plasma flows. The observed intensities of spectral lines are compared with the results of detailed kinetic calculations performed in a steady-state approximation. The calculations of the nitrogen and neon kinetics have shown that the electron temperature of a nitrogen plasma can be most conveniently determined from the intensity ratio of the resonance lines of He- and H-like nitrogen ions, while that of a neon plasma, from the intensity ratio between the resonance line of He-like Ne IX ions and the 3p?2s line of Li-like Ne VIII ions. In the experiments with plasma flows containing nitrogen ions, the electron temperature was found to be ≈120 eV, whereas in the experiments with plasma flows containing neon ions, it was 160–170 eV.     

Spectroscopic measurements of the parameters of the helium plasma jets generated in the plasma focus discharge at the PF-3 facility

The spectroscopic technique used to measure the parameters of the plasma jets generated in the plasma focus discharge and those of the plasma of the immobile gas through which these jets propagate is described. The time evolution of the intensities and shapes of spectral lines in experiments carried out with helium at the PF-3 facility was studied by means of electron-optical streak cameras. The plasma electron temperature, T ≈ 4–5 eV, was determined from the intensity ratio of two spectral lines, one of which (λ₁ = 5876 Å) belongs to neutral helium, while the other (λ₂ = 4686 Å), to hydrogen-like helium ions. The plasma density at different time instants was determined from the Stark broadening of these lines in the electric fields of different nature. The plasma density is found to vary from 4 × 10¹⁴ to 2 × 10¹⁷ cm^?3.     

Determination of the electron density and electric field in the plasma of a low-pressure microwave electrode discharge in hydrogen from the measured spectral line intensities

The parameters of the plasma of a microwave electrode discharge in hydrogen at pressures of 1–8 torr and incident powers of 20–80 W are measured by the so-called “relative intensity” method. The method allows one to determine the electron density and electric field in plasma by measuring the relative intensities of the H_α, H_β, and 763.5-nm Ar line emission and calculating the electron-impact rate constants from the homogeneous Boltzmann equation. The measurements show that there are regions in the discharge where the electron density is higher (a bright electrode sheath) and lower (a spherical region) than the critical density for the frequency 2.45 GHz (n_cr～7×10¹⁰ cm^?3). Inside the spherical region, the electric field varies slightly over the radius and the electron density increases as the discharge boundary is approached. The observed discharge structure can be attributed to the presence of a self-sustained discharge zone (electrode sheath); a non-self-sustained discharge zone (spherical region); and a decaying plasma region, which is separated from the active discharge zone by an electric double layer.     

X-ray spatial distribution of a low-inductance vacuum spark discharge

The paper presents the results of X-ray source spatial distribution measurements of a low-inductance vacuum spark discharge based on the analysis of helium-like ions spectral lines structure. The intensities of spectral lines of FeXXV ions and dielectronic satellites with spatial resolution were measured. The polarization of satellite line 1s2p ^{2 2} D _5/2-1s ²2p ² P _3/2 was found. The obtained dependence of maximum plasma parameters during micropinch events from initial plasma line density in discharge corresponds to the radiative collapse model.     

Laser-optical measurements of the velocities of the plasma jets formed from different gases in a kilojoule-range plasma focus facility

The velocities of the plasma jets formed from Ne, N₂, Ar, and Xe gases in plasma focus facilities were determined by means of laser-optical shadowgraphy of the shock waves generated at the jet leading edge. In spite of the almost tenfold ratio between the atomic weights of these gases, the outflow velocities of the plasma jets formed in experiments with these gases differ by less than twice, in the range of (0.7–1.1) × 10⁷ cm/s under similar discharge conditions. The energies of the jet ions were found to vary from 0.7 keV for nitrogen to 4 keV for xenon.     

Flow of a plasma of multielectron elements along a magnetic field

An analysis is made of a flow of Ar plasma imitating plasma flows in ion separation systems such as systems for processing spent nuclear fuel or ion cyclotron resonance isotope separation systems. It is found that the electron temperature is equalized along the flow by electron heat conduction. When the electron temperature is not too low (T _e ≥ E _ion/10, where E _ion is the ionization energy), multicharged ions are intensely produced along the entire flow. It is shown that this process is accompanied by the flow acceleration. Difficulties in describing a supersonic flow by hydrodynamic equations are pointed out.     

Laser-generated plasmas at INFN-LNS

Hot plasmas can be generated by fast and intense laser pulses ablating solids placed in vacuum. A Nd:Yag laser operating at the fundamental and second harmonics with 9-ns pulses (maximum energy of 900 mJ) focused on metallic surfaces produces high ablation yields of the order of μg/pulse and dense plasma that expands adiabatically at supersonic velocity along the normal to the target surface. The plasma emits neutral and charged particles. Charge states up to 10+ have been measured in heavy elements ablated with intensities of the order of 10¹⁰ W/cm². The ion temperature of the plasma is evaluated from the ion energy distributions measured with an ion energy analyzer. The electron temperature is measured through Faraday cups placed at the end of long drift tubes by using time-of-flight technique. The neutral temperature is measured with a special mass quadrupole spectrometer placed along the normal to the target surface. The plasma temperature increases with the laser pulse intensity. The ion temperature reaches values of the order of 400 eV, the electron temperature is of the order of 1 keV for hot electrons and 0.1 eV for thermal electrons, and the neutral temperature is of the order of 200 eV. The experimental apparatus, the diagnostic techniques, and the procedures for the plasma temperature characterization will be presented and discussed in detail. Published in Russian in Fizika Plazmy, 2006, Vol. 32, No. 6, pp. 558–564. The text was submitted by the authors in English.     

Study of the implosion of foam-wire loads at the Angara-5-1 facility

Results are presented from experiments on studying the compactness of compression of imploding nested foam-wire loads at currents of up to 4 MA at the Angara-5-1 facility. The degree of pinch compression was estimated from the dynamics of the spatial distribution of the current (magnetic field) and the shape of the soft X-ray pulse. The load consisted of nested cascades, one of which being a wire array and the other being a hollow or solid low-density cylinder made of agar-agar foam with a wall thickness of 100?C200 ??m. In some experiments, one of the cascades was made of C₂₀H₁₇O₆ solid-state organic acid foam. The radial distribution of the magnetic field inside the nested cascades of the imploding foam-wire load (both between the cascades and inside the inner cascade) was measured using tiny magnetic probes. The measured radial distributions of the magnetic field are compared with the magnetic field configuration calculated using a one-dimensional MHD code simulating the implosion of a nested foam-wire load. It is shown that the spatial structure of the current and magnetic field during the implosion of such a load is determined by the development of supersonic and subsonic magnetized plasma flows in its cascades. The specific features of pinch formation and methods for the compensation of the nonsimultaneous pinch compression between the anode and the cathode (the zipper effect) during the implosion of a nested foam-wire load are analyzed.     

Experimental investigation of the shock wave in a fast discharge with cylindrical geometry

The work is devoted to the registration and the study of the properties of cylindrical shock waves generated in the fast discharge (dI/dt ～ 10¹² A/s) inside the ceramic tube (Al₂O₃) filled by argon at pressures of 100 and 300 Pa. The shock wave appears at the inner wall of the insulator and moves to the discharge axis with a velocity of about (3?4) × 10⁶ cm/s with subsequent cumulation. The plasma behind the front is heated enough to produce radiation in the vacuum ultraviolet (VUV) region, which makes it possible to study its structure by means of a pinhole camera with a microchannel plate detector. The time resolution of the registration system was 10 ns. The analysis of VUV spectra of the plasma shows that the electron temperature behind the shock wave front is about several eV; after the moment of cumulation, its temperature increases to 20–30 eV.     

Associate Hydrations and Energies of Nucleotide Bases as Revealed by Low-Temperature Field Ionization Mass-Spectrometric Data

Abstract

The formation of water clusters, polyhydrates of nucleotide bases and their associates during simultaneous condensation of water and base molecules in vacuo onto a surface of a needle emitter cooled to 170 K was studied by field ionization mass spectrometry. It was found that different emitter temperatures are characterized by a specific distribution of intensities of cluster currents, depending on the number of water molecules in clusters. These distributions correlate with structural peculiarities and the relative energetics of formation of water clusters, polyhydrates of nucleotide bases and their associates at low temperature. The features observed in mass spectra for clusters m⁹Ade (H₂O)₅, m¹Ura (H₂O)₄ and m⁹Ade m¹Ura (H₂O)₂ are treated as a result of formation of energetically favorable structures stabilized by H-bonded bridges of water molecules.

The relative association constants and formation enthalpies of the noncomplementary pairs Ade Cyt, Gua Ura and the associates which model the aminoacid-base complexes m¹Ura Gin and m₂ ^1,3Thy Gin were determined from the temperature dependencies of the intensities of mass spectra peaks in the range 290–320 K.     

Ar + H<Subscript>2</Subscript> Plasma Interacting with Lithium-Filled Capillary Porous Structure

Ar + H₂ plasma interacting with liquid lithium was carried out on a one-cathode linear plasma device (SCU-PSI). The lithium sample was covered with capillary porous structure (CPS). It is found that the electron temperature of applied plasma ranged from ~0–1 eV and electron density ranged from 0.1 × 10²⁰ to 1 × 10²⁰ m^?3. The experimental results indicate that a reduction in the electron temperature and the lithium evaporation is found as the percentage of H₂ increases When the ratio of argon and hydrogen keeps constant, the electron temperature and lithium evaporation increase with applied input power, respectively. The retention of hydrogen atoms in lithium surface results in reducing the lithium evaporation. The XRD analysis result shows that during plasma radiation no LiH is formed.     

Parameters of the plasma of a dc pulsating discharge in a supersonic air flow

A dc discharge in a cold (T = 200 K) supersonic air flow at a static pressure of 200–400 Torr was studied experimentally. The excited unsteady pulsating discharge has the form of a thin plasma channel with a diameter of ≤1 mm, stretched downstream the flow. Depending on the discharge current, the pulsation frequency varies from 800 to 1600 Hz and the electron temperature varies from 8000 to 15000 K.     

Measurements of the temperature of subsonic CO<Subscript>2</Subscript> induction plasma flows by analyzing their emission spectra

Results are presented from measurements of the temperature characteristics of subsonic CO² plasma flows generated by a 100-kW induction plasmatron at the Institute for Problems of Mechanics, Russian Academy of Sciences. The atomic excitation temperature T _a and the population temperature T _e of the electronic states of C2 molecules (both averaged over the jet diameter) were measured from the absolute intensities of the atomic spectral lines and the spectrum of C2 molecules in different generation regimes at gas pressures of 25–140 hPa and anode supply powers of 29–72 kW. The longitudinal and radial profiles of the temperatures were determined for some of these regimes and compared to those obtained from numerical calculations of equilibrium induction plasma flows in the discharge channel. For some generation regimes, the dependences of the averaged (over the line of sight) rotational and vibrational temperatures (T _r and T _v) on the discharge parameters, as well as the radial profiles of these temperatures, were determined from the best fit of the measured and calculated spectra of C₂ molecules (Swan bands). The self-absorption of molecular emission was observed at sufficiently high temperatures and gas pressures, and its influence on the measured values of the molecular temperatures T _e, T _v, and T _r was examined.     

Interactions between soil properties,soil microbes and plants in remnant-grassland and old-field areas: a reciprocal transplant approach

Background and Aims

Biological soil crusts cover about one third of the terrestrial soil surfaces in drylands, fulfilling highly important ecosystem services. Their relevance to global carbon cycling, however, is still under debate.

Methods

We utilized CO₂ gas exchange measurements to investigate the net photosynthetic response of combined cyanobacteria/cyanolichen-, chlorolichen- and moss-dominated biocrusts and their isolated photoautotrophic components to light, temperature, and water. The results were compared with field studies to evaluate their compatibility.

Results

Different biocrust types responded similarly, being inhibited by limited and excess water, saturated by increasing light intensities, and having optimum temperatures. Cyanobacteria/cyanolichen-dominated biocrusts reached their water optimum at lowest contents (0.52–0.78 mm H₂O), were saturated at highest light intensities, and had a comparably high temperature optimum at 37 °C. Chlorolichen-dominated crusts had a medium water optimum (0.75–1.15 mm H₂O), medium saturating light intensities and a moderate temperature optimum of 22 °C. Moss-dominated biocrusts had the highest water optimum (1.76–2.38 mm H₂O), lowest saturating light intensities, and a similar temperature optimum at 22 °C. Isolated photoautotrophs responded similar to complete crusts, only isolated moss stems revealed much lower respiration rates compared to complete crusts.

Conclusions

In addition to their overall functional similarities, cyanobacteria/cyanolichen-dominated biocrusts appeared to be best adapted to predicted climate change of increasing temperatures and smaller precipitation events, followed by chlorolichen-dominated biocrusts. Moss-dominated biocrusts needed by far the largest amounts of water, thus likely being prone to anticipated climate change.