Study of the effect of preliminary wire explosion on X-ray generation during wire array Z-pinch implosion

Results are presented from experimental studies and numerical simulations of the effect of preliminary wire explosion on the parameters of X-ray emission generated during wire array Z-pinch implosion. The wire array implosion was driven by a current pulse with an amplitude of 0.5 MA and a rise time of 0.5 μs, while the preliminary wire explosion was produced by a current pulse with an amplitude of 0.5–1 kA per wire, a rise time of 100 ns, and a full width at half maximum of ～200 ns. The experiments showed that the current prepulse significantly impaired the parameters of X-ray pulses. In particular, along with a decrease in the amplitude and an increase in the duration of the X-ray pulse, its spiky structure became more pronounced. The results of numerical simulations with the use of a one-dimensional radiative MHD code are in good agreement with the parameters of Z-pinch emission in experiments with and without a current prepulse.     

Relation between the electric parameters of a Z-pinch discharge and plasma production in the load during the implosion of a cylindrical wire array

A study of the process of implosion of a cylindrical tungsten wire array by electrical and optical methods shows that it involves two phases. In the first phase, the plasma is produced from the dense wire cores under the action of the heat flux from the current-carrying plasma. This plasma then fills the internal space of the liner array. The measured inductance of the liner and its visible diameter vary only slightly in this phase. During the second phase, the total material of the liner is compressed toward the axis and the inductance of the discharge gap increases. The process of the implosion of wire arrays is studied by analyzing the electric parameters (current and voltage) of the load in the Angara-5-1 facility. The time behavior of the load inductance, the average current radius, and the start time of the liner compression are determined. The compression start time determined from the visible size of the liner is found to coincide with that determined from electric measurements. The compression ratio of the liner in terms of the average current radius turns out to be lower than that measured by optical and X-ray diagnostics. The reason is that, by the instant of maximum compression, only a portion of the current flows at the periphery of the initial wire array.     

Stratification and filamentation mechanism for a multicharged plasma of a Z-pinch

The thermal-ionizational instability that gives rise to axial and azimuthal inhomogeneities in the electron temperature and electron density in the form of striations and filaments in a multicharged plasma of a Z-pinch is studied in the linear approximation. The theoretical results on the rate at which the inhomogeneities grow and their characteristic spatial dimension agree with the experimental data.     

Dynamics of the implosion of a tungsten wire array onto the central aluminum wire

Results are presented from the studies of the magnetic implosion of a tungsten wire liner onto an aluminum wire at currents of 2.0–2.6 MA. The experiments were carried out in the S-300 high-power pulsed facility at the Russian Research Centre Kurchatov Institute. The liner is composed of 50 wires 6 μm in diameter and 1 cm in length, which are equally spaced on a circle 1 cm in diameter. An aluminum wire 120 μm in diameter is positioned at the array axis. The liner implosion was accompanied by the generation of VUV and soft X-ray emission. The parameters of the pinch plasma produced during the liner implosion onto the aluminum wire were determined from the time-resolved spectral measurements by a five-channel polychromator. The ion and electron densities turned out to be equal to n _i≈4×10¹⁹ cm⁻³ and n _e≈4×10²⁰ cm⁻³, respectively, and the electron temperature was T _e≈40 eV. The radiation energy measured in the range 50–600 eV was 2–10 kJ. The sources of soft X-ray emission in hydrogen-and helium-like aluminum lines were the bright spots and local objects (clouds) formed in the plasma corona at an electron temperature of 200–500 eV and electron density of 10²¹–10²² cm⁻³. The possibility of both the generation of an axial magnetic field during the liner implosion and the conversion of the energy of this field into soft X-ray emission is discussed. __________ Translated from Fizika Plazmy, Vol. 28, No. 6, 2002, pp. 514–521. Original Russian Text Copyright ? 2002 by Bakshaev, Blinov, Dan'ko, Ivanov, Klír, Korolev, Kravárik, Krása, Kubeš, Tumanov, Chernenko, Chesnokov, Shashkov, Juha.     

Influence of the current growth rate on the polarity effect in a wire array in the Angara-5-1 facility

It is shown that, at a sufficiently high current growth rate, the initial stage of implosion of a wire array is significantly affected by the radial electric fields. Due to the specific electrode configuration of wire arrays, the magnitude of the oppositely directed radial electric fields in different wire segments can reach 5 MV/cm. It is found that the process of plasma formation proceeds in different ways in segments with oppositely directed initial radial electric fields. The influence of this effect (the so-called “polarity effect”) on the implosion of cylindrical tungsten wire arrays in the Angara-5-1 facility becomes significant when the load voltage grows at a sufficiently high rate.     

Structural insight into lipopolysaccharide transport from the Gram-negative bacterial inner membrane to the outer membrane

Lipopolysaccharide (LPS) is an important component of the outer membrane (OM) of Gram-negative bacteria, playing essential roles in protecting bacteria from harsh environments, in drug resistance and in pathogenesis. LPS is synthesized in the cytoplasm and translocated to the periplasmic side of the inner membrane (IM), where it matures. Seven lipopolysaccharide transport proteins, LptA-G, form a trans‑envelope complex that is responsible for LPS extraction from the IM and transporting it across the periplasm to the OM. The LptD/E of the complex transports LPS across the OM and inserts it into the outer leaflet of the OM. In this review we focus upon structural and mechanistic studies of LPS transport proteins, with a particular focus upon the LPS ABC transporter LptB₂FG. This ATP binding cassette transporter complex consists of twelve transmembrane segments and has a unique mechanism whereby it extracts LPS from the periplasmic face of the IM through a pair of lateral gates and then powers trans‑periplasmic transport to the OM through a slide formed by either of the periplasmic domains of LptF or LptG, LptC, LptA and the N-terminal domain of LptD. The structural and functional studies of the seven lipopolysaccharide transport proteins provide a platform to explore the unusual mechanisms of LPS extraction, transport and insertion from the inner membrane to the outer membrane. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.     

Peripheral-type benzodiazepine receptors mediate translocation of cholesterol from outer to inner mitochondrial membranes in adrenocortical cells

In previous studies we demonstrated that peripheral-type benzodiazepine receptors (PBR) were coupled to steroidogenesis in several adrenocortical and Leydig cell systems (Mukhin, A.G., Papadopoulos, V., Costa, E., and Krueger, K.E. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 9813-9816; Papadopoulos, V., Mukhin, A.G., Costa, E., and Krueger, K.E. (1990) J. Biol. Chem. 265, 3772-3779). The current study elucidates the specific step in the steroid biosynthetic pathway by which PBR mediate the stimulation in steroid hormone production. The adrenocorticotropin (ACTH)-responsive Y-1 mouse adrenocortical cell line was used to compare the mechanisms by which ACTH and PK 11195 (a PBR ligand) stimulate steroidogenesis. The effects of these agents were studied at three stages along the steroid biosynthetic pathway: 1) secretion of 20 alpha OH-progesterone by Y-1 cell cultures; 2) pregnenolone production by isolated mitochondrial fractions; 3) quantities of cholesterol resident in outer and inner mitochondrial membrane fractions. Steroid synthesis stimulated by ACTH was blocked by cycloheximide, an effect documented by other laboratories characterized by an accumulation of mitochondrial cholesterol specifically in the outer membrane. In contrast, PK 11195-stimulated steroidogenesis was not inhibited by cycloheximide, and the magnitude of the stimulation was markedly enhanced when the cells were pretreated with cycloheximide and ACTH. When isolated mitochondria were used, stimulation of pregnenolone production by PK 11195 was largely independent of exogenously supplied cholesterol, indicating that PBR act on cholesterol already situated within the mitochondrial membranes. This phenomenon was found to be the result of a translocation of cholesterol from outer to inner mitochondrial membranes induced by the PBR ligand. These studies therefore suggest that mitochondrial intermembrane cholesterol transport in steroidogenic cells is mediated by a mechanism coupled to PBR.     

Investigation of the mechanism for rapid heating of nitrogen and air in gas discharges

A rapid heating of nitrogen-oxygen mixtures excited by gas discharges is investigated numerically with allowance for the following main processes: the reactions of predissociation of highly excited electronic states of oxygen molecules (which are populated via electron impact or via the quenching of the excited states of N₂ molecules), the reactions of quenching of the excited atoms O(¹ D) by nitrogen molecules, the VT relaxation reactions, etc. The calculated results adequately describe available experimental data on the dynamics of air heating in gas-discharge plasmas. It is shown that, over a broad range of values of the reduced electric field E/N, gas heating is maintained by a fixed fraction of the discharge power that is expended on the excitation of the electronic degrees of freedom of molecules (for discharges in air, η_E?28%). The lower the oxygen content of the mixture, the smaller the quantity η_E. The question of a rapid heating of nitrogen with a small admixture of oxygen is discussed.     

Visualization of domain formation in the inner and outer leaflets of a phospholipid bilayer

Large vesicles (5-10-micron in diameter) were formed in the presence of phospholipids fluorescently labeled on the acyl chain and visualized using a fluorescence microscope, charge-coupled-device camera and digital image processor. When such vesicles contained a fluorescent phosphatidic acid (PA) and were exposed to 2 mM CaCl2 or 0.5 mM PrCl3, it was possible to visualize PA-enriched domains within the vesicles. Calcium-induced domain formation was reversible in the presence of 4 mM EGTA. Vesicles were formed containing fluorescent PA on either the inner or outer leaflet of the bilayer and the patching and dissolution of patching were studied under conditions where calcium was present on the outside of the vesicle and where calcium was distributed across the bilayer. In addition, vesicles were formed with two different fluorescent PA's, one on the inner leaflet and a different one on the outer leaflet of the bilayer. The results of the experiments show that in vesicles formed primarily with naturally occurring phospholipids such as egg phosphatidylcholine or brain phosphatidylethanolamine, there was no coordinate action of the two leaflets of the bilayer. An exception to this was found, however, if the vesicles were formed in the presence of primarily dioleoyl phospholipids (greater than 95 mol %). In these vesicles there was a coordinate or coupled response to calcium by the two leaflets of the bilayer. In most cases, however, the two leaflets of the bilayer showed independent or uncoupled domain formation.     

Neutron emission during the implosion of a wire array onto a deuterated fiber

Results are presented from Z-pinch experiments performed in the S-300 facility (Kurchatov Institute) at a maximum current of 2 MA and current rise time of 100 ns. The Z-pinch load was a 1-cm-long 1-cmdiameter cylindrical array made of 40 tungsten wires with a total mass of 160 μg, at the axis of which a 100-μm-diameter (CD₂) _n deuterated fiber was installed. Hard X-ray and neutron signals were recorded using five scintillation detectors oriented in one radial and two axial directions. The maximum neutron yield from the DD reaction reached 3 × 10⁹ neutrons per shot. The average neutron energy was determined from time-of-flight measurements and Monte Carlo simulations under the assumption that the neutron emission time was independent of the neutron energy. The average neutron energy in different experiments was found to vary within the range 2.5–2.7 MeV. The fact that the average neutron energy was higher than 2.45 MeV (the energy corresponding to the DD reaction) is attributed to the beam-target collisional mechanism for the acceleration of deuterons to 100–500 keV.     

Determination of the plasma velocity in an imploding wire array from magnetic field measurements by a gradient probe

A method for measuring the gradient of the magnetic field in the plasma of an imploding wire array is described. Results from measurements of the magnitude and gradient of the magnetic field in a tungsten wire array on the Angara-5-1 facility at currents of ∼3 MA are presented. A novel method for calculating the velocity of the current-carrying plasma in the framework of MHD equations from data on the magnitude and gradient of the magnetic field at a certain point inside the array is proposed. It is demonstrated that a gradient magnetic probe can be used to investigate the plasma current sheath in plasma focus facilities.     

Gomori-positive elementary granules in inner and outer layer of the infundibulum

With the gallocyanine technique (Wittkowski, Bock and Franken, 1970) Gomori-positive substances of the infundibulum can be stained for light- and electron-microscopic examination. In various mammalian species, the size of Gomori-positive elementary granules in the outer layer is markedly different from that in the inner layer of the infundibulum. In general, the granules of the outer layer have less then half the diameter of those of the inner layer. In birds and fish, however, only small differences were found between the granules of both layers. The significance of the results is discussed.     

Metabolic regulation of organic osmolytes in tubules from rat renal inner and outer medulla

Glycerophosphorylcholine (GPC), sorbitol and inositol were quantitated in renal tubule suspensions from inner and outer medulla of untreated Sprague-Dawley rats to study the regulation of organic osmolyte concentrations under different metabolic conditions and varying extracellular osmolalities in vitro. Inner medullary tubules prepared in hypertonic saline (550 mosm/kg) contained osmolyte concentrations comparable to those found in the kidney in vivo. Incubation for up to 8 h at 5 mmol/l glucose increased sorbitol in the inner medullary tubules and medium in an osmolality-dependent fashion, whereas GPC and inositol remained constant. At a given glucose concentration the rate of sorbitol formation decreased linearly with increasing tubular sorbitol concentration, which was regulated by an osmolality-dependent export mechanism. Perturbation of tubular mechanisms by inhibition of glycolysis or oxidative phosphorylation did not change the tubular osmolyte content. In contrast to papilla outer medullary tubules contained only inositol. Lactate added as a metabolic substrate to the outer medullary tubules did not change the cellular inositol levels. In outer medullary tubules osmolality changes (320-710 mosm/kg) had no effect on tubular inositol. Addition of furosemide was without effect, when added in vitro. The results indicate that tubular sorbitol formation is regulated by glucose concentration, the level of tubular sorbitol, and an osmolality-dependent export mechanism. In contrast, cellular inositol and GPC levels seem to be independent of acute changes in tubular metabolism.     

Identification of a GTP-binding protein in the contact sites between inner and outer mitochondrial membranes

Whilst investigating whether GTP hydrolysis may be required for the import of preproteins into mitochondria we have found that a GTP-binding protein is located at the contact sites between mitochondrial inner and outer membranes. When mitochondrial outer membranes purified from rat liver were UV-irradiated in the presence of [alpha-32P]GTP, a 52 kDa protein was radiolabelled, whereas [alpha-32P]ATP did not label this protein. GTP-binding proteins were also labelled in the cytosolic and microsomal fractions, but the 52 kDa protein was concentrated in mitochondrial membranes and was the only protein specifically labelled by GTP in these membranes. Fractionation of mitochondrial membrane vesicles into outer membranes, inner membranes and contact sites between outer and inner membranes showed that the GTP-binding activity was highly enriched in contact sites, the location at which preprotein import is believed to occur. A protein of almost identical size was also found to be labelled in mitochondria from yeast.     

The transcript from a DNA puff of Rhynchosciara and its migration to the cytoplasm.

Electrophoretic analysis of 3H-RNA obtained from the proximal sections of Rhynchosciara salivary glands at two distinct developmental periods, one characterized by the presence and the other by the absence of the giant B-2 DNA puff, revealed that the appearance of a 14S poly(A)+ RNA is correlated with the opening of this puff. That this RNA species is transcribed from this puff is indicated by the fact that it is found in RNA extracted from B-2 puffs obtained by microdissection. This confirmed by the specific hybridization of the 14S poly(A)+ RNA to the B-2 locus. Our data indicate that the polyadenylation process takes place at the chromosome level, and that the nuclear sap is not an important compartment in the transport of polyadenylated RNA from the chromosome to the cytoplasm. The kinetics of migration of the 14S species to the cytoplasm were studied; the data indicate that this process is very rapid and, in addition, that the 14S RNA is unstable.     

Isolation and characterization of outer and inner envelope membranes of cyanelles from a glaucocystophyte, Cyanophora paradoxa

Envelope membranes were isolated by sucrose density gradient floatation centrifugation from the homogenate of cyanelles prepared from Cyanophora paradoxa. Two yellow bands were separated after 40 h of centrifugation. The buoyant density of one of the two fractions (fraction Y2) coincided with that of inner envelope membranes of spinach or plasma membranes of cyanobacteria. The other yellow fraction (fraction Y1) migrated to top of sucrose-gradient even at 0% sucrose. Pigment analysis revealed that the heavy yellow fraction was rich in zeaxanthin while the light fraction was rich in β-carotene, and the both fractions contained practically no chlorophylls. Another yellow fraction (fraction Y3) was isolated from the phycobiliprotein fraction, which was the position where the sample was placed for gradient centrifugation. Its buoyant density and absorption spectra were similar to outer membranes of cyanobacteria. We have assigned fractions Y2 and Y3 as inner and outer envelope membrane fractions of cyanelles, respectively. Protein compositions were rather different between the two envelope membranes indicating little cross-contamination among the fractions. H. Koike and Y. Ikeda contributed equally.     

A mechanism for sensing noise damage in the inner ear

Our sense of hearing requires functional sensory hair cells. Throughout life those hair cells are subjected to various traumas, the most common being loud sound. The primary effect of acoustic trauma is manifested as damage to the delicate mechanosensory apparatus of the hair cell stereocilia. This may eventually lead to hair cell death and irreversible deafness. Little is known about the way in which noxious sound stimuli affect individual cellular components of the auditory sensory epithelium. However, studies in different types of cell cultures have shown that damage and mechanical stimulation can activate changes in intracellular free calcium concentration ([Ca(2+)](i)) and elicit intercellular Ca(2+) waves. Thus an attractive hypothesis is that changes in [Ca(2+)](i), propagating as a wave through support cells in the organ of Corti, may constitute a fundamental mechanism to signal the occurrence of hair cell damage. The mechanism we describe here exhibits nanomolar sensitivity to extracellular ATP, involves regenerative propagation of intercellular calcium waves due to ATP originating from hair cells, and depends on functional IP(3)-sensitive intracellular stores in support cells.     

The dynamics of and radiation from a copper-wire corona in a megaampere Z-pinch

Evolution of the extreme ultraviolet (XUV) and soft X-ray (SXR) emission in the 50-to 2000-eV photon energy range from a plasma corona formed by loading a relatively thick Cu wire (with an initial diameter of 120 µm) was observed in a Z-pinch discharge with a maximum current of 2 MA and current rise time of 100 ns. A diagnostic complex consisting of a five-channel SXR polychromator, a four-frame X-ray pinhole camera, and a mica crystal spectrograph shows that double-humped emission pulses in the XUV and SXR spectral ranges are generated 70–130 ns after the onset of the discharge current. The total energy of the pulses is 5 kJ, and the maximum power is 60 GW. A part of the observed kiloelectronvolt X-ray emission from three to five spots with diameters of 1–2 mm consists of the Cu K-and L-shell lines.     

Stability of compression of the inner array plasma in nested arrays

The implosion of nested fiber/wire arrays was studied experimentally at the Angara-5-1 facility. The outer array consisted of kapron fibers, while the inner array was made of tungsten wires. The experiments were carried out at a discharge current of 3 MA. Stable compression of the inner array plasma was achieved by increasing the number of fibers in the outer array. In this case, a compact Z-pinch formed at the array axis. Near the pinch, no trailing plasma produced from the high-Z material of the inner array and capable of scattering and reradiating X-ray photons was observed. The trailing edge of the X-ray pulse was found to shorten in the absence of the trailing plasma around the pinch.