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S V Lebedev

Publications and source records attributed to S V Lebedev.

At least 19 recordsLinked to original sources
PubMed2006

Dynamics of cylindrically converging precursor plasma flow in wire-array Z -pinch experiments.

This paper summarizes the present understanding of the processes leading to precursor column formation in cylindrical wire arrays on the 1 MA MAGPIE generator at Imperial College London. Direct experimental measurements of the diameter variation during the collapse and formation phase of the precursor column are presented, along with soft x-ray emission, and quantitative radiography. In addition, data from twisted cylindrical arrays are presented which give additional information on the behavior of coronal plasma generated in wire array z pinches. Three stages in precursor column formation are identifiable from the data: broad initial density profile, rapid contraction to small diameter, and slow expansion after formation. The correlation of emission to column diameter variation indicates the contraction phase is a nonlinear collapse resulting from the increasing on-axis density and radiative cooling rate. The variation in the minimum diameter is measured for several array materials, and data show good agreement with a pressure balance model. Comparison of column expansion rates to analytical models allows an estimate of column temperature variation, and estimates of the current in the column are also made. Formation data are in good agreement with both fluid and kinetic modeling, but highlight the need to include collisionless flow in the early time behavior.

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PubMed2006

[Elimination of neurospecific proteins from CNS: pathogenetic and methodical aspects].

Appearance of neurospecific proteins (NSP) outside the brain plays a certain pathogenetic role in the development of autosensitization occurring in many kinds of CNS injuries and diseases. Analysis of modern views of cerebrospinal fluid (CSF) exchange allows us to suppose that NSP are eliminated from the brain tissue within CSF, moving from the subarachnoid space into cranial veins, and by lymphatic way, into deep cervical lymph nodes. Elevation of NSP level in CSF indicates an actual neudegenerative process. Serum levels of NSP are determined by the balance between the elimination of NSP from the brain, on the one hand, and their metabolism and the response of the immune system to the appearance of these autoantigenes in the blood stream, on the other. Basing on their own data on the dynamics of NSP (NSE, GFAR, and MBP) concentrations and the proportions of these proteins in CSF and serum (coefficient of elimination) in rats after ischemic, hypoxic, and autoimmune cerebral lesions, the authors offer an algorithm of pathogenetic evaluation, including, on the one part, a conclusion on the presence or absence of a neurodegenerative process, and, on the other, a conclusion on a normal or lowered rate of NSP elimination (metabolism). The results of such an analysis may have a clinical significance in terms of the development of a pathogenetic therapy, including, in every individual case, not only neuroprotectors, but also pharmaceuticals directed towards correction of the functional condition of the immune system.

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PubMed2005

Study of three-dimensional structure in wire-array z pinches by controlled seeding of axial modulations in wire radius.

Three-dimensional perturbations have been seeded in wire-array z pinches by etching 15 microm diameter aluminum wires to introduce 20% modulations in radius with a controlled axial wavelength. These perturbations seed additional three-dimensional imploding structures that are studied experimentally and with magnetohydrodynamics calculations, highlighting the role of current path nonuniformity in perturbation-induced magnetic bubble formation.

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PubMed2005

Demonstration of radiation pulse shaping with nested-tungsten-wire-array pinches for high-yield inertial confinement fusion.

Nested wire-array pinches are shown to generate soft x-ray radiation pulse shapes required for three-shock isentropic compression and hot-spot ignition of high-yield inertial confinement fusion capsules. We demonstrate a reproducible and tunable foot pulse (first shock) produced by interaction of the outer and inner arrays. A first-step pulse (second shock) is produced by inner array collision with a central CH2 foam target. Stagnation of the inner array at the axis produces the third shock. Capsules optimized for several of these shapes produce 290-900 MJ fusion yields in 1D simulations.

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PubMed2005

Effect of radial-electric-field polarity on wire-array Z-pinch dynamics.

The formation of plasma in wire-array Z-pinch experiments was found to depend upon the polarity of the radial-electric field near the wires. Reversing the radial-electric field midway along the length of an array resulted in the ablation rate of one-half of the array being reduced by 50%, significantly delaying the start of its implosion and altering its acceleration towards the axis. The observed phenomena cannot be explained by the standard magnetohydrodynamic models of array behavior, suggesting that effects such as electron emission may be important, especially during wire initiation.

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PubMed2005

Direct experimental evidence for current-transfer mode operation of nested tungsten wire arrays at 16-19 MA.

Nested tungsten wire arrays (20-mm on 12-mm diam.) are shown for the first time to operate in a current-transfer mode at 16-19 MA, even for azimuthal interwire gaps of 0.2 mm that are the smallest typically used for any array experiment. After current transfer, the inner wire array shows discrete wire ablation and implosion characteristics identical to that of a single array, such as axially nonuniform ablation, delayed acceleration, and trailing mass and current. The presence of trailing mass from the outer and the inner arrays may play a role in determining nested array performance.

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PubMed2005

Characteristics and scaling of tungsten-wire-array z -pinch implosion dynamics at 20 MA.

We present observations for 20-MA wire-array z pinches of an extended wire ablation period of 57%+/-3% of the stagnation time of the array and non-thin-shell implosion trajectories. These experiments were performed with 20-mm-diam wire arrays used for the double- z -pinch inertial confinement fusion experiments [M. E. Cuneo, Phys. Rev. Lett. 88, 215004 (2002)] on the Z accelerator [R. B. Spielman, Phys. Plasmas 5, 2105 (1998)]. This array has the smallest wire-wire gaps typically used at 20 MA (209 microm ). The extended ablation period for this array indicates that two-dimensional (r-z) thin-shell implosion models that implicitly assume wire ablation and wire-to-wire merger into a shell on a rapid time scale compared to wire acceleration are fundamentally incorrect or incomplete for high-wire-number, massive (>2 mg/cm) , single, tungsten wire arrays. In contrast to earlier work where the wire array accelerated from its initial position at approximately 80% of the stagnation time, our results show that very late acceleration is not a universal aspect of wire array implosions. We also varied the ablation period between 46%+/-2% and 71%+/-3% of the stagnation time, for the first time, by scaling the array diameter between 40 mm (at a wire-wire gap of 524 mum ) and 12 mm (at a wire-wire gap of 209 microm ), at a constant stagnation time of 100+/-6 ns . The deviation of the wire-array trajectory from that of a thin shell scales inversely with the ablation rate per unit mass: f(m) proportional[dm(ablate)/dt]/m(array). The convergence ratio of the effective position of the current at peak x-ray power is approximately 3.6+/-0.6:1 , much less than the > or = 10:1 typically inferred from x-ray pinhole camera measurements of the brightest emitting regions on axis, at peak x-ray power. The trailing mass at the array edge early in the implosion appears to produce wings on the pinch mass profile at stagnation that reduces the rate of compression of the pinch. The observation of precursor pinch formation, trailing mass, and trailing current indicates that all the mass and current do not assemble simultaneously on axis. Precursor and trailing implosions appear to impact the efficiency of the conversion of current (driver energy) to x rays. An instability with the character of an m = 0 sausage grows rapidly on axis at stagnation, during the rise time of pinch power. Just after peak power, a mild m = 1 kink instability of the pinch occurs which is correlated with the higher compression ratio of the pinch after peak power and the decrease of the power pulse. Understanding these three-dimensional, discrete-wire implosion characteristics is critical in order to efficiently scale wire arrays to higher currents and powers for fusion applications.

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PubMed2004

Selective accumulation of monoclonal antibodies against neurospecific enolase in brain tissue of rats with middle cerebral artery occlusion.

Preparations of I(125)-labeled monoclonal antibodies against neurospecific enolase and mouse plasma IgG1 were injected intravenously to rats immediately after unilateral occlusion of the middle cerebral artery. Radioactivity of I(125)-labeled monoclonal antibodies against neurospecific enolase in the brain tissue progressively increased, reached a maximum by the 48th hour, and remained practically unchanged after 72 h. At the same time radioactivity of labeled IgG1 in the brain tissue and radioactivity of both preparations in the blood, liver, spleen, kidneys, heart, and lungs decreased over 72 h. Selective accumulation of I(125)-labeled monoclonal antibodies against neurospecific enolase was less significant in the brain tissue of the contralateral hemisphere and cerebellum not exposed to ischemia.

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PubMed2004

Spatial characteristics of cisterna magna in rats and novel technique for puncture with a stereotactic manipulator.

The actual size and sites for puncture of the cisterna magna in adult Wistar rats were estimated on acrylic models. We proposed a new method for obtaining cerebrospinal fluid from narcotized rats. This method consists in controlled introduction of a needle into the cisterna magna through the skin and/or dura mater above this structure using a stereotactic micromanipulator. Experiments on rats with focal and global cerebral ischemia showed that this technique ensures single and repeated sampling of at least 50 microl liquor not containing blood admixtures.

Animals↗
PubMed2004

[Modeling of focal ischemia of the brain].

Methods of modeling and criteria of evaluating the pathological process in the central nervous system (CNS) as well as modern technologies of provoking the focal ischemia of the brain in experimental animals are under discussion. The results were analyzed comparatively from the viewpoint of efficiency and adequacy of certain models as well as of the clinical specificity of ischemic strikes in man and of set research goals. A literature analysis confirms that the existing arsenal of methodical schemes provides for choosing the most adequate model of focal ischemia of the brain and to ensure a cerebral infarction of a preset scope and localization; it makes possible also an objective evaluation of pathological processes occurring in the cerebral tissues both at the earliest stages of ischemic lesion and during a relatively long time period comparable with rehabilitation time period. Achievements in the sphere of experimental modeling of focal ischemia of the brain pave the way to further promotion of experimental therapy in acute stroke and open up new research priorities; it concerns primarily research of mechanisms timing the neurodegenerative process after ischemic stroke as well as searching-for and testing of means of stroke prevention and of patients' rehabilitation.

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PubMed2003

Ketamine-induced rotational asymmetry in evaluation of motor disturbances in rats with middle cerebral artery occlusion.

Intraperitoneal injection of ketamine in subanesthetic doses to Wistar rats with unilateral occlusion of the middle cerebral artery caused ipsilateral rotation (2-10 rpm), which was recorded in an automatic rotameter. The optimal dose of ketamine was 50 mg/kg. The animals were examined in an automatic rotameter for 40 min. Motor asymmetry persisted for no less than 2 months after surgery. According to the neurological test (Menzies scale) motor asymmetry in animals with focal brain ischemia persisted for no more than 30 days. The degree of ketamine-induced motor asymmetry in intact rats was 0.10 0.03 rpm.

Animals↗
PubMed2003

Monitoring of neurological deficit and disturbances in higher nervous activity in rats with focal cerebral ischemia.

Neurological, locomotor, and behavioral changes in 20 Wistar rats with permanent proximal occlusion of the middle cerebral artery and 18 control sham-operated animals were monitored for 2 months at 10-day intervals. Neurological deficit was maximum immediately after occlusion (3.0 0.6 points), then progressively decreased, but did not completely disappear (0.70 0.06 points on day 60). In control animals neurological status returned to normal on days 10-15. The degree of ketamine-induced rotational asymmetry was 4.0 0.7 rpm over 40 days after surgery and decreased to 2.5 0.6 rpm on day 60. In control rats this parameter only transiently increased to 1.00 0.03 rpm (tdelta=2.5, p<0.05). The time of stay on a rotarod and the latency of passive avoidance in rats with focal cerebral ischemia were lower than in control animals throughout the experiment. The results of complex tests can be used in the experimental search for new drugs for the therapy and rehabilitation of stroke patients.

Animals↗
PubMed2003

Neurological deficit and disturbances in higher nervous activity during modeling of perinatal hypoxic-ischemic damage to the central nervous system in rat pups.

Seven-day-old Wistar rat pups were subjected to unilateral occlusion of the common cerebral artery and maintained in oxygen-low atmosphere. Neurological and behavioral changes were monitored for 12 weeks. The survival rate of treated animals was 90%. Body weight gain in these rats was lower than in the control. Neurological deficit was maximum 1 week after treatment and slightly regressed by the 12th week. Locomotor activity in treated rats was higher than in controls. Administration of ketamine in subanesthetic doses caused permanent ipsilateral rotational asymmetry in animals. Spatial disorientation and cognitive deficit in rats with hypoxic-ischemic damage to the central nervous system were revealed in passive avoidance, Y-maze, and rotarod tests. The total area of the hemisphere decreased, while the area of the lateral cerebral ventricle increased at the side of occlusion over the first 4-5 weeks of postnatal development. The size of the ipsilateral hemisphere remained low in adult animals.

Animals↗
PubMed2003

Enzyme immunoassay of NSE and GFAP as the criterion of dynamic evaluation of the rat blood-brain barrier in perinatal hypoxic ischemic injury of the CNS.

Enzyme immunoassay of the serum neurospecific antigens (gliofibrillar acid protein and neurospecific enolase) was used for evaluation of the resistance of the blood-brain barrier in Wistar rats with perinatal hypoxia and ischemia of the CNS. Perinatal hypoxia and ischemia of the CNS was modeled by two methods: ligation of the common carotid artery in 7-day-old rats followed by 3.5-h hypoxic hypoxia or 15-min anoxic exposure of fetuses isolated via hysterectomy on day 21 of gestation. Enzyme immunoassay of serum gliofibrillar acid protein and neurospecific enolase in control an experimental rat pups was carried out once a week during 3 months. In controls serum levels of gliofibrillar acid protein and neurospecific enolase virtually did not change during postnatal development, while in animals with cerebral hypoxia and ischemia induced in fetuses by both methods serum concentration of neurospecific enolase sharply increased 1 week after the injury and increased on weeks 6 and 10. The content of gliofibrillar acid protein was maximum on week 1 and later considerably varied, the peaks of its concentrations observed on weeks 3 and 8 preceded the increase in neurospecific enolase activity in peripheral blood.

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PubMed2002

Indirect-drive inertial confinement fusion using highly supersonic, radiatively cooled, plasma slugs.

We present a new approach to indirect-drive inertial confinement fusion which makes use of highly supersonic, radiatively cooled, slugs of plasma to energize a hohlraum. 2D resistive magnetohydrodynamic simulations of slug formation in shaped liner Z-pinch implosions are presented along with 2D-radiation-hydrodynamic simulations of the slug impacting a converter foil and 3D-view-factor simulations of a double-ended hohlraum. Results for the Z facility at Sandia National Laboratory indicate that two synchronous slugs of 250 kJ kinetic energy could be produced, resulting in a capsule surface temperature of approximately 225 eV.

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PubMed2002

Comparison of the efficacy of cell preparations from embryonic ventral mesencephalon of various prenatal age transplanted intrastriatally to rats with 6-OHDA-induced Parkinsonism.

Cell preparations of ventral mesencephalon obtained from 8-, 14-, and 16-17-day rat embryos were stereotactically transplanted to homologous rats with 6-hydroxydopamine-induced hemiparkinsonism. Automated analysis of apomorphine-induced motor asymmetry for 3 months after neurotransplantation revealed higher efficacy of cell preparations from 8- and lower from 16-17-day-old embryos. These data correlated with histomorphological findings, in particular, with the size of grafts, glial reaction, and the number of dopaminergic neurons in the grafts.

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