Experimental study of a negative corona in atmospheric-pressure argon

Results are presented from experimental studies of a negative point-to-plane dc corona in atmospheric-pressure argon. Several operating modes are identified: a hysteresis region, a repetitive mode, and a glow-discharge mode. The effect of gas-dynamic parameters on the characteristics of a repetitive negative corona is investigated.     

Structure of the surface streamers of an AC barrier corona in argon

Results are presented from experimental studies of the structure of an ac surface discharge excited by a metal needle over a plane dielectric surface. A barrier corona discharge was ignited in atmospheric-pressure argon at frequencies of the applied sinusoidal voltage from 50 Hz to 30 kHz. In experiments, the area of a dielectric covered with the discharge plasma increased with applied voltage. The discharge structure in diffuse and streamer modes was recorded using a digital camera and a high-speed image tube operating in a frame mode. It is found that, in the positive and negative half-periods of the applied voltage, the structure of the surface discharge is substantially different. The statistical characteristics of the branching surface streamers in the positive and negative half-periods are determined as functions of the voltage frequency. The most intense lines in the emission spectrum of the barrier corona are determined for both half-periods. The correlation between the dynamics of the emission intensity and the dynamics of the discharge current and voltage is investigated.     

Characteristics of a DC discharge with a water cathode in argon

The characteristics of a dc discharge excited between a metal anode and a water cathode in argon were studied experimentally. The dimensions of the positive column and the electric field in it were measured, and the vibrational temperature in the positive column was determined from the N₂ C ³Π_u → B ³Π_g (0–2) emission band. It is shown that the power deposited in the positive column is almost entirely spent on gas heating. The obtained dependence of the reduced electric field on the gas pressure and the ionization frequencies calculated by solving the Boltzmann equation indicate that electrons are lost diffusively, whereas ionization proceeds in a stepwise manner via the lower metastable states of argon atoms.     

Simulation of ethylene conversion initiated by a streamer corona in an air flow

The conversion of ethylene (C₂H₄) at concentrations of 400 and 930 ppm in an air flow at a temperature of 295 K is simulated. Ethylene is added to air either upstream of the discharge chamber or in the reaction tube, downstream of a pulsed corona discharge. It is taken into account that the distribution of the gas components in the discharge zone is nonuniform due to the streamer nature of the discharge. In the reaction tube, all of the components are assumed to be uniform. Simulation results agree with the experiments carried out at voltage pulse amplitudes of 30 and 40 kV, a gas flow rate of 2–10 l/min, and a specific energy deposition of up to 0.15 J/cm³. It is shown that the ozone produced plays a governing role in the C₂H₄ conversion. It is found that it is possible to minimize the energy spent on conversion by choosing the optimum pulse repetition rate and the specific energy deposited per pulse. The presence of water vapor impedes the ethylene conversion and increases the concentration of formaldehyde and methane.     

Experimental and theoretical study of a quasi-steady electron-beam plasma in hot argon

An argon plasma produced by a quasi-steady high-energy electron beam was studied experimentally. The plasma density was measured using an open barrel-shaped microwave cavity. The gas temperature was shown to be a few times higher than room temperature. Electron beam propagation, as well as heat-transfer and kinetic processes in plasma, is modeled self-consistently for the actual experimental conditions. It is shown that the plasma density is largely governed by the conversion rate of the atomic ions into molecular ones. The calculated results are compared to the experimental data.     

Experimental study of the processes accompanying argon breakdown in a long discharge tube at a reduced pressure

Results are presented from experimental studies of the breakdown stage of a low-pressure discharge (1 and 5 Torr) in a glass tube the length of which (75 cm) is much larger than its diameter (2.8 cm). Breakdowns occurred under the action of positive voltage pulses with an amplitude of up to 9.4 kV and a characteristic rise time of 2–50 μs. The discharge current in the steady-state mode was 10–120 mA. The electrode voltage, discharge current, and radiation from the discharge gap were detected simultaneously. The dynamic breakdown voltage was measured, the prebreakdown ionization wave was recorded, and its velocity was determined. The dependence of the discharge parameters on the time interval between voltage pulses (the socalled “memory effect”) was analyzed. The memory effect manifests itself in a decrease or an increase in the breakdown voltage and a substantial decrease in its statistical scatter. The time interval between pulses in this case can reach 0.5 s. The effect of illumination of the discharge tube with a light source on the breakdown was studied. It is found that the irradiation of the anode region of the tube by radiation with wavelengths of ≤500 nm substantially reduces the dynamic breakdown voltage. Qualitative explanations of the obtained results are offered.     

Experimental investigation of X-ray lasing in a capillary argon discharge

Results are presented from experiments on the laser generation of X-ray radiation at the wavelength λ=469 ? (ε=26.4 eV) on the 3p(J=0)−3s(J=1) transition of Ne-like Ar ions. Experiments were carried out on the SIGNAL electrophysical facility with a 3.1-mm-diameter 157-mm-long Al₂O₃ ceramic capillary filled with argon at a pressure of 0.2–1.0 Torr. The discharge current amplitude was I ∼ 25–40 kA, the current rise rate being dI/dt ∼ 10¹² A/s. By a vacuum X-ray diode tuned to detect X-ray photons with energies in the range 10–40 eV, laser pulses with a duration of t ₁ ∼ 1 ns and maximum energy of E _1,max ∼ 1 μJ were recorded. The pulses were generated 35 ns after the discharge current was switched on. The line spectra in the wavelength range of 150–500 ? showed the bright λ=469 ? line. The angular divergence of the generated X-ray laser beam was estimated to be Δϑ ∼ 2 mrad. Original Russian Text ? O.N. Gilev, V.I. Afonin, V.I. Ostashev, V.Yu. Politov, A.M. Gafarov, A.L. Zapysov, A.V. Andriyash, é.P. Magda, L.N. Shamraev, A.A. Safronov, A.V. Komissarov, N.A. Khavronin, N.A. Pkhaĭko, L.V. Antonova, L.N. Shushlebin, 2006, published in Fizika Plazmy, 2006, Vol. 32, No. 2, pp. 160–165.     

Study of a DC gas discharge with a copper cathode in a water flow

A dc gas discharge between copper electrodes in the current range of 5–20 А was studied experimentally. The discharge gap length was varied within 45–70 mm. The cathode was a 10-mm-diameter rod placed in the water flowing out from a dielectric tube. Three discharge configurations differing in the position of the cathode upper end with respect to the water surface were considered: (i) above water; (ii) flush with the water surface, and (iii) under water. The electric and optical characteristics of the discharge in the second configuration were studied in more detail. It is established that the discharge properties are similar to those of an electric arc. Considerable cathode erosion was observed in the third configuration. It is revealed that fine-dispersed copper grains form in the course of erosion.     

Characteristics of a corona discharge with a hot corona electrode

The effect of the temperature of the corona electrode on the electrical characteristics of a corona discharge was studied experimentally. A modified Townsend formula for the current-voltage characteristic of a one-dimensional corona is proposed. Gasdynamic and thermal characteristics of a positive corona discharge in a coaxial electrode system are calculated. The calculated results are compared with the experimental data.     

Experimental study of the plasma effect on the generation of microwave radiation in systems with a virtual cathode

Results are presented from experimental studies of the plasma effect on the generation of microwave radiation in systems with a virtual cathode. Using a triode with a virtual cathode as an example, it is shown that the cathode and anode plasmas reduce the generation efficiency; in particular, the power of the generated microwave radiation decreases and the radiation frequency and the microwave pulse duration change appreciably. It is demonstrated that, at high microwave powers, the power radiated into free space can be reduced by the plasma generated at the surface of the output window. This plasma appears due to discharges developing on the window surface under the combined action of bremsstrahlung, UV radiation, electrons and ions arriving from the beam formation zone, and the microwave electric field.     

Dynamics of an Al wire corona of a megaampere Z-pinch

It is shown that the development of instabilities in a Z-pinch plasma formed by loading a relatively thick Al wire (an initial diameter of 120 μm and a maximum discharge current of 2–3 MA) is slowed down due to the high plasma density in the wire corona. A cylindrically symmetric, regular, and stable corona surrounding the wire contains a helical formation with a dense, cold, and magnetized plasma. X-ray pulses with a photon energy of several keV and an FWHM duration of 10–20 ns are generated by a few imploded neck structures in the pinch phase of the corona evolution (70–100 ns after the current onset). The main part of X radiation emitted by individual bright spots in the photon energy range 1.5–2.4 keV (up to 40 J at a peak power of 4 GW) consists of the continuum and the bound-bound transition radiation from H-and He-like Al ions. A possible scenario for the axial magnetic field evolution during an X-ray pulse is outlined. __________ Translated from Fizika Plazmy, Vol. 28, No. 4, 2002, pp. 329–336. Original Russian Text Copyright ? 2002 by Kubeš, Renner, Krousky, Kravárik, Bakshaev, Blinov, Chernenko, Gordeev, Dan’ko, Korolev, Shashkov.     

Calculation of multipoint likelihoods using flanking marker data: a simulation study

The calculation of multipoint likelihoods is computationally challenging, with the exact calculation of multipoint probabilities only possible on small pedigrees with many markers or large pedigrees with few markers. This paper explores the utility of calculating multipoint likelihoods using data on markers flanking a hypothesized position of the trait locus. The calculation of such likelihoods is often feasible, even on large pedigrees with missing data and complex structures. Performance characteristics of the flanking marker procedure are assessed through the calculation of multipoint heterogeneity LOD scores on data simulated for Genetic Analysis Workshop 14 (GAW14). Analysis is restricted to data on the Aipotu population on chromosomes 1, 3, and 4, where chromosomes 1 and 3 are known to contain disease loci. The flanking marker procedure performs well, even when missing data and genotyping errors are introduced.     

Experimental study of pulsatile and steady flow through a smooth tube and an atherosclerotic coronary artery casting of man

In vitro investigation of pulsatile and steady flows through a smooth, straight circular tube and a diseased human coronary artery cast was conducted with sugar-water solutions simulating the viscosity of blood. Time averaged pressure drops for pulsatile flows measured in the circular tube over a Reynolds number ranging from 50 to 1,000 were found to be identical to those for steady flows in the same tube, both of which were in excellent agreement with the Poiseuille flow prediction. For the polyurethane case (# 124) made from a human main coronary with significant but 'non obstructive' diffuse atherosclerotic disease, pressure drops for steady flows were found to be greater than Poiseuille flow predictions by a factor of 3-8 in the physiological Reynolds number range from about 100 to 400. Pulsatile flows in the same artery cast resulted in additional 30% increases in time averaged pressure drops, and thus flow resistance, compared to the steady flow data. Steady and pulsatile flow data measured in a straight, axisymmetric model of cast # 124 showed considerably smaller increases in flow resistance than those observed in # 124 casting.     

Comparative study of multipoint methods for genotype error detection

Several programs are currently available for the detection of genotyping error that may or may not be Mendelianly inconsistent. However, no systematic study exists that evaluates their performance under varying pedigree structures and sizes, marker spacing, and allele frequencies. Our simulation study compares four multipoint methods: Merlin, Mendel4, SimWalk2, and Sibmed. We look at empirical thresholds, power, and false-positive rates on 7 small pedigree structures that included sibships with and without genotyped parents, and a three-generation pedigree, using 11 microsatellite markers with 3 different map spacings. Simulated data includes 5,000 replicates of each pedigree structure and marker map, with random genotyping errors in about 4% of the middle marker's genotypes. We found that the default thresholds used by these programs provide low power (47-72%). Power is improved more by adding genotyped siblings than by using more closely spaced markers. Some mistyping methods are sensitive to the frequencies of the observed alleles. Siblings of mistyped individuals have elevated false-positive rates, as do markers close to the mistyped marker. We conclude that thresholds should be decided based on the pedigree and marker data and that greater focus should be placed on modeling genotyping error when computing likelihoods, rather than on detecting and eliminating genotyping errors.     

Analytic study of electron flows with a virtual cathode

Abstrac The formation of a virtual cathode is studied with one-dimensional analytic self-consistent dynamic models describing the pulsed injection of an electron beam into equipotential regions: half-space or a planeparallel gap. __________ Translated from Fizika Plazmy, Vol. 26, No. 5, 2000, pp. 439–444. Original Russian Text Copyright ? 2000 by Dubinov.     

Experimental study of laminar blood flow through an artery treated by a stent implantation: characterisation of intra-stent wall shear stress

The stimulation of endothelial cells by arterial wall shear stress (WSS) plays a central role in restenosis. The fluid-structure interaction between stent wire and blood flow alters the WSS, particularly between stent struts. We have designed an in vitro model of struts of an intra-vascular prosthesis to study blood flow through a 'stented' section. The experimental artery consisted of a transparent square section test vein, which reproduced the strut design (100x magnifying power). A programmable pump was used to maintain a steady blood flow. Particle image velocimetry method was used to measure the flow between and over the stent branches, and to quantify WSS. Several prosthesis patterns that were representative of the total stent strut geometry were studied in a greater detail. We obtained WSS values of between -1.5 and 1.5Pa in a weak SS area which provided a source of endothelial stimulation propitious to restenosis. We also compared two similar patterns located in two different flow areas (one at the entry of the stent and one further downstream). We only detected a slight difference between the weakest SS levels at these two sites. As the endothelial proliferation is greatly influenced by the SS, knowledge of the SS modification induced by the stent implantation could be of importance for intra-vascular prostheses design optimisation and thus can help to reduce the restenosis incidence rate.     

Comparison of a multipoint identity-by-descent method with parametric multipoint linkage analysis for mapping quantitative traits.

We previously developed a method of partitioning genetic variance of a quantitative trait to loci in specific chromosomal regions. In this paper, we compare this method--multipoint IBD (identical by descent) method (MIM)--with parametric multipoint linkage analysis (MLINK). A simulation study was performed comparing the methods for the major-locus, mixed, and two-locus models. The criterion for comparisons between MIM and MLINK was the average lod score from multiple replicates of simulated data sets. The effect of gene frequency, dominance, model misspecification, marker spacing, and informativeness are also considered in a smaller set of simulations. Within the context of the models examined, the MIM approach was found to be comparable in power with parametric multipoint linkage analysis when (a) parental data are unknown, (b) the effect of the major locus is small and there is additional genetic variation, or (c) the parameters of the major-locus model are misspecified. The performance of the MIM method relative to MLINK was markedly lower when the allele frequency at the trait locus was .2 versus .5, particularly for the case when parental data were assumed to be known. Dominance at the trait major locus, as well as marker spacing and heterozygosity, did not appear to have a large effect on the ELOD comparisons.     

Waves of states in an electron beam with a distributed virtual cathode

Results are presented from computer simulations of the dynamics of an electron beam injected into the drift space between two parallel conducting planes. The specific features of the competitive coexistence of states with and without a virtual cathode are investigated.     

Peculiarities of an electric discharge between an electrolytic cathode and a metal anode

Results from experimental studies of an electric discharge operating between a solid anode and an electrolytic cathode in a wide pressure range are presented. Specific features of the discharge ignition and discharge shape and peculiarities the structure of cathode spots on the electrolyte surface and anode spots on the surface of the solid electrode are revealed. The dependences of the current density on the electrolytic cathode and metal anode on the total current are measured, and the spatial distribution of the electric field is determined. A transition of a glow discharge into a multichannel discharge is investigated. The experimental data on the frequency and amplitude of the current and voltage pulsations are presented. Requirements for the maintenance of an electric discharge with an electrolytic cathode are formulated using the obtained experimental results.