Calculation of the field and current density distributions in a non-self-sustained discharge in the discharge chamber of a CO laser

Two-dimensional spatial distributions of the electric field and current density in a non-self-sustained discharge controlled by a fast electron beam were calculated in the quasineutral plasma approximation. The calculations were carried out for a gas-discharge chamber with an antistreamer electrode grid placed in parallel to the output window of the ionizer. The voltage drop near the grid surface that appears due to the inhomogeneity of the spatial distribution of the current density was calculated. The fraction of the discharge current that passes the grid and flows onto the foil separating the vacuum volume of the ionizer from the gas-discharge chamber was estimated. The dependence of the calculated values on the geometric parameters of the electrode grid and its position with respect to the output of the ionizer was analyzed.     

Dynamics of the structure of the plasma current sheath in a plasma focus discharge

The study is aimed at investigating the fine structure of the plasma current sheath (PCS) in the PF-3 plasma focus facility. The PCS dynamics in a deuterium discharge was studied. The PCS parameters were measured using absolutely calibrated magnetic probes installed at different positions with respect to the facility axis and the anode surface. A magneto-optical probe recording both the magnetic signal and the PCS optical luminosity was first applied to analyze the PCS structure. This made it possible to spatially resolve the current and shock-wave regions. It is demonstrated that the current distribution is different in different discharge stages. It is shown that the neutron yield is determined by the value of the current compressed toward the axis, rather then the amplitude of the total discharge current.     

Formation of a spark discharge in an inhomogeneous electric field with current limitation by a large ballast Resistance

Results of studies of a spark discharge initiated in argon in a point–plane electrode gap with limitation of the discharge current by a large ballast resistance are presented. It is shown that the current flowing through the plasma channel of such a low-current spark has the form of periodic pulses. It is experimentally demonstrated that, when a low-current spark transforms into a constricted glow discharge, current pulses disappear, the spatial structure of the cathode glow changes abruptly, and a brightly glowing positive plasma column forms in the gap.     

Effects of current pulses on the sustained discharge of visual cells of Limulus

Current pulses were used in the eccentric and retinular cells of the Limulus lateral eye to produce changes in the interspike interval of the discharge sustained by a constant light level. The effects on the interspike interval of hyperpolarizing and depolarizing perturbations, applied at various delays from the previous spike, were measured for different intensities and durations of the current pulse. The results show that when the perturbations were applied in the first part of the interval, effects contrary to what is normal were produced (i.e, hyperpolarizing pulses decreased the interspike interval instead of increasing it and vice versa for depolarizing pulses). Here we discuss briefly the implications on neural encoding models.     

Singlet oxygen in the low-temperature plasma of an electron-beam-sustained discharge

Results are presented from experimental and theoretical studies of the production of singlet delta oxygen in a pulsed electron-beam-sustained discharge ignited in a large (～18-1) volume at a total gas mixture pressure of up to 210 Torr. The measured yield of singlet oxygen reaches 10.5%. It is found that varying the reduced electric field from ～2 to ～11 kV/(cm atm) slightly affects singlet oxygen production. It is shown experimentally that an increase in the gas mixture pressure or the specific input energy reduces the duration of singlet oxygen luminescence. The calculated time evolution of the singlet oxygen concentration is compared with experimental results.     

Breakdown in helium in high-voltage open discharge with subnanosecond current front rise

Investigations of high-voltage open discharge in helium have shown a possibility of generation of current pulses with subnanosecond front rise, due to ultra-fast breakdown development. The open discharge is ignited between two planar cathodes with mesh anode in the middle between them. For gas pressure 6 Torr and 20 kV applied voltage, the rate of current rise reaches 500 A/(cm² ns) for current density 200 A/cm² and more. The time of breakdown development was measured for different helium pressures and a kinetic model of breakdown in open discharge is presented, based on elementary reactions for electrons, ions and fast atoms. The model also includes various cathode emission processes due to cathode bombardment by ions, fast atoms, electrons and photons of resonant radiation with Doppler shift of frequency. It is shown, that the dominating emission processes depend on the evolution of the discharge voltage during the breakdown. In the simulations, two cases of voltage behavior were considered: (i) the voltage is kept constant during the breakdown; (ii) the voltage is reduced with the growth of current. For the first case, the exponentially growing current is maintained due to photoemission by the resonant photons with Doppler-shifted frequency. For the second case, the dominating factor of current growth is the secondary electron emission. In both cases, the subnanosecond rise of discharge current was obtained. Also the effect of gas pressure on breakdown development was considered. It was found that for 20 Torr gas pressure the time of current rise decreases to 0.1 ns, which is in agreement with experimental data.     

Attainment of Optimum Health

The history of the worker in public health is reviewed, his present activities are assessed, and predictions are made concerning his future role. It is emphasized that the public health specialist is but one member of the total health team in the community. His interdependence with other disciplines must be accepted if optimal health care is to be provided.Although prepared specifically for public health workers, this article has direct relevance to the future of the medical profession as a whole. In view of the present intense interest in the future pattern of health care in Canada, the viewpoint of a physician with a dual background in public health and medical school administration and teaching is considered to be particularly pertinent.     

Evolution of the channel of a long leader in air after a sharp decrease in the discharge current

The dynamics of the plasma parameters in a given cross section of a long-lived leader channel in air after a jumplike decrease in the discharge current is simulated numerically with the help of a one-dimensional non-steady-state model constructed with allowance for the dynamics of the energy input into the channel, the expansion of the channel, and the nonequilibrium ionization kinetics in the leader plasma. It is shown that, after a decrease in the current, the electric field in the channel, first, rapidly decreases and, then, increases gradually as the gas cools. The higher the energy input into the discharge before the decrease in the current, the longer the time scale on which the electric field increases. The results of simulations of the electric field in the channel agree with the data from the experimental modeling of the actual leader channel by a short spark.     

Prebreakdown phase of an interelectrode discharge in wate

Results are presented from experimental studies of the prebreakdown phase of an electric discharge between point (anode) and plane (cathode) electrodes immersed in water with different initial conductivity. When a high-voltage pulse is applied, the induced conductivity is detected in the discharge gap. Its value is one order of magnitude higher than the initial conductivity. It is shown that the induced conductivity increases almost linearly with the initial conductivity. The induced conductivity correlates with the UV emission from the cathode surface. A qualitative analysis of the experimental results is performed.     

Numerical simulations of the development of an open discharge

Plasma Physics Reports - Results are presented from numerical simulations of the time evolution of open discharges in helium that are excited in the presence of an anode grid and generate electron...     

Evolution of a capillary discharge induced by a semiconductor current generator

Results are presented from the intermediate stage of work on creating a current generator in a circuit with an inductive energy storage and a semiconductor opening switch made of 40 SDL-800 diodes. A six-diode generator prototype has been created with a current pulse amplitude of ～4.5 kA and a leading edge duration of ～10–20 ns at an inductive load of 30–35 nH. The generator was used to study discharges in capillaries filled with argon or hydrogen. It is shown that, in a 2-mm-diameter capillary, the initial azimuthal asymmetry of a structure arising during the breakdown ceases as the discharge evolves, whereas in a 0.8-mm-diameter capillary, it is retained. Time-resolved spectroscopic studies of the plasma reveal the presence of line emission of highly ionized argon (ArVII and ArVIII) in the hottest phase of the discharge, which indicates that a temperature of 20–40 eV has been achieved.     

Correlation between the neutron yield from a plasma focus device and the jump in the discharge current

The correlation between the neutron yield Y from a plasma focus device and the jump in the discharge current ??I has been studied experimentally. The total yield of the DD neutron source was measured by the method of activation of silver isotopes. The time dependences of the neutron and ??-radiation yields were recorded using scintillation detectors. It is found that, for the given experimental device, the neutron yield depends linearly on the jump in the discharge current ??I at the instant of neutron generation. The influence of the initial deuterium pressure in the discharge chamber on this dependence is examined.     

Equality in Educational Policy and the Heritability of Educational Attainment

Secular variation in the heritability of educational attainment are proposed to be due to the implementation of more egalitarian educational policies leading to increased equality in educational opportunities in the second part of the 20^th century. The action of effect is hypothesized to be a decrease of shared environmental (e.g., family socioeconomic status or parents’ education) influences on educational attainment, giving more room for genetic differences between individuals to impact on the variation of the trait. However, this hypothesis has not yet found consistent evidence. Support for this effect relies mainly on comparisons between countries adopting different educational systems or between different time periods within a country reflecting changes in general policy. Using a population-based sample of 1271 pairs of adult twins, we analyzed the effect of the introduction of a specific educational policy in Spain in 1970. The shared-environmental variance decreased, leading to an increase in heritability in the post-reform cohort (44 vs. 67%) for males. Unstandardized estimates of genetic variance were of a similar magnitude (.56 vs. .57) between cohorts, while shared environmental variance decreased from .56 to .04. Heritability remained in the same range for women (40 vs. 34%). Our results support the role of educational policy in affecting the relative weight of genetic and environmental factors on educational attainment, such that increasing equality in educational opportunities increases heritability estimates by reducing variation of non-genetic familial origin.     

Microbial diversity in ultra-high-pressure rocks and fluids from the Chinese Continental Scientific Drilling Project in China

Microbial communities in ultra-high-pressure (UHP) rocks and drilling fluids from the Chinese Continental Scientific Drilling Project were characterized. The rocks had a porosity of 1 to 3.5% and a permeability of approximately 0.5 mDarcy. Abundant fluid and gas inclusions were present in the minerals. The rocks contained significant amounts of Fe2O3, FeO, P2O5, and nitrate (3 to 16 ppm). Acridine orange direct counting and phospholipid fatty acid analysis indicated that the total counts in the rocks and the fluids were 5.2 x 10(3) to 2.4 x 10(4) cells/g and 3.5 x 10(8) to 4.2 x 10(9) cells/g, respectively. Enrichment assays resulted in successful growth of thermophilic and alkaliphilic bacteria from the fluids, and some of these bacteria reduced Fe(III) to magnetite. 16S rRNA gene analyses indicated that the rocks were dominated by sequences similar to sequences of Proteobacteria and that most organisms were related to nitrate reducers from a saline, alkaline, cold habitat; however, some phylotypes were either members of a novel lineage or closely related to uncultured clones. The bacterial communities in the fluids were more diverse and included Proteobacteria, Bacteroidetes, gram-positive bacteria, Planctomycetes, and Candidatus taxa. The archaeal diversity was lower, and most sequences were not related to any known cultivated species. Some archaeal sequences were 90 to 95% similar to sequences recovered from ocean sediments or other subsurface environments. Some archaeal sequences from the drilling fluids were >93% similar to sequences of Sulfolobus solfataricus, and the thermophilic nature was consistent with the in situ temperature. We inferred that the microbes in the UHP rocks reside in fluid and gas inclusions, whereas those in the drilling fluids may be derived from subsurface fluids.     

Partially constricted glow discharge in an argon-nitrogen mixture

The characteristics of a glow discharge in Ar:N₂ mixtures with a low content of nitrogen (0.02–1%) are studied experimentally. Most studies were performed at pressures higher than 10 Torr, at which the discharge constriction goes by a jump and the hysteresis effect is well pronounced. It is found that the time during which the discharge switches from the diffuse to the constricted mode (and back) can reach ～1 s. The transition between these modes begins with the development of a constriction at one end of the positive column. Then, the constricted part of the discharge extends toward the other end until it occupies the entire column. The reverse transition occurs in a similar way. By varying the parameters of the electric circuit during the transition, the constriction front can be stopped to form a steady-state partially constricted discharge. It is shown that this type of discharge corresponds to points lying inside the hysteresis loop of the conventional I-V characteristic measured without affecting the discharge during a transition. A comparative analysis of the discharge characteristics in Ar:N₂ mixtures and in pure argon is performed.     

Coagulation of dust grains in the plasma of an RF discharge in argon

Results are presented from experimental studies of coagulation of dust grains of different sizes injected into a low-temperature plasma of an RF discharge in argon. A theoretical model describing the formation of dust clusters in a low-temperature plasma is developed and applied to interpret the results of experiments on the coagulation of dust grains having large negative charges. The grain size at which coagulation under the given plasma conditions is possible is estimated using the developed theory. The theoretical results are compared with the experimental data.     

Ordered dusty structures in the plasma of an RF electrodeless gas discharge

Results are presented from the experimental studies and numerical simulations of the behavior of dust grains in the plasma of an inductive RF discharge. The experiments were carried out with neon at a pressure of 25–500 Pa and with 1.87-μm melamine formaldehyde grains. The discharge was excited by a ring inductor supplied from a generator operating at a 100-MHz frequency. The effective dust-grain interaction potential used in numerical simulations involved the spatial dependence of the grain charge on the plasma floating potential, grain-interaction anisotropy resulting from the focusing of the drift ion current by the negatively charged grains, and specific features of the shielding of the dust grains by the plasma electrons and ions recombining both in the plasma bulk and on the grain surface. The results of Monte Carlo simulations show that the dust grains form specific filament structures observed experimentally in the plasma of an inductive electrodeless discharge. __________ Translated from Fizika Plazmy, Vol. 26, No. 5, 2000, pp. 445–454. Original Russian Text Copyright ? 2000 by Zobnin, Nefedov, Sinel’shchikov, Sinkevich, Usachev, Filinov, Fortov.