Electric breakdown during the pulsed current spreading in the sand

Processes of spreading of the pulsed current from spherical electrodes and an electric breakdown in the quartz sand are studied experimentally. When the current density on the electrode exceeds the critical value, a nonlinear reduction occurs in the grounding resistance as a result of sparking in the soil. The critical electric field strengths for ionization and breakdown are determined. The ionization-overheating instability is shown to develop on the electrode, which leads to the current contraction and formation of plasma channels.     

Development of a spark sustained by charging the stray capacitance of the external circuit in atmospheric-pressure nitrogen

Results are presented from experimental studies and numerical simulations of a spark discharge excited in a short point-plane gap filled with atmospheric-pressure nitrogen. The discharge was powered from a high-voltage source connected to the discharge gap through a large ballast resistance. In this case, a short-term spark develops only due to the charging of the stray capacitance of the external circuit; therefore, the energy released by the spark and its intensity are both low. Rapid current growth in a weak spark is accompanied by the contraction of the current channel rather than by its gasdynamic expansion, as it occurs in long-duration kiloampere sparks. Simulations show that, because of the very short spark lifetime, the plasma in a weak spark is substantially nonequilibrium and the gas temperature is fairly high.     

Evolution of the radial structure of a negative corona during its transformation into a glow discharge and a spark

With the proper stabilization of a negative corona, it is possible to increase the threshold current at which the corona discharge in the point-plane gap in air transforms into a spark. Then, in the current range corresponding to the transition region between the corona discharge and the spark, a new type of discharge arises—an atmospheric-pressure diffuse glow discharge. The transformation of the negative corona into a glow discharge and then into a spark is accompanied by the rearrangement of the discharge structure. The experiments show that, as the corona current increases, the radial current profile at the anode shrinks and the glow diameter near the anode increases. The radial profiles of the current and the corona glow during the transition to a glow discharge are measured. The longitudinal structure of the corona is computed using a 1.5-dimensional model that, unlike the other available models, includes gas ionization in the drift region of the corona. The experimental data are used to determine the effective cross section of the current channel at the anode. The radial glow profile near the anode is calculated using the measured current profile and assuming that the field profile is parabolic.     

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.     

Soil eukaryotic microorganism succession as affected by continuous cropping of peanut--pathogenic and beneficial fungi were selected

Peanut is an important oil crop worldwide and shows considerable adaptability but growth and yield are negatively affected by continuous cropping. Soil micro-organisms are efficient bio-indicators of soil quality and plant health and are critical to the sustainability of soil-based ecosystem function and to successful plant growth. In this study, 18S rRNA gene clone library analyses were employed to study the succession progress of soil eukaryotic micro-organisms under continuous peanut cultivation. Eight libraries were constructed for peanut over three continuous cropping cycles and its representative growth stages. Cluster analyses indicated that soil micro-eukaryotic assemblages obtained from the same peanut cropping cycle were similar, regardless of growth period. Six eukaryotic groups were found and fungi predominated in all libraries. The fungal populations showed significant dynamic change and overall diversity increased over time under continuous peanut cropping. The abundance and/or diversity of clones affiliated with Eurotiales, Hypocreales, Glomerales, Orbiliales, Mucorales and Tremellales showed an increasing trend with continuous cropping but clones affiliated with Agaricales, Cantharellales, Pezizales and Pyxidiophorales decreased in abundance and/or diversity over time. The current data, along with data from previous studies, demonstrated that the soil microbial community was affected by continuous cropping, in particular, the pathogenic and beneficial fungi that were positively selected over time, which is commonplace in agro-ecosystems. The trend towards an increase in fungal pathogens and simplification of the beneficial fungal community could be important factors contributing to the decline in peanut growth and yield over many years of continuous cropping.     

Laser radiation scattering from the wires and fibers of imploding arrays on the Angara-5-1 facility

A method is developed for measurements of laser radiation scattering by wires and fibers in different types of imploding arrays in the initial stage of plasma production at discharge currents per wire of up to 2 kA for aluminum arrays and up to 8 kA for tungsten arrays. The experiments were carried out on the Angara-5-1 facility at a current density in the wires of 10⁸ A/cm² and current growth rate of ～10¹³ A/s. It is found that the indicatrix of laser radiation reflected from the wires (fibers) in cylindrical and conical arrays is modified at currents of 0.1–10 kA per wire (fiber). The experimental data on the reflection and scattering of laser radiation from wires and fibers are compared with the results of numerical simulations of their electric explosion in vacuum. It is proposed that the change in the reflection indicatrix of laser radiation is caused by the onset of thermal instabilities. The typical size of density and temperature inhomogeneities on the wire surface is in a range of 10–20 μm, which probably results in a transition from specular to diffuse reflection of laser radiation. A simultaneous abrupt (over 2–3 ns) reduction in the reflection intensity from several wires of an array indicates a homogeneous distribution of the discharge current over the irradiated wires. This closes the issue of the quality of the contact between the wires and the electrodes. The obtained experimental information is of considerable importance for the development of numerical codes for simulations of the implosion of wire arrays and the refinement of the wire parameters in the initial stage of plasma production.     

Laboratory-scale evidence for lightning-mediated gene transfer in soil

Electrical fields and current can permeabilize bacterial membranes, allowing for the penetration of naked DNA. Given that the environment is subjected to regular thunderstorms and lightning discharges that induce enormous electrical perturbations, the possibility of natural electrotransformation of bacteria was investigated. We demonstrated with soil microcosm experiments that the transformation of added bacteria could be increased locally via lightning-mediated current injection. The incorporation of three genes coding for antibiotic resistance (plasmid pBR328) into the Escherichia coli strain DH10B recipient previously added to soil was observed only after the soil had been subjected to laboratory-scale lightning. Laboratory-scale lightning had an electrical field gradient (700 versus 600 kV m(-1)) and current density (2.5 versus 12.6 kA m(-2)) similar to those of full-scale lightning. Controls handled identically except for not being subjected to lightning produced no detectable antibiotic-resistant clones. In addition, simulated storm cloud electrical fields (in the absence of current) did not produce detectable clones (transformation detection limit, 10(-9)). Natural electrotransformation might be a mechanism involved in bacterial evolution.     

Laboratory-Scale Evidence for Lightning-Mediated Gene Transfer in Soil

Electrical fields and current can permeabilize bacterial membranes, allowing for the penetration of naked DNA. Given that the environment is subjected to regular thunderstorms and lightning discharges that induce enormous electrical perturbations, the possibility of natural electrotransformation of bacteria was investigated. We demonstrated with soil microcosm experiments that the transformation of added bacteria could be increased locally via lightning-mediated current injection. The incorporation of three genes coding for antibiotic resistance (plasmid pBR328) into the Escherichia coli strain DH10B recipient previously added to soil was observed only after the soil had been subjected to laboratory-scale lightning. Laboratory-scale lightning had an electrical field gradient (700 versus 600 kV m⁻¹) and current density (2.5 versus 12.6 kA m⁻²) similar to those of full-scale lightning. Controls handled identically except for not being subjected to lightning produced no detectable antibiotic-resistant clones. In addition, simulated storm cloud electrical fields (in the absence of current) did not produce detectable clones (transformation detection limit, 10⁻⁹). Natural electrotransformation might be a mechanism involved in bacterial evolution.     

Mechanisms for the formation and transport of ion fluxes in the plasma of a high-current vacuum spark

The processes of ion flux formation in the plasma of a high-current vacuum spark were investigated experimentally. It is shown that multicharged ions are generated in the neck formed in the erosion products of the inner electrode. The plasma escaping from the neck region plays a role of a piston dragging particles of the cold peripheral plasma into ambient space. As the discharge current increases, the flux of the evaporated electrode material grows, the degree of ionization of the plasma produced decreases, and the efficiency of plasma heating caused by the pinching effect is reduced.     

Gene Interactions between Potassium-Sensitive and Potassium-Resistant Mutations of PARAMECIUM TETRAURELIA

Two unlinked recessive mutations (ks-1 and ks-2 ) have been induced in Paramecium tetraurelia stock 51. Wild-type survives and grows when up to 30 mm KCl is added to the medium, but the mutants cease to grow and die when added KCl reaches 20-25 m m. These K(+)-sensitives have been crossed to stocks containing the K(+)-resistant genes, fA (very resistant) and kA (moderately resistant). All four genes are unlinked. Double mutants of ks-1 and either kA or fA are as resistant as the resistant member of the pair. Doubles of ks-2 and kA are like wild type, and doubles of ks-2 and fA are shifted from high resistance toward wild type. Gene ks-2 acts like a suppressor of kA and fA. This suppression can be understood in terms of the known biochemical defects of the mutants.     

Dynamics of the spatial structure of pulsed discharges in dense gases in point cathode−plane anode gaps and their erosion effect on the plane electrode surface

The dynamics of the spatial structure of the plasma of pulsed discharges in air and nitrogen in a nonuniform electric field and their erosion effect on the plane anode surface were studied experimentally. It is established that, at a nanosecond front of the voltage pulse, a diffuse discharge forms in the point cathode–plane anode gap due to the ionization wave propagating from the cathode. As the gap length decreases, the diffuse discharge transforms into a spark. A bright spot on the anode appears during the diffuse discharge, while the spark channel forms in the later discharge stage. The microstructure of autographs of anode spots and spark channels in discharges with durations of several nanoseconds is revealed. The autographs consist of up to 100 and more microcraters 5–100 μm in diameter. It is shown that, due to the short duration of the voltage pulse, a diffuse discharge can be implemented, several pulses of which do not produce appreciable erosion on the plane anode or the soot coating deposited on it.     

The water relations of the root–soil interface

The difference in hydrostatic pressure between the xylem of the leaf and the soil depends, for a given transpiration rate, on the series of hydraulic resistances encountered along this pathway. Many studies have shown that the sum of the resistances in the plant and the soil is too small to account for the fall in water pressure between the leaf xylem and the soil, especially when plants are growing in sandy soils, which are prone to dry rapidly. A resistance at the root–soil interface, caused possibly by poor contact between the roots and the soil, has been proposed to account for the discrepancy. We explored the resistance in the pathway from soil to leaf using a technique that allows precise and continuous non-destructive measurement of the hydrostatic pressure in the leaf xylem. When the soil was leached with water, the fall in leaf water status as the soil dried was reasonably well described by a simple physical model without the need to invoke an interfacial resistance. However, when the soil was flushed with a nutrient solution with an osmotic pressure of 70kPa, the hydrostatic pressure in the leaf xylem fell several times faster than that in the soil. We suggest that solutes accumulated either in the root or just outside it, creating large osmotic pressures, which gave the appearance of an interfacial resistance.     

Effect of the electron drift in a transverse electric field on the width of spark channels in a multichannel sliding discharge

The effect of electron drift on the transverse size of the spark channels in a multichannel sliding discharge on a dielectric surface is considered in a semianalytic approximation. The strength of an electric field transverse to the channel axis is estimated by the mirror image method. The estimate obtained is used to analyze the differential equation for the density of electrons with allowance for their drift from the channel into the surrounding layers. It is shown that the channel expands to a certain steady-state radius at which an increase in the local electron density due to the ionization of atoms is balanced by its decrease due to the electron drift from the surface channel layer into the surrounding layers. Numerical estimates are carried out for the conditions of earlier experiments with discharges in He, Ne, Ar, and Xe. The analysis applies to the initial nanosecond stage of the spark development, when the hydrodynamic expansion of the channels is still insignificant.     

Numerical simulations of spark channels propagating along the ground surface: Comparison with high-current experiment

A numerical model of a spark discharge propagating along the ground surface from the point at which an ∼100-kA current pulse is input into the ground has been developed based on experiments in which the velocity of a long leader was measured as a function of the leader current. The results of numerical simulations are in good agreement with the measured characteristics of creeping discharges excited in field experiments by using a high-power explosive magnetic generator. The reason why the length of a spark discharge depends weakly on the number of simultaneously developing channels is found. Analysis of the influence of the temporal characteristics of the current pulse on the parameters of the creeping spark discharge shows that actual lighting may exhibit similar behavior.     

耕作方式对黑土硬度和容重的影响

以吉林省德惠市8年黑土田间定位试验的小区土壤为研究对象,对不同耕作方式下土壤硬度和容重进行研究.结果表明:免耕增加了土壤硬度,主要表现在2.5 ～17.5 cm土层;在玉米连作和玉米-大豆轮作下,苗眼处免耕处理的最大土壤硬度分别为2816和1931 kPa,秋翻处理下分别为2660和2051 kPa,对作物生长均没有限制作用;秋翻处理的土壤硬度曲线随垄形而变化,免耕处理的土壤硬度曲线起伏较小.与秋翻相比,免耕显著增加了5～20 cm土层的土壤容重.5 ～ 30 cm土层的土壤容重在免耕处理下变化幅度较小,在秋翻处理下随土壤深度的增加逐渐增大.土壤容重与土壤硬度之间相关性不显著.     

Biofunctionalization of nanoparticle assisted mass spectrometry as biosensors for rapid detection of plant associated bacteria

This study is based on the application of matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) as biosensor to detect the plant associated bacteria (PAB) isolates from rhizospheric soil and root. The rapid bacterial detection via on particle ionization/enrichment technique using IgG functionalized Pt NPs (IgG-Pt NPs) assisted MALDI-TOF MS was successfully used to explore two PAB isolates, namely, Bacillus thuringiensis and B. subtilis from rhizospheric soil and roots of carrot plant. When these bacteria are used as bioformulations in agricultural as well as biotechnological applications, the plant growth promotion of economic crops was observed especially when the crops grow in less fertilize soil regions. This study proved that even at low concentrations, bacteria can also be directly detected without morphological, molecular and biochemical test. The current applied technique is simple, rapid and highly sensitive. Besides, it could be widely used for the detection of beneficially important PAB isolates in environmental samples.     

Assessing the applicability of the earth impedance method for in situ studies of tree root systems

Several electrical methods have been introduced as non-invasive techniques to overcome the limited accessibility to root systems. Among them, the earth impedance method (EIM) represents the most recent development. Applying an electrical field between a cormus and the rooted soil, the EIM measures the absorptive root surface area (ARSA) from grounding resistance patterns. Allometric relationships suggested that this method was a valuable tool. Crucial assumptions for the applicability of the EIM, however, have not been tested experimentally. Focusing on tree root systems, the present study assesses the applicability of the EIM. Six hypotheses, deduced from the EIM approach, were tested in several experiments and the results were compared with conventional methods. None of the hypotheses could be verified and the results allow two major conclusions. First, in terms of an analogue electrical circuit, a tree-root-soil continuum appears as a serial circuit with xylem and soil resistance being the dominant components. Allometric variation in contact resistance, with the latter being the proxy for root surface area, are thus overruled by the spatial and seasonal variation of soil and xylem resistances. Second, in a tree-root-soil continuum, distal roots conduct only a negligible portion of the electric charge. Most of charge carriers leave the root system in the proximal parts of the root-soil interface.     

The spark and its ember: separately gated local components of Ca(2+) release in skeletal muscle

Amplitude, spatial width, and rise time of Ca(2+) sparks were compared in frog fast-twitch muscle, in three conditions that alter activation of release channels by [Ca(2+)]. A total of approximately 17,000 sparks from 30 cells were evaluated. In cells under voltage clamp, caffeine (0.5 or 1 mM) increased average spark width by 28%, rise time by 18%, and amplitude by 7%. Increases in width were significant even among events of the same rise time. Spontaneous events recorded in permeabilized fibers with low internal [Mg(2+)] (0.4 mM), had width and rise times greater than in reference, and not significantly different than those in caffeine. The spark average in reference rides on a continuous fluorescence "ridge" and is continued by an "ember," a prolongation of width approximately 1 microm and amplitude <0.2, vanishing in approximately 100 ms. Ridge and ember were absent in caffeine and in permeabilized cells. Exposure of voltage-clamped cells to high internal [Mg(2+)] (7 mM) had effects opposite to caffeine, reducing spark width by 26% and amplitude by 27%. In high [Mg(2+)], the ember was visible in individual sparks as a prolongation of variable duration and amplitude up to 1.2. Based on simulations and calculation of Ca(2+) release flux from averaged sparks, the increase in spark width caused by caffeine was interpreted as evidence of an increase in radius of the release source-presumably by recruitment of additional channels. Conversely, spark narrowing suggests loss of contributing channels in high Mg(2+). Therefore, these changes in spark width at constant rise times are evidence of a multichannel origin of sparks. Because ridge and ember were reduced by promoters of Ca(2+)-dependent activation (caffeine, low [Mg(2+)]) and became more visible in the presence of its inhibitors, they are probably manifestations of Ca(2+) release directly operated by voltage sensors.     

Loading rate and external resistance control the electricity generation of microbial fuel cells with different three-dimensional anodes

The electricity generation, electrochemical and microbial characteristics of five microbial fuel cells (MFCs) with different three-dimensional electrodes (graphite and carbon felt, 2mm and 5mm graphite granules and graphite wool) was examined in relation to the applied loading rate and the external resistance. The graphite felt electrode yielded the highest maximum power output amounting up to 386Wm(-3) total anode compartment (TAC). However, based on the continuous current generation, limited differences between the materials were registered. Doubling the loading rate to 3.3gCODL(-1)TACd(-1) resulted only in an increased current generation when the external resistance was low (10.5-25Omega) or during polarization. Conversely, lowering the external resistance resulted in a steady increase of both the kinetic capacities of the biocatalyst and the continuous current generation from 77 (50Omega) up to 253 (10.5Omega)Am(-3)TAC. Operating a MFC at an external resistance close to its internal resistance, allows to increase the current generation from enhanced loading rates while maximizing the power generation.     

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.