Pulsed mode of a negative corona in nitrogen: II. Numerical calculations

A simplified model of a cathode sheath sustained by electron avalanches is presented. The model is used to calculate the pulsed mode of a negative corona in nitrogen in order to establish the physical picture of the processes occurring in a pulsed corona. The most important point is that, in the pulsed mode, both the averaged and dynamic current-voltage characteristics of a glow cathode sheath are found to have a negative slope. Lowering the degree to which the glow cathode sheath is subnormal (by sharply reducing the sheath area) or switching on additional ionization mechanisms (e.g., stepwise ionization) that force the cathode sheath to evolve into a prearc spot causes the negative slopes of the averaged and dynamic current-voltage characteristics of the sheath to become more gradual and even positive, thereby stabilizing the discharge current.     

Shape of the current tube of a negative point-to-plane corona in air

Results are presented from the experimental studies of a negative point-to-plane air corona discharge with controlled geometry. It is shown that, in a certain range of parameters, the steady-state current-voltage characteristic of the corona can be fitted by a parabola. The results obtained are used to determine the time-averaged shape of the current tube of a negative corona.     

Spatiotemporal evolution of the current and the integral and spectral emission characteristics of a negative corona in nitrogen during its transformation into a spark

Results are presented from experimental studies of the conversion of a steady-state negative corona into a spark. It is found that a spark in a negative corona in nitrogen and air is formed in the absence of fast primary streamers. It is shown that, in atmospheric-pressure nitrogen, the conversion of a corona into a spark begins with the propagation of a plasma channel (secondary streamer) from the point electrode (cathode) to the plane electrode (anode). In contrast, the plasma channel in air originates near the plane electrode and then propagates towards the point electrode. The propagation velocity of the secondary streamer is very low, V=10³–10⁴ cm/s. Two possible scenarios of the formation of the spark channel in a negative corona in nitrogen are described on the basis of the concept of a contracted volume glow discharge. Results are presented from time-resolved spectral measurements of plasma emission from different regions of the corona during its transformation into a spark.     

Phenomenology of a high-current negative point-to-plane corona in nitrogen

Results are presented from experimental studies of a steady-state negative point-to-plane corona in atmospheric-pressure nitrogen at anomalously high (milliampere) currents. The evolution of the corona glow with increasing discharge current is traced up to the corona conversion into a spark. It was found that the structure and current-voltage characteristic of the corona change significantly when the working gas is slowly blown trough the discharge cell. It is shown that, starting from a current of I≥0.5 mA, a negative corona in a nitrogen flow acts as a streamer corona, whereas there are no streamers in the absence of gas blowing. In contrast to the quadratic dependence of the current versus voltage in a low-current corona, the current-voltage characteristic of a high-current corona is linear. The time evolution of the radial profile of the current density at the anode is studied under different experimental conditions.     

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.     

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.     

Ion exposure chambers for small animals

Ion exposure chambers that have been designed and tested for use in biological and behavioral research with small animals are described in this report. The chambers exhibit an acceptable degree of uniformity in ion concentration, current density, and electric field within the exposure area. Gaseous by-products of corona discharge (O3 and NO2) have been measured and found to be less than .01 ppm and less than .1 ppm, respectively. Filtered air is fed to the individual exposure chambers, and temperature and humidity are well controlled. Noise due to corona and the air delivery system has been measured.     

Ozone produced by corona discharge in the presence of water

Significant levels of ozone have been detected in a reaction flask that was designed for studies using negative air ions generated from corona discharge. While diluting the gas in the vessel at the rate of 1.5 liters/min., more than 1.0 ppm O₃ was measured in the vessel when grounded water was present. Ozone levels were much less (15 ppb) in ambient air near the ion generator. Even in the absence of water, O₃ levels were only 7 ppb in the reaction flask. Without the corona discharge apparatus, levels of ozone in the gas phase (with water present) averaged 6 ppb. These results demonstrate that when negative air ions are generated from corona discharge in the presence of water, significant levels of ozone are produced. Therefore, O₃ could be directly responsible for many of the oxidizing effects that have been reportedly due to negative air ions.     

Growth and certain chemical constituents of tobacco plants exposed to air ions

Controlled experiments were performed in Faraday cages on the effects of positive and negative air ions on flue-cured tobacco plants. Continuous exposures for 15 days to air ions showed no significant differences in any plant growth characteristic between the treated and control plants. Standard errors in the measurement of the growth parameters for ion exposed plants were, however, consistently higher than those of control plants. Spatial variation in concentration gradients of air ions produced by corona discharge might have contributed to masking of the relatively small effects of air ions on biological organisms observed in previous experiments in this laboratory. No significant difference was observed between the experimental and control plants in nicotine, total alkaloid, and reducing sugar contents. Total nitrogen content was slightly higher for treated than control plants.     

Transition of a multipin negative corona in atmospheric air to a glow discharge

It is commonly accepted that, as the current increases, a diffuse negative corona inevitably goes over to a strongly nonuniform and nonsteady spark discharge. In this paper, a new effect—the transition of a negative corona to a diffuse glow discharge at atmospheric pressure—is studied experimentally and numerically. The evolution of the corona parameters during the transition to the regime of a glow discharge is traced.     

Comparison of the AC barrier corona with DC positive and negative coronas and barrier discharge

Results are presented from experimental studies of ac corona discharges between a point electrode and a dielectric-coated plate in nitrogen, argon, helium, and air in the voltage frequency range f=50 Hz–50 kHz. The characteristic features of this type of discharge are compared with the well-known features of dc positive and negative coronas and a barrier discharge between plane electrodes. It is shown that the presence of a dielectric barrier on the plane electrode significantly changes the electric characteristics and spatial structure of the corona, whereas the main phases of the discharge evolution remain unchanged as the voltage increases. With a point electrode, the breakdown voltage of the barrier corona decreases substantially as compared to the breakdown voltage of a barrier discharge with plane electrodes. This leads to softer conditions for the streamer formation in a barrier corona, which becomes more stable against spark generation.     

Simulation of nonstationary phenomena in atmospheric-pressure glow discharge

Nonstationary processes in atmospheric-pressure glow discharge manifest themselves in spontaneous transitions from the normal glow discharge into a spark. In the experiments, both so-called completed transitions in which a highly conductive constricted channel arises and incomplete transitions accompanied by the formation of a diffuse channel are observed. A model of the positive column of a discharge in air is elaborated that allows one to interpret specific features of the discharge both in the stationary stage and during its transition into a spark and makes it possible to calculate the characteristic oscillatory current waveforms for completed transitions into a spark and aperiodic ones for incomplete transitions. The calculated parameters of the positive column in the glow discharge mode agree well with experiment. Data on the densities of the most abundant species generated in the discharge (such as atomic oxygen, metastable nitrogen molecules, ozone, nitrogen oxides, and negative oxygen ions) are presented.     

Numerical simulations of Trichel pulses in a negative corona in air

Numerical simulations of a negative corona in air demonstrate that the experimentally observed regime of self-oscillations, known as Trichel pulses, is well described by a three-dimensional axisymmetric model that is based on the standard transport equations and in which only the ion-induced secondary electron emission at the cathode is taken into account. The quantitative difference between the measured and calculated values of the mean current and the pulse repetition rate most likely stems from the insufficiently large dimensions of the computation region and from the fact that the point shape adopted in simulations somewhat inexactly conforms to that used in experiments. It was found that the transverse discharge structure near the cathode radically changes during the pulse. Specifically, as the current grows, a cathode sheath forms at the discharge axis and expands over the cathode surface. When the current falls off, the cathode sheath is rapidly destroyed; as a result, the characteristic field structure is well defined only near the discharge axis and becomes virtually indistinguishable as the current decreases by an order of magnitude.     

Inactivation of possible micromycete food contaminants using the low-temperature plasma and hydrogen peroxide

The inhibition effect of hydrogen peroxide aerosol, low-temperature plasma and their combinations has been studied on several micromycetes spores. The low-temperature plasma was generated in corona discharges in the open air apparatus with hydrogen peroxide aerosol. Micromycete spores were inoculated on the surface of agar plates, exposed solely to the hydrogen peroxide aerosol, corona discharge or their combination. After incubation the diameter of inhibition zone was measured. The solely positive corona discharge exhibits no inactivation effect, the solely negative corona discharge and solely hydrogen peroxide aerosol exhibit the inactivation effect, however their combinations exhibit to be much more effective. Low-temperature plasma and hydrogen peroxide aerosol present a possible alternative method of microbial decontamination of food, food packages or other thermolabile materials.     

Two-component structure of the current pulse of a ranaway electron beam generated during electric breakdown of elevated-pressure nitrogen

Conditions are investigated at which two current pulses of ranaway electron beams are generated in elevated-pressure nitrogen during one voltage pulse. It is shown that the regime with two runaway electron beam current pulses takes place at decreased values of the electric field strength E in the gap (or decreased values of the parameter E/p, where p is the gas pressure). The regime with two runaway electron beam current pulses is observed both at high (1500?C3000 Torr) and low (below 100 Torr) pressures. It is shown that, for the second runaway electron beam current pulse to form, the voltage across the gap should be partially reduced during the first pulse. At low nitrogen pressures (~10 Torr), the regime in which two runaway electron beams are generated can be implemented by increasing the breakdown strength of the gap and/or increasing the value of E/p. In experiments carried out in atmospheric-pressure air with a picosecond time resolution, a rather complicated structure of the beam current pulse is observed at a voltage rise time of ~300 ps.     

Synergistic Combination of Electrolysis and Electroporation for Tissue Ablation

Electrolysis, electrochemotherapy with reversible electroporation, nanosecond pulsed electric fields and irreversible electroporation are valuable non-thermal electricity based tissue ablation technologies. This paper reports results from the first large animal study of a new non-thermal tissue ablation technology that employs “Synergistic electrolysis and electroporation” (SEE). The goal of this pre-clinical study is to expand on earlier studies with small animals and use the pig liver to establish SEE treatment parameters of clinical utility. We examined two SEE methods. One of the methods employs multiple electrochemotherapy-type reversible electroporation magnitude pulses, designed in such a way that the charge delivered during the electroporation pulses generates the electrolytic products. The second SEE method combines the delivery of a small number of electrochemotherapy magnitude electroporation pulses with a low voltage electrolysis generating DC current in three different ways. We show that both methods can produce lesion with dimensions of clinical utility, without the need to inject drugs as in electrochemotherapy, faster than with conventional electrolysis and with lower electric fields than irreversible electroporation and nanosecond pulsed ablation.     

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.     

Evaporation rate enhancement of water with air ions from a corona discharge

Space charges (air ions) produced by a single point-to-plane corona electrode system were used to study the enhancement in the evaporation rates of water at three ion current levels. The maximum evaporation rates of 0.019 and 0.017 g·min^?1 were observed at a 1 cm electrode gap for negative and positive air ions, respectively. The cumulative evaporation rates were linear with time and an ion-enhanced rate was about 4 times greater than the control. The current density distribution measurements agreed fairly well with those predicted from the Warburg law. The principal driving force for the observed evaporation enhancement was an ion drag phenomenon which created vortex motions in water when air ions were subjected to an externally applied electric field. Theoretical considerations from derived relationships in fluid mechanics demonstrate that the mass transfer coefficient is higher for positive than negative ions of the same current strength because of the mobility difference between the charges in the medium.     

An optical pulse modulation system for laser interference studies in the analytical ultracentrifuge

The mode of operation and construction of a high-speed laser light modulator is presented that is designed for use with the analytical ultracentrifuge. The system may be used with continuous wave lasers having optical powers not exceeding 100 W and has an optical bandwidth from 200 nm to 800 nm. The electronic modulation circuitry described is capable of producing optical pulses of 0.8 μsec duration at a firing frequency in excess of 100,000 pps and is designed to permit the laser source to operate in either a pulsed or nonpulsed mode. The unit is extremely versatile, and its characteristics permit full advantage to be taken of pulsed laser interferometry in the ultracentrifuge.     

Competition for pulsed resources: an experimental study of establishment and coexistence for an arid-land grass

In arid environments, episodically-pulsed resources are important components of annual water and nutrient supply for plants. This study set out to test whether seedlings have an increased capacity for using pulsed resources, which might then improve establishment when in competition with older individuals. A second aim was to determine whether there is a trade-off in competitive strategies when resources are supplied continuously at low concentrations, or as pulses with pronounced inter-pulse periods. A glasshouse experiment used a target-neighbour design of size-asymmetric competition, with juveniles of Panicum antidotale (blue panicgrass) introduced into contrasting densities of adult plants. Stable isotopes of nitrogen were used for measuring plant resource uptake from pulses, and tolerance to inter-pulse conditions was assessed as the mean residence time (MRT) of nitrogen. A higher root/shoot ratio and finer root system enhanced the capacity of juveniles to use resources when pulsed, rather than when continuously supplied. Higher resource uptake during pulses improved the establishment of juvenile Panicum in mixed cultures with older individuals. However, a trade-off was observed in plant strategies, with juveniles showing a lower MRT for nitrogen, which suggested reduced tolerance to resource deficit during inter-pulse periods. Under field conditions, higher utilization of pulsed resources would lead to the improved seedling establishment of Panicum adjacent to “nurse” plants, whereas mature plants with well-developed roots, exploiting a greater soil volume, maintain more constant resource uptake and retention during inter-pulse periods.