Positive Streamers In Air Research Articles

Temperature and pressure effects on positive streamers in air

In this paper we investigate temperature and pressure effects on the properties of a positive pulsed streamer plasma. We use a corona plasma setup in which we vary the temperature between 303 and 773 K at atmospheric pressure and the pressure between 37.9 and 100 kPa at room temperature to achieve the same neutral gas density and compare electrical and optical (ICCD imaging) measurements carried out in both experiments.From the measurements we found that the dissipated plasma energy and the average streamer propagation velocity are both higher for the temperature experiments at the same neutral gas densities as in the pressure experiments. This suggests the existence of a specific temperature effect. A possible explanation for this effect is a higher streamer conductivity at high temperatures, as was previously found in the literature.

Electron density fluctuations accelerate the branching of positive streamer discharges in air

Branching is an essential element of streamer discharge dynamics. We review the current state of theoretical understanding and recall that branching requires a finite perturbation. We argue that, in current laboratory experiments in ambient or artificial air, these perturbations can only be inherited from the initial state, or they can be due to intrinsic electron-density fluctuations owing to the discreteness of electrons. We incorporate these electron-density fluctuations into fully three-dimensional simulations of a positive streamer in air at standard temperature and pressure. We derive a quantitative estimate for the ratio of branching length to streamer diameter that agrees within a factor of 2 with experimental measurements. As branching without this noise would occur considerably later, if at all, we conclude that the intrinsic stochastic particle noise triggers branching of positive streamers in air at atmospheric pressure.

Probing photo-ionization: simulations of positive streamers in varying N2 : O2-mixtures

Photo-ionization is the accepted mechanism for the propagation of positive streamers in air though the parameters are not very well known; the efficiency of this mechanism largely depends on the presence of both nitrogen and oxygen. But experiments show that streamer propagation is amazingly robust against changes of the gas composition; even for pure nitrogen with impurity levels below 1 ppm streamers propagate essentially with the same velocity as in air, but their minimal diameter is smaller, and they branch more frequently. Additionally, they move more in a zigzag fashion and sometimes exhibit a feathery structure. In our simulations, we test the relative importance of photo-ionization and of the background ionization from pulsed repetitive discharges, in air as well as in nitrogen with 1 ppm O2. We also test reasonable parameter changes of the photo-ionization model. We find that photo-ionization dominates streamer propagation in air for repetition frequencies of at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition frequency has to be included above 1 Hz. Finally, we explain the feather-like structures around streamer channels that are observed in experiments in high purity nitrogen, but not in air.

Reconnection and merging of positive streamers in air

Pictures show that streamer or sprite discharge channels emerging from the same electrode sometimes seem to reconnect or merge though their heads carry electric charge of the same polarity; one might therefore suspect that reconnections are an artefact of the two-dimensional projection in the pictures. Here we use stereo photography to investigate the full three-dimensional structure of such events. We analyse reconnection, possibly an electrostatic effect in which a late thin streamer reconnects to an earlier thick streamer channel, and merging, a suggested photoionization effect in which two simultaneously propagating streamer heads merge into one new streamer. We use four different anode geometries (one tip, two tips, two asymmetric protrusions in a plate and a wire), placed 40 mm above a flat cathode plate in ambient air. A positive high voltage pulse is applied to the anode, creating a positive corona discharge. This discharge is studied with a fast ICCD camera, in many cases combined with optics to enable stereoscopic imaging. We find that reconnections as defined above occur frequently. Merging on the other hand was only observed at a pressure of 25 mbar and a tip separation of 2 mm, i.e. for a reduced tip distance of p·d = 50 µm bar. In this case the full width at half maximum of the streamer channel is more than 10 times as large as the tip separation. At higher pressures or with a wire anode, merging was not observed.

Positive streamers in air and nitrogen of varying density: experiments on similarity laws

Positive streamers in ambient air at pressures from 0.013 to 1 bar are investigated experimentally. The voltage applied to the anode needle ranges from 5 to 45 kV, the discharge gap from 1 to 16 cm. Using a ‘slow’ voltage rise time of 100–180 ns, the streamers are intentionally kept thin. For each pressure p, we find a minimal diameter dmin. To test whether streamers at different pressures are similar, the minimal streamer diameter dmin is multiplied by its pressure p; we find this product to be well approximated by p · dmin = 0.20 ± 0.02 mm bar over two decades of air pressure at room temperature. The value also fits diameters of sprite discharges above thunderclouds at an altitude of 80 km when extrapolated to room temperature (as air density rather than pressure determines the physical behaviour). The minimal velocity of streamers in our measurements is approximately 0.1 mm ns−1 = 105 m s−1. The same minimal velocity has been reported for tendrils in sprites. We also investigate the size of the initial ionization cloud at the electrode tip from which the streamers emerge, and the streamer length between branching events. The same quantities are also measured in nitrogen with a purity of approximately 99.9%. We characterize the essential differences with streamers in air and find a minimal diameter of p · dmin = 0.12 ± 0.02 mm bar in our nitrogen.

Positive Streamers in Ambient Air and a $\hbox{N}_{2}\!:\!\hbox{O}_{2}$ Mixture (99.8 : 0.2)

Photographs show distinct differences between positive streamers in air or in a nitrogen-oxygen mixture (0.2% O2). The streamers in the mixture branch more frequently, but the branches also extinguish more easily. Probably related to that, the streamers in the mixture propagate more in a zigzag manner, whereas they are straighter in air. Furthermore, streamers in the mixture can become longer; they are thinner and more intense.

Photoionization and Optical Emission Effects of Positive Streamers in Air at Ground Pressure

A positive streamer in a weak electric field in air at ground pressure is investigated by utilizing a recently developed photoionization model based on the radiative transfer theory. The modeling results on the streamer emissions demonstrate that blue emissions of the second positive band system of N2 dominate the streamer spectra, in contrast to streamers in predominately red sprite discharges observed at low air pressures at high altitudes in the Earth's atmosphere.

Circuit dependence of the diameter of pulsed positive streamers in air

The diameter and branching structure of positive streamers in ambient air are investigated with a fast iCCD camera. We use different pulsed power circuits and find that they generate different spatial streamer structures. The electrodes have a point-plane geometry and a distance of 40 or 80 mm, and the peak voltages over the discharge gap are up to 60 kV. Depending on circuit and peak voltage, we observe streamers with diameters varying gradually between 0.2 and 2.5 mm. The streamer velocity increases with the diameter, ranging from 0.07 to 1.5 mm ns−1, while the current density within the streamers stays almost constant. The thicker streamers extend much further before they branch than the thinner ones. The pulsed power supplies are a switched capacitor supply with an internal resistance of 1 kΩ and a transmission line transformer supply with an impedance of 200 Ω; additional resistors change the impedance as well as the voltage rise time in the case of the capacitor supply. We observe that short rise times and low impedance create thick streamers close to the pointed electrode, while a longer rise time as well as a higher impedance create thinner streamers at the same peak voltage over the discharge.

Three‐dimensional modeling of blue jets and blue starters

Blue jets and blue starters are considered as positive streamer coronas expanding from the streamer zones of conventional lightning leaders under conditions when large‐scale electric fields near the thundercloud tops exceed the minimum field required for the propagation of positive streamers in air. Results from a three‐dimensional fractal model based on a phenomenological probabilistic approach to the modeling of streamer coronas indicate, in particular, that blue jets and blue starters can be formed by a fast (∼1 s) accumulation of ∼110–150 C of positive thundercloud charge distributed in a volume with effective radius ∼3 km near the cloud top at ∼15 km. The model simulates the propagation of branching streamer channels constituting blue jets and blue starters as a three‐dimensional growth of fractal trees in the electric field created by thundercloud charges and self‐consistently accounts for the electric field effects due to the propagating streamers. Model results closely resemble blue jet and blue starter characteristics in terms of their altitude extents, transverse dimensions, and conical structure and indicate that blue starters are related to the initial phases of blue jets. The proposed model is supported by the recent spectroscopic observations of blue jets and blue starters, in particular, by the documentation of the 427.8 nm (first negative N2+) emission in blue starters as well as by the low‐pressure laboratory experiments on emission spectroscopy of corona streamers in air. The model results also appear to be in excellent agreement with the recent discovery of the streamer structure of blue jets.

Simulation of streamer discharges as finitely conducting channels

This paper presents a two-dimensional simulation of positive streamers in air at atmospheric pressure in a quasi-uniform electric field. A streamer is assumed to consist of a hemispherical tip and a finitely conducting cylindrical channel with a constant axial potential gradient. The model predicts that E/sub g//spl sim/450 kV/m and the radius is r/spl sim/50 /spl mu/m in air at standard atmospheric conditions. Moreover, the estimated number of positive ions in the streamer head of stably propagating streamers, agrees with estimations based on more advanced streamer models. The model is used to predict the behavior of streamers in electrical discharges in a semiuniform electric field and good agreement is found between experiment and theory. The computer simulation of such a simplistic model could be applied to predict the behavior of streamer discharges in complex electrode arrangements, including dielectric surfaces.

Simulation of positive streamers in air in weak uniform electric fields

The results of two-dimensional numerical simulations of single positive streamer development in atmospheric air in a uniform electric field superimposed on the nonuniform field of a spherical anode are presented. It is shown that the form of the streamer channel and the rate of growth of the streamer velocity with its length are determined by the value of the applied field.

The variation with temperature of positive streamer properties in air

Measurements are reported of the variation of the electric field required for propagation of positive streamers in air as a function of ambient temperature. Propagation velocities have also been measured as function of field and temperature. The results are discussed in terms of changes of air density consequent upon the temperature changes and comparisons are made with other work in which density has been changed by variation of pressure at constant temperature. There is some evidence that specific temperature effects exist. The variation of streamer velocity with air density indicates that a constant reduced total field E/N is maintained in the avalanches, replicating the streamer tip as the density is changed.

Energy transfer in positive streamers

A Schlieren optical system has been used to measure changes in neutral density in a point-plane gap, during and following positive streamers in air. The results show that energy is transferred to the air molecules by two processes. Electron-neutral elastic collisions during the streamer generate a shock-front which moves radially outwards at around 350 m s-1. Following the streamer current pulse,which lasts for around 0.1 mu s, positive ions drifting towards the negative plane transfer their energy to the neutrals over a period of tens of mu s. This positive ion transfer generates the corona wind and the schlieren technique allows the wind speed to be measured close to the point. Corona wind speeds were found to be in the range 8 to 12 m s-1.