S Sander Nijdam

Probing photo-ionization: experiments on positive streamers in pure gases and mixtures

Positive streamers are thought to propagate by photo-ionization; the parameters of photo-ionization depend on the nitrogen : oxygen ratio. Therefore we study streamers in nitrogen with 20%, 0.2% and 0.01% oxygen and in pure nitrogen as well as in pure oxygen and argon. Our new experimental set-up guarantees contamination of the pure gases to be well below 1 ppm. Streamers in oxygen are difficult to measure as they emit considerably less light in the sensitivity range of our fast ICCD camera than the other gases. Streamers in pure nitrogen and in all nitrogen–oxygen mixtures look generally similar, but become somewhat thinner and branch more with decreasing oxygen content. In pure nitrogen the streamers can branch so much that they resemble feathers. This feature is even more pronounced in pure argon, with approximately 10 2 hair tips cm −3 in the feathers at 200 mbar; this density can be interpreted as the free electron density creating avalanches towards the streamer stem. It is remarkable that the streamer velocity is essentially the same for similar voltage and pressure in all nitrogen–oxygen mixtures as well as in pure nitrogen, while the oxygen concentration and therefore the photo-ionization lengths vary by more than five orders of magnitude. Streamers in argon have essentially the same velocity as well. The physical similarity of streamers at different pressures is confirmed in all gases; the minimal diameters are smaller than in earlier measurements. S Online supplementary data available from stacks.iop.org/JPhysD/43/145204/mmedia (Some figures in this article are in colour only in the electronic version)

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 .W e 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. (Some figures in this article are in colour only in the electronic version)

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.

Probing background ionization: positive streamers with varying pulse repetition rate and with a radioactive admixture

Positive streamers need a source of free electrons ahead of them to propagate. A streamer can supply these electrons by itself through photo-ionization, or the electrons can be present due to external background ionization. Here we investigate the effects of background ionization on streamer propagation and morphology by changing the gas composition and the repetition rate of the voltage pulses, and by adding a small amount of radioactive 85Kr.We find that the general morphology of a positive streamer discharge in high-purity nitrogen depends on background ionization: at lower background ionization levels the streamers branch more and have a more feather-like appearance. This is observed both when varying the repetition rate and when adding 85Kr, though side branches are longer with the radioactive admixture. But velocities and minimal diameters of streamers are virtually independent of the background ionization level. In air, the inception cloud breaks up into streamers at a smaller radius when the repetition rate and therefore the background ionization level is higher. When measuring the effects of the pulse repetition rate and of the radioactive admixture on the discharge morphology, we found that our estimates of background ionization levels are consistent with these observations; this gives confidence in the estimates.Streamer channels generally do not follow the paths of previous discharge channels for repetition rates of up to 10 Hz. We estimate the effect of recombination and diffusion of ions and free electrons from the previous discharge and conclude that the old trail has largely disappeared at the moment of the next voltage pulse; therefore the next streamers indeed cannot follow the old trail.

Stereo-photography of streamers in air

Standard photographs of streamer discharges show a two-dimensional projection. Here, we present stereophotographic images that resolve their three-dimensional structure. We describe the stereoscopic setup and evaluation, and we present results for positive streamer discharges in air at 0.2–1bar in a point-plane geometry with a gap distance of 14cm and a voltage pulse of 47kV. In this case, an approximately Gaussian distribution of branching angles of 43°±12° is found; these angles do not significantly depend on the distance from the needle or on the gas pressure.

Investigation of positive streamers by double-pulse experiments, effects of repetition rate and gas mixture

Streamer discharges are often operated in a repetitively pulsed mode and are therefore influenced by species left over from the previous discharge, especially free electrons and ions. We have investigated these effects by applying two consecutive positive high voltage pulses of 200–700 ns duration to a point-plane gap in artificial air, pure nitrogen, other nitrogen–oxygen mixtures and pure argon at pressures between 67 and 533 mbar. The pulses had pulse-to-pulse intervals (Δt) between 200 ns and 40 ms. We imaged both discharges with two ICCD cameras and combined this to a compound image. We observe for values of Δt below 0.5–15 µs (at 133 mbar, with Δt depending on gas mixture) that during the second pulse the streamers continue the paths of the first-pulse streamers. We call the maximal time for which this continuation still occurs the continuation time. For N2–O2 mixtures, this time has a maximum at an oxygen concentration of about 0.2%. According to our plasma-chemical modelling this maximum is determined by the electron loss rate which has a minimum around this oxygen concentration. Depending on oxygen concentration the dominant recombining positive ion is , or where dominates around 0.2% O2 and recombines slowest.For increasing values of Δt we observe that after the continuation phase first no new streamers occur at all, then new streamers show up that avoid the entire pre-ionized region. Next we see new thin streamers that follow the edges of the old channels. For larger Δt (10–200 µs) the new streamers start to increase in size and move to the centre of the old channels. Finally, around millisecond timescales the new channels are completely independent of the old channels.Together this points to the combination of two mechanisms: streamers search the proximity of regions with increased electron density, but cannot penetrate regions with very high electron density.

Multiple scales in streamer discharges, with an emphasis on moving boundary approximations

Streamer discharges determine the very first stage of sparks or lightning, and they govern the evolution of huge sprite discharges above thunderclouds as well as the operation of corona reactors in plasma technology. Streamers are nonlinear structures with multiple inner scales. After briefly reviewing basic observations, experiments and the microphysics, we start from density models for streamers, i.e. from reaction–drift–diffusion equations for charged-particle densities coupled to the Poisson equation of electrostatics, and focus on derivation and solution of moving boundary approximations for the density models. We recall that so-called negative streamers are linearly stable against branching (and we conjecture this for positive streamers as well), and that streamer groups in two dimensions are well approximated by the classical Saffman–Taylor finger of two fluid flow. We draw conclusions on streamer physics, and we identify open problems in the moving boundary approximations.

Feather-like structures in positive streamers.

Electronic cooling for improving the signal‐to‐noise (S/N) performance of sensors was applied to the mechanically resonant Hughes rotating gravity gradiometer (RGG). An 8.3‐dB improvement in the S/N was obtained. The electronic cooling technique uses a negative feedback damping circuit between the two RGG transducers. The feedback circuit is electrically equivalent to a high‐impedance resistor cooled to cryogenic temperatures.Electronic cooling for improving the signal‐to‐noise (S/N) performance of sensors was applied to the mechanically resonant Hughes rotating gravity gradiometer (RGG). An 8.3‐dB improvement in the S/N was obtained. The electronic cooling technique uses a negative feedback damping circuit between the two RGG transducers. The feedback circuit is electrically equivalent to a high‐impedance resistor cooled to cryogenic temperatures.

Inception and propagation of positive streamers in high-purity nitrogen: effects of the voltage rise rate

Controlling streamer morphology is important for numerous applications. Up to now, the effect of the voltage rise rate was only studied across a wide range. Here we show that even slight variations in the voltage rise can have significant effects. We have studied positive streamer discharges in a 16cm point-plane gap in high-purity nitrogen 6.0, created by 25kV pulses with a duration of 130ns. The voltage rise varies by a rise rate from 1.9 to 2.7kVns −1 and by the first peak voltage of 22 to 28kV. A structural link is found between smaller discharges with a larger inception cloud caused by a faster rising voltage. This relation is explained by the greater stability of the inception cloud due to a faster voltage rise, causing a delay in the destabilization. Time-resolved measurements show that the inception cloud propagates slower than an earlier destabilized, more filamentary discharge. This explains that the discharge with a faster rising voltage pulse ends up being shorter. Furthermore, the effect of remaining background ionization in a pulse sequence has been studied, showing that channel thickness and branching rate are locally affected, depending on the covered volume of the previous discharge. (Some figures may appear in colour only in the online journal)

A Peculiar Streamer Morphology Created by a Complex Voltage Pulse

An unintended reflection of a voltage pulse from our Blumlein pulser leads to a peculiar streamer discharge morphology. A high negative voltage pulse, followed by a lower positive voltage pulse, creates positive streamers that run over the surface of the nearly spherical previously formed negative discharge.