Experimental investigations on the physics of streamers

Streamers are rapidly extending ionized fingers that can appear in gasses, liquids and solids. They are generated by high electric fields but can penetrate into areas where the background electric field is below the ionization threshold. Streamers occur in nature as a precursor to sparks and lightning, but also independently as sprites (large discharges high above thunderclouds) or St. Elmo’s fire. Their main applications are gas and water cleaning, ozone creation, particle charging and flow control. Streamers are very efficient in creating active chemical species as no energy is lost in heating of the background gas and surrounding materials. Furthermore, as streamers are the first phase of sparks, they are relevant for any application of sparks, e.g., in the ignition process in a combustion engine or a discharge lamp. Finally, streamers can occur in high voltage applications, like switch-gear. In this thesis, a number of aspects of the physics of streamers are investigated experimentally. In our study, we have created streamers by applying a high voltage pulse to a wire or sharp tip that is located 40 to 160 mm above a grounded plate. These experiments were conducted inside a vacuum chamber at various pressures between 25 and 1000 mbar, with various gasses and gas mixtures, most of high purity (up to less than 0.1 ppm contaminations). We create the voltage pulses by two different high voltage pulse sources. The C-supply can give pulses between 5 and 60 kV with a minimum risetime of about 15 ns and an exponential decay of varying duration. The newly built Blumlein pulser creates quasi-rectangular pulses with an amplitude between 20 and 35 kV, a duration of about 130 ns and a risetime of about 10 ns. Both pulse sources can produce pulses of positive and negative polarity but have primarily been used with positive polarity.First, the interaction of individual streamer channels and the streamer branching angles are analysed by stereo-photography. Then insight into the propagation mechanism of positive streamers (i.e., against the electron drift direction) is gained by changing the gas composition and the repetition frequency of voltage pulses. Finally, morphology, channel diameters, propagation velocities and spectra of laboratory streamer discharges in a variety of gasses and gas mixtures are studied. Some of these studies are used as a "simulation" of sprite discharges on earth as well as on other planets. Interaction and branching of streamers Pictures show that streamer or sprite discharge channels emerging from the same electrode sometimes seem to reconnect or merge even though their heads carry electric charge of the same polarity; one might therefore suspect that reconnections are an artifact of the two-dimensional projection in the pictures. We have used 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 photo-ionization effect in which two simultaneously propagating streamer heads merge into one new streamer. We find that reconnections as defined above occur frequently. Merging on the other hand was only observed with a double tip electrode at a pressure of 25 mbar and a tip separation of 2 mm, i.e., for a reduced tip distance of p . d = 50 mmbar. In this case the full width at half maximum of the streamer channel is more than 10 times as large as the tip separation. We have also investigated streamer branching with the stereo-photography method and have found that the average branching angle of streamers under the conditions that were investigated is about 42° with a standard deviation of 12°. The role of photo- and background ionization in streamer propagation Positive streamers in air are thought to propagate against the electron drift direction by photo-ionization whose parameters depend on the nitrogen:oxygen ratio. Therefore we study streamers in nitrogen with 20%, 0.2% and 0.01% oxygen and in pure nitrogen and argon. Our new experimental set-up guarantees contamination to be below 0.1 ppm for our purest nitrogen. Streamers in pure nitrogen and in all nitrogen/oxygen mixtures look generally similar, but become 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 102 hair tips/cm3 in the feathers at 200 mbar; this density can be interpreted as the density of free electrons that create 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. This is supported by recent modelling results byWormeester et al. in 2010. To study the effects of background ionization on streamers, we have used two methods: variation of pulse repetition frequency (0.01–10 Hz) and addition of about 9 parts per billion of radioactive 85Kr gas to pure nitrogen. We found that higher background ionization levels lead to smoother and thicker streamers. This is similar to the effect of increased photo-ionization close to the streamer tip, created by increasing the oxygen concentration. Again, we do not see any major effects on streamer properties, except that initiation probabilities go down significantly in pure nitrogen with low (0.01 Hz) repetition frequency. At 200 mbar, the estimated background ionization level from the 85Kr was about 4 ? 105 cm-3, which corresponds to the theoretical level in non-radioactive gas at a pulse repetition frequency of about 1 Hz under similar conditions. This fits with the observed variations in streamer morphology as function of repetition frequency for both pure nitrogen and the nitrogen-krypton mixture. Furthermore, we have found that streamers do not follow the paths of streamers in preceding discharges for pulse repetition frequencies around 1 Hz. This can be explained by the combination of recombination and diffusion of ionization after a discharge pulse which nearly flattens any leftover ionization trail in about 1 second. Streamers in other gasses and streamer spectra In order to get more insight in positive streamer propagation, we have studied more than just nitrogen-oxygen mixtures. We have studied pure oxygen, argon, helium, hydrogen and carbon dioxide. Each of these gasses has different properties like ionization levels, excitation levels, cross sections and electronegativity. Furthermore, we have studied streamers in binary gas mixtures that simulate the atmospheres of Venus (CO2–N2) and Jupiter (H2–He). Streamers in these gasses, as well as in air are physically similar to large scale sprite discharges on the corresponding planets. Therefore, the results of our measurements can be used to better equip (space) missions that study sprites on earth and other planets and can help in the interpretation of the observations of these missions. For all gasses and mixtures, overall morphology, velocities, diameters and emission spectra have been investigated. We have found that it is possible to create streamers in all gasses. Streamer diameters are more or less the same for all gasses, except for pure helium and the Jupiter atmosphere where minimal streamers are respectively 3 and 5 times thicker than in the other gasses. The physical similarity between streamers at different pressures has been confirmed for all gasses that enabled us to measure streamer diameters; the minimal diameters in air and other nitrogen-oxygen mixtures are smaller than in earlier measurements. Streamer velocities are even more similar; for a given combination of pressure and pulse voltage all propagation velocities are within a factor 2. Streamer brightness on the other hand is very different for the different gas mixtures. Streamers are brightest in nitrogen-oxygen mixtures, nitrogen, argon and helium and dimmest in oxygen, CO2 and the venusian mixture. The difference between the brightest and dimmest gasses is about three to four orders of magnitude in the optical range. Streamer spectra from molecular gasses are characterised by molecular bands. In gasses containing a significant amount of nitrogen (including the venusian mixture), the nitrogen second positive system dominates the emission spectrum. In contrast, spark-like discharges in the same gasses are dominated by radiation from neutral and ionized atoms. Spectra in atomic gasses (argon and helium) are different: the argon spectrum contains mainly atomic argon lines, but the helium spectrum also contains many lines of impurities, while we have no indication that the gas purity is below specification. The reason for the many impurity lines in helium are the high excitation and ionization levels of helium compared to the impurities. These high levels (and low cross sections for electron-atom collisions at low energies) may also explain the large diameter of streamers in pure helium.