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 16 cm point-plane gap in high-purity nitrogen 6.0, created by 25 kV pulses with a duration of 130 ns. The voltage rise varies by a rise rate from 1.9 kV/ns to 2.7 kV/ns and by the first peak voltage of 22 to 28 kV. 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 destabilisation. Time-resolved measurements show that the inception cloud propagates slower than an earlier destabilised, more filamentary discharge. This explains that the discharge with a faster rising voltage pulse ends up to be shorter. Furthermore, the effect of remaining background ionisation 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.Comment: 16 pages, 9 figure

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

Plasma particle lofting

In plasma particle lofting, macroscopic particles are picked up from a surface by an electric force. This force originates from a plasma that charges both the surface and any particle on it, leading to an electric force that pushes particles off the surface. This process has been suggested as a novel cleaning technique in modern high-tech applications, because it has intrinsic advantages over more traditional methods. Its development is, however, limited by a lack of knowledge of the underlying physics. Although the lofting has been demonstrated before, there are neither numerical nor experimental quantitative measures of it. Especially determining the charge deposited by a plasma on a particle on a surface proves difficult. We have developed a novel experimental method using a "probe force.'' This allows us to, for the first time, quantitatively measure the plasma lofting force. By applying this method to different plasma conditions we can identify the important plasma parameters, allowing us to tailor a plasma for specific cleaning applications. Additionally, the quantitative result can help in the development of new models for the electron and ion currents through a plasma sheath

Guiding of positive streamers in nitrogen, argon and N₂-O₂ mixtures by very low <i>n</i>_e laser-induced pre-ionization trails

In previous work we have shown that positive streamers in pure nitrogen can be guided by a laser-induced trail of low electron density. Here we show more detailed results from such measurements. We show the sensitivity of this laser-guiding on pressure p and found that the maximum delay between the laser pulse and voltage pulse for guiding scales with something between $1/p$ and $1/p^{2}$. We also show that when we use a narrower laser beam the laser guiding occurs less frequent and that when we move the laser beam away from the symmetry axis, guiding hardly is observed. Finally we show that laser guiding can also occur in pure argon

Simulating the charging of a particle on a surface in a plasma

It is common knowledge that a floating surface will charge negative when a plasma is applied over it. One can imagine that any macroscopic dust particle on the surface will also get charged. The theory that describes this charging is, however, underdeveloped. It has been suggested that the particle will share its charge with the surface, leading to roughly the same surface charge density. This is, however, only valid when both the surface and the particle are electrically conductive. In this contribution, we show a novel model to simulate the charge on a non-conducting particle on a surface in a plasma. It is based on balancing the ion and electron fluxes through the plasma sheath towards the particle. With this, we show that the charge on a particle on a surface can be five orders of magnitude higher than what was previously assumed. Knowledge of the charge on a particle on a surface is important, because it, combined with the plasma sheath electric field, will lead to an electric force on the particle. It has been proposed that this force is important in the lofting of dust from the surface of extra-terrestrial bodies. Additionally, it has been suggested, that it can be used for cleaning in high-tech applications, such as lithography machines and spacecrafts

Guiding of positive streamers in nitrogen, argon and N₂-O₂ mixtures by very low <i>n</i>_e laser-induced pre-ionization trails

In previous work we have shown that positive streamers in pure nitrogen can be guided by a laser-induced trail of low electron density. Here we show more detailed results from such measurements. We show the sensitivity of this laser-guiding on pressure p and found that the maximum delay between the laser pulse and voltage pulse for guiding scales with something between $1/p$ and $1/p^{2}$. We also show that when we use a narrower laser beam the laser guiding occurs less frequent and that when we move the laser beam away from the symmetry axis, guiding hardly is observed. Finally we show that laser guiding can also occur in pure argon

Streamers in air splitting into three branches

We investigate the branching of positive streamers in air and present the first systematic investigation of splitting into more than two branches. We study discharges in 100 mbar artificial air that is exposed to voltage pulses of 10 kV applied to a needle electrode 160 mm above a grounded plate. By imaging the discharge with two cameras from three angles, we establish that about every 200th branching event is a branching into three. Branching into three occurs more frequently for the relatively thicker streamers. In fact, we find that the surface of the total streamer cross-sections before and after a branching event is roughly the same.Comment: 6 pages, 7 figure

Exploring the temporally resolved electron density evolution in EUV induced plasmas

We measured for the first time the electron density in an Extreme Ultra-Violet induced plasma. This is achieved in a low-pressure argon plasma by using a method called microwave cavity resonance spectroscopy. The measured electron density just after the EUV pulse is $2.6\cdot10^{16}$ m$^{-3}$. This is in good agreement with a theoretical prediction from photo ionization, which yields a density of $4.5\cdot10^{16}$ m$^{-3}$. After the EUV pulse the density slightly increase due to electron impact ionization. The plasma (i.e. electron density) decays in tens of microseconds.Comment: 3 pages, 4 figure