Tmp Tanja Briels

Positive and negative streamers in ambient air: measuring diameter, velocity and dissipated energy

Positive and negative streamers are studied in ambient air at 1 bar; they emerge from a needle electrode placed 40 mm above a planar electrode. The amplitudes of the applied voltage pulses range from 5 to 96 kV; most pulses have rise times of 30 ns or shorter. Diameters, velocities and energies of the streamers are measured. Two regimes are identified; a low voltage regime where only positive streamers appear and a high voltage regime where both positive and negative streamers exist. Below 5 kV, no streamers emerge. In the range from 5 to 40 kV, positive streamers form, while the negative discharges only form a glowing cloud at the electrode tip, but no streamers. For 5–20 kV, diameters and velocities of the positive streamers have the minimal values of d = 0.2 mm and v ≈ 105 m s−1. For 20–40 kV, their diameters increase by a factor of 6 while the voltage increases only by a factor of 2. Above the transition value of 40 kV, streamers of both polarities form; they strongly resemble each other, though the positive ones propagate further; their diameters continue to increase with applied voltage. For 96 kV, positive streamers attain diameters of 3 mm and velocities of 4 × 106 m s−1; negative streamers are about 20% slower and thinner. An empirical fit formula for the relation between velocity v and diameter d is v = 0.5d2 mm−1 ns−1 for both polarities. Streamers of both polarities dissipate energies of the order of several millijoules per streamer while crossing the gap.

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.

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.

Time Resolved Measurements of Streamer Inception in Air

Images show how an ionization cloud forms at the needle electrode, how the cloud evolves into a shell, and how one or more streamers emerge from this shell and propagate.

Branching of positive discharge streamers in air at varying pressures

The formation of positive streamers in a 17-mm gap in air is studied at pressures varying in the range from 1010 to 100 mbar. An intensified charge coupled device camera is used to image the discharge. At high pressures, the discharge shows many branches, while at low pressure, fewer branches arise. The structure is not simply determined by the ratio of voltage over pressure.

Positive Streamers in Ambient Air and a

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.

\hbox{N}_{2}\!:\!\hbox{O}_{2}

Pulsed corona discharges can form very complex patterns. The properties of individual streamers are studied by determining their diameter and velocity in a pressure range of 0.015 to 1 bar. It appears that streamers have a minimum diameter, dmin, at a voltage just above inception. In air, the product of pressure and minimal diameter p-dmin is 0.20plusmn0.02 bar-mm for all pressures of the experiments. This value also matches the minimum diameter of sprite discharges above thunderclouds at 75 km altitude. Streamers produced by high and fastly rising voltages, on the other hand, can be as thick as 2.5 bar-mm and 20 times faster than minimal streamers. If the electrode gap is sufficiently large, the corona streamers branch during propagation. Branching is inherently a three dimensional effect. Stereo-photographic images are taken to resolve the three-dimensional structure . Such photos show an approximately Gaussian distribution of branching angles of 43degplusmn12deg; these angles do not significantly depend on the distance from the needle or on the gas pressure. Furthermore, the branching parameter D/d is determined, where D is the length of the streamer between two branching points and d is the diameter. Its value is 12plusmn4, so there is a considerable statistical fluctuation but this value also does not depend on pressure or voltage.