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

Measurement of the temperature of an ultracold ion source using time-dependent electric fields

We report on a measurement of the characteristic temperature of an ultracold rubidium ion source, in which a cloud of laser-cooled atoms is converted to ions by photo-ionization. Extracted ion pulses are focused on a detector with a pulsed-field technique. The resulting experimental spot sizes are compared to particle-tracking simulations, from which a source temperature $T = (1 \pm 2)$ mK and the corresponding transversal reduced emittance $\epsilon_r = 7.9 X 10^{-9}$ m rad $\sqrt{\rm{eV}}$ are determined. We find that this result is likely limited by space charge forces even though the average number of ions per bunch is 0.022.Comment: 8 pages, 11 figure

A comparison of 3D particle, fluid and hybrid simulations for negative streamers

In the high field region at the head of a discharge streamer, the electron energy distribution develops a long tail. In negative streamers, these electrons can run away and contribute to energetic processes such as terrestrial gamma-ray and electron flashes. Moreover, electron density fluctuations can accelerate streamer branching. To track energies and locations of single electrons in relevant regions, we have developed a 3D hybrid model that couples a particle model in the region of high fields and low electron densities with a fluid model in the rest of the domain. Here we validate our 3D hybrid model on a 3D (super-)particle model for negative streamers in overvolted gaps, and we show that it almost reaches the computational efficiency of a 3D fluid model. We also show that the extended fluid model approximates the particle and the hybrid model well until stochastic fluctuations become important, while the classical fluid model underestimates velocities and ionization densities. We compare density fluctuations and the onset of branching between the models, and we compare the front velocities with an analytical approximation

Nanosecond repetitively pulsed discharges in N 2

We evaluate the nanosecond temporal evolution of tens of thousands of positive discharges in \na 16 cm point-plane gap in high purity nitrogen 6.0 and in N2\xe2\x80\x93O2 gas mixtures with oxygen \ncontents of 100 ppm, 0.2%, 2% and 20%, for pressures between 66.7 and 200 mbar. The \nvoltage pulses have amplitudes of 20 to 40 kV with rise times of 20 or 60 ns and repetition \nfrequencies of 0.1 to 10 Hz. The discharges first rapidly form a growing cloud around the tip, \nthen they expand much more slowly like a shell and finally after a stagnation stage they can \nbreak up into rapid streamers. The radius of cloud and shell in artificial air is about 10% below \nthe theoretically predicted value and scales with pressure p as theoretically expected, while \nthe observed scaling of time scales with p raises questions. We find characteristic dependences \non the oxygen content. No cloud and shell stage can be seen in nitrogen 6.0, and streamers \nemerge immediately. The radius of cloud and shell increases with oxygen concentration. \nOn the other hand, the stagnation time after the shell phase is maximal for the intermediate \noxygen concentration of 0.1% and the number of streamers formed is minimal; here the cloud \nand shell phase seem to be particularly stable against destabilization into streamers