Slow decay of radiation after a pulsed streamer discharge in pure nitrogen

Light emission and electrical characteristics in the early post-discharge of a high purity nitrogen streamer have been investigated. Up to the millisecond regime, both light emission and current are significant, while the voltage has decayed after several tens of microseconds. The corresponding decay time constants are 240 µs and 580 µs for the current and radiance, respectively, versus 3.8 µs for the voltage decay. This suggests that energy transfer to high vibrational levels of N2 (X 1 Σ, ν) and high population of metastable N2 (A3 Σ+ ) species are important in sustaining the discharge

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 to 2.7 kV ns−1 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 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

Growth modes of Fe(110) revisited: a contribution of self-assembly to magnetic materials

We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and propose an overview or our contribution to the field. We show that the Stranski-Krastanov growth mode, recognized for a long time in these systems, is in fact characterized by a bimodal distribution of islands for growth temperature in the range 250-700°C. We observe firstly compact islands whose shape is determined by Wulff-Kaischev's theorem, secondly thin and flat islands that display a preferred height, ie independant from nominal thickness and deposition procedure (1.4nm for Mo, and 5.5nm for W on the average). We used this effect to fabricate self-organized arrays of nanometers-thick stripes by step decoration. Self-assembled nano-ties are also obtained for nucleation of the flat islands on Mo at fairly high temperature, ie 800°C. Finally, using interfacial layers and solid solutions we separate two effects on the preferred height, first that of the interfacial energy, second that of the continuously-varying lattice parameter of the growth surface.Comment: 49 pages. Invited topical review for J. Phys.: Condens. Matte

Feather-like structures in positive streamers.

In experiments positive streamers can have a feather-like structure, with small hairs connected to the main streamer channel. These feathers were observed in pure nitrogen (with impurities of 1ppm oxygen or less) but not in air. Based on results of numerical simulations, we provide a theoretical explanation for the emergence of these hairs as well as why the hairs are visible in nitrogen, but not in air

Feather-like structures in positive streamers interpreted as electron avalanches

In experiments positive streamers can have a feather-like structure, with small hairs connected to the main streamer channel. These feathers were observed in pure nitrogen (with impurities of 1 ppm oxygen or less) but not in air. We hypothesize that these hairs are individual electron avalanches moving towards the streamer channel. Based on results of numerical simulations, we provide a theoretical explanation why these hairs are visible in nitrogen, but not in air