Positive streamers in air and nitrogen of varying density: experiments on similarity laws

Abstract

Abstract. Positive streamers in ambient air at pressures from 0.013 to 1 bar are investigated experimentally. The voltage applied to the anode needle ranges from 5 to 45 kV, the discharge gap from 1 to 16 cm. Using a "slow" voltage rise time of 100 to 180 ns, the streamers are intentionally kept thin. For each pressure p, we find a minimal diameter d min . To test whether streamers at different pressures are similar, the minimal streamer diameter d min is multiplied by its pressure p; we find this product to be well approximated by p · d min = 0.20 ± 0.02 mm · bar over two decades of air pressure at room temperature. The value also fits diameters of sprite discharges above thunderclouds at an altitude of 80 km when extrapolated to room temperature (as air density rather than pressure determines the physical behavior). The minimal velocity of streamers in our measurements is approximately 0.1 mm/ns = 10 5 m/s. The same minimal velocity has been reported for tendrils in sprites. We also investigate the size of the initial ionization cloud at the electrode tip from which the streamers emerge, and the streamer length between branching events. The same quantities are also measured in nitrogen with a purity of approximately 99.9%. We characterize the essential differences with streamers in air and find a minimal diameter of p · d min = 0.12 ± 0.02 mm · bar in our nitrogen.

Positive and negative streamers in ambient air: modelling evolution and velocities

“…The minimal diameter of positive streamers in our simulations is about 0.2 mm, identical to the minimal diameter reported in experiments [30,31,2]. It should be noted, however, that the definition of the radius might differ between experiments and simulations as discussed in section 4.3.…”

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

“…[32]; a detailed drawing of the discharge chamber in these supplies including the current and voltage measurements is provided in Fig. 2 of [35]. The PM-supply was used in the experiments at the electrical engineering department and already discussed in [45] versions combine robustness, high efficiency and long life time [42,43].…”

“…2 of [35]. The current through the gap is measured with a Pearson current monitor (6585) for C 1 or via the voltage across a small series resistor R s = 2.75Ω between cathode and ground for C 2 .…”

“…2 of [35]. This vessel can be evacuated and filled with different gases with pressures in the range of 0.013 to 1 bar, as thoroughly described in [35].…”

“…At the physics department, the discharges generated with the C-supplies and the TLT-supply are photographed with a 4QuikE intensified CCD camera from Stanford Computer Optics as described in [35]. The camera has 1360×1024 pixels and a minimal exposure time of 2 ns.…”