Nonlinearity of electrical discharge dynamics

U. Ebert et al., used the 4 Quik E ICCD camera to study the dynamics of electrical discharges.

Feather-like structures in a positive streamer discharge in pure argon

Feather-like structures in a positive streamer discharge in pure argon at room temperature and atmospheric pressure. The image is recorded with a ultra fast ICCD camera and represents about 40mm of the discharge gap with the electrode tip in the top center. The blurred structures are out of focus. Reprinted figure with permission from U Ebert et al., 2011 Nonlinearity 24 C1. Copyright (2011) by IOP Publishing Ltd. The figure was published originally in figure 7 of S. Nijdam et al., 2010 J. Phys. D: Appl. Phys. 43 145204.

Feather-like structures in a positive streamer discharge

If a sufficiently high voltage is applied to a volume of ionizable matter, a electrical discharge channels its way through in the form of a tree with growing branches. The ionized branches that penetrate the ionized matter are called streamers. Everybody knows the momentary beauty and elegance of an randomly outstretched tree of streamer if a flash appears on the horizon. Using a high speed ICCD camera the evolution of electrical discharge can be studied and the beauty of feather-like structures in a positive streamer discharge can be captured.

Nonlinear nature of electrical discharges

Streamer discharges determine the very first stage of sparks or lightning. They govern in nature the evolution of huge sprite discharges above thunderclouds and in human made technology the operation of corona reactors in plasma technology. The electrical discharge can occur in stunning beauty as it can be seen in Figure 1. Furthermore, this picture shows the fact that the streamers are nonlinear structures with multiple inner scales. The captured discharge propagates through argon at standard temperature and pressure. At the upper middle of the picture, an invisible electrode needle supplies a positive voltage pulse and the photograph integrates the light emission over about a microsecond. Also, the streamer trees are three-dimensional objects and this structure of the electrical discharge can already be recognized as some branches on the picture are out of focus.

Positive discharges in artificial air in atmospheric pressure and 200mbar.

Positive discharges in artificial air (N2 : O2 = 80 : 20) at room temperature. Time evolution of a positive streamer discharge tree at atmospheric pressure (left column) and a zoom into the formation of positive streamers at 200 mbar (right column). Reprinted figure with permission from U Ebert et al., 2011 Nonlinearity 24 C1. Copyright (2011) by IOP Publishing Ltd.

Dynamics of electrical discharges

Although, lightning effects in nature seem to happen simultaneous the high shutter speed of an ICCD camera enables an insight to the process of lightning. This sequence of streamers can be seen in Figure 2 as a series of images shows the devolution of positive and negative streamer. The left column (panels a, b and c) shows positive streamer discharge trees which means the invisible needle on the top of the image is positive charged. Compared to the right column which shows negative streamer, hence with a negative charged needle. Panels (b) and (c) show that not the whole channel emits light, but only the pieces of the channel that grow during the exposure time of the camera. Another pedagogical illustration of the effect that only the actively growing tips of the channels emit light. Adjusting the opening and exposure time of the ICCD camera therefore has become an important tool in analyzing streamer dynamics. With exposure times as short as 300ps, even the shell type inner structure of the ionization front within the streamer tip could be reconstructed.

Title: Multiple scales in streamer discharges, with an emphasis on moving boundary approximations
Author: U Ebert, F Brau, G Derks, W Hundsdorfer, C-Y Kao, C Li, A Luque, B Meulenbroek, S Nijdam, V Ratushnaya, L Schäfer and S Tanveer
Institute: CWI Amsterdam, The Netherlands