HR: 16:25h
AN: AE44A-03 INVITED    [Abstracts]
TI: On the possibility of accelerating electrons to X-ray energies in the electric fields created during the meeting of positive and negative streamer fronts in laboratory electrical discharges
AU: * Cooray, V
EM: Vernon.Cooray@angstrom.uu.se
AF: Division of Electricity, Uppsala University, Uppsala, 751 21, Sweden
AU: Dwyer, J
EM: jdwyer@fit.edu
AF: Department of Physics and Space Physics, Florida Institute of Technology, Melbourne, FL 32901, United States
AU: Rahman, M
EM: Mahbubur.Rahman@angstrom.uu.se
AF: Division of Electricity, Uppsala University, Uppsala, 751 21, Sweden
AU: Rassoul, H
EM: rassoul@fit.edu
AF: Department of Physics and Space Physics, Florida Institute of Technology, Melbourne, FL 32901, United States
AB: In laboratory sparks X-rays are observed in two bursts, one occurring about 1 microsecond before the onset of the current and other taking place at the time of onset of the current [1, 2]. The location of the first burst in the discharge process suggests that the first burst of X-rays observed is probably generated by streamers. To investigate this further, streamers are modeled as finitely conducting resistive channels with potential gradients, 450 kV/m for positive and 1 MV/m for negative. A streamer has a radius of about 50 micrometers and in a streamer moving in the stability field (slightly above the potential gradient) the ion concentration at the head is such that the radius of the active region in which the electric field exceeds the breakdown value is about 200 micrometers. As the background field increases beyond the stability field the ion concentration at the head continues to increase until the streamer branches. At present, the exact streamer charge in the head when the streamer branching takes place is not known but it could be several times the typical charge. In the calculations presented here it is assumed that the streamer will branch when the head charge is about five times that of a streamer moving in stability field. We have evaluated the electric field configuration in space for a streamer moving in stability field, for a streamer just about to branch and for the encounter of two streamers of opposite polarity just about to branch. The resulting field configuration is utilized together with the energy dependent frictional force on electrons, as presented by Moss et al. [3], to evaluate the maximum energy an electron will receive in accelerating in the electric field of the streamers. The results of the calculation show that an encounter between two streamers of opposite polarity just about to branch can provide sufficient energy for the electrons to become runaways in 1.1 MV/m field, the average value measured in the gap just before breakdown. The photographs of sparks generated in the same gap configuration provide evidence that in these discharges the negative streamer front generated by the cathode is met by an oppositely moving streamer front somewhere in the vicinity of the mid gap making it possible for the runaways to gain more than 100 keV in crossing rest of the gap. Moreover, the calculations show that the field enhancement associated with the encounter of two streamers takes place when the streamers are within about few millimeters from each other. Since the speed of streamers range from about 20 cm/us to about 100 cm/us, the duration of the high electric field lasts only for a few nanoseconds and the duration of the X-ray burst generated by such an encounter cannot be longer than this time. This is in agreement with the experimental observations. [1] Dwyer et al., X-ray bursts produced by laboratory sparks in air, Geophys. Res. Lett., 32, L20809, doi:10.1029/2005GL024027, 2005. [2] Rahman et al., X-rays emitted by 80 cm long sparks in air, submitted to Geophys. Res. Lett., 2007. [3] Moss et al., Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders, J. Geophys. Res., vol. 111, AA02307, doi:10.1029/2005JA011350, 2006.
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 3399 General or miscellaneous
SC: Atmospheric and Space Electricity [AE]
MN: 2007 Fall Meeting