98_Denat_ELF_streame_spectrumr - I EEE Transactions on...

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382 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 5 NO. 3, June 1998 Spectral Analysis of the Light Emitted by - Streamers in Hvdrocarbon Liquids J I A. Denat, N. Bonifaci and M. Nur Laboratoire d’Electrostatique et de Materiaux Dielectriques CNRS and Joseph Fourier University, Grenoble, France ABSTRACT This paper presents the results of a spectroscopic study from 200 to 850 nm of the light emitted by streamers initiated in cyclohexane and n-pentane under step voltage in point-plane geome- try, Experimental spectra of the light emitted by bush-like and filamentary streamers are com- posed of the Ha-Balmer line, the Cz Swan band system and a background continuum. The diagnostic method we used for evaluating rotational and vibrational temperatures of excited C2 in streamers was first tested on high-pressure corona discharges in nitrogen. For streamers in cyclohexane and n-pentane, it was impossible to determine the rotational temperature of Ca and consequently the effective temperature of molecules in the streamer. Moreover, we found that vibrational populations of excited Cz do not follow Boltzmann statistics. This indicates that excitation processes are due to chemical reactions. Electron densities deduced of the Stark broadening of H, are in the range 4x1016 to 7x1Ol6 for filamentary streamers and 2 to 6x1Ol7 cmA3 during the breakdown phase. For slow bush-like streamers, the electron density is not measurable. 1 INTRODUCTION REVIOUS studies of streamer initiation and propagation into liq- uids have demonstrated that emitted light is systematically corre- lated to electric current; current and emitted light have similar shapes [l, 21. In hydrocarbons, spectral analysis of the light emitted by breakdown and prebreakdown phenomena has mainly revealed the presence of the H Balmer lines (Ha, Hp), the C2 Swan bands (d3 II,-a311,) and a back- ground continuum [3-61. This continuum has been attributed to disso- ciation and fragmentation of molecules and recombination of molecu- lar fragments. Moreover, it has been shown that stable decomposition products such as Ha, CH4, CzHs, etc. are formed in organic liquids sub- jected to a large number of streamers [7]. On this basis, the elucidation of the detailed decomposition mechanisms of hydrocarbons (C, H,) by streamers is impossible. Only the following reactions can be written C,H, + E -+ C2(d3II,),, + H(n = 3,4) +products cz(d3~,),, + c,(a3n,),,, + hvv,vrf (1) = 3,4) = 2) + hv,,p The energy E of the streamer discharge is transferred to molecules producing several types of excited and ionized molecules and, after competing and consecutive processes, the formation of excited radicals and finally the formation of stable decomposition products. Electrical discharges such as corona and streamers in high-pressure gases produce a plasma in which the different species do not have suf- ficient time to thermalize. This non-equilibrium plasma is character- ized by an electronic temperature T, much higher than the vibrational temperature of the molecules. The vibrational temperature is itself
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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98_Denat_ELF_streame_spectrumr - I EEE Transactions on...

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