Proc. V Intern. Conf. on Plasma Physics and Plasma Technology, Minsk, Belarus, September 18-22, 2006, Vol 1, p. 54-57.
THE TEMPERATURE DEPENDENCE OF THE CRITICAL ELECTRIC FIELD IN MIXTURES OF SF6 WITH C2H6 A.A. Belevtsev1, K.N. Firsov2, S.Yu. Kazantsev2, I.G. Kononov2 1
2
Institute for High Energy Densities, RAS, Izhorskaya 13/19, 125412 Moscow, Russia General Physics Institute, RAS, Vavilov st.38, 119991 Moscow, Russia, E-mail:
[email protected]
1. INTRODUCTION The critical reduced electric field (E/N)cr such that electron production equilibrates with electron attachment to neutral molecules is a fundamental characteristic of an electronegative gas. In strongly electronegative gases the (E/N)cr-values attain several hundred Td (1Td=10-17V⋅cm) thereby these gases are widely used in power industry as insulating media. In this connection the question arises as to how gas temperature affects the critical field because the operating temperatures typical of modern high-voltage installations are ordinary well above room temperature. Besides, new physical processes may be expected to occur in strongly electronegative gases at elevated temperatures. Of special interest are SF6 and SF6-based mixtures exhibiting both high dielectric strength and appropriate thermodynamic properties. The present day information on the temperature dependence of (E/N)cr in these media is very scarce and conflicting. In fact, there are only two papers /1,2/ dealing with this problem. In paper /1/, it is stated that the onset potential at constant gas density and, hence, the magnitude of (E/N)cr do not depend in SF6 on gas temperature within the range of 300 to 800 K. However the results of /2/ on determining (E/N)cr in a heated SF6 clearly manifest noticeable increase in the critical reduced fields in SF6 even at much less temperatures. As regards the SF6-based mixtures, no relevant information has there been in the literature. In our recent studies on a self-sustained volume discharge (SSVD) in mixtures of SF6 with hydrocarbons /3-6/ pre-irradiated by CO2-laser, it was established that the growth of the burning voltages with increasing the specific laser energy absorbed Wa is accompanied by an appreciable rise in the gas temperature. This would appear to allow deducing the temperature dependence of (E/N)cr in SF6-based mixtures. The present paper reports on determining the critical reduced electric fields in mixture of SF6 with C2H6 in the range of gas temperatures T=293-1400 K. The (E/N)cr-values are derived using the SSVD burning voltages in these mixtures vibrationally excited by a TEA CO2-laser radiation, while the gas temperatures are estimated by Wa.
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Proc. V Intern. Conf. on Plasma Physics and Plasma Technology, Minsk, Belarus, September 18-22, 2006, Vol 1, p. 54-57.
2. MEASURING TECHNIQUE The experimental set-up was similar to that described in /3,4/. An SSVD was triggered in pure SF6 mixture SF6:C2H6=5:1 after their being irradiated by a pulse TEA CO2-laser at the P(20) 10.6 μm band wavelength. The density of the laser radiation energy absorbed in the discharge development zone Wa was capable of being varied from 0.01 to 0.13 J/cm3. The method of measuring Wa is described in /4,5/ in greater detail. SSVD occurred in the needle (cathode), ∅15mm cylinder (anode) geometry with an interelectrode distance d=43 mm, the needle being directed perpendicularly to the cylinder axis. In order to obtain SSVD, a capacitor was discharged across the gap through a nitrogen-filled spark gap. The voltage pulse was applied to the discharge gap after a delay τ >3 μs relative to the laser pulse onset, the laser pulse duration τlas ~3 μs. 3. RESULTS Increasing the SSVD burning voltage Ulas can numerically be estimated by the quantity Δ=(Ulas-U)/U /3,5/ where U is the discharge voltage in the absence of laser illumination, both the voltages being taken at the amplitude magnitudes of the associated discharge currents. In the present work, the dependences Δ(Wa) have been obtained by analogy with /3,5/. Given those, the temperature dependence of (E/N)cr can then be found in the following way. The mixture heating occurs due to the vibrational relaxation of SF6 molecules, excited by a CO2-laser radiation, through the vibration-translation (V-T) energy exchange process. At sufficiently high Wa-values there takes place explosive-like temperature growth resulting in establishing the thermal equilibrium between the vibrational and translational degrees of freedom in a lapse of a characteristic time /6/ 1
τ exp
⎡ 2C (T )T Nτ τ ⎤ 2 = ⎢ V 0 0 VT las ⎥ . (α − 1)Wlas ⎣ ⎦
(1)
Here CV(T0) is the specific heat capacity at constant volume per one particle, N, a total number particle density. τVT is the characteristic V-T energy exchange time. T0≈300 K, α≈5. If τexp
