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Submitted on 1 Jan 1979
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A SUBSONIC ELECTRON FLUID AND THE FORMATION OF SMALL SPARKS
E. Barreto, H. Jurenka
To cite this version:
E. Barreto, H. Jurenka. A SUBSONIC ELECTRON FLUID AND THE FORMATION OF SMALL SPARKS. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-301-C7-302.
�10.1051/jphyscol:19797148�. �jpa-00219122�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au no?, Tome 40, JuiZZet 2979, page C7- 301
A SUBSONIC ELECTRON FLUID
AND
THE FORMATION OF SMALL SPARKSE. Barreto and H. Jurenka.
AtmospReric Sciences Research Center, State University of N e w York a t AZbany, AZbany, N e w York 22222.
I n a s m a l l high p r e s s u r e d i s c h a r g e gap, ava- l a n c h e s and s t r e a m e r s s t a r t i o n i z a t i o n b u t do n o t produce gas h e a t i n g o r a conducting b r i g h t f i l a - mentary channel. The e l e c t r o n d e n s i t y i s n o t high enough t o promote t h e i o n - e l e c t r o n i n t e r a c t i o n t h a t l e a d s t o r a p i d n e u t r a l gas h e a t i n g . Nevertheless, l o n g a f t e r i n i t i a l i o n i z a t i o n h a s subsided i n a gap, a v e r y r a p i d t r a n s i e n t s t a g e i s produced.
This c o n s t i t u t e s t h e s p a r k proper and changes a weakly i o n i z e d cold gas i n t o a h o t , h i g h l y c o n d u c t -
i n g luminous f l u i d . The s p a r k s t a g e may incorpor-
+
Image i n t e n s i f i e d p i c t u r e-
a t e v e r y r a p i d l y moving luminous f r o n t s , and de- pending on t h e power supply, may l e a d t o a much
c o o l e r a r c d i s c h a r g e . It h a s been suggested many 3 amp/div times t h a t t h e spark t r a n s i t i ~ n can be a s s o c i a t e d
w i t h e l e c t r o n f l u i d behavior: For i n s t a n c e , u s i n g
2 mm long gaps we have shown t h a t t h e i n i t i a l 1 0 n s e c / d i v i o n i z a t i o n produces a long-lived high temperature,
n o n - e q u i p a r t i t i o n e l e c t r o n f l u i d , and t h a t t h e propagation of f a s t luminous f r o n t s can be assoc- i a t e d w i t h non-linear e l e c t r o n waves.' Here we p r e s e n t evidence t o show t h a t , under a p p r o p r i a t e c o n d i t i o n s , subsonic e l e c t r o n f l u i d behavior can a l s o be e x h i b i t e d . It i s shown t h a t h e a t i n g of t h e gas c a n be accomplished by e l e c t r o n s t h a t be- have a s a f l u i d and undergo subsonic t u r b u l e n t mixing. From a p r a c t i c a l p o i n t of view, t h e s t u d y i s p e r t i n e n t t o problems i n l i g h t n i n g , explosion, and i g n i t i o n of combustible gas m i x t u r e s , vacuum d i s c h a r g e s and e l e c t r o d e e r o s i o n problems.
C u r r e n t t r a c e
-
1 urnE l e c t r o n microscope p i c t u r e of c a t h o d e s p o t
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797148
The figure shows an image intensified picture of a discharge produced in nitrogen using a 2 mm wide uniform field gap with a Cu cathode and a Au anode. The total energy used is 0.20 mJ and cor- responds to the minimum value required to ignite a propane-air mixture in the same gap. Npmolecules are heated because both discharges and current traces look the same. The gap is enclosed in a box with a flexible cover that allows a flow of N2 to be maintained across the discharge region.
Gas flows from top to bottom at a velocity around 100 m/sec. (The total volume flow is 58 cm3/sec.) The picture illustrates two distinct discharge, features that are supplemented with information from oscilloscopic recordings of the current:
a) There is an initial stage of avalanche ioniza- tion associated with the luminosity near theelec- trodes. The assynietry of the glow around the cathode spot shows that times are long enough to be affected by the transverse gas velocity. Cur- rent traces (procured with a large RC time) show that the initial ionization subsides in times of the order of a microsecond and consumes energy that is negligible compared to the stored 0.2 mJ.
The luminosity illustrates a well known fact:
the whole gap is full of ions and electrons be- fore the spark.
b) The conical luminous region is produced long after the avalanche current has died out. The cone apex is at the cathode spot and exhibits an angle that varies between 23O and 27'. This angle is much larger than angles of the order of 3O pro- duced by avalanches and photographed in cloud chambers. There are no streamers because of the short gap used. The current associated with the formation of the cone is recorded with a small RC.
time sec). It reaches a peak of the order of 10 A in a smooth pulse that lasts between 20
and 40 nsec. Cone luminosity is obviously un- affected by gas flow and the time intervals can only be accommodated by electron motion. The area under the current trace equals the charge in the capacitor. Hence, almost all the available energy goes into the gap during the cone formation stage.
Thus the cone is identified with the spark.
Finally, there are three pertinent facts associated with the formation of a cathode spot:
The first is that a spark is never produced with- out a spot. The second is that copious field emission is always produced by positive ions that accumulate near the metal cathode surface (e.g., in the oxide layer). The third is that time- resolved spectroscopic records2 show that even smaller sparks do not exhibit metal lines away from the cathode surface for times longer than those associated with cone formation. Withoutget- ting involved in the physics of the cathode spot, we may conclude that the cone is associated with field emission from the cathode. The cone angle recorded, and its range of variation, are exactly those for a subsonic submerged jet.3 We conclude that field emitted electrons interact as a fluid with those already in the gap. Turbulent diffu- sive mixing between collision dominated electrons raises the overall electron energy. Additional ionization is produced in the gap and leads to the critical density for thermalization.
*Work supported by the Office of Naval Research.
References
1. Barreto, E., et al, J.Appl.Phys.
65,
33.17-3327 (1974) and 48, 4510-4520 (1977) 2. Wiese, L.L. and J.A. Augis, J.Appl.Phys.
48,
4528-4535 (1977)
3. Abramovich, G.N., The-Theory of Turbulent Jets, MIT Press, Cambridge, Mass. (1963)
