AN INVESTIGATION OF ARC STARTING ON COLD CATHODES

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AN INVESTIGATION OF ARC STARTING ON COLD CATHODES

Harald L. Witting

To cite this version:

Harald L. Witting. AN INVESTIGATION OF ARC STARTING ON COLD CATHODES. Journal de

Physique Colloques, 1979, 40 (C7), pp.C7-451-C7-452. �10.1051/jphyscol:19797219�. �jpa-00219200�

JOURNAL DE PHYSIQUE CoZZoque C7, suppZ&nent au n07, Tome 40, J u i l l e t 1979, page C7- 451

A N INVESTIGATION OF ARC STARTING ON COLD CATHODES

Harald L.Witting.

Genera2 Electric Company, Corporate Research and Development Schenectady N.Y., 12345 U.S.A.

The phenomena t h a t occur when an a r c i s s a r t e d on a cold cathode a r e not well understood. ( l j Ye a r e interested in the brief period (a few s e c ) a f t e r breakdown when the electrode t r a n s f e r s from a cold cathode t o a hot cathode while being heated by the arc. We have investigated t h i s " s t a r t i n g period" by means of plasma probes and high speed photography.

Our electrode (Fig. 1) consists of a tungsten shank (1.2 mm d i a ) and a tungsten wire overwind (0.75 mm d i a ) . A f i n e tungsten plasma probe i s spaced 1 mm from the electrode t i p . The arc tube contains argon a t 2.6 kPa and mercury vapor a t 0.3 Pa.

Fig. 2 shows the discharge voltage and current f o r a symmetrical a r c tube (70 mm arc gap) in the f i r s t few ms of operation on a 60 Hz sine wave reactor, f o r the case where the tungsten electrodes a r e coated with an emission material (barium and thorium oxides). The discharge i s mostly i n the low-voltage a r c spot mode, with brief(40 micro-sec) re-ignition and de-ignition spikes near current zero. This pattern does not change s i g n i f i c a n t l y fok the f i r s t 1 or 2 sec of discharge operation.

Our plasma probe showed t h a t the cathode f a l l i s approximately 12v i n the a r c spot mode, and t h a t p r a c t i c a l l y a l l the re-igni tion and de-igni tion voltage drop occurs in the cathode f a l l .

High speed motion pictures of a r c s t a r t i n g were taken with a Idollensak Fastax camera i n 16 mm color film a t 3000 pictures/sec. These showed t h a t i n the a r c spot mode there i s a small, bright a r c spot t h a t moves rapidly ( 5 m/s) over the cathode sur- face. The s i z e of the arc spot has not been re- solved, i t appears t o be l e s s than 0.2 mm i n diam- e t e r . This implies a current density exceeding 15,000 ~/cm2. The arc spot i s surrounded by a l a r g e r (2 mm), r e l a t i v e l y d i f f u s e glow.

As the electrode heats up during s t a r t i n g (1-2 s e c ) , the re-ignition spike gradually length- ens in duration and becomes v i s i b l e in the motion pictures as a d i s t i n c t mode. In t h i s "cathode glow'! mode, an intense, t h i n glow hugs a substantial f r a c t i o n of the electrode surface without movement, and there i s a d e f i n i t e dark space between t h i s glow and the positive column. The cathode glow mode gradually decreases i n voltage and lengthens i n duration, and eventually (4-6 sec) i t t r a n s f e r s t o a diffuse, low-voltage thermionic a r c mode as the electrode reaches incandescence.

With a bare tungsten electrode (no emission m a t e r i a l ) , t h e a r c spot mode i s rarely observed on s t a r t i n g . Rather, the s t a t i o n a r y cathode glow mode dominates, i t has a high cathode f a l l (20Dv) thr,oughout each half-cycle and heats the electrodes rapidly.

The dominant and most puzzling feature of the s t a r t i n g period i s the a r c spot mode. W i t h a' cath- ode fa1 1 of l e s s than 20v, i t can t r a n s f e r a wide range of current (<O.lA t o 5A) t o cold electrodes.

This arc spot can move about a tungsten surface w i t h scattered oxide surface layers t h a t a r e l e s s than a few microns i n thickness as observed with v i s i b l e and scanning electron microscopy.

Fig. 1 Fig. 2

We analyze the cathode arc spot on the assump- tion t h a t the a r c maintains a small area on the cathode a t a high temperature SO t h a t thermionic emission, enhanced by strong e l e c t r i c f i e l d s , can

~ r o v i d e the required hiqh current density. The heat conduction from t h e hot spot t o the-bulk cathode i s approximately

Pth = (7r/2) T D K

where T i s the temperature and D the diameter of t h e hot spot, and K i s the thermal conductivity of the cathode.

We estimate the maximum hot spot s i z e by assum- ing t h a t the conduction heat loss i s supplied by a l l the available cathade heat, i . e . , by t h e product of current I and cathode f a l l Vc, so t h a t

D<

-

For example, with a hot spot temperature of 4000°K and with a tungsten substrate (K = 1.2 w/cm ^OK), a t 1A current and 12v cathode f a l l we find a maxi- mum hot spot diameter of 16 microns. The corre- sponding current density i s 500000 ~ / c m 2 . If 252 of this current i s carried by positive ions t h a t a r e neutralized and thermal ized a t the surface, then the pressure of the returning atom f l u x over the hot spot area A i s

P = (2n M k ~ ) l / ~ I14eA = 2500 kPa.

We conclude t h a t the s t a r t i n g of an oxide a c t i - vated, cold cathode i s dominated by an a r c spot mode t h a t has t h e e s s e n t i a l c h a r a c t e r i s t i c s of a vacuum a r c , namely an exceedingly small, dense, moving plasma region close t o the cathode surface.

The l o s s of ions and atoms from t h i s plasma t o the surrounding low-pressure discharge region i s bal- anced by a flow of evaporated atoms from the surface.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797219

Vacuum arc experiments have shown t h a t a r c i g n i t i o n i s p r a c t i c a l l y impossi ble a t very pure electrodes with u surface e f f e c t s such a s produced by oxides, e t c . ? ~ j This agrees with our observation t h a t a cathode a r c spot dominates the s t a r t i n g on electrodes t h a t contain an oxide emission material, while a pure tungsten electrode will s t a r t i n a high-vol tage glow mode.

References

1. Gaseous Electronics, M.N. Hirsh & H.J. Oskam Ed.

Vol. 1 , Academic Press. N.Y. 1978. o . 332.

2. Ci. ~ c k e r

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