EFFECT OF THERMIONIC CATHODES EVAPORATION PRODUCTS ON HIGH VOLTAGE VACUUM BREAKDOWN

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EFFECT OF THERMIONIC CATHODES

EVAPORATION PRODUCTS ON HIGH VOLTAGE VACUUM BREAKDOWN

M. Sinha, T. Lin

To cite this version:

M. Sinha, T. Lin. EFFECT OF THERMIONIC CATHODES EVAPORATION PRODUCTS ON

HIGH VOLTAGE VACUUM BREAKDOWN. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-

303-C9-307. �10.1051/jphyscol:1984951�. �jpa-00224437�

EFFECT OF THERMIONIC CATHODES EVAPORATION PRODUCTS ON HIGH VOLTAGE VACUUM BREAKDOWN

M.K. Sinha and T.Y. Lin

North Dakota State University, Fargo, North Dakota 58105, U.S.A.

Résumé - Nous avons étudié la conduction et le claquage électrique sous vide entre deux électrodes de molybdène en présence des produits d'évaporation délivrés soit par un "dispenser cathode" soit par un filament de tungstène thorié. Le facteur de renforcement de champ a été déterminé ainsi que le travail de sortie avant et après que les électrodes de molybdène soient exposées aux vapeurs. Les dégâts de surface dus au claquage ont été examinés au microscope électronique à balayage.

Abstract - We have studied electrical conduction and electrical breakdown in a vacuum gap between molybdenum electrodes in the presence of the evaporation products of a dispenser cathode and a thoria coated tungsten filament. The field enhancement factor was determined and the work function and the emit- ting area were determined before and after the Mo electrodes were exposed to the vapors. The surface damage on the electrodes due to arcing was examined in a scanning electron microscope.

We have studied the phenomenon of electrical conduction and breakdown in a vacuum gap between molybdenum electrodes in the presence of the evaporation products of a dispenser cathode and a thoria coated tungsten filament. The Fowler-Nordheim equation was used to determine the field enhancement factor. The work function of the molybdenum cathode and the emitting area of the field emission sites were measured before and after the Mo electrodes were exposed to the vapors. We have also examined in a scanning electron microscope the surface damage on the electrodes due to arcing. The critical voltages and local fields for breakdown have been measured.

Experiments were performed in a stainless steel base bell jar systeml.Varian model Vl-221. It is equipped with a sorption pump, a 140 -g ion pump and a titanium sub- limation pump. The flanged ports on the chamber were used with various feed- throughs. During experiments, the pressure in the system was =2xl0

9 torr. The molybdenum electrodes were cut from h inch diameter rod with a purity of 99.99%.

The edges were rounded to provide a smooth profile so that a high electric field would not be produced due to sharp edge. The electrodes were screwed on supporting molybdenum rods in the vacuum chamber. The front surfaces of the electrodes were mechanically polished using finally a 0.05 um AI2O3 abrasive powder in water sus- pension. The high voltage electrode was mounted on a stationary high voltage ceramic insulated feedthrough and the ground potential electrode was mounted on a linear motion feedthrough. Both the electrodes could be simultaneously outgassed at =1300K by electron bombardment heating. Electrodes temperature was measured with an optical pyrometer. A 2 HI resistor was used in the voltage-current measurement circuit to protect the power supply. The high voltage was applied by a Sorensen 0-30 KV dc power supply and measured by an electrostatic voltmeter. A Keithley picoammeter was used for measuring the current. During breakdown experiments the ammeter was removed and the voltage was increased until a spark occurred between the electrodes. The electrode surfaces were examined in a JEOL model JSM-35 scanning electron microscope (SEM).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984951

C9-304 JOURNAL DE PHYSIQUE

The Mo e l e c t r o d e s were o u t g a s s e d s e v e r a l t i m e s between 1250-1350 K t i l l s t a b l e v a l u e s o f t h e f i e l d enhancement f a c t o r , B and t h e e m i t t i n g a r e a were o b t a i n e d . The d i s p e n s e r c a t h o d e used was Semicon type-S i n which a p o r o u s t u n g s t e n m a t r i x i s im- p r e g n a t e d w i t h a m i x t u r e of BaO, CaO and A1203 i n t h e mole r a t i o 4 : l : l . The d i s - p e n s e r c a t h o d e was brought t o f a c e t h e s u r f a c e o f t h e Mo c a t h o d e and t h e n h e a t e d t o 10500C b r i g h t n e s s . The T a b l e 1 g i v e s t h e work f u n c t i o n and t h e e l e c t r o n e m i t t i n g a r e a o f t h e molybdenum e l e c t r o d e f o r d i f f e r e n t exposure t i m e s t o t h e v a p o r s which c o n s i s t o f Ba, BaO and some calcium2. The work f u n c t i o n d e c r e a s e d a s t h e e x p o s u r e time was i n c r e a s e d . The minimum v a l u e o b t a i n e d was 1.OleV. From t h e known e v a p o r a t i o n r a t e 2 o f t h e i m p r e g n a n t s we e s t i m a t e t h a t one monolayer (1015 %!ZE

cm2 ⁾ o f t h e vapor atoms i s d e p o s i t e d i n a p p r o x i m a t e l y 15 m i n u t e s .

TABLE 1

Molybdenum c a t h o d e exposed t o d i s p e n s e r c a t h o d e B = 121, d i s p e n s e r c a t h o d e t e m p e r a t u r e = 1050°c ( b r i g h t n e s s )

work

Exposure time (mins) f u n c t i o n (eV) E m i t t i n g a r e a cm2)

We a l s o exposed t h e Mo c a t h o d e s u r f a c e t o t h e v a p o r s o f a t h o r i a c o a t e d t u n g s t e n f i l a m e n t . The t h o r i a c o a t i n g was made by u s i n g t h e method g i v e n by lIanley3. The T a b l e 2 shows t h e e f f e c t of Tho2 v a p o r s on t h e molybdenum c a t h o d e . The i n c r e a s e i n work f u n c t i o n a t h i g h e r e v a p o r a t i o n tempures ($1800K) seems t o b e t h e r e s u l t of t h e a d s o r p t i o n o f contaminant g a s , s u c h a s oxygen o n t h e Mo s u r f a c e .

TABLE 2

Molybdenum c a t h o d e exposed t o T h o r i a c o a t e d f i l a m e n t

Enhancement E v a p o r a t i n g Exposure W o r k f u n c t i o n E m i t t i n g

f a c t o r @ ) temp.(OK) time(mins) ( e v ) a r e a (cm2)

Fig. 1 shows t h e a r c damage on a molybdenum c a t h o d e which was exposed t o t h e 4:1:1

d i s p e n s e r c a t h o d e e v a p o r a t i o n p r o d u c t s f o r 343 m i n u t e s b e f o r e t h e breakdown ex-

p e r i m e n t . Approximately 15 a r c s were observed a t 6.5KV. The gap d i s t a n c e between

t h e e l e c t r o d e was 0 . 5 1 mm. The F i g . l a shows t h a t t h e r e a r e many c i r c u l a r f e a t u r e s

on t h e s u r f a c e and some of them o v e r l a p . Each c i r c u l a r f e a t u r e i s b e l i e v e d t o b e

t h e r e s u l t o f one a r c . One s u c h f e a t u r e i s e n l a r g e d i n F i g . l b . The c e n t r a l r e -

gion i s a c r a t e r w i t h a m i c r o p a r t i c l e imbedded i n i t . Surrounding t h e r i m of t h e

c r a t e r a r e r e g i o n s showing f u r t h e r damage. T h i s a r c may have been i n i t i a t e d due

t o t h e impact o f t h e m i c r o p a r t i c l e o r i g i n a t i n g a t t h e anode. The h e a t g e n e r a t e d

a t t h e p o i n t of impact w i l l m e l t t h e f i l m and t h e m a t e r i a l w i l l d i f f u s e t o t h e

t h e f i l m m a t e r i a l h a s melted and p r o t r u s i o n f o r m a t i o n o c c u r s . Such p r o t r u s i o n s can themselves become s i t e s of i n t e n s e f i e l d e m i s s i o n and i n i t i a t e f u r t h e r break- downs.

The anode used w i t h t h e cathode of Fig. 1 i s shown i n Fig. 2. Some m i c r o p a r t i c l e s a r e s e e n i n Fig. 2a, and some f i l m m a t e r i a l d e t a c h e d from t h e c a t h o d e s u r f a c e due t o t h e h i g h f i e l d s t r e s s i s s e e n i n F i g . 2b. The f e a t u r e s shown i n F i g . 2c a r e per- haps due t o f o r m a t i o n of b l i s t e r s on t h e s u r f a c e and t h e i r subsequent r u t u r e . z The mechanism of t h e i r f o r m a t i o n has been s u g g e s t e d by S i n h a , Ku and Johnson . ^It can b e due t o t h e e l e c t r o n bombardment induced d i f f u s i o n of gas molecules which a r e a l r e a d y d i s s o l v e d i n t h e metal. Because of h i g h f i e l d s and h e a t i n g , t h e s e b u b b l e s may b u r s t and g i v e o f f g a s , and produce t h e f e a t u r e a s shown i n Fig. 2c. The Fig.2d a l s o shows a r u p t u r e d b l i s t e r . The wavy s u r f a c e i s q u i t e d i f f e r e n t from t h e c a t h o d e s u r f a c e . I t a p p e a r s t h a t t h e s u r f a c e was h e a t e d t o h i g h t e m p e r a t u r e due t o t h e f i e l d - e m i s s i o n c u r r e n t s .

We have c a l c u l a t e d t h e i n i t i a l l o c a l breakdown f i e l d Eb and t h e r e s u l t s a r e sum- marized i n t h e Table 3.

TABLE 3

Vapors Vb d vb/d 6 Eb

(kV) (ma) ( w / c m ) (MV/cm)

B a / ~ a 0 4.5 0.51 .088 12 1 10.65

Vb : breakdown v o l t a g e f3 : enhancement f a c t o r d : gap d i s t a n c e Eb : l o c a l f i e l d Vb/d : macroscopic f i e l d

From t h e a b o v e T a b l e w e s e e t h a t t h e l o c a l breakdown e l e c t r i c f i e l d f o r Ba/BaO c o a t e d s u r f a c e i s 10.95 W/cm and f o r t h e s u r f a c e exposed t o t h o r i a c o a t e d f i l a m e n t it i s between 37 t o "40 MY/cm.

References

1. M.K. Sinha and Yee-Gee Ku, V I I I I n t e r n a t i o n a l Symposium on Discharges and E l e c t r i c a l I n s u l a t i o n i n Vacuum, Sandia Labs, Albuquerque, New Mexico, 5-7, September 1978.

2. J.L. Cronin, IEE Proc. 128, P t I 19 (1981).

3. T.E. Hanley, J. Appl. Phys. 2, 583 (1948).

4. M.K. Sinha, Yee-Gee Ku and R a n d a l l P. Johnson, J. Appl. Phys. 520, 699 (1981).

C9-306 JOURNAL DE PHYSIQUE

F i g . 1 SEM PICTURES OF ?.fo CATHODE EXPOSED TO 4 : l : l DISPENSER CATHODE FOR 3 4 3 MINS