LOW-VOLTAGE ARC PLASMA IN THREE-ELECTRODE SYSTEM

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LOW-VOLTAGE ARC PLASMA IN THREE-ELECTRODE SYSTEM

V. Bakaht, A. Martsinovskiy, V. Yur’Yev

To cite this version:

V. Bakaht, A. Martsinovskiy, V. Yur’Yev. LOW-VOLTAGE ARC PLASMA IN THREE- ELECTRODE SYSTEM. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-497-C7-498.

�10.1051/jphyscol:19797241�. �jpa-00219225�

JOURNAL DE PHYSIQUE ColZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, page C7- 497

LOW-VOLTAGE ARC PLASMA IN THREE-ELECTRODE SYSTEM

V.G. Baksht, A.M. Martsinovskiy and V.G. Yur'yev.

A. F. I o f f e PhysicaZ-!7'eehnicaZ I n s t i t u t e , Academy o f Sciences o f the U.S.S. R., Leningrad, K-21 U . S. S. R.

I n v e s t i g a t i o n s of g r i d i n f l u e n c e on W e n c o n c e n t r a t i o n s /7, and V 2 becomes bas discharge plasma a r e few and not s y s t e - n e a r l y equal, t h e g r i d p o t e n t i a l drop

q,,

m a t i c a l /1/. There a r e no such i n v e s t i g a t i - ons f o r h i g h plasma c o n c e n t r a t i o n ( & - l D 12

- ^loi4

cmm3), though t h i s problem becorues s i g n i f i c a n t f o r p r a c t i c a l a p p l i c a t i o n s /2/.

The i n v e s t i g a t i o n of such problem i n s h o r t low-voltage a r c cesium plasma 2111s i n t h i s gap The experiments were perforlned a t pla-

ne geometry devices with s u r f a c e a r e a ⁴ 1 c 3 and i n t e r e l e c t r o d e d i s t a n c e

dac

^'(2-5)

m a .

The g r i d s with c e l l dlmtnsion (100- 2 0 0 ) ~ and perueab ili t y

d

^.r0.5-0.8 were utiliz'ed. Probe aud s p e c t r a l ylasrnu diagno- s t i c metbods were used together w i t h g a t i n g i n t e g r a t i o n technique. Time r e s o l u t i o n was

50 n s /3/. %hen a g r i d

i s

introduced i n t h e gap, ^' t h e d i s c h a r g e plasma d i s s o l v e s t o two r e g i o n s w i t h s i g n i f i c a n t l y v a r i o u s yro- p e r t i e s (cathode r e g i o n and anode r e g i o n

-

s e e ~ i g . 1 ) . I n t h e each r e g i o n e l e c t r o n temperature

,

c o n c e n t r a t i o n

II,

and plas- ma p o t e n t i a l

(P

change l i t t l e . But v e r y

sharp v a r i a t i o n of these paraiuet e r s t a k e s p l a c e i n r h e g r i d plane. i f d i s h a r g e i s de- veloped s i g n i f i c a n t l y and Lhe whole emissi- on c u r r e n t flows, t h e main p o t e n t i a l drop occurs I n t h e oathode sheath

(PI .

^{The ele-}

c t r o n beam f r o m cathode l o s e s i t s energy i n ,cathode-grid r e g i o n , !itherefore e l e c t r o n temperature and plasma c o n c e n t r a t i o n i n ca- thode r e g i o n a r e h i g h e r s i g n i f i c a n t l y than i n anode one (

G , >'Poz, n, >>nz I.

P o t e n t i - a l drop

S2

i n t h e g r i d p l a n e

i s

negative, i.e. r e t a r d i n g f o r e l e c t r o n flow from ea- thode t o anode region. Concentration

n2

es- t a b l i s h e s a t t h e l e v e l s u f f i c i e n t f o r cur- r e n t t r a n s f e r t o anode ( p o i n t

2

a t Fig.1).

When anode p o t e n t i a l

Ta

diminishes, ca- thode p o t e n t i a l drop

(P, , ;a,

^{17, and}

^q,;!

diminish a l s o , plaama par&@$krs i n anode r e g i o n being unchanged ( ~ i g . 1 , p o i n t 1 ) .

c h w e s i t s s i g n and t h e l o n g e n e r a t i o n be- cones g r e a t e r ~n anode r e k i o n than m ca-

thode one. Uischarge passes i n t o t a s i t r o n mode, discovered i n

/I/.

T h i s node of oke- r a t i o n i s t h e most ~conomic because of small v a l u e o f Ion c u r r e n t t o cathode (n,<

< h 2 ) . P o t e n t i a l drop

p12

a c c e l e r a t e s e l e -

c t r o n s and f a c i l i t a t e s t h e c u r r e n t flow in t h e most narrow space of d i s c h a r g e

-

^betwe-

e n f he g r i d wires.

When

7 / ,

i n c r e a s e s , enhancement of ffi

c e a s e s when p l a s u a r-uach the h i g h i o n i z a t i - on degree

.'

^lf

^ya

^continues

^{t o}

i n c r e a s e , t h e h o t ~ l e o t r o n s p e n e t r a t e i n t o anocie r e - gion. Theref ore e l e c t r o n tetuyerature

T ,

^g

and

concentrat i o n

n2

increase. Concentra- t i o n s i n cathode an& anode r e g i o n s draw to- getlier and

y,e

d i u i n i s h e s ( p o i n t

3 ) .

The diminution of g r i d p o t e n t i a l

Yg

cau- s e s t h e enhancement of t h e yre-electrode l a y e r diluension A near t h e g r i d wires ( ^{d 2 *}

vK).

^{I t} dliiiinisles the e l e c t r i c a l g r i d p e r f i e a b i l i t y

B dee

and causes t h e en- liancement of i o n g r i d c u r r e n t . T h e pre-ele-

c t r o d e l a y e r i s s u i f i c i e n . t l y narroiv f o r d i - scharge conciitions t h a t a r e i n t e r e s t i n g ,$or p r a c t i c a l a p p l i c a t i o n s ( 0 10'~ em-31, Theref ore perrneabili t y dilirinisbes slomly when

-5

i s depleated. Loreover, i f

y,2

2.q r e t a r d i n g ,

de/

d i ~ i n u t i o n

i s

comgensatd by s m a l l v a r i a f i o n of jO,z :

jQ det

x

4 ( . ~ , ( ~ $ j @ ) * h ~ - & ~ ] Therefore d i s c h a ge c u r r e n t and p l a s m parameters a r e near- l y independent, o f V

a( FI a ) 2

.It must be noticed t h a t the second reason of such independen- c e i s of t e n more s i g n i f i c a n t t h a n t h e f i r s t one. This phenotuenon ( r a t h e r than' e l e c t r o - s t a t i o screening) p r e v e n t s g r i d - c o n t r o l i n high d e n s i t y plasuia.

i t

is

e s s e n t i a l t h a t t h e discharge quen- ching occurs a t n e g a t i v e g r i d v o l t a g e Only

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797241

i f t h e p o t e n t i a l drop

i s

accelerating, f o r e l e c t r o n s . Near t h e quenohily p o i n t main i o n g e n e r a t i o n

is

concentrated

i n

mo- d e region, g r i d permeability i s small and anode c u r r e n t

Ja.

i s diuiiniuhed appreci- ably. This r e s u l t was obtained i n exgeri- ments a s well

as

i n t h e o r e t i c a l c a l c u l a t i - ons /4/.

When load r e s i s t a n c e Re i s i n anode c i r c u i t , diminution of jq cause6 ?/q

and(pl

i n c r e a s i n g (x+jdp=E,

⁼

c o n s t ) . Theref o r e c o n c e n t r a t i o n n, i n cathode r e g i o n incre- ases. l t prevents d , ~ diluinution and ja

i n t e r r u p t i o n .

T h e

load r e s i s t a n c e germit t o r e a l i z e

Llie

lower branch of voltage-

c u r r e n t c h a r a c t e r i s t i c - b r a n c h

with

negati- v e r e s i s t a n c e .

A t

t h i s branch uoaplicated o s c i l l a t i o n s occur

w i t h

frequency

^N

100 kcycles.

A t Fig.3

t h e t y p i c a l r e s u l t s of probe plasma parameters i n v e s t i g a t i o n i n cathode ( 1 ) and anode

( 2 )

r e g i o n s a r e shown f o r t h e conditions

^when

o s c i l l a t i o n s oacur. These

a r e

r e l a x a t i o n o s c i l l a t i o n s w i t h successive i o n g e n e r a t i o n t r a n s i t i o n s frooa

one

r e g i o n

co

another.

It

was a l s o

s h a m

t h a t

grid

influence6 upon d i s c h a r g e when J:

^{i s}

n e a r l y equal t o random c u r r e n t

J 2 =

# n & and g r i d p o t e n

t i a l drop d o e s n ' t c o n t r o l discharge. The c o n d i t i o n j, 2jt may b e r e a l i z e d by i o n c o n c e n t r a t i o n dimiiiution or

by

i n c r e a s i n g off anode c u r r e n t . The f i r s t s i t u a t i o n may be achieved by u t i l i z a t i o n of a &rid w i t h l a r g e s u r f a c e . But such g r i d c a u s e s t h e g r i d i o n c u r r e n t t o i n c r e a s e

and quenching

anode v o l t a g e t o i n c r e a s e a l s o . The second s i t u a t i o n

may

be achieved by ef f ec- t i v e e m i t t e r u t i l i z a t i o n , p a r t i c u l a r y

in

d i s c h a r g e with 6

^and

^8, vapours.

S ~ O P

taneous c u r r e n t i n t e r r u p t i o n s a r i s e i n t h i s s i t u a t i o n .

Possibility

of g r i d cont- r o l a r i s e s a l s o i f t h e dynamical e f f e c t s

i n plasma a r e u t i l i z e d . These two p o s s i b i - l i t i e s a r e considered i n corresponding r e - p o r t s .

/1/

B.0

.Johnson,

J . O l m s

tead,

^1Y

.H.Vebster.

PZ'OC~XRE 42, 1950 (1954)

/2/ G.U.

myaznov,

V

.B.Xaplan,

A

.&&iartai- novskiy,

V .I

.Serbin, V.G.Ywiev, Report on t h e

i V - t h

i n t 4 o n f .on Thermionic Po- wer Genaration ( ~ i n d c h o v e n , Netherlands, 1975)

/3/ V .B

.-plan,

A .N

&akarov,

A

.hl.hiartsinov- s k i y a.0. J o u r n . T e c h n . ~ h y s . ~ , 2 4 7 (1977).

= J ! . ~ o - ~ ~ o z z

Ea

⁼

^55v

Fig.

3

0 1

Pig

-10 -20