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FIELD EMISSION ELECTRIC PROPULSION : EMISSION CHARACTERISTIC OF SLIT EMITTERS
J. Mitterauer
To cite this version:
J. Mitterauer. FIELD EMISSION ELECTRIC PROPULSION : EMISSION CHARACTERIS- TIC OF SLIT EMITTERS. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-399-C7-404.
�10.1051/jphyscol:1986767�. �jpa-00225962�
JOURNAL DE PHYSIQUE
Colloque C7, supplément
au
n o 11, Tome 47, Novembre 1986FIELD EMISSION ELECTRIC PROPULSION : EMISSION CHARACTERISTIC OF SLIT EMITTERS
J. MITTERAUER
Institut für allgemeine Elektrotechnik und Elektronik,
Technische Universitat Wien, Gusshausstrasse
27-29,A-1040 Wien, Austria
Abstract
-
F i e l d emission e l e c t r i c p r o p u l s i o n i s t h e technological a p p l i c a t i o n of t h e p r i n c i p l e of l i q u i d metal i o n sources as t h r u s t e r s i n e l e c t r i c space propulsion. Research work aponsored by t h e European Space Agency (ES&) on t h e emission performance of a s l i t type f i e l d i o n t h r u s t e r i s reported and discussed.1 . 1 NTRODUCT 1 ON
Within ESR's research programme on e l e c t r i c propulsion, f i e l d emis- s i o n e l e c t r i c propulsion using 1 i q u i d caesium (CS) as f avoured propel- l a n t is o f s p e c i a l importance; research and development work perform- ed a t EÇTEC r e s u l t s i n t h e s l i t e m i t t e r / 1 / , / 2 / .
When t h e surface of a l i q u i d metal i s subjected t o a h i g h e l e c t r i c f i e l d i t i s d i s t o r t e d i n t o a cone o r a s e r i e s of cones which protrude more and more from t h e surface w i t h increasing f i e l d strength.
When t h e l o c a l f i e l d reaches values o f t h e order o f i09Vm-x, atoms o f t h e metal t i p a r e i o n i z e d e i t h e r by f i e l d desorption o r f i e l d i o n i - zation; w i t h t h e proper e l e c t r i c p o l a r i t y , t h e f r e e e l e c t r o n s a r e r e - jected i n t o t h e bulk o f the l i q u i d metal w h i l e t h e i o n s are acceler- ated and expelled from t h e e m i t t e r by t h e same e l e c t r i c f i e l d which has i o n i z e d them, c r e a t i n g t h e r e f o r e t h e t h r u s t .
Recent i n v e s t i g a t i o n s /3/,/4/ have demonstrated t h e outstanding importance of both t h e r e s i d u a l gas atmosphere and t h e e m i t t e r pre- p a r a t i o n technique on t h e emission performance of s l i t emitters. Be- sides t h e emission s i t e d i s t r i b u t i o n /4/,the beam divergence i s of considerable importance as i t i s d i r e c t l y connected t o t h e s p e c i f i c t h r u s t , t h e l a t t e r being a s t r i k i n g f i g u r e of m e r i t of a f i e l d emis- s i o n i o n t h r u s t e r .
I I . THE SLIT EMITTER MODULE
The s l i t e m i t t e r module i n p r i n c i p l e c o n s i s t s o f two symmetrical h i g h l y p o l i s h e d metal p l a t e s of t h e shape depicted i n F i g . l . I n one or both of t h e e m i t t e r halves t h e r e i s m i l l e d a recess t a be o f use as a r e s e r v o i r of t h e l i q u i d metal (mostly Cs) supplied t o t h e e m i t t e r mo- d u l e e i t h e r by an open funnel or a feeding c a p i l l a r y tube. On c e r t a i n regions of one of t h e i n s i d e faces t h e r e i s s p u t t e r deposited a l a y e r of n i c k e l w i t h a thickness of t h e order o f lO-'rn.
When t h e two halven are t i g h t l y clamped together they a r e separated by t h e thickness of t h i s l a y e r , t h u s forming a narrow s l i t of width w and depth d through which t h e l i q u i d metal can f l o w and be transported t o t h e edges of t h e s l i t by t h e a c t i o n of c a p i l l a r y f o r c e s iFig.2).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986767
JOURNAL DE PHYSIQUE
F i g u r e 1: E m i t t e r module w i t h beam d i vergence c h a r a c t e r i s t i c s
Cs
-
reser voir\
d *Emitter Slit
F i g u r e 2: S e c t i o n a l view o f t h e emi t t e r - a c c e l e r a t o r c o n f i g u r a t i o n
U n t i l now, a number o f e m i t t e r modules w i t h s l i t l e n g t h s o f 10, 30, 50 and 80mm have been produced. The e l e c t r o d e c o n f i g u r a t i o n w i d e l y used t o c r e a t e t h e praper e l e c t r i c f i e l d a t t h e e m i t t e r s l i t edge reg- i o n i s shown i n F i g . 2. A p l a n e a c c e l e r a t o r e l e c t r o d e w i t h an a p e r t u r e o f w i d t h 2b 15 mounted i n a l i n e a r d i s t a n c e a o f t h e e m i t t e r s l i t edge. Both e m i t t e r and a c c e l e r a t o r a r e k e p t a t v o l t a g e s +UE and -il,,, r e s p e c t i v e l y vs. ground p o t e n t i a l i n o r d e r t o accomplish f o r t h e e l e c t r i c f i e l d necessary t o c r e a t e and a c c e l e r a t e t h e i o n beam.
Under t h e assumption t h a t a l 1 o f t h e charge t r a n s p o r t e d from t h e e m i t t e r i s r e l e a s e d i n form o f s i n g l y i o n i z e d C s atoms o n l y (neglec- t i n g t h e r e f o r e t h e c o n t r i b u t i o n O+ malecular i o n s which a r e found t o c o n t r i b u t e o n l y about 115% t a t h e charge t r a n s p o r t ) and assuming f u r - thermore t h a t a l 1 these i o n s a r e e m i t t e d n o r m a l l y t o t h e p l a n e o f t h e a c c e l e r a t o r e l e c t r o d e , t h e t h r u s t F i s t h e n g i v e n
where vE i s t h e exhaust v e l o c i t y a f t h e i o n s ( i . e . t h e f i n a l v e l a c i t y i n f i e l d - f r e e space) r e s u l t i n g from c o n s e r v a t i o n o f energy
UE i s t h e e m i t t e r v o l t a g e and mi i s t h e i o n mass. R e l a t i n g t h e i o n mass f l o w r a t e
&+
t o t h e emission c u r r e n t IEthen t h e t h r u s t i s g i v e n by
C o n t r a r y t a t h e assumption made above o f an one-dimenaional beam one + i n d s t h a t f o r a s l i t e m i t t e r aome divergence o f t h e beam occurs i n b o t h t h e p l a n e normal t o t h e p l a n e o f t h e e m i t t e r s l i t as ne11 as i n t h e p l a n e o f t h e e m i t t e r , denoted by t h e azimuthal h a l f - a n g l e 9 and t h e t r a n s v e r s a l h a l f -angle w r e s p e c t i v e 1 y as b e i n g o u t 1 i n e d i n F i g . 1.
As t h e measured values o f t h e t h r u s t F always a r e below t h e theore- t i c a l ones d e f i n e d by eq.i4), i n an a t t e m p t t o account o f t h e beam d i - vergence R a r t o l i e t . a l . f l / proposed a m o d i f i e d f o r m u l a f o r t h e t h r u s t
where i t i s assumed t h a t t h e r e i s an u n i f o r m d i s t r i b u t i o n o f i o n t r a - j e c t o r i e s w i t h i n 9 and (p.
III.EXPERIMENTt3L RESULTS AND DISCUSSION
In order t o investigate the emission performance, a mini-emitter (emitter El) developed for this special purpose by ESTEC with a rather short emitter slit length 1=1.10-2m was inserted into an ultrahigh- vacuum equipment. The typical data for t h e emitter-accelerator geo- metry according Fig.2 are a s follows: a = 6.10d4m, 2b = 4.10-3m, d = 5.10-3m, w = 1,2.10-6m.
For the f irst time a slit emitter was f iring in vertical position, being supplied with C e of high purity by means of a completely closed Cs-feeding system; such a closed propellant supply system requiring in principle no gravitational forces and allowing for firing of the emit- ter in any optinal direction aleo claims general interest in yiew of future space applications of liquid metal field emission thrusters / 3 / .
In order t o obtain homogeneoua wetting of al1 interna1 surfaces of the Cs-feeding system and of the emitter itself, without emission an ultimate total background pressure of about 5.10-l0 mbar was main- tained within the vacuum chamber, the contribution of HzO and OH t o this pressure being about 20% and 5% respectively.
The common phenomenological picture of emission within a current range of some 10-3A always shows a rather large number (about 1200) of equidistant faint glowing zones of equal brightness spaced by about 7,5.10-6m and distributed homogeneously along t h e emitter slit edge, the only exception being two much brighter luminous regions at t h e slit ends a s well a s some few less brighter sites occuring always at the same places. As the brightness of t h e luminous zone in front of t h e apex of an emission site depends on t h e emission current, one may conclude from this feature a homogeneous distribution of equidistant spaced identical emission sites auperimpoaed by some few sites with higher emission current.
Beam di vergence
The profile of the ion beam emanating from the emission sites situ- ated along the emitter slit edge i s recorded in both azimuthal and transversal direction by either pivoting t h e emitter around an azi- muthal angle 3 and rotate an ion probe around a transversal angle <p.
Furthermore t h e azimuthal divergence of both the neutrals a s well a s atomic and molecular ions emanating from t h e emitter i s measured by means of a quadrupole mass analyzer ( @ M A ) , t h e axis of the latter being coincident t o the direction defined by the relations 9 = 0" and
w
= O-.The beam divergence in both azimuthal and transversal direction is outlined in the axionometric display shown in F i g . 3 .
Figure 3: Normalized azimuthal and transversal beam divergence profile of the emitter El a s a function of the emission current Ie
(Uncc = -4.103V, a = 6.10-4m)
C7-402
JOURNALDE
PHYSIQUEThe divergence prof iles in azimuthal direction
iw
= const) corres- ponds quite well t o the shape obtained hitherto for t h e case ip = 0'.while the profiles in transversal direction ( 9 = const> show a pro- nounced dip between two maxima of different height at values of the transversal angle 9 = 2b0, resulting f inally in a beam divergence characterized by a saddle surface. The two rather symmetric maxima in the transversal divergence profile may be attributed t o pronounced side emission typically for this emitter.
According t o t h e different possibilities of mass spectroscopie ana- lysis of t h e constituents of the ion beam, the beam divergence was measui-ed conaidering the thermal neutral particle flux (CsoTH) wi thout ion emission, t h e neutral particle flux (Cs°FI) during ion emission a s well a s the ionized particle flux (Cs+FI), t h e results being shown in Fig. 4.
j(N
-
il01
f
Figure 4: Normalized azimuthal beam divergence profiles of t h e emitter
El
for t h e different constituent5 of the beam(1s = 5.10-=A, Uncc = -4.1OSV, a = 6.10-4m)
As the neutral particle flux (CsoTH) emanates from t h e whole +ree Cs-surf ace at t h e emi tter slit orifice, a cosine-distribution (cos3) according t o Lambert's law may be expected for t h e beam divergence.
Measurements reiult in a slightly broader beam divergence profile;
this feature i a attributed t o t h e meniscus of C s protruding out of t h e emitter slit orifice also without any applied voltage and t o the wet- ting of t h e rounded emitter %lit edges, allowing a certain fraction of vapour emanating perpendicular t o t h e plane of the emitter slit.
The most striking result i s obtained for the neutral particle flux (Cs°FI)
.
T h e divergence profile of t h e beam t o a certain extent is between t h e rather narrow distribution of the ionized particle flux(Cs+FI) and the cosine distribution; t h e results of different measure- ments show a rather large scatter, t h e values of t h e FWHM being be- tween about 97- and 107O. The divergence profile shows a cut off at 9 = 60" due t o the shadow effect of t h e accelerator electrode.
If one assumes t h e neutral particle flux <Cs°FIf being created mainly due t o dissociation of microdroplets by electric forces, t h e observed beam profiles possibly may be explalned by a superposition of the strictly directed velocity components of initially charged micro- particles in the accelerating electric field with t h e rather unif orm distributed velocity components of neutral particles due t o the dis- sociation process i t s e l f ~ also elastic collision processes between ions and neutral atoms may play some distinct role.
The beam profile of the ionized atomic particle flux (Cs+FI) final- ly i s nearly identical t o the divergence profiles obtained by ion pro- be measurements; similar results were obtained for molecular ions Cs=+ and Cs=+.
Thrust and s p e c i f i c power
I n t h e present experiment t h e r e i s no p o s s i b i l i t y of a d i r e c t mea- surement of t h e t h r u s t of t h e e m i t t e r modules. I n order t o o b t a i n a comparative value, t h e t h r u s t i s c a l c u l a t e d according eq.(4), açsuming f o r s i m p l i c i t y s i n g l y i o n i z e d atoms Cs+ o n l y . * >
With mce = 2,208.10-=%kg, eq.(4) i s given
Considering f u r t h e r t h e i n f l u e n c e of t h e beam divergence one f i n d s
where 6 i s a divergence f a c t o r given according eq.i5).
From F i g i 3 one fi nds f o r a w i de range o f t h e emissi on c u r r e n t near- l y constant values o f t h e azimuthal h a l f angle ( w = 50°) and of t h e t r a n s v e r s a l h a l f angle (Et=3S0)
.
For a comparable e m i t t e r (ESTEC1,l) y e t having an e m i t t e r s l i t l e n g t h 1 = 5.10-"m, B a r t o l i &.al . / l / measured t h e t h r u s t by means of a balance a t a constant value o f t h e emission c u r r e n t IE = 1.10-"A;
f o r t h i s c u r r e n t t h e beam divergence was evaluated t o be about -35"
and ~ 1 5 I n a v o l t a g e range of ~ . Ue between
&.los
and 1,2.104V, good agreement w a s found between t h e measured t h r u s t and t h e c a l c u l a t e d values according eq.(7).I n order t o compare t h e emission perIormance o f t h e present e m i t t e r E l and t h e e m i t t e r ESTEC1,1, f o r t h e same values of t h e s p e c i f i c emis- s i o n c u r r e n t I,'= I r / l = 2.10-=%cm-+ and an e m i t t e r voltage UE = l.104V, c h a r a c t e r i s t i c emission parameter a r e summarized i n Tab.1.
Besides t h e divergence f a c t o r 8, t h e r e a r e i n d i c a t e d i n Tab.1 as follows: The t h r u s t F,I t h e t o t a l power i n p u t P, t h e s p e c i f i c t h r u s t Fe' normalized t o e m i t t e r u n i t l e n g t h
F', = F s / l ( 8 )
and f i n a l l y t h e s p e c i f i c power P ' which i s given by
The t o t a l power i n p u t P t o t h e e m i t t e r , n e g l e c t i n g t h e almost t i n y c o n t r i b u t i o n o f t h e a c c e l e r a t o r power 105s then i s given by
For cornparison, +urthermore w i t h 6 = 1 and 1 = 5.10-Zm t h e o r e t i c a l values o f both t h e t h r u s t Fe and t h e s p e c i f i c t h r u s t F'I a r e i n d i c a t - ed; t h e s p e c i + i c power P ' f o l l o w s from eqs. (6) and (10)
Table 1
Theory
- -
1.00 1,66. I O - =-
3,3.10-~ 6,00. 104ESTEC1,l 35" 15" 0.93 1,55. I O - = 100 3,l. 6,47. I O 4 E l 50" 35" 0,87 2,88.1tP4 20 2,9.10-4 6,94.104
I D Considering the a c t u a l percentage o f dimers and t r i m e r s (Cs=+, Css') i n t h e i o n beam, t h e share of t h e l a t t e r i s about 16% on t h e charge t r a n s p o r t , about 30% on t h e mass t r a n s p o r t and about 22% on t h e t h r u s t .
C7-404 J O U R N A L
DE
PHYSIQUEMeasurements on t h e e m i t t e r ESTEC1,I were performed up t o a maximum emission c u r r e n t of IE = 1,5.10-"f3 / l / j t h i s value, correspon'ding t o a s p e c i f i c emission c u r r e n t IE' = 3.10-3A~m-1, p o s s i b l y i s due t o cur- r e n t s a t u r a t i o n . On the other side, t h e e m i t t e r E l operates up t o a s p e c i f i c emission c u r r e n t IE' = 5.10-sAcm-+ without any s a t u r a t i o n e f f e c t , t h e l a t t e r value only be due t o t h e l i m i t e d c u r r e n t capacity of t h e h i g h voltage power supply. For t h i s l i m i t i n g value and w i t h Ue = l.104V, t h e emission parameter according Tab.1 f o r the e m i t t e r E l now are
I n conclusion, w i t h regard t o t h e s p e c i f i c power or t h e power t o t h r u s t r a t i o , which i s one of t h e most d e c i s i v e emission parameter i n view of a f u t u r e a p p l i c a t i o n of l i q u i d metal f i e l d emission t h r u e t e r s i n space, t h e c a l c u l a t e d emission performance of t h e e m i t t e r E l i s on- l y f a i r l y below t h a t of t h e e m i t t e r EÇTEC Ill; t h i s discrepancy i s due t o a somewhat higher beam divergence.
On t h e other side, t h e c a p a b i l i t y of t h e e m i t t e r E l t o operate a t r a t h e r h i g h values o f s p e c i f i c emiasion c u r r e n t and specif i c t h r u s t r e s p e c t i v e l y successfully was demonstrated. This outstanding emission performance may be c o n t r i b u t e d t o t h e e x c e l l e n t w e t t i n g p r o p e r t i e s and the n e a r l y homogeneous emission s i t e d i s t r i b u t i o n inherent t o t h e l a t - t e r e m i t t e r .
ACKNOWLEDGEMENTS
This research work waa performed f o r t h e European Space Agency (ESA) under ESTEC-Contract No.5051/82.
REFERENCEÇ
/1/ C. B a r t o l i
,
H-von Rohden, S.P.Thompson and J. Blommers: " A l i q u i d caesium f i e l d i o n source f o r space propulsion"J. Phys. D: Appl
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Fhys. l7, 2473-2483 (19841/2/ D. Valentian, J. P. Bugeat, C - B a r t o l i and H. von Rohden: " F i e l d emis- s i o n e l e c t r i c propulsion development s t a t u s "
,
17th I n t . E l e c t r i c Propulsion Conf erence ,Tokyo, Paper IEPC 84-34 ( 1984)/3/ J.Mitterauer, " F i e l d emiçsion e l e c t r i c propulsion: Çpectroscopic i n v e s t i g a t i o n s on s l i t emi t t e r s "
F i n a l Report on ESTEC-Contract 5051/82 (1986)
f 4/ J. M i t t e r a u e r
,
" F i e l d emission e l e c t r i c propulsion: Emission s i t e d i s t r i b u t i o n o f s l i t emi t t e r s " , Proc. X I 1. I n t . Symposium 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, Shoresh, I s r a e l (1986)( t o be published)
