LIQUID METAL ION SOURCES AS THRUSTERS FOR ELECTRIC SPACE PROPULSION

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LIQUID METAL ION SOURCES AS THRUSTERS FOR ELECTRIC SPACE PROPULSION

J. Mitterauer

To cite this version:

J. Mitterauer. LIQUID METAL ION SOURCES AS THRUSTERS FOR ELECTRIC SPACE PROPULSION. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-171-C6-176.

�10.1051/jphyscol:1987628�. �jpa-00226831�

Colloque C6, suppl6ment au n O 1 l , Tome 48, novembre 1987

LIQUID METAL ION SOURCES AS THRUSTERS FOR ELECTRIC SPACE PROPULSION

J. Mitterauer

Institut fiir Allgerneine Elektrotechnik und Elektronik Technische Universitgt Wien, Gusshausstrasse 27-29, A-1040 Wien, Austria

Abstract: Electric space propulsion systems are characterized accord- 1.1g their inherent features and specified domains of application in space missions. Particular1 y t h e Field Emission Electric Propulsion (FEEf )-system using a liquid metal ion source a s a thruster i s des- cribed and recent results on i t s emission performance are presented.

I. INTRODUCTION

Space propulsion by mass expulsion actually can be performed by conversion of chemically stored thermal energy on t h e o n e side or el- ectric energy on the other side into kinetic energy of t h e expelled mass. In t h e case of chemical propulsion, the amount of energy releas- ed i s limited by the energy content of the chemical propellants. Che- mical propulsion is therefore energy limited but not power limited;

compared with electric propulsion, chemical propulsion i s characteriz- ed by relative high thrust and low exhaust velocity.

In the case of electric propulsion, electric energy normally being converted from solar radiation energy is u s e d - t o supply or augment the kinetic energy necessary for mass expulsion. Bearing in mind the prac- tical limitations of electric power and propellant mass on board of the spacecraft, electric propulsion i s therefore power limited but not energy 1 imi ted. Compared with chemical propulsion, electric propulsion i s characterized by relative low thrust and high exhaust velocity.

The application of electric propulsion t o space missions i s re- stricted by its inherent characteristics, i.e. above all high exhaust velocity, high specific electric power and high operating voltage;

From these primary characteristics there result derived characterist- ics for space applications: Low propellant mass (typically about 1OX of that required with advanced chemical propellants), low thrust lev- el, long operating duration due t o t h e low thrust (typicall y 5. loJ h )

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electrical power conversion and finally t h e necessity of ion beam neu- tralization in t h e case of electrostatic systems in order t o avoid electrostatic charging of the spacecraft.

On the other hand, the inherent characteristics of electric pro- pul si on of f er 'specified domains of thei r appl ication in space missions:

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987628

C6-172 JOURNAL DE PHYSIQUE

The slit emitter module

The slit emitter module in principle consists of two symmetrical highly polished metal plates of t h e shape depicted in Fig.1. In one or both of t h e emitter halves there i s milled a recess t o be of use a s a reservoir of t h e liquid metal supplied t o t h e emitter module by a feeding capillary tube. On certain regions of one of the inside faces there i s sputter deposited a layer of nickel with a thickness of the order of 10-'m. When t h e t w o halves are tightly clamped together they are separated by t h e thickness of this layer, t h u s forming a narrow slit through which the liquid metal can flow and be transported t o t h e edges of t h e slit by the action of capillary forces. The material used for t h e emitter fabrication i s stainless steel and, more recently,

Inconel X 7 5 0 which has shown better wetting characteristics for caesi- um, the liquid metal used for most of t h e emitter tests because of its superior wetting characteristics, low melting point and high atomic mass.

Thruster, propellant system and power conditioning unit

The FEEP thruster basically comprises t h e emitter which i s kept at a positive voltage, the accelerator, kept at negative voltage and which i s used a s a counter-electrode t o establish the field needed for ionization and finally t h e neutralizer in order t o avoid electrostatic charging of t h e spacecraft. As illustrated in Fig.2, t h e complete FEEP system comprises:

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The proposed neutralizer for t h e future development i s a plasma bridge neutralizer fed also by caesium which can be operated with very low mass flow rates of approximately only 4% of t h e main flow rate or less. The propellant tank i s a metallic bellows device pressurized for caesium expulsion by heated zeolite. The power conditioning unit consits of a thruster dedicated power supply module utilizing advanced technology in power conversion.

The main advantages of a propulsion system based on t h e field emis- sion principle are:

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Fig. 1: Slit emitter module in original size

Fig.2: Basic design of the FEEP-system

In 1972, t h e European Space hgency (ESA) started t h e development of an advanced concept of electric propulsion based on t h e field emission principle; the application of this principle in liquid metal ion sour- ces (LMIS) has resulted in a high specific impulse ion thruster called FEEP (Field Emission Electric Propulsion). This activity has been car- ried out at t h e European Space Research and Technology Centre (ESTEC) in Noordwijk under t h e ESR Technological and Research Programme /I/.

Major activities in research and development sponsored by ES& further- more are being pursued at the SocigtB EuropBenne d e Propulsions (F), at t h e Fulmer Research Institute (UK)

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The field emission principle

In a field emission electric propulsion system the ions being ex- pelled are created directly from t h e surface of a liquid metal expas- ed t o vacuum by means of a high electric field resulting from suitable voltages applied t o an emitting electrode geometry. When the surface of a liquid metal i s subjected t o a high electric field it i s distort- ed into a cone or a series of cones which protrude more and more from the surface with increasing field strength. The onset of ion emission occurs when the applied total voltage between the emitter and accel- erator electrode is in excess of a critical value which is called the onset voltage

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Considering one of the most advanced physical models of the ion emission process / 2 / , field evaporation i s claimed t o b e t h e do- minant mechanism of ion f ormation, but it also was found that thermal evaporation followed by field ionization may make a significant con- tribution t o t h e total ion current. Field ionization, unlike field evaporation, can occur also in a region of t h e order o+ SO-% wide above the apex of an emitting cone, where thermally evaporated neutral atoms and clusters (also emanating from t h e shanks of t h e cone) were attracted by polarization forces. Contrary t o field evaporation, the field ionization mechanism gives rise t o singly charged ions only.

Bearing in mind finally t h e obviously inherent feature of this mechan- ism t o emit beneath atomic ions and molecular ions of minor percentage /3/ even charged clusters and microdroplets (the diameter of the lat- ter ranging between 10-Tm t o 10-7m), a new scope for speculation oh vapor production processes i s offered. Experimental evidence on neu- tral emission support t h e view that virtual1 y all of t h e neutral mass i s emitted a s microparticles /3/,/4/,/5/. These are interpreted a s aggregates of neutral atoms carrying a surface charge such that t h e inward surface tension forces approximately are balanced by t h e out- ward electric forces. It i s known that for microparticles with dia- meters less than about 2.10-Tm t h e effective surface tension decreases a 5 a result of t h e gross curvature of the liquid surface / 5 / . One would expect therefore that microparticles in this range would be un- stable and would dissociate under t h e outwards electric forces, con- tributing probably t o the supply of neutral atoms and clusters men- ti oned above.

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T h e typical F E E P thruster performance i s summarized in Table I:

Range of emitter length Emitter slit width Range of emitter voltage Range of accelerator voltage Thrust per unit length Power t o thrust ratio Specific impulse Mass. efficiency Power ef f i ci ency

111. EMISSION PERFORMANCE OF THE FEEP SYSTEM

Recent investigations have demonstrated t h e outstanding importance of both t h e residual g a s atmosphere and t h e emitter preparatian tech- nique o n t h e emission performance of F E E P thrusters /4/,/6/. In addit- ion, very recently, t h e capability o+ F E E P thruster operation in t h e pulsed mode h a s been demonstrated, t h u s opening t h e possibility of utilising t h i s system f o r n e w d o m a i n s i n application like extreme f i n e positioning of spacecraft /7/.

Emission s i t e distribution and specific thrust

If t h e slit emitter i s filled with liquid metal and t h e surfaces of t h e slit a r e uniformly wetted, a homogeneous linear liquid tip of semicylindrical s h a p e i s assumed t o exist at onset conditions. En- hancing t h e electric field, t h i s t i p i s distorted; equidistant spaced emitting s i t e s a r e originating along t h e extension of t h e emitter slit, each of them contributing t o t h e ion beam.

In order t o maintain t h e s e conditions, t h e emitter i s heated and outgassed at a n ultimate background pressure of about 5.10-xombar and then i s supplied with caesium of extreme purity. Complete and perfect wetting i s obtained, resulting in an instantaneous firing of t h e emit- ter and being characterized by a nearly homogeneous distribution of equidistant emission ei tes, t h e 1 inear density of t h e latter being ab- out 1200 cm-*. T h e measured spacing of about

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Fig. 3: Equilibrium configuration of a liquid metal conductor in a n electric field of cylindrical symmetry

(a) Electrode configuration

(b) &xially symmetrical varicose mode of t h e liquid metal

w i t h no t i m e v a r i a t i o n s . Among these, t h e a n a l y s i s of a c y l i n d r i c a l case seems t o be o f s p e c i a l relevance t o t h e s l i t e m i t t e r , where i t i s assumed t h a t t h e i n i t i a l shape of t h e l i n e a r l i q u i d metal t i p j u s t be- f o r e emission onset i s of s e m i c y l i n d r i c a l geometry. One considers an i n f i n i t e l y long , c i r c u l a r c y l i n d e r of r a d i u s r made of l i q u i d metal t o be placed i n a hollow metal c y l i n d e r of r a d i u s R as i t i s o u t l i n e d i n Fig.S(a). The i n i t i a l pre-perturbation e q u i l i b r i u m c o n d i t i o n i s given

where E, i s t h e i n i t i a l l y uniform e l e c t r i c f i e l d a t t h e surface of t h e l i q u i d c y l i n d e r and d t h e surface tension of the l i q u i d metal. A small p e r t u r b a t i o n r ' of t h e r a d i u s i s a p p l i e d which i s assumed t o be s i - nusoidal i n both 9 and z ; Laplace's equation then i s solved f o r t h e r e s u l t i n g perturbed p o t e n t i a l

a'

where H ( r ' ) i s a Hankel function. For t h e mode m = 0 which i s t h e normal a x i a l l y symmetrical varicose mode shown i n Fig.S(b) t h e r e e x i s t two s o l u t i o n s

k r r = 2 and knr = 0,2 ( 3 ) Considering t h e r e 1 a t i on

t h e r e s u l t i n g wavelengths of t h e varicose mode are

XI = m- and XI

l a m

I n order t o decide which o f t h e two p e r t u r b a t i o n wavelengths may apply, a comparison was made between t h e r e s u l t s of a standing wave approach f o r the plane case / l o / and t h e present values; as t h e smaller wave- l e n g t h Xi corresponds q u i t e w e l l w i t h t h e former r e s u l t s , t h i s value was selected f o r t h e f u r t h e r considerations.

The c a p a b i l i t y o f such a t h r u s t e r t o operate a t r a t h e r h i g h values of t h e s p e c i f i c emission c u r r e n t o f 5. 10-3Acm-i and more without any c u r r e n t s a t u r a t i o n was demonstrated s u c c e s f u l l y /6/. The c a l c u l a t e d s p e c i f i c t h r u s t of about 7.10-4Ncm-i r e s u l t i n g f o r t h i s s p e c i f i c cur- r e n t i s considerably above t h e values measured f o r a comparable thrua- t e r a t ESTEC.

Pulsed o p e r a t i on mode

The need f o r u l t r a - f i n e a t t i t u d e c o n t r o l of spacecraft f o r f u t u r e s c i e n t i f i c missions and t h e unique combination of low t h r u s t and ex- tremely f a s t switching c a p a b i l i t y o f f e r e d by a FEEP t h r u s t e r had s t i - mulated t h e i n t e r e s t on t h e FEEP system performance when operated i n t h e pulsed mode; t h e r e f o r e several dedicated experiments have been c a r r i e d out t o assess t h e emission c h a r a c t e r i s t i c s of a FEEP t h r u s t e r operating w i t h a pulsed power supply u n i t / 7 / :

The p u l s e c h a r a c t e r i s t i c s o f voltage and c u r r e n t f o l l o w q u i t e c l o s e l y t h e corresponding values of FEEP t h r u s t e r s operated i n t h e continous mode. Several pulse frequencies ranging from 50 t o 5.10-4Hz have been explored; t h e minimum p u l s e l e n g t h which can be achieved i s i n t h e range of lOms w i t h emission c u r r e n t s o f about 3.10-'A; t h i s i s equivalent t o a t h r u s t l e v e l o f 3.10-&N and an impulse b i t of 3.10-mNs which i s f a r beyond t h e c a p a b i l i t i e s of any chemical propulsion system.

JOURNAL DE PHYSIQUE

REFERENCES

/1/ C. Bartoli and W. Berry: "Review of European electric propulsion developments", 19th Int. Electric Propulsion Conference, Colorado Springs, Paper AIAA-87-1099 (1987)

/2/ D. R. Kingham and L. W. Swanson: "Mechanism of ion formation in liquid metal ion sources", J.Physique

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/3/ J.Mitterauer: "Spectroscopic investigations of a field emission generated radiative zone; Mass spectroscopic measurements", Final Report on ESTEC-Contract 2734/76 (1981)

ESA Report Reference CR (P) 1589

/4/ J. Mi tterauer: "Field emission electric propulsion: Spectroscopic investigations on slit emitters", Final Report on ESTEC-Contract 5051/82 (1985), ESA Report Reference CR(P)2330

/5/ S. P. Thompson: "Neutral emission from 1 iquid metal ion sources"

Vacuum

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/6/ J. Mi tterauer: "Fie1 d emission electric propulsion: Emission site distribution of slit emitters", Proc. 12th Int. Conf

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/7/ D. Laurini

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19th Int-Electric Propulsion Conference, Colorado Springs, Paper AIAA-87-1046 (1987)

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Basic investigations of the ion emitting region", Final Report on ESTEC-Contract 4462/80 (1983)

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/lO/G.I.Taylor and A.D.McEwan: "The stability of a horizontal fluid inter+ace in a vertical electric field", J.Fluid Mech.,ZZ,l (1965)