EMISSION PERFORMANCE OF A SLIT TYPE CAESIUM FIELD ION SOURCE

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EMISSION PERFORMANCE OF A SLIT TYPE CAESIUM FIELD ION SOURCE

J. Mitterauer

To cite this version:

J. Mitterauer. EMISSION PERFORMANCE OF A SLIT TYPE CAESIUM FIELD ION SOURCE.

Journal de Physique Colloques, 1989, 50 (C8), pp.C8-191-C8-196. �10.1051/jphyscol:1989833�. �jpa-

00229931�

COLLOQUE DE PHYSIQUE

Colloque C8, supplkment au no 11, Tome 50, novembre 1989

EMISSION PERFORMANCE OF A SLIT TYPE CAESIUM FIELD ION SOURCE

J

.

Institut fiir Allgemeine Elektrotechnik und Elektronik Technische Universitat Wien, A-1040 Wien, Austria

ABSTRACT

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1

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When t h e surface of a liquid metal exposed t o vacuum is subjected to a high electric field resulting from suitable voltages applied t o an emit- ting electrode geometry, it i s distorted i n t o a c o n e or a series of c o n e s which protrude more and more from t h e surface with increasing field

strength. When t h e field reaches values of s o m e 10TVm-x, according t o t h e polarity of t h e field generating v01 t a g e field emission or field ioniza- tion o c c u r s from t h e apex region of t h e s e cones. Because t h e r a d i u s of curvature at t h e apex of such a c o n e i s of t h e order of 10-'m or less, applied voltages of s o m e 10JV a r e sufficient t o obtain t h e required high electric f i e l d s with macroscopic interelectrode distances of about 10-=m.

Electron and ion beams can be created from liquid metal wetted needles or from capillaries i n t o which t h e liquid metal i s allowed t o flow: electron emission a s we1 l a s ion emission from caesium-wetted tungsten needles and stainless steel capillaries h a s been demonstrated successfully /l/.

O n e of t h e most advanced field ion sources i s t h e slit emitter originally developed at t h e European S p a c e Agency (€S&) as an ion thruster for elec- t r i c s p a c e propulsion with liquid caesium a s propellant /2/. In t h i s case, t h e capillary i s elongated t o a slit of s o m e la-=m length with nearly rectangular c r o s s section, allowing therefore t h e occurrence of a s e r i e s of equally spaced emitting cones. T h i s t y p e of ion s o u r c e represents a n ultimate development in precision mechanics, demonstrated by actual nu- merical values of about 10-&m for both t h e emitter slit width W and t h e round-o+f radius r e of t h e emitter slit e d g e fFig.2).

Besides t h e oriqinal concept a s an ion thruster, t h e slit emitter gener- ally may b e employed a s a high current liquid metal field ion source for terrestrial applications in science and technology.

2 - EXPERIMENT

T h e slit emitter in principle consists of t w o symmetrical highly polished metal plates of t h e s h a p e depicted in Fig.1. In o n e or both of t h e emitter halves t h e r e i s milled a recess t o b e of u s e a s a reservoi? of t h e liquid metal supplied t o t h e emitter module by a feeding capillary tube. On cer- tain regions of o n e of t h e inside f a c e s t h e r e i s sputter deposited a layer of nickel with a thickness of about 10-'m. When t h e t w o halves a r e tightly clamped together they a r e separated by t h e thickness of t h i s layer, t h u s forming a narrow slit of length 1, width W and depth d through which t h e liquid metal can f l o w and be transported t o t h e e d g e s of t h e slit by t h e action of capillary forces. T h e material used for t h e emitter fabrication i s stainless steel and, more recently, Inconel.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989833

The electrode configuration used t o create t h e proper electric field at thm emitter slit edge region i s shown in Fig. l. A plane accelerator elec- trode with a n aperture of width 2b i s mounted in a distance a of the emitter slit edge. Emitter and accelerator are kept at voltages +U= and -U&- rerpectively vs. ground potential in order t o create the electric -field necessary t o produce and accelerate t h e ion beam. Actual data for t h e emi tter-accelerator geometry are a s f 01 lows8 l=l ,S. 10-*m; a-6. l W 4 m ; Zb=4.10-=m; d=1.10-3m# ~=l,Z.lO-~rn and l,S.lO-Lm, respectively. Typical voltagas for U= range between 0 and 6kV at a constant value Uac==-SkV, with a maximum emission current It=5m6 for Ul=6kV.

Emitter body

t '

CS

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B

Emitter slit

Arcderaior electrode

Fig.1

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The ultra high vacuum (UHVj-system is equipped with t w o diffusion pumps connected in series, t h e first of them operating a s a booster stage; the working fluid i s Santovac'with a vapour pressure at normal conditions be- low 1.10-lombar. The use of liquid nitrogen cooled baffles together with a n additional titanium sublimation pump allows of an ultimate total back- ground pressure of the order of 10-xOmbar. Ultra clean and reproducible operating conditions within the ultra high vacuum chamber are highly de- sirable especial l y during t h e initial phase of emitter preparation and Cs- supply. A totally closed CS-f eeding system i s used, involving CS-supply t o t h e emitter from a separated CS-reservoir through a metal capillary tube either by gravitational and capillary forces or by means of additional positive pressure feeding. The interior surfaces of both the emitter and the CS-feeding system are outgassed under UHV-conditions at temperatures a s high a s 450-C by radiation and resistive heating, respectively.

In order t o avoid condensation of CS on uncooled surfaces and t o reduce therefore t h e background CS-vapour pressure, the ion beam of t h e emitter i s directed towards e cylindrical cold shroud at ground potential cooled t o liquid nitrogen temperature. Secondary electrons released by the impact of high energy CS-ions were suppressed by a cage-shaped wire grid mounted beneath t h e interior surf ace of t h e cold shroud, being biased negative towards ground.

Monitoring of the residual g a s atmosphere a s well a s mass spectroscopic analysis of both t h e constituents of t h e ion beam and of the azimuthal distribution of t h e latter i s carried out by means of a fixed quadrupole mass analyzer (BMA), tilting t h e emitter together with the accelerator electrode around an axis of rotation parallel t o and coincident with the edge of t h e emitter. Visual and photographic observation of the emitter- accelerator region is by means of a high resolution microscope, a1 lowing for linear scanning along t h e emitter slit edge.

3

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If the emitter i s filled with liquid Cs and t h e surfaces of t h e slit until t h e slit edges are uniformly wetted by t h e liquid metal, a homogeneous linear liquid tip of semicylindrical shape i s assumed t o exist at t h e slit edges at onset conditions of emission. The sectional view of this emitter region schematically i s outlined in Fig.2, showing also different idea- lized assumptions on t h e equilibrium shape of t h e liquid tip; t h e tip radius r, dependens on both the emitter slit width W and t h e radius of curvature r~ of t h e emitter slit edges, which are both of the w d e r of 10-Lm. Enhancing the electric field

,

Due t o electrohydrodynamic effects, equidistant spaced emitting sites ori- ginate along the extension of the emitter slit. The existence of a matrix arrangement of wavelike instabilities on electrically stressed surfaces of fluids i s a well known phenomenon / 3 / . The linear disposition of emitting features observed on slit emitters /4/,/5/ obviously i s related t o these surface instabilities.

Fig.2

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The mathematical treatment of this problem has been discussed in some de- tail / 6 / , considering also standing wave perturbations with no time vari- ations. For wavelike instabilities occuring on t h e electrical l y stressed CS-surface at the orifice of the emitter slit, a simple relation exists between the wavelength X of these instabilities, which i s identical with the spacing of the emission sites, and the radius of curvature r, of the semicylindrical liquid metal tip

Within t h e theoretical figures out1 ined in Fig.2, t h e case (a) seems t o b e most unlikely for an emitter slit edge properly wetted with CS. The equi- librium configurations according (b) and (c) are more realistic a s these depend mainly on t h e actual wetting properties at the emitter slit edge.

Considering actual wetting features, even values of t h e radius of curva- ture ro much in excess of t h e case r-=2w may occur.

With values of X measured for two emitter with slit widths of w=1,2. 10-6m and ~ = 1 , 5 . 1 0 - ~ m , respectively, there result for t h e radius of curvature re

1 , Z w 7 , 3J.m 2,4W l ,2wn lbwn Slun l ,Sum l Own 3 , 2 ~ m

The occurence of a nearly constant value of X verified experimentally for a wide range of emission currents obviously may not be explained in terms of a simple standing wave theory neglecting hydrodynamic effects but has rather t o be treated by a flow field theory assuming also mutual interac- tions of adjacent emission sites. A s a qualitative approach, one may as- sume an emission site distribution mainly impressed by the onset condi- tions which i s not changed considerably with increasing emission current d u e t o t h e concentration of t h e hydrodynamic flow of CS t o the apices of t h e emission sites. Considering t h e instabilities of liquid columns, a similar idea even has been proposed /6/, i.e. in a viscous fluid the positions of the emitting sites are likely t o be stabilized by the fluid flow.

Wetting of t h e emitter slit and t h e adjacent areas i s the most important but even contradictory feature in the analysis of the emission site dis- tribution. On the one side, perfect wetting of t h e interior of t h e slit until t h e slit edge i s essential for the occurrence of a homogeneous Cs- surface which is distorted by t h e applied field t o a linear array of stable emission sites with rather small spacings X of t h e order of 10-'m. On the other side, stimulated by t h e emission process itself, wetting of much larger areas outside the emitter slit edge i s a favoured process.

Due t o t h e resulting large values of r, and X, rather large emission sites with enhanced inherent instability towards microdroplet formation may oc- cur; final l y

,

4

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For a slit emitter some divergence of t h e ion beam occurs in both the plane normal t o t h e plane of t h e emitter slit a s well a s in the plane of t h e emitter, denoted by the azimuthal half-angle 9 and the transversal ha1 f -angle W, respective1 y. In t h e present experiment no direct measure- ment of t h e transversal beam divergence was possible; the azimuthal ion beam divergence i s measured either by t h e quadrupole mass analyzer (BMA), where t h e axis of t h e latter i s defined by %Do, or by an ion probe which i s turned into the axis of t h e @MA.

In order t o compare t h e experimental results, t h e beam divergence profil- e s are normalized, defining the intensity at 3=Oo t o be unity. A common figure t o characterize t h e angular dependence of the shape of t h e beam divergence profile i s defined by t h e 'full width at half maximum' (FWHM)

,

i.e. t h e angular distance of t w o points of the symmetrical curve at 50% of t h e maximum amplitude.

For an emitter with a slit width ~=1,5.10-~m, the ion beam divergence for different values of the emission current I= i s shown in Fig.3. The diver- gence characteristics outlined in Fig.3a were obtained directly after the initial CS-feeding of t h e emitter; CS-wetting does not yet reach the ex- ternal areas adjacent t o the emitter slit edge-The divergence profiles are quite similar, following rough1 y a cosine distribution especial l y below 3=30°. These cosinusiodal profiles with a FWHM of about 70- for IE=SmR are typical for an even new and unconditioned emitter.

Only 5 days later, after rather intense ion emission experiments, the ge- neral beam divergence has changed considerably, a s it i s shown in Fig.36.

The most striking feature now i s the tendency towards a lateral maximum of t h e beam divergence profile at -300; this tendency i s more pronounced for higher emission currents, resulting in an even enhanced broadening of t h e

ilt& with a FWHM of about 90- far Iro5mR.

An obvious explanation of this feature follows from the corresponding wetting status. Due t o numerous emission events, the nearly homogeneos Cs- coverage, which i s evident for t h e beginning of t h e experimental period, now is superimposed by initiation of CS-creeping or droplet formation outside t h e emitter slit edge; this may initiate enhanced spurious lateral emission from emission sites occuring there preferential l y with increasiny emission current.

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Fig.3

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(a) Divergence characteristics direct1 y after t h e initial CS-f eeding

( b ) Divergence characteristics after ion emission during an experimental

period lasting for 5 days

5

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Imperfect wetting of t h e CS-feeding system and the emitter slit itself at the initial CS-supply i s irreversible; CS-handling at UHV-conditions and outgassing of the complete vacuum facility i s obligatory. If the emitter and t h e feeding capillary are properly degasaed, the liquid CS immediately runs through the capillary into the emitter until t o the slit edge. Initi- ally there occurs no continous exterior wetting along t h e whole length o+

t h e slit edge. hlthough the vacuum conditions before CS-supply generally are in favour of excellent wetting, obviously there exist some obstacles for t h e C s t o surmount the rather sharp edges of the emitter slit.

Complete wetting of t h e slit edge occurs gradually only by the emission process itself; thermal stress of t h e emitter slit edge i s assumed t o be the main reason for this wetting process.

Finally, condensation of CS at t h e emitter surface adjacent t o t h e emitter slit edge result in an irreversible CS-creeping, stimulating sometimes even CS-overflow.

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Once t h e emitter i s filled with liquid C s and t h e surfaces of the slit until t h e slit edge are uniformly wetted, a linear array of equidistant spaced emission sites exists at t h e slit edge which i s contributed t o wavelike instabilities on t h e electrically stressed CS-surface at t h e emitter sl.it edge.

The occurence of a n e a r l y constant value of X v e r i f i e d experimentally f o r a wide range o f emission c u r r e n t s obviously may not be explained i n terms o f a simple standing wave theory n e g l e c t i n g hydrodynamic e f f e c t s b u t has r a t h e r t o be t r e a t e d by a f l o w f i e l d theory assuming a l s o mutual i n t e r a c - t i o n s of adjacent emission s i t e s .

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For an e m i t t e r not yet f u l l y conditioned immediately a f t e r t h e i n i t i a l Cs- supply, t h e azimuthal i o n beam divergence roughly obeys a cosine d i s t r i b u - t i o n , w i t h a f u l l width a t half-maximum (FWHM) of about 70°.

With increasing c o n d i t i o n i n g and a homogeneous CS-coverage a t t h e e m i t t e r s l i t edge, t h e tendency of t h e beam divergence p r o f i l e s t o show maxima a t an azimuthal angle of approximately 30- and a d i p a t Oo more and more becomes evident; t h e FWHM increases towards 80°-1000. F i n a l l y , i f pronoun- ced CS-creeping and CS-overflow had occured, the maxima even are s h i f t e d towards higher values o f t h e azimuthal angle.

This research work was supported by t h e Austrian Science Foundation (FWF) under P r o j e c t No.5284.

REFERENCES

/ l / Mitterauer, J., J.Physique45, C9/185 (1984)

/2/ B a r t o l i , C., von Rohden H., Thompson, S.P. and Blommers, J., J.Phys.D:Appl .Phys.&z, 2473 (1984)

/3/ Melcher, J.R., J - F l u i d Mech.22, 1 (1965)

/4/ Aitken, K.L. and Prewett, P.D., F i n a l Report on ESTEC-Contract No. 4462/80, ( 1983)

/5/ Mitterauer, J., IEEE-Trans-Plasma Sci.P=F'z, 593 (1987) / 6 / Thomas, C.Ll., L i t t l e , P.F., Mahony, C. and Banks, C., F i n a l

Report on ESTEC-Contract No.3686/78, (1980)