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Submitted on 1 Jan 1979
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H- PRODUCTION BY INTERACTION OF A H+2 - ION BEAM WITH A SURFACE
P. Massmann, H. Hopman, J. Los
To cite this version:
P. Massmann, H. Hopman, J. Los. H- PRODUCTION BY INTERACTION OF A H+2 - ION BEAM WITH A SURFACE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-435-C7-436.
�10.1051/jphyscol:19797211�. �jpa-00219192�
JOURNAL DE PHYSIQUE CoZZoque C7, suppzdment au n07, Tome 40, JuiZzet 1979, page ~ 7 - 435
H- PRODUCnON BY INTERACTION OF A H;
-
ION BEAM WITH A SURFACEP. Massmann, H.J. Hopman and J. Los.
Association EURATOH-FOM, FOM-lnstitut voor Atoom- en MoZeccZfysiea, KruisZaan 407, Amsterdam-Wgm., The Netherlands.
A b s t r a c t
We p r e s e n t measurements concerning t h e n e g a t i v e io- n i z a t i o n of a *2-ion beam w i t h e n e r g i e s between 50 and 300 eV impinging on a ThOz-surface. We f i n d ne- g a t i v e i o n i z a t i o n e f f i c i e n c i e s between 2 and 7%.
R e t a r d a t i o n measurements show t h a t t h e n e g a t i v e i o n s a r e r e f l e c t e d from t h e s u r f a c e w i t h an energy of about 0.4 times t h e i n c i d e n t beam energy. The ap- p e a r i n g energy s p r e a d s u g g e s t s t h a t t h e n e u t r a l i z a - t i o n of t h e i n c i d e n t
HZ
-ions takes p l a c e v i a reso- n a n t e l e c t r o n c a p t u r e i n t o a r e p u l s i v e s t a t e occuring b e f o r e c o l l i s i o n w i t h t h e s u r f a c e . I n t r o d u c t i o nAn important method t o h e a t thermonuclear plasmas i s t h e i n j e c t i o n of h i g h energy atomic hydrogen beams. I n t h e c a s e of beam e n e r g i e s above 200 k e V i t
i s g e n e r a l l y b e l i e v e d t h a t atomic beams must be pro- duced s t a r t i n g from n e g a t i v e i o n s o u r c e s [ I ] . One of t h e two p r e s e n t l y favoured l i n e s of development i s based on d i r e c t e x t r a c t i o n o f n e g a t i v e IxyGroge~l i o n s from s u r f a c e plasma sources (SPS). There i s evidence t h a t t h e a l k a l i admixture i n t h e d i s c h a r g e of t h e SPS l e a d s t o a lowering of t h e workfunction of t h e s o u r c e s u r f a c e s and t h a t t h e n e g a t i v e i o n s a r e formed by i o n i z a t i o n on t h e cathode s u r f a c e .
The high conversion e f f i c i e n c y p r e d i c t e d even f o r low i n c i d e n t e n e r g i e s C21 and t h e measured e f f i c i e n - cy v a l u e s o f 10% 131 s u g g e s t however a l s o a n o t h e r a p p l i c a t i o n o f t h e n e g a t i v e s u r f a c e i o n i z a t i o n pro- c e s s . A low workfunction s u r f a c e may b e used a s io- n i z i n g element i n an a n a l y s e r f o r charge exchange n e u t r a l s emerging from a hydrogen plasma. Such a n e u t r a l p a r t i c l e a n a l y s e r should o p e r a t e i n t h e low energy range <300 eV where t h e c u r r e n t l y u s e d s t r i p - ping p r o c e s s becomes i n a p p l i c a b l e . A s i m i l a r i d e a has r e c e n t l y been proposed by othersC47.
F o r both a p p l i c a t i o n s fundamental knowledge o f t h e n e g a t i v e s u r f a c e i o n i z a t i o n p r o c e s s i s d e s i r a - b l e . I n one p r o c e s s d e s c r i b e d by t h e i o n i z a t i o n mo-
d e l according t o K i s h i n e v s k i i , reviewed i n C21, t h e i n c i d e n t p a r t i c l e s a r e n e g a t i v e l y i o n i z e d during t h e i r r e f l e c t i o n from t h e s u r f a c e . There i s eviden- ce C51 t h a t a l s o s p u t t e r i n g o f hydrogen p a r t i c l e s adsorbed on t h e s u r f a c e could p l a y a r o l e i n nega- t i v e i o n production. C r u c i a l f o r a n e u t r a l p a r t i c l e a n a l y s e r i s a good knowledge of t h e energy d i s t r i - b u t i o n o f t h e secondary p a r t i c l e s . Since t h e energy r e f l e c t i o n d e c r e a s e s w i t h i n c r e a s i n g (normal) i n c i - d e n t v e l o c i t y [61 due t o p e n e t r a t i o n e f f e c t s one would l i k e t o know a t which i n c i d e n c e a n g l e an op- t i m u m o f energy c o n s e r v a t i o n and n e g a t i v e i o n f o r - mation can be o b t a i n e d .
The p r e s e n t measurements have been done w i t h Tho2 a s low workfunction s u r f ace m a t e r i a l . It was chosen because of i t s e a s y a v a i l a b i l i t y and i t s d u r a b i l i t y a g a i n s t a i r exposure.
Experimental
A p o s i t i v e hydrogen i o n beam, c o n s i s t i n g of 70% H:, 20% ~ 3 + and 10% H+
,
w i t h energy between 50 and 320 eV and a t o t a l c u r r e n t of about 5 x 1 0 - ' O ~ i s c r e a t e dby a modified s p u t t e r i o n gun. The undesired beam components can b e e l i m i n a t e d by means of a Wien f i l - t e r . The i n t e r a c t i n g s u r f a c e i s formed by a Tho2
-
I r r i b b o n . The s u r f a c e temperature i s 1000 K, t h e working (hydrogen) p r e s s u r e 2x1K6 Torr. The ribbon i s mounted a t t h e c e n t e r of a t u r n t a b l e allowing f o r adjustment of t h e wanted i n c i d e n c e a n g l e 8. I f t h e s u r f a c e i s p o s i t i o n e d t o normal i n c i d e n c e t h e p r i - , mary beam c u r r e n t can b e measured by means of a Faraday cup which i s p l a c e d i n f r o n t of t h e ribbon on t h e same t u r n t a b l e . C o a x i a l l y around t h e t u r n t a - b l e d i f f e r e n t c y l i n d r i c a l s h i e l d s can be p l a c e d with e n t r a n c e s l i t s o f same dimensions i n theprrma-
r y beam l i n e and e x i t s l i t s of v a r i o u s widths d e f i - ning t h e d e t e c t i o n a n g l e width A 8 around t h e a n g l e of s p e c u l a r r e f l e c t i o n . The e n t r a n c e s l i t i s chosen i n such a way t h a t f o r i n c i d e n c e a n g l e s 20°< 0<65' t h e beam f r a c t i o n p a s s i n g t h e s l i t i s i n t e r c e p t e d by t h e ribbon and f o r 8=0° i s c o l l e c t e d by t h e Fara- day cup. A second cup w i t h two g r i d s f o r p o s i t i v e VR+ and n e g a t i v e VR- b i a s placed behind t h e e x i t s l i t allows d e t e c t i o n and energy a n a l y s i s of t h e s e - condary p a r t i c l e s .
With t h i s s e t up two s e r i e s of measurements have been done. The f i r s t a t 8=45O and A 8"90°, t h e se- cond a t 8=62.5' and A 8=b0. I n t h e f i r s t c a s e t h e undiscriminated beam h a s been used t o measure t h e t o t a l n e g a t i v e i o n i z a t i o n e f f i c i e n c y 17- a s a func- t i o n of i n c i d e n t beam v o l t a g e Vb ( F i g . 1 ) . Since 0-
Fig. 1
-
Negative i o n i z a t i o n e f f i c i e n c y vs i n c i d e n t beam v o l t a g e . Conditions: 8 = 4 5 ' ; ~ 8 =go0 ;VR-=-35V;VR+=320V; from t h e low t o t h e high energy p a r t t h e e r r o r s vary between 55 and 25%.
i s d e f i n e d a s t h e r a t i o of secondary over primary c u r r e n t t h e y i e l d p e r i n c i d e n t nucleon would be about a f a c t o r of 2 lower. For t h e s e measurements VR+ h a s been a p p l i e d d i r e c t l y t o t h e second Faraday cup and only one n e g a t i v e l y b i a s e d g r i d h a s been used t o s u p p r e s s secondary e l e c t r o n s . The workfun- t i o n @=3.5
+
0.5 V h a s been determined from measlre- ments of t h e thermal e m i s s i o n c u r r e n t v e r s u s s u r f a c e temperature.I n t h e second c a s e i n f o r m a t i o n about t h c energy d i s t r i b u t i o n of t h e n e g a t i v e Gens h a s been o b t a i n e d
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797211
f o r Vb = 200, 300 V by applying a r e t a r d i n g v o l t a g e OrVR->-300 V t o t h e second g r i d i n f r o n t of t h e secondary Faraday cup (Fig. 2 ) . The r e t a r d a t i o n measurements have been done w i t h a HZ+ -beam.
F i g . 2
-
R e f l e c t e d n e g a t i v e i o n c u r r e n t vs n e g a t i v e r e t a r d i n g v o l t a g e . Conditions : Vb = 200V;0 Vb = 300 V, 4- = 0.06%; 8=62.$;
A e =
6O;yi+'Vb;t h e arrows i n d i c a t e t h e t h e o r e t i c a l cut-off v a l u e . Discussion
From F i g . 1 we s e e t h a t i n s p i t e of t h e r e l a t i v e - l y high workfunction a n e g a t i v e i o n y i e l d of seve- - - r a l p e r c e n t s can b e reached w i t h Th0z. These v a l u e s a r e o n l y about a f a c t o r of 2 l e s s than t h o s e measured by Schneider e t a l . C31 with a t h i c k Cs-
l a y e r ($ = 1.9 V) who a l s o f i n d an i n c r e a s e of i o n i - z a t i o n e f f i c i e n c y w i t h i n c i d e n t energy i n t h e con- s i d e r e d energy range. Checks w i t h f i x e d e x i t s l i t and v a r i e d i n c i d e n c e angle i n d i c a t e however t h a t t h e a n g u l a r s p r e a d of t h e secondary p a r t i c l e s must b e l a r g e . This i s t h e main r e a s o n why t h e , e f f i c i e n -
cy i n t h e case of F i g . 2 appears much lower t h a n i n F i g . 1 .
Assuming c l a s s i c a l c o l l i s i o n s between Hz and t h e oxygen atom o f t h e ThOz an i d e a l r e t a r d a t i o n curve would have a s h a r p cut-off a t VR- = 0.45 Vb. The a p p r e c i a b l e energy s p r e a d around t h i s cut-off v a l u e i s n o t caused by a p p a r a t u s e f f e c t s . This has been confirmed by measurements w i t h d i r e c t l y r e f l e c t e d
HZ
y i e l d i n g a s p r e a d of l e s s than 10%. I t i s propo- s e d t h a t r e s o n a n t n e u t r a l i z a t i o n o f t h eHZ
i n t o a r e p u l s i v e s t a t e l e a d s t o t h e measured energy spread.I n t h i s p r o c e s s t h e d i s s o c i a t i o n energy of t h e mo- l e c u l e i s s p r e a d o v e r t h e two H-atoms b e f o r e c o l l i - s i o n w i t h t h e s u r f a c e . R e f l e c t i o n of t h e atoms from an i d e a l l y f l a t s u r f a c e p r e d i c t s a width f o r t h e energy d i s t r i b u t i o n which i s i n reasonable agree- ment w i t h t h e measurements. The f l a t p o r t i o n of t h e r e t a r d a t i o n curves s u g g e s t s t h a t no e x c e s s i v e pene- t r a t i o n of t h e i n c i d e n t p a r t i c l e s i n t o t h e s u r f a c e occurs and t h a t the d e t e c t e d p a r t i c l e s a r e n e g a t i v e i o n s r e f l e c t e d from t h e s u r f a c e .
References
[ I ] K.H. Berkner, R.V. P y l e , J.W. S t e a r n s , Nucl.
Fusion (1975) 249
C21 Yu.1. Bel'chenko, G . I . Dimov, V.G. Dudnikov, I C A T r a n s l a t i o n s o f P r e p r i n t IYaF-77-57, P h i l a d e l p h i a , U.S.A. (1977)
C31 P.J. Schneider, K.H. Berkner, W.G. Graham, R.V. Pyle, J . W . S t e a m s , Symp. Prod. Neutsali- z a t i o n o f Neg. Hydr. I o n s and Beams, S e p t . (1977) p . 6 3
141 J.R. Hiskes, A.M. Karo, LLL-abstract, UCRL
80687, submitted t o t h e S a n t a Fe Plasma Diag- n o s t i c s meeting o f t h e APS, March (1978) C51 K. Wiesemann, K. P r e l e c , Th. S l u y t e r s , J. Appl.
Phys. 48 (1977) 2668
C61 O.S.
oZ,
M.T. Robinson, Nucl. I n s t r . & Meth.132 (1976) 647
