WORKSHOP REPORT ON POLARIZED PROTON ION SOURCES

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WORKSHOP REPORT ON POLARIZED PROTON ION SOURCES

P. Schmor

To cite this version:

P. Schmor. WORKSHOP REPORT ON POLARIZED PROTON ION SOURCES. Journal de Physique Colloques, 1985, 46 (C2), pp.C2-683-C2-690. �10.1051/jphyscol:1985285�. �jpa-00224604�

WORKSHOP REPORT ON POLARIZED PROTON ION SOURCES

P.W. Schmor

TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. VST 2AS, Canada

Résumé - Nous présentons ici le résumé d'un atelier qui a examiné les performances actuelles et les possibilités futures des sources de protons polarisés en mai 1983. Les meilleures sources basées sur l'effet Lamb ont atteint 4 uA mais les perspectives d'accroissement sont limitées par la luminosité de la source. Les sources atomiques ont atteint environ un niveau de 8 pA en H- et on a proposé des techniques pour accroître leur niveau jusqu'à 1 mA. On décrit des techniques pour réaliser des

faisceaux puisés intenses à partir de sources atomiques ultra-froides.

On présente les résultats obtenus avec une source d'ions H- à pompage optique ainsi que d'autres méthodes pour produit de l'hydrogène atomique polarisé par pompage optique.

Abstract - The results from a workshop, during May 1983, which examined the capabilities and future possibilities of polarized proton sources are summarized. The best Lamb-shift sources have achieved 4 uA but the poten- tial for further increase is limited by the proton source brightness.

Atomic sources have reached the ~ 8 uA H~ range and techniques for

increasing this to ~ 1 mA are proposed. Techniques for achieving intense pulsed beams with ultra cold atomic sources are described. Results from an optically pumped H- ion source at KEK are given along with alternative schemes for producing polarized atomic hydrogen by optical pumping.

INTRODUCTION

A workshop on high intensity polarized proton Ion sources, sponsored by the International Committee for Symposia on High Energy Spin Physics was held at TRIUMF from May 23rd to 28th, 1983. There were forty-two registered participants special- izing in the areas of accelerator, ion source, atomic, fusion and nuclear physics reflecting a growing interest in polarized proton sources. The proceedings of the workshop have been published as an AIP conference proceedings / l / . The program was split equally into formal lecture and informal working sessions. Topics covered included Lamb-shift (metastable) sources, atomic (ground state) sources, ultra cold atomic beams and targets, optically pumped sources and ionizers.

The workshop was primarily concerned with improving the performance of polar- ized proton ion sources. In principle, however, any nucleus with a non-zero spin can be polarized by similar techniques. In existing polarized proton sources, the procedure can be summarized in four basic steps. Initially a high quality atomic beam is formed. The electron spin of the atoms is then aligned through the use of magnetic fields, RF or photon induced transitions, spin exchange or charge exchange.

Next the nuclear spin is aligned through the hyperfine interaction using appropriate magnetic fields and/or RF induced transitions. Finally the nuclear-polarized atoms are ionized through either charge exchange or electron stripping. Although the technical details are quite different for each type of source and each has a unique set of problems, there Is also a considerable area of common interest in such topics as the choice of the ideal ionizer or the techniques for the formation of an atomic beam of high brightness.

In the Lamb-shift source, metastable (2S) atoms of a particular spin orienta- tion are selectively quenched to the ground state. The remaining metastable atoms, now aligned, are selectively ionized. In the atomic (ground state) source the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985285

C2-684 JOURNAL DE PHYSIQUE

unwanted s p i n a l i g n m e n t is removed from a slow a t o m i c beam through t h e u s e of non- u n i f o r m magnetic f i e l d s (Stern-Gerlach s e p a r a t i o n ) . The r e m a i n i n g , p o l a r i z e d , beam

i s t h e n i o n i z e d e i t h e r d i r e c t l y t o form H- o r H+ o r i n a two s t e p p r o c e s s f i r s t producing H+ and t h e n H- by double c h a r g e exchange. For t h e o p t i c a l l y pumped s o u r c e one can e i t h e r pump t h e atom d i r e c t l y (lie3 and a l k a l i s ) w i t h a s u i t a b l e l a s e r o r pump an a l k a l i and t r a n s f e r t h e s p i n t o an a t o m i c o r i o n beam by e i t h e r s p i n o r c h a r g e exchange. I n t h e u l t r a c o l d atomic s o u r c e t h e atoms r e l a x a t low tempera- t u r e s i n a h i g h m a g n e t i c f i e l d , f i r s t t o an e l e c t r o n s p i n a l i g n e d t a r g e t and eventu- a l l y t o a n u c l e a r s p i n a l i g n e d t a r g e t . An i o n i z e r a n d / o r e x t r a c t o r i s r e q u i r e d t o form a p o l a r i z e d beam. The workshop a d d r e s s e d t h e s p e c i f i c problems of each s o u r c e i n s e p a r a t e working s e s s i o n s . Common c o n c e r n s such a s i o n i z e r s were d i s c u s s e d i n j o i n t working s e s s i o n s .

LAMB-SHIFT SOURCES

The p h y s i c s and t e c h n o l o g i c a l d e t a i l s of Lamb-shift type p o l a r i z e d i o n s o u r c e s have been t h o r o u g h l y reviewed / 2 / . N e v e r t h e l e s s , t o remind t h e non-source s p e c i a l - i s t s of t h e e s s e n t i a l d e t a i l s , a block diagram showing t h e i m p o r t a n t p r o c e s s e s i s g i v e n i n Fig. 1. I n b r i e f o u t l i n e , p r o t o n s from a n i o n s o u r c e a r e t r a n s p o r t e d a t -550 eV t h r o u g h a cesium vapour t o produce a m e t a s t a b l e (Bqs) a t o m i c beam.

Nuclear p o l a r i z a t i o n i s a c h i e v e d through t h e u s e of m a g n e t i c a n d / o r RF e l e c t r i c f i e l d s which c a u s e mixing between m e t a s t a b l e 2 S I l 2 s t a t e s of one s p i n alignment and t h e s h o r t l i v e d 2 P I l 2 s t a t e s . F i n a l l y a s e l e c t i v e i o n i z e r i s used t o p r e f e r e n t i a l l y i o n i z e o n l y t h e remaining m e t a s t a b l e s , now p o l a r i z e d .

EXCHANGE

IONIZER

Fig. I

The H- c u r r e n t from t h e b e s t s o u r c e s h a s i n c r e a s e d from t h a t r e p o r t e d two y e a r s e a r l i e r by 100% t o -2 VA p e r h y p e r f i n e s t a t e , a l b e i t w i t h s l i g h t l y l e s s p o l a r i z a - t i o n . The working g r o u p concluded t h a t t h e Lamb-shift s o u r c e t e c h n o l o g y i s mature and f u r t h e r l a r g e i n c r e a s e s i n c u r r e n t w i l l be d i f f i c u l t t o a c h i e v e . A major d i f f i - c u l t y w i t h t h e Lamb-shift s o u r c e h a s been i n t h e m e t a s t a b l e beam f o r m a t i o n r e g i o n where s p a c e c h a r g e f o r c e s due t o t h e r e l a t i v e l y slow 550 eV p r o t o n s have l i m i t e d t h e u s e f u l p r o t o n c u r r e n t n e u t r a l i z e d i n t h e cesium vapour c a n a l . There have been two a p p r o a c h e s t o t r y t o r e d u c e t h e s e v e r i t y of t h e s p a c e c h a r g e problem. I n t h e f i r s t , t h e cesium h a s been p l a c e d a s c l o s e a s p o s s i b l e t o t h e p r o t o n s o u r c e , b e f o r e t h e 550 eV p r o t o n beam h a s s i g n i f i c a n t l y d i v e r g e d . I n t h e s e c o n d , t h e p r o t o n beam i s expanded and t r a n s p o r t e d up t o 30 cm p r i o r t o being r e f o c u s s e d through t h e cesium c a n a l . Both a p p r o a c h e s a r e producing v e r y s i m i l a r 8- c u r r e n t s . The primary l i m i t a - t i o n i s now t h e b r i g h t n e s s of t h e p r o t o n s o u r c e . D i s c u s s i o n c e n t r e d on p o s s i b l e t e c h n i q u e s t o circumvent t h i s l i m i t a t i o n t h r o u g h t h e u s e of m u l t i - h o l e e x t r a c t i o n e l e c t r o d e s and u n c o n v e n t i o n a l p r o t o n s o u r c e s , such a s t h e e l e c t r o n - c y c l o t r o n -

r e s o n a n c e type s o u r c e . It was f e l t t h a t with c a r e f u l l y o p t i m i z e d d e s i g n s , a f a c t o r of two i n c r e a s e i n c u r r e n t might be p o s s i b l e . A f u r t h e r f a c t o r of two i n c r e a s e i n c u r r e n t would be e x p e c t e d i f t h e cesium vapour c o u l d be e l e c t r o n i c a l l y p o l a r i z e d by l a s e r pumping, removing t h e need f o r e i t h e r t h e r f s p i n f i l t e r o r t h e d i a b a t i c f i e l d r e v e r s a l . It was p o i n t e d o u t t h a t l a r g e c u r r e n t s ( 5 mA!!) of I?+ c o u l d be pro- duced i f t h e m e t a s t a b l e beam could be e f f i c i e n t l y photo-ionized. I n c o n c l u s i o n , even though s u b s t a n t i a l i n c r e a s e s i n e i t h e r H- o r D- c u r r e n t a r e not expected i n t h e n e a r f u t u r e , t h e Lamb-shift s o u r c e w i l l c o n t i n u e t o be used on a c c e l e r a t o r s f o r some time y e t b e c a u s e o f ; 1 ) t h e good e m i t t a n c e , 2 ) t h e e a s e of choosing e i t h e r pure v e c t o r o r pure t e n s o r deuterium s t a t e s and 3 ) t h e s u i t a b i l i t y ( l o n g d r i f t l e n g t h )

ATOMIC BEAM SOURCES

Atomic beam sources have also been thoroughly reviewed in the literature /3,4/.

A block diagram outlining the important processes in a source is given in Fig. 2. A

Fig. 2

rl

MAGNET

dissociater, nozzle and skimmer are used to form a slow collimated beam of atom hydrogen (or deuterium). Spin selection is accomplished with Stern-Gerlach style separation magnets and RF induced transitions. Electron ionizers are used to pro- duce $+, followed by alkali vapour targets if 8- is desired. Alternatively

8- is also produced directly with the reaction 80 + ^{C g + C+} + 8-. In the two years since the previous workshop the ft- currents have increased from 3 PA to 6 ~ J A dc and to 7.5 VA pulsed. The dc &+ currents are already at the 125 PA level and do not have the same demands for improvement.

The atomic beam source has the potential for substantially higher currents if the source is optimized. In the atomic beam formation region, for instance, it is important to efficiently cool the beam to less than 20 K as the beam density is proportional to T-I/~. It was pointed out that this gain will not be seen unless the surface of the accommodator is coated with a material having a low recombination coefficient in the low temperature regime. Finally, in order to realize the larger currents, it is also necessary to redesign the separation magnet3 to match the slower velocity beam. The acceptance of the system would be increased since these magnets can be shortened and their vertical aperature increased. Thus the beam density after the separation magnets is also sensitive to the atomic beam velocity and is expected to be proportional to ~ - 3 ~ ~ . It was estimated for an ideally designed beam formation region, the current could be increased a factor of 20 to 30 over existing atomic sources.

Improvements to the ionizers were also considered. For ions, a modern electron impact ionizer is capable of ionizing 3 to 5% of the 80 beam. It was felt that an electron-cyclotron-resonance (ECR) ionizer would be much more efficient but might also lead to a loss in olarization. For $- ions, it was expected

that sources using the 60 + ^C: + I!- + C$ reaction would shortly be achieving -20 !JA due to improvements made in the cesium beam. A novel suggestion by J. Alessi was

ALKALI VAPOR G-

ADDER CANAL

3'

R f

TEONs

) e+H+H++2ex ^IONIZER

-

C2-686 JOURNAL DE PHYSIQUE

the use of a ring magnetron, sh wn in Fig. 3, to take advantage of the much larger cross sections in the reaction % ^{O+ D- + 8- + D o /5/.} The ring magnetron design should avoid the space charge problems with the D- beam that led to failure in earlier attempts to use this reaction. At least an order of magnitude increase in

8- current would be expected.

The workshop concluded that the atomic source has a large potential for improvement. The projected , if realized, could in the future result in ft+

currents exceeding 2 m* and

Fins

CATHODE

~ 0 0 0 0 0

SOLENOID

DEFLECTION

00000000

-(

a

H- Fig. 3

ULTRA COLD ATOMIC SOURCES

A thorough discussion on the physics and possibilities of ultra cold atomic sources is the subject of another contribution to this conference / 6 / . The source has many design variations depending on the application. Figure 4 shows a

particular scheme that received a great deal of interest at the Vancouver workshop.

Hydrogen atoms are cooled in stages down to - ^{0.3 K} and then stored in a confinement cell within an axial magnetic field. Only atoms in the two hyperfine states having the proper electron spin alignment are trapped. These atoms can be extracted by using p-waves to induce transitions to either one of the two other hyperfine states.

Ionization techniques would be similar to those used in convential atomic sources.

The v-wave extraction technique is particularly convenient for applications requir ing intense pulsed beams, such as syncrotrons or linacs since it would be possible to accumulate atoms between pulses. Furthermore the extracted atoms would be both focussed and accelerated by the magnetic field of the solenoid. Laboratory experi- ments have demonstrated storage densities

-

-

atoms/sec. It was estimated that for low-duty cycle accelerators, such as the AGS, it should be possible to reach peak intensities

-

Three y e a r s ago a t t h e Ann Arbor workshop Anderson reviewed h i s p r o p o s a l f o r an o p t i c a l l y pumped p o l a r i z e d proton s o u r c e 1 7 1 . A t t h e Vancouver workshop Mori of KEK d e s c r i b e d t h e c h a r a c t e r i s t i c s of a working s o u r c e based on t h e Anderson p r o p o s a l / 8 / . The e s s e n t i a l d e t a i l s of t h i s scheme a r e i n d i c a t e d i n t h e b l o c k diagram of F i g . 5. P r o t o n s , a t an energy of

-

s o u r c e h a s allowed Mori t o circumvent t h e a n t i c i p a t e d problem of e m i t t a n c e blow-up i n t h e f i r s t c h a r g e exchange c a n a l . The beam p o l a r i z a t i o n was measured a t 355 keV

H INLET

I / T U B E I

Fig. 4

H + No POLARIZATION PRODUCTION

PRODUCTION PLUS OF NUCLEAR PRODUCTION

ti0 PRODUCTION POLARIZED

-

(ELECTRON P O L A R I Z E D ) Ho

-

DYE LASERS

Fig. 5

- ^{E C R}

ION SOURCE

OPTICALLY PUMPED SODIUM ATOMS

IHO

SONA

TRANSIT ION - zo IONIZER H-

c2-688 JOURNAL DE PHYSIQUE

a s a f u n c t i o n of beam c u r r e n t . The i n i t i a l r e s u l t s a r e shown i n F i g . 6. It i s b e l i e v e d t h a t t h e p o l a r i z a t i o n c a n be i n c r e a s e d w i t h g r e a t e r l a s e r power. High background g a s p r e s s u r e i s a l s o s u s p e c t e d t o be r e s p o n s i b l e f o r a s u b s t a n t i a l u n p o l a r i z e d background. The workshop f e l t t h a t t h e s o u r c e h a s a l a r g e p o t e n t i a l f o r improvement o v e r i t s a l r e a d y i m p r e s s i v e performance. It was s u g g e s t e d t h a t p o t a s s i - um i n s t e a d of sodium would r e s u l t i n h i g h e r l a s e r p o l a r i z a t i o n e f f i c i e n c y . Also n o t e d was t h e f a c t t h a t s i n c e most of t h e a n g u l a r momentum was b e i n g l o s t t o t h e w a l l s , a w a l l c o a t i n g t o r e d u c e t h i s l o s s r a t e would a l s o enhance t h e e f f i c i e n c y . Development work on s u c h s o u r c e s i s b e i n g a c t i v e l y p u r s u e d a t KEK, TRIUMF, and t h e U n i v e r s i t y of W i s c o n s i n .

Happer of P r i n c e t o n U n i v e r s i t y proposed a s y s t e m t o make p o l a r i z e d fl u s i n g s p i n exchange c o l l i s i o n s . I n t h i s scheme r u b i d i u m and hydrogen atoms a r e combined i n a c e l l . The r u b i d i u m i s e l e c t r o n - s p i n p o l a r i z e d by o p t i c a l pumping, r a t h e r t h a n t h e hydrogen. ( I n t e n s e l a s e r s a r e a v a i l a b l e a t w a v e l e n g t h s s u i t a b l e f o r t h e a l k a l i s

I I I I I I

5 10 15 20 25 30

H

' C U R R E N T (PA) 100

-

F i g . 6

I I I

L A M P

P O L A R I Z E R G L A S S C E L L CONTAINING H + Rb

"ti

Fig. 7 Z

0

-

LASER ON -

-

- -

5 ⁵⁰

2 E - a J

B

0

1 :

LASER OFF -

-

Rb(C) + H(4) h a s a r e l a t i v e l y l a r g e c r o s s s e c t i o n , a=2x10-l4 cm2. A s c h e m a t i c of a p o s s i b l e e x p e r i m e n t a l s e t u p i s shown i n F i g . 7. F r e q u e n t s p i n e x c h a n g e c o l l i s i o n s l e a d t o n u c l e a r p o l a r i z a t i o n . The power r e q u i r e m e n t s a r e a b o u t 4 w a t t s of o p t i c a l power p e r ampere of p r o t o n s . The g r o u p a t P r i n c e t o n h a v e c a r r i e d o u t e x p e r i m e n t s p r o d u c i n g h i g h l y p o l a r i z e d 1 2 9 ~ e by s p i n e x c h a n g e r e a c t i o n s w i t h o p t i c a l l y pumped Rb 1111. The c r o s s s e c t i o n f o r t h i s r e a c t i o n i s o n l y = 1 x 1 0 - ~ ~ cm2. C o n s e q u e n t l y o n e would e x p e c t a much h i g h e r t r a n s f e r e f f i c i e n c y when hydrogen is u s e d , i f t h e l o w e r w a l l r e l a x a t i o n t i m e f o r HC c a n b e i n c r e a s e d .

SUMMARY

A f t e r many y e a r s of e f f o r t t o r e a c h t h e 1 0 r a n g e , p o l a r i z e d i o n s o u r c e b u i l d e r s a r e c o n f i d e n t t h a t a t l e a s t two of t h e e x i s t i n g t e c h n o l o g i e s s h o u l d b e c a p a b l e of d e m o n s t r a t i n g d c I?c u r r e n t s of

-

-

F i n l l y , f o r t h e more d i s t a n t f u t u r e , schemes h a v e b e e n p r o p o s e d which would l e a d t o 8- s o u r c e s e q u a l l i n g t o d a y ' s u n p o l a r i z e d H- s o u r c e s . Anderson e t a l . i n two r e c e n t p u b l i c a t i o n s 1 1 4 , 1 5 1 c a l c u l a t e t h a t s u c c e s s i v e s p i n - d e p e n d e n t c h a r g e - t r a n s f e r c o l l i s i o n s c a n b e u t i l i z e d t o p r o d u c e n u c l e a r - p o l a r i z e d H- c u r r e n t s by a p r o c e s s t h e y r e f e r t o a s " c o l l i s i o n a l pumping". The p r o c e s s r e q u i r e s a s p i n - p o l a r i z e d t a r - g e t t h i c k n e s s - 1 0 1 7 atoms/cm2, a l m o s t two o r d e r s of m a g n i t u d e t h i c k e r t h a n t h e e x i s t i n g o p t i c a l l y - p u m p e d s p i n - a l i g n e d t a r g e t s . A g r o u p a t B e r k e l e y i s p l a n n i n g t o e x p e r i m e n t a l l y examine t h e problems a s s o c i a t e d w i t h o p t i c a l pumping a t t h e r e q u i r e d t a r g e t t h i c k n e s s .

The f u t u r e f o r p o l a r i z e d p r o t o n i o n s o u r c e s i s v e r y p r o m i s i n g . T h e r e a r e s e v e r a l new g r o u p s a c t i v e l y b u i l d i n g s o u r c e s and new i d e a s a r e b e i n g pursued.

REFERENCES

P o l a r i z e d P r o t o n I o n S o u r c e s , TRIUMF, G. Roy and P. Schmor, e d s . , AIP Conf.

P r o c . No. 117 (AIP, New York, 1984).

T.3. C l e g g , P o l a r i z e d P r o t o n I o n S o u r c e s , Ann A r b o r , A.D. K r i s c h and A.T.M. L i n , e d s . , (AIP, New York, 1 9 8 2 ) 80, p.21.

W. G r i i e b l e r , P o l a r i z e d P r o t o n I o n S o u r c e s , Ann A r b o r , A.D. K r i s c h and A.T.M.

L i n , e d s . , (AIP, New York, 1982) 80, p.53.

W. H a e b e r l i , P o l a r i z e d P r o t o n I o n S o u r c e s , Ann A r b o r , A.D. Y r i s c h andd A.T.M.

L i n e , e d s . , (AIP, New York, 1 9 8 2 ) 80, p.85.

J.G. A l e s s i , Th. S l u y t e r s and A. H e r s h c o v i t c h , P o l a r i z e d P r o t o n Ion S o u r c e s , TRIUMF, G. Roy and P. Schmor, e d s . , (AIP, New York, 1 9 8 4 ) , 117, p.32.

D. K l e p p n e r , C o n t r i b u t i o n t o t h i s c o n f e r e n c e .

L.W. Anderson, P o l a r i z e d P r o t o n I o n S o u r c e s , Ann A r b o r , A.D. K r i s c h and A.T.M.

L i n , e d s . , (AIP, New York, 1982) 80, p.155.

Y. Mori, K. I k e g a m i , 2. I g a r a s h i , A. T a k a g i , and S. Fukumoto, P o l a r i z e d P r o t o n I o n S o u r c e s , TRIUMF, G. Roy and P. S c h m o r , , e d s . , (AIP, New York, 1 9 8 4 ) , 117,

p.123.

P.G. Sona, E n e r g i a N u c l e a i r e , 14, 295 ( 1 9 6 7 ) .

R. G e l l e r , B. J a c q u o t and C. J a c q u o t , P o l a r i z e d P r o t o n I o n S o u r c e s , TRIUMF, G.

Roy and P. Schmor, e d s . , (AIP, New York, 1 9 8 4 ) , 117, p.162.

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11. N.D. Bhaskar, W. Happer, M. Larsson, and X. Zeng, Phys. Rev. Letters 50, 105 (1983).

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