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HEAVY ION ACCELERATORS OF THE FUTURE
Ch. Schmelzer
To cite this version:
Ch. Schmelzer. HEAVY ION ACCELERATORS OF THE FUTURE. Journal de Physique Colloques,
1972, 33 (C5), pp.C5-195-C5-206. �10.1051/jphyscol:1972515�. �jpa-00215117�
JOURNAL DE PHYSIQUE Colloque C 5 , suppldment au no 8-9, Tome 33, AoQt-Septembre 1972, page C5-195
HEAVY ION ACCELERATORS OF THE FUTURE Ch. Schmelzer
GSI , Darmstadt
,
GermanyRBsud
-
Apr&s avoir discut& les proprietbs des faisceaux d'ions lourds, nous decrivons les moyens dont nous disposons avec les acc814rateurs conventionnels. Nous discutons en outre 1'8tat actuel et les d8veloppements futurs, les nouvelles sources d'ions, les acc8- lerateurs supra-conducteurs et l'acc818ration collective.Abstract
-
After a discussion of beam properties for heavy ion nuclear research, present possibilities of using conventional accelerating systems are described. The present status of future developments, new ion sources, superconducting accelerator structures and collective acceleration,is reviewed.i, Introduction
The desire of nuclear physicists and chemists to use heavier and heavier projectiles has increased rapidly during the past few years. The papers read during this conference give not only an impressive demonstration of this tendency, but they also point out that experimental nuclear research with heavy projectiles opens new ways toward a deeper' under-
standing of nuclear matter.
A new generation of particle accelerators specifi- cally designed for the production of sufficiently energetic beams of heavy ions will, however, not only 1er.d important services to nuclear research.
The study of interactions of fast, heavy ions with matter opens new ways to solve quite different problems in numerous fields of natural philosophy.
Besides that, several interesting technological applications can be envisaged.
Atomic physics is one of these fields. Beam foil spectroscopy allows the investigation of the elec- tronic spectra of highly ionized atoms, as well as the determination of the life-tines of the involved excited states under especially clear experimental
conditions. This material may prove, for instance, valuable for theoretical astrophysics and stellar spectroscopy with extraterrestrial instruments.
Spectra of atoms of very high atomic number can be simulated within the short time interval
-1 9
(about 10 s), during which two colliding heavy ions form one "molecular ion".
The high specific energy loss of heavy particles slowed down in condensed matter causes considerable radiation damage. Very heavy ions produce excita- tion energies up to a few lo2 eV/atom. Aside from possible applications in fundamental soiid state research, the production of measurable changes of the macroscopic properties of material Irradiated with heavy ions becomes possible in times that are orders of magnitude shorter than by irradiation with light particles. Thus the testing of, e.g., reac- tor materials can be speeded up considerably.
finally, the high ionization density at the end of the track of a heavy ion stopped in condensed matter allows typical chemical reactions to occur, which are of particular interest in radiation biology and may eventually lead to applications in medical therapy.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972515
C5-196 Ch. SQnvlELZER
T h i s n e c e s s a r i l y i n c o m p l e t e l i s t o f a p p l i c a t i o n s o f e n e r g e t i c heavy i o n beams may s e r v e t o d e m o n s t r a t e t h e b r e a d t h o f what may b e c a l l e d heavy i o n r e - s e a r c h and t o show t h a t t h e j u s t i f i c a t i o n f o r con- s t r u c t i n g heavy i o n a c c e l e r a t o r s r e s t s n o t o n l y on t h e demands o f n u c l e a r p h y s i c i s t s and c h e m i s t s . 2. Beam p a r a m e t e r s
The f o l l o w i n g l i s t o f beam p a r a m e t e r s and p r o p e r - t i e s o f a heavy i o n a c c e l e r a t o r i s based on t h e n e e d s f o r n u c l e a r r e s e a r c h b u t c o v e r s t h e ( g e n e r a l - l y l e s s s e v e r e ) r e q u i r e m e n t s f o r heavy i o n e x p e r i - ments i n many o t h e r f i e l d s a s w e l l .
The mass r a n g e s h o u l d c o v e r a l l e l e m e n t s w i t h t h e p o s s i b l e exemption o f t h e v e r y l i g h t s p e c i e s (A 5 1 0 t o 201, which a l r e a d y c a n b e a c c e l e r a t e d t o u s e f u l e n e r g i e s by many e x i s t i n g machines.
r e a c t i o n s w i t h t h e h e a v i e s t t a r g e t e l e m e n t s , i . e . (W/A)max ; 8 MeV
f o r t h e h e a v i e s t p r a j e c t i l e s . For l i g h t e r p a r t i - c l e s (A i 601, v a l u e s a b o u t t w i c e a s l a r g e a r e d e s i r a b l e , e . g . , f o r work i n n u c l e a r s p e c t r o s c o p y . The e n e r g y must b e c o n t i n u o u s l y a n d
-
c o n s i d e r i n g t h e narrow e n e r g y windows o f many i n t e r e s t i n g heavy i o n r e a c t i o n s-
r e p r o d u c i b l y v a r i a b l e . A l o w e r l i m i t o f(W/A)min ? 2 MeV
i s d e s i r a b l e t o s t a y s u f f i c i e n t l y f a r below t h e Coulomb b a r r i e r , e.g. f o r t h e s t u d y o f Coulomb e x c i -
t a t i o n .
The p r i m a r y e n e r g y r e s o l u t i o n s h o u l d c o r r e s p o n d t o t h e s t a t e o f t h e a r t ,
The maximum s p e c i f i c e n e r g y , W/A, s h o u l d r e a c h X/AW ; 200
s u f f i c i e n t l y f a r a b o v e t h e Coulomb b a r r i e r f o r b e i n g a t y p i c a l v a l u e . T h i s s t a t e m e n t d e s e r v e s two comments. F i r s t l y , i t seems, a s can b e s e e n from i n s p e c t i n g F i g . 1, t h a t t h e l a r g e s p e c i f i c e n e r g y l o s s o f heavy i o n s makes h i g h e n e r g y r e s o l u t i o n un- n e c e s s a r y . N e v e r t h e l e s s , good e n e r g y r e s o l u t i o n must b e p r o v i d e d f o r e x p e r i m e n t s w i t h l i g h t e r i o n s a n d / o r e x t r e m e l y t h i n , e . g . g a s e o u s , t a r g e t s . The second comment c o n c e r n s t h e t e r m "primary" e n e r g y r e s o l u - t i o n . I t r e f e r s i n p a r t i c u l a r t o r a d i o f r e q u e n c y ac- c e l e r a t o r s , where n o t t h e e n e r g y s p r e a d , b u t t h e d i s t r i b u t i o n o f p a r t i c l e s i n L o n g i t u d i n a l p h a s e s p a c e forms a c o r r e c t measure o f beam q u a l i t y . A s i n d i c a t e d i n F i g . 2 , t h e e n v e l o p e o f t h e p a r t i c l e d i s t r i b u t i o n s h o u l d b e s l e n d e r , b e c a u s e t h e n t h e beam can b e m a n i p u l a t e d by de- o r r e b u n c h i n g t o ap- p e a r a t t h e t a r g e t w i t h o u t l o s s o f beam i n t e n s i t y e i t h e r i n s h a p e b (good e n e r g y r e s o l u t i o n and l o n g m i c r o p u l s e ) f o r c o i n c i d e n c e e x p e r i m e n t s , o r i n
F i g . 1: Energy d e c r e a s e of 10 MeV/nucleon i o n s of Zn, Sn and U w i t h i n a N i t a r g e t . The shaded r e g i o n s i n d i c a t e t h e p e n e t r a t - i o n d e p t h , where t h e e n e r g y o f t h e i o n s i s c l o s e
t o t h e Coulomb b a r r i e r (1). F i g . 2
HEAVY ION MACHINES
. . .
C5-197shape a (larger energy spread and short pulse dura- tion) tor time-of-flight experiments. The effici- ent use of the latter case demands the highest tech- nically possible "on/off" ratio of beam intensity.
Finally, particle currents of
should be provided for the heaviest projectiles, and higher values are desirable for lighter par- ticles.
The physical limitation of useful beam intensity is given by the power dissipation in the targets.
Fig. 3 shows a rather extreme case. A 6 MeVlnu- cleon uranium beam irradiates 1 cm2 of a 1 rng cm -2
Fig. 3: Target temperatures for pulsed (a) and continuous (b) irradiation of fixed and rotating (c,d) targets. See text for details (2).
uranium target. The particle current is
1.2 x 1012 s-I, corresponding to a target dissipa- tion of 10 Watts. With a continuous beam (curve b) the target equilibrium temperature is already very close zo the melting point of uranium, whereas the instantaneous peak temperatures caused by a pulsed beam (curve a, 5 ms on, 15 ms off) are well above this value. The use of targets rotating in syn- chronism with the pulse frequency becomes essential (curves c ,dl.
That, nevertheless, higher beam intensities are desirable in some cases, e.g. for producing strongly collimated beams by throwing away a large
fraction of the particles, nay be illustrated by the following example. A 10 MeV/nucleon lead beam is scattered in a lead target. Seen under 45O, the angular distance between scattered projectiles leaving the target nucleus in the ground state and the first excited state (2.6 MeV) amounts only to roughly 1 mrad. A separation of the two scattering peaks thus demands a primary beam parallel within at least 0.3 mrad. (It should be noted that charge change processes occuring at the collimation dia- phragms greatly help in producing clean, highly collimated beams, provided that the last defining aperture is followed by a suitable clearing field.) 3. The ideal heavy ion accelerator
'She ideal heavy ion accelerator is shown schemati- cally in the upper part of Fig. 4. The box at the
OC Stripper
Fig. 4: Schematic diagrams of an "ideal"
(top) and a combined heavy ion accelerator (below).
right represents a conventional, variable energy accelerator. The essential part is the ion source S. It must produce sufficiently intense beams of ions of a charge-to-mass-number ratio which, over the entire range, must be
Thus the desired specific energies can be obtained with a relatively moderate total accelerating voltage. With such an ideal ion source, many existing machines could be used for heavy ion work,
" doch die Verhaltnisse,
sie sind nicht so! " (3)
C5-198 Ch. SCHMELZER
An inspection of Fig. 5 shows what can actually be The velocity dependence of the mean charge after expected from conventional arc ion sources, stripping is today sufficiently well known for the
present purpose4). The results are presented in form of the two nonograms, Fig. 6 and Fig. 7, which
Duoplosrnatron PIG
Fig. 5: Performance of present arc sources for heavy ions.
assuming particle currents of at least 1013 s-I- The "ideal" ratio of c/A can be obtained only up to A = 20. A total accelerating voltage of 160 MV had to be applied to bring u12+ ions to 8 MeV/nu- cleon. This is to be compared to 27 MV for the
"ideal" case, g/A = 0.3.
The way out of this situation is to raise c / A by stripping at elevated velocities. The accelerator then takes the form shown in the lower part of Fig. q: a preaccelerator raises the energy, typically to 1 to 2 MeV/nucleon, where the ions are stripped in a gaseous or solid target. Values of
</A up to 0.5 and more can be obtained by this procedure. The useful beam intensity, however, decreases. Typical relative yields for the most abundant charge state decrease from 50% for N- to 15% for U-ions.
are correct within about
+
1 to 2 charge units.Gaseous Targets
Fig. 6: Nonogram for
56,~)
after stripping In gaseous targets ( 5 ) .10 10
I
The superiority of foil strippers is impaired by intensity limitations. Stripping foils, typically
-
2of g cm
,
can hardly stand particle currents of very heavy ions (A ; 10 2 ) of more than 10 11 s-L There is hope that this situation can be im- proved.1:
-
0.1--
'I.
Summing up, it is not yet possible ta produce beams of proper intensity and in sufficiently high charge states of high-Z atoms in ion sources that are suitable for accelerator purposes. Therefore, it is necessary to enhance 5 by stripping. The "conven- tional" heavy ion accelerator thus becomes a combination of two accelerating systems separated
-
0.1- --
0.01-0.001
-0.0001
yrv
amu
HEAVY ION MACHINES
. . .
by a s t r i p p e r .
4. Conventional a c c e l e r a t i o n methods Solid Targets
omu
@
w/A
F i g . 7 : Nomogram f o r F ( 0 , z ) a f t e r i n s o l i d t a r g e t s ( 5 ) .
s t r i p p i n g
Before d i s c u s s i n g s e v e r a l t y p i c a l combined heavy ion a c c e l e r a t o r s , a few more g e n e r a l p o i n t s d e s e r v e mentioning.
The dependence of s p e c i f i c p a r t i c l e energy, W/A, on t h e charge-to-mass r a t i o of t h e a c c e l e r a t e d i o n s d i f f e r s f o r d i f f e r e n t a c c e l e r a t i o n methods, a s shown i n Table 1.
The d i s t i n c t i o n between "common f i e l d " and "sepa- r a t e f i e l d " RF-Linacs i n Table 1 d e s e r v e s explana- t i o n . I n common-field l i n a c s ( e , g - of t h e
Alvarez t y p e ) , t h e a c c e l e r a t i n g s t r u c t u r e i s em- bedded i n an e l e c t r o m a g n e t i c f i e l d common t o a l l a c c e l e r a t i n g gaps. The l e n g t h o f t h e d r i f t t u b e s determines t h e v e l o c i t y p r o f i l e a l o n g t h e beam p a t h . I n s e p a r a t e - f i e l d l i n a c s , t h e a c c e l e r a t i n g gaps a r e e x c i t e d i n d i v i d u a l l y , and by p r o p e r s e t t i n g s o f h a s e and amplitude o f t h e gap f i e l d s , t h e energy g a i n of t h e s t r u c t u r e can be v a r i e d .
The l a s t remark l e a d s t o t h e q u e s t i o n o f energy v a r i a t i o n . Table 2 summarizes t h e d i f f e r e n t p o s s i b i l i t i e s . An a d d i t i o n a l e x p l a n a t i o n i s neces- s a r y i n t h e c a s e of common f i e l d l i n a c s . There, energy v a r i a t i o n i s only p o s s i b l e by deforming t h e a x i a l e l e c t r i c f i e l d ( t i l t ) i n such a way t h a t a t a c e r t a i n a x i a l p o s i t i o n t h e p a r t i c l e s drop o u t of synchronism and begin t o d r i f t . The disadvan- t a g e of t h i s method i s , b e s i d e s needing r a t h e r
TABLE 1
Dependence on g/A o f s p e c i f i c energy g a i n AW/A
A c c e l e r a t i n g system
I
AW governed byI
Spec. energy g a i n A W l A DCS e p a r a t e d f i e l d Linac Common f i e l d Linac
I
p a r t i c l e v e l o c i t yI
= c o n s t a n t C i r c u l a r a c c e l e r a t o r s P a r t i c l e mag. r i g i d i t y afl; / A ) ~Ch. C?aMELrnR
TABLE 2
V a r i a t i o n of p a r t i c l e energy
Separated f i e l d Linac phase o r v o l t a g e
I
( w = c o n s t a n t ) Accelerating systemCommon f i e l d l i n a c a x i a l f i e l d d i s t r i b u t i o n ( w = c o n s t a n t )
(beam q u a l i t y s u f f e ~ s ) Parameters o f v a r i a t i o n
C i r c u l a r a c c e l e r a t o r s Guide f i e l d and frequency
I
f R m = c o n s t a n t ) t e d i o u s though r e p r o d u c i b l e adjustment, t h a tt h e beam q u a l i t y s u f f e r s and, i n p a r t i c u l a r , t h e time s t r u c t u r e more o r l e s s vanishes.
Vacuum requirements f o r heavy ion a c c e l e r a t o r s a r e more s e v e r e (by about one o r d e r of magnitude) than f o r machines a c c e l e r a t i n g l i g h t p a r t i c l e s i n o r d e r t o minimize charge changes due t o c o l l i s i o n s o f i o n s with r e s i d u a l g a s atoms. The e f f e c t o f charge- changing c o l l i s i o n s during a c c e l e r a t i o n d i f f e r s c h a r a c t e r i s t i c a l l y f o r t h e t h r e e major a c c e l e r a t i n g methods: For e l e c t r o s t a t i c machines, t h e p a r t i c l e o f wrong charge simply g a i n s a d i f f e r e n t energy.
In RF-Linacs, it i s f u r t h e r synchronousLy accele- rated,and only t h e beam q u a l i t y s u f f e r s somewhat.
In c i r c u l a r machines, t h e c e n t e r o f t h e p a r t i c l e o r b i t a f t e r a charge change no l o n g e r c o i n c i d e s with t h e c e n t e r o f t h e guide f i e l d , a n d t h e p a r t i c l e is l o s t .
5 . T y p i c a l heavy i o n a c c e l e r a t o r combinations
S e v e r a l accelera-cor combinations, p a r t l y i n opera- t i o n o r c o n s t r u c t i o n , p a r t l y i n t h e s t a t e of pro- p o s a l s , are shown s c h e m a t i c a l l y i n Fig. 8.
Combination
1
shows a l i n e a r a c c e l e r a t o r o f t h e HILAC-type, A t y p i e a l example w i l l be d i s c u s s e d i n more d e t a i l a t t h e end of this s e c t i o n .Fig. 8: Typical heavy i o n a c c e l e r a t o r cornbi- n a t i o n s . S = Ion s o u r c e ; L = R f Linac;
S t = S t r i p p e r ; F = e/m-Filter; 3 Buncher;
h = frequency m u l t i p l i c a t i o n .
HEAVY ION LUY\CHINES
. . .
The n e x t two c o m b i n a t i o n s u s e e l e c t r o s t a t i c ma- c h i n e s a s i n f e c t o r s . Tandem van-de-Graaff machines a r e shown, which i n t h e m s e l v e s can a l r e a d y b e con- s i d e r e d combined a c c e l e r a t o r s . I n i t s p r e s e n t s t a t e o f development, t h e i r u s e f o r heavy i o n r e s e a r c h i s l i m i t e d t o e x p e r i m e n t s i n t h e mass r a n g e A 2 30. A Tandem, p r o v i d i n g a 6 MeV/nucleon beam o f uranium would n o t o n l y need a t e r m i n a l v o l t a g e o f a b o u t 50 MV, b u t a l s o two f o i l s t r i p p e r s , one i n t h e t e r m i n a l f o l l o w e d by a s e c o n d i n t h e a c c e l e r a t i n g column. Thus, The e x p e c t e d beam i n - t e n s i t y w i l l b e low f o r v e r y heavy i o n s . F o r uranium, t h e o v e r a l l t r a n s m i s s i o n t u r n s o u t t o b e o n l y a few p e r c e n t . The c o n s t r u c t i o n o f s u c h a super-tandem may p r e s e n t c o n s i d e r a b l e t e c h n i c a l d i f f i c u l t i e s .
On t h e o t h e r hand, e x t e n d i n g t h e e n e r g y and mass r a n g e o f e x i s t i n g tandem f a c i l i t i e s by RF p o s t - a c c e l e r a t o r s i s o f c o n s i d e r a b l e i n t e r e s t .
The c o m b i n a t i o n shown i n F i g . 8 , 2 u s e s a n RF l i n e a r p o s t a c c e l e r a t o r . A h e l i c a l a c c e l e r a t i n g s t r u c t u r e i s shown. Using a b o u t f i f t e e n s e c t i o n s e a c h 1 m l o n g , bromine i o n s c a n b e a c c e l e r a t e d t o
5.5 MeV/nucleon. The e x p e c t e d p a r t i c l e c u r r e n t s a r e o f t h e o r d e r o f 10" s-l. The n e c e s s a r y mean RF power i s a b o u t 300 kW, i f t h e p a s t a c c e l e r a t o r is p u l s e d w i t h 25% d u t y c y c l e . An e x t e n s i o n o f t h e mass r a n g e o f t h i s c o m b i n a t i o n much above A = 1 0 0 seems uneconomical, b o t h f o r r e a s o n s o f beam i n t e n - s i t y and power consumption.
The c o m b i n a t i o n i n F i g . 8 , 3 r e p r e s e n t s a n o t h e r w i d e l y d i s c u s s e d s o l u t i o n . A s p l i t - s e c t o r , i s o - c h r o n o u s c y c l o t r a n r e p l a c e s t h e l i n e a r p o s t a c c e l e - r a t o r of F i g . 8.2. Using a tandem w i t h 20 MV ter- m i n a l v o l t a g e , C / A 0.15 f o r t h e h e a v i e s t i o n s , which c a n b e a c c e l e r a t e d t o 8 t o 1 0 MeV/nucleon i n a maximum g u i d e f i e l d o f a p p r o x i m a t e l y 16 KG. The maximum o r b i t r a d i u s w i l l b e a b o u t 3.5 m. The s p l i t - s e c t o r t y p e a l l o w s placement o f t h e s t r i p p e r S t 2 o u t s i d e t h e c y c l o t r o n , which is o f c o n s i d e r a b l e a d v a n t a g e from a n o p e r a t i o n a l p o i n t o f view. ( I n c o n v e n t i o n a l c y c l o t r o n s , t h e s t r i p p e r must b e p l a c e d i n s i d e t h e machine, a s i n d i c a t e d i n F i g . 8 , 5 ) A s i n t h e p r e c e d i n g example, d o u b l e s t r i p p i n g Leads
t o c o n s i d e r a b l e i n t e n s i t y l i m i t a t i o n s f o r high-i:
b e a n s .
Thus, one comes t o c o n s i d e r o t h e r i n j e c t o r s y s t e m s , c y c l o t r o n s ( F i g . 8.Q) o r l i n e a r a c c e l e r a t o r s ( F i g . 8,s). I n b o t h c a s e s , a s i n g l e s t r i p p e r is n e c e s s a r y and t h e o v e r a l l t r a n s m i s s i o n f o r heavy i o n s c o r r e s p o n d i r r g l y l a r g e r . F o r r e a s o n s d i s c u s s e d below, a l i n a c i n j e c t o r would a p p e a r p r e f e r a b l e , were it n o t f o r t h e a d d i t i o n a l c o m p l i c a t i o n s a r i s i n g from t h e n e c e s s i t y o f m a t c h i n g a c o n s t a n t - f r e q u e n c y i n j e c t o r t o a v a r i a b l e - f r e q u e n c y p a s t - a c c e l e r a t o r ( s e e a l s o T a b l e 2 ) .
S i z e and c o s t o f t h e p o s t a c c e l e r a t o r depend, u n d e r o t h e r w i s e e q u a l c i r c u m s t a n c e s , l a r g e l y upon t h e charge-to-mass r a t i o 5 / A . t / A a f t e r s t r i p p i n g i n c r e a s e s w i t h p a r t i c l e v e l o c 4 t y . Hence, t h e mass dependence o f s p e c i f i c e n e r g y (WfA) o f t h e p r e - a c c e l e r a t o r becomes i m p o r t a n t . The f u l l c u r v e s i n F i g . 9 show t h e c h a r a c t e r i s t i c b e h a v i o r o f a
f i g . 9: Dependence o f maximum s p e c i f i c e n e r g y W/A on mass number A f o r d i f f e r e n t heavy i o n machines ( 6 ) . See t e x t f o r d e t a i l s .
Ch. SCHMELZER
UNILAC t o y o u t ~ r m
P L m m
Fig. 10: Layout of the UNILAC
tandem (MP), a linear accelerator (CF-Linac) and a cyclotron (CY), based on realistic ion source pro- perties (see Fig. 5) and, for the tandem, the stripper performance. Considering the slopes of these curves, the common field linac shows the best properties.
The dashed lines in Fig. 9 give typical values of the overall mass dependence of W/A for the first three examples of combined accelerators shown in Fig, 8. The decision as to which combination one may select depends, besides considerations of con- structional and operating cost, mainly on the envisaged experimental program.
The accelerator combinations described so far appear to be realistic solutions for specific energies up to about 10 MeVlnucleon for uranium.
For much higher energies, synchrotrons are by far the most economical postaccelerators (Fig. 8,6).
The use of a fast cycling synchrotron with a corres- pondingly short beam path is advisable, and speci- fic injection energies above about 5 MeV/nucleon should be used. Both facts help to reduce the vacuum requirements necessary for obtaining a good beam transmission.
The necessarily rather sketchy remarks made so far may deserve a few more detailed comments. The .
UNILAC, now under construction at Darmstadt, may serve as an example (Fig. TO).
The ion sources will be duoplasmatrons for A 2 80 and Penning sources for the heavier projectiles.
Flexibility considerations and a certain pessimism with respect to source life-times led to the instal- lation of two separate 320 kV DC injectors, each with space for two sources.
The beam transport system to the accelerator proper has a mass resolution of M/AM = 250 for a 10 cm mrad beam. This precaution is considered essential, since, contrary to cyclotrons, linear accelerators show poor separating properties.
The beam is then injected at 12 keV/nucleon into a chain of four WiderGe accelerators, all together 27 m long. The low particle velocity at injection (I3 = 0.005) dictates the low frequency of 27 MHz in the preaccelerator. The beam is accelerated to 1.4 MeV/nucleon and then enters the stripper region. Stripping is accomplished either by a transverse C02 jet or by foils, and a short helix
HEAVY I O N MACHINES
...
s e c t i o n s e r v e s t o compensate f o r t h e e n e r g y l o s s i n 6. T e c h n o l o g i c a l improvements t h e s t r i p p e r s . A m a g n e t i c c h a r g e s e p a r a t o r s e l e c t s
t h e d e s i r e d c h a r g e s t a t e f o r f u r t h e r a c c e l e r a t i o n , The c o n v e n t i o n a l a c c e l e r a t i n g s y s t e m s d e s c r i b e d and i n a d d i t i o n a l l o w s t h e s e l e c t i o n o f a p a r a s i t i c h o v e a r e p r e t t y f a r from t h e " i d e a l " heavy i o n beam f o r e x p e r i m e n t s a t low e n e r g y . machine d i s c u s s e d i n i t i a l l y , b o t h w i t h r e s p e c t
t o s i z e , a s w e l l
-
e x p e c i a l l y i n t h e c a s e o f The f o l l o w i n g p o s t a c c e l e r a t o r works a t 108 MHz and l i n e a r a c c e l e r a t o r s-
a s by c o n s i d e r i n g t h e power c o n s i s t s o f t h r e e main e l e m e n t s : a f i r s t A l v a r e z consumption. The improvement o f i o n s o u r c e s and a c c e l e r a t o r r a i s i n g t h e s p e c i f i c e n e r g y t o 3.8 MeV/n u c l e o n , a second A l v a r e z g o i n g t o 5.9 MeV/nucleon.
Both A l v a r e z t a n k s a r e 29 m l o n g . F i n a l l y , a 20 m l o n g c h a i n o f t w e n t y s e p a r a t e l y e x c i t e d s i n g l e g a p c a v i t i e s a l l o w f o r c o n t i n u o u s e n e r g y v a r i a t i o n . F o r s p e c i f i c e n e r g i e s between 3 . 8 and 6 . 2 M e V / n u c l e o n t h e second A l v a r e z s t a g e i s s w i t c h e d o f f w i t h t h e e x c e p t i o n of t h e c e n t r a l c e l l , which t h e n s e r v e s a s a r e b u n c h e r . If n e c e s s a r y , t h e l o w e r e n e r g y l i m i t c a n be e x t e n d e d t o 1 . 8 MeV/
n u c l e o n by d e c e l e r a t i n g i n t h e s i n g l e g a p s t r u c - t u r e .
With 2 MW mean RF power i n s t a l l e d , 8.5 MeV/nucleon uranium i o n s a r e o b t a i n e d i n p u l s e d o p e r a t i o n
( m a c r o s c o p i c d u t y c y c l e o f 25%, i . e . 8 M W p u l s e power) by u s i n g t h e g a s s t r i p p e r . With f o i l s t r i p p i n g , o n l v 3 . 2 MW p u l s e power i s n e c e s s a r y f o r t h e same e n e r g y , and 1 0 . 2 MeV/nucleon a r e r e a c h e d w i t h 5.5 M W p u l s e power. The dependence o f o b t a i n a b l e s p e c i f i c e n e r g i e s f o r o t h e r e l e m e n t s i s shown i n F i g . 11.
t h e i n t r o d u c t i o n o f s u p e r c o n d u c t i n g RF s t r u c t u r e s open v e r y p r o m i s i n g p o s s i b i l i t i e s t o improve t h i s s i t u a t i o n .
The main aim i n p r o d u c i n g s o u r c e s f o r i o n s o f h i g h e r c h a r g e t h a n a v a i l a b l e t o d a y i s t o i n c r e a s e t h e c o n t a i n m e n t t i m e o f t h e i o n s w i t h i n t h e s o u r c e i n o r d e r t o r e a c h h i g h c h a r g e s t a t e s by s t e p w i s e i o n i z a t i o n u n d e r i n t e n s e e l e c t r o n bombardment. The EBIS ( E l e c t r o n Beam I o n S o u r c e ) shown s c h e m a t i c a l l y i n F i g . 12 may s e r v e as a t y p i c a l example. A s t r o n g e l e c t r o n beam fe.g.3 keV, 1 0 A cm -2 ), f o c u s e d by a s o l e n o i d , t r a v e r s e s t h e s o u r c e and is c o l l e c t e d a t t h e e l e c t r o d e C. A s e t o f a u x i l i a r y e l e c t r o d e s , 1 t o 3 , c r e a t e s a p o t e n t i a l w e l l ( a i n F i g . 1 2 ) i n which i o n s a r e t r a p p e d and i o n i z e d
.iy
;. A; XI1 iD . ' 5
.
10 . e5 , t >.10 , 120 , + i'O , IF0 , 280 , IUI ,"-
1,o-
1'0 Y F i g . 1 2 : EBIS s o u r c e , a f t e r S e p t i e r ( 7 ) .Fi
.
11: specific energies W/A versus s t e p w i s e . A f t e r a c o n t a i n m e n t t i m e o f t h e o r d e r mass numberA f o r t h e UNILAC. F u l l c u r v e s : s , t h e p o t e n t i a l o f e l e c t r o d e 3 is lowered, g a s s t r i p p e r , d a s h e d c u r v e s : f o i l s t r i p p e r .6 = m a c r o s ~ o p i c d u t y c y c l e . t h e w e l l opened (b i n F i g . 1 2 ) a n d t h e i o n s c a n b e e x t r a c t e d by e l e c t r o d e E. With t h i s t y p e o f s o u r c e
CS -204 Ch. SCHMELZER
~ o n e t s ~ ) o b t a i n e d beams of AU"' w i t h good i n t e n - s i t y (10" s - I ) . Vacua below 10 -8 Torr a r e neces- s a r y t o o b t a i n a s u f f i c i e n t l y l a r g e r a t i o of recom- bination-to-containment-time. Work i s i n
p r o g r e s s 9 ) t o a l l o w f o r continuous i n s t e a d of pulsed e x t r a c t i o n .
The second t e c h n o l o g i c a l improvement a i ~ s a t pro- ducing high a c c e l e r a t i n g f i e l d s a t low power, using superconduct5ng s t z ' u c t u r e elements. The RF s u r f a c e conddctance of copper a t room t e m p e r a t u r e i s about
-
810-* Ohms a t 7 GHz, b u t drops t o 1 0 Ohns f o r a niobium s u r f a c e a t 1 . 8 K. Thus, t h e RF power t o produce a given a c c e l e r a t i n g f i e l d i s reduced by s i x o r d e r s of magnitude and hence n e g l i g i b l e com- pared t o t h e p w e r n e c e s s a r y t o a c c e l e r a t e t h e p a r t i c l e . The consequent extreme bean l o a d i n g i n t h e superconducting c a s e p r e s e n t s some problems, which, however, appear s u f f i c i e n t l y understood t o be mastered.
Work on superconducting s t r u c t u r e s f o r e l e c t r o n l i n a c s began a decade ago a t S t a n f o r d . The Karlsruhe Group c o n c e n t r a t e d a l a r g e p a r t of t h e i r e f f o r t on "slow" a c c e l e r a t i n g s t r u c t u r e s which e v e n t u a l l y w i l l be s u i t a b l e f o r heavy i o n work 10)
.
R e p r o d u c i b i l i t y o f t h e p r o p e r t i e s o f superconduct- i n g r e s o n a t o r s was s o l v e d by u s i n g niobium con- d u c t o r s covered by a t h i n f i l m o f niobium pento- x i d e .
For slow p a r t i c l e s , t h e s m a l l dimensions of h e l i x r e s o n a t o r s make them e s p e c i a l l y s u i t e d f o r a superconducting s t r u c t u r e . S u p e r f l u i d helium, flowing through t h e hollow conductor, c o n s i d e r a b l y s i m p l i f i e s t h e c o o l i n g problem. So f a r , a x i a l f i e l d g r a d i e n t s ar: I? MV/m have been o b t a i n e d , and i n a p r o t o t y p e a c c e l e r a t o r , u s i n g a s i n g l e h e l i x about 4Ocm i n Pength,protons were a c c e l e r a t e d from 750 t o 1130 keV. To e s t i m a t e t h i s accomplishment, it should be remembered t h a t t h e band width o f t h e h e l i x r e s o n a t o r i s o n l y a few Hz a t 90 MHz r e s o - nance ~ 5 e q u e n c y . To overcome d i s t u r b i n g e f f e c t s caused by mechanical v i b r a t i o n s a s w e l l a s by pondeI'omotoric e f f e c t s ( t h e h e l i x c a r r i e s c u r r e n t s of s e v e r a l hundreds o f amperes), an e l a b o r a t e , f a s t servo-system had t o be developed. The n e x t s t e p t o
m a s t e r i s t o lock two and more a c c e l e r a t i n g elementt i n phase. When t h i s problem, which i s being
s t u d i e d i n t h e meantime i n s e v e r a l l a b o r a t o r i e s , i s s o l v e d s u c c e s s f u l i y , p r o j e c t s o f superconducting hcavy i o ~ l i n e a r * c c e l e r a t o ~ s should be s e r i o u s l y c o n s i d e r e d . Although it is premature t o p r e d i c t a c t u a l c o n s t r u c t i o n c o s t o f superconducting ma- c h i n e s , t h e o p e r a t i n g c o s t ( ( i n c l u d i n g r e f r i g e r a t i o n ) o f a superconducting l i n a c i s c e r t a i n l y lower by a f a c t o r between f i v e and t o n compared t o "warm"
machines o f e q u a l performance.
7. Coll.ective A c c e l e r a t i o n
Through a l l t h e preceding d i s c u s s i o n s , t h e charge/
mass problem went l i k e a "red t h r e a d " . The p r e s e n t i n a b i l i t y t o produce s u f f i c i e n t l y l a r g e < / A v a l u e s f o r heavy ions complicates a l l c o n v e n t i o n a l a c c e l e - r a t i n g systems, a l l of which use e x t e r n a l e l e c t r i c f i e l d s a c t i n g on i n ~ i v i d u a l charged p a r t i c l e s . T h i s < / A d i f f i c u l t y could be overcome, a s was shown i n p r i n c i p l e by Alfbn and Wernholml1) two decades ago, i f t h e a c c e l e r a t i n g f i e l d would a c t on a l a r g e assembly of e l e c t r o n s c o n t a i n i n g a s m a l l a d m i x t w e of p o s i t i v e i o n s , such t h a t t h e g l o b a l charge-to-mass r a t i o of t h e assembly always s t a y s c l o s e t o t h a t o f a s i n g l e e l e c t r o n . S i n c e d u r i n g a c c e l e r a t i o n a l l members o f t h e assembly move with t h e same v e l o c i t y , t h e energy o f t h e i o n s
where w i s t h e energy o f t h e e l e c t r o n s , bl t h e i o n i c , m t h e e l e c t r o n i c r e s t mass and
9
-112Y = ( I - ~ )
.
The main o b s t a c l e t o producing s t a b l e a s s e m b l i e s which can be c o l l e c t i v e l y a c c e l e r a t e d was overcome by Veksler12), whose concept of t h e r e l a t i v i s t i c e l e c t r o n r i n g r e s t s on e a r l i e r work of Budker 13
.
Consider a r i n g o f H e l e c t r o n s , moving w i t h t h e v e l o c i t y B . Then t h e Coulomb r e p u l s i o n f e l t by
2 -2
an e l e c t r o n i s reduced by t h e f a c t o r 1 - B = y due t o t h e Lorentz a t t r a c t i o n of t h e e l e c t r o n c u r r e n t s . The remaining r e p u l s i o n can be compen- s a t e d f o r by adding a number < N of p o s i t i v e
i o n c h a r g e s . I f
HEAVY ION MACHINES
.. .
t h e r i n g i s s t a b i l i z e d . The maximum e l e c t r i c f i e l d , t h e l ~ o l d i n g power, o f t h e r i n g is p r o p o r - t i o n a l t o l / a , I h e i n g The c i r c u l a t i n g c u r r e n t , a t h e r a d i u s o f t h e c u r r e n t t h r e a d . One o f t h e methods o f p r o d u c i n g e l e c t r o n r i n g s i s s k e t c h e d i n F i g . 13. A s h o r t , v e r y i n t e n s i v e b u r s t o f w e l l - c o l l i m a t e d , f a s t e l e c t r o n s is i n j e c t e d i n t o a mag- n e t i c m i r r o r f i e l d formed by two s o l e n o i d s , t h e r e b y f o r m i r ~ g a r e l a t i v e l y l a r g e r i n g . Then t h e m a g n e t i c f i e l d is r a p i d l y r a i s e d ( f o r 10 t o 100 p s ) . The r i n g d i a m e t e r and i t s t h i c k n e s s s h r i n k , and a t t h e same time t h e e l e c t r o n s g a i n f u r t h e r e n e r g y a s i n a B e t a t r o n . A s h o r t p u l s e of atoms i s t h e n i n - j e c t e d , which a r e i o n i z e d by e l e c t r o n c o l l i s i o n and s t a b i l i z e t h e r i n g . T y p i c a l d i n e n s i o n s of r i n g s compressed i n t h i s manner a r c 6 cm major and 0 . 3 cm minolb d i a m e t e r . With l d 3 c i r c u l a r i n g e l e c t r o n s and y
-
40, t h e h o l d i n g power i s a b o u t 150 I.iV/m. A s l i g h t a x i a l asymmetry o f t h e magne- t i c f i e i d a l l o w s t h e r i n g t o b e moved from t h e compr3essor i n t o an a x i a l f i e l d f o r a c c e l e r a t i o n . B e s i d e s a x i a l e l e c t r i c f i e l d s , a c o n s t a n t , v e r y s l o w l y d e c r e a s i n g a x i a l m a g n e t i c f i e l d (E3'10 g a u s s ) c a n b e u s e d f o r a c c e l e r , r t t i o n ( F i g . 14). Thee n e r g y u s e d f o r t h e a c c e l e r a t i o n o f t h e r i n g is t a k e n from t h e e n e r g y o f s h e c i r c u l a t i n g e l e c t r o n s . The r i n g d i m e n s i o n s t h u s i n c r e a s e , and t h e h o l d i n g power, f a l l s c o r r e s p o n d i n g l y .
So f a r , s e v e r a l l a b o r a t o r i e s have produced com- p r e s s e d . e l e c t r o n r i n g s , b u t o n l y S a r a n t s e v ' s g r o u p i n Dubna s u c c e e d e d i n a c c e l e r a t i n g a l p h a - p a r t i c l e s t o 30 MeV by t h e m a g n e t i c a c c e l e r a t i o n method ( 1 4 ) .
P e t e r s o n ( 1 5 ) h a s e s t i m a t e d t h a t a m a g n e t i c a c c e l e - r a t i o n f i e l d 1 . 7 rn l o n g i s s ~ i f f i c i e r i t t o a c c e l e r a t e 1 0 uranium i o n s t r a p p e d i n a n e l e c t r o n r i n g of t h e 9
d i m e n s i o n s g i v e n above t o 10 MeV/nucleon. The d u r a t i o n of- t h e beam p u l s e i s a b o u t 10-" s , and r e p e t i t i o n r a t e s o f l o 2 s - I a p p e a r p o s s i b l e .
S i n c e t h e t e c h n i q u e o f c o l l e c t i v e a c c e l e r a t i o n o f e l e c t r o n r i n g s i s s t i l l i n a n e a r l y p h a s e o f development, it a p p e a r s p r e m a t u r e t o compare t h e p o s s i b l e performance o f e l e c t r o n r i n g a c c e l e r a t o r s w i t h t h a t af more c o n v e n t i o n a l machines. I t a p p e a r s t h a t s e v e r a l y e a r s o f r e s e a r c h a r e n e c e s - s a r y t o p r o v e t h e p r a c t i c a l a p p l i c a b i l i t y o f t h i s b r i l l i a n t new method.
- l o 3 A , 5MeV e - beam for
'
-20 n st
l o w B high 8
F i g . 13: E l e c t r o n r i n g compressor
Fig. 14: M a g n e t i c e x p a n s i o n a c c e l e r a t i o n o f e l e c t r o n r i n g s . Bp
-
10 g a u s s .Ch. SCHMELZER
REFERENCES
1 ) GSI R e p o r t 72-4 (1972). 9 ) R. BECKER e t a l . , IEEE Trans. Nucl. S c i . NS-19,
2 ) F. NICKEL a n d H. ElVALD, t o b e p u b l i s h e d . 125 (1972).
1 0 ) A. CITRON, p r i v a t e communication 3 ) B. BRECHT, " ~ i e ~ r e i ~ r o s c h e n o ~ e r " , B e r l i n (1 928 ).
4 ) H.D. BETZ, MIT Lab. Nucl. S c i . Techn. R e p o r t 1 1 ) H . &FEN and 0. WERNHOLM, A r k i v f o r F i s i k 5,
COO
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3069-1 (1971 ). 175 ( 1 9 5 2 )5) The nomograms were p r e p a r e d by B. FRANZKE, GSI. 1 2 ) V.I. VEKSLER e t a l . , P r o c . 6 t h I n t . Conf, on High En. Acc., Cambridge r e p o r t CEAL-2000, 289 6 ) D a t a a f t e r R.S. LIVINGSTON and J . A . MARTIN,
(1967).
P a r t i c l e Accel. 2 , 189 (1971
).
1 3 ) G.I. BUDKER, P r o c . CERN Symp. on High En. Acc.
7 ) A. SEPTIER, IEEE T r a n s . Nucl. S c i . NS-19, 22
6 8 ( 1 9 5 6 ) (1972).
1 4 ) V.P. SARANTSEV e t a l . , Dubna r e p o r t JINR 8 ) E.U. DONFTS et a l . , Proc. 1 s t I n t . Conf. on I o n P9-5558 (1971 1.
S o u r c e s , S a c l a y , 635 (1969).
1 3 ) J . M . PETERSON, IEEE T r a n s . Nucl. S c i . NS-19, 276 (1972).
DISCUSSION
M. LEFORT ( O r s a y )
As P r o f . Schmelzer s a i d i n h i s i n t r o d u c t i o n , i t would b e v e r y d e s i r a b l e t o d e v e l o p h i g h l y c h a r g e d i o n s o u r c e s , s i n c e s t r i p p e r s wont
never
b e a good s o l u t i o n f o r h i g h i n t e n s i t y beams of m u l t i c h a r g e d i o n s . A t t h e p r e s e n t t i m e , t h e more p r o m i s i n g s t u d y c o r r e s p o n d s t o D o n e t s ' s p r o p o s a l . However i t seems t h a t t h e d u t y c y c l e is. f o r t h e . moment.-very s m a l l . S i n c e . i t i s . .so i m p o r t a n t f o r t h e f u t u r e of heavy i o n machines, i t would b e v e r y u s e f u l t o o r g a n i z e a j o i n t e f - f o r t i n Europe f o r d e v e l o p i n g new m u l t i c h a r g e d i o n s o u r c e s of Donets t y p e .
Ch. SMMELZER ( D a r m s t a d t )
I a g r e e w i t h P r o f . L e f o r t , t h a t we s h o u l d a v o i d s t r i p p i n g whenever p o s s i b l e . The c o n t a i n m e n t s o u r c e s , l i k e D o n e t s ' s s o u r c e , w i l l be f u r t h e r developed. The F r a n k f u r t g r o u p , f o r i n s t a n c e , works on a m o d i f i e d D o n e t s s o u r c e w i t h c o n t i -
l a r g e e l e c t r o n c a p t u r e c r o s s s e c t i o n s of m u l t i - c h a r g e d h e a v y i o n s . A l r e a d y 1'0' i o n s have, a t a b o u t 2 0 keV/n, c a p t u r e c r o s s s e c t i o n s of t h e o r d e r of c m 2
,
and i t seems t h a t t h e c r o s s s e c t i o n s r i s e p r o p o r t i o n a l t o i o n i c c h a r g e J , i f n o t , a s i n d i c a t e d by measurements o f Ryding and Wittkower, w i t h a b o u t t h e 3/2 power o f.
Xf t h i s t e n d e n c y i s c o r r e c t , t h e n e x t r e m e va- cuum c o n d i t i o n s w e r e n e c e s s a r y t o p r o d u c e - a n d t r a n s p o r t i o n s o f v e r y h i g h c h a r g e .
J. STEYAERT ( Univ. d e Louvain-la-Neuve)
Appearance of new a c c e l e r a t o r s p r o d u c e s much e n t h u s i a s m b u t up-grading of p r e s e n t l y a v a i l a - b l e o n e s i s c e r t a i n l y an o t h e r way t o go. What c a n CERN (European O r g a n i z a t i o n o f N u c l e a r R e - s e a r c h ) d o f o r heavy-ion p h y s i c i s t s a s it is w e l l known t h a t to-day heavy-ion a c c e l e r a t o r pro- j e c t s a r e a t t h e l i m i t of n a t i o n a l budgets.
Can we h a v e heavy i o n s i n P S a n d ISR ? nuous e x t r a c t i o n . But I l i k e t o p o i n t t o t h e
