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Multiply charged heavy ions produced by energetic plasmas
Volume 31A. number 4
PHYSICS L E T T E R S
23 February 1970
MULTIPLY CHARGED HEAVY IONS PRODUCED BY ENERGETIC PLASMAS* H. POSTMA Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Received 22 December 1969
Intense multiply charged heavy ion beams may be produced through single-impact ionization with subsequent shakeoff of electrons when heavy elements are introduced into presently available m i r r o r contained energetic electron plasmas. Uniform steady-state currents of 6 mA of Xe12+ over a 300 cm 2 area appear likely.
T h e r e i s broad and c o n s i d e r a b l e i n t e r e s t in the p o s s i b i l i t y of p r o d u c i n g f a r - t r a n s u r a u i c e l e m e n t s through the fusion of heavy nuclei. E s s e n tial to all the suggested a c c e l e r a t o r s for a c h i e v ing this goal i s the r e q u i r m e n t for a s o u r c e for the copious production of highly s t r i p p e d heavy nuclei. Two [1,2] r e c e n t i d e a s have been suggested in which a high d e g r e e of e l e c t r o n s t r i p p i n g is obtained through the l o n g - t e r m c o n f i n e m e n t of ions in the e l e c t r o s t a t i c potential well of an e n e r getic e l e c t r o n p l a s m a . In both s c h e m e s , a s t e p b y - s t e p i o n i z a t i o n of the heavy ion r e q u i r e s long c o n f i n e m e n t t i m e s . In the c a l c u l a t i o n s by Daughterty et al. [1] the c o m b i n a t i o n of ion conf i n e m e n t t i m e and e l e c t r o n energy and d e n s i t y led to the elegant concept of the t o r o i d a l c o n t a i n e r HIPAC; yet much work r e m a i n s before the concept will be applicable a s an ion s o u r c e . On the other hand, the suggestion of Stix [2] for the long t e r m e l e c t r o s t a t i c c o n f i n e m e n t of ions in an openended g e o m e t r y a w a i t s the d e m o n s t r a t i o n of stable p l a s m a c o n f i n e m e n t having n e c e s s a r i l y l a r g e (keV) negative space potentials. We suggest in this l e t t e r another method which uses present state-of-the-art energetic electron p l a s m a s to c r e a t e multiply charged heavy ions through single i m p a c t ionization. The physical p r o c e s s of i n n e r shell i o n i z a t i o n and the s u b s e quent Auger t r a n s i t i o n s and shakeoff of many e l e c t r o n s [3] through the single i m p a c t of keV e l e c t r o n s with heavy e l e m e n t s has been the subj e c t of c o n s i d e r a b l e r e s e a r c h [3-6]. F o r the p r o d u c t i o n of highly charged ions this p r o c e s s has c o n s i d e r a b l e appeal since it does not r e q u i r e the l o n g - t e r m c o n f i n e m e n t of ions i n o r d e r to
s t r i p the e l e c t r o n s away. When this p r o c e s s takes place in dense e n e r getic e l e c t r o n p l a s m a s such as a r e p r e s e n t l y a v a i l a b l e , the production r a t e s of multiply charged ions appear to be high enough to obtain useful s t e a d y - s t a t e s o u r c e c u r r e n t s for heavy ion a c c e l e r a t o r s . In the following n u m e r i c a l example we will u s e the p a r a m e t e r s of the p l a s m a c r e a t e d in a device known as ELMO [7] at the Oak Ridge National L a b o r a t o r y . However, t h e r e a r e s e v e r a l other p l a s m a d e v i c e s of d i v e r s e types that also appear a t t r a c t i v e a s ion s o u r c e s u s i n g the single i m p a c t ionization p r o c e s s ; for example, Burnout IV [8] and C i r c ~ - P l e i a d e [9], to m e n t i o n two. The ELMO device s e e m s to have m o r e i m m e d i a t e applicability. In brief, hot e l e c t r o n s a r e c r e a t e d and confined in a s i m p l e m a g n e t i c m i r r o r c o n t a i n e r through the coupling of a high-power m i c r o w a v e s o u r c e to the cyclot r o n r e s o n a n c e of e l e c t r o n s in the c o n t a i n e r ' s m a g n e t i c field. The p l a s m a thus produced i s dense, stable and steady state with the typical o p e r a t i n g c h a r a c t e r i s t i c s given in table 1. The most complete e x p e r i m e n t a l absolute c r o s s section data on highly charged ions r e s u l t ing f r o m single i m p a c t i o n i z a t i o n i s the work of Schram [4] on argon, krypton and xenon. Since xenon at this point being h e a v i e r is one of the m o r e i n t e r e s t i n g ions, we can u s e the v a l u e s r e a d f r o m S c h r a m ' s fig. 5. As an example, we can calculate the p r o d u c tion r a t e of Xe 12+ a s s u m i n g the ELMO p l a s m a can be r u n a s well on xenon as has been done in the past on d e u t e r i u m , h e l i u m and argon. The production r a t e of Xe 12+ can be calculated through the s i m p l e r e l a t i o n :
* Research sponsored by the US Atomic Energy Commission under contract with the Union Carbide Corp.
dN + volume dt = no ne (ri Ve m i r r o r a r e a
196
in particles sec
PHYSICS
Volume 31A, n u m b e r 4 Table 1
C h a r a c t e r i s t i c s of ELMO [7] p l a s m a A mirror-confined energetic electron plasma Parameter
Symbol
E l e c t r o n density Electron temperature Volume Operating p r e s s u r e for deuterium Neutral density Microwave power Microwave wavelength Mirror throht area Central magnetic field M i r r o r t h r o a t magnetic field
Value
nc Te V p
3 × 1012/cm 3 10 keV 2 liters 10 -4 T o r r
no P A A Bo BM
3 × 1012/cm3 1 kW 8 mm 300 cm 2 10 kG 20 kG
w h e r e v e ~ 1010 c m / s e c a n d t h e o t h e r p a r a m e t e r s c a m e f r o m t a b l e 1 a n d 8 ¢ h r a m ' s v a l u e of a~ = = 2 × 1 0 - 2 2 c m" 2 f o r X e 12+ . F o r X e 12+ w e g " e t a p r o d u c t i o n r a t e of 20 # ^ A / c m 2 e x i t i n g u n i f o r m l y f r o m o n e of t h e 300 c m Z ~ m i r r o r t h r o a t s . In u s i n g this device as an ion source one mirror coil can b e m a d e w e a k e r t h a n t h e o t h e r to c a u s e p r e f e r e n tial loss through the weaker mirror. After leaving t h e m i r r o r , p r e a c c e l e r a t i o n a n d s e p a r a t i o n of i o n s for the desired charge state can be made prepara t o r y to i n j e c t i o n i n t o t h e f i n a l a c c e l e r a t o r stages. For use as an ion source, the ELMO device could be made more compact, the pressure perh a p s r a i s e d a n o t h e r f a c t o r of 10, t h e t h r o a t m a d e smaller a l l d i r e c t i o n s of i n c r e a s i n g t h e f l u x of i o n s . In a d d i t i o n to t h e u s e of t h i s a s a n i o n s o u r c e f o r i m p l e m e n t i n g t h e e x p l o r a t i o n of f a r transuranic elements, there are shorter-range uses in other heavy ion investigations. -
LETTERS
23 F e b r u a r y 1970
There may be higher charge states for other e l e m e n t s w i t h h i g h e r c r o s s s e c t i o n s t h a t a r e of greater interest, but many more measurements w i l l n e e d to b e m a d e b e f o r e t h e p a t t e r n s a r e f o u n d . H o w e v e r , w i t h t h e a d v a n t a g e s of p r e s e n t day feasibility, continuous operation, intense c u r r e n t s a n d a c h o i c e of c h a r g e s t a t e s , t h e c o n cept presented in this letter might be considered for incorporation into present and projected heavy ion accelerator plans. I w i s h to t h a n k A. H. S n e l l , R i c h a r d L e v y , C. F. B a r n e t t a n d R. A. D a n d l f o r f r u i t f u l d i s c u s sions.
RS/~r~'i~CP~S 1. J. D. Daughterty, L. Grodzins, G. S. J a n e s and R. H. Levy, Phys. Rev. L e t t e r s 20 (1968} 369. 2. T. H, Stix, Phys. Rev. L e t t e r s 23 (1969} 1093. 3. F. Pleasonton and A. H. Snell, Proc. Roy. Soc. A241 (1957) 141. 4. B. L. Schram, Physica 32 (1966} 197. 5. M. J, Van d e r Wiel, Th. M. EI-Sherbini and L. Vriens, P h y s i c a 42 (1969) 411. 6. T.A. Carlson and M. O. Krause, Phys. Rev. L e t t e r s 14 (1965} 3 9 0 . 7. W. B. Ard, R.A. Dandl, A. C. England, G.M. Haas and N. H. L a z a r . Proc. Conf. on P l a s m a physics and controlled n u c l e a r fusion r e s e a r c h , Culham, 1965 (IAEA, Vienna, 1966), Vol. H, p. 153. 8. L Alexeff, W. D. Jones, R. V. Neidigh, W.F. Peed and W. L. Stirling, Proc. Conf. on P l a s m a physics and controlled n u c l e a r fusion r e s e a r c h , Culham (IAEA, Vienna, 1966), Vol. II, p. 781. 9. T. Consoli, Proc. 3rd Int. Conf. on P l a s m a physics and controlled n u c l e a r fusion r e a s e a r c h , Novosibirsk, 1969, (IAEA, Vienna, 1969), Vol. II, p. 361.
197
PHYSICS L E T T E R S
23 February 1970
MULTIPLY CHARGED HEAVY IONS PRODUCED BY ENERGETIC PLASMAS* H. POSTMA Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Received 22 December 1969
Intense multiply charged heavy ion beams may be produced through single-impact ionization with subsequent shakeoff of electrons when heavy elements are introduced into presently available m i r r o r contained energetic electron plasmas. Uniform steady-state currents of 6 mA of Xe12+ over a 300 cm 2 area appear likely.
T h e r e i s broad and c o n s i d e r a b l e i n t e r e s t in the p o s s i b i l i t y of p r o d u c i n g f a r - t r a n s u r a u i c e l e m e n t s through the fusion of heavy nuclei. E s s e n tial to all the suggested a c c e l e r a t o r s for a c h i e v ing this goal i s the r e q u i r m e n t for a s o u r c e for the copious production of highly s t r i p p e d heavy nuclei. Two [1,2] r e c e n t i d e a s have been suggested in which a high d e g r e e of e l e c t r o n s t r i p p i n g is obtained through the l o n g - t e r m c o n f i n e m e n t of ions in the e l e c t r o s t a t i c potential well of an e n e r getic e l e c t r o n p l a s m a . In both s c h e m e s , a s t e p b y - s t e p i o n i z a t i o n of the heavy ion r e q u i r e s long c o n f i n e m e n t t i m e s . In the c a l c u l a t i o n s by Daughterty et al. [1] the c o m b i n a t i o n of ion conf i n e m e n t t i m e and e l e c t r o n energy and d e n s i t y led to the elegant concept of the t o r o i d a l c o n t a i n e r HIPAC; yet much work r e m a i n s before the concept will be applicable a s an ion s o u r c e . On the other hand, the suggestion of Stix [2] for the long t e r m e l e c t r o s t a t i c c o n f i n e m e n t of ions in an openended g e o m e t r y a w a i t s the d e m o n s t r a t i o n of stable p l a s m a c o n f i n e m e n t having n e c e s s a r i l y l a r g e (keV) negative space potentials. We suggest in this l e t t e r another method which uses present state-of-the-art energetic electron p l a s m a s to c r e a t e multiply charged heavy ions through single i m p a c t ionization. The physical p r o c e s s of i n n e r shell i o n i z a t i o n and the s u b s e quent Auger t r a n s i t i o n s and shakeoff of many e l e c t r o n s [3] through the single i m p a c t of keV e l e c t r o n s with heavy e l e m e n t s has been the subj e c t of c o n s i d e r a b l e r e s e a r c h [3-6]. F o r the p r o d u c t i o n of highly charged ions this p r o c e s s has c o n s i d e r a b l e appeal since it does not r e q u i r e the l o n g - t e r m c o n f i n e m e n t of ions i n o r d e r to
s t r i p the e l e c t r o n s away. When this p r o c e s s takes place in dense e n e r getic e l e c t r o n p l a s m a s such as a r e p r e s e n t l y a v a i l a b l e , the production r a t e s of multiply charged ions appear to be high enough to obtain useful s t e a d y - s t a t e s o u r c e c u r r e n t s for heavy ion a c c e l e r a t o r s . In the following n u m e r i c a l example we will u s e the p a r a m e t e r s of the p l a s m a c r e a t e d in a device known as ELMO [7] at the Oak Ridge National L a b o r a t o r y . However, t h e r e a r e s e v e r a l other p l a s m a d e v i c e s of d i v e r s e types that also appear a t t r a c t i v e a s ion s o u r c e s u s i n g the single i m p a c t ionization p r o c e s s ; for example, Burnout IV [8] and C i r c ~ - P l e i a d e [9], to m e n t i o n two. The ELMO device s e e m s to have m o r e i m m e d i a t e applicability. In brief, hot e l e c t r o n s a r e c r e a t e d and confined in a s i m p l e m a g n e t i c m i r r o r c o n t a i n e r through the coupling of a high-power m i c r o w a v e s o u r c e to the cyclot r o n r e s o n a n c e of e l e c t r o n s in the c o n t a i n e r ' s m a g n e t i c field. The p l a s m a thus produced i s dense, stable and steady state with the typical o p e r a t i n g c h a r a c t e r i s t i c s given in table 1. The most complete e x p e r i m e n t a l absolute c r o s s section data on highly charged ions r e s u l t ing f r o m single i m p a c t i o n i z a t i o n i s the work of Schram [4] on argon, krypton and xenon. Since xenon at this point being h e a v i e r is one of the m o r e i n t e r e s t i n g ions, we can u s e the v a l u e s r e a d f r o m S c h r a m ' s fig. 5. As an example, we can calculate the p r o d u c tion r a t e of Xe 12+ a s s u m i n g the ELMO p l a s m a can be r u n a s well on xenon as has been done in the past on d e u t e r i u m , h e l i u m and argon. The production r a t e of Xe 12+ can be calculated through the s i m p l e r e l a t i o n :
* Research sponsored by the US Atomic Energy Commission under contract with the Union Carbide Corp.
dN + volume dt = no ne (ri Ve m i r r o r a r e a
196
in particles sec
PHYSICS
Volume 31A, n u m b e r 4 Table 1
C h a r a c t e r i s t i c s of ELMO [7] p l a s m a A mirror-confined energetic electron plasma Parameter
Symbol
E l e c t r o n density Electron temperature Volume Operating p r e s s u r e for deuterium Neutral density Microwave power Microwave wavelength Mirror throht area Central magnetic field M i r r o r t h r o a t magnetic field
Value
nc Te V p
3 × 1012/cm 3 10 keV 2 liters 10 -4 T o r r
no P A A Bo BM
3 × 1012/cm3 1 kW 8 mm 300 cm 2 10 kG 20 kG
w h e r e v e ~ 1010 c m / s e c a n d t h e o t h e r p a r a m e t e r s c a m e f r o m t a b l e 1 a n d 8 ¢ h r a m ' s v a l u e of a~ = = 2 × 1 0 - 2 2 c m" 2 f o r X e 12+ . F o r X e 12+ w e g " e t a p r o d u c t i o n r a t e of 20 # ^ A / c m 2 e x i t i n g u n i f o r m l y f r o m o n e of t h e 300 c m Z ~ m i r r o r t h r o a t s . In u s i n g this device as an ion source one mirror coil can b e m a d e w e a k e r t h a n t h e o t h e r to c a u s e p r e f e r e n tial loss through the weaker mirror. After leaving t h e m i r r o r , p r e a c c e l e r a t i o n a n d s e p a r a t i o n of i o n s for the desired charge state can be made prepara t o r y to i n j e c t i o n i n t o t h e f i n a l a c c e l e r a t o r stages. For use as an ion source, the ELMO device could be made more compact, the pressure perh a p s r a i s e d a n o t h e r f a c t o r of 10, t h e t h r o a t m a d e smaller a l l d i r e c t i o n s of i n c r e a s i n g t h e f l u x of i o n s . In a d d i t i o n to t h e u s e of t h i s a s a n i o n s o u r c e f o r i m p l e m e n t i n g t h e e x p l o r a t i o n of f a r transuranic elements, there are shorter-range uses in other heavy ion investigations. -
LETTERS
23 F e b r u a r y 1970
There may be higher charge states for other e l e m e n t s w i t h h i g h e r c r o s s s e c t i o n s t h a t a r e of greater interest, but many more measurements w i l l n e e d to b e m a d e b e f o r e t h e p a t t e r n s a r e f o u n d . H o w e v e r , w i t h t h e a d v a n t a g e s of p r e s e n t day feasibility, continuous operation, intense c u r r e n t s a n d a c h o i c e of c h a r g e s t a t e s , t h e c o n cept presented in this letter might be considered for incorporation into present and projected heavy ion accelerator plans. I w i s h to t h a n k A. H. S n e l l , R i c h a r d L e v y , C. F. B a r n e t t a n d R. A. D a n d l f o r f r u i t f u l d i s c u s sions.
RS/~r~'i~CP~S 1. J. D. Daughterty, L. Grodzins, G. S. J a n e s and R. H. Levy, Phys. Rev. L e t t e r s 20 (1968} 369. 2. T. H, Stix, Phys. Rev. L e t t e r s 23 (1969} 1093. 3. F. Pleasonton and A. H. Snell, Proc. Roy. Soc. A241 (1957) 141. 4. B. L. Schram, Physica 32 (1966} 197. 5. M. J, Van d e r Wiel, Th. M. EI-Sherbini and L. Vriens, P h y s i c a 42 (1969) 411. 6. T.A. Carlson and M. O. Krause, Phys. Rev. L e t t e r s 14 (1965} 3 9 0 . 7. W. B. Ard, R.A. Dandl, A. C. England, G.M. Haas and N. H. L a z a r . Proc. Conf. on P l a s m a physics and controlled n u c l e a r fusion r e s e a r c h , Culham, 1965 (IAEA, Vienna, 1966), Vol. H, p. 153. 8. L Alexeff, W. D. Jones, R. V. Neidigh, W.F. Peed and W. L. Stirling, Proc. Conf. on P l a s m a physics and controlled n u c l e a r fusion r e s e a r c h , Culham (IAEA, Vienna, 1966), Vol. II, p. 781. 9. T. Consoli, Proc. 3rd Int. Conf. on P l a s m a physics and controlled n u c l e a r fusion r e a s e a r c h , Novosibirsk, 1969, (IAEA, Vienna, 1969), Vol. II, p. 361.
197