Revista Brasileira de Física, Vol. 7, Nº 1, 1977
Production of Negative Helium ions
A. SZANTO DE TOLEDO and 0. SALA
Instituto de Física *, Universidade de São Paulo, São Paulo SP
Recebido e m 7 de Janeiro de 1977
A n e g a t i v helium i o n source u s i n g potassium charge exchange vapor has
been developed
ratorl.
to
be used as an i n j e c t o r f o r t h e
P e l l e t r o n accele-
3 ~ and
e a bem c u r r e n t s o f up t o 2pA have been e x t r a c t e d
with
75% p a r t i c l e t r a n s m i s s i o n through the machine.
Uma f o n t e de íons negativos de hél i o u t i l i zando vapor de potáss i o
par a t r o c a de carga f o i desenvolvida para s e r u t i l izada j u n t o com o Ace1
lerador Pelletron
.
Feixes de p a r t í c u l a s de 3 ~ ee a,
e n t r e 1 e 2 pA são e x t r a í d o s .
com
intensidade
Uma transmissão de 75% das
partículas
a t r a v é s do a c e l e r a d o r é consegui da
1. INTRODUCTION
Two-stage p a r t i c l e a c c e l e r a t o r s r e q u i r e t h e i n j e c t i o n o f negative ions.
I n t h e case o f helium, due t o t h e e l e c t r o n i c c o n f i g u r a t i o n
small a f f i n i t y o f an e x t r a e l e c t r o n , v e r y low e f f i c i e n c y
and
is
the
obtained
i n t h e f o r m a t i o n o f a n e g a t i v e ion.
The d e s i g n o f the n e g a t i v e h e l i u m i o n source, t o be used t o g e t h e r w i t h
t h e P e l l e t r o n a c c e l e r a t o r o f t h e U n i v e r s i t y o f São Paulo, was based on
t h e two s t e p process o f forming He- ions and on t h e
c h a r a c t e r i s t i c s o f t h e beam.
*
P o s t a l address: C.P.
20516, 01000 -São Paulo SP.
r e q u i red
optical
2. PRODUCTION OF He-
H o l 4 i e n and M i d t d a l
showed t h a t t h e (1s 2 s 2p) "p5,/,
s t a t e o f He- i s
3
~
rnetastable w i t h an e s t i m a t e d mean l i f e o f 1 . 7 ~ 1 0 - sec.
I n t h i s 4~,,2
s t a t e , t h e b i n d i n g energy o f t h e 2p e l e c t r o n i s o f t h e o r d e r o f
0.075
eV, Ref. 4, and a11 t h e o t h e r p o s s i b l e s t a t e s o f He- undergo decay by
auto- ionization,
i.e.,
decay by emission o f a f r e e e l e c t r o n .
Based o n t h i s argument, Donnal l y
e t ai!.
suggested t h a t negative
1iurn ions c o u l d be produced by a nearly- resonant two-step
he-
charge
ex-
change r e a c t i o n , as f o l lows:
( i ) Forrnation o f HeO (1s 2s)
3 ~ 1
f
rom He+ (1s) by a charge
exchange
t r a n s f e r c o l l i s i o n w i t h an a1,kali atom;
( i i) Forrnation o f He- from HeO (1s 2s)
3 ~ 1 by
elec-
pick- up o f a 2p
t ron:
He+(ls)
+ x O - + H e ( 1 ~ 2 s )+ x~ + ~ ~and
Charge exchange r e a c t i o n s a r e n e a r l y-resonant when t h e change
IAEl
t h e i n t e r n a 1 energy o f t h e c01 1 i d i n g system i s very smal 1. A
qual i t a -
of
t i v e i n t e r p r e t a t i o n o f t h i s f e a t u r e rnay be given i n terms o f t h e e x t e n t
t o which t h e c o l l i s i o n approximates t o a d i a b a t i c c o n d i t i o n s . This f a c t
i s a consequence o f t h e "Massey c r i t e r i o n ' I 6 which
states
that
the
c r o s s - s e c t i o n f o r a charge exchange r e a c t i o n i s maximum when t h e c o l l i s i o n t i m e i s o f t h e o r d e r o f t h e e l e c t r o n i c t r a n s i t i o n time.
C a l c u l a t i o n s based on t h i s c r i t e r i o n 7
and
s u ~ t s ~
i n d' i~c a t e t h a t , i n t h e case o f a
+
He
further
experimental
re-
primary beam, a s a t i s f a c -
t o r y e f f i c i e n c y can be reached when an a l k a l i element (vapor) i s
used
as t h e medium f o r t h e charge t r a n s f e r .
I n Table 1 a r e shown t h e c a l c u l a t e d
d i v i d u a l p i c k - u p r e a c t i o n s , i .e.,
ion.
7
resonance energy f o r t h e two
f o r m a t i o n o f t h e Heo-atom and
As b o t h r e a c t i o n s occur i n t h e same chamber they a r e n o t
inHe-
obser-
a
>
Ci
e-
l
i
n
.-m
N
a
-
r
ved i n d i v i d u a l l y , and t h e experimental resonance energy f o r
the
com-
o l e t e process i s shown i n t h e l a s t column o f Table 1 .
T d L e I . Resonance energy ( K ~ v )f o r :
lst
step:
znd
individual
'ormation o f ( l s 2 s )
Element
3
~
step:
1f o r m a t i o n
individual
*PIj2 ~ e -
]eo
Among t h e a l k a l i elements, potassium seems t o be t h e
because i t has t h e necessary vapor pressure12
(-140°c),
Exper i rnent a l
o f (ls2s2p)
at
more
convenient
low
temperature
i s e a s i e r t o handle, cheaper, and l e s s t o x i c .
3. THE APPARATUS
The n e g a t i v e h e l ium i o n source ( F i g . 1) c o n s i s t s e s s e n t i a l l y o f a
v e n t i o n a l R.F. p o s i t i v e h e l ium i o n source13 ( F i g . 1 :
ions a r e e x t r a c t e d when a p o s i t i v e b i a s V
P
3)
con-
+
He
from which
i s applied (Fig. 1 : 2).
e x t r a c t i o n e l e c t r o d e ( ~ i ~ . l : 6t )o g e t h e r w i t h an e i n z e l l e n s
An
(Fig.l:8)
i s used t o focus the posi t i v e beam i n t h e charge exchange chamber ( F i g .
:
l
O
T h i s chamber i s f i l l e d w i t h the a l k a l i vapor o b t a i n e d by
t i n g t h e potassium i n a s t a i n l e s s s t e e l oven ( ~ i g . 1:I 1 ) .
The
hea-
exchan-
ge chamber i s e l e c t r i c a l l y connected t o t h e externa1 e l e c t r o d e s o f the
einzel lens.
+
I n t h i s way, i t i s p o s s i b l e t o c o n t r o l t h e He beam ener-
gy so as t o maximize t h e e f f i c i e n c y o f t h e charge exchange process.
A
v e r y srnall c o n t a c t area between t h e chamber and i t s support reduces t h e
heat l o s s .
Fig. 2. He-/He
+
y i e l d and beam i n t e n s i t y vs. He
+
energy.
To prevent the condensation
of
potassium i n the high e l e c t r i c a l grad i aph ragms
d i e n t region o f the i o n source, two pai r s o f copper
are
used i n the entrance ( F i g . l :9) and e x i t (Fig.1 :lO) o f the charge
change chamber.
vapor,
To increase the e f f i c i e n c y o f trapping
of
ex-
potassium
t h e diaphragms are cooled by a freon exchanger c i r c u i tl',
:l4)
A second e i n z e l lens ( F g.l
magnetic analyser (Fig.
:17) i s employed t o separate
qmponents o f the beam, mainly
trons.
i s used i n the He- beam t r a n s p o r t . A
+
HeO, He
the
different
ions, contaminants
and
elec-
I n t h i s case, a 30° d e f l e c t i o n i s used, defined by a s l i t sys-
tem (Fig.l:19).
The negative helium
ions
o f 85
are then accelerated up t o an energy
KeV by means o f a t e n e l e c t r o d e a c c e l e r a t i n g tube.
This
final
beam
energy was chosen t o g e t the l a r g e s t possible overlap between the emittance15 o f the i o n source and t h e acceptance o f the Pel l e t r o n accelerator.
Thus a s a t i s f a c t o r y p a r t i c l e transmission through the machine
can
be
obtained.
As the charge exchange process i s nearly-resonant,
there e x i s t s an opI n Fig. 2, one
timum energy a t which the e f f i c i e n c y i s maximum.
see the dependence on t h e i n c i d e n t energy o f the y i e l d ( i .e
He-/He+ beam i n t e n s i t i e s )
.
c o l l i s i o n energy o f 11 KeV.
A maximum r a t i o o f
5% can be
can
the r a t i o
reached a t a
This r e s u l t i s i n agreement w i t h the
li-
m i t s predicated by the Massey c r i t e r i o n .
+
The t o t a l He- beam i n t e n s i t y as a f u n c t i o n o f He
sented i n Fig. 2.
energy i s a l s o
One can see t h a t t h e energy corresponding
highest beam i n t e n s i t y does not correspond t o the highest
He+
.
to
ratio
This i s due t o the f a c t t h a t the charge exchange chamber
prethe
~e-/
bias
i s the same as the e x t r a c t i o n e l e c t r o d e b i a s which a f f e c t s the extracted He+ beam i ntens i t y
.
F i g . 3; ~ e beam
i n t e n s i t y vs. potassium temperature.
The dimensions o f the charge exchange charnber a l l o w
a
95%
positive
beam transrnission. Negative heliurn bearn i n t e n s i t i e s o f the o r d e r o f 1-2
VA can be reached, depending on the c p e r a t i o n c o n d i t i o n s .
The potassium vapor pressure i n s i d e t h e charge exchange c e l l
c r i t i c a l ; w i t h a v e r y low pressure, two c o l l i s i o n s o f t h e He
is
+'
r i n g i t s t r a n s i t i n t h e c e l l a r e irnprobable; and w i t h a h i g h
ion
very
du-
pressure
t h e r e i s a l a r g e p r o b a b i l i t y o f l o s i n g the 2p e l e c t r o n (which
i s very
.
weakl y bound t o t h e He- atom: 0 .O75 e ~ )
The potassium vapor pressure i s c o n t r o l l e d by t h e oven heating c u r r e n t .
The oven and c e l l ternperatures a r e monitored by means o f an iron-const a n t a n thermocouple.
The r e l a t i o n between t h e n e g a t i v e bearn c u r r e n t and
the potassium c o n t a i n e r i s shown i n F i g u r e 3.
t h e temperature o f
The potassium
consurnp-
t i o n i s estirnated i n 100 mg per hour.
I t i s found necessary t o process t h e potassium b e f o r e use
i n the
ion
source.
until
the
The oven i s heated o u t o f the source, i n vacuum,
potassium m e l t s and a11 t h e o i l bubbles o u t .
I n normal o p e r a t i o n , a good vacuum
source.
Torr) i s required inside the
A change o f 50% i n t h i s pressure can be r e s p o n s i b l e f o r a r e -
d u c t i o n o f 20% i n t h e He- beam i n t e n s i t y .
The i o n source described i n t h i s a r t i c l e has been i n normal
operation
i n t h e Pel l e t r o n A c c e l e r a t o r o f t h e U n i v e r s i t y o f São Paulo since 1973,
4 ~ e -beams.
f o r t h e i n j e c t i o n o f 3 ~ e and
A p a r t i c l e t r a n s m i s s i o n o f 75% f o r 3 ~ and
e a beams i s normal l y obtained
The i o n source i s a b l e t o o p e r a t e d u r i n g a continuous p e r i o d
week.
of
When t h e potassiurn i n t h e oven i s exhausted, t h e c o n t a i n e r
be replaced e a s i l y w i t h o u t dismounting the i o n source.
one
can
The a u t h o r s would l i k e t o thank Joel P e r e i r a , V i c t o r H. Rotberg,
Fung Chen and A. T. Mendes f o r t h e i r c o l l a b o r a t i o n
in
the
Wang
project,
c o n s t r u c t i o n and t e s t s o f t h e i o n source.
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&
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