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Triply charged argon and neon ion beams from ion beams rf sources
NUCLEAR
INSTRUMENTS
AND
METHODS
(1978)
157
607-609
;
©
NORTH-HOLLAND
PUBLISHING
CO.
L E T T E R S TO T H E E D I T O R T R I P L Y C H A R G E D A R G O N AND N E O N ION BEAMS F R O M RF S O U R C E S E. J. KNYSTAUTASand R. LAPO1NTE Laboratoire de I'Acc~l~rateur Van de Graq[l~ D~partement de physique, Universit~ Laval, Quebec, Canada GIK 7P4
Received 15 September 1978 Slight modification of a classical rf ion source allows routine production of triply charged neon and argon ion beams in an upgraded (7 MV) CN Van de Graaff accelerator. Absolute current measurements yield analysed beam intensities in the 100-500 nA range. Identification of beam particles is verified by observing their far UV emission spectrum after passage through a thin carbon exciter foil. Essential requirements for producing such beams are a small but ~ritical reduction of the exit canal diameter and close monitoring of rf oscillator power. Since its inception, the radio frequency ion source has found a wide use in accelerators due in part to its simplicity and ease of operation. While it was first used for protons, deuterons, alphas and helium ions for nuclear reaction studies, needs for more exotic beams in other domains such as accelerator-based atomic physics and ion implantation extended its use to heavier ions, both singly arid doubly charged. For several years now, we have been using dou-
bly ionised beams of neonl), argon2), bromine 3) (from HBr) and krypton 3) produced by an rf source in the terminal of an upgraded (7 MV) CN Van de Graaff accelerator. Designed primarily for the production of copious beams ( - 1 g A ) of ~z-particles, the terminal is equipped with a 30 ° preselecting magnet, a feature which has greatly facilitated work with multiply charged ions. Sputtering of the exit canal by heavy ions, especially noble gas ions which have a high sputtering
Ar "I'++ ~
C
18 MeV
~r r,*-
-
__
-~
R
N
I
Fig. 1. Far UV spectrum showing known lines of highly ionised foil-excited argon, using an incident Ar + + + beam at 18 MeV.
608
E.
J.
KNYSTAUTAS
AND
yield, limits the useful life of an rf source in such applications to about a month of constant operation. Our sources are home-made and basically duplicate the standard long-bottle design made by High Voltage Engineering Corp. The metal flanges are first glued to the Pyrex tube with vinyl acetate, then sealed from the outside with Aremco 515. The first serves to reduce outgassing, the second to prevent leaks during the intense heating of the glass during operation. A series of tests has shown that reduction of the exit canal diameter from the standard 0.081" to 0.070" optimises the output of the source for multiply charged ions. Further experimentation showed that liners of tantalum in an aluminium housing give best results, although some .success was also obtained with stainless steel liners. This is evidently related to the sputtering rate, which gradually erodes the exit canal. In fact, a gradual deterioration of beam quality is often noted over several weeks of operation. Gas pressure in, the ion source is controlled by the,rmo-leaks, and its adjustment is critical for
R,
LAPOINTE
doubly charged ions and even more so for triply charged ones. The gas pressure for peak yield becomes more and more sharply defined as the desired ion charge increases. As no direct measure of source pressure exists, the power drawn by the discharge from the rf oscillator must be very closely monitored. The current drawn by the oscillator tubes is in fact the principal and most sensitive parameter determining the operating condition of the source. A 250 W, 125 MHz oscillator is used, and for maximum yield of multiply charged ions, the total (screen and plate) oscillator current must be maintained at 280-300 mA (plate voltage is 700 V). Although triply charged beams take 2-3 h of adjustment due to the extremely sensitive dependence on gas pressure mentioned above (due in part to the slow response of the thermo-leaks), once obtained, they will remain stable indefinitely without further operator intervention. Extraction voltage is generally -2.5 kV. A Faraday cup, based on the HVEC design, is used for absolute current measurements. Secondary electrons are suppressed by applying a nega-
18 M e V
Ne *++
* C
QD
d N
et
,t,J,
co
-"
t,
~l
C o<
I'--
0 ¢.3
]| /[
N CO
N ,.~1~I . < i:,,
o~ ~1"
I~
"
~ -
o~[~
~
[/ q ,~!I 8
o< q
z
e
h l
~ x
--
_ ,+
o< ,,
!
z
~
d
:
,~1
~5o Fig.
I~. *+
2. F o i l - e x c i t e d
2oo tar UV
spectrum
for
]8 MeV
neon.
x(J)
,,
zso
ARGON
AND
NEON
,TABLE 1 Results o f typical absolute current measurements of analysed neon and argon beams at 6 MV terminal voltage. Ne + Ne 2+ Ne 3+ Ne 4+
2.2/IA 3.5/2A 120 nA --
Ar + Ar 2+ Ar 3+ Ar 4+
a 1.5/~A 500 nA 5 nA
a Exceeds capacity of 90 ° analysing magnet (145 MeV.amu).
tive potential ( - 50 V) to a cylindrical shield which encloses the Faraday cup. Typical, though not necessarily maximum, yields for neon and argon beams are shown in table 1. Beams are analysed by a 90 ° magnet which has a maximum mass-energy product of 145MeV.amu. Although the resolving power of the analysing magnet is - 2 0 0 , care must be taken to ensure that a small uncertainty in terminal voltage does not lead to transmission of a beam with a nearby, but incorrect, mass-to-charge ratio. For example, Ar +++ beams have a mass-to-charge ratio of 13.3, close to 14 (that of N+), a plausible contaminant. The same applies to Ne +++ at an m / e of 6.7, which is very close to C ++ and N ++
ION BEAMS
609
To avoid this possibility, identification of the triply charged beams was verified by observing their far UV emission spectra after passage through a thin ( - 2 0 p g / c m 2) carbon foil. Figs. 1 and 2 show such spectra obtained with a 2.2 m grazing-incidence monochromator. The known lines of highly stripped argon and neon which appear leave no doubt as to the identity of the incident ions. An attempt was made to obtain quadruply charged neon and argon ions from the source, but intensities were of the order of a few nA, insufficient for our purposes. In conclusion, the useful energy range of a CN Van de Graaff accelerator has been extended from 7 to 21 MeV by a surprisingly small but apparently crucial reduction of the rf ion source exit canal diameter. References l) L. Barrette, E. J. Knystautas and R. Drouin, Nucl. Instr. and Meth. 110 (1973) 29. 2) E. J. Knystautas, R. Drouin and M. Druetta, Abstract, Annual Meeting, Optical Society of America (Toronto, Canada, Oct. 1977). 3) E. J. Knystautas and R. Drouin, J. Quant. Spectrosc. Radiat. Transfer 17 (1977) 551.
INSTRUMENTS
AND
METHODS
(1978)
157
607-609
;
©
NORTH-HOLLAND
PUBLISHING
CO.
L E T T E R S TO T H E E D I T O R T R I P L Y C H A R G E D A R G O N AND N E O N ION BEAMS F R O M RF S O U R C E S E. J. KNYSTAUTASand R. LAPO1NTE Laboratoire de I'Acc~l~rateur Van de Graq[l~ D~partement de physique, Universit~ Laval, Quebec, Canada GIK 7P4
Received 15 September 1978 Slight modification of a classical rf ion source allows routine production of triply charged neon and argon ion beams in an upgraded (7 MV) CN Van de Graaff accelerator. Absolute current measurements yield analysed beam intensities in the 100-500 nA range. Identification of beam particles is verified by observing their far UV emission spectrum after passage through a thin carbon exciter foil. Essential requirements for producing such beams are a small but ~ritical reduction of the exit canal diameter and close monitoring of rf oscillator power. Since its inception, the radio frequency ion source has found a wide use in accelerators due in part to its simplicity and ease of operation. While it was first used for protons, deuterons, alphas and helium ions for nuclear reaction studies, needs for more exotic beams in other domains such as accelerator-based atomic physics and ion implantation extended its use to heavier ions, both singly arid doubly charged. For several years now, we have been using dou-
bly ionised beams of neonl), argon2), bromine 3) (from HBr) and krypton 3) produced by an rf source in the terminal of an upgraded (7 MV) CN Van de Graaff accelerator. Designed primarily for the production of copious beams ( - 1 g A ) of ~z-particles, the terminal is equipped with a 30 ° preselecting magnet, a feature which has greatly facilitated work with multiply charged ions. Sputtering of the exit canal by heavy ions, especially noble gas ions which have a high sputtering
Ar "I'++ ~
C
18 MeV
~r r,*-
-
__
-~
R
N
I
Fig. 1. Far UV spectrum showing known lines of highly ionised foil-excited argon, using an incident Ar + + + beam at 18 MeV.
608
E.
J.
KNYSTAUTAS
AND
yield, limits the useful life of an rf source in such applications to about a month of constant operation. Our sources are home-made and basically duplicate the standard long-bottle design made by High Voltage Engineering Corp. The metal flanges are first glued to the Pyrex tube with vinyl acetate, then sealed from the outside with Aremco 515. The first serves to reduce outgassing, the second to prevent leaks during the intense heating of the glass during operation. A series of tests has shown that reduction of the exit canal diameter from the standard 0.081" to 0.070" optimises the output of the source for multiply charged ions. Further experimentation showed that liners of tantalum in an aluminium housing give best results, although some .success was also obtained with stainless steel liners. This is evidently related to the sputtering rate, which gradually erodes the exit canal. In fact, a gradual deterioration of beam quality is often noted over several weeks of operation. Gas pressure in, the ion source is controlled by the,rmo-leaks, and its adjustment is critical for
R,
LAPOINTE
doubly charged ions and even more so for triply charged ones. The gas pressure for peak yield becomes more and more sharply defined as the desired ion charge increases. As no direct measure of source pressure exists, the power drawn by the discharge from the rf oscillator must be very closely monitored. The current drawn by the oscillator tubes is in fact the principal and most sensitive parameter determining the operating condition of the source. A 250 W, 125 MHz oscillator is used, and for maximum yield of multiply charged ions, the total (screen and plate) oscillator current must be maintained at 280-300 mA (plate voltage is 700 V). Although triply charged beams take 2-3 h of adjustment due to the extremely sensitive dependence on gas pressure mentioned above (due in part to the slow response of the thermo-leaks), once obtained, they will remain stable indefinitely without further operator intervention. Extraction voltage is generally -2.5 kV. A Faraday cup, based on the HVEC design, is used for absolute current measurements. Secondary electrons are suppressed by applying a nega-
18 M e V
Ne *++
* C
QD
d N
et
,t,J,
co
-"
t,
~l
C o<
I'--
0 ¢.3
]| /[
N CO
N ,.~1~I . < i:,,
o~ ~1"
I~
"
~ -
o~[~
~
[/ q ,~!I 8
o< q
z
e
h l
~ x
--
_ ,+
o< ,,
!
z
~
d
:
,~1
~5o Fig.
I~. *+
2. F o i l - e x c i t e d
2oo tar UV
spectrum
for
]8 MeV
neon.
x(J)
,,
zso
ARGON
AND
NEON
,TABLE 1 Results o f typical absolute current measurements of analysed neon and argon beams at 6 MV terminal voltage. Ne + Ne 2+ Ne 3+ Ne 4+
2.2/IA 3.5/2A 120 nA --
Ar + Ar 2+ Ar 3+ Ar 4+
a 1.5/~A 500 nA 5 nA
a Exceeds capacity of 90 ° analysing magnet (145 MeV.amu).
tive potential ( - 50 V) to a cylindrical shield which encloses the Faraday cup. Typical, though not necessarily maximum, yields for neon and argon beams are shown in table 1. Beams are analysed by a 90 ° magnet which has a maximum mass-energy product of 145MeV.amu. Although the resolving power of the analysing magnet is - 2 0 0 , care must be taken to ensure that a small uncertainty in terminal voltage does not lead to transmission of a beam with a nearby, but incorrect, mass-to-charge ratio. For example, Ar +++ beams have a mass-to-charge ratio of 13.3, close to 14 (that of N+), a plausible contaminant. The same applies to Ne +++ at an m / e of 6.7, which is very close to C ++ and N ++
ION BEAMS
609
To avoid this possibility, identification of the triply charged beams was verified by observing their far UV emission spectra after passage through a thin ( - 2 0 p g / c m 2) carbon foil. Figs. 1 and 2 show such spectra obtained with a 2.2 m grazing-incidence monochromator. The known lines of highly stripped argon and neon which appear leave no doubt as to the identity of the incident ions. An attempt was made to obtain quadruply charged neon and argon ions from the source, but intensities were of the order of a few nA, insufficient for our purposes. In conclusion, the useful energy range of a CN Van de Graaff accelerator has been extended from 7 to 21 MeV by a surprisingly small but apparently crucial reduction of the rf ion source exit canal diameter. References l) L. Barrette, E. J. Knystautas and R. Drouin, Nucl. Instr. and Meth. 110 (1973) 29. 2) E. J. Knystautas, R. Drouin and M. Druetta, Abstract, Annual Meeting, Optical Society of America (Toronto, Canada, Oct. 1977). 3) E. J. Knystautas and R. Drouin, J. Quant. Spectrosc. Radiat. Transfer 17 (1977) 551.