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Operation of a radio-frequency ion source in a tandem electrostatic accelerator
Nuclear Instmments end Methods in Physics Research A 372 (19%) 280-282
NUCLEAR
INSTRUMENTS & METHODS IN PHYSICS RESEARCH Se&on A
ELSEVfER
Operation of a radio-frequency ion source in a tandem electrostatic accelerator E.G. Myers*, J.K. Thompson, P.A. Allen, I?W.E. Barber, G.A. Brown, V.S. Griffin, B.G. Schmidt, S.W. Trimble Depomnent
of Physics.
Florida State University, Tallahassee, FL 32306, USA
Received 28 August 1995
Abstract A radio-frequency ion source using electrostatic inflection has been operated in the terminal of a model FN tandem Van de Graaff accelerator. This atrangement is useful for high resolution resonance experiments in atomic and low energy nuclear physics and for hydrogen profiling using the the 6.4 MeV ‘H( 15N,cwy)‘*C reaction.
1, Introduction
2. Ion source and inflection
The single stage Van de Graaff electrostatic accelerator, using a radio-frequency discharge source in the high voltage terminal, preceded the development of the tandem accelerator [ 1] which uses a foil or gas stripper and an external negative ion source. Because of the advantages of higher beam energies, larger range of ions that can be accelerated and the greater convenience of maintaining the ion source, most larger and many modem smaller electrostatic accelerators use the tandem principle. However, for certain applications, single stage machines have adv~~ges of higher beam currents and reduced energy spread. This is particularly true for the acceleration of nitrogen which does not form a stable negative ion. To produce nitrogen beams with a tandem requires the injection of a molecular ion such as CN- or NH-. This results in ioss of transmission and an increase in energy spread due to the “Coulomb explosion” [ 21 following stripping. Hence, in order to produce a low energy spread 6-7 MeV N+ beam for a precision atomic spectroscopy experiment [ 31, we have constructed and operated an RF source with an electrostatic deflector which can be temporarily installed in the terminal of the Florida State Universi~ EN taudem accelerator. With a gas feed of “Na the arrangement is useful for hydrogen profiling using the 6.4 MeV ‘H( 15N,c~y)‘~C reaction with depth resolutions of a few nm [4,5]. The source is also useful for nuclear physics experiments requiring the acceleration of noble gas ions including 3He.
A simplified drawing of the ion source, extraction and inflection arrangement is shown in Fig. 1. The source and inflector assembly are attached to either side of a 24 cm diameter flange which mounts to the side of the stripper foil
* Corresponding author. 0168~90@2/96/$1.5.00@ 1996 Elsevier Science B.V. All rights reserved SSDlO168-9002(95)01389-X
-
E 0
ii?
RF-Source
Fig. 1. Simplified drawing of the ion source and inflection system.
E.G. Myers et ol./Nucl.
Instr. and Meth. in Phys. Res. A 372 (1996) 280-282
assembly housing, the foil assembly having been removed. The source axis is horizontal, at 90” to the accelerator axis, and the ions are inflected onto the axis using an electrostatic deflector. This is more practical than removing the foil housing and mounting the source directly to the end of the acceleration tube since, in this case, it would be necessary to greatly increase the axial support of all the acceleration tubes. The ion source consisted of a standard rebuilt HVEC ion source tube [6] fitted with a 1.8 mm i.d., 12.5 mm long aluminum exit canal. The source was supplied with gas from a 1 liter at STP capacity cylinder through a mechanical leak controlled via a plexiglass rod. The source was mounted on a short column consisting of two 7.5 cm long ceramic spacers in between which was sandwiched a flange which held the focus electrode. This was modeled on the HVEC design used in single ended machines and had an aperture of 7.6 mm. The 90°, 15 cm radius of curvature, 2.5 cm gap electrostatic deflector had electrodes with concentric spherical inner surfaces. Such an arrangement is double focusing and has object and image planes a distance equal to the radius of curvature from the entrance and exit planes respectively [ 71. The plates were milled out of aluminum and were mounted to the main support flange using side pieces of G-10 fiberglass. Also mounted to the side pieces were two grounded electrodes which terminated the fringe fields at the entrance and exit of the main electrodes, and also two pairs of trim deflector electrodes, to produce small vertical deflections and displacements of the ion beam. Instead of using an additional set of deflectors, small horizontal displacements of the ion beam were produced by varying the balance between the nearly equal and opposite voltages applied to the main 90” electrodes. The distance from the exit plane of the spherical deflector to the first active plane of the acceleration tube was 50 cm. In order to improve the matching of the ion beam into the tubes, which in our machine are of the spiral inclined plane type, the resistive grading of the first ten planes was halved. 3. Power supplies and control electronics The power supplies and control electronics for the source in the accelerator terminal are required to operate in 7 atm of SF6 insulating gas and to withstand pressure cycling down to 100 mTorr, as occurs during gas transfer. The electmnics must withstand voltage surges due to high voltage sparks up to terminal voltages of 9 MV. They must also run from 400 Hz, 110 vat as supplied by the terminal alternator. The ion source discharge was sustained with a nominally 40 W RFoscillator similar to the standard HVEC design [ 61. It used two 6146B tubes and was completely reliable. The anode pin was biased to a fixed voltage of 4.4 kV with respect to the source body using a 15 W, switching regulated power supply [ 81. Both this supply and the RF oscillator were mounted on a small subplatform electrically connected
281
to the source tube base plate and received power via a 30 kV isolation transformer. All the other high and low voltage supplies, and the control electronics, were installed in an aluminum protective housing 60 cm x 55 cm x 17.5 cm. Four more 15 W HV supplies [S] were used to respectively bias the source, the focus electrode, and the inner and outer 90” deflector electrodes. Qpical operating voltages were -l-15 kV, +17 kV, -3.2 kV and +3.3 kV respectively. ?ivo pairs of positive and negative 2 kV, 4 W supplies were used for the two pairs of trim deflectors. Polarity reversal was obtained using a pair of high voltage vacuum relays. These eight HV power supplies, and the high voltage relays, were controlled using a commercial control interface [9], and an RS-232 fiber optic link, from an external IBM compatible personal computer. The interface consisted of a controller card together with an eight channel, 12 bit DAC card, and an eight channel relay card. The fiber optic ran along the “low energy” column of the tandem, and through a feedthrough in the accelerator tank wall. The program which allowed the operator to control the source was written in QuickBasic. Considerable effort was exerted to protect the electronics in the terminal from damage due to accelerator sparks. The main housing received its power from the alternator via a pair of commercial surge suppressors. The only other electrical penetrations through this housing were co-axial high voltage output leads whose shields were continuous to the supplies and solidly grounded at the housing wall. The interface electronics, whose microprocessors were exceptionally sensitive to spark induced damage, and the fiber optic modem, were installed in a second aluminum housing located inside the main housing. All low voltage power supply and control output leads were fed through the wall of this inner housing using low pass filter feedthroughs. All power supplies were solidly grounded and all cabling was kept close to a grounded plane. The fiber optic ran fmm the modem in the inner housing, along the inside of the corona rings at the bottom of the “low energy” column and then through a feedthrough in the accelerator tank wall. A commercial fiber optic cable was found to be unsuitable due to high voltage breakdown in the cladding. However, the bare, unclad, plastic coated 60 ,um core, 125 pm total diameter fiber remained undamaged provided it was maintained under light tension. This prevented electrostatic forces causing a loop of the fiber to protrude through the rings where it is subject to damage by sparks. The tank wall feedthrough consisted of a 10 cm long plug through which a tapered hole was drilled. The fiber was run through the hole and sealed with epoxy. 4. Operational experience The terminal ion source has been operated with both 14N2 and 15Nz supply gas for a total of over 200 h at terminal voltages from 2 to 8 MV. N+ beam currents of up to 500 nA,
282
E.G. Myers et d/Nucl.
Instr. and Meth. in Phys. Res. A 372 (1996) 280-282
after analysis around a 90” magnet, were obtained from a total, unanalysed current measured at the exit of the accelerator of 15 PA. With the source,gas pressure optimized the vacuum near the electrostatic deflector in the terminal was approximately 5 x lo-’ Ton-, while that at the accelerator exit, which was pumped by a 15 cm diameter cryopump, was approximately 5 x 10B7 Tom At these pressures, loading of the acceleration tubes was found to correlate more with increased beam current than with increased gas flow. At the end of the run period there was no significant degradation in the beam output and quality. However, on source disassembly the aluminum exit canal was found to be somewhat eroded, its internal diameter having increased by about 10%. In order to produce beams of N5+ for Doppler tuned laser spectroscopy on helium-like nitrogen [ 33, the beam was stripped with a 5-10 pg carbon foil before the analysing magnet. Energy resonance widths as small as 4 keV FWHM at 6 to 7 MeV were measured, which can be ascribed to instability in the terminal voltage stabilization and energy loss straggling in the foil. We aim to further reduce this spread by improved voltage stabilization and using gas stripping. The energy spread from the source, from off-line measurements using the electrostatic deflector as an energy analyser, was observed to be less than 100 eV, which is consistent with other measurements with this type of source [ 121. We also used a “N beam to test the calibration of the 90” analysing magnet by exciting the ‘H( r5N, cuy)‘*C resonance near 6.4 MeV [ 10,111 in a thin (approx. 20 mTorr) hydrogen gas target. Although we did not apply the system for hydrogen depth profiling using this reaction, we note that our energy resolution is well below the 5-15 keV width of the resonance which is dominated by thermal motion of the hydrogen [ 131.
Acknowledgement We acknowledge useful conversations with G. Harper (University of Washington) and G. Norton (NRC Inc.), and also the work of the FSU Nuclear Research Machine Shop. This work was partly supported by the National Science Foundation and the State of Florida.
References [ 11 RJ. Van de Graaff, Nucl. Iostr. and Meth. 8 (1960) 195. [2] R Middleton, in: Proc. Symp. North Eastern Accelerator Personnel, Los Alamos, 1977, ed. R. Woods, p. 15. [3] E.G. Myers, J.K. Thompson,E.P. Gavathas,N.R. Claussen, J.D. Silver and D.J.H. Ho&, Phys. Rev. Lett. 75 (1995) 3637. 141 W.A. Lanford, X.S. Guo and K. Rodbell, Proc. Workshop on High Energy and Heavy Ion Beams in Materials Analysis, Albuquerque, New Mexico, 1989, eds. JR. Tesmer et al. (Materials Research Society, Pittsburgh,PA) p. 203. [5] W.A. Lanford, HP. Trautvetter,J.F. Ziegler and Keller, Appl. Phys. I&t. 28 (1976) 566. 161 See, for example, High Voltage Engineering Corp. literature on the model AK or KN Van de GraaIf accelerators. Such ion sources can be. obtained from various suppliers. [7] H. Wollnik, in: Focusing of ChargedParticles, Vol. 2, ed. A. Septier (Academic Press, London, 1967) p. 164. [S] Glassman, Inc., MJ series. [ 91 Keithley-Metrabyte“Metrabus”. IlO] E.J. Evers, J.W. De Vries, G.A.P. Engelbertinkand C. Van der Leun, Nucl. In&r.and Meth. A 257 (1987) 91. Nucl. Insu. and Medr. I1 l] T. Osipowics, K.P. Lieb and S. B~es~ermann, B 18 (1987) 232. [ 121 L. Valyi. Atom and Ion Sources (Wiley-Interscience, London, 1977) p. 334. [ 131 K.M. HON and W.A. Lanford, Nucl. Instr. and Meth. B 29 (1988) 609.
NUCLEAR
INSTRUMENTS & METHODS IN PHYSICS RESEARCH Se&on A
ELSEVfER
Operation of a radio-frequency ion source in a tandem electrostatic accelerator E.G. Myers*, J.K. Thompson, P.A. Allen, I?W.E. Barber, G.A. Brown, V.S. Griffin, B.G. Schmidt, S.W. Trimble Depomnent
of Physics.
Florida State University, Tallahassee, FL 32306, USA
Received 28 August 1995
Abstract A radio-frequency ion source using electrostatic inflection has been operated in the terminal of a model FN tandem Van de Graaff accelerator. This atrangement is useful for high resolution resonance experiments in atomic and low energy nuclear physics and for hydrogen profiling using the the 6.4 MeV ‘H( 15N,cwy)‘*C reaction.
1, Introduction
2. Ion source and inflection
The single stage Van de Graaff electrostatic accelerator, using a radio-frequency discharge source in the high voltage terminal, preceded the development of the tandem accelerator [ 1] which uses a foil or gas stripper and an external negative ion source. Because of the advantages of higher beam energies, larger range of ions that can be accelerated and the greater convenience of maintaining the ion source, most larger and many modem smaller electrostatic accelerators use the tandem principle. However, for certain applications, single stage machines have adv~~ges of higher beam currents and reduced energy spread. This is particularly true for the acceleration of nitrogen which does not form a stable negative ion. To produce nitrogen beams with a tandem requires the injection of a molecular ion such as CN- or NH-. This results in ioss of transmission and an increase in energy spread due to the “Coulomb explosion” [ 21 following stripping. Hence, in order to produce a low energy spread 6-7 MeV N+ beam for a precision atomic spectroscopy experiment [ 31, we have constructed and operated an RF source with an electrostatic deflector which can be temporarily installed in the terminal of the Florida State Universi~ EN taudem accelerator. With a gas feed of “Na the arrangement is useful for hydrogen profiling using the 6.4 MeV ‘H( 15N,c~y)‘~C reaction with depth resolutions of a few nm [4,5]. The source is also useful for nuclear physics experiments requiring the acceleration of noble gas ions including 3He.
A simplified drawing of the ion source, extraction and inflection arrangement is shown in Fig. 1. The source and inflector assembly are attached to either side of a 24 cm diameter flange which mounts to the side of the stripper foil
* Corresponding author. 0168~90@2/96/$1.5.00@ 1996 Elsevier Science B.V. All rights reserved SSDlO168-9002(95)01389-X
-
E 0
ii?
RF-Source
Fig. 1. Simplified drawing of the ion source and inflection system.
E.G. Myers et ol./Nucl.
Instr. and Meth. in Phys. Res. A 372 (1996) 280-282
assembly housing, the foil assembly having been removed. The source axis is horizontal, at 90” to the accelerator axis, and the ions are inflected onto the axis using an electrostatic deflector. This is more practical than removing the foil housing and mounting the source directly to the end of the acceleration tube since, in this case, it would be necessary to greatly increase the axial support of all the acceleration tubes. The ion source consisted of a standard rebuilt HVEC ion source tube [6] fitted with a 1.8 mm i.d., 12.5 mm long aluminum exit canal. The source was supplied with gas from a 1 liter at STP capacity cylinder through a mechanical leak controlled via a plexiglass rod. The source was mounted on a short column consisting of two 7.5 cm long ceramic spacers in between which was sandwiched a flange which held the focus electrode. This was modeled on the HVEC design used in single ended machines and had an aperture of 7.6 mm. The 90°, 15 cm radius of curvature, 2.5 cm gap electrostatic deflector had electrodes with concentric spherical inner surfaces. Such an arrangement is double focusing and has object and image planes a distance equal to the radius of curvature from the entrance and exit planes respectively [ 71. The plates were milled out of aluminum and were mounted to the main support flange using side pieces of G-10 fiberglass. Also mounted to the side pieces were two grounded electrodes which terminated the fringe fields at the entrance and exit of the main electrodes, and also two pairs of trim deflector electrodes, to produce small vertical deflections and displacements of the ion beam. Instead of using an additional set of deflectors, small horizontal displacements of the ion beam were produced by varying the balance between the nearly equal and opposite voltages applied to the main 90” electrodes. The distance from the exit plane of the spherical deflector to the first active plane of the acceleration tube was 50 cm. In order to improve the matching of the ion beam into the tubes, which in our machine are of the spiral inclined plane type, the resistive grading of the first ten planes was halved. 3. Power supplies and control electronics The power supplies and control electronics for the source in the accelerator terminal are required to operate in 7 atm of SF6 insulating gas and to withstand pressure cycling down to 100 mTorr, as occurs during gas transfer. The electmnics must withstand voltage surges due to high voltage sparks up to terminal voltages of 9 MV. They must also run from 400 Hz, 110 vat as supplied by the terminal alternator. The ion source discharge was sustained with a nominally 40 W RFoscillator similar to the standard HVEC design [ 61. It used two 6146B tubes and was completely reliable. The anode pin was biased to a fixed voltage of 4.4 kV with respect to the source body using a 15 W, switching regulated power supply [ 81. Both this supply and the RF oscillator were mounted on a small subplatform electrically connected
281
to the source tube base plate and received power via a 30 kV isolation transformer. All the other high and low voltage supplies, and the control electronics, were installed in an aluminum protective housing 60 cm x 55 cm x 17.5 cm. Four more 15 W HV supplies [S] were used to respectively bias the source, the focus electrode, and the inner and outer 90” deflector electrodes. Qpical operating voltages were -l-15 kV, +17 kV, -3.2 kV and +3.3 kV respectively. ?ivo pairs of positive and negative 2 kV, 4 W supplies were used for the two pairs of trim deflectors. Polarity reversal was obtained using a pair of high voltage vacuum relays. These eight HV power supplies, and the high voltage relays, were controlled using a commercial control interface [9], and an RS-232 fiber optic link, from an external IBM compatible personal computer. The interface consisted of a controller card together with an eight channel, 12 bit DAC card, and an eight channel relay card. The fiber optic ran along the “low energy” column of the tandem, and through a feedthrough in the accelerator tank wall. The program which allowed the operator to control the source was written in QuickBasic. Considerable effort was exerted to protect the electronics in the terminal from damage due to accelerator sparks. The main housing received its power from the alternator via a pair of commercial surge suppressors. The only other electrical penetrations through this housing were co-axial high voltage output leads whose shields were continuous to the supplies and solidly grounded at the housing wall. The interface electronics, whose microprocessors were exceptionally sensitive to spark induced damage, and the fiber optic modem, were installed in a second aluminum housing located inside the main housing. All low voltage power supply and control output leads were fed through the wall of this inner housing using low pass filter feedthroughs. All power supplies were solidly grounded and all cabling was kept close to a grounded plane. The fiber optic ran fmm the modem in the inner housing, along the inside of the corona rings at the bottom of the “low energy” column and then through a feedthrough in the accelerator tank wall. A commercial fiber optic cable was found to be unsuitable due to high voltage breakdown in the cladding. However, the bare, unclad, plastic coated 60 ,um core, 125 pm total diameter fiber remained undamaged provided it was maintained under light tension. This prevented electrostatic forces causing a loop of the fiber to protrude through the rings where it is subject to damage by sparks. The tank wall feedthrough consisted of a 10 cm long plug through which a tapered hole was drilled. The fiber was run through the hole and sealed with epoxy. 4. Operational experience The terminal ion source has been operated with both 14N2 and 15Nz supply gas for a total of over 200 h at terminal voltages from 2 to 8 MV. N+ beam currents of up to 500 nA,
282
E.G. Myers et d/Nucl.
Instr. and Meth. in Phys. Res. A 372 (1996) 280-282
after analysis around a 90” magnet, were obtained from a total, unanalysed current measured at the exit of the accelerator of 15 PA. With the source,gas pressure optimized the vacuum near the electrostatic deflector in the terminal was approximately 5 x lo-’ Ton-, while that at the accelerator exit, which was pumped by a 15 cm diameter cryopump, was approximately 5 x 10B7 Tom At these pressures, loading of the acceleration tubes was found to correlate more with increased beam current than with increased gas flow. At the end of the run period there was no significant degradation in the beam output and quality. However, on source disassembly the aluminum exit canal was found to be somewhat eroded, its internal diameter having increased by about 10%. In order to produce beams of N5+ for Doppler tuned laser spectroscopy on helium-like nitrogen [ 33, the beam was stripped with a 5-10 pg carbon foil before the analysing magnet. Energy resonance widths as small as 4 keV FWHM at 6 to 7 MeV were measured, which can be ascribed to instability in the terminal voltage stabilization and energy loss straggling in the foil. We aim to further reduce this spread by improved voltage stabilization and using gas stripping. The energy spread from the source, from off-line measurements using the electrostatic deflector as an energy analyser, was observed to be less than 100 eV, which is consistent with other measurements with this type of source [ 121. We also used a “N beam to test the calibration of the 90” analysing magnet by exciting the ‘H( r5N, cuy)‘*C resonance near 6.4 MeV [ 10,111 in a thin (approx. 20 mTorr) hydrogen gas target. Although we did not apply the system for hydrogen depth profiling using this reaction, we note that our energy resolution is well below the 5-15 keV width of the resonance which is dominated by thermal motion of the hydrogen [ 131.
Acknowledgement We acknowledge useful conversations with G. Harper (University of Washington) and G. Norton (NRC Inc.), and also the work of the FSU Nuclear Research Machine Shop. This work was partly supported by the National Science Foundation and the State of Florida.
References [ 11 RJ. Van de Graaff, Nucl. Iostr. and Meth. 8 (1960) 195. [2] R Middleton, in: Proc. Symp. North Eastern Accelerator Personnel, Los Alamos, 1977, ed. R. Woods, p. 15. [3] E.G. Myers, J.K. Thompson,E.P. Gavathas,N.R. Claussen, J.D. Silver and D.J.H. Ho&, Phys. Rev. Lett. 75 (1995) 3637. 141 W.A. Lanford, X.S. Guo and K. Rodbell, Proc. Workshop on High Energy and Heavy Ion Beams in Materials Analysis, Albuquerque, New Mexico, 1989, eds. JR. Tesmer et al. (Materials Research Society, Pittsburgh,PA) p. 203. [5] W.A. Lanford, HP. Trautvetter,J.F. Ziegler and Keller, Appl. Phys. I&t. 28 (1976) 566. 161 See, for example, High Voltage Engineering Corp. literature on the model AK or KN Van de GraaIf accelerators. Such ion sources can be. obtained from various suppliers. [7] H. Wollnik, in: Focusing of ChargedParticles, Vol. 2, ed. A. Septier (Academic Press, London, 1967) p. 164. [S] Glassman, Inc., MJ series. [ 91 Keithley-Metrabyte“Metrabus”. IlO] E.J. Evers, J.W. De Vries, G.A.P. Engelbertinkand C. Van der Leun, Nucl. In&r.and Meth. A 257 (1987) 91. Nucl. Insu. and Medr. I1 l] T. Osipowics, K.P. Lieb and S. B~es~ermann, B 18 (1987) 232. [ 121 L. Valyi. Atom and Ion Sources (Wiley-Interscience, London, 1977) p. 334. [ 131 K.M. HON and W.A. Lanford, Nucl. Instr. and Meth. B 29 (1988) 609.