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Extraction of variable energy neutral hydrogen beams from the University of Manitoba Cyclotron
302
Nuclear Instruments and Methods in Physics Research A253 (1987) 302-304 North-Holland, Amsterdam
Letter to the Editor
E X T R A C T I O N OF VARIABLE ENERGY N E U T R A L H Y D R O G E N B E A M S FROM THE UNIVERSITY OF MANITOBA CYCLOTRON J.J.G. D U R O C H E R , G.R. S M I T H , V.P. D E R E N C H U K a n d J.R. A N D E R S O N The University of Manitoba Accelerator Laboratory, Department of Physics, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Received 5 August 1986
Neutral hydrogen beams with energies between 30 and 50 MeV and fluxes on target of up to 4×1012 H°/(s cm2) have been successfully extracted from the University of Manitoba Cyclotron. Upon completion in October 1986, the facility is expected to deliver 10 to 50 MeV H ° beams with fluxes in excess of 10TM H°/s cm2).
The hydrogen atom is undoubtedly the most widely investigated of all atomic species. The atom and its ions have been studied extensively from both theoretical and experimental view points. Beams of hydrogen are also widely used as probes in the investigation of other atomic and nuclear systems. In the past such beams have been made up of either positive (i.e. protons) or negative hydrogen ions accelerated to the desired energy. Recently, interest has arisen in the investigation of neutral hydrogen atoms moving at relativistic velocities, Such particles could be used in studies including investigation of: fundamental electromagnetic interactions with materials [1], the basic study of the relativistic atoms' behavior [2], in-flight beam polarization [3], high energy ion implantation. A major collaborative experimental program is presently under way, in conjunction with members of Oak Ridge National Laboratory, studying optical radiation emitted from metallic and nonmetallic surfaces bombarded with such H ° beams, A program was initiated at the University of Manitoba Accelerator Laboratory in late 1984 to develop a 10 to 50 MeV variable energy neutral hydrogen beam facility for use in such experiments. The successful implementation of the second stage of development of such a facility was completed in March 1986. The University of Manitoba Spiral Ridge Cyclotron is an H - accelerator capable of extracting 20 to 50 MeV protons through use of a stripping foil Therefore, production of a beam of neutral hydrogen atoms requires that one electron be removed from the H - ion. This extra electron is bound with art energy of 0.75 eV [4]. The method adopted for the removal of the electron was to pass the H - beam through a stripping medium which would not strip the second, more tightly bound (13.65 eV [4]) electron. 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The project has entered the final stage of a three stage development program that is scheduled for completion in October 1986, and that is summarized in table 1. The development stages consist of different methods of stripping the circulating H - beam in the cyclotron. Initially, the beam was stripped using the residual gas within the cyclotron. The efficiency of this process was of the order of 0.01% for a 10 6 Torr residual gas pressure in the cyclotron. An extensive study was carried out at this stage to determine the effects of residual gas pressure and the cyclotron's magnetic field on the H ° extraction efficiency. The first experiments using neutral hydrogen beams were also performed at this time using the approximately 10 9 H ° / ( s cm2) available from residual gas stripping. The second stage of development involved stripping the H - beam with a thin carbon foil (thickness = 5 /zg/cm 2) which yielded a stripping efficiency of approximately 5% at 40 MeV. Leading up to this stage extensive calculations of the H - beam dynamics within the cyclotron were carried out to determine the optimum H - focussing parameters and stripping foil positions. These calculations were carried out using a FORTRAN program, NEUTRA-STRIP, written by one of the authors (JJGD) and based on programs used to calculate the central region H - orbits in the cyclotron
Table 1 Scheduleof development program Stage
Stripping medium
1 2 3
Residual gas Carbon foil Metal vapour
H° f l u x s-lcm-2 1.3 X 1 0 9 4 × 1012 > 1014
Operational as of December1985 March1986 October 1986
303
J.J.G. Durocher et al. / Extraction of hydrogen beams
[5]. The program made use of the magnetic field data obtained in a recent field mapping and upgrading of the University of Manitoba Cyclotron [6]. The beamline and associated experimental structures used in stage 2 are illustrated in fig. 1. Also shown are the stripping foil probe assembly and the calculated stripping points within the cyclotron for production of 20, 35 and 50 MeV neutral beams, along with equilibrium orbits for H - ions of those energies. The H ° extraction beamline is placed alongside the proton exit beamline and sits within the cyclotron vault. Auxiliary structures are required within the cyclotron to intercept the H + beam that also emerges from the carbon stripping foil (identified as the proton probe in the diagram), The energy of the extracted H ° beam is varied by moving the carbon stripping foil to the previously calculated position on the desired energy H - orbit. The stripping point chosen is that which results in the neutral beam (which of course cannot be focussed or steered once created) proceeding in a straight line to the exit port on the cyclotron. Since the H ° beam will arrive at different angles with respect to the exit port for different energies, the experimental beam line is pivoted about the exit port in order to provide the straight line path needed by the emerging particles, The extracted H ° particles pass through 12 mm diameter collimators located at the entrance and exit of the transport pipe and which define the horizontal
extent of the beam. The collimators allow passage of some 25% of the generated H ° flux. The height of the beam is determined by the vertical momentum component of the circulating H - beam within the cyclotron at the stripping point. The vertical momentum at a point may be minimized by adjusting the cyclotron magnetic field to optimize the vertical focussing of the H - beam. This resulted in a 4 by 12 mm elliptically shaped beamspot at the target site. The target site is located in an experimental chamber some 1.85 m from the stripping point in the cyclotron. While passing through the target the H ° atoms are stripped of their remaining electron and the resultant proton beam is monitored to determine the initial H ° flux. This is accomplished through the use of two current monitors: a secondary electron emission monitor and a Faraday cup. The stage 2 neutral beam facility has also been used to conduct a number of experiments. An active collaboration between the University of Manitoba and Oak Ridge National Laboratory has been involved in using the H ° beam as a probe to study optical radiation emitted from metallic and nonmetallic surfaces [1]. An extensive experimental system has been assembled for these experiments. Software has been written which monitors the H ° beam on target, controls the monochrometer apparatus during the scans and collects, displays and does a preliminary analysis on the experimental data. To date some 400 spectra have been accumulated for optical radiation emitted from the surfaces of some twenty different targets bombarded with H ° beams. These data are currently being analysed in preparation for publication. In the third stage of development of the Neutral Hydrogen Beam Facility the carbon stripping foil is to be replaced by a variable thickness metal vapor stripper. With the use of such a system stripping efficiencies greater than 80% are anticipated for all H - energies. Since the cyclotron is potentially capable of accelerating
up o 0
Fig. 1. The University of Manitoba H ° Facility. Layout of the University of Manitoba Cyclotron showing H ° facility: 1, probe assembly for stripping H- beam; 2, 3, 4, equilibrium orbits for 50, 35 and 20 MeV H- beams. Stripping points are indicated by +. 5, proton pickoff probe; 6, bellows; 7, 12 mm diameter collimators; 8, experimental chamber; 9, beam current monitors,
o
we xpe oac
ve °
intensities of greater than 1014 H ° / ( s cm2). Work is well under way on this stage of the facility development. The metal vapor stripper is under construction and software is being written and tested to implement computer control of the H ° production system. A microcomputer system ( P C / X T type) will be used to monitor and control some 15 parameters affecting the beam and its use. In summary, atomic hydrogen beams have been extracted from the University of Manitoba Cyclotron using two different H ° production techniques. Fluxes of
1012
up to 4 × H ° / ( s cm2) for 35 to 50 MeV particles have been registered on target in a number of experimerits. Further development of the facility will be cornpleted by October 1986. The Accelerator Laboratory is presently considering a number of internal experiments
304
J.J.G. Durocher et al. / Extraction of hydrogen beams
which propose to make use of this facility and is anticipating considerable interest from external users. [2] Acknowledgements
[3]
The authors wish to thank Mr. C.S. Bohun for preparing the figure illustrating the facility. This work is supported by the Natural Science and Engineering Research Council of Canada.
[4] [5]
References [1] O.H. Crawford, Luminescence from Metals and Insulators, Proc. of the Wemer Brandt Workshop on Penetration
[6]
Phenomena: Photon Emission from Irradiated Surfaces, ORNL, Oak Ridge, Tenn. (April 15-16, 1985). B.S. Bhakar, private communication, Department of Physics, University of Manitoba (April 1986). A.N. Zelenskiy, S.A. Kokhanovskiy, V.M. Lobashev, N.M. Sobolevskiy and E.A. Volferts, Nucl. Inst. and Meth. 227 (1984) 429. C.L. Perkaris, Phys. Rev. 112 (1958) 1649. M. Yoon, Ph.D. Thesis, Department of Physics, University of Manitoba (1986). I. Gusdal, J. Anderson, J. Bruckshaw, V. Derenchuk, F. Konopasak, J. Lancaster, S. Oh, C.A. Smith, H. Uzat, M. Yoon and J.S.C. McKee, IEEE Trans. Nucl. Sci. NS-32 (1985) 2724.
Nuclear Instruments and Methods in Physics Research A253 (1987) 302-304 North-Holland, Amsterdam
Letter to the Editor
E X T R A C T I O N OF VARIABLE ENERGY N E U T R A L H Y D R O G E N B E A M S FROM THE UNIVERSITY OF MANITOBA CYCLOTRON J.J.G. D U R O C H E R , G.R. S M I T H , V.P. D E R E N C H U K a n d J.R. A N D E R S O N The University of Manitoba Accelerator Laboratory, Department of Physics, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Received 5 August 1986
Neutral hydrogen beams with energies between 30 and 50 MeV and fluxes on target of up to 4×1012 H°/(s cm2) have been successfully extracted from the University of Manitoba Cyclotron. Upon completion in October 1986, the facility is expected to deliver 10 to 50 MeV H ° beams with fluxes in excess of 10TM H°/s cm2).
The hydrogen atom is undoubtedly the most widely investigated of all atomic species. The atom and its ions have been studied extensively from both theoretical and experimental view points. Beams of hydrogen are also widely used as probes in the investigation of other atomic and nuclear systems. In the past such beams have been made up of either positive (i.e. protons) or negative hydrogen ions accelerated to the desired energy. Recently, interest has arisen in the investigation of neutral hydrogen atoms moving at relativistic velocities, Such particles could be used in studies including investigation of: fundamental electromagnetic interactions with materials [1], the basic study of the relativistic atoms' behavior [2], in-flight beam polarization [3], high energy ion implantation. A major collaborative experimental program is presently under way, in conjunction with members of Oak Ridge National Laboratory, studying optical radiation emitted from metallic and nonmetallic surfaces bombarded with such H ° beams, A program was initiated at the University of Manitoba Accelerator Laboratory in late 1984 to develop a 10 to 50 MeV variable energy neutral hydrogen beam facility for use in such experiments. The successful implementation of the second stage of development of such a facility was completed in March 1986. The University of Manitoba Spiral Ridge Cyclotron is an H - accelerator capable of extracting 20 to 50 MeV protons through use of a stripping foil Therefore, production of a beam of neutral hydrogen atoms requires that one electron be removed from the H - ion. This extra electron is bound with art energy of 0.75 eV [4]. The method adopted for the removal of the electron was to pass the H - beam through a stripping medium which would not strip the second, more tightly bound (13.65 eV [4]) electron. 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The project has entered the final stage of a three stage development program that is scheduled for completion in October 1986, and that is summarized in table 1. The development stages consist of different methods of stripping the circulating H - beam in the cyclotron. Initially, the beam was stripped using the residual gas within the cyclotron. The efficiency of this process was of the order of 0.01% for a 10 6 Torr residual gas pressure in the cyclotron. An extensive study was carried out at this stage to determine the effects of residual gas pressure and the cyclotron's magnetic field on the H ° extraction efficiency. The first experiments using neutral hydrogen beams were also performed at this time using the approximately 10 9 H ° / ( s cm2) available from residual gas stripping. The second stage of development involved stripping the H - beam with a thin carbon foil (thickness = 5 /zg/cm 2) which yielded a stripping efficiency of approximately 5% at 40 MeV. Leading up to this stage extensive calculations of the H - beam dynamics within the cyclotron were carried out to determine the optimum H - focussing parameters and stripping foil positions. These calculations were carried out using a FORTRAN program, NEUTRA-STRIP, written by one of the authors (JJGD) and based on programs used to calculate the central region H - orbits in the cyclotron
Table 1 Scheduleof development program Stage
Stripping medium
1 2 3
Residual gas Carbon foil Metal vapour
H° f l u x s-lcm-2 1.3 X 1 0 9 4 × 1012 > 1014
Operational as of December1985 March1986 October 1986
303
J.J.G. Durocher et al. / Extraction of hydrogen beams
[5]. The program made use of the magnetic field data obtained in a recent field mapping and upgrading of the University of Manitoba Cyclotron [6]. The beamline and associated experimental structures used in stage 2 are illustrated in fig. 1. Also shown are the stripping foil probe assembly and the calculated stripping points within the cyclotron for production of 20, 35 and 50 MeV neutral beams, along with equilibrium orbits for H - ions of those energies. The H ° extraction beamline is placed alongside the proton exit beamline and sits within the cyclotron vault. Auxiliary structures are required within the cyclotron to intercept the H + beam that also emerges from the carbon stripping foil (identified as the proton probe in the diagram), The energy of the extracted H ° beam is varied by moving the carbon stripping foil to the previously calculated position on the desired energy H - orbit. The stripping point chosen is that which results in the neutral beam (which of course cannot be focussed or steered once created) proceeding in a straight line to the exit port on the cyclotron. Since the H ° beam will arrive at different angles with respect to the exit port for different energies, the experimental beam line is pivoted about the exit port in order to provide the straight line path needed by the emerging particles, The extracted H ° particles pass through 12 mm diameter collimators located at the entrance and exit of the transport pipe and which define the horizontal
extent of the beam. The collimators allow passage of some 25% of the generated H ° flux. The height of the beam is determined by the vertical momentum component of the circulating H - beam within the cyclotron at the stripping point. The vertical momentum at a point may be minimized by adjusting the cyclotron magnetic field to optimize the vertical focussing of the H - beam. This resulted in a 4 by 12 mm elliptically shaped beamspot at the target site. The target site is located in an experimental chamber some 1.85 m from the stripping point in the cyclotron. While passing through the target the H ° atoms are stripped of their remaining electron and the resultant proton beam is monitored to determine the initial H ° flux. This is accomplished through the use of two current monitors: a secondary electron emission monitor and a Faraday cup. The stage 2 neutral beam facility has also been used to conduct a number of experiments. An active collaboration between the University of Manitoba and Oak Ridge National Laboratory has been involved in using the H ° beam as a probe to study optical radiation emitted from metallic and nonmetallic surfaces [1]. An extensive experimental system has been assembled for these experiments. Software has been written which monitors the H ° beam on target, controls the monochrometer apparatus during the scans and collects, displays and does a preliminary analysis on the experimental data. To date some 400 spectra have been accumulated for optical radiation emitted from the surfaces of some twenty different targets bombarded with H ° beams. These data are currently being analysed in preparation for publication. In the third stage of development of the Neutral Hydrogen Beam Facility the carbon stripping foil is to be replaced by a variable thickness metal vapor stripper. With the use of such a system stripping efficiencies greater than 80% are anticipated for all H - energies. Since the cyclotron is potentially capable of accelerating
up o 0
Fig. 1. The University of Manitoba H ° Facility. Layout of the University of Manitoba Cyclotron showing H ° facility: 1, probe assembly for stripping H- beam; 2, 3, 4, equilibrium orbits for 50, 35 and 20 MeV H- beams. Stripping points are indicated by +. 5, proton pickoff probe; 6, bellows; 7, 12 mm diameter collimators; 8, experimental chamber; 9, beam current monitors,
o
we xpe oac
ve °
intensities of greater than 1014 H ° / ( s cm2). Work is well under way on this stage of the facility development. The metal vapor stripper is under construction and software is being written and tested to implement computer control of the H ° production system. A microcomputer system ( P C / X T type) will be used to monitor and control some 15 parameters affecting the beam and its use. In summary, atomic hydrogen beams have been extracted from the University of Manitoba Cyclotron using two different H ° production techniques. Fluxes of
1012
up to 4 × H ° / ( s cm2) for 35 to 50 MeV particles have been registered on target in a number of experimerits. Further development of the facility will be cornpleted by October 1986. The Accelerator Laboratory is presently considering a number of internal experiments
304
J.J.G. Durocher et al. / Extraction of hydrogen beams
which propose to make use of this facility and is anticipating considerable interest from external users. [2] Acknowledgements
[3]
The authors wish to thank Mr. C.S. Bohun for preparing the figure illustrating the facility. This work is supported by the Natural Science and Engineering Research Council of Canada.
[4] [5]
References [1] O.H. Crawford, Luminescence from Metals and Insulators, Proc. of the Wemer Brandt Workshop on Penetration
[6]
Phenomena: Photon Emission from Irradiated Surfaces, ORNL, Oak Ridge, Tenn. (April 15-16, 1985). B.S. Bhakar, private communication, Department of Physics, University of Manitoba (April 1986). A.N. Zelenskiy, S.A. Kokhanovskiy, V.M. Lobashev, N.M. Sobolevskiy and E.A. Volferts, Nucl. Inst. and Meth. 227 (1984) 429. C.L. Perkaris, Phys. Rev. 112 (1958) 1649. M. Yoon, Ph.D. Thesis, Department of Physics, University of Manitoba (1986). I. Gusdal, J. Anderson, J. Bruckshaw, V. Derenchuk, F. Konopasak, J. Lancaster, S. Oh, C.A. Smith, H. Uzat, M. Yoon and J.S.C. McKee, IEEE Trans. Nucl. Sci. NS-32 (1985) 2724.