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Proposal for direct conversion of a polarized atomic beam to polarized negative ions
NUCLEAR
INSTRUMENTS
AND METHODS
62 0 9 6 8 ) 3 5 5 - 3 5 7 ; © N O R T H - H O L L A N D
PUBLISHING
CO.
PROPOSAL FOR DIRECT CONVERSION OF A POLARIZED ATOMIC BEAM TO POLARIZED NEGATIVE IONS W. HAEBERLI
University of Wisconsin, Madison, Wisconsin, U.S.A. Received 8 April 1968 It is proposed to produce negative polarized ions of hydrogen or deuterium for injection into tandem accelerators by crossing a polarized beam of thermal hydrogen or deuterium atoms with a beam of fast neutral atoms or with a beam of negative ions. Polarized beams of high ion-optic quality with at least one # A intensity can be expected.
Experiments which showed that negative hydrogen ions with large nuclear polarization can be produced were carried out at the University of Wisconsin some years agog). The motivation for the experiments was to provide a polarized beam for injection into a tandem accelerator. The method consisted of passing a 50 keV polarized beam of deuterons through a thin carbon foil. The polarized deuteron beam in turn was obtained from ionization of a polarized atomic beam. Later, the beam was accelerated in a tandem machine and it was demonstrated 2) that no depolarization results provided that the strippling of electrons from the negative ions in the centre electrode of the accelerator is done with a thin foil rather than with the more commonly used gas stripper. It was proposed 3) that in general the charge exchange (attachment or stripping of electrons) causes no depolarization if either the charge exchange process happens in a time short compared to a Larmor period (which is the case for a thin foil) or if a strong magnetic field is applied to decouple the nuclear spin from the electrons. The latter method has recently been demonstrated to work as expected4'5). It is thus fair to say that there is no difficulty in maintaining a large polarization during the formation, acceleration and stripping of the negative ions. The problem of obtaining adequate intensities of polarized negative ions has been a more difficult one. The early experiments at Wisconsin gave a beam intensity somewhat less than 0.001 pA of which, however, only a small fraction was available at the target. It was thought that the poor transmission through the machine was connected with scattering of the beam in the first charge exchange foil. Recently, the group at Erlangen 5) obtained excellent transmission when the first charge took place in nitrogen gas in a differentially pumped canal. In this case the beam current on target approached 0.001 pA. Larger beam currents can be expected if the charge exchange is carried out in an alkali vapour. It has been shown that at 1 keV proton energy, about 10% of a proton beam can be converted to
negative ions by charge exchange in potassium vapour6). In view of the fact that as much as 6 #A of polarized proton beam has been produced at SaclayV), it is quite possible that polarized negative beams of several tenth of a pA can be produced by the conventional charge exchange method. The ionizer and charge exchange canal would have to be at a potential near about - 50 kV in order to obtain the necessary tandem injection energy. In this note we wish to consider other methods to convert polarized hydrogen atoms to negative ions. The direct conversion of the atomic beam to negative ions is proposed by processes of the type H° + X ° - o H - + X +, or
H °+Y-~H-
+yo.
The necessary relative velocity between the collision partners is obtained by crossing a fast beam of X ° or Y - with the thermal atomic beam. In particular one might consider H ° + C s ° ~ H - + Cs +. As the relative velocity v between the collision partners decreases the cross section increases and reaches about a = 3 x 10 - 1 6 cm 2 at v = 4.4 x 10 ~ cm/s which is the lowest velocity for which measurements have been madeS). The peak cross section is expected to occur 9) at about v = 2 x 107 cm/s which corresponds to an energy of the Cs ° atoms of 28 keV. In order to estimate the expected beam intensity, assume that a Cs + beam of current density j is produced e.g. by surface ionization ~o), and that this beam is subsequently neutralized by passage through a Cs vapour celll~). The fast Cs ° beam is crossed with a polarized beam of slow H ° atoms. If the volume density of H ° target atoms is n, the beam current I of negative polarized ions is
I =jnaV, where V is the volume c o m m o n to both beams. The
355
356
w. HAEBERL! \ c,°
, ,
/
polarized H ° atoms
,
~
"-
\\ \\\
interaction region
,::.;! ;;:i:) .::::"'-
cesium vapour
Cs* beam
H-
a value 14) a = 2.2 x 10-15 cm 2 at 4.4 × 107 cm/s (2 k e y D - ) . Since negative hydrogen ion beams of several mA have been produced with dc arc sourceslS), one can anticipate negative polarized beam intensities of several pA. It may be of interest to point out, in passing, that a similar technique could also be used to give microampere beams of polarized electrons by the process
Fig. 1. Schematic diagram of arrangement for direct conversion of a polarized atomic beam to polarized negative ions.
atomic beam obtained a t Saclay 12) has a velocity of about 4 x 105 cm/s and an intensity near 1017 atoms/s over a diameter of 1 cm, giving n ~ 3 x 1011/cm 3. According to recent work with Cs beams ~°) a current density of j = 10 m A / c m 2 is readily obtained at the energies required here, giving I = 1 #A, even if we assume an interaction volume of only V = = 1 cm 3 and using the value of the charge transfer cross section given above. A possible arrangement is shown in fig. 1. The Cs + beam is accelerated from a surface ionization source, which may be at ground potential, to a Cs vapour canal e.g. at - 3 0 kV. The Cs ° beam emerging from the canal is collinear with the atomic beam. The H - created in the cross-hatched region are extracted by a weak electric field, and are deflected by a suitable magnetic field or by an electrostatic mirror. Subsequently the H - ions are accelerated to ground potential and injected into the tandem accelerator. The desired strength of the magnetic guide field B in the interaction region is either about a k G or a few G, depending on whether or not the hydrogen atomic beam is exposed to rf transitions13). It is expected that a negative ion beam of very high ion optic quality can be obtained because there is virtually no space charge in the ionization region so that the field configuration and ion trajectories can be accurately controlled. The transverse m o m e n t u m which the hydrogen atom obtains in the charge transfer collision is certainly less than is obtained in the more conventional charge exchange arrangement where multiple scattering may cause additional divergence of the beam. Instead of a neutral atom, a negative ion may also be useful as a donor of electrons. One may consider, for instance, a negative hydrogen or deuterium ion. The process H°+D-~H - +D ° is a resonant charge transfer process. The cross section increases with decreasing relative velocity and reaches
H°+H+~H
+ +H ++e,
which has a cross section 16) o f a ~ 2 × 10 - 1 6 c m 2 at an energy of about 60 keV. Thus, under the above assumptions, a proton current density of 20 m A / c m z would yield 1.2 pA of polarized electrons for an interaction volume of 1 cm 3. It would be useful to compare the method of negative ion formation proposed here with the type of ion source proposed by DonnallyW), i.e., the L a m b shift ion source which uses charge exchange in cesium to form metastable hydrogen atoms with subsequent conversion of the metastable atom to negative ions in argon. Polarized negative ion beam currents up to 0.1 pA have been reported for L a m b shift ion sources18). A device of this type has been installed on the Wisconsin tandem accelerator 19) and it has been shown that good transmission through the machine is obtained. At present it is not clear which type of negative ion source will be the more useful one. Clearly, ease of operation and reliability have to be considered in addition to beam intensity. There is no question that the Lamb-shift source already now is a useful device and that it will be improved further. The purpose of the present note was to point out that the atomic beam method of negative ion production may also be capable of very substantial improvement. Referenees 1) W. Gruebler, W. Haeberli and P. Schwandt, Phys. Rev. Letters 12 (1964) 595. 2) W. Haeberli, W. Gruebler, P. Extermann and P. Schwandt, Phys. Rev. Letters 15 (1965) 267. 3) W. Haeberli, Proc. Second Intern. Conf. Polarization Phenomena, Karlsruhe 1965 (ed. P. Huber and H. Schopper); (Birkhfiuser, Basel, 1966) p. 64. 4) W. Griiebler, V. K6nig and P. Marmier, Phys. Letters 24B (1967) 335. 6) G. Clausnitzer, W. Dfirr, R. Fleischmann, G. Graw, E. Salzborn and J. Witte, Phys. Letters 25B (1967) 267. 8) B. L. Donnally and R. Becker, Bull. Am. Phys. Soc. 12 (1967) 29. A conversion yield to negative ions of 1 4 ~ has been reported for 1 keV protons in Cs by P. J. Bjorkholm, D. H. Loyd, A. S. Schlachter, L. W. Anderson and W. Haeberli, Bull. Am. Phys. Soc. 13 (1968) 67 and a conversion yield of 2 5 ~ was reported by L. W. Drake and R. Krotkov, Phys. Rev. Letters 16 (1966) 848, for 1 keV deuterons in Cs.
D I R E C T C O N V E R S I O N OF A P O L A R I Z E D A T O M I C BEAM 7) j. Thirion, Bull. Am. Phys. Soc. 12 (1967) 1170. s) p. j. Bjorkholm, D. H. Loyd, A. S. Schlachter, L. W. Anderson and W. Haeberli, private communication. 9) The velocity at which the peak cross section was estimated on the basis of fig. 12 of the paper by Ya. M. Fogel, Sov. Phys. Uspekhi 3 (1960) 390. 10) M. LaChance, G. Kuskevics and B. Thompson, American Institute of Astronomy and Aeronautics Journal, A I A A 3 (1965) 1498. 11) J. Perel, R. H. Vernon and H. L. Daley, Phys. Rev. 138 (1965) A937. 12) R. Beurtey, R. Maillard, A. Papineau, C. Re and J. Thirion, Saclay Progress Report CEN-N-621, 81 (CEA, France, 1966).
357
13) See e . g . J . M . Dickson, Progr. Nucl. Tech. Instr. 1 (1965) 103, or W. Haeberli, Ann. Rev. Nucl. Sci. 17 (1967) 373. 14) D. G. Hummer, R. F. Stebbings, W. L. Fire and L. M. Branscomb, Phys. Rev. 119 (1960) 6681. 15) K. W. Ehlers, Nucl. Instr. and Meth. 32 (1965) 309. 16) j. V. Ireland and H. B. Gilbody, in Atomic collision processes (ed. M. R. C. McDowell; Wiley, New York, 1964) p. 666. 17) B. L. Donnally, T. Clapp, W. Sawyer and M, Schultz, Phys. Rev. Letters 12 (1964) 502; B. L. Donnally and W. Sawyer, Phys. Rev. Letters 15 (1965) 439. 18) H. Briickmann and L. Friedrich, Zentrum fiir Kernphysik, Karlsruhe, Germany, private communication. 19) T. B. Clegg, G. R. Plattner, L. G. Keller and W. Haeberli, Nucl. Instr. and Meth. 57 (1967) 167.
INSTRUMENTS
AND METHODS
62 0 9 6 8 ) 3 5 5 - 3 5 7 ; © N O R T H - H O L L A N D
PUBLISHING
CO.
PROPOSAL FOR DIRECT CONVERSION OF A POLARIZED ATOMIC BEAM TO POLARIZED NEGATIVE IONS W. HAEBERLI
University of Wisconsin, Madison, Wisconsin, U.S.A. Received 8 April 1968 It is proposed to produce negative polarized ions of hydrogen or deuterium for injection into tandem accelerators by crossing a polarized beam of thermal hydrogen or deuterium atoms with a beam of fast neutral atoms or with a beam of negative ions. Polarized beams of high ion-optic quality with at least one # A intensity can be expected.
Experiments which showed that negative hydrogen ions with large nuclear polarization can be produced were carried out at the University of Wisconsin some years agog). The motivation for the experiments was to provide a polarized beam for injection into a tandem accelerator. The method consisted of passing a 50 keV polarized beam of deuterons through a thin carbon foil. The polarized deuteron beam in turn was obtained from ionization of a polarized atomic beam. Later, the beam was accelerated in a tandem machine and it was demonstrated 2) that no depolarization results provided that the strippling of electrons from the negative ions in the centre electrode of the accelerator is done with a thin foil rather than with the more commonly used gas stripper. It was proposed 3) that in general the charge exchange (attachment or stripping of electrons) causes no depolarization if either the charge exchange process happens in a time short compared to a Larmor period (which is the case for a thin foil) or if a strong magnetic field is applied to decouple the nuclear spin from the electrons. The latter method has recently been demonstrated to work as expected4'5). It is thus fair to say that there is no difficulty in maintaining a large polarization during the formation, acceleration and stripping of the negative ions. The problem of obtaining adequate intensities of polarized negative ions has been a more difficult one. The early experiments at Wisconsin gave a beam intensity somewhat less than 0.001 pA of which, however, only a small fraction was available at the target. It was thought that the poor transmission through the machine was connected with scattering of the beam in the first charge exchange foil. Recently, the group at Erlangen 5) obtained excellent transmission when the first charge took place in nitrogen gas in a differentially pumped canal. In this case the beam current on target approached 0.001 pA. Larger beam currents can be expected if the charge exchange is carried out in an alkali vapour. It has been shown that at 1 keV proton energy, about 10% of a proton beam can be converted to
negative ions by charge exchange in potassium vapour6). In view of the fact that as much as 6 #A of polarized proton beam has been produced at SaclayV), it is quite possible that polarized negative beams of several tenth of a pA can be produced by the conventional charge exchange method. The ionizer and charge exchange canal would have to be at a potential near about - 50 kV in order to obtain the necessary tandem injection energy. In this note we wish to consider other methods to convert polarized hydrogen atoms to negative ions. The direct conversion of the atomic beam to negative ions is proposed by processes of the type H° + X ° - o H - + X +, or
H °+Y-~H-
+yo.
The necessary relative velocity between the collision partners is obtained by crossing a fast beam of X ° or Y - with the thermal atomic beam. In particular one might consider H ° + C s ° ~ H - + Cs +. As the relative velocity v between the collision partners decreases the cross section increases and reaches about a = 3 x 10 - 1 6 cm 2 at v = 4.4 x 10 ~ cm/s which is the lowest velocity for which measurements have been madeS). The peak cross section is expected to occur 9) at about v = 2 x 107 cm/s which corresponds to an energy of the Cs ° atoms of 28 keV. In order to estimate the expected beam intensity, assume that a Cs + beam of current density j is produced e.g. by surface ionization ~o), and that this beam is subsequently neutralized by passage through a Cs vapour celll~). The fast Cs ° beam is crossed with a polarized beam of slow H ° atoms. If the volume density of H ° target atoms is n, the beam current I of negative polarized ions is
I =jnaV, where V is the volume c o m m o n to both beams. The
355
356
w. HAEBERL! \ c,°
, ,
/
polarized H ° atoms
,
~
"-
\\ \\\
interaction region
,::.;! ;;:i:) .::::"'-
cesium vapour
Cs* beam
H-
a value 14) a = 2.2 x 10-15 cm 2 at 4.4 × 107 cm/s (2 k e y D - ) . Since negative hydrogen ion beams of several mA have been produced with dc arc sourceslS), one can anticipate negative polarized beam intensities of several pA. It may be of interest to point out, in passing, that a similar technique could also be used to give microampere beams of polarized electrons by the process
Fig. 1. Schematic diagram of arrangement for direct conversion of a polarized atomic beam to polarized negative ions.
atomic beam obtained a t Saclay 12) has a velocity of about 4 x 105 cm/s and an intensity near 1017 atoms/s over a diameter of 1 cm, giving n ~ 3 x 1011/cm 3. According to recent work with Cs beams ~°) a current density of j = 10 m A / c m 2 is readily obtained at the energies required here, giving I = 1 #A, even if we assume an interaction volume of only V = = 1 cm 3 and using the value of the charge transfer cross section given above. A possible arrangement is shown in fig. 1. The Cs + beam is accelerated from a surface ionization source, which may be at ground potential, to a Cs vapour canal e.g. at - 3 0 kV. The Cs ° beam emerging from the canal is collinear with the atomic beam. The H - created in the cross-hatched region are extracted by a weak electric field, and are deflected by a suitable magnetic field or by an electrostatic mirror. Subsequently the H - ions are accelerated to ground potential and injected into the tandem accelerator. The desired strength of the magnetic guide field B in the interaction region is either about a k G or a few G, depending on whether or not the hydrogen atomic beam is exposed to rf transitions13). It is expected that a negative ion beam of very high ion optic quality can be obtained because there is virtually no space charge in the ionization region so that the field configuration and ion trajectories can be accurately controlled. The transverse m o m e n t u m which the hydrogen atom obtains in the charge transfer collision is certainly less than is obtained in the more conventional charge exchange arrangement where multiple scattering may cause additional divergence of the beam. Instead of a neutral atom, a negative ion may also be useful as a donor of electrons. One may consider, for instance, a negative hydrogen or deuterium ion. The process H°+D-~H - +D ° is a resonant charge transfer process. The cross section increases with decreasing relative velocity and reaches
H°+H+~H
+ +H ++e,
which has a cross section 16) o f a ~ 2 × 10 - 1 6 c m 2 at an energy of about 60 keV. Thus, under the above assumptions, a proton current density of 20 m A / c m z would yield 1.2 pA of polarized electrons for an interaction volume of 1 cm 3. It would be useful to compare the method of negative ion formation proposed here with the type of ion source proposed by DonnallyW), i.e., the L a m b shift ion source which uses charge exchange in cesium to form metastable hydrogen atoms with subsequent conversion of the metastable atom to negative ions in argon. Polarized negative ion beam currents up to 0.1 pA have been reported for L a m b shift ion sources18). A device of this type has been installed on the Wisconsin tandem accelerator 19) and it has been shown that good transmission through the machine is obtained. At present it is not clear which type of negative ion source will be the more useful one. Clearly, ease of operation and reliability have to be considered in addition to beam intensity. There is no question that the Lamb-shift source already now is a useful device and that it will be improved further. The purpose of the present note was to point out that the atomic beam method of negative ion production may also be capable of very substantial improvement. Referenees 1) W. Gruebler, W. Haeberli and P. Schwandt, Phys. Rev. Letters 12 (1964) 595. 2) W. Haeberli, W. Gruebler, P. Extermann and P. Schwandt, Phys. Rev. Letters 15 (1965) 267. 3) W. Haeberli, Proc. Second Intern. Conf. Polarization Phenomena, Karlsruhe 1965 (ed. P. Huber and H. Schopper); (Birkhfiuser, Basel, 1966) p. 64. 4) W. Griiebler, V. K6nig and P. Marmier, Phys. Letters 24B (1967) 335. 6) G. Clausnitzer, W. Dfirr, R. Fleischmann, G. Graw, E. Salzborn and J. Witte, Phys. Letters 25B (1967) 267. 8) B. L. Donnally and R. Becker, Bull. Am. Phys. Soc. 12 (1967) 29. A conversion yield to negative ions of 1 4 ~ has been reported for 1 keV protons in Cs by P. J. Bjorkholm, D. H. Loyd, A. S. Schlachter, L. W. Anderson and W. Haeberli, Bull. Am. Phys. Soc. 13 (1968) 67 and a conversion yield of 2 5 ~ was reported by L. W. Drake and R. Krotkov, Phys. Rev. Letters 16 (1966) 848, for 1 keV deuterons in Cs.
D I R E C T C O N V E R S I O N OF A P O L A R I Z E D A T O M I C BEAM 7) j. Thirion, Bull. Am. Phys. Soc. 12 (1967) 1170. s) p. j. Bjorkholm, D. H. Loyd, A. S. Schlachter, L. W. Anderson and W. Haeberli, private communication. 9) The velocity at which the peak cross section was estimated on the basis of fig. 12 of the paper by Ya. M. Fogel, Sov. Phys. Uspekhi 3 (1960) 390. 10) M. LaChance, G. Kuskevics and B. Thompson, American Institute of Astronomy and Aeronautics Journal, A I A A 3 (1965) 1498. 11) J. Perel, R. H. Vernon and H. L. Daley, Phys. Rev. 138 (1965) A937. 12) R. Beurtey, R. Maillard, A. Papineau, C. Re and J. Thirion, Saclay Progress Report CEN-N-621, 81 (CEA, France, 1966).
357
13) See e . g . J . M . Dickson, Progr. Nucl. Tech. Instr. 1 (1965) 103, or W. Haeberli, Ann. Rev. Nucl. Sci. 17 (1967) 373. 14) D. G. Hummer, R. F. Stebbings, W. L. Fire and L. M. Branscomb, Phys. Rev. 119 (1960) 6681. 15) K. W. Ehlers, Nucl. Instr. and Meth. 32 (1965) 309. 16) j. V. Ireland and H. B. Gilbody, in Atomic collision processes (ed. M. R. C. McDowell; Wiley, New York, 1964) p. 666. 17) B. L. Donnally, T. Clapp, W. Sawyer and M, Schultz, Phys. Rev. Letters 12 (1964) 502; B. L. Donnally and W. Sawyer, Phys. Rev. Letters 15 (1965) 439. 18) H. Briickmann and L. Friedrich, Zentrum fiir Kernphysik, Karlsruhe, Germany, private communication. 19) T. B. Clegg, G. R. Plattner, L. G. Keller and W. Haeberli, Nucl. Instr. and Meth. 57 (1967) 167.