Method For Producing Ions Utilizing A Charge-transfer Collision

Abstract

Ionization of a first beam of polarized neutral atoms is accomplished by a charge-transfer collision with a second beam of atoms.

Description

CONTRACTUAL ORIGIN OF THE INVENTION

The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION

This invention relates to polarized ions and more particularly to a method for producing polarized ions utilizing a charge-transfer collision.

Polarized ion beams are used in particle accelerators to strike targets of interest. They are presently produced by polarizing atoms in the beam and then ionizing the polarized atoms. Ionization to produce polarized positive-ion beams is generally effected in the manner of either the strong field ionization method disclosed by Glavish and Collins, Proceedings of the 2nd International Symposium on Polarization Phenomena or Nucleons, Editors: Huber and Schopper, Birkhauser Verlag Basel und Stuttgart, page 86, or by weak field ionization as disclosed by Rudin et al., "Helvetica Physica Acta," Volume 34, page 58, 1961. With these methods, a beam of polarized positive ions approximately 1 microampere in intensity is produced. If polarized negative ions are required as for Tandem accelerators and some cyclotrons, such ions are presently produced by accelerating the aforedescribed positive-ion beam and passing it through a suitable foil or gas. This method results in a polarized negative-ion beam which is relatively weak in intensity, approximately 0.01 microampere.

Accordingly, it is one object of the present invention to provide an improved method of ionizing polarized atoms.

It is another object of the present invention to provide a polarized ion beam of greater intensity than heretofore.

It is another object of the present invention to provide a method for directly converting polarized atoms to polarized negative ions.

It is another object of the present invention to provide a polarized negative ion beam having an intensity of 1 microampere.

It is another object of the present invention to provide a polarized positive-ion beam having an intensity of 10 microamperes.

Other objects of the present invention will become more apparent as the detailed description proceeds.

SUMMARY OF THE INVENTION

In general, the method of the present invention comprises effecting a charge-transfer collision between a first beam of polarized neutral atoms to be ionized and a generated second beam of atoms whereby the polarized atoms in said first beam are ionized.

BRIEF DESCRIPTION OF THE DRAWING

Further understanding of the present invention may best by obtained by consideration of the accompanying drawings wherein:

FIG. 1 is a schematic diagram of an apparatus for practicing the method of the present invention.

FIG. 2 is a schematic diagram of an alternate apparatus for the practice of the method of the present invention.

As previously described, in the production of polarized ions neutral atoms are formed into a beam and polarized. These polarized neutral atoms are subsequently ionized. In the present invention, the direct conversion of the neutral polarized atoms to polarized negative ions is effected by a charge-transfer collision with a second atomic beam, which collision may be described by the general equation

X.sup.0 + Z.sup.0 -- X.sup..sup.- + Z.sup..sup.+ .

This equation is illustrative of a charge-transfer collision between neutral polarized atoms X.sup.0 and a generated beam of neutral atoms Z.sup.0 to produce polarized negative ions X.sup..sup.- and positive ions Z.sup..sup.+. Further understanding of this general relationship to produce polarized negative ions may be achieved by considering a particular case wherein polarized neutral hydrogen atoms are converted to polarized negative hydrogen ions by a charge-transfer collision with neutral cesium atoms, which relationship may be described by

H.sup.0 +Cs.sup.0 -- H.sup..sup.- + Cs.sup..sup.+.

In this charge-transfer collision to effect the polarized negative hydro hydrogen ions, the relative velocity of the neutral cesium atoms with respect to the neutral hydrogen atoms must be such as to effect a charge transfer upon collision. For maximum ion intensity production, the relative velocity V between the neutral cesium and polarized hydrogen atoms should be approximately 2.times.10.sup.7 cm./sec.

The method of the charge-transfer collision may be achieved utilizing the apparatus shown in FIG. 1. A neutral cesium atomic beam 10 is produced at a predetermined velocity by the conventional technique of passing a positive cesium ion beam through a cesium vapor cell (not shown) to neutralize the ions in the beam. The neutral cesium atomic beam 10 is then transmitted collinear with the polarized neutral hydrogen atomic beam 14 to effect an interaction therewith in the interior 16 of a solenoid 18 which is coaxial with the apparatus producing the polarized hydrogen beam 14. The solenoid 18 has a winding 20 disposed thereabout which, when excited, produces a magnetic field B within the solenoid parallel to and in the direction of the polarized hydrogen atomic beam 14. This magnetic field B maintains and defines the spin alignment direction of the polarized hydrogen atoms. As previously described, the neutral cesium atoms are generated and injected into the interior 16 of the solenoid 18 with a velocity sufficient to effect a charge-transfer collision with the polarized neutral hydrogen atoms of atomic beam 14. For the case of hydrogen and cesium, the relative velocity therebetween should be 2.times.10.sup.7 cm./sec. to achieve maximum ionization intensity output. In the charge-transfer collision between the neutral cesium atoms and the polarized neutral hydrogen atoms, the cesium atom gives up an electron which is acquired by the hydrogen atom, thereby producing polarized negative hydrogen ions in the interaction region within the interior 16 of solenoid 18.

An electrode 22 having a positive potential thereon extracts the polarized negative hydrogen ions from the interior 16 of solenoid 18. Subsequently, electrode 24 and grid 26 act to form a focus lens 28 and focus the extracted polarized negative hydrogen ions into a beam 29. The focused polarized negative hydrogen ions are then reflected by an electrostatic mirror 30 comprising two grids 32 and 34 for use in the accelerator.

It will be appreciated that the aforedescribed method is accomplished in a partial vacuum. With a partial vacuum of 10.sup..sup.-7 mm. Hg., a magnetic field B of approximately 1 kilogauss in the interaction region of the interior 16 of solenoid 18, a relative velocity V between the neutral cesium and polarized hydrogen atoms of 2.times.10.sup.7 cm./sec. and electrode and grid biases as follows, an output polarized negative hydrogen ion beam intensity of 1 microampere may be achieved.

Electrode 22 +100 volts

Electrode 24 +1600 volts

Grid 26 +5000 volts

Grid 32 +5000 volts (same potential as grid 26)

Grid 34 electrical ground.

It is not intended that the aforedescribed hydrogen-cesium reaction be a limitation on the general equation

X.sup.0 + Z.sup.0 -- X.sup..sup.- + Z.sup..sup.+.

It is to be understood that material other than hydrogen and cesium may be substituted in this general equation to effect the production by charge-transfer collision of polarized negative ions of high intensity. For example, polarized neutral lithium or deuterium atoms may be substituted for the polarized neutral hydrogen atoms in the aforedescribed example to produce polarized negative lithium or deuterium ions. Further neutral lithium, deuterium or hydrogen atoms may be substituted for the neutral cesium atoms to act as an electron donor.

The charge-transfer collision method of the present invention may further be described by the general equation

X.sup.0 + Z.sup..sup.- -- X.sup..sup.- + Z.sup.0.

This equation is illustrative of a charge-transfer collision between neutral polarized atoms X.sup.0 and a generated beam of negative ions Z.sup..sup.- to produce polarized negative ions X.sup..sup.- and neutral atoms Z.sup.0. Further understanding of this general relationship to produce polarized negative ions may be achieved by considering a particular case wherein polarized neutral hydrogen atoms are converted to polarized negative hydrogen ions by charge-transfer collision with negative deuterium ions. The relationship may be described by

H.sup.0 + D.sup..sup.- -- H.sup..sup.- + D.sup.0.

In this charge-transfer collision to effect the polarized negative hydrogen ions, the relative velocity of the negative deuterium ions with respect to the neutral hydrogen atoms must be such as to effect a charge transfer upon collision. For maximum polarized ion intensity production, the relative velocity V between the neutral polarized hydrogen atoms and negative deuterium ions should be approximately 4.times.10.sup.7 cm./sec. This charge-transfer collision may be achieved utilizing the apparatus shown in FIG. 2. The apparatus of FIG. 2 is the same as the apparatus of FIG. 1 except that the negative-ion beam enters the interaction region 16 from the same direction as the polarized neutral atom beam. In the embodiment of FIG. 2, the negative-deuterium-ion beam 40 enters the interaction region 16 after being reflected by an electrostatic mirror 42 comprising grids 44 and 46 maintained at potentials of approximately -2000 volts and electrical ground, respectively. The neutral polarized hydrogen atom beam 14 is not affected by the mirror 42 and also enters the interaction region 16 wherein a charge-transfer collision is effected between the neutral hydrogen atoms and negative deuterium ions. The remaining operation of the apparatus of FIG. 2 is the same as that of FIG. 1. The negative deuterium ions may be produced by conventional DC arc source techniques. Further, it will be appreciated that other materials may be substituted to produce polarized negative ions for the practice of the charge-transfer collision according to the general formula

X.sup.0 + Z.sup..sup.- -- X.sup..sup.- + Z.sup.0.

For example, neutral lithium or deuterium atoms may be substituted for the neutral hydrogen atoms to produce polarized negative lithium or deuterium ions. Further, negative lithium or hydrogen ions may be substituted for the negative deuterium ions as an electron donor. When negative hydrogen ions are substituted for negative deuterium ions, the same approximate relative velocity of 4.times.10.sup.7 cm./sec. is maintained with respect to the polarized neutral hydrogen ions.

The charge-transfer collision method of the present invention may be further expressed by a third general equation

X.sup.0 + Z.sup..sup.+ -- X.sup..sup.+ + Z.sup..sup.+ +e.

This equation is illustrative of a charge-transfer collision between neutral polarized atoms X.sup.0 and a generated beam of positive ions to produce polarized positive ions X.sup..sup.+ and positive ions Z.sup..sup.+ and free electrons. This general relationship may be specifically illustrated by the process

H.sup.0 + D.sup..sup.+-- H.sup..sup.+ + D.sup..sup.+ +e.

In this case, polarized neutral hydrogen atoms are converted to polarized positive hydrogen ions by charge-transfer collision with positive deuterium ions. This process may be carried out by the aforedescribed and illustrated apparatus of FIG. 2 with the polarity of the various grid and electrode biases adjusted to effect extraction of polarized positive hydrogen ions. As for the aforedescribed embodiments, the relative velocity between the polarized neutral hydrogen atoms and the positive deuterium ions must be sufficient to effect a charge-transfer collision. For the specific example utilizing polarized neutral hydrogen atoms and positive deuterium ions, a relative velocity of 4.times.10.sup.8 cm./sec. will achieve maximum polarized positive hydrogen ion intensity output (approximately 10 microamperes). It will also be appreciated that the third general charge-transfer collision relationship X.sup.0 + Z.sup..sup.+ -- X.sup..sup.+ + Z.sup..sup.+ +e may be effective using materials other than those recited in the specific example and is not to be limited thereto. For example, polarized neutral lithium, cesium or deuterium atoms can be substituted for the polarized neutral hydrogen atoms to produce polarized positive lithium, cesium or deuterium ions. Further, positive hydrogen, lithium or cesium ions can be substituted for the positive deuterium ions.

It will be appreciated that, in the practice of the present invention according to the aforedescribed methods, it is not requisite that the atomic beams effecting the charge-transfer collision be collinear. The present invention is operable with apparatus wherein the atomic beams have other relative propagation directions. For example, the beams may be oriented to cross at right angles with the resultant charge-exchange collision still being accomplished. Further, where the present invention is applied to ionization in a weak field then the magnetic field in the interaction region need not be parallel to either atomic beam but may be oriented at will.

Persons skilled in the art will, of course, readily adapt the general teachings of the invention to embodiments far different from the embodiments illustrated. Accordingly, the scope of the protection afforded the invention should not be limited to the particular embodiment illustrated in the drawings and described above but should be determined only in accordance with the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

Claims

I claim:

1. A method for ionizing a first beam of polarized neutral atoms comprising generating a second beam of atoms and effecting a charge-transfer collision between said first polarized atomic beam and said second atomic beam to ionize said polarized atoms.

2. The method according to claim 1 wherein said second beam generation and charge-transfer collision comprises generating a magnetic field and generating and transmitting said second beam to intercept said first beam at a velocity relative thereto to effect a charge-transfer collision therewith in said magnetic field.

3. The method according to claim 1 wherein said second beam generation and charge-transfer collision comprises generating said second atomic beam at a predetermined velocity, transmitting said second atomic beam collinear with said first polarized atomic beam in a direction opposite thereto to effect a charge transfer therewith in a partial vacuum to ionize said polarized atoms, and extracting said ionized polarized atoms.

4. The method according to claim 3 wherein said charge transfer between said first and second atomic beams is effected in a magnetic field parallel to the direction of said polarized atomic beam.

5. The method according to claim 1 wherein said second beam generation and charge-transfer collision comprises generating a second beam of neutral cesium atoms at a velocity relative said first beam atoms of approximately 2.times.10.sup.7 centimeters/second, transmitting said second beam to collide with said first beam atoms and effect a charge transfer therewith in a partial vacuum to ionize atoms of said first beam, maintaining a magnetic field in the region of said charge-transfer collision, and extracting from said charge-transfer collision region polarized ionized atoms.

6. The method according to claim 1 wherein said second beam generation and charge-transfer collision comprises generating a second beam of either hydrogen or deuterium atoms at a velocity relative said first beam atoms of approximately 4.times.10.sup.7 centimeters/second, transmitting said second beam to collide with said first beam atoms and effect a charge transfer therewith in a partial vacuum to ionize atoms of said first beam, maintaining a magnetic field in the region of said charge-transfer collision, and extracting from said charge-transfer collision region polarized ionized atoms.

7. The method according to claim 1 wherein said first beam comprises neutral atoms of hydrogen and said second atomic beam generation and charge-transfer collision comprises generating a magnetic field, generating in a partial vacuum at a predetermined velocity a second beam of either lithium or cesium atoms to effect a charge-transfer collision with said first beam in said magnetic field and form polarized ionized hydrogen atoms, and extracting said polarized ionized hydrogen atoms.

8. The method according to claim 7 wherein said cesium atoms of said second beam are neutral cesium atoms generated to provide a relative velocity between the atoms of said first and second beams of approximately 2.times.10.sup.7 centimeters/second.

9. The method according to claim 1 wherein said first beam comprises neutral atoms of hydrogen and said second atomic beam generation and charge-transfer collision comprises generating a magnetic field, generating in a partial vacuum at a predetermined velocity a second beam of either deuterium or hydrogen atoms to effect a charge-transfer collision with said first polarized atomic beam in said magnetic field and form polarized ionized hydrogen atoms, and extracting said polarized ionized hydrogen atoms.

10. The method according to claim 9 wherein either of said deuterium or hydrogen atoms of said second beam are ionized atoms generated to provide a relative velocity between the atoms of said first and second beams of approximately 2.times.10.sup.7 centimeters/second.

11. The method according to claim 1 wherein said first beam comprises neutral atoms of either lithium, cesium, or deuterium and said second atomic beam generation and charge-transfer collision comprises generating a magnetic field, generating in a partial vacuum at a predetermined velocity a second beam of either lithium or cesium atoms to effect a charge-transfer collision with said first polarized atomic beam in said magnetic field and form polarized ionized atoms, and extracting said polarized ionized atoms.

12. The method according to claim 1 wherein said first beam comprises neutral atoms of either lithium, cesium, or deuterium and said second atomic beam generation and charge-transfer collision comprises generating a magnetic field, generating in a partial vacuum at a predetermined velocity a second beam of either deuterium or hydrogen atoms to effect a charge-transfer collision with said first polarized atomic beam in said magnetic field and form polarized ionized atoms, and extracting said polarized ionized atoms.