US 3659236 A
A magnet to be utilized in systems for the magnetic focusing of charged particles including a pair of iron pole pieces having a variable width gap narrowest at the point of entry of the particles, further having a protrusion at the same point and a yoke for supporting the pole pieces and a pair of symmetrically arranged electromagnetic coils between the yoke numbers for generating a magnetic field.
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Description (OCR text may contain errors)
United States Patent Whitehead, Jr.
 INHOMOGENEITY VARIABLE 1 51 Apr. 25, 1972 [5 6] References Cited MAGNETIC FIELD MAGNET UNITED STATES PATENTS [721 lnvenm" f 3,517,191 6/1970 Liebl ..250/49.s ux 73 Assignee; The United states f America as 2,777,958 1/1957 Poole ..250/49.5 UX
represented by the Secretary of the Air Force Primary Examiner-George Harris Attorneyl-larry A. Herbert, Jr. and Henry S. Miller. Jr.  F1led: Aug. 5, 1970 [211 App]. No.: 61,158  ABSTRACT A magnet to be utilized in systems for the magnetic focusing of  US. Cl. ..335/2l0, 250/495 D charged P including a pair Of iron pole pieces having a 51 1 1111. C1. ..n01r 7/00 variable width W narrowest at the P of entry of the P  Field of Search "250/49 5 CD, 495 D 419 ME; cles, further having a protrusion at the same point and a yoke 335/210 296, 297, 298 for supporting the pole pieces and a pair of symmetrically arranged electromagnetic coils between the yoke numbers for generating a magnetic field.
2 Claims, 5 Drawing Figures 3% g" 3.4 L 2 L PATENTED APR 2 5 I972 SHEET 2 OF 5 PATENTEDAPRS m2 SHEET 3 [IF 5 INVENTOR. 744419.? M Wm 7-4-1154 PATENTEUAPR 25 I972 3,659,236 SHEET 58F 5 INVENTOR. 72mm? wwwmmm l INHOMOGENEITY VARIABLE MAGNETIC FIELD MAGNET BACKGROUND OF THE INVENTION This invention relates generally to magnetic focusing devices for charged particles and more particularly to a focusing device having application in mass spectrometry and electromagnetic isotope separation where a beam of particles possessing a widely distributed momentum spectrum is trans mitted and focused. Known mass spectrometers or isotope separators can accept, transmit, and focus an ion beam with momentum components dispersed over a range of only to percent. These limitations are imposed by mechanical restrictions of the magnetic system and deterioration of the quality of the focused image resulting from geometric lens aberrations. A configuration known as the Mattauch-Herzog design has been used to good advantage to gain an extended range for particle focusing. This design however suffers from the disadvantagesof inadequate range for particular applications and also the ion beam focal plane in this design is located inside the gap of the magnet which is a disadvantageous and inconvenient position for many applications.
Ideally, for many purposes it would be desirable to devise a magnetic focusing system which would accept charged particles from a source (object point) located outside the magnet, transmit the beam through the lens, and focus the image of all masses from I to 1,000 along a straight focal plane outside the magnet with equal separation between each mass position. Where a monoenergetic source of ions is utilized it has been suggested that a uniform magnetic field would provide the desired result. However, the mass separation from a monoenergetic source of ions using a uniform magnetic field depends on the inverse square root of the masses, and this nonlinear scale provides less desirable results than does a linear relationship.
Applicant's invention substantially provides the sought after results by using a magnet with variable magnetic field inhomogeneity in a new and novel manner.
SUMMARY OF THE INVENTION The invention consists of a new and improved magnet that is designed for focusing charged particles. The invention is particularly suited for use in systems where a beam of particles possessing a spectrum of widely distributed momentum components is transmitted and focused or defocused. Specific examples of such systems would include electromagnetic isotope separators and mass spectrometers.
The magnet is made up of a plurality of iron pole pieces held together in any suitable fashion, e.g., bolted together. The shape of the pole pieces is entirely unique in that the inside faces of the pole pieces are aligned so as to be slightly closer at one end than at the other and thereby provide an inhomogeneous field between them. The critical magnet edge is generally straight with a protrusion at one end having a generally concave-convex shape. The protrusion is referred to as the leading edge and has a high field gradient. The straight edge is in an area of diminishing field gradient.
. The yoke of the magnet consists of a plurality of iron plates separated by two electromagnets for generating the magnetic field.
It is therefore an object of this invention to provide a new and improved magnet with a variable magnetic field where the transmitted mass range is much greater than any other practical magnetic system, permitting the simultaneous recording and comparison of widely separated mass peaks.
It is another object of the invention to provide a focusing magnet with variable magnetic field inhomogeneity where the entire focal plane is outside the main magnet and readily accessible and magnetic shielding can be provided for detectors if needed.
It is a further object of the invention to provide a focusing magnet for charged particles of extensive range with a relatively straight focal plane composed of a small number of straight segments.
It is still another object of the invention to provide a new and improved magnet with variable magnetic field inhomogeneity with a uniform focusing quality over the entire focal plane.
It is still a further object of the invention to provide a new and improved magnet for focusing charged particles with a more linear function of mass dispersion than any similar device known.
It is another object of the invention to provide a focusing magnet which is economical to produce and utilizes conventional, currently available components that lend themselves to practical mass-production manufacturing techniques.
These and other advantages, features and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiment in the accompanying drawings.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the focusing distance as a function of magnetic field inhomogeneity for three magnet deflection angles.
FIG. 2 is a graph of the relative field strength in the magnet gap along the axis of variation.
FIG. 3 is a graphic representation of mass separation in the invention.
FIG. 4 is a perspective view of the magnet of the invention; and
FIG. 5 shows the magnet of the invention including mag netic field sensing devices.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT To properly understand the operation of the invention it is necessary to understand the effects of inhomogeneous field magnetic focusing devices in general. Consider a conventional, well-known inhomogeneous magnet having a pair of generally disk-shaped iron pole pieces. The inner faces of the pole pieces are parallel near the center and each slopes gradually away from the other near the outside edge of the disk.
The field inside the magnet is circularly symmetric and decreases with increasing radius approximately as l/r where 1; is the constant of inhomogeneity and 0 s n s 1. This type of field is produced by tapering the iron pole pieces so that the gap increases with increasing radius. It is known that in a magnetic field of this type, for symmetric object-image locations,
[ /2 (1-11) r/21 (kw) 5 1,
where do" is the distance from the object or image to the front edge of the magnet, ro" is the basic radius of the magnet, and 1 is the magnet deflecting angle. By referring to FIG. 1, the equation is evaluated showing the increase in focusing distance for smaller magnet deflection angles and higher field inhomogeneities. Sector magnet angles of I, and 60 using symmetric object-image position are evaluated with do/ r0 as a function of the inhomogeneity n.
The instant invention iepresen'ts a completely new application of the inhomogeneous magnetic field focusing principle. In FIG. 1 it is shown that as the inhomogeneity increases, the relative focusing distances increase, and a sharp increase occurs above 'fl=0.5. Since it is desirable to have a low mass or energy particle, (small radius of curvature in the magnet) focused at about the same distance from the magnet as the high mass particle (large radius), this effect can be obtained by allowing the low mass to traverse a region of high field inhomogeneity and the high mass to pass through a region of comparatively weak field gradient.
FIG. 2 shows the magnetic field distribution produced by the iron pole pieces which characterize the instant invention. The leading edge of the magnet has the narrowest gap between pole pieces. Where the gap is narrowest the field gradient and field strength are high and charged particles, e.g.,
do tan ions traveling through this part of the magnet, are in the region of relatively long focal length.
With regard to FIG. 3, ion beam trajectories are shown for the magnet of the invention, when analyzing or separating isotopes from a monoenergetic source with 10,000 ev. assumed. Within the magnet the field is a function only of the vertical dimension. The axes of the figure are graduated in inches. The ions are generated from the monoenergetic source at 10 and pass through the field of the magnet 12. The particles pass through the narrowest gap initially where a strong gradient field generates the long focal length relative to the radius of curvature. It can be seen then that mass being the lightest particle passes through the field and turns sharply and focuses just outside the maget approximately 5 inches on the horizontal scale from the source. Similarly the heavier mass 40 focuses at a minus 5 inches on the vertical scale, approximately 25 inches from the source. The same holds true for the other illustrated particles. The remaining mass numbers will fall in the appropriate place in the figure.
The trajectories shown are only examples and it is possible to adjust the given parameters to accommodate other situations. For example, the mass range 20 to 1,000 could be analyzed by doubling the field strength or reducing the beam energy by a factor offour.
Concerning FIG. 4, the magnet of the invention is shown consisting of a pair of iron pole pieces 20 and 22. The pole pieces are generally supported and joined to a yoke consisting of the plate-like numbers 24 and 26. A pair of wound wire coils 28 (one not shown) are symmetrically arranged between the yoke numbers and generate the magnetic field in the magnet.
The pole pieces 20 and 22 are generally of a rectilinear shape, however at one corner there is an extension having a concave-convex shape shown at 30. The pole pieces have a gap 32 between them which tapers gradually toward the extension 30 so that the gap is the narrowest at this point. The pole pieces and yoke numbers may each be made of a number of plates held together as shown in the figure by the bolts 34,
or unitized construction is possible using other conventional means of assembly.
. FIG. 5 shows the variable inhomogeneity magnet. The system shown is used to determine the magnetic field distribution and from these data ion trajectories can be computed with good accuracy. characteristically an ion source 40 will feed a supply of ions 42 into the magnet 44 where they are separated according to mass number (or in some applications, according to energy). This is illustrated in FIG. 3.
An appropriate magnetic field sensing element 46 is mounted on an adjustable carriage 48 and may be moved toward and away from the magnet by the screw thread 50. Calibration means 52 located adjacent to the carriage provides for accurate adjustment in this direction. A second carriage 54 is mounted on a threaded screw 56 for lateral movement of the sensor. The second carriage is likewise calibrated (58) for accurate placement of the sensor.
The cable 60 connects to an amplifier and digital recording device as is currently well known and understood in the art.
Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.
What is claimed is: v
l. A magnetic focusing system for charged particles comprising in combination: a pair of identical iron pole pieces, each of the pole pieces having a flat surface on one face and a flat stepped portion on the other face, said pole pieces being mounted in a spaced relationship, closer at one end than at the other; a yoke means for supporting the pole pieces and a plurality of electromagnets mounted in the yoke and adjacent to the pole pieces for generating an inhomogeneous field gradient between the pole pieces.
2. A magnetic focusing system for charged particles according to claim 1 wherein the pole pieces have at least one straight edge, and said straight edge forming a parabolic extension to the pole piece at one end.