US 3379911 A
Description (OCR text may contain errors)
April 23, 1968 H. A. ENGE 3,379,911
PARTICLE ACCELERATOR PROVIDED WITH AN ADJUSTABLE 370 NON-DISPERSIVE MAGNE CHARGED-PARTICLE BEAM BEND Filed June 11. 1965 2 Sheets-Sheet 1 Apnl 23, 1958 ENGE 3,379,911
PARTICLE ACCELERATOR PROVIDED WITH AN ADJUSTABLE 2 0 NON-DISPERSIVE MAGNETIC CHARGED-PARTICLE BEAM BENDER Filed June 11, 1965 2 Sheets-Sheet B United States Patent M ABSTRAGT 6F THE DISCLGflURE A uniform magnetic field is produced between poles of a magnet. The poles include shim pole pieces which permit variation of the field boundaries and also introduction of a nonuniformity into the field, so as to minimize dispersion of the beam. In this way, the beam from a charged-particle accelerator may be deflected to a variety of beam-utilization areas without altering the position of the accelerator itself.
This invention relates to the deflection of beams of charged particles and in particular to a novel method for effecting such deflection without substantial dispersion of the beam and to novel magnetic apparatus for carrying out the method. The method comprehends providing a uniform magnetic field with well defined boundaries and adjusting the strength of the magnetic field and the position of the boundaries with respect to a given beam of charged particles, such that when the beam of charged particles is injected into the magnetic field through one boundary of the field the particles will describe a generally circular trajectory or series of trajectories and emerge from the magnetic field through another boundary thereof after having been deflected through an angle of approximately 270". At a portion of this trajectory, a nonuniformity is introduced into the magnetic field and the exit boundary is adjustable so that the angle between the exit boundary and the emergent trajectories may be adjusted.
In connection with particle accelerators for the acceleration of charged particles to high velocity, it is frequently desirable to have additional apparatus for changing the direction of the charged particle beam which has been accelerated by the particle accelerator. For example, a need to bend the charged particle beam may be caused by geometrical limitations of the room in which the particle accelerator is located, or it may be desired to direct the charged particle beam alternatively into one of several beam utilization areas. In electron irradiation installations, wherein products are irradiated on a conveyor by an electron beam from an electron accelerator, space limitations frequently require that the accelerator be mounted horizontally, but the need for a horizontal product conveyor may make it desirable to have the emergent beam travelling in the vertical direction as it strikes the product. Again, it will occasionally be desirable in radiographic installations to cause the electron beam to be deflected before striking the target for the production of X-rays. In the case of large accelerators for the acceleration of charged particles to high energy for studies in nuclear physics and the like, the cost of operating the accelerator and the original capital cost of the accelerator are so large that such an accelerator will be used for a variety of purposes, so that generally there will be several beam utilization areas associated with such an accelerator, and some sort of beam bending device is required (such as a switching magnet), in order to direct the beam into one of the beam utilization areas.
It will be apparent from the foregoing that the inven- Patented Apr. 23, T1968 tion is not limited to any particular deflection angle. *However, for the purposes indicated, a very common angle of deflection is and so the invention will be described with particular reference to an effective deflection angle in the vicinity of 90 (the actual deflection angle being in the vicinity of 270), but it will be apparent from the following that the invention is not limited to this range of defiection angles.
Of course, it is well known that a magnetic field trans verse to the direction of motion of a charged particle will exert a deflecting force on the charged particle. However, the deflection action itself always introduces some focusing or defocusing action which may have an undesirable result unless proper precautions are taken. In particular, the inherent energy spread in the beam from a microwave linear accelerator, introduces problems which are not encountered in magnetic systems used with accelerators producing a monoenergetic beam. For a monoenergetic beam one speaks of median-plane focusing and normalplane or vertical focusing. If focusing inboth planes is positive (is. the rays are converging at the exit and coming to a focus at the same point), one calls this point a stigmatic image point. When the beam contains particles of different momenta, the dispersive power of the magnet in the median plane will in general produce a momentum spectrum at the image. If, by special arrangements the dispersion is zero at the image the magnet may be said to be triple-focusing: i.e., it focuses particles of different momenta and particles following different orbits from an object point to the same image point. All such focusing is firstorder focusing, and therefore no vertical momentum focusing action is required, since any dispersion introduced by the magnet owing to differing momenta among the particles is a second-order effect.
The invention may best be understood with reference to the following detailed description thereof, having reference to the accompanying drawing in which:
FIGURE 1 is a top view of the lower pole face of a horizontally disposed bending magnet constructed in accordance with the principles of the invention;
FIG. 2 is a sectional view along the line 2-2 of FIG- URE 1,
FIG. 3 is a view similar to that of FIG. 1 showing a slight modification of the embodiment of the invention shown in FIGS. 1 and 2; and
FIG. 4 is a sectional view along the line 44 of FIG. 3.
Referring to the drawing and first to FIGS. 1 and 2 thereof, a magnet 1 having a pair of pole faces 2, 3 is positioned in the path of a charged particle beam 4 which may be produced, for example, by a suitable charged particle accelerator 5 so that the charged particle beam 4 enters between the pole faces 2, 3 of the magnet 1. Of course, this charged particle beam 4 will in general have to travel in an evacuated region and accordingly a suitable vacuum chamber 6 must surround the beam 4 at all times during its trajectory, Charged particles travelling through the magnetic field between the pole faces 2, 3 of the magnet 1 will travel along a circle having a radius of curvature which is proportional to the momentum of the particle and inversely proportional to the strength of the magnetic field, as is well known. The strength of the magnetic field is so related to the energy of the incoming beam that the beam is deflected by the magnetic field through an angle of approximately 270 before emerging therefrom. The over-all effect, therefore, is to deflect the beam so that it emerges from the magnetic field on a trajectory which is at an angle of 90 to the incoming trajectory. Before the beam enters the magnetic field, it is said to be in object space; after the beam leaves the magnetic field, it is said to be in image space. Magnetic shim pole pieces 7, 8 may be provided respectively at the entrance and exit boundaries of the magnetic field as shown, so that the angle between the beam trajectory and the magnetic field boundary may be adjusted both at the entrance and at the exit. In addition, at the central portion of the trajectory, additional shim pole pieces 9, are introduced so that a nonuniformity may be introduced into the uniform magnetic field. These shim pole pieces are referred to as gradient shim pole pieces and are so positioned in order that they may have maximum effect upon momentum focusing and much smaller effect upon spatial focusing in either plane. The function of the gradient shim pole pieces is to provide momentum focusing and they will be adjusted so that the magnetic field increases at the appropriate rate towards the outer periphery of the family of nearcircular trajectories. That this will provide momentum focusing is apparent since particles having greater than average momentum will have a trajectory of greater than average radius of curvature and hence will travel through a region of stronger magnetic field as it traverses the gradient shim pole pieces. This greater magnetic field will bend the particles so to speak back towards the trajectory of those particles having average momentum.
Thus each pole face 2, 3 consists of five sections. The bottom pole face 2 comprises magnetic shim pole piece 8, stationary piece 18, gradient shim pole piece 9, stationary piece 17, and magnetic shim pole piece 7. The upper pole face 3 comprises magnetic shim pole piece 8', stationary piece 18, gradient shim pole piece 10, another stationary piece (not shown) and another magnetic shim pole piece (not shown).
Momentum focusing is defined as convergence towards a point (the momentum focus) in image space of particles of different momenta but the same charge entering the magnetic field along the center line of the incident trajectory. Alternatively, momentum focusing may be defined as convergence of particles of different magnetic rigidities, where magnetic rigidity is defined as the product of the magnetic fiux density and the radius of curvature of the trajectory and is therefore equal to the momentum divided by the charge. At the momentum focus, there is no momentum dispersion, where dispersion means the spatial separation caused by differing magnetic rigidities in a plane perpendicular to the center line of the trajectory in the image space.
Actually, in accordance with the invention, momentum focusing may be achieved even without the gradienp shim pole pieces, but the gradient shims may be set to make the magnet achromatic, which means that there is a momentum focus at any point along the center line of the trajectory in image space: in other words, particles of differing magnetic rigidity entering the magnetic field along the center line of the incident trajectory will all leave the magnetic field along the center line of the exit trajectory.
In order to understand the invention, consider first the trajectory of a particle on the center line of the incoming beam and having average momentum. The particle first passes by the target 11 missing it by a distance X at the point P of nearest approach to the target. The magnetic field deflects the particle through an angle of 270 and back onto the target 11. The point P is located a distance R from both the entrance boundary and the exit boundary, where R is the radius of curvature of the particles trajectory in the magnetic field. The angle which this trajectory makes to the normal to the entrance boundary is designated A, and the angle which this trajectory makes with the normal to the exit boundary is designated B. These two angles are variable by varying the appropriate shims. The distance between the point at which this particle leaves the magnetic field and the point at which the charged particle beam is to be focused is equal to R-i-X (where X may be either negative, as shown in FIGURE 1, or positive), and this distance is also a variable until the focal point (i.e. the target location) becomes fixed by the requirements of the user of the machine. Once the users requirements have fixed the value of R+X, the central pair of shim pole pieces may then be initially adjusted to provide momentum focusing at the selected focal spot, and the other two pairs of shim pole pieces may then be adjusted so as to provide spatial focusing in both planes at the focal spot. Since the action of all three pairs of shim pole pieces is interdependent, they will in general be adjusted concurrently. For the (or rather 270) deflection angle indicated, a representative angle A is 45 and the angle B has a value which depends upon the image distance R-l-X. For a value of R-l-X equal to 2.73 R, and the gradient shim pole pieces set flush with the pole faces (or with gradient shim pole pieces omitted) so that there is no non-uniformity within the magnetic field, the angle B has a value of 32.4". This calculation assumes that the object is at infinity, the object being the point from which the incoming charged particles diverge or appear to diverge. For the conventional microwave linear accelerator, the incoming charged particle beam is practically parallel so that this asumption is justified. The significant relationship is the ratio between the object distance and the radius of curvature R so that if the object distance is, for example, several yards this represents approximately infinity with respect to the conventional bending magnet for such a device. The foregoing calculation also assumes that the pole-face separation D is small with respect to R: i.e., that D/R is very small, If D/R is not small (e.g. /5 or so), R-l-X must be greater than 2.73 R to prpvide a triple focus, and may even become infinitely large for a large enough value of D/R.
The case in which R-l-X is equal to 2.73 R is shown in FIGURES 3 and 4, which are similar to FIGURES 1 and 2, respectively, and more complete in that the return yoke and coils are shown in FIGURES 3 and 4. Referring to FIGS. 3 and 4, a magnet 21 having a pair of pole faces 22, 23 is positioned in the path of a charged particle beam 24 which may be produced, for example, by a suitable charged particle accelerator 25 so that the charged particle beam 24 enters between the pole faces 22, 23 within a suitable vacuum chamber 26. Magnetic shim pole pieces 27, 23 may be provided respectively at the entrance and exit boundaries of the magnetic field as shown. In addition, at the central portion of the trajectory, gradient shim pole pieces 29, 30 are introduced so that a nonuniformity may be introduced into the uniform magnetic field. As in the embodiment of FIGS. 1 and 2, each pole face 22, 23 consists of five sections. The bottom pole face 22 comprises magnetic shim pole piece 28, stationary piece 38, gradient shim pole piece 29, stationary piece 37, and magnetic shim pole piece 27. The upper pole face 23 comprises magnetic shim pole piece 28', stationary piece 38, gradient shim pole piece 30, another stationary piece (not shown) and another magnetic shim pole piece (not shown). The return yoke is shown at 32, and the coils for excitation of the magnet 21 are shown at 33, 34. Since X is positive, the beam 24 crosses itself before reaching the target 31.
Returning now to the case where D/R is very small, R+X may be reduced below 2.73 R by tilting the gradient shim pole pieces to introduce a nonuniformity in the magnetic field so as to provide momentum focus at image dis tances less than 2.73 R, down to as low as 0.5 R. For one position of the gradient shim pole pieces, which position may readily be determined experimentally by measuring the properties of the emergent beam in the image space while varying the tilt of the gradient shim pole pieces, the magnet becomes achromatic. When the gradient shim pole pieces are set in this achromatic position, X generally is negative so that the target must be placed closer to the exit boundary than is the incident beam.
It will be noted that measurements and angles are taken in the drawing at a line (as at 12 and 13) which is displaced from the edge of the pole pieces. These displaced lines represent the virtual field boundaries. The virtual field boundaries I2, 13 are somewhat external to the edges of the pole pieces 7, 8 respectively, for the reason that the magnetic field does not drop to zero immediately in the vicinity of the edges of the pole pieces, but rather drops off fairly rapidly. The location of the virtual field boundary is well known in the art, and for the usual pole gap D said location is a distance of between .7 and .8 D outside the edge of the pole piece.
It should be noted that the boundaries can and may be curved in order to make second order corrections in focusing.
Having thus described the principles of the invention, together with an illustrative embodiment thereof, it is to be understood that, although specific terms are employed, they are used in a generic and descriptive sense, and not for purposes of limitation, the scope of the invention being set forth in the following claims.
1. Apparatus for bombarding a target with a beam of charged particles, comprising in combination: (1) means for producing a beam of charged particles, (2) means for directing said beam towards said target soas to miss said target by a distance 1.73 R at the point P of nearest approach to said target, (3) means for injecting said beam into a uniform magnetic field perpendicular to the beams trajectory at an incident angle of 45, in the plane defined by the beams trajectory, between the beams trajectory and the normal to the boundary of the field and at a distance R from P, where R is equal to the radius of curvature in said magnetic field of particles in said beam, and (4) means for ejecting said beam from said magnetic field at an exit angle of 324, in the plane defined by the beams trajectory, between the beams trajectory and the normal to the boundary of the field and at a distance R from the incoming trajectory.
2. Apparatus for momentum-focusing a beam of charged particles onto a target and, in so doing, also spatially focusing said beam onto said target in the median plane and in the vertical plane, comprising in combination: (1) means for producing a beam of charged particles, (2) means for directing said beam towards said target so as to miss said target by a distance X at the point P of nearest approach to said target, (3) means for injecting said beam into a uniform magnetic field perpendicular to the beams trajectory at an incident angle A, in the plane defined by the beams trajectory, between the beams trajectory and the normal to the boundary of the field and at a distance R from P equal to the radius of curvature in said magnetic field of particles in said beam, (4) means for ejecting said beam from said magnetic field at an exit angle B, in the plane defined by the beams trajectory, between the beams trajectory and the normal to the boundary of the field and at a distance R from the incoming trajectory, and (5) means for introducing a non-uniformity into said magnetic field at a portion of said trajectory such that the magnetic field increases towards the outer periphery of said trajectory.
3. Apparatus for deflecting a beam of charged particles, comprising a magnet having pole faces and being adapted to produce a uniform magnetic field between said pole faces, a vacuum chamber between said pole faces and enclosing a planned trajectory, said magnet having two boundaries of the magnetic field across said trajectory, a pair of shim pole pieces at each of said boundaries for adjusting the angular position of each of said boundaries, and a pair of shim pole pieces flanking said trajectory at a location between said boundaries for adjusting the angular position of each of the pole-face areas at that location.
4. Apparatus for deflecting a beam of charged particles, comprising a magnet having pole faces and being adapted to produce a uniform magnetic field between said pole faces, a vacuum chamber between said pole faces and enclosing a planned trajectory, said magnet having two boundaries of the magnetic field across said trajectory, and a pair of shim pole pieces flanking said trajectory and moveable with respect to the rest of said pole faces at a location between said boundaries for adjusting the angular position of each of the pole-face areas at that location.
References Cited UNITED STATES PATENTS 2,651,000 9/1953 Linder 313- X 2,777,958 1/1957 Le Poole 31377 3,243,667 3/1966 Enge 335210 2,511,728 6/1950 Long 31363 2,551,544 5/1951 Nier et al 31363 2,593,508 4/1952 Washburn 335-212 3,280,816 10/1966 Priore 328--233 JAMES W. LAWRENCE, Primary Examiner.
DAVID J. GALVIN, Examiner.
V. LAFRANCHI, Asistant Examiner.