US2882396A

US2882396A - High energy particle accelerator

Info

Publication number: US2882396A
Application number: US389508A
Authority: US
Inventors: Ernest D Courant; Hartland S Snyder; Livingston Milton Stanley
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-10-30
Filing date: 1953-10-30
Publication date: 1959-04-14
Anticipated expiration: 1976-04-14

Description

April.14, 1959 E. D. (iZOURANT EI'AL 2,882,396 HIGH ENERGY PARTICLE ACCELERATOR Filed Oct; 30, 1953 2 Sheets-Sheet 1 INVENTOR. ERNE5T a C0 ANT MILTON STA/V4 LIVINGSTON HHETLA/VD 6f .FA/YDEE E. 15'. COURANT ETAL HIGH ENERGY PARTICLE ACCELERATOR April 14, 1959 2 Sheets-Shea 2 Filed Oct. 30, 1953 FIG. 6.

' INVENTORS ERNEST 0. (Oak/4N7" Y LII M76570 SNYDER 7 ATTORNEY United States Patent HIGH ENERGY PARTICLE ACCELERATOR Ernest I). Cannot, Blue Point, and Hartland S. Snyder,

Bayport, N.Y., and Milton Stanley Livingston, Belmont, Mass, assignors to the United States of America as represented by the United States Atomic Energy Commission Application October 30, 1953, Serial No. 389,508

2 Claims. (Cl. 250-27) The present invention relates to a new and improved method and apparatus for focusing and accelerating moving charged particles. More particularly, apparatus in corporating the improved method of the persent invention can be used for accelerating charged particles to energy levels as high as 'l00 billion electron volts (100 b.e.v.).

It is .well known that at the present time the highest energy particle accelerator is the Cosmotron at Brookhaven National Laboratory which has attained 2.3 b.e.v. The present invention can therefore be used to provide a particle accelerator having almost 50 times the highest energy attained today. This can be accomplished without aproportionate increase in the cost of the accelerator.

The history of high energy particle accelerators indicates that higher and higher energy levels are needed by nuclear physicists to obtain information relating to properties of matterand energy. Information about nuclei are obtained by bombarding them with high energy particles such as protons and detecting the particles emitted from the nuclei. In order to learn about particles produced in nuclear reactions, it is necessary to have instruments which will produce these particles in the laboratory under controlled conditions.

Many techniques have been used during the past years employing a variety of equipment ranging from Van de'GraatI generators to cyclotrons, betatrons, synchrotrons and proton synchrotrons. It has been determined that increasing the energy of proton synchrotrons using conventional .methods will lead to prohibitive cost factors which will prevent a practical machine for more than 15 b.e.v. from being constructed. For example, the present day Cosmotron uses a magnet which weighs 2000 tons. .The Bevatron, which is the proton synchrotron under construction at the University of California, has a design level 0136 b.ev. and has a magnet weighing 10,000 tons. In the: Cosmotron or Bevatron, which use known proton synchrotron techniques, the weight of the machine increases faster than the square of the desired energy. Thus, a machine of times the diameter of the Cosmotron would produce some 30 b.e.v. and would weigh 200,000 tons.

On the other hand, apparatus incorporating the method of the present invention can accelerate charged particles 110-100 b.e.v. using magnets of total weight of about 8,000 tons.

Accordingly, it is an object of the present invention to provide a new and improved method and apparatus for focusing moving charged particles.

Another object of the present invention is to provide a new and improved apparatus for accelerating charged particles to energies substantially higher than those presently attainable While keeping the apparatus within reasonably attainable dimensions.

vAnother object of the present invention is to provide a high energy particle accelerator incorporating the strong focusing principle and using magnet sectors which are separated by fieldfree regions.

2,882,396? Patented Apr. 14, 1.959

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An object of the present invention is also to provide a new and improved particle accelerator wherein the gradient of the magnetic field varies azimuthly so that adjacent ends of successive magnetic sectors have the same gradients.

An object of the present invention is to provide a particle accelerator having discrete magnet sectors wherein the gradient of the magnetic field is reversed within each magnet sector. I

More particularly, the present invention includes means for establishing a magnetic field in discrete sectors along the path of the moving charged particles, the magnetic field varying in each sector in accordance with the relation:

where B is the value of the magnetic field at the cool: librium orbit of radius r of the path of the particles, B equals the magnetic field at the radius r of the chamber and n equals the magnetic field gradient index, the polarity of n being abruptly reversed a plurality of times as the particles travel along their arcuate path, whereby the particles are alternately converged towards the .axis of their equilibrium orbit and diverged therefrom i-n snc: cessive sectors with a resultantfocusing efiect. i

The many objects and advantages of the present invention may best be appreciated by reference to the ac: companying drawings, the figures of which illustrate apparatus incorporating preferred embodiments of the present invention and capable of carrying out the method of the invention. v

In the drawings: j

Figure !l is a plan view of a portion of thecircumference of a particle accelerator incorporating the strong focusing principle of the present invention.

Figure 2 is a transverse cross-sectional view of a magnet sector taken along the line 2-2 of Figure 1.

Figure 3 is a transverse cross-sectional view of amagnet sector taken along the line 3-3 of Figure 1.

Figures 4 and 5 are diagrammaticv representations used to explain the strong focusing principles of the present invention.

Figure 6 is a transverse cross-sectional view taken along the line 66 of Figure 1.

Figure 7 is a diagrammatic representationof target insertion apparatus taken generally along the line 7-- -7.Of Figure 1.

The operation of the present invention will be more easily understood if a short review is included of the method of operation of present day proton synchrotrons. The Cosmotron will be chosen as the typical proton synchrotron in operation today, as a complete discussion thereof can be found in entire issue of the Reviewof Scientific Instruments for September 3.

In the Cosmotron, a beam of high energy particles is injected, by means of a Van de Graaff generator, into an evacuated annular tube of rectangular cross-section. This annular chamber rests between the poles of. a C shaped magnet, so that the magnetic field permeates the evacuated space within the tube in-a direction perpendicular to the path of the injected protons.

The protons are accelerated once every revolution by a pulsed electric field and the magnetic field is propon tionately increased to keep the protons in a'prescribed closed orbit. Many protons do not follow this equilib rium orbit because of angular and energy spread in the injected beam, scattering by the residual-gas, magnetic inhomogeneities and frequency errors in the accelerating electric field. In the Cosmotron, the total path traversed by a particle is 150,000 miles. Therefore, any particle that is aimed incorrectly will wander far from the ideal orbit long before the final energy is reached unless it is towards the ideal orbit whenever it deviates therefrom.

To prevent the loss of these protons, which do not follow the prescribed equilibrium orbit, it is, common practice to shape the magnetic field to provide restoring forces to the protonsthat stray from the equilibrium orbit. These restoring forces give rise to the well-known betatron oscillations about the equilibrium orbit. These protons now oscillate in stable orbits which are displaced radially and vertically from the equilibrium orbit. They are not exactly in phase with the main beam, but on the average they receive the proper amount of energy from the accelerating field to remain in a stable orbit and increase in energy.

The aperture of the tube through which the beam passes must be large enough to accommodate the maximum amplitude of the oscillating protons in order to prevent loss thereof. The resulting increase in the size of the tube naturally carries with it an increase in the magnet size in addition to the high vacuum problems present in evacuating large volumes. Inthe conventional proton synchroton the magnetic restoring forces for the protons are produced by shaping the magnetic field so that the field changes as the radius of the'proton orbit varies. In a circular machine, the primary function of the magnetic field is to provide a centripetal'force directed toward the center of the accelerator to bend the particles into a circular orbit. To provide focusing forces, the magnet is shaped so that when a particle is just above the ideal orbit, the force on it has a small downward component in addition to the main bending force. Similarly, a particle below the ideal orbit experience an upward component of force. Such restoring forces are produced by a magnetic field having lines of force concave toward the center of the orbit. That is, the poles of the magnet are shaped so that the magnetic field strength decreases as the orbit radius increases.

Thus, the above-described field will provide a vertical focusing or restoring force. This vertical restoring force could be made as strong as desired, except for the fact that the beam must also be restored in a horizontal direction. That is, the beam may move inward or outward 1 radially while remaining in the same horizontal plane as theequilibrium orbit. It will be shown below that in conventional proton synchrotrons it is not possible to where B is the magnetic field at the equilibrium orbit of radius r and B is the magnetic field at the radius r. Differentiating this equation, we derive:

n B E (2) It is known in the art that the magnetic field gradient index n in conventional proton synchrotrons must have a magnitude between 0 and 1 in order to maintain the oscillating protons in stable orbits. A value of n outside of these limitswill result in the loss of protons which are not traveling in the equilibrium orbit. Conventional synchrotrons are therefore designed with many magnetic sectors, each having the same magnetic field gradient index n. The actual magnitude of n" is chosen from other design considerations.

In the conventional synchrotron, the frequency of the bctatron" oscillation is given by the following equations:

where fz is the frequency of the vertical oscillation, f, is the frequency of the radial (horizontal) oscillation and f is the frequency of revolution of the moving charged particles. The amplitudes of the oscillation frequencies are inversely proportional to the frequencies for a given angular deviation. Therefore, it is apparent that the aperture of the chamber through which the charged particles pass can only be decreased in one direction at the expense of the other and thatthe minimum aperture for both occurs when n equals 0.5. That is, if n is increased, the vertical oscillation frequency, f will increase,..de creasing the vertical amplitude and therefore the required vertical aperture. On the other hand, the same increase in n will decrease the radial oscillation frequencyand thereby increase the amplitude of the horizontal'o'scillations and the required horizontal aperture.

Therefore, in conventional synchrotrons, the strength of each of the two focusingforces (horizontal and vertical) is limited by the requirement that the other must also be present. The vertical and horizontalvfocusing forces each have a maximum strength jbeyond. which the other will not exist. Accordingly, a delicate balance must be maintained between the two focusing conditions.

The above description of a conventional ,proton syn chrotron indicates the difiiculties presented in attempting to obtain higher energy particles using the conventional arrangement of the magnetic restoring forces. By contrast, an apparatus incorporating the method of the present invention can use the strongest obtainable magnetic fields as restoring forces for each of the vertically and horizontally displaced particles without adversely affecting the other restoring force.

In a synchrotron using the principles of the present invention as illustrated by Figures 1, 2 and 3, the betatron oscillation frequencies are: 1.

where the coeflicient n is a function of n, and N is the number of magnetic sectors. From Equation 4, it can be seen'that the betatron oscillation frequencies can be increased by using a large number of magnetic sectors and correspondingly large values of n. a

The above result is brought about in the present invention by abruptly reversing the magnetic field gradient index from a positive n value to a negative n value and conversely a plurality of times as the particles travel along their arcuate path. Y

In Figure 1, which represents apparatus incorporating the method of the present invention, the particles tobe accelerated are injected into the annular chamber 10 by means of a conventional Van de Graafi generator or linear accelerator 11 .and a conduit 12. Uniformly spaced along the annular chamber 10 are

magnetic sectors

20, 40, etc. For purposes of clarity, only four of the magnetic sectors and a portion of the annular cham her are illustrated, but it is apparent that the annular chamber extends in the form of a circle of 'very large radius and the magnetic sectors are uniformly spaced along the entire circumference of the annular'charnber 10. Since the circle is of very large radius, the sections of the chamber 10 between the magnet sectors may be straight without unduly affecting the orbit of the particles.

These straight sections are very important in the operation of a practical particle accelerator which uses the method of the present invention. It is apparent that the magnetic field established by sectors 20, 40'etc. doe's'n'ot substantially extend intothestraight sections. There fore importantequipment may be coupled to'the annular chamber at-the straight sections without'having-to take into account the etfect of the magnetic field. Forexample, conduits 13'and'14 are shown connected to the chamber 10 between sectors 20 and '40. These conduits are in turn connected to vacuum pumps '(not shown) ating units 30, shown in diagrammatic form. These radio-frequency units include large ferrite toroids which surround the annular chamber and to which is connected a source of radio-frequency power. Units of this type are more fully described in the Review of Scientific Instruments (September 1953), pp. 779805. It should be appreciated that each straight section of the chamber 10 is not surrounded by an RF. unit. The number of R.-F. units required depends on the size of the accelerator and the amount of energy imparted to the particles by each unit.

In other straight sections may be inserted a target insertion apparatus which places the target into the path of the particles when they have reached their desired energy level. Referring to Figure 7., a diagrammatic representation of the target insertion apparatus is shown connected to a portion of the chamber 10. A cylindrical flanged insert 51 is mounted on the wall of chamber 10 and extends through an opening 52 in the side thereof. Insert 51 is hermetically sealed by means of a gasket 53.

Reciprocally mounted within said insert 51 is a piston 54 which supports the target 55 at the internal end thereof. The external end of piston 54 travels through and is controlled by a solenoid 56. Piston 54 is hermetically sealed within insert 51 by O-rings 57 and 58.

It can be seen that energization of solenoid 56 forces piston 54 into the chamber so that target 55 intercepts the circulating particles.

Referring now to Figure 2, it is seen that the magnetic sector 20 has a generally O-shaped cross-section. The

pole tips

21 and 22 of the magnet 20 are shown in Figure 2 to provide a negative field gradient in the chamber 10. That is, the magnetic field decreases with increasing radius of the particle orbit. For the shape of

pole tips

21 and 22 illustrated in Figure 2, it is seen that the magnetic field is strongest at the most inward point of the annular chamber 23 and weakest at the outermost point 24. The pole tips are shaped so that this magnetic field decreases uniformly with orbit radius from points 23 to 24 of the annular chamber 10. The magnet coil windings 26 are shown wound about opposite poles of the magnetic sector 20. It is apparent that as these coils are energized, a magnetic field will be set up as shown by the dotted lines 27.

Referring now to Figure 3, a transverse cross-section of magnetic sector 40 is shown. Sector 40 has

poles

41 and 42 which are shaped to provide a positive magnetic field gradient, that is, the magnetic field increases with orbit radius from point 43 to point 44 of the chamber 10.

It is therefore seen that as the particles proceed through the annular chamber 10, they are first subjected to a negative field gradient in the magnetic sector 20 and upon emerging therefrom they travel through a straight magnetic field-free section. They are then subjected to a positive magnetic field gradient of sector 40. The particles continue to be subjected to successive alternate field gradients as they proceed around the annular chamber. Therefore, the field gradient abruptly reverses a plurality of times as the particles travel along their arcuate path. 7

As indicated above, the negative field gradient is best for restoring vertically displaced particles, while the positive field gradient is best for restoring radially displaced particles. However, while particles in sector 20 are subjected to a strong radial focusing force, they are also subjected to a vertical defocusing force. Similarly, in magnetic sector 40, while the particles are subjected to a strong vertical focusing force, they are simultaneously subjected to a strong radial defocusing force. This is similar to sending a beam of light through a sequence of equally strong converging and diverging lenses.

At first, it would appear that subjecting the particles to such equal and opposite magnetic lenses would not necessarily have a resultant converging effect. However, .the restoring force acting ona particle is proportional to the distance of that particle from the equilibriumorbit. Therefore, the farther away from the equilibrium orbit it is located, the stronger the focusing or defocusing force.

To illustrate the above, refer to Figures 4 and 5. In Figure 4 is shown a particle at position A which is radially displaced from the equilibrium orbit O in sector 20. As this is the negative field gradient sector, the .particle will be subjected to a strong vertical focusing force and a strong radial defocusing force. The latter force tends to move the particle to a position A, further away from the equilibrium orbit than its ori'ginalposition'A, while traveling through the magnetic sector 20. Upon leaving the sector 20, the particle passes through a straight field-free section where its velocity is relatively unaffected and enters magnetic sector 40. The particle is now subjected to the focusing forces of the magnetic field of the magnetic sector 40 as shown in Figure 5. In this positive field gradient sector, the particle is subjected to a strong vertical defocusing force and a strong radial focus ing force. However, now at position A, the particle is farther away from the equilibrium orbit 0 than it was at position A. It will therefore be subjected to a stronger radial focusing force in magnetic sector 40 than the previous radial defocusing force to which it was subjected in sector 20. Accordingly, the particle will now tend to move to a new position A which is closer to the equilibrium orbit than the original position A.

Similarly, returning to Figure 4, a particle at position B, which is vertically displaced from the equilibrium orbit 0 in sector 20, will be subjected to a strong vertical focusing field and move to new position B. In the magnetic sector 40, as shown .in Figure 5, the particle will be subjected to a strong vertical defocusing force. However, since the vertically displaced particle is now closer to the equilibrium orbit at B than it was at B in sector 20, this vertical defocusing force will tend to move the particle to a new position B" which is closer to the equilibrium orbit 0 than its original position.

It is therefore seen that a particle subjected to magnetic fields of such alternate gradient configurations will have a resultant convergent elfect toward the equilibrium orbit O regardless of whether the particle is originally vertically or radially displaced. It is apparent that particles that are both radially and vertically displaced will be affected in a similar manner.

In operation, therefore, the moving charged particles are injected into the annular chamber 10 by means of the accelerator 11 while the magnetic field is established at a low level. The particles enter the magnetic sector 20 and are curved by the effect of the magnetic field as well as being subjected to strong vertical focusing forces and strong radial defocusing forces. On emerging from the magnetic sector 20, the energy of the particles is increased upon passing through the 'R.-F. accelerating unit 30 in a manner described in the above-mentioned Review of Scientific Instruments article. The particles now enter the magnetic sector 40 at an increased energy. The magnetic field is proportionally increased to curve the particles a sutficient amount in accordance with their increased energy.

In magnetic sector 40, the particles are subjected to strong radial focusing and strong vertical defocusing. Upon emergence from magnetic sector 40, they may again pass through an R.-F. accelerating unit 30 which further accelerates the particles, thereby increasing their energy. The particles then enter'a magnetic sector of negative field gradient similar to magnetic sector 20 and the cycle is repeated. It is therefore seen that each time the particle makes a complete orbit, its energy has been increased by each R.-F. accelerating unit and the magnetic field has been proportionately increased to maintain the orbit of the particles at a constant value. This continues until the particles reach the desired energy level, at which time solenoid 56 of Figure 7 is energized. This inserts target 7 55 into the path ofthe -particles.=.which bombard the target and produce the desired nuclear reactions.

Instead of such target insertion apparatus, box 50 may be used to diagrammatically illustrate ejection apparatus with which it ispossible to eject the particles from the annular chamber at the desired energy level. One such apparatus is described in Patent #2,599,l88, issued to Dr. M. Stanley Livingston.

The means for increasing the magnetic field in proportionto the increased momentum of the moving charged particles can be carried out by a combination generatorinverter set. This unit is shown diagrammatically in Figure 1 by box 70 which is connected to a magnet sector by conductors 71 and 72. It is apparent that the various magnet sectors can be connected in parallel to the same or similar units. The operationtof the generator-inverter set-is more fully described in the Review of Scientific Instruments (September 1953),pp. 769-772. Also, for most efficient operation of the accelerator, it is desirable that all of the R.-F. accelerating units 30 be properly phased so that a suitable amount of energy is transferred to the particles as they pass through the R.-F. units.

It has been determined that a'particle accelerator, using the method of the strong focusing principle herein described, constructed to accelerate particles to 100 b.e.v., could have 176 magnet sectors and an orbit radius of approximately 1000 feet The magnitude of n would be 1200 and each magnet sector would be about 28 feet long. The straight sections of the annular chamber in the field-free regions (between magnetic sectors) would be 7 feet long. The polarity of n would of course vary abruptly from 1200 to -1200 as the particles proceed along their orbit path. However, it is not essential'that the magnitude of n always be the same for both the negative and positive field gradients. For certain purposes it may be desirable to have magnetic field gradient indexes of different magnitudes in the same accelerator.

It is apparent that field-free regions between magnetic sectors are essential to the practical, economical operation of a synchrotron incorporating the strong focusing principle. By'having such field-free regions, it is possible to insert the RAF. accelerating without distorting the magnetic field. Similarly, the conduits leading to the high vacuum pumps, can be located in the field-free regions. It is also apparent that injection and ejection of the particles'are greatly simplified by the use of such straight field-free regions. In addition, with field-free regions, each magnet sector can be Wound individually with a large saving in cost.

The performance of a particle accelerator incorporating the method of the present invention may be improved by further usage of the straight field-free sections of the annular chamber. Referring to Figure 6, a transverse cross-sectional view of a-quadruple magnet 60 is shown.

Magnet 60 has two north poles 61 and two south poles 62, all with pole faces shaped as rectangular hyperbolas. Equal energizing coils 63 are wound about the magnet poles and are spaced therebetween as illustrated. When coils 63 are energized, a magnetic field is set up as shown by dotted lines 64. The established field is a doublydivergent magneticfield varying uniformly and equally from the axis of chamber 10 in both the horizontal and vertical directions. Also, along the axis of chamber 10, the magnetic field is zero.

If a second lens is similarly set up in the next succeed ing straight section, the energizing coils 63 can be arranged so that poles 61 are south poles while poles 63 are north poles. The direction of the established magnetic field of this second magnet will therefore be the reverse of that in the first magnet. Accordingly, particles that travel through these magnets will be successively subjected to diverging and converging magnetic fields which will introduce an additional focusing mechanism in the operation of the particle accelerator. These magnets also can be used for a difierent purpose if it is so desired. In Equation 4 above the betatron oscillation frequencies f and f, are indicated. If either f (vertical frequency) or f, (horizontal frequency) is an integral or half-integral multiple of the revolution frequency of the particlesor if the sum of these oscillation frequencies is a multiple of the revolution frequency, apossibility of beam instability exists. If that occurs, the second function of the quadrupole magnet lenses becomes important. By properly phasing the energization of these quadrupole lenses, it is possible to change the betatron oscillation frequencies and thereby avoid the beam instability difficulties.

A number of these magnet lenses may be used at various straight sections as they are needed. For example, two out of every five of the straight sections may be utilized in this fashion.

Therefore the term magnetic field-free region is meant to indicate a'straight section of the annular chamber 10 between the magnet sectors. That is, these straight sections are field-free" only in the sense that the magnetic field established by the magnet sectors does not substantially extend into the area of the straight sections. Obviously, if a magnet lens of the type described herein ?bove is included, the straight section will not be fieldree.

Accordingly, it is desirable to have the largest possible length of straight sections along the annular chamber for the use of the above-described essential equipment. However, the size of these straight field-free sections cannot be increased indefinitely without affecting the orbit of the particles. That is, the particles are curved and focused only during the time that they are within the region of the annular chamber permeated by the magnetic field. We have determined that the fieldfree regions between magnetic sectors should be no more than 25 percent of the length of the magnet, sectors in order to have proper operation of the embodiment illustrated in Figures 1, 2 and 3.

It will be noted that the magnetic sector 20 had a negative field gradient, while the magnetic sector 40 had a positive field gradient. The particles in the abovedescribed embodiment therefore were transmitted successively through a a (0) (field-free) and a field gradient. That is, the particles proceeded through the following configurations:

etc., with the (0) field-free region limited to a maximum of 25% of the length of a magnet sector.

We have found that the length of the field-free region can be increased in a second embodiment of the strong focusing principle. This second embodiment subjects the moving particle to the following polarity field gradient:

etc. That is, the positive and negative field gradients are combined in one composite magnet sector and the polarity scheme of each sector is thus azimuthally reversed as the particle proceeds along its orbit through that com posite sector. I

Figures 1, 2 and 3 may be used to illustrate this second embodiment. If a transverse cross-sectional view is taken through a composite magnet sector 40 employing this second embodiment and on a plane corresponding to the line 2' 2.' in one extremity of magnet sector 40 of Figure 1, it will appear as Figure 2. Therefore, in one composite magnet sector such as 40, the first half would have poles such as 41 and 42 illustrated in Figure 3, while the shape of the pole tips in the second half would be as shown in Figure 2 taken along line 2- This configuration is then reversed in the next succeeding composite sector with the first half being negative as in Figure 2 and the second half being positive as in Figure 3. That is, the magnetic field gradient index It would be the same polarity in adjacent halves of successive magnet sectors.

For example, the polarity of the magnetic field gradient index n in the half of sector 40 closest to R.-F. unit 30 in Figure 1 would be positive. The polarity of the magnetic field gradient index n in that half of the sector 20 closest to R.-F. unit 30 would also be positive, so that the adjacent halves of

sectors

20 and 40 would have the same polarity magnetic field gradient indexes.

By use of this configuration, we have determined that the field-free regions may be increased to one-half the length of the magnetic sectors rather than the 25% of magnet sector length possible with the first-mentioned embodiment. This gives more room for the auxiliary apparatus as well as the end windings of the coils wound about the poles of the C-shaped magnets.

It is apparent that various other configurations can be used incorporating the same strong focusing principle. For example, the plan View of Figure 1 indicates that the back leg (closed portion) of the C-shaped magnet of all the magnetic sectors is always placed radially inward from the annular chamber. This would make the ejection and use of negative particles more convenient. However, if the back legs of several successive magnet sectors are placed radially outward from the annular chamber, it will be possible also to eject and use positive particles that may be emitted from the target inserted into the proton beam.

Another configuration would be to use one magnet sector for curving the accelerated particles and another magnet sector for focusing the accelerated particles, instead of these features being combined in each magnet sector.

While the salient features of this invention have been described in detail with respect to several embodiments, it will of course be apparent that numerous modifications can be made within the spirit and scope of this invention and it is therefore not desired to limit the invention to the exact details shown, except in so far as they may be defined in the following claims.

We claim:

1. Apparatus for accelerating moving charged particles to extremely high energy levels which comprises, in combination, an evacuated annular chamber, means for injecting said particles into said chamber, means for establishing a magnetic field in discrete circumferential sectors said chamber to cause said particles to follow an arcuate path in said chamber, adjacent sectors being separated by a region free of said magnetic field, said means for establishing said magnetic field including a plurality of electro-magnets having a C-shaped cross section and hyperbolic pole tips to provide a magnetic field in each sector in accordance with the equation:

where B is the value of the magnetic field at the equilibrium orbit of radius r of the path of the particles, B is the magnetic field at radius r of the path of the particles and n is the magnetic field gradient index, said magnet means including a pole-tip arrangement in which, in a predetermined distribution about said path, the hyperbolae of said pole tips are open and closed in the radial direction in difierent sectors whereby the polarity of n is reversed abruptly a plurality of times as the particles travel along their arcuate path, the shape of each pole tip being constant along its length of the annular chamber, radio frequency accelerating units positioned in regions between said circumferential sectors, each of said accelerating units increasing the energy of said particles as they pass 10 therethrough, means for varying the magnitude of said magnetic field in proportion to the increased momentum of said particles and means for utilizing said particles when they reach the desired energy.

2. Apparatus for accelerating moving charged particles to extremely high energy levels which comprises, in combination, an evacuated annular chamber having an axis, means for injecting said particles into said chamber, means for establishing a magnetic field in discrete circumferential sectors of said chamber to cause said particles to follow an arcuate path in said chamber, adjacent sectors being separated by a region free of said magnetic field, said means for establishing said magnetic field including a plurality of electromagnets having a C-shaped cross section and hyperbolic pole tips to provide a magnetic field in each sector in accordance with the equation:

where B is the value of the magnetic field at the equilibrium orbit of radius r of the path of the particles, B is the magnetic field at radius r of the path of the particles and n is the magnetic field gradient index, said magnet means including a pole-tip arrangement in which, in a predetermined distribution about said path, the hyperbolae of said pole tips are open and closed in the radial direction in different sectors whereby the polarity of n is reversed abruptly a plurality of times as the particles travel along their arcuate path, the shape of each pole tip being constant along its length of the annular chamber, radio frequency accelerating units positioned in regions between said circumferential sectors, each of said accelerating units increasing the energy of said particles as they pass therethrough, means for varying the magnitude of said magnetic field in proportion to the increased momentum of said particles, focusing means positioned about the axis of said chamber in field-free regions to stabilize the arcuate path of said particles, said focusing means including a plurality of quadrupole magnetic lenses having rectangular hyperbolic pole faces to establish a magnetic field that varies uniformly in both the horizontal and vertical directions from the axis of said chamber, the polarity of said poles of said lenses being arranged so that each lens is effectively rotated with respect to the next succeeding lens, and means for utilizing said particles when they reach the desired energy.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Crane: The Racetrack: A Proposed Modification of the Synchrotron; Dennison et al.: The Stability of Orbits in a Racetrac Physical Review, vol. 69, pp. 242- 243, 1946.