US3016458A - Resonator particle separator - Google Patents

Description

3,015,458 Patented Jan. 9, 1962 This invention relates to radio frequency resonators and more particularly to resonators for exerting preferential forces on high energy particles.

In high energy particle physics, it is often desirable to have a means of separating particles of equal momentum but ditferent mass and having energies on the order of several billion electron volts. In the case of particles having energies of about one billion electron volts (1 bev.) relatively simple techniques are available for effecting the desired separation. But in the case of particles having about bev. and greater energies the problem heretofore has not been readily soluble.

Waveguide resonators have heretofore been considered for separation of high energy particles; however, an inherent difliculty with ordinary waveguide resonators arises because they usually have field patterns which include an associated magnetic field component normal to the axis and to the transverse electric field. Because of the interaction of the magnetic and electrical fields in such waveguide resonators, their usefulness in nuclear physics is limited. If an ordinary TE waveguide is used in a resonant system, the waveguide must be operated at cut-off to insure that there is no variation in the instantaneous field pattern in the beam direction (along the so-called z axis), and it is necessary to introduce drift tubes within which the particle beam can be shielded from the magnetic field during alternate half periods. But if use as a deflector is contemplated, it is difficul-t to realize an aperture greater than about a quarter wavelength of the radio frequency used to energize the waveguide because of the restricting eifect of the drift tubes. Various configurations of electrodes supported in resonant cavities can yield the desired results, but always with rather severe aperture limitations.

This invention accomplishes the preferential deflection of high energy particles in a resonator by utilizing a radio frequency energized waveguide system in which there is established for deflection purposes, a field pattern including a traveling wave of deflecting electric field free from the usual magnetic field component whose effect if present, would be to cancel the electricdeflection. Separation of the particles is accomplished by this traveling-wave transverse electric field. The velocity of the Wave down the waveguide is chosen such that the desired particles gradually slip backward or forward along the deflecting wave. The length of the separator is such that the total slip between these particles and wave is exactly a full wavelength. The net eflect is that the desired particles spend equal times in fields deflecting them in the two opposite directions and they emerge therefore with their velocity parallel to the axis. At the same time the wave velocity has been chosen equal. to that of an undesired particle type; thus the field continues to deflect that particle in one or the other direction throughout the length of the separator. Since particles arrive at all phases, both :beams are fanned out with maximum intensity at the extremes of the deflection. On emergence, the desired particles form a parallel beam as compared to the nude sired particles which have velocity components away from the axis. It can be shown that, if the particles, upon emergence from the resonator, are allowed to drift a distance equal to half of the separator length and then are passed through another separator the same as the first, the desired particles will be restored to their original configuration and the deflections of the undesired particles will be multiplied by a substantial factor, such as five.

One form of this invention comprises a resonator having two opposed conducting surfaces positioned axially exterior to two axially opposed radio frequency powered spaced chargeable boundary surfaces. The charged opposite boundary surfaces are defined by a plurality of discrete, charged members arranged in parallel array in two lines parallel to the long axis of the resonator. The cross sectional contour of each charged member has a shape defined by the equation C0 w sinh E y sin z constant to satisfy a boundary condition imposed by 'Maxwells equations. The length of each member (transverse to the long axis of the resonator), and the least surface-tosurface axial separation spacing of the axially opposed counterpart members are equal to h/ 2 where )t is the wavelength of the radio frequency used to energize the resonator. A full theoretical discussion and mathematical analysis forming the basis of this invention is given in my paper A Radio-Frequency Mass Separator for Complete Separation of High Energy Particle Beams which appears in the Procedings of the International Conference on High-Energy Accelerators and Instrumentation, pages 422427, published by the European Organization for Nuclear Research.

A particular object of this invention thus is to provide versatile apparatus which can be used as a traveling wave tube for the preferential deflection of charged nuclear particle beams.

A further object of this invention is to provide a resonator in which previous limitations in the use of resonators to deflect high energy particles are removed.

A still further object is to provide a resonator which can be used for particle beam acceleration, deceleration or deflection.

A still further object is to provide apparatus for the deflection of composite particle beams in which one species of particles having a particular velocity emerges without deflection and all other species of particles having velocities greater or less than the preferred velocity experience deflection and emerge from the resonator with transverse velocity.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the accompanying drawing in which:

FIG. 1 is an isometric view of one portion of a resonant cavity constructedaccording to the principles of this invention;

FIGS. 2a, 2b and 20 show in cross-section alternate shapes of the boundary surface members which can be used in the resonator of FIG. 1;

FIG. 3 is a schematic side view of a section of a resonator according to this invention illustrating how the boundary members are charged at any particular instant to accomplish the particle separation desired;

FIG. 4 is a schematic side view of a section of a resonator according to this invention illustrating the manner of energizing when the resonator is to he used as an accelerator or decelerator; and

PEG. 5 shows an embodiment of this invention employing two spaced resonators in series to effect the separation of particles in a high energy mixed particle beam into a tight beam of the desired particles and avwidely deflected beam of the undesirable particles.

Assume a resonator which has a sinusoidal travelingwave transverse electric field with the direction of wave and particle travel along the z axis (when expressed in the familiar x, y, z three-dimensional coordinate system), a transverse electric field along the y axis, and the magnetic field component in the x direction equal to zero. The deflecting electric field E has a sinusoidal pattern in the Z direction. The magnetic field component B which would normally cause a deflection opposite to that due to the electric field is set equal to zero.

The form of the Maxwells equations satisfying these conditions is shown in my aforementioned paper. In those equations the symbols have their conventional meanings: x, y and z refer to the coordinate axes, E is the electric field strength, 33 is the magnetic field strength, is the phase velocity, is the wave number which is not determined by the field equations but is fixed only by the boundary conditions, and a: (:22-1' times frequency), t, and the trigonometric functions have their conventional meanings as when used in the description of sinusoidal wave pattern.

Since both E components vanish for x=: \/4, boundary walls can be erected in the planes through this value of x. The other boundaries are given by 7 When is chosen equal to w/c the equation for the ideal boundary sh ape has the final form C0 O) sinh E y.sin z=constant To terminate this resonator in the y direction, a field pattern analagous to that described by the above equations with the hyperbolic functions replaced by decaying exponentials may be utilized since this latter is also a solution OfMaXWells equations. At some reasonable value of y, the boundaries are cut off. and continue in the y direction with a mirror image of the boundary surface structure. To prevent radiation from this resonator which is not enclosed, a grounded conducting boundary at a distance of a quarter to half a wave length in the y direction beyond the end of the boundary surface structure is provided.

Referring now to FIG. 1 wherein such a resonator is illustrated, there is shown a rectangular standing wave guide or resonator it formed by a first pair of opposite parallelboundary'walls,12 and 14 and a second pair of opposite, parallel boundary walls 16 and 18. The walls are made of conductive material and may be grounded as is understood in the art. Walls 12 and 14 are preferably plane shapes but can be cambered for improved structural rigidity if required for mechanical stability. The conducting surfaces can be entirely of metallic construction or of a metallic foil supported on a suitable nonmetallic structural material such as plywood or pressed board. Aluminum or copper are suitable materials for all metallic construction and aluminum foil is suitable for the supported foil construction.

To establish the field pattern in which magnetic field components do not cancel the deflecting effects of the transverse'electric fields, walls 16 and 18 are each provided with two rows of spaced connections 26, 25' and 28, 28' respectively, to support the ends of a plurality of boundary surface elements 32 and 34-. Walls 16 and 13 are spaced apart a distance M 2 where A is the wavelength of the radio frequency signal used to energize resonator 10. The row of elements 32 is separated from the row of elements 34- by a distance of 2, measured from the outer surfaces of elements 32 and 34. The spaces between adjacent elements 32 and between adjacent elements 34 is M2, these distances beingmeasured from axes passing through the geometric centers of the elements. The separation between wall 12 and the row of elements 52 is of the order of V4 to M2. Wall 14 is separated a similar distance from the row of elements 34. A suitable cross sectional outline of each element 32 and 34 would be the shape of a hypcrbola with the outwardly facing vertices in alignment along the y axis to obtain the proper shape of the boundary structure as called for by Equation 9. FIG- URES 2a, 2b and 20 show in cross-section some other various contours which can be used as close approximations to hyperbolic shape shown in FIG. 1, for the convenience of manufacturing the elements 32. and 34. FIG. 2a shows a circle, FlG. 2b is an ellipse having a major axis passing through the surface at points 52; and 44, and FIG. 20 shows a contour made up of a parabola and its identical mirror image with each of the vertices at 46, 48 and the line of intersection of the mirror images represented by the vertical axis 49.

The RF energizing field'and dimensions of resonator it) are selected so that the speed of the traveling wave therein is matched to the speed of the undesired particle. The desired particle slips behind or ahead of the traveling wave. The length of the resonator is selected so that the desired particle will slip behind exactly one full wavelength as it-traverses the resonator. The RF wavelength to be imposed is related to the resonator length, and the remaining dimensions of resonator iii are calculated based upon the nature of the particles to beseparated, as will be shown below. The effect produced thereby-is that the desired particles spend equal times in fields deflecting it in two opposite directions and they emerge with their velocity parallel to the z axis while the field continuesto deflect the unwanted particles in one or the other directions through out the length of resonator 10. On emergence from resonator 10, the desired particles form a parallel beam and the undesired particles. have velocity components away from the axis. A shield with an. approximate aperture (not shown) would be mounted adjacent the exit end of resonatorll) to block-oft the deflected particles and pass the parallel beam of the desired particles. Resonator 10 may be open at both ends and mounted within an evacuated chamber (not shown) to prevent interference with the particles.

As already noted, the exact dimensions of resonator 10 are determined by the nuclear particles which are to be separated. For example, resonator 10 can be used for the separation of negative protons and negative pious in a composite beam which has undergone momentum analysis in a conventional magnetic deflection such that the particles have essentially the same energy and velocity. Protons and pionswith momenta of 10 bev./c., such as can be produced readily by a 25 bev. proton accelerator, can be separated because the rest energy of protons is 938 mev. and that of pions is only 141 mev. although their total energies are 10.044bev. and 10.001 bev. respectively. The ratio of the particle velocity to the velocity of light for these particles at the momentum of 10 bev./c is 0.99563 for protons and 0.99990 for pious. Assuming the one period diiference, then, from my paper,

where L is the length of resonator 10, c is the velocity of themmesons, and v is the velocity of the anti-protons.

The negative protons emerge as a beam parallel to the z axis but spread out in the y direction, whereas the negative pious emerge with angular distribution and with somewhat greater spacial spread than that of the negative proton beam. When the mixed component beam entering the resonator is uniformly distributed in time-- radio frequency signal, the value of E is fixed and the angular spread of the negative pion beam can be calculated. The results of calculations for three different wavelength which can be used for energizing the resonator are shown in Table l.

Table 1 shows that there will be about a four-tenths degree angular separation at the resonator exit and that the length of the resonator and size of the aperture vary directly with the wavelength of the radio frequency energizing power source although the total power used to energize the resonator is substantially independent of the radio frequency.

When it is desired to improve the separation between particles by about a factor of 150 greater than the separation achieved by using a single resonator of this invention, it is possible to use two resonators arranged in series as shown in the embodiment pictured in FIG. 5.

FIGURE is a schematic drawing, exaggerated along the y axis to illustrate the principle of a side view of a pair of identical resonators and 10' according to this invention used for the separation of a mixed component nuclear particle beam which has previously been subjected to momentum analysis. Resonators 10 and 10 are arranged in axial alignment as shown by their common axis zz. The mixed component nuclear particle beam enters at one end 4-8 of resonator 10 along the axis zz and exits resonator 10 at 50 resolved into a single particle species beam parallel to the z axis and somewhat spread out in a vertical direction as shown by the solid lines labeled 52 and a pair of the second particle species having angular velocity as shown by the broken lines labeled 54. The length between the entrance 48 and the exit 50 of the resonator 10 and consequently the corresponding length between the ends 56 and 58 of resonator 10' is determined a previously outlined so that the particle species beam labeled 52 will emerge parallel from resonator It The entrance 56 of the second resonator 10' is spaced in relation to the exit 50 of the first resonator 10 so that the separation distance is equal to one half the resonator length 10 and 10. Since there is no deflecting force acting on the particle in this space, the particles of both species continue to travel in straight paths thus increasing the separation between the beams of particles of differing species. By correct choice of RP. phase in the second resonator 10', the desired particle beam i refocused whereas the undesired particle beams are caused to deflect further from the Z1 axis. Shielding (not shown) may be placed between resonators 10 and 10' with an aperture to block the unwanted particles taking the path 54 having a spread beyond the beam of the wanted particles.

In the embodiment of FIGURE 5 both resonators 10 and 10 are energized with a radio frequency signal placed as described above. In another embodiment, not shown, still greater separation between the desired and undesired particle beams can be realized by spacing apart the two identical resonators only one-fourth a resonator length instead of one-half a resonator length and using a radio frequency energizing source for the second resonator which is one-quarter cycle out of phase with the radiofrequency energizing source for the first resonator. Substantially complete separation can be achieved with this construction.

FIGURE 3 is a schematic view of the instantaneous pattern of charges on elements 32 and 34 shown in FIG. 1 illustrating graphically how resonator 10 functions.

The individual chargeable boundary surface member elements are identified by 32a, 32b, 320, etc. and 34a, 34b, 340, etc. for convenience. Alternate and staggered elements 32a, 34b, 32c and 3401 are positive while elements 34a, 32b, 34c, and 32d are shown as negative, at a particular instant. These charges are reversed in accordance with the reversals at the imposed R.F. field. Thus a particular particle moving through resonator 10 along arrow A at exactly the wave velocity therein will always be subject to a bending force in the same direction as the opposite elements 32 and 34 reverse their charges as the particle passes between each pair. The particle slipping behind will be subject to bending forces in both directions so that in slipping behind one full wavelength as it traverses resonator 10, the bending force will effectively be cancelled. While chargeable members 32 and 34' may assume the electrical polarities as indicated, it is understood that straps may be used to connect the ends of like charged members 32 and 34 if needed to obtain the proper charging or to improve efficiency of the unit. The use of straps is now established in the art as shown in vol. 6, Radiation Laboratory Series (1942), published by McGraw-I-Iill Book Company.

In FIGURE 4 is shown a schematic view similar to that of FIG. 3 except that resonator 10 is to be used as an accelerator or decelerator of particles, rather than as a separator. In this arrangement opposite pairs of elements 32 and 34 are charged alike and straps are used to obtain this relationship. Thus the elements are effectively pulsed to either accelerate or decelerate the charged particles coming along arrow B. When resonator 14 is used as an accelerator with relativisitic particles, the velocity of the particles will not change although their masses will increase.

It should be understood, of course, that the foregoing relates to only preferred embodiments of this invention and that numerous modifications and alterations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

I claim:

1. A wave guide having x, y and z coordinate mutually perpendicular axes for use with a particular radio fre quency signal with the direction of Wave travel along the z axis of said guide, the transverse electric field of said signal being along the y axis of said guide and the magnetic field component of said signal being along the x axis of said guide, comprising, means for terminating said guide along the x-axis to establish boundary conditions of zero electric potential, and means for establishing the boundary shapes of said guide along the y axis in accordance with the relation w O) sinh 2 Y sin 7; Z =constant where w is 21r times frequency of said signal, 0 is the phase velocity of said signal and Y and Z are the values of the cordinate dimensions of said guide along the y and z axes respectively.

2. The wave guide of claim 1 in which two rows each consisting of spaced parallel chargeable membersestablish said boundary shapes.

3. A radio frequency wave guide resonator elongated along a z coordinate axis, the transverse electric field of the imposed radio frequency signal disposed along the y coordinate axis and the magnetic field component of said signal along the x coordinate axis, said coordinate axes being mutually perpendicular, comprising, a pair of parallel, co-planar conductive wall members spaced apart a distance of one half the Wave length of said signal along said x axis, and two rows, each row consisting of spaced chargeable members extending between said wall members parallel to said x axis defining the boundaries of said reso nator along the y axis, said rows being spaced from each other along the y axis substantially one half the wave length of said signal measure from the adjacent outer surfaces of said members, with each member in one row having an opposite member in the other row, said members in each row being spaced apart a distance of substantially one half the wave length of said signal measured from the central axes'or" said members:

4. Apparatus for separating desired from undesired high energy nuclear charged particles Comprising a Wave guide resonator having x, y and z coordinate mutually perpendicular dimensions utilizing'a particular radio frequency signal with the direction of Wave travel along the z dimension of said guide, the transverse electric field of said signal along the y dimension of said guide, and the magnetic field of said signal alongthe x dimension, comprising a pair of co-planar parallel conductive walls perpendicular to the x dimension for terminating said guide along this dimension, and means for establishing the boundary shapes of 'said guide along the y dimension in accordance with the relation 0 U) (O smh 3 Y S111 T: Z =constant where w is 21r times the frequency of said signal, c is the phase velocity of said signal and Y and Z' are the coordinate dimensions of said guide along the y and 2 dimensions, respectively, the length of said guide selected for producing a standing wave within said guide having a velocity equal to the velocity of the undesired particles.

5. The apparatus of claim 4 in which the length of saidguide is also selected so that the undesired particle will shift in phase by one wave length of said Wave as said undesired particle transverses said guide, in accordance with the relationship References Cited in the file of, this patent UNITED STATES PATENTS 2,679,585. Drazy May 25, 1954 2,745,984 Hagelbarger et al. May 15, 1956 2,746,036 Walker May 15, 1956

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