US3324325A - Dielectric wall stabilization of intense charged particle beams - Google Patents

Description

June 6, 1967 J. BRlGGs 3,324,325

' DIELECTRIC WALL STABILIZATION OF INTENSE CHARGED PARTICLE BEAMS Filed Sept. 10, 1965 2 Sheets-Sheet 1 INVENTOR RICHARD J. BRIGGS BY Km 4- Q ATTORNEY June 6, 1967 R. J. amass 3,324,325

DIELECTRIC WALL STABILIZATION OF INTENSE CHARGED PARTICLE BEAMS Filed Sept. 10, 1965 2 Sheets-Sheet 2 I PARTICLE ORBITING REQUENCY ENERGY INVENTOR. RICHARD J. BRIGGS BY M 0. W

' ATTORNEY United States Patent 3,324,325 DIELECTRIC WALL STABILIZATIQN 0F INTENSE CHARGED PARTICLE BEAMS Richard .I. Briggs, Dan /idle, Calif assignor to the United States of America as represented by the United States Atomic Energy Commission Fiied Sept. 10, 1965, Ser. No. 486,577 15 Claims. {(11, 3l362) ABSTRACT OF THE DISCLOSURE Apparatus for guiding an intense charged particle beam along a path defined by a guide or housing in which an effective dielectric structure is disposed between the guide and beam to stabilize the beam against negative mass instability disturbances.

The present invention relates generally to the stabilization of the trajectory of a beam of charged particles. More particularly, it appertains to inductive wall stabilization of a charged particle beam traveling along an arcuate path.

Beams of charged particles are extensively employed for a variety of purposes in nuclear research, creation of plasmas, and various related fields. Often it is desirable to provide an intense, i.e., of the order of one ampere, beam of charged particles to investigate, for example, the structure of the nucleus of atoms. Moreover, in many nuclear experiments it is generally preferred that a well focused beam of particles be provided characterized by a minimum spread in particle energy to increase precision, to eliminate spurious effects as well as to simplify interpretation of experimental results.

Generally, particle beams are produced by linear or circular path accelerators. While circular path accelerators are able to generate a focused low intensity, i.e., milliamperes or less, beam of energetic charged particles, with a minimum particle energy spread, the beam intensity is limited to low levels by instability effects. Similarly, in raising the current levels of beam storage rings instabilities are encountered which limit the intensity of the beams which may be stored. Beam storage ring techniques are often used in linear accelerators as Well as circular accelerators.

A predominant beam intensity limiting instability is the negative mass instability of beams directed along an arcuate path. This type of instability gives rise to an unwanted density modulation perturbation of the particle beam along the axis of travel and in some cases, results in a loss of particles to the walls of the beam guide. As will be set forth in more detail hereinbelow, when the particles of the beam are influenced by the negative mass instability, they collect and form highly dense bunches spaced at intervals along the beam axis. As the bunch density increases, for example, as a result of increasing the intensity of the beam, space charge effects eventually prevent the further collection of particles in the bunches. Particle beams up to one ampere in intensity have been generated by circular type accelerators by introducing betatron oscillations and/ or particle energy spread in the beam to offset the negative mass instability influences at the expense of beam definition and exactness in particle energy.

The present invention provides apparatus which accomplishes the stabilization of beams of particles having a narrow span of energies at high intensities and thereby overcomes many of the limitations and disadvantages characteristic of prior art energetic particle beam arcuate type generators and beam path housing structures or beam 3,324,325 Patented June 6, 1967 guides. More specifically, stable confinement of charged particle beams magnetically directed along arcuate paths of generators and beam guides is achieved by providing the particle beam guide structure with a magnetically permeable wall member interposed between the beam and beam facing surface of the beam guide structure at locations on diametrically opposite sides of the arcuate section of the beam. The material and dimension of the wall member is selected for a given size beam guide to define at predetermined beam energies an inductive impedance along the arcuate beam path by modifying the electric field component therealong to allow the inductive magnetic field component to predominate. By constructing the particle beam guide structure to present such an inductive impedance to the charged particle beam as it traverses an arcute path, forces which otherwise are present to drive the negative mass instability are eliminated. In the absence of instability inducing forces, it is unnecessary to rely on particle energy spread and/or betatron oscillation effects to stabilize the beam with concomitant increased particle energy spread and loss of beam definition.

Accordingly, an object of the invention is the provision of apparatus capable of stably confining an intense beam of charged particles directed along an arcuate path.

Another object of the invention is to provide a stably confined well focused beam of charged particles having a minimum spread of particle energizes. A further object of the invention is to eliminate the factors giving rise to negative mass instabilities in charged particle beams directed along arcuate paths.

These and other objects and advantages of the present invention will become apparent from the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a cross sectional view of a beam guide structure constructed in accordance with the present invention illustrating the orientation of the electric and magnetic field vectors;

FIGURE 2 is a normalized graphical plot of particle orbiting frequency as a function of beam energy for different classes of charged particle accelerators, as follows:

Curve a representing weak focusing particle accelerators; and curve b representing strong focusing particle accelerators;

FIGURE 3 is a sectional view of a segment of a circular beam guide tube including an arcuate section constructed in accordance with the present invention;

FIGURE 4 is an isometric view of a toroidal charged particle accelerator;

FIGURE 5 is a cross sectional view taken along lines 55 of FIGURE 4 portraying a dielectric type inductive inner Wall;

FIGURE 6 is a cross sectional view of a toroidal charged particle accelerator with spaced apart inductive wall members;

FIGURE 7 is a sectional view of a beam guide with an inductive inner wall defined by spaced conductive annular fins;

FIGURE 8 is an isometric view of a section of one embodiment of a two-beam type charged particle accelerator constructed in accordance with the present invention;

And FIGURE 9 is an isometric view of a section of the beam guide of an alternate embodiment of the two-beam charged particle accelerator of FIGURE 8.

In charged particle beam accelerators and beam guides where a beam of particles is magnetically directed along an arcuate path, the beam of particles can be subject to the negative mass instability when the effective orbital frequency of the particles is a decreasing function of energy. Orbital frequency is herein defined as the number of times per second that a charged particle beam would revolve about a closed circular path having a radius of curvature equal to that of the arcuate path through which they are magnetically directed. Hence, charged particles directed through a particle beam guide having both straight and arcuate sections 11 and 12 respectively as illustrated in FIGURE 3 have an effective orbital frequency in the same sense as charged particles directed around a toroidal particle beam accelerator 13 (or storage ring) illustrated in FIGURE 4.

The negative mass instability driving forces arises from the inherent irregularities in the particle density distribution along the arcuate beam axis, and the nature of the beam guide wall facing the particle beam. Conventional beam guides of circular charged particle accelerators and guides are constructed of conductive material with a smooth surface facing the beam so that the irregular particle density distribution, i.e., longitudinal density modulation of the beam, establishes an electric field which has a longitudinal vector component that serves to drive the negative mass instability. The physical explanation of this instability can best be described with reference to FIG- URE 1. As illustrated therein, the charged particles of the beam 14 gather in bunches 16 at spaced intervals along an arcuate path 17 traversed by beam 14. Hence, beam 14 is defined by particle bunches 16 spaced apart by void spaces 18, with the bunches 16 and voids 18 represented by positive and negative signs respectively. As illustrated in FIGURE 1, beam 14 is composed of positively charged particles such as protons or positive ions. However, it is to be noted that the beam 14 could be composed of negatively charged particles such as electrons or negative ions. In such cases, the negative signs would represent the particle bunch and the positive signs the particle voids. In either case, i.e., positively or negatively charged particle beams, the negative mass instability grows in the same manner.

Considering now the particle bunch 16 in conventional arcuate beam guides, the positive particles at the head 19 of bunch 16 experience a forward longitudinal force i.e., in the direction of arrow 17. Moreover, the positive particles at the tail 21 of bunch 16 experience a backward longitudinal force. Where the particles orbiting frequency is a decreasing function of energy, for example, above the transition energy in strong focusing particle accelerators, the longitudinal forces increase the orbiting radius and decrease the angular velocity of the particles at the head 19 of beam 16 so that such particles move backward towards the region of increased density of bunch 16. Simultaneously, the longitudinal force experienced by the particles at the tail 21 of bunch 16 decreases the orbiting radius and increases the angular velocity of the particles and the particles move forward towards the region of increased density of bunch 16. As the bunch 16 becomes denser, the tendency of the particles to bunch becomes greater. Consequently, in magnetically directed charged particle accelerators and beam guides having an arcuate beam path segment, beam intensity limitations are encountered at particle beam energies where the particle orbiting frequency decreases with an increase in energy.

Circular type charged particle accelerators are generally classified into two groups, weak and strong focusing types. With reference to FIGURE 2, it is seen that the weak focusing accelerators generate energetic charged particle beams under conditions favoring the existence of the negative mass instability. (See curve a.) This principally arises out of the fact that such machines are characterized by a radial magnetic field gradient in which, as the beam energy is increased, the beam moves towards weaker magnetic field regions at larger radial orbits. Hence, the orbital frequency becomes lower.

In the case of strong focusing circular beam accelerators and guide tubes, e.g., for proton and electron beams, relativistic effects are encountered at high beam energies,

A. i.e., of the order of one mev. for electrons and one bev. for protons, which result in the orbital frequency being lower, i.e., the cycles/ second are fewer, as the beam energy is increased. (See curve b.)

As noted hereinbefore attempts to offset the effects of the negative mass instability encountered by both classes of accelerators and guides has involved particle energy spread and betatron oscillation techniques. As further noted hereinbefore, with the present invention an entirely new concept for offsetting the effects of the negative mass instability is contemplated. In fact, with the apparatus of the present invention, the negative mass instability is eliminated.

It has been found that if the segments of the beam guide structure of circular charged particle accelerators and beam guides proximate the arcuate beam path represent an inductive longitudinal impedance to the charged particle beam magnetically directed therethrough, the conditions giving rise to the negative mass instability will be eliminated; hence the negative mass instability itself is eliminated. From Poyntings theorem, it is found that such an inductive impedance at the beam results whenever the magnetic field energy storage outside the beam is greater than the electric field energy. By shorting out or substantially reducing the electric field component of the electromagnetic field in the region of the inner wall of the beam guide, the energy stored in the magnetic field component will be greater than that stored in the electric field component of the electromagnetic field. Shorting out the electric field, i.e., reducing the electric field potential to zero, at a point between the beam guide and charged particle beam insures the establishment of such an inductive impedance field.

Referring now to FIGURE 1, negative mass instability stabilization is accomplished by disposing a magnetically permeable electric field intensity reducing wall member 22 of a selected wall thickness, t, proximate the inner surface 23 of arcuate beam guide 24. Such a wall member 22 is defined herein as an inductive Wall since the modification of the electric field, as described above, results in an inductive impedance stabilizing effect upon the charged particle beam. Beam guide 24 is an arcuate segment of, for example, a toroidal charged particle accelerator of the type portrayed in FIGURE 4, positioned within a magnetic field represented by arrows 26 which bends the charged particle beam 14 along path 17 curving out of the plane of the drawing. The inductive wall 22 is interposed between beam 14 and inner surface 23 of guide 24 at locations on diametrically opposite sides of the beam.

The configuration and composition of inductive wall 22 required may take several forms. As noted hereinbefore, stabilization of the negative mass instability will be accomplished by a proper disposition of appropriately dimensioned magnetically permeable electric field intensity reducing structure of selected wall thickness. The particular wall thickness selected is dependent upon the energy range of the charged particle beams to be directed therethrough two embodiments of the inductive wall of the present invention are illustrated respectively in FIG- URES 5 and 7. Referring first to FIGURE 5, a dielectric material 27, e.g., aluminum oxide or titanium dioxide having typical dielectric constants of 5.5 and respectively, of predetermined thickness t is disposed to be supported by the inner surface 28 of a toroidal beam guide tube 29 of toroidal charged particle accelerator 13, the toroidal tube 29 having a selected minor radius r The dielectric material 27 is positioned as a film or layer at locations on diametrically opposite sides of a beam 31 magnetically directed through tube 29 by magnetic field 32 to define an inductive wall. For a toroidal tube 29 of a given minor radius r, and a charged particle beam 31 of a given energy and beam radius r the required thickness, i of the dielectric material 27 is defined by the inequality expression 1)" i' an where v is the particle velocity and c is the speed of light.

As shown in FIGURE 5, the dielectric material 27 is simply disposed to cover the entire inner surface of tube 29. However, the stabilizing condition is satisfied merely if spaced segments 33 and 34 of dielectric having a thickness r are positioned at locations on diametrically opposite sides of a beam 36 as portrayed in FIGURE 6, so that a suitable inductive longitudinal electric field impedance will be present at the beam.

Attention is now directed to FIGURE 7 wherein there is portrayed a conductive fin inductive wall embodiment of the present invention. As will be set forth in detail hereinbelow, the conductive fin inductive wall shorts out, i.e., reduces to a negligible level the electric field in the vicinity thereof thereby serving to insure that the electric field component of the electromagnetic field is substantially reduced. As shown in FIGURE 7, a plurality of conductive annular fins 37 of selected annular thickness t are disposed spaced apart in coaxial relation to the beam axis 38. In one embodiment contemplated, fins 37 are secured to the inner surface 39 of the arcuate section of a beam guide 41. Beam guide 41 may be a toroidal tube of a circular type charged particle accelerator, for example, as illustrated in FIGURE 4, a toroidal tube of a charged particle storage ring device, or an arcuate beam guide section connecting two angularly disposed linear beam guides, etc. In any case, for a toroidal tube of a given minor redius r and a charged particle beam of a given energy and beam radius r the required annular thickness, I of the fins 37 is defined by the inequality expression Furthermore, it is preferable that the fin spacing s be adjusted so that the following inequality expression is satisfied m v m where E is the electric field in the medium, E is the electric field in a vacuum, and c is the dielectric constant of the medium. Hence referring again to FIGURE 1, since in the case of the dielectric type inductive wall, c is substantially greater than unity, the electric field therein, i.e., E will be substantially reduced from the magnitude of the electric field 42 in the region between the inductive wall 22 and beam 14. Furthermore, in the case of the fin type inductive wall, the electric field lines terminate at the edge thereof facing the beam. Consequently, in the region between the fins the electric field is essentially zero. This results in an effective fin dielectric constant approaching infinity. Therefore, from the foregoing it is seen that a magnetically permeable electric field intensity reducing wall member interposed between the beam guide and a charged particle beam magnetically directed along an arcuate path results in eliminating the negative mass instability.

Although the inductive wall of the present invention will find application in any circular charged particle beam accelerator or circular beam guide through which a beam is magnetically directed along an arcuate path, the following discussion will describe the use of the inductive wall in two distinct charged particle circular type beam accelerators. 1 Referring again to FIGURES 4 and 5, toroidal charged particle beam accelerator 13 comprising stainless steel toroidal beam guide tube 29 is provided with a beam entrance port 45 for injecting therein from a charged particle source (not shown) selected charged particles, e.g., protons, to form a positive ion type beam 31. To evacuate tube 29 a port 43 is adapted thereto to provide suitable communication to a vacuum pump (not shown) whereby the tube may be evacuated to at least l() millimeters of mercury. An aluminum oxide material of thickness t is disposed in surface covering relation to the interior surface 23 of tube 29 to define inductive wall 27. The tube 29 is positioned in a magnetic field 32 so that the lines thereof permeate the tube 29 in a direction perpendicular to the plane of the tube. In strong focusing type charged particle accelerators, the magnetic field 32 is modulated to accelerate the proton beam 31 to relativistic energies to, for example, impinge on a target (not shown) disposed in communication with the interior of tube 29.

For a proton beam accelerated from a non-relativistic to a relativistic energy equivalent to =25 where 7 is defined by the equation the accelerator 13 is constructed preferably in accordance with the following parameters:

With reference to curve b of FIGURE 2, it is seen that as the energy of beam 31 is increased from non-relativistic to relativistic energies, both positive and negative mass effects are encountered. In the positive mass regime, the efiects of the electromagnetic waves on the charged particles is exactly contra to those previously described as encountered in the negative mass regime. That is to say, in the presence of an inductive wall, a beam of charged particles below the transition energy, E may encounter positive mass instability forces. However, by adjusting the thickness, r of dielectric 27 to make the inequality expression 1) an equality at the transition energy, both positive and negative mass instabilities will be eliminated, i.e., a null point region of stability is in effect provided. In the strong focusing charged particle accelerator constructed in accordance with the above noted parameters, the thickness of the aluminum dioxide inductive wall 27 defined by equation is 3.2 millimeters.

In the case where the proton particle beam is to be stored in a toroidal storage ring constructed in accordance with the above noted parameters of the toroidal accelerator 29, magnetic field 32 is maintained constant and means 44 for accelerating beam 31 is adapted to tube 29. The beam accelerating means 44 is operated to maintain the beam energy constant. Since single energy beams are directed through storage rings, the thickness of the dielectric inductive wall need only be adjusted to eliminate the negative mass instability at that energy. Hence, from the inequality expression (1), it is found that an aluminum oxide inductive wall thickness of 0.32 millimeter or greater will suppress the negative mass instability at a relativistic beam energy equivalent to 'y=25.

Energetic charged particle beam accelerated or stored in apparatus other than toroidal types also can be stabilized by an inductive wall. For example, referring to FIGURES 8 and 9, an annular beam guide 46 of rectangular cross section of a two-beam strong focusing charged particle accelerator, such as the MURA machine, is positioned between the poles 47 and 48 of a radial-sectored magnet 49. Adjacent radial sectors 51 of magnet 49 are energized to generate opposite magnetic fields having a gradient which increases radially outward. The charged particles which are to be accelerated and formed into a beam are injected through an inlet port 52 communicating to the interior of guide 46 at its radially innermost wall 53. In order to evacuate the interior of guide 46, a port 54 is provided at a wall thereof for communication of the interior to a suitable vacuum pump (not shown).

In using the MURA accelerator to accelerate electrons to 40 mev., the accelerator is constructed in accordance with the following parameters:

Cm. Inner radius of guide 120 Outer radius of guide 200 Injection radius of particles 125 Interior vertical height of wall 53 7.0

The radial sectors 51 of magnet 49 are energized to cause the electrons to execute orbits of increasing radius whereby at 35 mev. the electrons orbit at a radius of 195 centimeters. The transition energy of the beam is 1.1 mev. and i reached by the beam as it accelerates to a radius of 145 centimeters. Since the negative mass instability is encountered above the transition energy, it is only necessary to provide an inductive wall at the top and bottom walls, 57 and 58 respectively, of the annular guide proximate the regions wherein the energy of the beam is above the transition energy. Hence, in the accelerator described above, an inductive wall 59 is positioned within region bounded by top and bottom *wall 57 and 58 and radii of 145 centimeters and 195 centimeters. Either a dielectric material or conductive ring shaped fins disposed at opposite surfaces 61 and 62 of top and bottom walls 57 and 58 respectively would adequately serve as inductive wall 59.

For example, in FIGURE 8 an aluminum oxide layer 63 is coated on surfaces 61 and 62 of walls 57 and 58 respectively between radii of 145 centimeters and 195 centimeters. The thickness of the layer 63 decreases radially outward from a maximum of at least 0.7 centimeter at region 64 to a minimum of at least 3.5 1() centimeters at region 66.

With reference to FIGURE 9, it is seen that inductive Wall 59 also could be formed by disposing radially spaced ring shaped aluminum oxide layers 67 of varying thickness along surfaces 61 and 62. The thickness of layers 67 varies inversely as the radius of the layers. The thickness of the inductive wall 59 at point locations along surfaces 61 and 62 corresponding to instantaneous values of beam energy and beam orbiting radius, can be determined approximately from the inequality expression 1.

It is further noted that the intensity of energetic charged particle beams directed along arcuate paths can be further increased by combining inductive wall beam stabilization with energy spread and betatron oscillation techniques. As noted hereinbefore, the beam definition and exactness of beam energy is affected by such techniques. However, it should be appreciated that heretofore unattainable very intense beams of charged particles can be generated by combining such beam stabilizing techniques with the inductive wall beam stabilization of the present invention.

While the present invention has been hereinbefore described with respect to specific embodiments, it will be apparent that numerous modifications and variations are possible within the spirit and scope of the invention and thus it is not intended to limit the invention except by the terms of the following claims.

What is claimed is:

1. In charged particle beam directing apparatus including means for generating a magnetic field to direct a charged particle beam of a predetermined energy along an arcuate path, the combination comprising, a housing having at least a section thereof constructed of magnetically permeable material to allow said magnetic field to penetrate therethrough adapted to receive said charged particle beam therethrough in a direction angularly related to said penetrating magnetic field, said penetrating magnetic field directing said beam along an arcuate path so that said beam induces electric field regions between said path and housing effective to reduce negative mass instabilities in said beam, and a magnetically permeable wall structure mounted between said beam and said magnetically permeable section of said housing at locations on diametrically opposite sides of said beam, said structure having a thickness correlated with the energy of said beam to provide a dielectric effect for offsetting said regions of induced electric field, thereby stabilizing said beam against negative mass instability.

2. The charged particle beam directing apparatus as recited in claim 1 further defined by said housing being a magnetically permeable toroidal tube having an inner surface and adapted with a charged particle entrance port, said tube adapted to be supported in said magnetic field in a plane perpendicular to said magnetic field.

3. The charged particle beam directing apparatus as recited in claim 2 further defined by said wall structure being disposed in covering relation to the entire inner surface of said toroidal tube.

4. The charged particle beam directing apparatus as recited in claim 1 further defined by said wall structure being a dielectric material.

5. The charged particle beam directing apparatus as recited in claim 1 further defined by said wall structure being comprised of a plurality of spaced apart conductive fins mounted to said magnetically permeable section of said housing, said fins extending within said housing a selected distance.

6. The charged particle beam directing apparatus as recited in claim 1 further defined by said housing being a magnetically permeable annulus of rectangular cross section defined by radially extending top and bottom walls terminating at inner and outer end walls having an inner surface and adapted with a charged particle entrance port, said annulus adapted to be supported in said magnetic field in a plane perpendicular to said magnetic field.

7. The charged particle beam directing apparatus as recited in claim 6 further defined by said wall structure being a dielectric material disposed on said top and bottom walls at opposite locations, the thickness of said dielectric material decreasing in the radial direction.

8. The charged particle beam directing apparatus as recited in claim 7 further defined by said dielectric material disposed to cover the entire surface of said top and bottom walls between the outer wall and approximately a point one third the distance from said inner Wall.

9. An energetic charged particle storage ring device comprising, a magnetically permeable toroidal tube having an inner surface and adapted with a charged particle entrance port for introduction and circulation of a charged particle beam wherein said beam induces electric field regions within said storage tube effective to reduce negative mass instabilities in said beam, and a magnetically permeable wall structure mounted between said beam and said inner surface of said tube at locations on diametrically opposite sides of a beam directed 9 through said tube, the thickness t of said Wall defined by the inequality expression Where v is the velocity of the particles of said beam, is the speed of light, r is the minor radius of the toroidal tube at the inner surface, r is the radius of the beam, and e is the dielectric constant of the wall structure thereby offsetting said regions of induced electric field.

10. The storage ring recited in claim 9 further defined by said wall structure being a layer of dielectric secured in covering relation to the inner surface of said tube.

11. The storage ring recited in claim 9 further defined by said dielectric selected from the group of materials consisting of aluminum oxide and titanium dioxide.

12. The storage ring as recited in claim 9 further defined by said wall structure being comprised of a plurality of spaced apart annular conductive rings secured Within said toroidal tube in coaxial relation with its minor axis.

13. A beam guide structure for a circular charged particle beam strong focusing accelerator for circulating a charged particle beam wherein said beam induces electric field regions within said accelerator effective to reduce negative mass instabilities comprising, a magnetically permeable toroidal tube having an inner surface and provided with a charged particle entrance port, and a layer of dielectric material of selected thickness secured in covering relation to the inner surface of said tube, the thickness r of said dielectric defined by the equation where v, is the velocity of the particles of said beam at the transitional energy of said beam, c is the speed of light, r is the minor radius of the toroidal tube at the inner surface, 1' is the radius of the beam, and e is the dielectric constant of the dielectric thereby offsetting said regions of induced electric field to stabilize said beam against negative mass instabilities.

14. The beam guide structure as recited in claim 13 further defined by said dielectric selected from the group of materials consisting of aluminum oxide and titanium dioxide.

15. Apparatus for stabilizing a charged particle beam against negative mass instability while said beam is traveling in a beam guide wherein charged particle bunches of increasing size create induced electric field regions of increasing magnitude comprising a magnetically permeable wall structure interposed between said beam and said beam guide at locations on diametrically opposite sides of said beam, said wall structure having a thickness correlated with the energy of said beam to provide a dielectric constant effect when interacting with said beam which is of a magnitude effective to attenuate said electric field regions of increasing magnitude.

References Cited UNITED STATES PATENTS 2,825,833 3/1958 Yanagisawa 3l362 JAMES W. LAWRENCE, Primary Examiner. STANLEY D. SCHLOSSER, Examiner.

Claims (1)

1. IN CHARGED PARTICLE BEAM DIRECTING APPARATUS INCLUDING MEANS FOR GENERATING A MAGNETIC FIELD TO DIRECT A CHARGED PARTICLE BEAM OF A PREDETERMINED ENERGY ALONG AN ARCUATE PATH, THE COMBINATION COMPRISING, A HOUSING HAVING AT LEAST A SECTION THEREOF CONSTRUCTED OF MAGNETICALLY PERMEABLE MATERIAL TO ALLOW SAID MAGNETIC FIELD TO PENETRATE THERETHROUGH ADAPTED TO RECEIVE SAID CHARGED PARTICLE BEAM THERETHROUGH IN A DIRECTION ANGULARLY RELATED TO SAID PENETRATING MAGNETIC FIELD, SAID PENETRATING MAGNETIC FIELD DIRECTING SAID BEAM ALONG AN ARCUATE PATH SO THAT SAID BEAM INDUCES ELECTRIC FIELD REGIONS BETWEEN SAID PATH AND HOUSING EFFECTIVE TO REDUCE NEGATIVE MASS INSTABILITIES IN SAID BEAM, AND A MAGNETICALLY PERMEABLE WALL STRUCTURE MOUNTED BETWEEN SAID BEAM AND SAID MAGNETICALLY PERMEABLE SECTION OF SAID HOUSING AT LOCATIONS ON DIAMETRICALLY OPPOSITE SIDES OF SAID BEAM, SAID STRUCTURE HAVING A THICKNESS CORRELATED WITH THE ENERGY OF SAID BEAM TO PROVIDE A DIELECTRIC EFFECT FOR OFFSETTING SAID REGIONS OF INDUCED ELECTRIC FIELD, THEREBY STABILIZING SAID BEAM AGAINST NEGATIVE MASS INSTABILITY.

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