US3660658A - Electron beam deflector system - Google Patents

Abstract

A magnetic system for deflecting the beam in an electron accelerator, comprising two sets of electromagnets which deflect the beam through a total angle of 90* and are located within in a single mounting, pivotally mounted about the axis of the entry beam.

Description

United States Patent Leboutet et al. May 2, 1972 [541 ELECTRON BEAM DEFLECTOR 3,287,558 11/1966 Bly et al ..250/49.5

SYSTEM 2,999,185 9/1961 Lubcke 2,602,751 7/1952 Robinson... [72] Inventors: Hubert Leboutet; Jean Jaouen; Jeanne Au- 2.7777958 H1957 Le poolemn couturier, all of Paris, France 293L738 M1960 Bruck U 73 Assignee; csp 3,358,239 l2/l967 Franke et al 3,360,647 12/1967 Avery ..250/49.5 [22] Filed: Mar. 4, 1970 v [211 pp NO: 16,529 OTHER PUBLICATIONS Experimental Evaluation of the Physical Characteristics of a 30] F A H ti P it D t 45- MEV Medical Linear Electron Accelerator" by C. L. 0 pp ca on n r y a a Hsieh et al.from Radiology," Vol. 67, No. 2,Aug. 1956, pp.

Mar. 12, 1969 France ,.6906976 2 [52] U.S. Cl. ..250/49.5 D, 250/495 R, 313/79, Prim y Ex minerWil1iam F. Lin quist 328/230 At/orney-Kurt Kelman 51 1111. C1. ..G0ln 23/00 [58] Field of Search ..250/41.9 ME, 49.5 R, 49.5 D, [57] ABSTRACT 250/49'5 313/79 328/230 A magnetic system for deflecting the beam in an electron accelerator, comprising two sets of electromagnets which deflect [56] References cued the beam through a total angle of 90 and are located within in UNITED STATES PATENTS a single mounting, pivotally mounted about the axis of the entry beam. 3,331,978 7/1967 Brown et al ..328/230 X 3,031,596 4/1962 Leboutet et a1 ..250/41.9 X 4 Claims, 11 Drawing Figures PATENTEDMM 2 I972 SHFET l UF 5 ELECTRON BEAM DEFLECTOR SYSTEM The present invention relates to the deflection of electron beams. In numbers of applications electron beams are exploited either by impact on metal targets with resultant production of X-rays, which are used in industrial radiography or in radiotherapy, or directly by impact upon the zone being treated as in electron radiotherapy.

The apparatus concerned generally comprise electron accelerator devices, and devices, generally of a magnetic nature, for guiding and deflecting accelerated electrons.

In certain applications the angle of incidence of the electron beam upon the object being irradiated or bombarded, should be readily varied, and therefore the radiography or radiotherapy machine has to rotate around the object. Due to the dimensions and weight of electron accelerators, bulky and expensive mountings have thus to be provided.

This may be difficult, or well-nigh impossible.

It is an object of the invention to avoid these difficulties by providing a system for deflecting the electron beam instead of rotating the accelerator itself.

According to the invention there is provided an arrangement for deflecting the electron beam of a particle accelerator, comprising a first and a second deflector system, said first deflector system comprising means for deflecting said beam through a first predetermined angle in a given plane, and said second deflector system comprising means for deflecting said beam through a second angle in said given plane so predetermined that the final direction of said beam intersects the initial direction thereof.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the ensuing description and in which FIG. 1 illustrates an overall diagram of the system according to the invention FIG, 2 illustrates a detailed view of one of the electromagnets belonging to a first part of the magnetic deflection system FIG. 3 illustrates a detailed view of a variant embodiment of one of the above electromagnets;

FIGS. 4a and 4!: illustrate an electromagnet belonging to a second part of the magnetic deflection system FIG. 5 illustrates a modified embodiment of the electromagnet shown in FIGS. 4a and 4b FIGS. 6a, 6band 6c illustrate a detailed section through an electromagnet belonging to the second part of the magnetic deflection system, in accordance with a second embodiment.

FIG. 7 illustrates a magnetic deflection system for imposing an angular scan motion on the beam, as a function of time, and

FIG. 8 illustrates a magnetic deflection system for producing a second type of angular scan as a function of time.

In FIG. 1, a schematic overall view of the pivoting device in accordance with the invention, has been given, and in order to simplify description, this system will be referred to in the ensuing text as a rotary deflection system.

The electron beam 1, which has been previously given the desired acceleration in the direction XX, is introduced into the input of a high vacuum tubular enclosure 2 pivotally mounted about an axis XX.

It is subjected to a first deflection under the action of a first set of electromagnets D which deflect it through an angle a in the clockwise direction in a plane containing the axis XX, and then to a second deflection, under the action of a second system of electromagnets D which deflect it in the reverse direction, in the same plane, through an angle [3 such that the direction of the beam at the exit from the second system makes an angle of substantially 90 with that which it has at the input of the first electromagnet system, and consequently with the axis XX, the displacement being in the direction of this axis.

To give some idea of the orders of magnitude involved, the angle a can be, for example, of 37 and the angle B of 127.

The plane in which the directions of the two deflections are contained, is a plane of symmetry for the rotary deflection system and the electron beam, and, in order to simplify the ensuing description, will be referred to in the following as the plane of the figure.

FIG. 2 shows in perspective one of the three deflection electromagnets, of the assembly D The magnetic circuit comprises a yoke 3 connecting the two pole-pieces 4, which take the form of sectors, and only one of which has been illustrated for the sake of clarity, and two magnetic coils 5.

These three electromagnets, which create magnetic fields in a direction perpendicular to the plane of the figure, are arranged successively in the path of the electron beam so that the intermediate electromagnet is head-to-tail with the first and third ones, and so that the direction of its magnetic field is opposite to that of the outside electromagnets, thus producing electron trajectories which alternately curve in one direction and then the other.

This has been fully described in the US. Pat. No. 3,031,596 assigned to the same Assignee as the present patent applicamm.

This device has been improved in the present Application in that, in order to compensate for the divergence of the electron beam, the latter has been made convergent, in the electronoptical sense, in the two directions corresponding respectively to the plane of the Figure and to the direction perpendicular to the plane of the Figure.

This double convergence is obtained, for example, by

respectively modifying the angle of the planes which respectively define the input and exit faces of the pole-pieces of the input and exit electromagnets, in relation to the axis of the electron beam.

In a preferred embodiment shown in FIG, 3, this angle can be adjusted by giving the terminal faces of the corresponding pole-pieces, the form of cylinders 6, rotatably mounted on axes perpendicular to the plane defined by the trajectories of the beam.

While the arrangement hereinbefore described is a preferred embodiment for achieving the desired convergence, it goes without saying that this convergence may be achieved by any other suitable means, and in particular by giving the faces of the pole-pie es forms which locally modify the gradient of the magnetic field.

The supply of electric current to the three electromagnets of the deflection device D can be effected in series, so that the magnetization of the system can be regulated in an overall fashion moreover, individual secondary windings can be used to adjust the strength of the magnetic field created by the main current. By way of example, since the second electromagnet is located substantially at the horizontal object focus of the exit electromagnet, as described in the patent referred to hereinbefore, when the current through said second electromagnet is varied the exit beam will be displaced in the plane of the figure, parallel to itself similarly, if the current in the third electromagnet is varied, the beam will pivot, in the plane of the figure, substantially about the center of the third electromagnet.

In FIG. 4, a detailed illustration has been given of the structure and disposition of the electromagnets which make up the electromagnet system D This system is formed by a single deflector electromagnet E however, in accordance with a modification, it may be followed by a complementary system of magnetic lenses. Hereinafter, the respective characteristics and modes of operation are described.

The purpose of the electromagnet is to deflect the electron beam as indicated hereinbefore, and the way in which it operates is quite obvious however, in a preferred embodiment, it can be given a secondary function, namely that of an optically convergent element. For this purpose, the polepieces will be given a generally frusto-conical form, so that the electromagnet air-gap varies as a function of the distance from the center of curvature of the electromagnet. The magnetic field thus produced consequently exhibits a gradient and a particular value of this gradient is advantageous.

This value corresponds to an n index equal to +0.5, this index here having the usual meaning, and being defined by where B is the magnetic flux in the electromagnet gap corresponding to the radius R of the particle trajectories.

The preferred index n of +0.5 gives the electromagnet identical convergence properties in the plane of the figure and in the plane perpendicular thereto.

This particular value, is however, not critical.

The entry and exit faces of the electromagnet E, exhibit the following advantageous features The entry face 7 is not perpendicular to the beam but inclined in relation thereto in such a fashion that it produces convergence in the plane of the figure any electron of the beam differing from the mean electron either by its energy or by a transverse distance therefrom, and arriving at said face parallel to the general axis of the beam and in the plane of the figure will thus appear, when it reaches the exit face, to have emanated from a point source F. By way of example, for an angle of 35 on the part of the entry face with the normal to the beam, this source will be situated at about 1.4 times the radius of curvature of the beam trajectory, on the straight line projecting the exit axis.

The exit face 8 likewise has an inclination to the plane normal to the beam in order to produce a convergent effect in the plane of the Figure, similar to the effect obtained by inclination of the entry face. The angle at the exit face is not critical and the convergence produced there is essentially designed to counteract the natural divergence of the beam emanating from the virtual source F, and this result will be achieved if the angle of the exit face is 35, for which value the system, in the optical sense, is afocal and has a magnification of l in a plane perpendicular to the plane of the figure.

If it is desired to introduce a greater degree of convergence into the beam in the plane of the figure then the angle of the exit face can be increased up to 45 at the expense of a slight divergence in the system in the plane perpendicular to that of the figure.

In a preferred embodiment, the convergence can be adjusted by utilizing rotational electromagnet entry and exit faces, as described hereinbefore.

The beam will thus have convergence in the plane of the figure and divergence in the plane perpendicular thereto, this leading to an elliptical section beam with the major axis perpendicular to the plane of the figure. However, since the electromagnet E does not ensure passage at the same point, i.e focusing, of the electrons of different velocities, and spreads their trajectories in the plane of the figure the nett result is an elliptical section the major axis of which is contained in the plane ofthe figure.

In certain applications, it is necessary to obtain a beam section of substantially circular form.

FIG. illustrates a variant embodiment which will be described hereinafter and which comprises, in addition to the electromagnet E.,, a magnetic device which will enable the above end to be achieved.

This embodiment comprises, beyond the exit fact of the electromagnet E.,, as illustrated in FIG. 3, a system of three quadripolar magnetic lenses forming a symmetrical triplet located in the path of the beam. Systems of this kind are vwell known in the art and those skilled in the art will appreciate that by varying different parameters, such as the currents involved or certain dimensions such as the axial length of the poles, it is possible to vary the magneto-optical characteristics of a triplet separately in the two principal mutually perpendicular planes, i.e. the plane of the figure and the plane perpendicular thereto.

The triplet considered is made up of three quadrupolar magnetic lenses located in the path of the beam. Those skilled in the art will be well aware that his kind of lens has properties of convergence in the particular plane passing through its optical axis, and of divergence in a plane perpendicular to the first and passing likewise through the optical axis.

The preferred order of the three quadripolar magnetic lenses making up the triplet, is contrived so that, in the plane of the figure, they are successively and respectively convergent, divergent and convergent.

For appropriate values of the geometric dimensions and of the control parameters of the triplet, it is thus possible to obtain at the output thereof a circular cross-section beam.

In a certain number of applications, it is possible to avoid having to use the aforesaid triplet and still have only a small residual amount of ellipticity, by improving the energy focusing through modification of the electromagnet 15,.

FIG. 6 illustrates a variant embodiment in which the air-gap of the electromagnet E is divided into two parts trough which the beam successively passes. The first part 25 has the form of pole-pieces which create a field gradient of index n +0.5 as explained already hereinbefore in the second part 26, the field gradient, on the contrary, is negative and the index is high, 2 or 3 for example.

The entry face 27 is normal to the beam, while the exit face 28 on the other hand makes a substantial angle with the plane normal to the beam the angle is variable in order to enable the convergence to be regulated, this by employing a rotary cylindrical component 21 similar to those already described in relation to FIG. 3.

A final set of characteristic features of the present invention is intended to achieve a geometric scanning of the irradiation zone along two mutually perpendicular cartesian coordinates, by the point of impact of the electron beam, or of the X-ray beam produced by the bombardment of an appropriate metal target. This allows a much larger area than that defined by the beam section alone to be bombarded.

FIG. 7 illustrates in the case of an electron beam, an arrangement which will enable the desired result to be achieved. In the path of the beam and beyond the triplet magnetic system already described, a quadripolar electromagnet is arranged comprising coils 12 and pole-pieces 13 extending through the coils and magnetized by same so that the beam is deflected in two perpendicular directions and can thus reach any point on the area to be scanned. The preferred location of the quadripole device is such that its center of deflection coincides with the image focus of the triplet beyond which it is situated, and the preferred form of the polepieces is one in which the air-gap faces 14 are inclined so that a conical taper is formed to permit the free inclination of the trajectories without any interception of the electrons.

FIG. 8 relates to the X-ray scanning. The advantage of this kind of scanning resides in the possibility of obtaining uniform irradiation over an extensive area; if, in order to equalize the radiation, recourse is had to the conventional solution employing an equalizer disc of variable thickness operating by absorption of part of the radiation, then it is possible to give this disc smaller variations in thickness. Consequently, its construction is simplified and a smaller fraction of the radiation is absorbed therely.

It has been indicated hereinbefore that X-rays are produced by impact of the electron beam upon an appropriate metal target, the peak radiation being orientated along the axis of the incident electron beam. The introduction of geometric scanning therefore leads to the variation in the angle of impact of the incident electron beam on the metal target.

This result can be obtained by a purely magnetic arrangement. One very simple embodiment consists in exploiting the specific deflecting characteristics of the electromagnet E,. Depending upon the magnitude of the current used to supply it, this electromagnet produces differential deflection of the electrons which leave its exit face at different points for a field which is stronger than a field corresponding to exit along the mean axis, the electrons are deflected further and exit at 20 for a weaker field, they are not deflected so much and exit at 21. It will therefore readily be appreciated that when the particles are focused by the triplet of quadripolar lenses which then follows, they leave on trajectories which are the more oblique the further away they are from the mean axis of the beam leaving the magnetic E At the exit from the triplet system, the target 22 which produces the X-rays, is thus struck by the beam at variable angles, and this is a result which, as we have shown hereinbefore, is necessary in order for the main direction of radiation itself to be variable as a function of time.

The variation in magnetic field in E is simple to obtain by modulation of the magnetizing current. For this purpose, in a preferred embodiment there is provided, in addition to the main winding normally provided, an additional winding through which an electric current, variable in magnitude and sign, flows.

By way of example, for an electromagnet E having a mean radius of curvature of 28cm, the total angular amplitude in the plane of the figure is in the order of 3 for an amplitude of 1 percent on the part of the modulation of the magnetic field. The period of the oscillations is in the order of a fraction of a second. I

In a direction perpendicular to the plane of the figure, geometric scanning is likewise obtainable by arranging, for example at the exit from the triplet system D,, an electromagnet 24 which produces in the plane of the figure a magnetic field which is variable in time, in magnitude and sign.

Of course the invention is not limited to the embodiments described and shown which were given solely by way of example.

What is claimed is:

1. An arrangement for deflecting the electron beam of a particle accelerator which comprises in combination, a mounting pivotally mounted at the accelerator exit for rotation about the axis of the beam, said mounting securing a first and a second deflector system, said first deflector system having means for deflecting said beam through a first predetermined angle in a given plane, and said second deflector system having means for deflecting said beam through a second angle in said given plane so predetermined that the final direction of said beam intersects the initial direction thereof at an angle at least equal to said first deflector system having means for deflecting said beam and comprising three electromagnets forming an achromatic system, the respective entry and exit faces of the pole-pieces of said electromagnets making angles different from zero with planes perpendicular to the axis of said beam whereby said deflector system is made convergent, said second deflector system comprising an electromagnet having pole pieces whose entry and exit faces respectively make angles other than zero with planes perpendicular to the beam axis whereby the system is convergent in the plane of the beam, the exit of said second deflector system being a triplet arrangement of quadrupolar magnetic lenses which in the plane of the beam are respectively convergent, divergent and convergent whereby said beam is focused at a point beyond said second deflector system; a target for producing X-rays disposed at said point beyond said second deflector system; a scanning electromagnet having an air-gap disposed in the path of the electron beam between said first and second deflector systems and having coils with means for varying as a function of time the current flowing in said coils; a scanning winding around an electromagnet forming part of said second deflector system; means for varying as a function of time the current flowing in said scanning winding, said scanning electromagnet and said scanning winding disposed normal to each other for producing a cyclical variation of the electron beam impact angle on said target, achieving an X-ray scanning.

2. The arrangement for deflecting of claim 1 wherein the pole-pieces of the electromagnet of the second deflector system have an air-gap, in a first part of which air-gap the width is at any point thereof proportional to its distance to the curvature center of the mean beam path and said distance.

3. The arrangement for deflecting of claim 1 wherein said electromagnets formin the fir st deflector system have airgaps with a wldth w rch rs inversely proportional to its distance to the curvature center of the mean beam path through said electromagnets.

4. The arrangement for deflecting of claim 1 wherein entry and exit faces of said second deflector system carry plates of ferromagnetic material pivotally mounted about an axis parallel to the magnetic field, whereby said angles are adjustable.

Claims (4)

1. An arrangement for deflecting the electron beam of a particle accelerator which comprises in combination, a mounting pivotally mounted at the accelerator exit for rotation about the axis of the beam, said mounting securing a first and a second deflector system, said first deflector system having means for deflecting said beam through a first predetermined angle in a given plane, and said second deflector system having means for deflecting said beam through a second angle in said given plane so predetermined that the final direction of said beam intersects the initial direction thereof at an angle at least equal to 90*, said first deflector system having means for deflecting said beam and comprising three electromagnets forming an achromatic system, the respective entry and exit faces of the pole-pieces of said electromagnets making angles different from zero with planes perpendicular to the axis of said beam whereby said deflector system is made convergent, said second deflector system comprising an electromagnet having pole pieces whose entry and exit faces respectively make angles other than zero with planes perpendicular to the beam axis whereby the system is convergent in the plane of the beam, the exit of said second deflector system being a triplet arrangement of quadrupolar magnetic lenses which in the plane of the beam are respectively convergent, divergent and convergent whereby said beam is focused at a point beyond said second deflector system; a target for producing Xrays disposed at said point beyond said second deflector system; a scanning electromagnet having an air-gap disposed in the path of the electron beam between said first and second deflector systems and having coils with means for varying as a function of time the current flowing in said coils; a scanning winding around an electromagnet forming part of said second deflector system; means for varying as a function of time the current flowing in said scanning winding, said scanning electromagnet and said scanning winding disposed normal to each other for producing a cyclical variation of the electron beam impact angle on said target, achieving an X-ray scanning.

2. The arrangement for deflecting of claim 1 wherein the pole-pieces of the electromagnet of the second deflector system have an air-gap, in a first part of which air-gap the width is at any point thereof proportional to its distance to the curvature center of the mean beam path and said distance.

3. The arrangement for deflecting of claim 1 wherein said electromagnets forming the first deflector system have air-gaps with a width which is inversely proportional to its distance to the curvature center of the mean beam path through said electromagnets.

4. The arrangement for deflecting of claim 1 wherein entry and exit faces of said second deflector system carry plates of ferromagnetic material pivotally mounted about an axis parallel to the magnetic field, whereby said angles are adjustable.

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