US2721954A - Electrostatic apparatus for bending beams of charged particles - Google Patents

Description

1955 J. c. NYGARD ELECTROSTATIC APPARATUS FOR BENDING BEAMS OF CHARGED PARTICLES Filed Nov. 5, 1952 FIG. 4 A FlG.4 C

INVENTOR John C.Ngqm:d MwfifLJ/ ATTYS United States Patent ELECTROSTATIC APPARATUS BENDING BEAMS 0F CHARGED PARTICLES Application November 5, 1952, Serial 318,913

16 Claims. (Cl. 315-18) This invention relates to apparatus for bending beams of charged particles, and in particular torsimple electro static means for deflecting a beam of charged particles through any desired angle of deflection, comprising apair of arcuate, parallel, conducting plates, defining an arcuate passageway therebetween, and means for establishing an electrostatic field between said plates and across said passageway of such magnitude and direction as to cause a beam of charged particles, which is directed into said passageway at one extremity thereof, to follow saidpassageway and emerge therefrom at the opposite extremity thereof. f

In order that the principle and operation of my invention may be clearly understood, ,1 have illustrated in the drawings one form of apparatus embodying my invention. However, as will be apparent from the disclosure acceleration tube 2 is to be used as a source of X-rays, a target of some appropriate material such as. lead may be provided at the lower extremity 7 of the tube extension 4. Or, if the acceleration tube 2 is to be used for analyzing a beam of positive ions, the lower extremity 7 of the tube extension 4 may communicate directly with a beamanalyzing chamber or other apparatus. My invention is not limited to any particular use of the beam of charged particles, nor is my invention limited to any particular means for creating said beam.

Within the tube extension'4 I provide, in accordance with my invention, a pair of arcuate, parallel, conducting plates, as shown in detail in Figs. 2 and '3, consisting of an outer plate 9 and an inner plateltl which define an arcuate passageway 11 therebetween. The outer platej9 is supported on the wall of the tube extension 4 by in sulators 12, and the inner plate 10 is supported by con duct-ive members 13, the plates 9, 10 being so positioned that the beam 1 enters said passageway ll-at the upper extremity 6 thereof. At the opposite, or lower, extremity 7 of said passageway 11 I provide a slit edge 14 on the hereinafter, my invention is not limited to use with Fig. l is a vertical, longitudinal, central section, with an intermediate part broken away, taken through one form of an acceleration tube embodying my invention;

Fig. 2 is a view in perspective of a pair of conducting plates for the'.deflection of a beam of charged particles through a desired angle, in accordance with my inven tion;

Fig. 3 is a diagram, illustrating the electrical connec tions of the pair of conducting plates illustrated in Fig. 2;

Fig. 3A is a diagram illustrating a modification of' the electrical connections illustrated in Fig. '3; and

Figs. 4A, 4B, and 4C are. diagrams illustrating the initial and subsequent deflection of a beam of charged particles, in accordance with my invention.

- Referring to Fig. 1, a beam 1 of charged particlesmay be created by an acceleration tube 2 such asthat disclosed in U. S. Patent No. 2,521,426 to Trump and Cloud, but wherein the lower end 3 of the acceleration tube has been modified, as shown, by the provision of an arcuate tube extension 4. Charged particles emitted by the source 5 are accelerated down the tube in a manner not necessary to set forth herein in detail and enter the tube extension 4 at the upper extremity 6 thereof as a concentrated beam. In accordance with my invention and by means of apparatus to-be described in detail hereinafter, the beam of charged particles isconstrained to follow an arcuate path within the tube extension and to issue therefrom at the lower extremity 7 thereof.

The entire internal'volume of the acceleration tube 2 is evacuated, from thesource 5 to the lower extremity 7 of the tube extension 4, and accordingly the lower extremity 7 of the tube extension 4 is covered by some appropriate means, indicated merely diagrammatically at 8. For example, if the acceleration tube 2 is to be used as a source of cathode rays, the lower extremity 7 of the tube extension 4 may. be covered by an electron window consisting of a thin aluminum foil. Or, if the outer plate 9, which extends into said passageway 11, as shown, for purposes to be described in detail hereinafter.

Referring now to Fig. 3, the inner p'late 10 is electrically grounded, 'as shown at 15. For example,= said inner plate 10 may be electrically connected to the wall of the tube extension 4 (which is maintained at ground potential) by conductive members 13, as shown in Fig. l. The'outer plate 9 is connected to ground through any suitable high impedance path, as,by connecting a resistor (which may be of either the linear or non-linear type), shown in Fig. 3 at 16, between said outer plate 9 and the wall of the tube extension 4. Alternatively, a set of corona points, as shown in Fig. 3A at 17, or any other high impedance path may be provided;

In accordance with my invention, I create an electrostatic field across the arcuate passageway 11 by causing a potential difference of E volts to be impressed be tween the plates 9, 10. The electrostatic field is of such magnitude and direction that the beam 1 of charged particles is caused to follow an arcuate path through the passageway 11, so that the beam emerges at theiower extremity 7 thereof.

The electrostatic field is preferably, although not nece's sarily, of uniform intensity at the region traversed by the beam, and so the width of the plates 9, 10 should be suffiicently great with respect to the distance therebetween and with respect to the diameter of the beam 1 so that fringe effects are minimized. Suitable dimensions would be 5 cm. for the'width of the plates 9, 10 and 2cm. for the distance between the plates 9, 10. h

The length of the plates 9, 10 should be such that the angle subtended thereby is equal to the desired angle of deflection. In the drawings, this angle is shown as merely by way of illustration.

As will be shown hereinafter, the potential difference E automatically assumes the proper magnitude to de: flect thebeam through the passageway ll'between the plates 9, 10. If the radius of curvature of the are described by the path of the beam 1 through the passage-v way 11 is R cm., and if the acceleratingpotentiai of the acceleration tube 2 is V volts, then the magnitude of E in volts is related to R by the equation Vslc R where s is the distance (in cm.) between the plates 9, 10 and where k is in general approximately equal to More precisely, I

V (m) 9 10 y,

(Ve) l.6 10- +(m) 9 10 where m is the rest mass (in gms.) of and e the charge (in coulombs) on the charged particles.

However, the permissible magnitude of E is limited by the insulating ability of the vacuum within the tube extension 4. A suitable value of E would be 120 kv. if s is 2 cm. To obtain this value of E, the plates 9, should be constructed so that Vk 6O,O00 For a 2 MeV electron beam, V=2 l0 volts and k=1.2,

so that R=40 cm.

The radius of curvature of the outer plate 9 would thus be 41 cm., and that of the inner plate 10 would be 39 cm.

The operation of the apparatus is as follows. Initially, there is no electrostatic field between the plates 9, 10 and the beam 1 is undeflected so that it strikes the outer plate 9, as shown in Fig. 4A. This causes a charge of the same polarity as that of the charged particles in the beam 1 to be distributed over the outer plate 9. Since the inner plate 10 is grounded, an electrostatic field appears between the plates 9, 10 which tends to deflect the beam 1 away from the outer plate 9, as shown in Fig. 4B. As long as the beam 1 continues to strike the outer plate 9, charge is continuously being deposited on said outer plate 9. At the same time, however, the electric charge on the outer plate 9 is continuously leaking ofi through the resistor 16 (or other high impedance path). If the beam current intercepted by the outer plate 9 is i amperes and the resistor 16 (or other high impedance path) has a resistance (or impedance) or r ohms, the potential difference between the plates 9, 10 will be E=ir volts.

As long as the entire beam 1 strikes the outer plate 9, i will be equal to the total beam current and E will assume its maximum value and will remain at that value until the beam 1 is deflected sufficiently to impinge upon the slit edge 14. As the beam 1 is deflected still further, some of the beam current will clear the slit edge 14, so that the value of i and hence the value of E will fall from their respective maximum values. The more the beam 1 is deflected, the lower the values of i and E, so that the deflecting force on the beam 1 is diminished. Consequently, the beam 1 will assume an equilibrium position such that the beam current i intercepted by the slit edge 14 will cause the electrostatic field to deflect the beam 1 by an amount just sufficient to maintain the beam 1 in such equilibrium position, as shown in Fig. 40.

For example, suppose that a 2 MeV, 250 microampere electron beam is to be deflected by means of plates 9, 10 spaced 2 cm. apart and having radii of curvature of 39 cm. and 41 cm. If the resistance or impedance r of the resistor 16 or other high impedance means is 12,000 megohms, the maximum value of E, in the absence of breakdown, would be 3 megavolts. In order to prevent breakdown between the plates 9, 10 which are only a short distance apart, a spark gap 18 may be connected in parallel with the resistor 16 (or other high impedance means) in order to limit the voltage E, as shown in Fig. 3. For example, a spark gap 18 may be provided such that the voltage E is limited to 200 kv.

Since E=l20 kv. for the equilibrium position of the beam 1, the beam current i intercepted by the slit edge 14 at equilibrium position will be 10 microamperes. If the beam 1 should suddenly be underdeflected, as by a sudden increase in V, the beam current i will increase by an amount i', and the time rate of change of B will be where C is the capacitance (in farads) of the plates 9, 10. On the other hand, if the beam 1 should suddenly be overdeflected, as by asudden decrease in V, the beam 4 current i will decrease by an amount i, and the initial time rate of change of B will be t dE E Initial Assuming that the capacitance of the plates 9, 10 is 100 micro-microfarads, and that i is 10 microamperes, the time rate of change of E in the case of underdeflection will be 100 kv./sec. In the case of overdeflection, the time rate of change of E for small values of t (that is, for values of t less than rC) will be 100 kv./sec.

It is to be understood that by the term outer plate" I refer to that plate which has the greater radius of curvature, and by the term inner plate I refer to that plate which has the lesser radius of curvature. Both plates, 9 and 10, are positioned wholly within the arcuate extension 3 of the acceleration tube 2.

It is further to be understood that by the term arcuate plate I refer to a plate which is longitudinally arcuate and transversely flat. Similarly, by the term arcuate passageway I refer to an arcuate passageway whereof the curvature is wholly in the lengthwise direction of said passageway.

The term slit edge refers merely to a narrow strip of metal 14 which is connected to the lower extremity of the outer plate 9, as by welding or soldering; or, the slit edge may be formed simply by bending the lower extremity of the outer plate 9 so that it extends a short distance into the passageway 11 at the lower extremity 7 thereof.

Having thus disclosed a preferred embodiment of my invention, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

1. Apparatus for electrostatic deflection of a beam of v charged particles through a desired angle of deflection comprising means for creating a beam of charged particles, a pair of arcuate, parallel, conducting plates, defining an arcuate passageway therebetween which sub-' tends an angle which is equal to the desired angle of deflection, said plates being supported so that said beam enters said passageway at one extremity thereof, and high impedance means electrically connecting said plates to each other.

2. Apparatus for electrostatic deflection of a beam of charged particles through a desired angle of deflection comprising means for creating a beam of charged particles, a pair of arcuate, parallel, conducting plates, consisting of a first plate and a second plate defining an arcuate passageway therebetween which subtends an angle ,which is equal to the desired angle of deflection, said second plate having a'smaller radius of curvature than said first plate, said plates being supported so that said beam enters said passageway at one extremity thereof, a conducting slit edge extending from said first plate into said passageway at the opposite extremity thereof so as to intercept a small portion of said beam, and high impedance means electrically connecting said first plate to said second plate.

3. Apparatus for electrostatic deflection of a beam of charged particles through a desired angle of deflection comprising means for creating a beam of charged particles, a pair of arcuate, parallel, conducting plates, consisting of a first plate and a second plate defining an arcuate passageway therebetween which subtends an angle which is equal to the desired angle of deflection, said second plate having a smaller radius of curvature than said first plate, said plates being supported so that said beam enters said passageway at one extremity thereof, a con-. ducting slit edge extending from said first plate into said passageway at the opposite extremity thereof so as to intercepta small portion of said beam; high impedance means electrically connecting said first plate to said second plate, and a spark gap connected in parallel with said resistor.

4. Apparatus in accordance with claim 2, wherein said high impedance means comprises a resistor.

5. Apparatus in accordance with claim 2, wherein said high impedance means comprises a set of corona points.

6. Apparatus for electrostatically bending a beam of charged particles, comprising a first conducting, arcuate plate, a second conducting, arcuate plate, said second plate being parallel to and spaced from said first plate, said second plate having a smaller radius of curvature than said first plate, said first plate and said second plate defining an arcuate passageway therebetween, means for creating a beam of charged particles, means for directing said beam into said passageway at one extremity thereof, and high impedance means electrically connecting said first plate to said second plate.

7. Apparatus for electrostatically bending a beam of charged particles, comprising a first conducting, arcuate plate, a second conducting, arcuate plate, said second plate being parallel to and spaced from said first plate, said second plate having a smaller radius of curvature than said first plate, said first plate and said second plate defining an arcuate passageway therebetween, means for creating a beam of charged particles, means for directing said beam into said passageway at one extremity thereof, a slit edge extending from the outer plate into said passageway at the opposite extremity thereof, so as to intercept a small portion of said beam, said slit edge being electrically connected to said first plate, and a high impedance electrical path between said first plate and ground, said second plate being grounded.

8. In a high-voltage high-vacuum acceleration tube, having a source of charged particles at one extremity thereof and an arcuate tube extension at the opposite extremity thereof, means for forming charged particles emitted from said source into a beam which enters said arcuate tube extension at high velocity, a pair of arcuate, parallel, conducting plates supported within said arcuate tube extension to define an arcuate passageway therebetween along the longitudinal axis of said arcuate tube extension, and high impedance means electrically connecting said plates to each other.

9. In a high-voltage high-vacuum acceleration tube, having a source of charged particles at one extremity thereof and an arcuate tube extension at the opposite extremity thereof, means for forming charged particles emitted from said source into a beam which enters said arcuate tube extension at high velocity, a pair of arcuate, parallel, conducting plates supported within said arcuate tube extension to define an arcuate passageway therebetween along the longitudinal axis of said arcuate tube extension, a slit edge on that plate which has the greater radius of curvature at that extremity of said plate which is the more remote from said source, said slit edge extending a short distance into said passageway, and a high impedance path electrically connecting said plates.

10. Electrostatic apparatus for deflecting beams of charged particles, comprising in combination an acceleration tube the wall whereof is composed of a multiplicity of electrode disk-like rings and insulation disk-like rings alternating therewith, the said acceleration tube having a source of charged particles at one end thereof and at the other end, beyond the said electrode rings and the insulation rings, having connected thereto an arcuate tube extension, that at its end the more remote from said tube Wall is covered, so that the entire internal volume of the acceleration tube may be evacuated, the said tube extension having a pair of arcuate, parallel conducting plates that are transversely flat from edge to edge, the said two plates defining an arcuate passageway therebetween, and high impedance means electrically connecting said plates to each other.

11. Electrostatic apparatus in accordance with claim 10, wherein the outer plate of said pair of plates is supported on the inner wall of the said arcuate tube extension by insulators, and the inner plate of said pair of plates is supported by conductive members, the said plates being positioned so that the beam of charged particles enters the passageway between said plates at their ends nearest the said acceleration tube wall.

12. Electrostatic apparatus in accordance with claim 10, wherein the outer plate of said pair of plates is supported on the inner Wall of the said arcuate tube exten sion by insulators, and the inner plate of said pair of plates is supported by conductive members, the said plates being positioned so that the beam of charged particles enters the passageway between said plates at their ends nearest the said acceleration tube wall, the said outer plate having at its exit end a slit edge extending transversely of said plate.

13. Electrostatic apparatus in accordance with claim 10, wherein the acceleration tube is a source of cathode rays, and the exit end of the arcuate tube extension is covered by an electron window.

14. Electrostatic apparatus in accordance with claim 10, wherein the acceleration tube is a source of X-rays and wherein a target is provided at the exit end of the said tube extension.

15. Electrostatic apparatus in accordance with claim 10, wherein the acceleration tube is a source of positive ions.

16. Electrostatic apparatus in accordance with claim 10, wherein the said inner plate is electrically grounded and wherein the said outer plate is connected to ground through a high impedance path.

References Cited in the file of this patent UNITED STATES PATENTS 2,193,602 Penney Mar. 12, 1940 2,219,902 Myers Oct. 29, 1940 2,355,191 Vance Aug. 8, 1944 2,464,562 Diemer Mar. 15, 1949 2,474,224 Clark June 28, 1949 2,533,859 Wideroe Dec. 12, 1950 2,556,166 Clark June 12, 1951 2,680,814 Robinson June 8, 1954

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