US2383751A

US2383751A - Electron lens

Info

Publication number: US2383751A
Application number: US442129A
Authority: US
Inventors: Spangenberg Karl
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1942-05-07
Filing date: 1942-05-07
Publication date: 1945-08-28
Anticipated expiration: 1962-08-28
Also published as: GB564003A

Description

Aug. 28, 1945. K.SPANGENBERG 2,383,751

ELECTRON LENS Filed m '1, 1942 Z-Sheets-Sheet 1 INVENTOR KARL SPANGENBEEG ATTORNEY Aug. 28,1945. K. SP ANGEN BERG 2,383,751

ELECTRON LENS I I Filed May 7, 1942 2 Sheets-Sheet 2 I 2 as: 4- .56169 0 '5" :0 40800010609000! INVENTOR KARL Smuaszvesea' A'I'I'OR EY niflcation and focal length.

Patented At 2.8, 1945 UNlTED STATES PATENT OFFICE ELECTRON LENS Karl Spangenberg, Palo Alto, Calif., assignor to International Standard Electric Corporation,

New York, N. Y., a corporation of Delaware Application May 7, 1942, Serial No. 442,129

' 8 Claims;

This invention relates to electron lenses and more particularly to equal diameter cylinder lenses;

In the. past various types of electron lenses have been proposed and constructed. These lenses have generally varied quite widely in mag- It is generally desired to make an electron lens with the smallest transverse dimension practicable in order that the neck portion of a tube retaining the lens maybe kept small in diameter.

Furthermore, for certain types of apparatus such as velocity modulated tubes or cathoderay tubes used for some types of scanning or commutating purposes, for example, it is desirable thatthe cathode beam be concentrated by a lens having a long focal length. Such a lens serves to maintain the beam relatively small in cross-section over along path and generally produces a smaller spot at the remote end thereof.

With these requirements in mind it is an object of my invention to provide an electron lens havture opening and, therefore, whenever the aperture is enlarged the entire diameter must be quite order that they may widely increased. Accordingly, a weak lens con-.

struction utilizing this type of concentrating arrangement has not generally been adopted.

The, other type of lens most commonly used, is one composed of two cylindrical electrodes. In the case of cylindrical electrodes, it has been considered that the lens action takes place at a plane of discontinuity of field representing the adjacent ends of these cylinders.- when the cylinders are made of different diameters, the lens arrangement has been made so thatthe ends of the cylinders are substantially in the same plane. However, when equal diameter cylindrical electrodes were used, a small amount of spec is required in be insulated from one another so that diiferent potentials may be applied thereto. According to the prior art-small spacings were therefore provided between these cylinders, generally in the order of, about one-tenth of the cylinder diameter although the spacing was ing a small transverse dimension and along focal length.

According to a feature of my invention these and other objects are accomplished by providinga lens comprising two axially spaced equal diameter cylinders, the spacing between the cylinders being relatively great with respect to the diameter thereof.

A better understanding of my invention and the objects and features thereof may be had from the particular description thereof made with reference to the accompanying drawings in which:

Fig. 1 is an illustration of a typical electron lens incorporating the features of my invention, and- Fig. 2 is a graph illustrating the features of my invention.

Before entering into a specific the apparatus it will help to clarify the understanding of my invention to discuss generally the properties and construction of electron lens of the prior art.

Usually one of three types of electron lens systems have been generally adopted. One of these types is called an aperture lens and consists of two spaced 'discs to which positive'potentials are.

applied, the discs being provided with central apertures through which the electron beam passes. Inzthis type of lens, the lens action is generally considered as taking'place in the space between the spaced discs and apertures. The disadvantage of this type'of lens wherever small dimensions are required is that'the diameter of the disc must be quite large compared tothe aperdescription of varied slightly depending upon the potential differences at which the cylinders were designed. to

be polarized. Thus, if a large potential diiferensce I was contemplated a. slightly greater spacing was producing a great spreading of the electron beam.

required. However, all .of these spacings between cylinders were generally made less than a quarter of the cylinder diameter.

A third type of lens-may be mentioned as consisting of one cylinder element and one aperture element. This type of arrangement. ofcourse, suffers from the same disadvantage of small aperture lenses since it requires a large transverse dimension to accommodate the apertured disc. It should be realized that with a. lens made of cylinder elements the smallest overall diameter with respect to aperture openings is provided. Accordingly, in order to retain small dimensions this is the preferable type of lens.

I have discovered that equal diameter cylinder lenses need not be made with the ends closely spaced but may beprovided with relatively wide spacings between the cylindrical elements. I When this is done it is found that such a lens has a much longer focal length and therefore is capable of focusing a. beam at a distant point, without Ithas been found that when the spacing is increased while maintaining the same object distance and voltage ratio of energy applied to the cylinders of the lens, the focal length also increases. This increase in focal length is not very great as the separation between "the cylinders is increased up to aboutv half the diameter of the lens cylinders. However, beyond this half diameter relationship it is found that the diilerence is quite great. At a separation equal to the cylinder diameters, it is found that the focal length of the lens is in the order of six times that of the known forms of lens with the same object distance and voltage ratio applied to the lens electrodes.

In Fig. 1 is illustrated a typical structural arrangement of an electron beam producing arbe provided instead of the specific arrangement illustrated in Fig. l.

Spaced longitudinally in the direction of travel of the electron beam are arranged two equal diameter cylinders 55 and t. A positive voltage Vi is applied to cylinder 5 and a positive voltage V2 is applied to cylinder 6. All of the electrodes 3, d, 5 and t are supported on insulating rods 7 and 8, which in turn are fastened firmly to the base 9, which also serves to support the cathode i and heat shield electrode 2.

At 0 is shown an arrow representing the point of smallest cross-section of the accelerated electron beam. This may be considered as the ob ject of the lens. This object O is reproduced as an image I on the far side of the

electron lenses

5, 6. The center of the lens is indicated by the dotted line C. The distance from O to C indicated by p, is known as the object distance and the distance q from the center of'the lens to image I is known as the image distance. An electron lens of the type shown, or of any other type must be considered equivalent to a thick lens in optics.

Accordingly, there are to be considered two different rays shown as m and rb in the drawings. The ray Ta proceeds from the arrow 0 at an angle crossing the center line axis of the lens at point F. At plane P the ray ra. is bent and travels in a substantially parallel ray to the corresponding pointon the image. The point F whereray ra crosses the longitudinal center line of the lens is known as the first principal focus. Ray rb may be considered as starting from the point on image I to a plane known as the second p pal plane Pl, where it is bent and traverses parallel to the lens axis to the corresponding point on object O. The distance between the second principal plane and the point Fl, where ray 1b crosses the longitudinal center line is known as the second principal focus.

In all the electron lenses where any particular structure, has been determined, there are four variables any two of which takes the characteristics of the lens. Thus, when two lens electrodes of diameter D' and spacing S are fixed in a tube structure the four remaining variables are object distance 2), image distance q, magnification m and voltage ratio V2 /Vl'. When any two of these parameters are fixed the others are also fixed, and only can be used in the particular relation to produce focus. Thus, if the object and image distance are once fixed, only one voltage ratio will produce a proper focus and the magnification is determined thereby. I have discovered, however, that a fifth element, namely the spacing S, can be used to provide an electron lens of small diameter which is relatively weak. This is accomplished by increasing the spacing between the lens elements.

In Fig. 2 is shown a graph on a log-log scale demonstrating the difference in properties of the prior art equal diameter cylinder lens, and the equal diameter cylinder lens in accordance with my invention. The solid line graph illustrams the pattern for a prior art lens in which the spacing between the equal diameter cylinders is equal to 0.1 of the cylinder diameter. In this graph the abscissa is shown as the object distance p and the ordinates as the image distance q. The lens pattern then takes the form of a curvilinear rectangle. Along the line substantially of 45 positive slope is represented the voltage ratio.

On this is the voltage ratio of V2 to Vi as indicated in Fig. l. The magnification then follows the other side of the rectangle. As indicated magnifications of 0.1, 1 and ,5 are shown, the broken line graph represents the corresponding pattern for equal diameter cylindrical lenses spaced apart a distance equal to thediameter of the cylinder. I

By reference to this graph the features of my invention may be readily determined. For example, at object distance it the prior art lens has an image distance of substantially 1.4:, while the lens with the 1 to i spacing has an image distance approximately 2.6 at the same voltage ratio 10. At the lower voltage ratio of ,5, the image distance for the same object distance is 4 for the prior art lens, and about 16 for the 1 to 1 ratio lens. At 20 object distance the prior art lens shows an image distance of approximately 6.2 at a voltage ratio of 3, while the lens with a 1 to 1 spacing has an image distance of approximately 80. a

From the above consideration anddiscussion it is clear that an electron lens constructed in accordance with my invention has a much greater object to image distance than is possible with the prior art type of lens. Preferably, the specing in accordance with my invention is made greater than half the diameter of the cylinders involved and between this dimension and about three times the diameter. Results have shown that the most useful spacing considering the space requirements, as well as the other features thereof, occurs when the spacing S is maintained between the limits of about .75 to 1.25 times the diameter of the cylinders involved. Also, better results are obtained when the voltage ratio VZ/Vl is between the limits of 1 to 1, and 5 to 1. However, the advantages of my invention are not limited to' these optimum values as is clear from the examples shown in the plotted curves.

In all electron lenses I have found that the magnification may be expressed by the formula where K is a factor determined by the type of lens. In the known form of lens of the. prior art,

this factor Kis generally in the order. of .8. Howof the present invention, with its open end cylinders, differs widely from the prior art devices using apertured barrier electrodes, even where in some cases the apertures in the barrier electrodes are lined with tubes, since such tubes not only do not serve the purpose of the cylinders used in the cylinder type of, electron lens, in bringing to a junction at the image plane all those trajectories of the electrons which intersect at the same point in the object area, but serve chiefly to aid in trapping and barring from the aperture those electrons whose trajectories make too great an angle with the axis of the aperture tubes.

It may be said that in all electron lens devices employing cylinders, in the sense in which that term is used in this art, the internal diameter of the end of the cylinder which first receives the electron beam is so located and of such size that if the tangents, to the trajectories of all the electrons emerging from the emission aperture of the beam forming device, be determined at the plane of the emission aperture and be considered as' extended 'in the direction of'theelectron flow, all such extensions of said tangents will intersect the plane of the open end of the cylinder nearest the emission aperture within the circumference of said open end of the cylinder. In other words, all the electrons emitted from the emission aperture can, without the action of any deviating force, enter and pass into the open end of the first cylinder, so that thereafter both cylinders enclose the paths of substantially all electrons emitted by the emission device.

This is entirely difierent from the aperture type of electron lens, even where the apertures are provided with tubes, since such apertures and tubes are much smaller than the cylinders and, hence, the devices trap and hold back all those electrons whose extended trajectory tangents, determined at the plane of the emission aperture, do not intersect the plane of the aperture, or of the open end of its aperture tube, within the circumference of such aperture or tube, unless some additional deviating force such as a strong magnetic field be applied to the electrons in advance of the first aperture tube.

An electron lens in accordance with my invention in which the spacing is made equal to the diameter of the lens element, has been found to of said open end within the circumference thereof said cylinders being spaced apart in the direction of electron flow a distance between one and three times said diameter, and means for continuously maintaining positive potentials on said cylinders, the ratio of the potentials on the two cylinders, taken in the direction of electron flow, being not less than one and not more than ten. r

2. An electron supply means for producing an electron beam with relatively small spot size, comprising an electron emitting element, means for accelerating and concentrating electrons from said electron supply means into an electron beam, said means having an emission aperture from which the electrons of said beam emerge and an electron lens in the path-of said beam comprising two open ended conductive cylinders of the same diameter arranged end to end in the path of said electron beam, the open end near-1 est the emission aperture, of the first cylinder, being arranged so that the tangents, atthe plane oi the emission aperture, to the trajectories of all the emerging electrons, if extended in the direction of electron flow would intersect the plane of said open end within the circumference thereof said cylinders being spaced apart in the direction of electron flow a distance at least as great as said diameter and means for continuously "maintaining positive potentials on said cylinders.

3. An electron supply means for producing an electron beam with relatively small spot size; comprising an electron emitting element, means for accelerating and concentrating electrons from said electron supply means into an electron beam, said means having an emission aperture from which the electrons of said beam emerge and an electron lens in the path of said beam .comprising two open ended conductive cylinders of the same diameter arranged end to-end in the path of said electron beam, the open end nearest 1 the emission aperture, of the first cylinder, being in general have a magnification about 20% below that of all known prior art lens, and the focal length at least six times as great.

While Ihave described above the principal features of my invention, it is considered that this is given merely by way of example. Certain departures from my inventionmay be had-within the scope of t e invention.

What I claim is; 1;. An electron supply means for producing an electron beam with relatively small spot size, comprising an electron emitting element, means for accelerating and concentrating electrons from said electron supply means into an electron beam,

said means having an emission aperture from, which the electrons'oi said beam emerge and an theemerging. electrons, if-extended in the directionoi electron flow wouldintersect the plane arranged so that the tangents, at the plane of the emission aperture, to the trajectories of all the emerging electrons, if extended in the direction oi electron flow would intersect the plane of said open end within the circumference thereof said cylinders being spaced apart in the direction of electron flow a distance between one and three times said diameter, and means for continuously maintaining on said cylinders positive potentials of such ratio, taken in the direction of electron flow as to give the desired spot size at the image plane.-

4. An lectron lens for controlling the focus of an electron beam of electrons emerging at I various angles from an emission aperture to provide a relatively long focal length, comprising a pair of open ended cylindrical elements located substantially coaxially in the path of the beam and arranged to receive all the electrons directly from the emission aperture over the paths they were following when emerging from said emission aperture, said elements being of substantially equal diameter andlongitudinally spaced apart a distance equal to at least about said diameter and means for maintaining positive potentials on said'elements.

5. An electron lens according to claim 4 where-.- in the ratio of the potential applied to the cylindrical element furthest in the direction of electron flow of said beam and that applied to the other element is not less than one and not more I 6. An electron lens according to claim 4 in which the ratio of the potential applied'to the cylinder furthest in the direction of electron trons directly from the emission aperture, over,

the paths they were following when emerging from the emission aperture, said elements being of substantially equal diameter and longitudinally spaced'apart a distance which is about the same'as the diameter of said cylinders, and means for maintaining on each cylinder 2. positive potential, the ratio of the potentials, taken in the direction of electron flow, being not less than one and not more than five.

8. An electron supply means'for producing a beam with relatively small spot size at the image plane and with a relatively long focal length,

1 path of said beam and comprising two openended conductive cylinders of the same diameter, arranged end to end in the path of said beam, the open end, nearest the emission aperture, of the first cylinder, being arranged so that the tangents, at th plane of the emission aperture, to the trajectories of all the emerging electrons, if extended in the direction of the electron flow, would intersect the plane of said open end within the circumference thereof, said cylinders being spaced apart, in the direction of electron flow, a distance not less than equal to one diameter of the cylinders and not more than three times said diameter, and means for continuously inaintaining a positive potential on each cylinder, the ratio of the potentials, taken in the .direction of electron flow, being not less than one and not more than fifteen.

KARL SPANGENBERG.