US3050653A - Magnetic focusing device - Google Patents

Description

1962 H. w. ca. SALINGER 3,050,653

MAGNETIC FOCUSING DEVICE Filed July 28, 1955 B (FLUX DENSITY) H MAGNE T/ZING F ORCE INVENTOR. HANS W G. SALINGER A TTORN'E Y United States Patent 3,056,653 MAGNETIC FGCUSlNG DEVIQE Hans W. G. Salinger, Fort Wayne, Ind, assignor to In ternational Telephone and Telegraph Corporation Filed July 28, 1955, Ser. No. 525,041 8 Claims. (Cl. 31368) The present invention relates to a magnetic focusing device, and more particularly to a lens for use in electron discharge tubes. Such a lens may be used for focusing either electron or ion particles. While an electron lens is specifically treated in the following, it will be understood that the same principles apply to an ion lens.

Almost all cathode ray tubes utilize small apertures through which a beam or bundle of electrons pass. Very often focusing means in the form of either magnetic or electrostatic lens arrangements serve to reduce the crosssectional area of the beam at predetermined points, and it is with this latter feature that this invention is primarily concerned.

In general, this invention comprises a magnetic lens of desired configuration which may be used in various types of cathode ray devices, for example, in electron guns, electron microscopes, as a part of the storage grid of memory tubes, or in shadow masks of color television tubes. Other applications of the principles of this invention will at once become apparent to persons skilled in the art upon understanding the description to follow.

It is therefore an object of this invention to provide a magnetic lens for focusing an electron beam.

It is another object of this invention to provide a magnetic lens having an extremely short focal length and high magnification power, this lens being the ultimate in simplicity of construction and not requiring any power or external excitation for producing the focusing elfects.

The general rule relating the magnetic fields in air and iron is that the component of flux density B which is perpendicular to the air-iron interface passes without change through the boundary. The same is true of the components of the magnetizing force H which are parallel to said interface.

In accordance with this invention there is provided a plate-like electrode having at least one electron-receiving aperture, this electrode being composed of hard magnetic material which possesses considerable coercive force, the electrode being permanently magnetized and so thin that residual induction is substantially zero.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagrammatic illustration in section of a typical cathode ray tube which utilizes an embodiment of this invention;

FIG. 2 is an enlarged fragmentary section of the tube of FIG. 1;

FIG. 3 is a fragmentary section of permanent magnet material of the type used in this invention;

FIG. 4 is an enlarged fragmentary section of a portion of this invention as used in the tube of FIG. 1; and

FIG. 5 is a hysteresis curve used in explaning the principles of this invention.

Referring to the drawings, and more particularly to FIGS. 1 and 2, the cathode ray tube indicated generally by the reference numeral 1 is of conventional construction and corresponds to the Iatron storage tube as manufactured by Farnsworth Electronics Division of International Telephone and Telegraph Corporation. Typical storage tubes which may utilize the magnetic lens of 3,05%,553 Patented Aug. 21, 1962 this invention are disclosed in Farnsworth Patent No. 2,228,388 and in Davis application Ser. No. 479,329, entitled Storage Tube Construction, and filed January 3, 1955, now Patent No. 2,932,764. The tube 1 comprises an electron gun 2 which emits a beam 3 of electrons which may be scanned in a conventional television raster pattern by means of deflection coils 4, and a phosphor screen 5 disposed adjacent an electrostatic storage screen 6. A flood electron gun 18 positioned in the rear of the tube 1 emits a flood beam of electrons as represented by the reference numeral 10, this flood beam covering the active area of the storage screen 6 in the usual manner.

In operation, a video signal modulates the beam 3 as it is scanned over the active surface of the storage screen 6 by means of the deflecting coils 4. An electrostatic charge image is impressed on the screen 6 which corresponds to the video image used for modulating the beam 3, this charge image serving to modulate electrons from the flood beam 10 which pass therethrough to be accelerated onto the phosphor screen 5. The screen 5 thereupon luminesces in a pattern corresponding to the charge image on the screen 6 which in turn corresponds to the video image used to modulate the beam 3. Exact details of construction and operation of a storage tube of this character as well as suitable circuitry for operating the tube are well-known and need not be further elaborated here.

In FIG. 2 is illustrated in magnified detail the phosphor and storage screens 5 and 6, respectively. Basically, the storage screen 6 is conventional in the sense that it comprises a conductive backing 7 which carries an insulator coating 8. The assembly 7, 8 is provided with a plurality of apertures 9, the number of apertures corresponding to a fine mesh screen of, for example, 10-0 to 500 mesh. The material 8 in the present instance is conventional and may consist of the usual silicon monoxide or quartz which has a secondary emission ratio greater than unity. In this invention, the backing '7, preferably, is from 1 to 3 mils thickness, and is composed of a hard magnetic material which may be permanently magnetized.

When the backing 7 is permanently magnetized, each aperture becomes a magnetic electron lens and thereby exercises a focusing influence on a stream of electrons, such as 1%, as it passes therethrough. In an operating embodiment of the invention, the spacing between the phosphor and storage screens 5 and 6 respectively is such that the beam 10 will focus at or near the surface of the phosphor 5.

Since it is Well-known that a magnetic field exerts a deflecting influence on electrons, it is evident that magnetism emanating from the plane surfaces of the backing 7 will deflect or interfere with the desired path of the electrons of both. of the beams 3 and 119 of the tube of FIG. 1. This elfeot is undesired and requires, in order for the screen 6 to be properly operable, that the magnetic field be limited only to the vicinity of each aperture 9. This is accomplished in this invention as explained in the following. FIG. 5 illustrates a typical hysteresis curve for a hard magnetic material. Assuming that the material is in the form of a closed ring, with no free poles or air gaps, magnetization of the ring by applying a large magnetic field which is later removed leaves the material permanently magnetized. The remanence or residual induction in the material is represented by point 11 which lies on the ordinate of the graph. However, if this magnetic material is in the form of a rod, the effective m ag netizing force H is not zero after the external magnetizing field is removed, because the free ends of the rod provide magnetic poles which themselves generate a magnetizing force. The residual induction or flux density in such a rod is indicated by the point 12 on the hysteresis curve. As the length of the rod is shortened, the residual induction correspondingly diminishes as indicated by the point 13, so that if the rod is eventually reduced to a thin foil, point 14 on the abscissa is reached. This point 14 corresponds to zero residual induction or flux density B inside the material, while the true field or magnetizing force H (coercive force) is large and opposite to the direction of the original magnetizing force.

Such a thin toil will exhibit no magnetic field externally thereof, since no flux can emerge. It will be recalled earlier that one of the requisites of this invention was that the magnetic field be limited to the vicinity of the apertures 9 of the screen 6 in order to prevent deleterious defiection or dispersion of the electron stream approaching the screen.

In FIG. 3, the solid arrow 15 represents the original direction of magnetization applied to a foil do which is greatly exaggerated in thickness for purposes of clear illustration. The dashed line arrows 1'7 inside the foil represent the direction of the internal field. However, if a hole or aperture is punched through the foil as represented by FIG. 4, the internal field 17 will be present in the space of the hole as well as in the solid material. The general rule relating the magnetization in air and in iron is that the perpendicular components of flux B pass without change through the iair-toair inter-face. The same is true of the parallel components of the magnetizing force H.

It is seen from the well-known relation in which ,0. is the permeability factor having a value of one 1) for air, B and H are numerically equal in air, or in other Words are numerically equal in the space of the screen aperture 9. These conditions being true, at point a on the surface of the foil 16 (FIG. 4) the magnetizing force H is weak, because it is essentially perpendicular to the foil surface, and at the same time the magnetization B is almost zero in the iron. By contrast, at point 17 on the Wall of the aperture 9, the field H is parallel to the aperture wall whereupon the fields of the magnetizing force H are the same in the aperture and in the iron. In other Words, the field strength in the aperture will numerically be about equal to the coercive force of the magnetic material, i.e., in the order of 100 to 1,000 oersteds. It should be noted, the direction of the field is opposite to the direction 15 of the original mag netization. Because of the presence of this field inside of the aperture 9, electrons directed therethrough will be focused as previously explained.

Suitable materials for the foil are known by the trademarks Alnico, Sil-ma-nal, Cunife, and similar alloys which possess a high coercive force, or in other words are magnetically hard. Pressed sheets or foils made of finely subdivided iron possess the desired magnetic property as do some of the well-known ferrite materials. Among these substances, the ones that can be shaped into thin foils are best adapted to the invention, such as those used for magnetic recording ribbons, computer memories and the like. The thicknesses of the backing or foil 7 ('FIG. 2) may (for the storage tube of FIG. 1) range from 1 to 3 mils, with the aperture diameter about equal to the foil thickness.

While this invention has been described in connection with a storage type cathode ray tube, as stated previously, the invent-ion may be adapted to ordinary electron guns and to single aperture electron microscopes. When used in connection with electron guns or microscopes, the in sulator coating 8 of course is not used, the permanently magnetized foil itself with a suitably sized aperture constituting the complete electrode.

Considering a single aperture through the foil, the focal length of the resultant, magnetic lens is given by the equation V is the electron velocity in volts, H the field strength in oersteds, and the integral is extended over the length of the aperture (thickness of foil). For the following example, a somewhat thicker foil, say one (1) millimeter thickness (a'z=.l cm.) may be chosen. Selecting values of V=1 volt, H 400, fdz=.l cm. gives then a focal length of 2.8 mm.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. A cathode ray tube comprising a plate-like electrode having at least one electron-receiving aperture, said electrode being composed of a thin foil of magnetic material which possesses coercive force, said electrode further being permanently magnetized in its thickness direction whereby its residual induction is substantially zero, said aperture having approximately the same diameter as the thickness of said foil so that the magnetizing force from said material appears in said aperture to exercise a focusing action on a stream of electrons passing therethrough.

2. A cathode ray tube comprising a plate-like member having a plurality of tiny apertures therein, said member being composed of a thin foil of magnetic material permanently magnetized in its thickness direction so that when an external magnetizing force is removed the residual induction is substantially zero, said apertures having a diameter approximately equal to the thickness of said foil so that a focusing magnetizing force is present therein after said external magnetizing force is removed, and a layer of insulating material on said member and having secondary emission characteristics greater than unity.

3. A cathode ray tube comprising a plate-like electrode having a at least one electron-receiving aperture, said electrode being composed of magnetic material which possesses coercive force, said electrode further being permanently magnetized in its thickness direction and being so thin that residual induction is substantially zero, the thickness of said electrode being from one to three (1 to 3) mils and said aperture being from one to three (1 to 3) mils in diameter.

4. A cathode-ray tube comprising an evacuated envelope containing a beam source of electrons, a screen mounted in position to be scanned by electrons from said source, and a magnetizable apertured mask through which beam electrons from said source pass in their transit to said screen, said mask being magnetized to form a magnetic lens at each aperture thereof to magnetically focus the electrons passing through said aperture.

5. A cathode ray tube comprising an evacuated envelope containing a beam source of electrons, a screen mounted in position to receive electrons from said source, and a platelike electrode positioned between said beam source and said screen and having at least one electron-passing aperture formed therein through which beam electrons from said source pass in their transit to said screen, said electrode being composed of a thin toll of magnetic material which possesses coercive force, said electrode further being permanently magnetized in its thickness direction whereby its residual induction is substantially zero thereby forming a magnetic lens at said aperture magnetically to focus the electrons passing therethrough.

6. A cathode ray tube comprising an evacuated envelope containing a beam source of electrons, a screen mounted in position to receive electrons from said source, and a plate-like electrode positioned between said beam source and said screen and having at least one electron-passing aperture formed therein through which beam electrons pass in their transit to said screen, said electrode being composed of a thin foil of magnetic material which possesses coercive force, said electrode further being pennanently magnetized in its thickness direction whereby its residual induction is substantially zero, said aperture having approximately the same diameter as the thickness of said foil so that the magnetizing force from said material appears in said aperture to exercise a focusing action on the stream of electrons passing therethrough.

7. A cathode ray tube comprising an evacuated envelope containing a beam source of electrons, a screen mounted in position to receive electrons from said source, and a plate-like member positioned between said beam source and said screen and having a plurality of tiny apertures formed therein through which beam electrons from said source pass in their transit to said screen, said member being composed of a thin foil of magnetic material which may be permanently magnetized in its thickness direction so that when an external magnetizing force is removed the residual induction is substantially zero, said apertures having a diameter approximately equal to the thickness of said foil so that a focusing magnetizing force is present therein after said external magnetizing force is removed thereby magnetically to focus the electrons passing through said apertures, and a layer of insulating material on the side of said member toward said beam source and having secondary emission characteristics greater than unity.

8. The invention as set forth in claim 4, in which said mask is permanently magnetized, with the side facing said screen constituting one pole, and the opposite side constituting the other pole.

References Cited in the file of this patent UNITED STATES PATENTS 2,149,101 Ploke "Feb. 28, 1939 2,547,638 Gardner Apr. 3, 1951 2,555,850 Glyptis June 5, 1951 2,659,026 Epstein Nov. 10, 1953

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