US3223871A

US3223871A - Electron optical system

Info

Publication number: US3223871A
Application number: US133221A
Authority: US
Inventors: Schlesinger Kurt
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1961-08-22
Filing date: 1961-08-22
Publication date: 1965-12-14
Anticipated expiration: 1982-12-14

Description

Dec. 14, 1965 K. SCHLESINGER 3,223,871

ELECTRON DPTICAL SYSTEM Filed Aug. 22, 1961 3 Sheets-Sheet 1 FIG.I.

so IIIIIIMII- MODULATING susmu.

INVENTORI KURT SCHLESINGER SATTORNE Dec. 14, 1965 K. SCHLESINGER 3,223,871

ELECTRON OPTICAL SYSTEM Filed Aug. 22, 1961 3 Sheets-Sheet 2 I3 Ill/111/1 1 1 1,

INVENTORI KURT SCHL SINGER fig;

HIS

TORNEY.-

Dec. 14, 1965 K. SCHLESINGER 3,223,871

ELECTRON OPTICAL SYSTEM Filed Aug. 22, 1961 3 Sheets-Sheet 3 INVENTOR: URT SCHLESINGER United States Patent C 3,223,871 ELECTRON OPTHIAL SYSTEM Kurt Schlesinger, Fayetteville, N .Y., assignor to General Electric Company, a corporation of New York Filed Aug. 22, 1961, Ser. No. 133,221 13 Claims. (Cl. 31383) The present invention relates to ultra high resolution electron optical systems, and particularly to improved cathode ray tubes embodying such electron optical systerns.

One object of the invention is to provide an electron optical system for producing an electron beam having a tar-get spot size which is extremely small, e.g., a few microns in diameter, at target beam current levels providing desirable energy transfer to the target, e.g., several microamperes.

Another object is to provide such an electron optical system wherein the electron beam can be readily deflected through a substantial target-scanning angle, e.g., of the order of 40 degrees, and where-in the overall physical length of the electron optical system is conveniently small.

Another object is to provide an electron optical system of the character described wherein defiection-defocusing of the electron beam is minimized.

Another object is to provide an electron optical system of the character described including convenient means for precisely controlling the cross-sectional shape of the target scanning electron beam.

Another object is to provide an electron optical system of the character described which does not require critical mechanical alignment of the structural elements thereof.

Another object is to provide an improved cathode ray tube embodying an electron optical system of the foregoing character.

These and other objects of the invention will be apparent from the following description and the accompanying drawings wherein:

FIGURE 1 is an axial sectional view of a cathode ray tube constructed according to my invention;

FIGURE 2 is an enlarged fragmentary view of a portion of the structure of FIGURE 1;

FIGURE 3 is an enlarged transverse sectional view of the structure of FIGURE 1 taken on the line 33 thereof;

FIGURE 4 is an enlarged transverse sectional view of the structure of FIGURE 1, taken on the line 4-4 thereof;

FIGURE 5 is a view similar to FIGURE 4 showing an alternative form of one feature of the invention;

FIGURE 6 is a schematic diagram of circuitry associated with the apparatus of FIGURE 5.

An electronic optical system constructed in accordance with my invention is shown in FIGURE 1 embodied in a cathode ray tube which includes an envelope having an elongated neck 2, and an enlarged funnel portion 4 closed by a face plate 6 on the interior of face plate 6 is a target in the form of a luminescent screen 8. The screen 8 may consist preferably of a continuous transparent film of luminescent material which may be of the vapor-phase deposited type such as taught by US. Patents 2,675,331, 2,685,530, and 2,887,401, commonly assigned herewith.

Arranged in order coaxially Within the neck portion of the envelope from the base or rearward end of the neck toward the target screen 8 are a plurality of sections hereinafter to be described in greater detail. These sections include an electron beam generating section 10, a prefocusing lens section 20, an electrically neck-elongating section 30, and a main focus lens section 40.

Referring to FIG. 2 in the electron beam generating section 10 of the tube, electrons are emitted from an 3,223,871 Patented Dec. 14, 1965 axially located cathode 11 which is preferably of the high current-density type such as a dispenser cathode. The emitted electrons pass through axially apertured collimator electrode 12, first anode 13, beam intensity modulating gate electrode 14, and a meniscus electrode 15. As best shown in FIGURE 2, anode 13 has a forwardly projecting (towards screen 8) convex surface 16 which, together with the confronting rearwardly directly or facing concavity 17 of the gate electrode 14 which, as illustrated, is in receiving relationship to the projecting surface 16, form a focusing electrostatic field, the equi-potential surfaces of which are hyperboloids of revolution symmetrical with the axis of the tube neck and asymptotic to readwardly concave conical surface 17 of approximately 109 apex angle whose apex faces screen 8 or away from said emitter and lies on the tube neck axis in substantial coincidence with the central aperture 18 in the meniscus electrode. The characteristics of such a focusing electrostatic field are described in more detail in my copending application Serial No. 16,523, now U.S. Patent 2,995,676 commonly assigned herewith.

The field between the anode 13 and the gate electrode 14 forms in the aperture 18 an effective virtual cathode of demagnified size relative to the actual cathode 11 and thus illuminates the aperture 18 with an electron beam of density many times that of the emission density from the actual cathode 11, for resulting maximum brightness at the screen 8. In a successfuly operated tube constructed as herein described, the cathode 11 was operated at ground potential, collomator 12 at about +10 volts, anode 13 and meniscus 15 at about 500 volts, the beam intensity modulation was achieved by varying the potential of gate 14 between 5 and +10 volts, and current densities of about 25 amperes per square centimeter at aperture 18 were achieved with emission density at the cathode of only about 2 amperes per square centimeter.

The electron beam emerges from the aperture 18 into the prefocusing lens section 20, within which is formed an electrostatic field whose equipotentials are hyperboloids of revolution coaxial with the tube neck axis, and asymptotic to and within a coaxial forwardly concave conical surface 21 of approximately 109 apex angle having its apex facing emitter 11 and located in substantial coincidence with aperture 18. The forward end of the prefocusing lens section is terminated by a transverse axially apertured conductive wall 22 having a coaxial opening 23 and spaced from the conical surface 21 by a supporting insulating cylinder 24.

The two

surfaces

21 and 22 serve to form the hyperbolic electrostatic field within the prefocusing lens section 20, and, if desired, the formation of such field may be augmented by further electrode means, such as resistive coatings on the cylinder 24 having local potentials corresponding to the local space potential of the prefocusing lens field.

In FIG. 1, adjacent transverse wall 22, and closed at its rearward end thereby, is the electrically neck-elongating section 30 which includes an accelerating cylindrical spiral electrode 31 arranged coaxially with the neck axis. The spiral electrode 31 may conveniently consist, as shown, of a conductive spiral coating of uniform pitch on the interior surface of an insulating support cylinder 32. Desirably the spiral electrode 31 has a high impedance of the order of 3050 megohms to minimize power consumption. Wall 22 and the rearward end of the spiral electrode 31 are connected by a lead 35 to a suitable adjustable potential source, shown schematically as potentiometer 50, which may provide to lead 35 a relatively low potential of the order of to 800 volts. A conductivecoating 33 on the exterior of the insulating cylinder 32 serves as an electrostatic shield for the spiral electrode and also conveniently provides part of a conductive path from the forward end of electrode 31 to a conductor 53 of substantially higher potential, which may be for example of the order of 7,000 volts, so as to provide a substantial accelerating field within spiral electrode 31. Annular

conductive caps

27, 36 are provided at each end of the cylinder 32 to facilitate mechanical support and provide convenient electrical connections to the ends of spiral electrode 31.

The forward end of the cylinder 32 is closed by a transverse conductive wall 37 having a central aperture 38. Forward of the aperture 38 and partially supported by wall 37 is the main focusing lens section at) of the tube, which may be of any suitable type including a unipotential lens, but is here shown as a bipotential lens. Section 40 includes as one element of the bipotential lens a coaxial conductive cylinder 41 of diminished diameter relative to electrode 31 and having an enlarged mouth 42 at its forward end. Cylinder 41 preferably has the same potential as the forward end of spiral electrode 31. The aperture 38 serves as a limiting aperture minimizing aberration through the lens section 40, and another transverse wall 43 intermediate the ends of cylinder 41 has an axial aperture 44 and serves as a shield to cut down emission of stray electrons from the lens section 40. The forward end of the lens section 40 is spaced by an annular insulator 45 from a supporting conductive sleeve 46 which in turn is connected by conductive support fingers 47 to the neck wall. Sleeve 46 forms the second element of the bipotential lens and is electrically connected by fingers 47 and an internal conductive coating 48 at the front of the neck and on the inner surface of the envelope funnel 4 to the high voltage terminal 51 of the tube, which may have a potential several times that of cylinder 41, for example 20 kv. A suitable deflection yoke 49 is provided for scanning the electron beam on the screen 8.

In the operation of the electron optical system, an electron beam of high current density, for example of the order of 25 amperes per square centimeter, substantially demagnified in cross-section by the focusing field between anode 13 and gate 14, and modulated in intensity by a control signal applied to gate 14, is supplied to aperture 18, and forms there a virtual cathode. The prefocusing lens section 20 operates to provide a virtual image of the virtual cathode at aperture 18, which virtual image serves as the object for the main focusing lens 40. It has been found that the accelerating section 30 has the electrical effect of elongating the distance from the principal plane of the main focusing lens 40 to its effective object plane. Thus, since the magnification of the main focusing lens, as is well known to those skilled in the art, is proportional to the image distance divided by the effective object distance, for a fixed image distance between the screen 8 and the main focusing lens 40 the image size or electron beam spot size on the screen is correspondingly decreased by the neck-elongating or object distance-elongating action of spiral accelerating electrode 31.

Dynamic focusing as well as control of resolution is provided conveniently by varying the potential of the wall 22 relative to the potential of surface 21 of the meniscus electrode 15, which in turn modifies the action of the prefocusing lens section 20. For example, it has been found that as the potential of wall 22 is increased above that of surface 21, the overall effect of the prefocus lens becomes positive or converging, and the virtual image of the aperture 18 produced by the prefocus lens section moves rearwardly and increases in size. Since this virtual image serves as the effective object for the main focusing lens 40, the overall effect at the screen is one of moderate spot size enlargement with a considerable increase in beam current, such that the current density observed at the screen remains substantially constant. When the potential of wall 22 equals that of surface 21 of meniscus electrode 15, the space between meniscus electrode 15 and wall 22 becomes substantially field-free, and the location of the effective object for the main focusing lens is at the aperture 18. When the potential of wall 22 is decreased below that of the meniscus electrode, however, it has oeen found that the overall effect of the prefocusing lens section 20 is diverging or negative, and the virtual image of aperture 18 which the prefocus lens forms moves forward of the aperture 18 and is diminished in size. The result of this at screen 8 is that the electron beam spot size gets smaller and beam current decreases somewhat. Thus convenient control of resolution, as well as dynamic focusing, is obtained merely by adjusting the potential of Wall 22.

I have found that, with a potential at terminal 51 of 20 kv., with cylinder 41 and the forward end of spiral electrode 31 at 7 kv., and with meniscus electrode 15 at 500 volts, in a tube of the type described, excellently small spot sizes of about .00033 inch (8 microns) at screen 8 with 1.5 microampere beam current can be obtained. This spot size is obtained when the field in the prefocusing lens 20 is decelerating, and the rearward end of spiral electrode 31 and wall 22 have a potential of about 0.6 that of electrode 15, i.e., 300 volts.

Optionally, the neck-elongating section 30 may be arranged to act as a moderate converging lens for the electron beam, as well as an accelerator, for example by changing the spiral 31 from a uniform pitch to one having a pitch progressively increasing toward wall 37. However in such a case, care must be taken that the converging lens action of the section 30 is not made so strong as to produce a second crossover of the electron beam before it reaches the screen 8, since such a result would cause undesired enlargment of the spot size at the screw.

Since proper centering of the electron beam at aperture 18 and at the limiting aperture 38 at the forward end of the spiral electrode 31 is important to preserve efficient transmission of beam current through the electron optical system, to alleviate problems of exact coaxial alignment of the various parts of the system as well as to correct for the effect of the earths magnetic field, adjustable centering coils are provided external to the tube neck in the vicinity of the electron beam generating section 10, to provide centering at aperture 18. A similar set of coils 67 is provided in the vicinity of the spiral electrode 31 to provide centering at aperture 38. To avoid repetition, only coils 60 will be described in detail, it being understood that coils 67 are similar in all respects except for increased length, as is apparent from FIG- URE 1.

As shown in the sectional view of FIGURE 3 the centering coil assembly 60 outside section 10 consists of an insulating cylindrical support 61 on which are mounted two pairs of electromagnetic coils, the windings of the coils being designated AA, BB, CC, and DD far clarity. Coils AA and BB are wound in series and supplied from a remote source of reversible-polarity, adjustable amplitude direct current, which may be for example a battery 62 and a potentiometer 63, and are for centering the beam in the horizontal direction. Coils CC and DD are likewise wound in series and supplied from a remote source of reversible-polarity adjustable amplitude direct current, which as shown may be battery 62 and a potentiometer 64, and are for centering the beam in the vertical direction.

For the purpose of permitting precise Vernier control of the cross-sectional shape of the electron beam, in order to enhance spot roundness or if desired to produce spot ellipticity at the screen, an assembly of beam shaping coils is provided external to the tube neck in the vicinity of the main focus lens section 40. This beam shaping coil assembly '70 is shown in detail in the cross-sectional view of FIGURE 4. It consists of an insulating support cylinder 71 which is rotatably adjustable about the neck axis and on which are mounted four coils EE, FF, GG, and HH each arranged to subtend a angle in a plane transverse to the neck axis. The coils are wound in series in a sense such as to develop two mutually perpendicular pairs of magnetic poles wherin adjacent poles are of opposite polarity and diametrically opposed poles have the same polarity, as shown in FIGURE 4. The coils are connected to a remote source of reversible-polarity adjustable-amplitude direct current, which may be for example a potentiometer 64 and battery 62 as shown in FIG- URE 3.

Referring to FIGURE 4, the N and S negative poles there shown, which result from the current in coils EE, FF, GG, and HH, indicate how the mutually orthogonal forces of an electron beam passing in an axial direction through the field generated by coils EE, FF, GG and HH tend to correct ellipticity of the beam. A beam having an undesired degree of allipticity, for example with a major axis 95, can be conveniently rendered circular in cross-section merely by properly rotating the coil assembly 70 so that the beam compressing transverse magnetic fields thereof, as shown by poles N and S and vectors 72, coincide with the compress the major axis of the ellipse, and the beam expanding transverse magnetic forces thereof, as shown by vectors 73, coincide with and expand the minor axis of the ellipse, and adjusting the current through the coils of assembly 70 for the desired degree of roundness of the beam. Conversely, if for any reason a desired degree of ellipticity is required, such may be obtained even from a beam of perfectly circular cross-section by suitably angularly adjusting the coil assembly and properly adjusting the amount of current through the coils of assembly 70.

It will thus be appreciated from FIGURE 4 that the coil assembly 70 provides magnetic fields through the tube neck at the main focus lens section 40 which cannot provide any deflection of the beam in a manner such as would affect its centering but which do distort or change as desired the cross-sectional shape of the electron beam.

Alternative means for Vernier control of the crosssectional shape of the electron beam are shown in FIG- URES-5 and 6. The apparatus of FIGURES 5 and 6 has the advantage that it effects control of beam shape entirely electrically, eliminating any need for mechanical rotation of a coil assembly around the tube neck. This alternative beam shaping means includes an assembly of coils arranged external to the tube neck in the vicinity of the main focus lens section 40, and shown in detail in FIGURE 5. The coils of FIGURE 5 are arranged and energized to provide magnetic fields which produce pairs of force vectors which act along the major and minor ellipse axes of the electron beam, similar to the action of

vectors

72 and 73 of FIGURE 4, except that with the apparatus of FIGURES 5 and 6 the force vectors can be electrically rotated through 360 to any desired angle of orientation, and the amplitude of each pair of vectors can be electrically varied from a maximum in one sense through zero to a maximum in the opposite sense. Thus the apparatus of FIGURES 5 and 6 can correct for, or introduce, any degree of ellipticity of the beam at any angular orientation of the ellipse axes.

Turning to FIGURE 5, eight identical coils are provided in two sets of four-coils each, designated A1, A2, A3, A4, and B1, B2, B3, B4. The coils of the A set subtend adjacent angles of 90 at the neck axis, and the coils of the B set likewise subtend adjacent angles of 90 but are displaced 45 relative to the A coils. Each of the two current paths of each coil which extends parallel to the neck axis is centered in a sector subtending an angle of 22 /2" at the neck axis. Designating the current flow directions by plus and minus signs, the coil arrangement will be evident from FIGURE 5. All four A coils are wounds in series and are supplied by direct current of controllable amplitude and reversible polarity, and all four B coils are wound in series and supplied by direct current of controlla'ble amplitude and reversible polarity, separately from the A coils. The diametrically spaced coils of each set, e.g., coils A2 and A4, provide magnetic poles of one polarity, say the North poles designated NA in FIGURE 5,

while the remaining coils A1 and A3 of the same set provide poles of opposite polarity, shown as SA in FIGURE 5. Such poles NA and SA correspond in function and eflect to the N and S poles of FIGURE 4. Likewise the B coils provide poles SB and NB of FIG- URE 5. Thus the eight coils are capable of providing eight magnetic poles spaced 45 about the axis of the electron beam. The polarity of any diametrically spaced pair of poles is always identical, and the polarity of any spaced poles is always different, so that regardless of the eifect on ellipticity there is no net effect on the centering of the electron beam. Moreover, by the circuit of FIGURE 6 hereafter to be described, the strength and polarity of each such pole may be adjusted from maximum in one sense through zero to maximum in the opposite sense, and thus the summed values of the mutually perpendicular pairs of force vectors exerted on the electron beam by such poles may be made to rotate to any angle and have any desired amplitude.

The circuit of FIGURE 6 includes a pair of field

intensity control potentiometers

105, 113, which provide for control of the amplitude of the current fed to the A and B coils, respectively, and a pair of field orientation control potentiometers 103, 111. As will be explained in detail hereinafter, the potentiometers 103 and 111 serve to impose a sine-cosine relationship on the variations in current amplitude and polarity supplied the respective A and B coil sets, and this enables control of the angular orientation of the summed force vectors produced by the magnetic poles of FIGURE 5, throughout the entire 360.

As shown in FIGURE 6, one side of the A coil series is grounded, and the other side is connected through a current limiting resistor 101 and field orientation control potentiometer 103 to the field intensity control potentiometer 105 which with battery 107 provides a source of reversible polarity adjustable amplitude direct current. Likewise one side of the B coil series is grounded and the other side is connected through resistor 109 and orientation control potentiometer 111 to a source of reversible polarity adjustable amplitude direct current at intensity control potentiometer 113. The movable contacts 140, 141 of

potentiometers

105, 113 are ganged, and their stationary windings are connected to battery 107 with opposite polarity.

Potentiometer 103 has four equal segments separating five

terminals

115, 123, 124, 125, 126, and potentiometer 111 likewise has four equal segments separating five

terminals

116, 127, 128, 129, 130. The

movable contacts

135, 136 of potentiometers 103 and 111 are ganged.

The terminals 115', 124 and 126- of potentiometer 103 and the

terminals

128 and 130 of potentiometer 111 are grounded, while

terminals

123 and 129 are joined, and 127 are joined, and 116 and 126 are joined.

Intensity control potentiometer 113 determines the maximum current of one polarity available to the B coils from the terminal 129 of potentiometer 111 and available to the A coils from terminal 123 of potentiometer 103, while potentiometer 105 controls the maximum current of the opposite polarity available to the A coils from terminal 125 of potentiometer 103 and available to the B coils from terminal 127 of potentiometer 111.

As the

movable contacts

135, 136 of potentiometers 103 and 111 are moved the currents to the A and B coil sets available from

contacts

135, 136 vary from zero to a maximum in one sense, through zero again and to a maxim-um of the opposite sense with a 90 phase dilference and hence have a continuous relative approximately sinecosine relationship. These sine-cosine related currents produce vector summed magnetic fields which can be rotated to any angular orientation in FIGURE 5, and thus the setting of the movable contacts of potentiometers 103 and 111 enables the summed force vectors affecting the ellipticity of the electron beam to be rotatively oriented at any angle throughout the 360 range. Moreover the settings of

potentiometers

103, 113 control the maximum values of the currents available from

contacts

135, 136, and hence control the amplitude of the force vectors affecting the ellipticity of the electron beam. Thus merely by adjusting the pairs of

moveable contacts

140, 141, and 135, 136, any degree of ellipticity may be introduced into or removed from the electron beam, without affecting its centering.

Thus there has been show-n and described a high resolution electron optical system for a cathode ray tube or the like which permits the generation of extremely fine electron beam spot sizes of as little as a few microns diameter and within an overall length which is conveniently small. The mechanical structure of a cathode ray tube embodying such electron optical system is relatively simple and rugged, and associated means surrounding the axis of the system is provided for obtaining precise centerlng of the electron beam and thereby relaxing the tolerances of alignment of the parts. Additionally precise vernier control of the beam cross-sectional shape is provided so that substantial scanning angles of the order of 40 can be obtained with minimum defocusing.

A cathode ray tube constructed as above described has been found to provide very large screen current density of the order up to 2.0 amperes per square centimeter, with screen power loadings of up to approximately 50 kw. per square centimeter at 20 kw. screen potential and with a resolution of up to the equivalent of 10,000 raster lines on a tube face. Such resolution is the equivalent of up to 400 complete TV pictures arranged in a square or up to 100 million dots or 25 million hits of four increments per bit.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than those illustrative embodiments heretofore described. Accordingly it is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron optical system comprising, arranged in coaxial relation, an electron beam source, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, an electron beam prefocusing electron lens disposed between said electron beam source and said target for providing an electron optical image of said electron beam source for imaging on said target by said main focusing lens, and accelerating spiral electrode means providing an axial electron accelerating field between said prefocusing lens and said main lens, and means for varying the strength of said accelerating field to vary the effective object distance of said main lens to reduce spot size on said target.

2. An electron optical system comprising, arranged in coaxial relation, an electron beam source, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, cylindrical spiral accelerating electrode means providing an axial electron accelerating field between said main focusing lens and said electron beam source, a prefocusing electron lens between said electron beam source and said accelerating electrode means for providing an electron optical image of said electron beam source for imaging on said target by said main focusing lens, said accelerating electrode means having a planar transverse apertured wall portion at one end thereof next adjacent said prefocusing electron lens, and electron beam deflecting means between said main focusing lens and said target for scanning said beam across said target.

3. An electron optical system comprising, arranged in coaxial relation along a reference axis, an electron beam source including an emitter, electrode means forming an electrostatic field having equipotential surfaces which are hyperboloids asymptotic to a conical surface of approximately 109 apex angle coaxial with said reference axis and with said apex facing away from said emitter, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, spiral accelerating electrode means providing an axial electron accelerating field between said main focusing lens and said electron beam source, and a prefocusing electron lens between said source and said accelerating electrode means for providing an electron optical virtual image of said electron beam source for imaging on said target by said main focusing lens.

4. An electron optical system comprising, arranged in coaxial relation, an electron beam source, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, axially extending accelerating electrode means providing an axially extending uniform axial electron accelerating field between said main focusing lens and said electron beam source, and a prefocusing electron lens between said source and said accelerating electrode means for providing an electron optical image of said electron beam source for imaging on said target by said main focusing lens, said prefocusing electron lens forming an electrostatic field having equipotential surfaces which are hyperboloids asymptotic to a forwardly concave conical surface of approximately 109 apex angle coaxial with said axis with said apex facing said electron beam source.

5. In an electron optical system including a reference electron beam axis, coaxial prefocus lens means forming an electrostatic field having equipotential surfaces which are hyperboloids asymptotic to a forwardly concave conical surface of approximately 109 apex angle coaxial with said axis, coaxial cylindrical spiral electrode means extending axially from said prefocus lens from the larger opening of said conical surface and forming a uniform axial electron accelerating field, and an axially apertured transverse electrode electrically connected to said spiral electrode separating said prefocus lens and said spiral electrode, and means for adjusting the potential of said transverse electrode.

6. In an electron optical system including an electron beam source and a target spaced from said source along a reference axis, said electron beam source including an anode electrode and a gate electrode, said anode electrode having a forwardly projecting convex forward surface coaxial with said reference axis and said gate electrode having a rearwardly directed concavity coaxial with said axis and centrally apertured for passage of said electron beam, said gate electrode being adapted to have applied thereto modulation signals for varying the flow of electrons in said beam, variable refractivity prefocus lens means coaxially disposed between said target and said beam source, and including an electrode having a forwardly concave conical surface coaxial with said reference axis and with the apex thereof facing said electron beam source, means forming an axial opening in said conical surface through which the electron beam from said source is adapted to .pass, electrode means forming a coaxial main focus lens intermediate said .prefocus lens and said target, and axially extending spiral electrode means forming an axially extending uniform axial electron accelerating field between said prefocus lens and said main focus lens, said axially extending electrode means having a transverse wall electrode portion adjacent said prefocusing lens means.

7. In an electron optical system including an electron beam source and a target spaced from said source along a reference axis, said electron beam source including an anode electrode and a gate electrode, said anode electrode having a forwardly projecting convex surface coaxial with said reference axis and said gate electrode having a rearwardly directed concavity and coaxial with said axis and centrally apertured for passage of said electron beam, said gate electrode being adapted to have applied thereto modulations signals for varying the flow of elec- 9 trons in said beam, variable refractivity prefocus lens means coaxially disposed between said target and said beam source, electrode means forming a coaxial main focus lens intermediate said prefocus lens and said target, and axially extending electrode means defining an axially extending electron accelerating field between said prefocusing lens in said main lens for electrically varying the effective object distance of said main lens, said means including an electrode effective for extending the electronic length of said system between said prefocusing lens and said main lens.

8. In an electron optical system including an electron beam source and a target spaced from said source along a reference axis, prefocus lens means forming between said target and said beam source an electrostatic field having equipotential surfaces which are hyperboloids asymptotic to a forwardly concave conical surface of approximately 109 apex angle coaxial with said reference axis with the apex of said conical surface facing said electron beam source, means for directing said electron beam axially into said prefocus lens field, electrode means forming a coaxial main focus lens intermediate said prefocus lens and said target, and axially extending electrode means forming an axially extending electron accelerating field between said prefocus lens and said main focus lens, said axially extending electrode means being effective to electronically extend the distance in said optical system between said prefocusing and said main lens, the said axially extending electrode having a transverse apertured wall electrode portion on at least one end thereof.

9. In an electron optical system including an electron beam source and a target spaced from said source along a reference axis, said electron beam source including an anode electrode and a gate electrode, said anode electrode having a forwardly projecting convex surface coaxial with said reference axis, said gate electrode having a rearwardly directed concavity coaxial with said axis, said concavity being centrally apertured for passage of said electron beam, said gate electrode being adapted to have applied thereto modulation signals for varying the flow of electrons in said beam, prefocus lens means forming between said target and said beam source an electrostatic field having equipotential surfaces which are hyperboloids asymptotic to a forwardly concave conical surface of approximately 109 apex angle coaxial with said reference axis with the apex of said conical surface facing said electron beam source, means for directing said electron beam axially into said prefocus lens field, electrode means forming a coaxial main focus lens intermediate said prefocus lens and said target, and axially extending cylindrical electrode means forming a uniform axially extending electron accelerating field between said prefocus lens and said main focus lens, said axially extending cylindrical electrode means having a coaxially apertured transverse Wall portion at each end thereof.

10. In an electron optical system including an electron beam source and a target spaced from said source along a reference axis, said electron beam source including an anode electrode and a gate electrode, said anode electrode having a forwardly projecting convex surface coaxial with said reference axis, said gate electrode having a rearwardly directed concavity coaxial with said axis, said concavity being centrally apertured for passage of said electron beam, said gate electrode being adapted to have applied thereto modulation signals for varying the flow of electrons in said beam, prefocus lens means coaxi-ally disposed between said target and said beam source and including an electrode having a forwardly concave conical surface of approximately 109 apex angle coaxial with said axis and with the apex thereof facing said electron beam source, an axially apertured transverse electrode axially spaced forward of said conical surface, means forming an axial virtual cathode opening in said conical surface, means for directing said electron beam axially through said virtual cathode opening, electrode means forming a coaxial main focus lens intermediate said prefocus lens and said target, and a coaxial cylindrical spiral electrode extending between said prefocus lens and said main focus lens for forming a uniform axial accelerating field.

11. An electron optical system comprising, arranged in coaxial relation, an electron beam source, a target spaced from said source, a main electron beam focusing lens disposed between said source and said target, prefocusing electron lens means between said source and said m-ain lens for providing an electron optical virtual image of said electron beam source for imaging on said target by said main lens, electrical means for controlling the effective object distance of said main lens, electron beam ellipticity control means including electromagnetic coil means for forming a plurality of transverse magnetic fields in the path of said beam and having diametrically spaced pairs of magnetic poles quadrilaterally spaced in a plane transverse to said beam, the poles of each diametrically spaced pair being of the same polarity and the adjacent poles of each quadrilaterally spaced set being of opposite polarity, and means for varying the strength and polarity of said poles.

12. Apparatus as defined in claim 11 wherein said pole strength and polarity varying means includes a support for said coil means rotatable about said reference axis, and a source of variable magnitude reversible polarity direct current for said coil means.

13. Apparatus as defined in claim 11 wherein said magnetic coil means includes eight coils arranged to provide said quadrilaterally spaced poles in two sets of four each with the poles of one set being rotatively displaced 45 relative to the poles of the other set, and wherein field intensity control means are provided to control the magnitude of the current maximum current available to each coil, and wherein field orientation control means is provided to impose a substantially sinecosine relationship on the variation of pole strength and polarity between the coils of one set relative to the coils of the other set.

References Cited by the Examiner UNITED STATES PATENTS 2,318,423 5/1943 Samuel 31514 X 2,520,813 8/1950 Rubenberg 31515 2,630,544 3/1953 Tiley 315--3.6 2,914,675 11/1959 Van Dorsten. 2,986,668 5/1961 Haflinger et al. 31383 X 2,995,676 8/ 1961 Schlesinger 31515 3,040,205 6/ 1962 Walker 31383 X FOREIGN PATENTS 735,463 8/ 1955 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

JOHN W. HUCKERT, GEORGE N. WESTBY,

Examiners.

Claims

1. AN ELECTRON OPTICAL SYSTEM COMPRISING, ARRANGED IN COAXIAL RELATION, AN ELECTRON BEAM SOURCE, A TARGET SPACED FROM SAID SOURCE, A MAIN ELECTRON BEAM FOCUSING LENS DISPOSED BETWEEN SAID SOURCE AND SAID TARGET, AN ELECTRON BEAM PREFOCUSING ELECTRON LENS DISPOSED BETWEEN SAID ELECTRON BEAM SOURCE AND SAID TARGER FOR PROVIDING AN ELECTRON OPTICAL IMAGE OF SAID ELECTRON BEAM SOURCE FOR IMAGING ON SAID TARGET BY SAID MAIN FOCUSING LENS, AND ACCELERATING SPIRAL ELECTRODE MEANS FOR PROVIDING AN AXIAL ELECTRON ACCELERATING FIELD BETWEEN SAID PREFOCUSING LENS AND SAID MAIN LENS, AND MEANS FOR VARYING THE STRENGTH