US3904913A

US3904913A - Focusing means for cathode ray tubes

Info

Publication number: US3904913A
Application number: US391916A
Authority: US
Inventors: Gordon R Spencer
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1973-08-27
Filing date: 1973-08-27
Publication date: 1975-09-09
Anticipated expiration: 1992-09-09

Abstract

A cathode ray type tube wherein an electron gun is operatively disposed for directing an electron beam toward an axially spaced target surface, the gun being provided with means for producing a sharply focused beam having a focal spot suitable for scanning the target surface in a writing mode and alternatively producing a less sharply focused beam having a higher electron density and a larger focal spot suitable for scanning and uniformly charging the target surface.

Description

United States Patent Spencer [45] Sept. 9, 1975 [54] FOCUSING MEANS FOR CATHODE RAY 2,942,128 6/[960 Johnson .i 313/82 R TUBES 3,320,457 5/l967 Burdick ct al, 3 l 3/82 BF 3,354,335 ll/l967 Corpcw .l 313/86 KM X [75] Inventor: Gordon R. Spencer, Westwood,

Ma55- Primary Examiner-Robert Segal [73] Assignee: Raytheon Company, Lexingtonq Attorney, Agent, or Firm-.lohn T. Meaney; Harold A.

Massv Murphy; Joseph D. Pannone [22] Filed: Aug. 27, 1973 [57] ABSTRACT [2l] APPL N0- 391,916 A cathode ray type tube wherein an electron gun is operatively disposed for directing an electron beam [52] U.s. C1 313/449; 313/391 toward a axially spaced target Suffaehe gu being [5l] lm. CL2 HOU 29/02; HOU 31,58 provided with means for producing a sharply focused [58] Field of Search I 313/71, 68 D` 86 82 R1 beam having a focal spot suitable for scanning the tar- 33/82 BF get surface in a writing mode and alternatively producing a less sharply focused beam having a higher [56] References Cited electron density and a larger focal spot suitable for scanning and uniformly charging the target surface.

2 Claims, 5 Drawing Figures PATENTEU SEP 9 i975 .air 1 of 2 FOCUSING MEANS FOR CATHODE RAY TUBES BACKGROUND OF THE INVENTION This invention is related generally to electron tubes of the cathode ray type and is concerned more particularly with an electron gun having means for uniformly charging a raster area of a target surface.

An electron tube of cathode ray type generally comprises an evacuated envelope wherein an electron gun disposed adjacent one end of the envelope emits an electron beam which scans a target surface of an electrode disposed adjacent the other end of the envelope. Tubes of the described type may include orthicons, vidicons, storage tubes and the like which may be operated in different modes, such as priming, writing, reading and erasing, for examples.

The electron gun usually comprises an electron emitting cathode and a coaxially aligned series of spaced grid, anode, and focusing elements. Aberrations introduced by these elements generally are minimized by restricting the diameter of the beam such that it includes only an axial central portion of the beam. This may be achieved by an axially aligned anode having a transverse portion wherein an aperture is centrally disposed. Thus` a central axial portion of the beam passes through the aperture, and a surrounding annular portion of the beam impinges on the transverse portion of the beam limiting anode. Accordingly, a sharply focused beam is provided for scanning the target surface, but the electron current of the beam is reduced considerably.

The sharply focused scanning beam is advantageously used when high resolution is required, as in the writing and reading modes, for examples. However, when the target is being uniformly' charged to a desired potential, as in the priming and erasing modes, for example, the entire electron beam generally is preferred for scanning the target surface. Consequently, preceding the beam limiting anode member in the electron gun, there may be disposed an enhancement electrode which directs substantially all thc electrons in the beam through the aperture in the anode when desired. Due to the necessarily short focal distance involved, a large portion of these electrons follow diverging trajectories after passing through the aperture. Accordingly, the enhanced beam produces at the target surface an excessively large scanning spot, which may be as large as 2U percent of the target surface` for example. This increase in the scanning spot size generally is tolerated, because the associated increase in electron current provides means for charging the target surface in a shorter time interval than required when using the sharply focused beam.

However, when the excessively large scanning spot is used for charging the target surface, it generally is found that the effective raster area of the surface is Charged non-uniformly and a surrounding marginal area is charged unnecessarily. Thus, non-uniform priming of the target surface causes inaccuracies to occur in subsequent writing and reading modes. Also, nonuniform erasing of the target surface results in residual stored data being carried over into a subsequent frame of stored information. As a result, confusing and erroneous information is obtained from the tube during subsequent reading modes.

Therefore, it is essential that electron tubes of the cathode ray type be provided with means for controlling the size of the scanning spot when the entire electron beam is being used for uniformly charging a target surface.

SUMMARY OF THE INVENTION Accordingly, this invention provides a cathode ray type tube wherein an electron gun is operatively disposed for directing an electron beam toward an axially spaced target surface, the gun including means for producing a beam having a focal spot suitable for scanning the target surface in a writing mode and alternatively producing a beam having a higher electron current and a focal spot suitable for scanning and uniformly charging the target surface.

The electron gun comprises a beam generating means disposed adjacent one end of the gun for forming an electron beam directed toward the other end of the gun, and beam control means disposed adjacent the other end of the gun for varying the cross-sectional Size and focusing of the beam. The beam generating means includes a series of beam forming elements axially aligned with an electron emitting cathode, and the beam control means includes a plurality of beam size determining means, each associated with a respective focusing means. One of the beam size determining means may include a beam limiting electrode axially aligned with the series of beam forming elements, and the associated focusing means may include a subsequent axially aligned focusing electrode. The beam limiting electrode may comprise an apertured anode which permits the passage of only a central axial portion of the beam formed by the series of beam forming elements. The associated focusing electrode is maintained at an appropriate potential for focusing the electrons in the limited beam onto a focal spot which is suitable for scanning the target surface in a writing mode of operation. Another one ofthe beam size determining means may comprise an enhancement electrode disposed in preceding axial alignment with the apertured anode. The enhancement electrode may be pulsed to a potential suitable for directing substantially the entire electron beam formed by the series of elements through the apertured anode. The focusing means associated with the enhancement electrode may comprise a focusing electrode disposed in subsequent axial alignment with the focusing electrode associated with the apertured anode. The enhancement focusing electrode is pulsed to the same potential as the enhancement electrode and, consequently, may be electrically coupled directly to the enhancement electrode. When the enhancement focusing electrode is maintained at the potential of the enhancement electrode, it is found that the greater number of electrons in the enhanced beam passing through the apertured anode are focused onto a focal spot of suitable size for scanning and uniformly charging the target surface.

The preferred embodiment, shown herein, comprises a storage tube having an evacuated envelope wherein a mesh-like storage electrode is disposed between a collector electrode and an axially spaced electron gun. The storage electrode is provided with a dielectric target surface which is scanned by an electron beam emanating from the gun during operation of the tube. By applying suitable voltage to the storage electrode and regulating the electron beam accordingly, the storage tube may be operated in a well-known sequence of priming` writing` reading and erasing modes.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, the following, more detailed, description makes reference to the accompanying drawings wherein:

FIG. l is an axial view. partly in section, of a preferred embodiment of this invention-` FlG. 2 is a schematic axial view of a prior art type of electron gun',

FIG. 3 is a diagrammatic view of a target surface scanned by an enhanced electron beam emanating from the gun shown in FIG. 2;

FIG. 4 is a schematic axial view of the electron gun shown in FIG. l; and

FIG. 5 is a diagrammatic view of a target surface scanned by an enhanced electron beam emanating from the gun shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings wherein like characters of reference designate like parts` FIG. l shows a storage tube l0 comprising a tubular envelope l2 which preferably is made of dielectric material, such as glass, for cxample, and is provided with a neck portion 14. The neck portion I4 tapers outwardly at one end to merge with an end of a relatively large diameter portion 16 of envelope I2. Pcripherally sealed to the other end of portion 16 is a transversely disposed end plate 18 which closes one end of the envelope I2. The other end of neck portion I4 is peripherally sealed to a transversely disposed disk 20 which closes the other end of envelope l2. The end disk 20 is provided with an evacuation tubing 22 which may be made of glass, for example, and which is sealed-off after processing of the tube 10 is completed. Extending axially through the end disk 20, in a vacuum-tight manner, is a plurality of insulatingly spaced terminal pins 23. The pins 23 provide means for applying voltages to respective electrodes of an electron gun 24 which is axially disposed within the neck portion 14 of envelope l2.

The electron gun 24 includes an electron emitting cathode 26 which constitutes the generating end of the gun and is disposed adjacent the end disk 20. The cathode 26 may be directly heated, or may be indirectly heated, as by means of filament 28, for example. Cathode 26 is axially disposed in spaced relationship within a first grid cup 30 which has an aperture 32 centrally disposed in its closed end. Axially spaced from the closed end of grid cup 30 is a similar closed end of a second grid cup 34 having a centrally disposed aperture 36 which is aligned with the aperture 32. The first and second grid cups, 30 and 34, respectively, regulate the flow of electrons from the cathode 26 and aid in forming the emitted electrons into a beam. The open end of grid cup 34 is disposed in spaced relationship with one end of a coaxially disposed, hollow cylinder 38 which serves as the first anode of gun 24. The first anode cylinder 38 is maintained at a suitable potential for accelerating the electrons emerging from second grid cup 34 and further forming them into a beam.

The opposing end of anode cylinder 38 is disposed in spaced relationship with one end of a coaxial hollow cylinder 40 which functions as an enhancement electrode when desired. Axially spaced from the opposing end of the enhancement electrode is an open end of a coaxially disposed sleeve 42 having therein a transverse wall 44 which is provided with a central aperture 46. The sleeve 42 functions as a beam limiting anode which permits only an axial central portion of the electron beam to pass through the aperture 46 when a sharply focused beam is required. As a result, a surrounding annular portion of the beam impinges on the transverse wall 44 and is absorbed by the beam limiting anode member 42. In this mode of operation, the enhancement electrode 40 may be maintained at a suitable potential for functioning as an electron accelerating anode. However, when use of the entire electron beam is desired, the enhancement electrode 40 is pulsed to a suitable potential for directing substantially all the electrons in the beam through the aperture 46 of the beam limiting anode 42. Due to the necessarily close spacing of the enhancement electrode 40 to the aperture 46, a large portion of the electrons in the enhanced beam follow mutually' diverging trajectories after passing through the aperture.

The opposing end portion of beam limiting anode 42 may taper inwardly to form a reduced diameter end, and is disposed in spaced relationship with one end of a coaxial hollow cylinder 48. The cylinder 48 comprises a focusing electrode which is maintained at a suitable potential for sharply focusing the central axial portion of the electron beam defined by the aperture 46. As such, the focusing electrode 48 is inadequate for focusing the diverging electrons in the enhanced beam. Preferably, focusing of the enhanced beam is achieved in a manner which does not require radical structural changes or redesign of external circuitry connected to the electrodes of the gun 24.

The other end of focusing electrode 48 is disposed in spaced relationship with a preferably inward tapering end portion ofa coaxially disposed, hollow cylinder 50. The cylinder 50 constitutes a second anode cylinder of the gun 24 and has an opposing end disposed in spaced relationship with a coaxial hollow cylinder S2. The cylinder 52 comprises an enhancement focusing electrode which functions as an anode when only the axial central portion of the beam is passing through the aperture 46. However, when substantially the entire electron beam is directed through the aperture 46, the enhancement focusing electrode 52 is pulsed to the same potential as the enhancement 40. Axially spaced from the opposing end of the enhancement focusing electrode 52 is an end of a hollow coaxial cylinder 54 which constitutes a third anode cylinder of the gun 24. By positioning the enhancement focusing electrode 52 between the two anode cylinders, 50 and 54, respectively, the electrode 52 appears electrostatically` when functioning as an anode, to be an extension of the anode cylinder S0 and 54. Thus, the enhancement focusing electrode 52 is readily introduced into the structure of gun 24 and does not require a redesign of external circuitry. Since the enhancement focusing electrode 52 is pulsed to the same potential as the enhancement electrode 40, it may conveniently be connected electrically thereto, by conventional means, within the envelope l2. The enhancement focusing electrode S2 is suitably positioned with respect to aperture 46 in beam limiting anode 42 to produce the desired focusing effect on the electrons in the enhanced beam.

The described electrodes of the electron guri 24 may be supported in insulating spaced relationship with one another by suitable attachment, to a plurality of spaced side rods, such as 56 and 58 for examples. The

side rods

56 and 58, respectively, extend axially within the neck portion 14 of envelope 12 and are preferably made of dielectric material, such as glass, for example. Disposed in spaced coaxial relationship with the third anode 54 is a hollow cylinder 60 which constitutes the fourth anode cylinder of gun 24. Cylinder 60 may comprise a continuous band of conductive material, such as graphite, for example, which is adherringly deposited by conventional means on the inner surface of the neck portion 14. The third anode 54 is electrically coupled to the fourth anode 60 by suitable means, such as radial flange 62 extending outwardly from the third anode cylinder S4 and supporting a plurality of resilient tabs 64 in electrical contact with an adjacent end portion of cylinder 60, for example. The opposing end portion of fourth anode cylinder 60 may extend onto the outwardly tapering wall of neck portion 14 and constitutes the exit end of electron gun 24.

The gun 24 is eoaxially aligned with the axis of tube and projects a beam of electrons, as described, into the large diameter portion 16 of envelope 12. The electron beam may be deflected in a predetermined manner by suitable means, such as external deflection yoke 66, for example, which encircles the fourth anode cylinder 60 within neck portion 14. Within the large diameter portion 16 of envelope 12, the electron beam passes through a pair of axially spaced, hollow cylinders which constitute first and second collimating electrodes, 68 and 70, respectively. Each of the

electrodes

68 and 70, respectively, may comprise a continuous band of conductive material, such as graphite, for example, which is adherringly deposited by conventional means 0n the inner surface of large diameter portion 16. The

collimating electrodes

68 and 70 are electrically connected to respective terminal buttons 69 and 7l which extend in a vacuum-tight manner through the wall of envelope l2. The terminal buttons 69 and 71 provide means for maintaining the

collimating electrodes

68 and 70, respectively, at suitable potentials for aligning the electron beam with the axial centerline of tube l0.

Axially spaced from the second collimating electrode 70 is a transversely disposed decelerator electrode 72 which generally comprises a fine mesh screen stretched across the opening of a conductive support ring. The decelerator electrode 72 is disposed in coaxial alignment with the axial centerline of tube l0, and is insulatingly secured to a parallel storage electrode 74. The storage electrode 74 also generally comprises a fine mesh screen stretched across the opening Of a conductive support ring, but it is provided with a target surface 76 by coating one surface with a film of dielectric material, such as glass, for example. The decelerator electrode 72 and the storage electrode 74 are electrically connected to respective

terminal buttons

73 and 75 which extend through the wall of envelope l2 in a vacuum-tight manner. The storage electrode 74 is insulatingly coupled to a parallel collector electrode 78 which may comprise a thin conductive plate made of titanium, for example. The collector electrode 78 is electrically connected to a supporting terminal member 79 which may extend axially through the end plate 18 in a vacuum-tight manner.

ln operation, the deflection yoke 66 produces a varying magnetic field which causes the electron beam emanating from gun 24 to scan a desired raster area of the dielectric target surface 76, in a well-known manner. The decelerator electrode 72 generally is maintained at a lower positive potential than the exit end of gun 24 for the purpose of establishing a retardation field which decelerates the beamed electrons as they approach the target surface 76. As a result, the first and second collimating electrodes, 68 and 70, respectively, are enabled to direct the electrons electrostatically into a perpendicular approach to the target surface 76, while the beam is scanning the raster area of the target surface. ln this manner, the storage tube l0 may be operated in a well-known sequence of priming, writing, reading and erasing modes.

Briefly, in the priming mode, the scanning electron beam deposits electrons on the raster area of the dielectric target surface 76 to charge it uniformly to the potential of the cathode 26 in gun 24. Then, for the writing mode, the potential of target surface 76 ge nerally is increased to a considerably higher positive value, such that the scanning electron beam causes localized emission of secondary electrons from discrete elements of the raster area. However, in the writing mode, the scanning beam is modulated by an information signal which usually is applied to the control grid 30 of gun 24. Consequently, the signal is stored on the raster area of target surface 76 in a varying pattern of positively charged elements which correspond to modulations introduced in the scanning beam by the information signal, Subsequently, for the reading mode, the potential of target surface 76 generally is decreased to a value slightly below cathode potential whereby the elements least charged in the writing mode may repel electrons from the scanning beam. Thus, in the reading mode varying amounts of electrons from the scanning beam are allowed to pass through respective apertures in the mesh-like storage electrode 74 depending on the charged elements adjacent the apertures. Accordingly, the collector electrode 80 receives a sequentially varying electron current which constitutes a nondestructive reading of the signal stored on the raster area of target surface 76. This signal may be erased from the raster area by increasing the potential of target surface 76 to a relatively high positive value with respect to the cathode 26. As a result, the scanning beam produces a saturation emission of secondary electrons from successive discrete elements of the raster area thereby charging it uniformly to a higher positive potential with respect to cathode 26.

Thus, it may be seen that a sharply focused electron beam is desirable for the writing and reading modes where resolution is required in the storing and reading of information data. However, in the priming and erasing modes where fast uniform charging of the target surface is desired, it is more advantageous to utilize the full beam current. Consequently, as shown in FIG. 2, a prior art electron gun for a similar tube of the cathode ray type may comprise a eoaxially aligned series of spaced electrodes including a cathode 26a, respective first and second grid electrodes 30a and 34a, first anode cylinder 38a, enhancement electrode 40a, beam limiting anode 42a having a transverse portion 44a wherein an aperture 46a is centrally disposed, focusing electrode 48a, and second anode cylinder 50a. The anodes 38a, 42a and 50a may conveniently be connected electrically to one another internally of the tube envelope, since they are generally maintained at the same electrical potential.

In order to minimize aberrations, the electron beam produced by the described prior art gun is reduced diametrically to include only an axial central portion of the beam. Consequently, the desired axial central portion of the beam is permitted to pass through the aperture 46a in beam limiting anode 42a and the surrounding annular portion of the beam impinges on the transverse wall portion 44a. ln this manner, a sharply focused electron beam is obtained for the reading and writing modes, but the beam current is reduced considerably. When full beam current is required for the priming and erasing modes, a suitable potential is applied to the enhancement electrode 40u for focusing substantially the entire electron beam through the aperture 46a in beam limiting anode 42a. However, a large portion of the electrons passing through the aperture 46a follow mutually diverging paths which are not adequately corrected by the focusing electrode 48a. Consequently, as shown in FIG. 3, the enhanced beam produces at the target surface 76a an excessively large scanning spot 80a. As a result, only a central portion 84a of a desired raster area 82a is charged uniformly. A surrounding area 86a ofthe raster area 82u is insufficiently charged, and an outer marginal area 88a of the target surface 76u is charged unnecessarily.

As shown in FIG. 4, the electron gun 24 of this invention is provided with an enhancement focusing electrode 52 which is connected electrically to the cnhancement electrode 40 internally of the envelope l2. Consequently, additional external connection or associated changes in external circuitry are not required. Also, when a suitable pulse voltage is applied to the enhancement electrode 40 for focusing substantially the entire electron beam through aperture 46 the same pulse voltage is applied simultaneously to enhancement focusing electrode 52. As a result, the enhancement focusing-electrode 52 is established at an optimum potential for refocusing the enhanced beam onto a desired focal spot 80, as shown in FIG. 5. The focal spot 80 is relatively smaller than the excessively large scanning spot 80u and` therefore. is more suitable for scanning and uniformly charging a well-defined raster area 82 of target surface 76, as required for the priming and erasing modes, for examples. However, the focal spot 80, preferably, is slightly larger than the scanning spot utilized for the writing mode in order to ensure the erasure of line structure produced during the previous writing modes.

From the foregoing, it will be apparent that all of the objectives of this invention have been achieved by the structure shown and described. lt also will be apparent, however, that various changes may be made by those skilled in the art without departing from the spirit of the invention as expressed in the appended claims. lt is to be understood, therefore, that all matter shown and described is to be interpreted as illustrative and not in a limiting sense.

l claim:

l. An electron gun comprising:

an electron emitting cathode disposed adjacent one end of the gun for directing a beam of electrons toward an opposing output end of the gun;

a beam limiting anode cylinder operatively aligned with the cathode and having therein a transverse wall provided with a central aperture for permitting a limited electron beam therethrough;

a first focusing sleeve electrode operatively disposed in alignment between the cathode and the beam limiting anode for intermittently focusing an enhanced electron beam through the aperture;

a second focusing sleeve electrode operatively disposed between the beam limiting anode and the output end of the gun, and positioned a predetermined axial distance from said wall member for focusing said limited electron beam;

a third focusing sleeve electrode operatively disposed between the second focusing sleeve electrode and the output end of the gun, and positioned a predetermined axial distance from said wall member for focusing said enhanced electron beam, the third focusing electrode being electrically coupled to the first focusing electrode by an interconnecting conductive member;

an intermediate anode cylinder operatively disposed in alignment between the second and third focusing electrode', and

an output anode cylinder operatively aligned with the third focusing electrode and disposed adjacent the output end of the gun.

2. An electron tube of the cathode ray type comprising:

an evacuated envelope;

an electrode having a target surface transversely disposed within the envelope; and

an electron gun operatively disposed within the envelope for directing toward the target surface a sharply focused electron beam and, alternatively, a less sharply focused, higher density electron beam, each beam having a focal spot suitable for scanning thc target surface, the gun including;

an electron emitting cathode disposed adjacent one end of the gun for directing a beam of electrons toward on opposing output end of the gun;

a beam limiting anode cylinder operatively aligned with the cathode and having therein a transverse wall member provided with a central aperture for permitting a limited electron beam therethrough;

a first focusing sleeve electrode operatively disposed in alignment between the cathode and the beam limiting anode, and spaced a sufficient axial distance from the wall member for intermittently focusing a higher density electron beam through said aperture; second focusing sleeve electrode operatively disposed between the beam limiting anode and the output end of the gun, and spaced a sufficient axial distance from the wall member for sharply focusing the limited electron beam onto said target surface; a third focusing sleeve electrode operatively disposed between the second focusing electrode and the output end of the gun, and spaced a sufficient axial distance from the wall member for focusing the higher density electron beam less sharply onto said target surface, the third focusing electrode being electrically coupled directly to the first focusing electrode by an interconnecting conductive member within the envelope; an intermediate anode cylinder operatively disposed in alignment between the second and third focusing electrodes; and an output anode cylinder operatively aligned with the third focusing electrode and disposed adjacent the output end of the gun.

Claims

1. An electron gun comprising: an electron emitting cathode disposed adjacent one end of the gun for directing a beam of electrons toward an opposing output end of the gun; a beam limiting anode cylinder operatively aligned with the cathode and having therein a transverse wall provided with a central aperture for permitting a limited electron beam therethrough; a first focusing sleeve electrode operatively disposed in alignment between the cathode and the beam limiting anode for intermittently focusing an enhanced electron beam through the aperture; a second focusing sleeve electrode operatively disposed between the beam limiting anode and the output end of the gun, and positioned a predetermined axial distance from said wall member for focusing said limited electron beam; a third focusing sleeve electrode operatively disposed between the second focusing sleeve electrode and the output end of the gun, and positioned a predetermined axial distance from said wall member for focusing said enhanced electron beam, the third focusing electrode being electrically coupled to the first focusing electrode by an interconnecting conductive member; an intermediate anode cylinder operatively disposed in alignment between the second and third focusing electrode; and an output anode cylinder operatively aligned with the third focusing electrode and disposed adjacent the output end of the gun.

2. An electron tube of the cathode ray type comprising: an evacuated envelope; an electrode having a target surface transversely disposed within the envelope; and an electron gun operatively disposed within the envelope for directing toward the target surface a sharply focused electron beam and, alternatively, a less sharply focused, higher density electron beam, each beam having a focal spot suitable for scanning the target surface, the gun including: an electron emitting cathode disposed adjacent one end of the gun for directing a beam of electrons toward on opposing output end of the gun; a beam limiting anode cylinder operatively aligned with the cathode and having therein a transverse wall member provided with a central aperture for permitting a limited electron beam therethrough; a first focusing sleeve electrode operatively disposed in alignment between the cathode and the beam limiting anode, and spaced a sufficient axial distance from the wall member for intermittently focusing a higher density electron beam through said aperture; a second focusing sleeve electrode operatively disposed between the beam limiting anode and the output end of the gun, and spaced a sufficient axial distance from the wall member for sharply focusing the limited electron beam onto said target surface; a third focusing sleeve electrode operatively disposed between the second focusing electrode and the output end of the gun, and spaced a sufficient axial distance from the wall member for focusing the higher density electron beam less sharply onto said target surface, the third focusing electrode being electrically coupled directly to the first focusing electrode by an interconnecting conductive member within the envelope; an intermediate anode cylinder operatively disposed in alignment between the second and third focusing electrOdes; and an output anode cylinder operatively aligned with the third focusing electrode and disposed adjacent the output end of the gun.