WO2000028566A1

WO2000028566A1 - Direct heating cathode unit and electron gun using the same

Info

Publication number: WO2000028566A1
Application number: PCT/KR1999/000601
Authority: WO
Inventors: Jong Seo Choi; Yoon Chang Kim; Nikolay Osaulenko; Vladislav Shutovsky
Original assignee: Samsung Sdi Co., Ltd.; Nikos-Eco, Ltd.
Priority date: 1998-11-09
Filing date: 1999-10-04
Publication date: 2000-05-18
Also published as: UA28130C2; KR100310686B1; CN1333916A; KR20000034945A; EP1129464A1; JP2002529899A

Abstract

A direct heating cathode unit (100) including a metal alloy emitter (110) having at least two parallel lateral surfaces (113, 115) extending crosswise with respect to a longitudinal geometrical axis penetrating a base, two heater wires (120, 150) having central parts (121, 131) welded on both lateral surfaces (113, 115) of the emitter (110), the lateral surfaces (113, 115) being parallel to and facing each other, a cathode holder (150) having current supply holder pins (143, 144), and heater supports (141, 142) to which both ends of the heater wires (120, 130) in a pair are connected to the current supply holder pins (143, 144), and connected to the current supply holder pins (143, 144) such that the vertical axis of the cathode holder (150) coincides with the vertical axis of the emitter (110). An electron gun using the direct heating cathode unit (100) includes a control unit, a screen electrode and a plurality of focusing electrodes, sequentially disposed from the cathode unit (100).

Description

DIRECT HEATING CATHODE UNIT AND ELECTRON GUN USING THE SAME

Technical Field The present invention relates to an electron gun for an electronic device, and more particularly, to a direct heating cathode unit used as electron emission sources for various electronic devices and an electron gun using the cathode unit.

Background Art An electron gun is equipped in electro-vacuum devices including color picture tubes, oscilloscopes and radiolocation devices, and other tubes in which high-density electron beams are used in which images are formed using the electron beams emitted from the electron gun.

The electron gun for use in such an electro- vacuum device includes a cathode unit used as an electron emission source, and a plurality of focusing electrodes which constitute an electronic lens for focusing and accelerating electron beams emitted from the cathode unit.

In the electron gun, the cathode unit which is an electron emission source is one of important elements affecting screen brightness, power requirement, readiness time, and life of electro-vacuum devices.

Cathode units are classified into a direct heating type and an indirect heating type. Currently, direct heating oxide cathodes employing a method of directly heating an electron emitting material are widely used as electron emission sources.

However, the emission abilities of the direct heating oxide cathodes are limited and it is not possible to consistently attain rich current density of more than 1 A/cm².

Thus, the direct heating oxide cathodes are not suitable for modern electro-vacuum devices which must have high brightness.

In indirect heating oxide cathodes, heat transmission between a heater and an emitter is delayed, which is due to the absence of direct thermal contact between the heater and the emitter. The delayed heat transmission from the heater to the emitter results in additional power consumption, which is caused by preliminary heating of the emitter.

An example of a cathode unit of a direct heating type for an electro-vacuum device is disclosed in U.S. Patent No. 4,137,747.

A direct heating cathode unit 10, as shown in FIG. 1 , includes current supply holders 13 and 14, and two filaments (heating wires) 15 and 16 whose first ends are fixed to the holders 13 and 14 and whose second ends are fixed on lateral surfaces of the emitter 11.

In the cathode unit 10 having the aforementioned construction, since the ends of the filaments 15 and 16 are welded on the lateral surfaces of the emitter 11, the strength of supporting the emitter 11 using the filaments 15 and 16 is unstable. In particular, asymmetry of the cathode unit 10 is observed in a direction perpendicular to the linear axis of the same. Also, although the strength of supporting the cathode unit 10 is high due to the two holders 13 and 14, support for the cathode unit 10 in a perpendicular direction is weak. Thus, even under a small mechanical shock, vibration of the cathode unit 10 or displacement of the emitter 11 supported on the filaments 15 and 16 may occur.

Another example of a direct heating cathode unit is disclosed in Russian Federation Patent No. 2,052,856.

As shown in FIG. 2, a direct heating cathode unit 20 includes two filaments 22 and 23 connected to the body of an emitter 21 by their central parts , cross arms (current feeders) 24 and 25 to which the ends of the filaments 22 and 23 are connected in pairs, and current supply supports 26 and 27 to which the cross arms 24 and 25 are fixed and supported on the insulator of a cathode holder. Here, the resistance of the cross arms 24 and 25 are smaller than that of the filaments 22 and 23. The lengths of four branches of the filaments 22 and 23 positioned between the emitter 21 and the cross arms 24 and 25 are equal to each other. Grooves 21a into which the filaments 22 and 23 are mounted are formed in the lower portion of the emitter 21.

In the cathode unit 20 having the aforementioned construction, the grooves 21a into which filaments 22 and 23 are mounted are very small and the distance therebetween is very small. Therefore, it is difficult to manufacture the cathode unit 20 and much time is required for manufacture thereof.

An example of a cathode and heater assembly for electron-beam devices is disclosed in Canadian Patent Publication No. 2,034,919.

As shown in FIG. 3, a cathode unit assembly includes a thermionic emitter 31 having a longitudinal geometrical axis; at least two filaments 32 and 33 with each of the filaments 32 and 33 having a middle part, whereon the thermionic emitter 31 is mounted, a first peripheral section having an end, and a second peripheral section of a length equal to that of the first peripheral section and having an end, wherein the first and second peripheral sections are positioned at an equal acute angle to the longitudinal geometrical axis of the thermionic emitter 31, and their ends are positioned at the apexes of a polygon with two mutually orthogonal axes of symmetry, the intersection point whereof is positioned on the geometrical axis of the thermionic emitter 31; first current-conducting leads 34 and 35 each having an axis and connected to the ends of the first peripheral sections of the filaments 32 and 33; second current-conducting leads 36 and 37 each having an axis and connected to the ends of the second peripheral sections of the filaments 32 and 33, wherein one of the axes of symmetry of the polygon passes through the axes of the first and second current-conducting leads 34, 35, 36 and 37; and a base 38 rigidly mounting the first and second current-conducting leads 34, 35, 36 and 37. In the cathode and heater assembly having the aforementioned construction, since the ends of the filaments 32 and 33 are positioned at the apexes of the longitudinal geometrical axis of the thermionic emitter 31, the amount of thermal expansion of the filaments 32 and 33 cannot be absorbed. Thus, the emitter 31 may move in the direction of the longitudinal geometrical axis due to the thermal expansion of the filaments 32 and 33. Also, the construction of the cathode and heater assembly is complicated. Further, due to interface reaction between the emitter 31 and the filaments 32 and 33, the emitter 31 and the filaments 32 and 33 may be separated or disconnected from each other. Also, the efficiency of heat transmission between the emitter 31 and the filaments 32 and 33 is low. Disclosure of the Invention

To solve the above problems, it is an objective of the present invention to provide a direct heating cathode unit which can be applied to electro-vacuum devices in which brightness, durability and reliability are required, and which can reduce the man-hour and manufacturing cost in manufacturing an emitter body and connecting heater wires (filaments) and an emitter.

It is another objective of the present invention to provide a direct heating cathode unit which can improve the capability of consistently emitting electrons.

It is still another objective of the present invention to provide an electron gun using the direct heating cathode unit.

Accordingly, to achieve the first objective, there is provided a direct heating cathode unit including a metal alloy emitter having at least two parallel lateral surfaces extending crosswise with respect to a longitudinal geometrical axis penetrating a base, two heater wires having central parts welded on both lateral surfaces of the emitter, the lateral surfaces being parallel to and facing each other, a cathode holder having current supply holder pins, and heater supports to which both ends of the heater wires in a pair are connected to the current supply holder pins, and connected to the current supply holder pins such that the vertical axis of the cathode holder coincides with the vertical axis of the emitter. In the present invention, the emitter is rectangular and the thickness thereof may vary within a range of 1/5 to 1/2 the size of the base. Also, the lengths of the projections of the heat wires are preferably 2 to 5 times the diameters of the heater wires. The heater supports are preferably formed of high-resistance iron-nickel alloy. According to another aspect of the present invention, there is provided a direct heating cathode unit including an emitter having at least two parallel lateral surfaces extending crosswise with respect to a longitudinal geometrical axis penetrating a base, and made of an electron-emitting metal alloy including 0.5 to 9.0% by weight of a rare-earth metal of the cerium group, 0.5 to 15.0% by weight of tungsten and/or rhenium, 0.5 to 10% by weight of hafnium and the balance of iridium, two heater wires having central parts welded on both lateral surfaces of the emitter, the lateral surfaces being parallel to and facing each other, a holder body having current supply holder pins, and heater supports to which both ends of the heater wires in a pair are connected to the current supply holder pins, and connected to the current supply holder pins such that the vertical axis of the cathode holder coincides with the vertical axis of the emitter.

Alternatively, there is provided an electron gun including a cathode unit having a metal alloy emitter having at least two parallel lateral surfaces extending crosswise with respect to a longitudinal geometrical axis penetrating a base, two heater wires having central parts welded on both lateral surfaces of the emitter, the lateral surfaces being parallel to and facing each other, a cathode holder having current supply holder pins, and heater supports to which both ends of the heater wires in a pair are connected to the current supply holder pins, and connected to the current supply holder pins such that the vertical axis of the cathode holder coincides with the vertical axis of the emitter, a control unit and a screen electrode sequentially disposed from the cathode unit, constituting a triode together with the cathode unit, and a plurality of focusing electrodes installed in the vicinity of the screen electrode, constituting a main lens and an auxiliary lens.

In the present invention, the electron-emitting metal alloy includes 0.5 to 9.0% by weight of a rare-earth metal of the cerium group, 0.5 to 15.0% by weight of tungsten and/or rhenium, 0.5 to 10% by weight of hafnium and the balance of iridium.

Brief Description of the Drawings

The above objectives and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:

FIG. 1 is a side view illustrating a conventional direct heating cathode unit; FIG. 2 is a perspective view illustrating another conventional direct heating cathode unit; FIG. 3 is a perspective view illustrating still another conventional direct heating cathode unit; FIG. 4 is a side view illustrating an electron gun employing a direct heating cathode unit according to the present invention;

FIG. 5 is an exploded perspective view illustrating the direct heating cathode unit shown in FIG. 4; FIG. 6 is a side view illustrating the direct heating cathode unit according to the present invention;

FIG. 7 is an exploded perspective view illustrating heater wires mounted into an emitter; and

FIGS 8 A through 8C illustrate the resistance contact welding of the emitter and heater wires.

Best mode for carrying out the Invention

FIG. 4 illustrates an example of an electron gun employing a direct heating cathode unit according to the present invention. As shown in FIG. 4, the electron gun includes a direct heating cathode unit

100 for emitting electron beams, a control electrode 200 and a screen electrode 300, sequentially installed from the direct heating cathode unit 100, the direct heating cathode unit 100, the control electrode 200 and the screen electrode 300 constituting a triode, a plurality of focusing electrodes 400 and a final focusing electrode 500, for focusing and accelerating the electron beams. Electron beam passing holes for forming an electronic lens are formed in the respective electrodes, and the shapes thereof may be changed in various forms according to types of electronic lenses to be formed.

FIGS. 5 and 6 illustrate an example of a direct heating cathode unit mounted in the electron gun for an electro-vacuum device to be used as an electron emission source.

As shown in the drawings, a direct heating cathode unit 100 includes an emitter 110 having a predetermined shape, two heater wires 120 and 130 having central parts 121 and 131 welded to both lateral surfaces 113 and 115 of the emitter 110, the lateral surfaces being parallel to and facing each other, and branches 122, 123, 132 and 133, and a pair of heater supports 141 and 142 to which ends of the branches 122, 123, 132 and 133 of the heater wires 120 and 130 are welded. The heater supports 141 and 142 are supported by current supply holder pins 143 and 144 perpendicularly supported on an insulator 151 of a cathode holder 150. The insulator 151 is supported by a cylindrical sleeve 152. The shapes of the insulator 151 and the cylindrical sleeve 152 can be changed and adjusted in various forms when they are mounted on the cathode unit for an electro-vacuum device or the electron gun for a cathode ray tube.

The heater supports 141 and 142 to which the ends of the heater wires 120 and 130 are welded and fixed to the current supply holder pins 143 and 144 through their central parts so that the heater supports 141 and 142 and the current supply holder pins 143 and 144 are disposed crosswise. The heater supports 141 and 142 are preferably formed of a material which allows a minimal amount of heat to be transmitted from the heater wires 120 and 130 to the heater supports 141 and 142. Suitable materials include high-resistance iron-nickel alloy having low electrical conductivity and thermal conductivity. By choosing the heater supports having the above-described shape and material, the exothermic efficiency can be improved, despite the use of heater wires with short lengths.

FIG. 7 illustrates the welding and supporting state of the parallel lateral surfaces of the emitter of the aforementioned direct heating cathode unit and the central parts of the heater wires.

As shown in FIG. 7, the central parts 121 and 131 of the heater wires 120 and 130 are welded to the emitter 110 along the entire length of each of the parallel lateral surfaces 113 and 115, and projections 124, 125, 134 and 135 projecting in either side by a predetermined length (L) are provided. The branches 122, 123, 132 and 133 whose ends are supported to the heater supports 141 and 142 from the projections 124, 125, 134 and 135. Here, the length of each of the projections 124, 125, 134 and 135 is preferably 2 to 5 times the diameter of each of the heater wires 120 and 130. This limit in the length is obtained in consideration of the depth in which constituent atoms of the electron-emissive metal alloy can easily diffusively penetrate into the heater wires 120 and 130 during high-temperature operation of the cathode unit. The limit in the projection length can be chosen by the preservation requirement of a high level in view of a design dimension stability. The shape in which the corresponding parallel lateral surfaces of the emitter 110 are welded on the central parts of the heater wires 120 and 130 and the ends of the branches of the heater wires 120 and 130 are fixed on the heater supports 141 and 142, is a truncated tetrahedral pyramid.

The central parts of the heater wires 120 and 130 are connected with the emitter 110 by resistance contact welding. In other words, as shown in FIGS. 8A through 8C, a heater wire 120 or 130 is disposed between the lateral surfaces of the emitter 110 supported on a die 601 and a welding electrode 600. In this state, current is supplied to the emitter 110 and an electrode rod 600 so that the heater wire 120 or 130 generates heat. Also, the heater wires 120 and 130 are pressed toward the lateral surfaces of the emitter 110 using the welding electrode 600 so as to be embedded into the lateral surfaces of the emitter 110, as shown FIG. 7. Here, the central parts of the heater wires 120 and 130 welded on the lateral surfaces of the emitter 100 must be linearly positioned with respect to the central axes of the lateral surfaces of the emitter 110.

The emitter 110 supported by the heater wires 120 and 130 is of a tetrahedral shape, having four lateral surfaces 113, 114, 115 and 116 extending crosswise with respect to a longitudinal geometrical axis C which penetrates the central portion of a base 112 of the emitter 110 and the central portion of a cathode holder 150. At least two facing surfaces 113 and 115 of the four lateral surfaces 113, 114, 115 and 116, that is, the surfaces on which the central parts 121 and 131 of the heater wires 120 and 130 are welded, are parallel to each other. The thickness of the emitter 110 preferably varies within a range of 1/5 to 1/2 the size of the base in view of the narrow lateral surfaces of the emitter 110 welded on the heater wires 120 and 130 made of tungsten and the technological processing requirements in manufacturing the emitter 110.

The emitter 110 consists of an electron-emitting metal alloy including 0.5 to 9.0% by weight of a rare-earth metal of the cerium group, 0.5 to 15.0% by weight of tungsten and/or rhenium, 0.5 to 10% by weight of hafnium and the balance of iridium. If the content of the rare-earth metal of the cerium group is less than 0.5% by weight, the life of the cathode is shortened due to a deficiency in the rare-earth metal of the cerium group which is an active component. On the other hand, if the content of the rare-earth metal of the cerium group is greater than 9.0% by weight, a compound such as Ir₂Ce or Ir₂La having a low emissive property may be formed on the surface of the cathode. Here, the rare-earth metal of the cerium group is preferably at least one selected from the group consisting of lanthanum, cerium, praseodymium, neodymium and samarium.

The content of tungsten and/or rhenium is selected within a limit at which the plasticity and emissive capability of the alloy are not deteriorated, and if the content thereof is less than 0.5% by weight, the melting point of the alloy is lowered, which makes it impossible for the cathode to operate at high working temperatures. If the content of tungsten and/or rhenium is greater than 15.0% by weight, the plasticity and emissive capability of the cathode are deteriorated. Also, the electron-emitting metal alloy contains 0.5 to 10% by weight of hafnium. If the content of hafnium is less than 0.5 % by weight, the work function and working temperature increase, thereby resulting in a negligible effect of increasing the life of the cathode and increasing the brittleness of the alloy. If the content of hafnium is greater than 10% by weight, the content of iridium is comparatively reduced, thereby deteriorating the emissive capability of the alloy and lowering the melting point of the alloy.

Now, an example of the above-described direct heating cathode unit of a practicably applicable dimension will be described.

The emitter 110 is manufactured to have a dimension of 0.6 x 0.6 x 0.2 mm and the diameters of the heater wires 120 and 130 made of tungsten-rhenium alloy (80% by weight of tungsten and 20% by weight of rhenium) are 50 μm. The lengths of the heater wires 120 and 130 are equal to each other, that is, 4 mm. As the heater supports 141 and 142 formed of a corrosion-proof metal, wires having a diameter of 0.23 mm are employed. The lengths of the heater supports 141 and 142 are equal to each other, that is, 3 mm. The current supply holder pins 143 and 144 are formed of Kovar wire having a diameter of 0.4 mm. The insulator 151 of the cathode holder 150 is formed of glass C48-2. The cylindrical sleeve 152 of the cathode holder 150 is formed of a Kovar roll having a shell thickness of 0.2 mm and an outer diameter of 4.25 mm. The lengths of the parallel parts of the heater wires 120 and 130, that is, the central parts thereof, are equal to each other, that is, 0.9 mm. The lengths of the projections extending from the emitter 110 are equal to each other, that is, 3 times the diameter of the heater wire 120 or 130.

The aforementioned direct heating cathode unit and the electron gun employing the same operate as follows.

First, heater wire voltage and current of 1.2 V and 1.2 A are applied to the current supply holder pins 143 and 144 of the cathode holder 150. If the above- defined voltage is applied, heat is generated from the heater wires 120 and 130 whose branches are supported to the heater supports 141 and 142 through the ends of the branches so that the emitter 110 is heated to a temperature of 1400° C or higher, at which the density of current emission from the emissive surface of the emitter 110 of the cathode unit is 5 A/cm² or higher.

As described above, the electron beams having a high density current emitted from the cathode unit are focused and accelerated by a pre-focus lens formed between a screen electrode and a focusing electrode and then finally focused and accelerated by a main lens formed between the focusing electrode and a final accelerating electrode.

The direct heating cathode unit having the above-described construction according to the present invention and the electron gun employing the same have the following advantages.

First, since the efficiency of heat transmission between heater wires and an emitter is high, power consumption can be minimized.

Second, heater wires are not separated or disconnected from an emitter due to the interface reaction between the heater wires and the emitter.

Third, since thermal stability during high-temperature operation is excellent, compared to the conventional direct heating type metal cathode unit, the direct heating cathode unit according to the present invention is barely deformed.

Fourth, the direct heating cathode unit according to the present invention is very resistant to external shock due to its a high mechanical stability.

Fifth, due to the small size of the emitter as well as the above-mentioned advantages, the electron gun employing the direct heating cathode unit according to the present invention is excellent in view of focus or cut-off characteristics. Although the present invention has been described with reference to a specific embodiment, it should be understood that the invention is not limited thereto, and that various changes and modifications may be effected therein by one skilled in the art without departing from the spirit and scope of the present invention as defined in the appended claims.

Industrial Applicability

A direct heating cathode unit according to the present invention is applicable to electron emission sources for electron guns provided in electro-vacuum devices including color picture tubes, oscilloscopes and radiolocation devices, and other tubes in which high-density electron beams are used.

Claims

What is claimed is:

1. A direct heating cathode unit comprising: an emitter having at least two parallel lateral surfaces extending crosswise with respect to a longitudinal geometrical axis penetrating a base, and made of an electron-emitting metal alloy; two heater wires having central parts welded on both lateral surfaces of the emitter, the lateral surfaces being parallel to and facing each other; a cathode holder having current supply holder pins; and heater supports to which both ends of the heater wires in a pair are connected to the current supply holder pins, and connected to the current supply holder pins such that the vertical axis of the cathode holder coincides with the vertical axis of the emitter.

2. The direct heating cathode unit according to claim 1, wherein the emitter is rectangular and the thickness thereof varies within a range of 1/5 to 1/2 the size of the base.

3. The direct heating cathode unit according to claim 1, wherein the parallel parts of the heater wires extend from a portion welded on the body of the emitter.

4. The direct heating cathode unit according to claim 1, wherein the length of each of the projections of the heat wires is 2 to 5 times the diameter of each of the heater wires.

5. The direct heating cathode unit according to claim 1, wherein the central parts of the heater wires are connected to the central parts of the lateral surfaces of the emitter.

6. The direct heating cathode unit according to claim 1, wherein the heater supports are formed of high-resistance iron-nickel alloy.

7. The direct heating cathode unit according to claim 1, wherein the central parts of the heater supports are connected to the current supply holder pins.

8. The direct heating cathode unit according to claim 1, wherein the heater supports are connected to the current supply holder pins to be disposed crosswise with respect to the current supply holder pins.

9. The direct heating cathode unit according to claim 1 , wherein the heater wires supported to the heater supports are shaped of truncated tetrahedral pyramids.

10. The direct heating cathode unit according to claim 1, wherein the lateral surfaces of the emitter and the central parts of the heater wires are connected by resistance contact welding.

11. The direct heating cathode unit according to claim 10, wherein the resistance contact welding is performed such that the heater wires are made to generate heat by supplying current in a state where the heater wires are interposed between a welding electrode and the emitter, and such that the heater wires are pressed toward the lateral surfaces of the emitter using the welding electrode to be embedded into the lateral surfaces of the emitter.

12. A direct heating cathode unit comprising: an emitter having at least two parallel lateral surfaces extending crosswise with respect to a longitudinal geometrical axis penetrating a base, and made of an electron-emitting metal alloy including 0.5 to 9.0% by weight of a rare-earth metal of the cerium group, 0.5 to 15.0% by weight of tungsten and/or rhenium, 0.5 to 10% by weight of hafnium and the balance of iridium; two heater wires having central parts welded on both lateral surfaces of the emitter, the lateral surfaces being parallel to and facing each other; a cathode holder having current supply holder pins; and heater supports to which both ends of the heater wires in a pair are connected to the current supply holder pins, and connected to the current supply holder pins such that the vertical axis of the cathode holder coincides with the vertical axis of the emitter.

13. The direct heating cathode unit according to claim 12, wherein the emitter is rectangular and the thickness thereof varies within a range of 1/5 to 1/2 the size of the base.

14. The direct heating cathode unit according to claim 12, wherein the parallel parts of the heater wires extend from a portion welded on the body of the emitter.

15. The direct heating cathode unit according to claim 14, wherein the length of each of the projections of the heat wires is 2 to 5 times the diameter of each of the heater wires.

16. The direct heating cathode unit according to claim 12, wherein the central parts of the heater wires are connected to the central parts of the lateral surfaces of the emitter.

17. The direct heating cathode unit according to claim 12, wherein the heater supports are formed of high-resistance iron-nickel alloy.

18. The direct heating cathode unit according to claim 12, wherein the central parts of the heater supports are connected to the current supply holder pins.

19. The direct heating cathode unit according to claim 12, wherein the heater supports are connected to the current supply holder pins to be disposed crosswise with respect to the current supply holder pins.

20. The direct heating cathode unit according to claim 12, wherein the heater wires supported to the heater supports are shaped of truncated tetrahedral pyramids.

21. The direct heating cathode unit according to claim 12, wherein the lateral surfaces of the emitter and the central parts of the heater wires are connected by resistance contact welding.

22. The direct heating cathode unit according to claim 21, wherein the resistance contact welding is performed such that the heater wires are made to generate heat by supplying current in a state where the heater wires are interposed between a welding electrode and the emitter, and such that the heater wires are pressed toward the lateral surfaces of the emitter using the welding electrode to be embedded into the lateral surfaces of the emitter.

23. An electron gun comprising: a cathode unit having an emitter having at least two parallel lateral surfaces extending crosswise with respect to a longitudinal geometrical axis penetrating a base, and made of an electron-emitting metal alloy, two heater wires having central parts welded on both lateral surfaces of the emitter, the lateral surfaces being parallel to and facing each other, a cathode holder having current supply holder pins, and heater supports to which both ends of the heater wires in a pair are connected to the current supply holder pins, and connected to the current supply holder pins such that the vertical axis of the cathode holder coincides with the vertical axis of the emitter; a control unit and a screen electrode sequentially disposed from the cathode unit, constituting a triode together with the cathode unit; and a plurality of focusing electrodes installed in the vicinity of the screen electrode, constituting a main lens and an auxiliary lens.

24. The electron gun according to claim 23 , wherein the electron-emitting metal alloy comprises 0.5 to 9.0% by weight of a rare-earth metal of the cerium group, 0.5 to 15.0% by weight of tungsten and/or rhenium, 0.5 to 10% by weight of hafnium and the balance of iridium.