US3772560A

US3772560A - Thermionic cathode

Info

Publication number: US3772560A
Application number: US00198595A
Authority: US
Inventors: R Orthuber
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1971-11-15
Filing date: 1971-11-15
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13
Also published as: FR2160481A1; GB1412148A; DE2255754A1; FR2160481B1

Abstract

A thermoinic cathode including two conductive metal supportleads and a plurality of parallel segments of thoriated tungsten wire for producing an electron stream of a very large cross sectional area required, e.g., in tubes utilizing channel-type electron multipliers. The support-leads are formed in the shape of a comb with spaced interleaved teeth. Each of the wire segments extends perpendicularly over and is conductively bonded to each tooth.

Description

United States Patent [191 Orthuber Nov. 13, 1973 Primary ExaminerRonald L. Wibert Assistant Examiner-Paul A. Sacher Attorney-C. Cornell Remsen, Jr. et al.

[57] ABSTRACT A thermoinic cathode including two conductive metal support-leads and a plurality of parallel segments of thoriated tungsten wire for producing an electron stream of a very large cross sectional area required, e.g., in tubes utilizing channel-type electron multipliers. The support-leads are formed in the shape of a comb with spaced interleaved teeth. Each of the wire segments extends perpendicularly over and is conductively bonded to each tooth.

9 Claims, 3 Drawing Figures THERMIONIC CATHODE BACKGROUND OF THE INVENTION This invention relates to the vacuum tube art, and

more particularly, to a thermionic cathode.

Recently it has become desirable to provide an electron stream of a very large cross sectional area. For example, flood electron sources for providing such large electron streams are disclosed and illustrated in U. S. Pat. No. 3,541,254. However, although a thoriated tungsten filament has a number of advantages, if such a filament is long, the electrons are emitted therefrom at widely varying potentials. This causes disruption of the picture produced by the tubes of said patent. Further, it is not possible to use a continuous thoriated tungsten filament of a reasonable wire size for the electron emission called for by the said tubes. Conventional cathodes of this type would also require an exhorbitant amount of power.

SUMMARY OF THE INVENTION In accordance with the device of the present invention, the above-described and other disadvantages of the prior art are overcome by providing two combshaped conductive support-leads for a plurality of parallel, resistive wire-like segments.

The support-lead teeth may be highly conductive and are spaced and insulated, but interleaved and the wirelike segments conductively bonded thereto. The segments may be made of thoriated tungsten. They extend in a direction approximately perpendicular to the teeth.

* In accordance with the foregoing, the electron stream emitted from this arrangement will be nearly mono-energetic because the potential drop along parallel wire segments can be made small and identical for all segments.

The arrangement permits flooding of a large area by means of a small total emitting area and, thus, high temperature of the collective emitting segments necessary for efficient electron emission can be obtained with modest power requirements.

The above-described and'other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings which are to be regarded as merely illustrative:

FIG. 1 is a diagrammatic view of a television picture tube of the type disclosed in said patent;

FIG. 2 is a broken away perspective view of a flood cathode for the tube of FIG. 1 and constructed in accordance with the present invention; and

FIG. 3 is a graph illustrating parameters helpful in constructing the cathode of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, a television picture tube is indicated at including an evacuated envelope 11, a flood cathode 12, a multichannel array including a channel-type electron multiplier 13 and a phosphor screen 14. A control circuit 15 is connected to cathode 12, multiplier 13 and screen 14.

The structure shown in FIG. 1 may all be identical to that disclosed in said patent except cathode l2.

Cathode 12 is a thoriated tungsten thermionic cathode as shown in FIG. 2. Cathode 12 includes a dielectric plate 16. A pair of comb-shaped conductive support-leads 17 and 18 have interleaved teeth 19 and 20, respectively. Both support-leads 17 and 18 are insulated from each other and are fixed to plate 16. However, plate 16 may be omitted if the structure otherwise has adequate physical integrity. Support-leads l7 and 18 have end portions 21 and 22, respectively, which serve as conductive buses to a heater source of potential 23.

A plurality of thoriated tungsten wire segments 24 are positioned in contact with teeth 19 and 20 and are conductively bonded thereto thereat. Each segment 24 may be welded or otherwise conductively bonded to each tooth over the entire width of the tooth.

Support-leads 17 and 18' may be made of any conductive material suitable for acting as a lead from a tungsten filament or otherwise.

Segments 24 need not extend upwardly above plate 16, as shown in FIG. 2. This has been done only for clarity of illustration.

In the display of height H and width W, shown in FIG. 2, assume series of n,, horizontal, highly conductive teeth is provided with a tooth width d and mutually spaced by S Crossing this grid of teeth perpendicularly, a set of n,, vertical and parallel thoriated tungsten wire segments 24 are arrayed. These wire segments have a diameter d and are spaced by S,,. They are in good electric contact with the highly conductive horizontal teeth so that the wire segments are shorted out within the entire intersecting area. Voltages V and V are alternatingly applied to the horizontal teeth which are considered conductive enough to make ohmic heat development negligible. The emission obtainable from the unsupported and, thus, heated segments of the vertical wires is given by the temperatureand the total area of the segments. This area is:

wherein n is the total number of unsupported wire segments,

' v X n v n) Thus,

A m v b)/ v h] (3) Introducing the length-to-d iameter ratio of the unsupported wire segments A n b)/ w and choosing S,, 8,, for a square array of emitters A 1rd,, xd /sfi 11w a /s WW.

(5) The value of A is maximized if d, 0 or n/ ial (6) Thus,

AIXIHBX) 7r IO/ U) Besides, d cannot exceed 8,, and, thus, S, which yields a maximum for the emitting surface [for vanishing tooth width and tightly packed wires (d,,, S,,)] of The factor 1r reflects, of course, the surface increase caused by the cylindrical surface configuration of an H X W sheet formed by the contiguous wires.

From Figure 4.1 l of Spangenberg, Vacuum Tubes, the total emission density from thoriated tungsten is approximately 1 1 amp./cm. Thus, the total emission available from the structure is J 1, ,4 n w (d,,,/S,,) A HW amps.

Assuming now a display area HW= 1.2 X 10 cm.

and a wire tooth spacing S, S,, 1 cm., the available emitted current .l is plotted as a function of wirediameter d,,, and length-to-diameter ratio A of the unsupported segments of the emitting wire in FIG. 3. The power consumption, neglecting end cooling effects and thus only due to radiation and emission-cooling, is also proportional to the unsupported surface area A and, according to Stephan-Boltzmans law, given by With K 5.73 X 10' watts/cm. degree and e, 0.25 at 1,900 K, according to Spangenbergs Figure 4.8 and T= l,900 K, the radiative power dissipation becomes P (watts) 18.6 J (amps.).

In order to obtain a white display level of a tricolor tube of the type disclosed in said patent of 100 foot lamberts, with a channel-type electron multiplier gain of G 100 and a storage gain of 40, it will generally be necessary to apply a flood current of I44 ma. in a inch diagonal display. Thus, assuming an efficiency of rt; in the collection of the emitted current, an J 3 X 144 432 ma. is required.

The condition J, 432 ma. is indicated in FIG. 3 as a vertical line which shows some suitable combinations of w,, and )t resulting in J, 432 ma. and according to (12) P 8 watts neglecting end cooling. This neglection of end cooling does, however, not appear permissible for small length-to-diameter ratio, such as A I. It should, however, cause no significant increase in heater power for A between 10 and 100.

The flood cathode structure of FIG. 2 can generally be made with thoriated tungsten wire segments 24 having a 0.003 to 0.001 cm. or 1.3 to 0.4 mils diameter and having an unsupported length (between teeth) of 0.03 to 0.1 cm. or 13 to 40 mils, respectively.

By uniformly distributing the unsupported portions of segments 24 between teeth 19 and 20, it is possible to obtain a substantially equipotential electron source. The same is also true because all the teeth 19 may be at the same potential V, whereas all the teeth 20 may be at the same potential V,. This means that the entire difference of potential is supplied to each said unsupported portion and the lower velocity electrons accelerated from cathode 12 are thoroughly intermingled with the higher velocity electrons. Still further, due to the very short lengths of the said unsupported portions, a very large majority of electrons will be emitted from the centers thereof. A greatmajority of the electrons emitted will thus be emitted at a point that has a potential very close to or exactly equal to (V, V,)/2.

Uniform spacing for teeth 19 and 20 and segments 24 is not necessary, but preferred.

The tooth spacing is uniform in all directions. Such a uniform spacing is not absolutely necessary, but it is preferred. For example, preferably, S 8,. See FIG. 2.

By the use of the cathode 12, it is also possible to use segments of a reasonable size. Lower power consumption is also an advantage.

The cathode 12 may be employed in a tube of the type disclosed in said patent or any other tube requiring a thermionic cathode for providing a stream of electrons relatively large in cross sectional area.

Support-leads 17 and 18 may be made in any conventional way. They may be evaporated onto plate 16 through a stencil. Alternatively, they may be stamped, machined or etched.

The phrase channel-type electron multiplier as used herein and in the claims is hereby defined in include, but not be limited to, a multiplier of the type disclosed in said patent.

As stated previously, in some cases, plate 16 may be omitted. In these cases, cathode 12 may be supported by coventional conductive support-leads, for example, conductively bonded by any conventional method to end portions 21 and 22 of support-leads 17 and 18, respectively.

The phrase means to maintain at a potential" is hereby defined for use herein and in the claims to mean either a single conductor or a single conductor plus structure in addition thereto.

The segments 24 are preferably made of thoriated tungsten wire, but the material of segments 24 is not necessarily limited to thoriated tungsten. Further, each of the teeth 19 and 20 has a rectangular cross section uniform throughout its length, but the shape of teeth 19 and 20 may be changed without departing from the invention. The same is true of end portions 21 and 22, which may or may not be identical, as desired. Each of the support-leads l7 and 18 has a uniform thickness throughout its extent, but this need not always be true in practicing the invention. All of the teeth 19 and 20 are shown to be identical, but this need not always be true in order to practice the invention. The same is applicable to the uniform thickness of plate 16.

Each of the segments 24 has a circular cross section uniform throughout its length. However, different nonuniform cross sections may be used while practicing the invention.

What is claimed is:

1. A thermionic cathode comprising: first and second sets of elongated conductive teeth; holder means to support said sets in fixed positions insulated from each other, all of said teeth being substantially straight and parallel, at least one tooth in one set being positioned between two teeth in the other set, all of the teeth in said first set being spaced from each other and from all the teeth in said second set; a plurality of elongated, parallel resistive segments extending substantially perpendicularly to all of said teeth and conductively bonded to said one and said two teeth; first conductive means fixed to all the teeth in said first set to maintain them at a first predetermined potential; and second conductive means fixed to all the teeth in said second set to maintain them at a second predetermined potential different from said first predetermined potential, said teeth all having a conductivity substantially greater than that of said segments.

2. The invention as defined in claim 1, wherein each of said teeth has a thickness smaller than its width, and a length longer than its width, each tooth being rectangular in cross section, each tooth having a cross section uniform along its length, said first conductive means being a first conductive busat one end of said first set integral with the teeth thereof, said first bus having a thickness equal to that of said teeth, all of said teeth having the same thickness, said second conductive means being a second conductive bus adjacent to, but spaced from, the other end of said first set and integral with the teeth of said second set, said second bus having a thickness equal to that of said teeth, said holder means including a dielectric board having a flat face, said teeth and said buses being fixed to said flat face with their thickness dimension perpendicular thereto, each of said resistive segments being a thoriated tungsten wire segment of a circular cross section uniform throughout its length, the teeth of said first set being positioned alternately between the teeth of said second set, and vice versa.

3. The invention as defined in claim 2, including an evacuated envelope, said holder means being fixed relative to said envelope in a position to support said strips therein, and a channel-type electron multiplier array having an input side facing the sides of said teeth, said multiplier input side being approximately parallel to said tooth sides.

4. The invention as defined in claim 1, wherein the teeth of said first set are positioned alternately between the teeth of said second set, and vice versa.

5. The invention as defined in claim 4, wherein each of said sets includes at least two teeth.

6. The invention as defined in claim 5, wherein each of said sets includes at least three teeth.

7. The invention as defined in claim 6, wherein the spacing of two adjacent ones of said segments is approximately equal to one tooth width plus the spacing between two adjacent teeth, said teeth and said segments having uniform spacings.

8. The invention as defined in claim 4, including an evacuated envelope, said holder means being fixed relative to said envelope in a position to support said strips therein, and a channel-type electron multiplier array having an input side facing the sides of said teeth, said multiplier array input side being approximately parallel to said tooth sides.

9. The invention as defined in claim 1, including an evacuated envelope, said holder means being fixed relative to said envelope in a position to support said strips therein, and a channel-type electron multiplier array having an input side facing the sides of said teeth, said multiplier array input side being approximately parallel to said tooth sides.

Claims

2. The invention as defined in claim 1, wherein each of said teeth has a thickness smaller than its width, and a length longer than its width, each tooth being rectangular in cross section, each tooth having a cross section uniform along its length, said first conductive means being a first conductive bus at one end of said first set integral with the teeth thereof, said first bus having a thickness equal to that of said teeth, all of said teeth having the same thickness, said second conductive means being a second conductive bus adjacent to, but spaced from, the other end of said first set and integral with the teeth of said second set, said second bus having a thickness equal to that of said teeth, said holder means including a dielectric board having a flat face, said teeth and said buses being fixed to said flat face with their thickness dimension perpendicular thereto, each of said resistive segments being a thoriated tungsten wire segment of a circular cross section uniform throughout its length, the teeth of said first set being positioned alternately between the teeth of said second set, and vice versa.