US2988658A - Electron gun for cathode ray tube - Google Patents

Description

June 1961 A. M. SKELLETT ELECTRON GUN FOR CATHODE RAY TUBE Filed July 2. 1959 'IIIIIVIIIIA 'II INVENTOR. ALBERT M. SK ELLETT A T TORNEYS.

United States Patent 6 2,988,658 ELECTRON GUN FOR CATHODE RAY'TUBE Albert M. Skellett, Madison, N.J., assiguor to Tuug-Sol Electric Inc., a corporation of Delaware Filed July 2, 1959, Ser. No. 824,496 2 Claims. (Cl. 313- 82) This invention relates to an electron gun with a cold cathode which may be used in a television picture tube, a cathode ray oscillograph, or any other type of apparatus which requires the use of a focused beam of electrons. The invention has particular reference to the electrode means for generating and directing a stream of electrons through a small opening where the beam may be focused by either magnetic or electrostatic focusing means.

Present electron guns derive their electrons from a hot cathode which requires a heater, subject to burn-out, and a layer of barium and strontium oxides. The heater uses considerable power and takes some time to heat the cathode material after the device is first turned on. Also, the emissive material has a rather short life and is subject to flaking oif, blistering, and evaporating to other electrodes.

The present invention eliminates all the above mentioned disadvantages and makes possible an electron gun which will start almost immediately after the voltages are applied. The efliciency is increased because there is no heater power necessary and the life of the gun is increased substantially over the average life of heated cathodes.

One of the objects of this invention is to provide an improved electron gun with a cold cathode which avoids within a first anode 15 which is provided with a small one or more of the disadvantages and limitations of prior art arrangements.

Another object of the invention is to eliminate the heater which is necessary in prior art hot cathodes.

Another object of the invention is to reduce the warmup period after the device has been turned on and to start the electron emission almost immediately.

Another object of the invention is to increase the efficiency of electron guns by eliminating the power required for the cathode heater.

Another object of the invention is to increase the life of all devices using electron guns.

The invention includes an electron gun having a cold cathode. The cathode includes a conductive base with a layer of cold emissive material such as magnesium oxide in porous sponge-like form. An external source of electrons is employed to start electron emission and a sustainmg grid electrode is used to maintain the electron emission after the device has been put into operation. The electron gun also includes control electrodes for modulating the electron beam intensity and focusing electrodes for focusing the beam onto a target or fluorescent screen. For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings.

FIG. 1 is a cross sectional view of a portion of a cathode ray tube showing the cold cathode and a magnetic focusing coil.

FIG. 2 is a cross sectional view of an electron gun showing an alternate form of cathode arrangement in connection with an electrostatic focusing means.

FIG. 3 is a cross sectional view similar to FIG. 2 but showing a diiferent arrangement of control electrodes.

Referring now to FIG. 1, the electron gun is housed within a glass envelope 10 which may have an enlarged portion 11 terminating in a mount for a fluorescent screen (not shown). The cold cathode 12 comprising a metal sleeve with a porous sponge-like coating containing magnesium oxide thereon, is mounted within a sustaining grid 13 and a control grid 14. This assembly is mounted opening 16 where electrons emitted from the cathode coating are directed along the axis of the envelope. Starting means must be provided to initiate electron emission from the magnesium oxide coating of the cathode. Such means, in the particular embodiments of the invention illustrated in the drawing, comprise a filament 17 which when energized gives off electrons to cause bombardment of the coating and consequent emission of electrons therefrom. T he filament 17 is mounted outside the anode 15 and adjacent to slot 18 in the anode structure. When the device is first started, a sustaining voltage of about 300 volts positive with respect to the cathode is impressed upon grid 13 through a stabilizing resistor (not shown). Then filament 17 is lighted and electrons from the filament are drawn into the anode space and start electron emission from the cathode 1 2. Once electron emission from the cathode is initiated, it becomes self-sustaining and therefore the circuit of filament 17 may be opened. Electrons emitted by the cathode surface pass through hole 16 and then are attracted by a second anode 20 having a hole 21. The combination of holes 16 and 21 at a potential dilferen'ce produces a collimating action and sends the stream of electrons along the axis toward the target'or fluorescent screen under the influence of the field produced by another anode 19. The electron beam is divergent as it leaves the second anode 20 but an electromagnetic coil 22 produces an axial magnetic field and alters the direction of the electrons causing them to converge and come to a focus at a desired plane. [If the device is to be used as an oscillograph or as a television picture tube, a deflection means is provided such as a magnetic yoke 23. Electrostatic means, such as deflector plates for deflecting the beam may also be used.

The cold cathode described above emits electrons preferentially in one direction only and for this reason it can be coated with electron emitting material on one side only. This material may be porous magnesium oxide as described in patent application, Serial No. 808,618, now Patent No. 2,945,151, filed April 24, 1959 by Bernard G. Firth, or it may be a mixture of magnesium oxide and other metallic oxides which enhance the electron emitting characteristics.

It should be noted that the starter filament 17 is not in line of sight of the cathode material. The starting electrons emitted from the starter filament must pass through hole 18 and then move around supporting wire 24 before hitting the cathode coating. The support 24 acts as a mask for stopping the heavy molecules which are always emitted by the filament. These molecules may be many kinds of impurities existing in the tungsten filament and if they are allowed to strike the cathode coating they will poison it by lowering its electron emitting characteristics.

In the alternate device shown in FIG. 2 the cathode material 25 is mounted on the end of a metal cylinder 26. This view shows a cathode heater 27 but this heater is employed only during the initial assembly and exhaust procedures. The cathode cylinder 26 may be secured within a sustaining grid electrode 28 by means of an annular insulator 30. In this arrangement a mask 31 is mounted adjacent to a hole 32 in the sustaining grid structure for shielding the cathode material from the impurities which are given off by the starter filament 17. The sustaining grid 28 is generally maintained about 200 volts above the potential of the cathode by a potential supplied through a stabilizing resistor.

The control grid for modulating the intensity of the electron beam may be a flat electrode 33 with a central opening 34 as shown in FIG. 2 and the first anode 35 which is maintained at a relatively high potential above the cathode is a hollow cylindrical electrode having flat electron beam passes through both holes and in doing so, many of the more divergent electrons are eliminated from the beam by striking the exit barrier adjacent to hole 37.

Focusing electrodes 38 and 40 constitute electrostatic focusing means for providing the focusing action to produce a small spot on the fluorescent screen. In addition to the focusing electrodes shown in FIG. 2, the final anode 19 is deposited on the inside surface of the envelope and aids in focusing and increasing the electron speed.

The invention is not restricted to this particular type of focusing means. Details of the construction and operation of other similar focusing means may be found in a book Vacuum Tubes by K. R. Spangenberg, published by McGraw-Hill Company of New York in 1948, chapter 15, pages 412 to 474.

The electron gun shown in FIG. 3 is similar to that shown in FIG. 2 except that two control electrodes 41 and 42 are provided. The sustaining electrode 43 is mounted between the two control electrodes and serves its usual purpose of maintaining a continuous flow of electrons from the cathode coating 25. In this form of electron gun, the first control electrode 41 is generally coarse having only a few but relatively large openings. The second control electrode 43 is made of much finer mesh and exercises a much greater control over the electron fiow.

The control grid 41 modulates the intensity of the electron stream by space charge action and by modifying the potential gradient at the surface of the cathode coating. The second control grid 43 modulates the intensity of the electron stream by virtual cathode action. The two control grids are electrically connected together and positively biased with respect to the cathode.

It has been found by experiment that the electron stream given off by a cold cathode as described herein contains electrons with a rather wide range of velocities. This wide range makes the resultant beam more difficult to control and focus. It has also been found that the electrons leave the cathode surface nearly at right angles to it, a fact that aids in focusing. Those electrons which emerge through opening 36 at an angle to the axis and which have high velocities are the most difi'icult to focus and these are caught by wall 35 and prevented from entering the beam. Thus the arrangement of FIG. 3 excludes the hardest-to-focus electrons and allows only those which are relatively easy to focus to enter the beam. This reduction is not substantial and most of the electrons go through aperture 37 and are focused.

The electron gun described above can be operated with any type of deflecting means including the magnetic deflection yoke 23 or an electrostatic system of deflecting plates.

In one practical embodiment of the electron gun as shown in FIG. 2, the following voltages were applied to the gun electrodes:

Volts Cathode 26 0 Sustaining electrode 28 (in series with resistor of 30,000 ohms) 350 Control electrode 33 First anode 35 2000 Focusing electrode 38 400 Focusing electrode 40 2000 Aquadag coating 19 14,000

The foregoing disclosure and drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense. The only limitations are to be determined from the scope of the appended claims.

For the purpose of clarity the support means for the various electrodes of the new gun and the electrical connection thereto, have not been illustrated in the drawing. Obviously the new gun could include conventional means insulatedly supporting the various electrodes in alignment. Also, conventional means would be employed for connecting the electrodes to suitable pin terminals or the like of a conventional base for application of operating voltages to the electrodes.

What is claimed is:

1. An electron gun comprising a cold cathode having a conductive base with a porous sponge-like oxide coating having the characteristic of self-sustained electron emission once emission is initiated, an anode spaced apart from the cathode and having an opening for the passage of electrons, a sustaining electrode mounted in the space between the cathode and the anode for maintaining electron emission from the cathode coating, a first control electrode mounted in the space between the sustaining electrode and the cathode, and a second control electrode electrically connected to said first control electrode to be at the same potential therewith positive with respect to that of the cathode and mounted in the space between the sustaining electrode and the anode, said first and second control electrodes and said sustaining electrode all formed with openings for the passage of electrons, the openings in said first control electrode being wider than the openings in said sustaining electrode and in said second control electrode.

2. An electron gun as set forth in claim 1 including an electron emitting filament heated by an external source of potential and positioned externally of the cathode for initiating electron emission from the coating thereof and a mask mounted between the starter filament and the cathode coating for shielding the coating from impurities given otf by the filament.

, References Cited in the file of this patent UNITED STATES PATENTS 2,509,053 Calbick May 23, 1950 2,802,127 Dobischek Aug. 16, 1957 2,842,706 Dobischek July 8, 1958 2,867,687 Glenn Jan. 6, 1959 FOREIGN PATENTS 465,266 Great Britain May 3, 1937

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