US2160021A - Electrode arrangement for cathode ray tubes - Google Patents

Description

May 30, 1939. H. A. IAMS ELECTRODE ARRANGEMENT FOR CATHODE RAY TUBES Filed June 29, 1937 INVENTOR HARLEY A. JAMS ATTORN EY Patented May 30, 1939 iJNiTED STATES PATENT GFFEQE ELECTRODE ARRANGEMENT FOR CATHODE RAY TUBES Delaware Application June 29, 1937, Serial No. 150,921

5 Claims.

My invention relates to cathode ray tubes, and particularly to an electrode arrangement for focusing the electron beam which is used in such tubes.

Cathode ray tubes of the usual type have an electron gun comprising an electron emitting cathode, means for controlling the intensity of electron emission from the cathode usually referred to as a control electrode or grid, and two anodes operating at difierent potentials for accelerating and focusing the electron beam. A fluorescent screen is positioned to intercept the beam to produce a spot of light at the point of impact of the beam on the screen.

The accelerating and focusing anodes of the electron gun are usually so constructed as to generate an electrostatic field to focus an electron beam and produce a sharply defined spot on the fluorescent screen when the potentials on the two anodes are maintained at a definite voltage ratio, one to the other. Thus, if a change is made in the voltage of the first anode, which is near the cathode, a proportional change must be made in the higher potential on the second anode which is further from the cathode, in order to maintain the cross-section of the beam and the area of the resulting light spot on the fluorescent screen of the desired minimum size. Usually the two anodes are supplied with different potentials from a single source of anode potential through a voltage divider network exterior to the tube. The electrons after striking the fluorescent screen are collected by the second anode and returned to the cathode through the voltage divider network. The amount of current collected by the second anode varies with varying conditions of beam current. Such variations in current unbalance the normal distribution of current in the voltage divider thereby changing the de- 5 sired ratio of voltage supplied to the two anodes.

As a result the point at which the diameter of the beam is a minimum, or the focal point of the beam, may shift to either side of the fluorescent screen, defocusing the beam on the screen and increasing the size of the spot of light.

One object of my invention is to provide structure for use in a cathode ray tube wherein defocusing due to variations in the source of anode potential is minimized and substantially eliminated. Another object of my invention is to pro vide a tube in which the electron beam is in fixed focus and no manual adjustment is required.

A further object of my invention is to provide a i cathode ray tube in which the electron beam cannot be defocused by varying the anode potential during the operation of the device.

A better understanding of my invention will be obtained, and other objects, features, and advantages of my invention will appear from the following description taken in connection with the accompanying drawing in which the figure is a diagrammatic View illustrating one form of cathode ray tube incorporating my invention.

The tube shown in the drawing comprises a highly evacuated envelope or bulb I with a tubular arm or neck section enclosing the electron gun, and a frusto-conical section closed at one end by a transparent glass wall 2 on which is deposited a layer of fluorescent material, such as finely divided willemite, to form the target or fluorescent screen 3. The electron gun assembly includes a cathode 4 for generating an electron beam, the cathode enclosing a heating element or filament 5, a centrally apertured control electrode 6 connected to the usual biasing battery, a first anode l of cylindrical shape having two apertured discs 8 and 9, with the apertured disc 8 located at the end of the anode nearer the cathode and the apertured disc 9 displaced from the disc 8 by about A; of the anode length, and a second anode l0 which is preferably a conductive coating on the inner surface of the envelope I-and extending from a point on the neck wall slightly removed from the end of the first anode to a point on the frusto-conical section close to the fluorescent screen 3. The two anodes enclose separate portions of the path of the electron beam and are preferably coaxial with the oathode 4.

Conventional electron beam deflection means such as coils II and I2 are located near the end of the neck of the tube to generate deflection fields which sweep the beam in mutually perpendicular directions to trace any desired pattern on the fluorescent screen. It is obvious that electrostatic deflection plates may be substituted for either one or both sets of the deflection coils, if desired.

In accordance with my invention I provide an auxiliary electrode l3 between the first and second anodes and overlapping the adjacent ends of the two anodes of the electron gun. Further, in accordance with my invention I connect the first and second anodes together and to the posi- 15.0 tive terminal of a direct current source, such as the battery I4, so that both anodes are at the same potential, instead of at difierent potentials as in the conventional tube, and also connect the auxiliary electrode l3 to the cathode 4 and to the 55 negative terminal of the same direct current source. The electrical connections between the two anodes and between the cathode and the auxiliary electrode are non-inductive and of low resistance and may be made either within the envelope l or by way of the external connectors as shown in the drawing.

The auxiliary electrode I3 is preferably in the form of a metallic cylinder located between and co-axial with the first and second anodes and extending for a short distance over the adjacent ends of the two anodes this being in telescopic relation with respect to the two anodes, and for this reason will be hereinafter referred to as the intermediate electrode. The position of the intermediate electrode and its length and diameter are functions of the diameters of the first and second anodes and of the distance from the intermediate electrode to the fluorescent screen. Thus, by maintaining the dimensions and spaced relationship of the two anodes the same, except for those of the intermediate electrode, it is possible to increase or decrease the length of the intermediate electrode with corresponding increase or decrease of its diameter. Likewise the distance between the fluorescent screen and the first anode may be increased or decreased by decreasing or increasing the length of the intermediate electrode. With such a design the beam is properly focused under all conditions of operation. Thus the common potential of the two anodes may be varied between the usual maximum and minimum allowable voltage, such as from 200 to 10,000 volts, without destroying the focus of the tube. It is, however, desirable that the diameter of the intermediate electrode 93 be greater than that of the first anode in order to minimize collection of current on this electrode. For example, I have obtained good results with a tube having the control electrode, first anode and auxiliary electrode constructed from nickel tubing having a wall thickness of 0.15 inches, with the other dimensions of the tube as follows:

Cathode (4) diameter inch 0.125 Control electrode (6) diameter do Control electrode aperture diameter do 0.030 Cathode to control electrode aperture distance inch 0.010 Control electrode aperture to front edge of electrode cylinder distance mm 4.0 Control electrode to 1st anode distance mm 1.0 First anode (1) diameter inch First anode (1) length mm 28 Anode aperture (8) diameter inch 0.075 Anode aperture (9) diameter do 0.040 Thickness of apertured discs "do--- 0.005 First anode (1)intermediate electrode (l3) overlap distance mm 6.8 Intermediate electrode (l3) diameter inch" /2 Intermediate electrode (I3) length rnn 12.0 Intermediate electrode (l3) to screen (3) distance "inches" 8 A; Intermediate electrode (l3)second anode l0) overlap distance -.rnnn. 5.2 Second anode (l0) diameter inches 1% While I do not desire to be limited to any particular theory of operation of my new and improved device it seems probable that the electrons emitted by the cathode 4 are drawn away by the electrostatic field which is generated by the anode 1 and which extends into the region of the cathode through the aperture of the control electrode 6. The form of the electrostatic field near the aperture of the accelerating electrode is such as to cause the electrons to be accelerated in paths which cross the longitudinal axis of the electron gun near the apertured disc 8 of the anode l. The electrons then move through the first portion of the anode I, that is, the portion between the apertured discs 8 and 9, in straight lines radiating from the point of intersection with the axis inasmuch as the region between the apertured discs 8 and 9 is substantially a fieldfree space. In order to bring this beam of electrons to a focus on the fluorescent screen 3 two conditions must be fulfilled. First, the electrostatic field at the end of the anode I must be so shaped that the electrons are given a velocity toward the axis of the electron gun which is proportional to their distance from the axis, and second, it is necessary that the velocities which the electrons acquire toward the axis. be varied inproportion to their velocity along the axis. In my improved electron gun structure these two requirements are fulfilled as follows: As the electron beam passes through the apertured disc 9 in the form of a diverging bundle of electron rays it approaches the end of the anode I and from the end of the anode l to the screen 3 the electrons pass through three separate and distinct electrostatic fields. The field just within the end of the anode i, set up by the coaction of the potentials applied to the anode 'i' and the inter mediate electrode l3 produces a radial component of velocity which gives the electrons a slight additional outward velocity, that is, away from the electron gun axis. passes beyond the end of the anode I and into the region enclosed by the auxiliary electrode l3, the electrons are given a strong radial component of velocity toward the axis. At the entrance to the anode It the field is of such a nature that.

the electrons are again given a slight radial component of velocity away from the axis. However, the inward component of velocity is larger than the sum of the two outward components and the final result is to direct the electrons toward a point on the electron gun axis at the surface of the fluorescent screen. The electrostatic fields acting on the electrons of the beam further removed from the axis are stronger than the fields acting upon the electrons near the axis so that the outer electrons of the beam are given greater radial components of velocity toward the axis than the inner electrons so that all of the electrons of the beam are directed toward the axis at a single point corresponding to the position of the fluorescent screen.

The velocity with which an electron is moving at any given point depends upon the electrostatic potentials of that point with respect to the cathode and are determined by the relative potentials impressed upon the control electrode 6 and the anodes l and Ill which are connected together. The intermediate electrode l3, as previously noted, is operated at cathode potential and since the focusing field is established between this intermediate electrode and the anodes I and [0 any change in anode potential is accompanied by a proportional change in the focusing field. Thus, if the speed of the electrons along the axis is raised by increasing the common anode potential on anodes l and ill, the speed of the electrons toward the axis is increased by a proportionate amount. Therefore, the paths taken by the in dividual electrons in the beam remain the same regardless of the common anode potential so As the electron beam that all of the electrons contained in the beam are directed toward a single point which intersects the electron gun axis at the fluorescent screen. It is thus possible by the use of my invention to vary the potential applied to the electrostatic focusing anodes without defocusing the beam.

From the foregoing description it will be apparent that various other modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim:

1. In a cathode ray tube, an electron emitting cathode, a fluorescent screen, a cylindrical electrode of equal diameter throughout its length between said cathode and said screen adapted to be maintained at a positive potential for projecting the electrons toward said screen, a second electrode adjacent the end of the said first electrode which is nearer the screen, and electrically connected to said cathode through a lowresistance non-inductive path for maintaining said second electrode at cathode potential, and a third electrode connected to said first electrode, each of said second and third electrodes being of progressively larger diameter from said first electrode to said screen.

2. A cathode ray tube comprising an evacuated envelope enclosing an electron emitting cathode, a screen element, a plurality of electrodes between said cathode and said screen element comprising a tubular first anode of a greater length than its diameter and having two apertured diaphragms for projecting the electrons towards said screen, a second tubular anode co-axial with and electrically connected to said first anode, and an intermediate cylindrical electrode between and in telescopic relation with said first and second anodes and electrically connected to said cathode through a low resistance non-inductive path for maintaining said electrode at the potential of said cathode so that with the cooperative action of the first and second anodes an electrostatic field is developed and the projected electrons are focused to a sharply defined point upon the said screen element.

3. In an electron discharge device, a source of electrons, means to control the intensity of electrons from said source, a screen, an elongated tubular first anode element for acelerating the electrons toward said screen, a second elongated tubular anode element of larger diameter thansaid first anode and further removed from said source than said first anode, said second anode being adapted to be maintained at the same potential as said first anode, and a cylindrical electrode intermediate and in telescopic relation with said first and second anodes and electrically connected to said source of electrons through a low resistance non-inductive path so as to generate with the cooperative action of said anodes an electrostatic field serving to focus the projected electrons to a sharply defined point on said screen.

4. A cathode ray tube comprising an evacuated envelope, an electron emitting cathode within the envelope, a target in the path of the electrons emitted by said cathode, and three substantially cylindrical focusing electrodes within the envelope in the space separating the cathode from the target, and of progressively increasing diameter from said cathode to said target, the outer two of said electrodes being connected together and adapted to receive a high positive potential with respect to said cathode, the intervening electrode being in telescopic relation with said outer two electrodes and connected to said cathode through a low resistance non-inductive path.

5. In a cathode ray tube, an electron emitting cathode to generate an electron beam, a fluorescent screen, two substantially cylindrical electrodes of unequal diameter interposed between said cathode and said screen and surrounding separate portions of the path of said beam, the electrode of smaller diameter being nearer the cathode than the other of said electrodes, means to maintain said electrodes at the same positive potential with respect to said cathode, an auxiliary electrode of a diameter intermediate those of said two cylindrical electrodes between the said electrodes and overlapping the adjacent ends of said electrodes and a low resistance non-inductive electrical connection between said auxiliary electrode and said cathode for maintaining said auxiliary electrode at cathode potential.

HARLEY A. IAMS.

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