US2185378A - Electron gun for cathode ray tubes - Google Patents

Description

Jan, 2, 194a.

l. G. MALOFF ET AL.

ELECTRON GUN FOR CATHODE RAY TUBES Filed June 26, 1937 IN VENTORS IOURY G MALOFF DA wo W. EPSTE/N ATTORN EY Patented Jan. 2, 1940 stein, Mcrchantville,

and Karl lit. Wendi, Au-

dubon, N, 1., assignors to Radio Corporation of America, a corporation of Delaware Application June 26,

2 Claims.

This invention relates to an improvement in electron tubes and particularly cathode ray'tubes such as those used in television transmitting and receiving apparatus and for oscillograph observations. Tubes of this type have the general characteristics of an electron beam and a target upon which the beam is caused to impinge either to produce optical or electrical effects. The gun structure includes an electron emitter (which may be separately heated if desired) and an ac celerating electrode or anode. In the usual gun structure the accelerating electrode is provided with a beam limiting aperture and because of the presence of this small aperture many of the electrons of the beam are stopped with the result that the beam, by the time it reaches the target, is materially reduced in intensity. Furthermore considerable current is drawn by the accelerating electrode.

In order to utilize all of the contrast that a particular tube is capable of giving, or, in other words, to be able to use the intensities of the beam within the entire range between minimum (usually zero beam current) and a maximum beam current and in order to be able to reduce or focus the size of the beam to a small point at the plane of the target, the present invention proposes to eliminate the customarily used beam limiting aperture in order that the beam shall not be permitted to contact the accelerating electrode or anode so that no current will be drawn by this element. Any substantial amount of current on the accelerating electrode or first anode limits the extent to which the contrast capabilities of a particular tube can be utilized and the extent to which the beam can be converged or focused at the target.

One main purpose or object of the invention is, therefore, to produce a cathode ray tube having no beam limiting aperture and with the electron gun so constructed that the electron stream will not strike the sides of the first anode whereby the first anode current will thereby be reduced or completely eliminated;

Other objects and advantages of the invention will be apparent from the description which follows but will include among other features, those of providing an electron tube of high eiiiciency which is easily controlled in its operation.

The invention in certain of its preferred forms will best be understood by referring to the drawing in which:

Figure 1 represents a section of a cathode ray tube constructed in accordance with the invention;

1937, Serial No. 150,432

Figure 2 shows a curve of the angle of divergence of a cathode ray as a function of the length of the skirt on the control electrode, and

Figure 3 shows a section of a cathode tube embodying a modified form of the invention.

Referring to the drawing, in Figure 1, character H3 represents the glass or vitreous envelope of a cathode ray tube. Within the tube are positioned'and supported by any appropriate means a cathode or emit er ii and a filament or heater ii for supplying heat to the cathode. Surround ing the cathode is a control electrode or grid 13 having a skirt portion l5 and a beam limiting aperture M. An accelerating electrode or first anode it is also provided which is in the form of a cylinder and is positioned in such a manner that the axis of the cylinder coincides with the axis of the tube and the produced cathode ray beam.

The inside portion of the envelope is provided with a conducting coating ii and connection is made thereto by way of the terminal it where by a source of positive potential be applied to the coating in order that it function as a second anode or accelerating electrode. finch sec ond anode voltage is usually of the order of three to five times that applied to the anode 56'. A fluorescent screen or target it (such, for instance, as the well known mosaic electrode) is also provided at the end of the tube.

As the electrons are drawn from the cathode or emitter by the positive potentials of the first and seccndaccelerating anodes it and ii, they diverge, and this angle of divergence is a function of the length of the skirt portion lb of the control electrode l3. A curve of this relationship is shown in Figure 2 wherein it may be seen that if no skirt portion is provided the electrons will iverge at an angle of approximately 45 and as the skirt length is increased to about" 6 mm. this angle of divergence is reduced to approximately 5.

In the tube shown in Figure 1, however, the grid skirt I5 is made to have a length of approximately 6 mm. so that the beam of electrons is more concentrated as it leaves the control electrode beam limiting aperture. The first accelerating electrode or anode it, as stated above, has no current or beam limiting aperture and the diameter of this electrode or anode, as well as its length, is so chosen that none of the electrons as emitted from the cathode or emitter will strike the walls of the anode. By reason of such construction, no secondary electrons are produced by beam impact on the first anode electrode and substantially no first anode current is present since no primary electrons strike the first anode. The first anode 16, therefore, functions first as a means for accelerating the electron stream and secondly, due to its inter-action with the second anode l1 and the difference of potentials of the two anodes, as an electron beam condensing lens whereby the beam may be brought to a small focal point on the target IS.

The first and second accelerating electrodes being at different positive potentials, envelopes of equi-potential points are, of course, present between the two anodes and these envelopes are represented at 20.

Figure 3 of the disclosure represents a modified form of the invention wherein the same results may be accomplished by slightly different construction. In this modification no skirt or a very short skirt is provided for the control electrode or grid l3 and in order to prevent the electrons from striking the first accelerating electrode or anode l6 this anode is made to have a greater diameter than in the tube of Fig. 1 or in presently used commercial tubes of the cathode ray type. This increased diameter is necessary in view of the fact that the angle of divergence 0 is increased due to the decrease in the grid skirt length. With this construction, the same results are accomplished as are accomplished in the construction shown in Figure 1, namely, the reduction of a first anode current to zero, or substantially zero, in view of the fact that no primary electrons of the beam come in contact with the anode 16.

Where it is mechanically impossible to increase the diameter of the first accelerating anode the modification shown in Figure 1 may then be used and it is also to be understood that various combinations of skirt length and first anode diameter may also be used. In each case the conducting coating I! of the tube which forms the second anode extends into the neck of the tube at least to the plane at the end of the first anode. Obviously, the second accelerating anode I! may be a metallic element appropriately supported within the tube instead of in the form of a conducting coating on the inside surface of the tube.

By using either of the above described methods, or any combination of these methods, it is clear that no first anode current is present and no secondary electrons are created since the electron beam does not at any time come in contact with the walls of the first anode, and furthermore, no secondary electrons are caused by the primary electron stream striking a first anode beam limiting aperture since no such aperture is in either case used. The absence of these secondary electrons causes the focal point of the electron beam at the target to be more easily controlled, to be reduced in diameter, and to be more uniform in intensity, the fringe present in some of the older type tubes being eliminated.

While no specific means for deflecting the developed cathode ray beam across the luminous screen or target has been shown it is, naturally, to be understood that any known means, such as electro-magnetic or electrostatic (or a combination of each) may be used although, in general, the electro-magnetic form is preferred. Further, while the invention has been herein described and shown particularly in connection with electronic tubes of the type wherein a luminous effect is produced by the impact of the electron beam upon the target it is, of course, intended that the invention is equally applicable to electronic tubes for translating optical values into electric signalling impulses, and particularly so when high detail scanning requires an especially sharp focusing of the scanning beam upon the mosaic electrode target of those tubes known by the trade names Iconoscope and/or Iconotron.

Various modifications may be made in my device without departing from the general features and embodiments thereof, and it is to be understood that any and all of such modifications which fall fairly within the spirit and scope of the hereinafter appended claims may be utilized.

We claim as our invention:

1. A cathode ray tube comprising an envelope, electron means for developing a cathode ray beam comprising an electron emitter, a control electrode and an accelerating electrode, said accelerating electrode having a greater diameter than the control electrode, a centrally perforated disc member positioned coaxially within said control electrode and spaced from one end thereof a distance such that the maximum diameter of the developed beam in diverging is maintained less than the diameter of the accelerating electrode, the accelerating electrode comprising a cylindrical member of uniform bore throughout its length and positioned coaxial with respect to the developed beam and overlapping at least a portion of said control electrode.

2. A cathode ray tube comprising an envelope, electrode structure adapted to have voltages applied thereto for developing a cathode ray beam comprising an electron emitter, a control elec trode and an accelerating electrode, said accelerating electrode having a diameter greater than that of the control electrode, means in said control electrode for limiting the intensity of the cathode ray beam, said means being spaced from one end of the control electrode whereby a cylindrical skirt portion extends beyond the plane of said means, said accelerating electrode being tubular in form and of a uniform internal bore, and means for supporting said accelerating electrode coaxial with respect to the developed beam and overlapping at least a portion of said control electrode, the length of said skirt portion and the diameter of the accelerating electrode being so chosen that the beam is prevented from contacting any part of said accelerating electrode.

IOURY G. MALOFF. DAVID W. EPSTEIN. KARL R. WENDT.

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