US3032674A - Electron gun structure for cathode ray tube - Google Patents

Description

y 1962 J. w. SCHWARTZ 3,032,674

ELECTRON GUN STRUCTURE FOR CATHODE RAY TUBE Filed Oct. 50, 1957 2 Sheets-Sheet 1 IN V EN TOR.

ZTORNEX May 1, 1962 J. w. SCHWARTZ ELECTRON GUN STRUCTURE FOR CATHODE RAY TUBE 2 Sheets-Sheet 2 Filed Oct. 50, 1957 AT RNEK 3,032,674 ELECTRON GUN STRUCTURE FOR CATHODE RAY TUBE James W. Schwartz, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Oct. 30, 1957, Ser. No. 693,467 11 Claims. (Cl. 3113-82) This invention is directed to an electron gun structure for use in a cathode ray tube.

A cathode ray tube utilizes an electron gun for providing a narrowbeam of electrons which is scanned over a fluorescent screen Within the tube. The electrons of the beam are derived from a heated thermionic cathode electrode and are urged through an apertured control grid electrode by appropriate positive fields. The control grid electrode may be of a single aperture or a plurality of apertures positioned closely together over a portion of the emitting surface of the cathode. There is a range of operating voltages applied to the control grid electrode, the lowest voltage of which prevents electrons from passing through the control grid aperture. This potential is known as the cut-ofl potential of the tube and is usually a potential negative to that of the cathode electrode. The other potentials, at which the control grid is operated, are more positive than the cut-off potential and are applied to the control grid to vary the electron flow through the control grid aperture.

In normal tube operation, incoming signal pulses are applied between the cathode electrode and the control grid to supply a varying potential diiference between these two electrodes so as to form a corresponding variation in the electron fiow through the control grid aperture. This produces a modulated electron beam which by appropriate electrode structures is focused on the fluorescent screen of the tube. As the beam isdeflected across the surface of the fluorescent screen, its modulation produces a variable light pattern which is an optical reproduction of the incoming signals applied to the tube.

Cathode ray tubes of the type described, which are used for television viewing purposes and which use a cathode control grid arrangement, may require an oper-.

ative potential range of control grid voltage relative to the cathode voltage of 100 volts from cut-01f potential to maximum beam current. A normal control voltage range, however, is in the order of 50 to 70 volts from cut-ofi to maximum beam current. These voltage ranges are excessive and require considerable amplification of the incoming video signal to operate the tube effectively.

Conventional cathode ray tube guns are basically similar to ordinary triode or tetrode amplifier tubes. A negative grid located between the cathode and a positive accelerating electrode controls the electron flow from the cathode by altering the electric field near the cathode. Conventional electron guns for cathode ray tubes exhibit a transconductance of approximately 20 mho. Many vacuum tube amplifier structures produce a transconductance of 100 to 500 times this figure. However, a direct application of receiving tube structure for controlling an electron gun current can not be made. This is due to the additional requirement that the electron gun must form a dense narrow angle beam, which can be imaged to a small spot. Some cathode ray tube guns have been built which employ a fine metal mesh screen on the cathode side of the usual control grid. Such electron guns required only 5 volts of drive and produced peak currents as much as 800 microamperes. However,

3,932,574 a Patented May 1, 1962 close tolerance requirements and critical design considerations and limited resolution have restricted the application of guns of this type for television picture tubes.

It is thus an object of this invention to provide a novel electron gun for a cathode ray tube which has low voltage requirements for the control grid operation.

It is another object of the invention to provide a novel electron gun for a cathode ray tube requiring a minimum change in control grid voltage from cut-ofi to full beam current.

It is a further object of this invention to provide a novel electron gun structure which will exhibit a high transconductance.

It is another object of this invention to provide a novel electron gun design which will exhibit high currents with low voltage control grid drive.

It is a further object of the invention to provide an electron gun having a high transconductance and a picture resolution essentially equivalent to present commercial television picture tubes.

The invention is in a novel electron gun structure for a cathode ray tube utilizing a conventional cathode and two screen covered apertured grids adjacent to the cath ode. The screen-covered grid closer to the cathode is operated at a positive potential and the second screencovered grid is operated at a small negative potential. Thus, a dense virtual cathode forms in the region between the two grids resulting in a very high tube transconductance. Incoming signals are applied to the second screen covered grid to modulate the electron flow through this second grid. Skirt portions adjacent to the second screen covered grid form focusing fields, which converge the electron flow into a cross-over region which may be then imaged on the phosphor screen of the tube. This design has provided peak beam currents of 800 to 1600 microamperes with 2 to 4 volts control grid drive and without noticeably impairing the required resolution for the tube.

FIGURE 1 is a schematic showing of a cathode ray tube utilizing a novel gun structure in accordance with the invention.

FIGURE 2 is an enlarged sectional view of a portion of the tube of FIGURE 1 including the electron gun structure.

FIGURE 3 is a further enlarged sectional view of the cathode control grid region of the electron gun of FIG- URE 2.

FIGURE 3a is a further enlarged section of a portion of the structure of FIGURE 3.

FIGURE 4 is a graphic showing of the operation of the novel gun structure of FIGURES 2 and 3 and under certain specific conditions of gun operation.

As shown in FIGURE 1, a cathode ray tube utilized for television purposes, consists of an evacuated envelope having a bulb portion 10 and a tubular neck portion 12 extending from the smaller end of the bulb 19. Within the neck portion there is mounted a novel electron gun aosaora 3 formed from appropriately designed coils mounted within a neck yoke structure 20.

FIGURE 2 shows the gun portion of the cathode ray tube of FIGURE 1 in greater detail. A cathode-grid electrode assembly is indicated at 22. This structure is shown in greater detail in FIGURE 3 and will be de scribed below. The cathode-grid assembly 22 provides an electron flow which is accelerated through an aperture 24 in the bottom of a cup-like accelerating electrode 26. The electron beam is further accelerated along the axis of the tubular neck portion 12 by a tubular electrode 28 having an apertured wall 30 partially closing the end of the electrode 28 adjacent to electrode 26. The electron beam passes through the aperture 32 in the wall 30 at high velocity toward the fluorescent screen 16 within the bulb portion 10 of the tube. Electrode 23 is operated at substantially 20 thousand volts and provides the ultimate accelerating potential of the tube. A conductive wall coating 34 is electrically tied by spring spacer elements 36 to electrode 28 to provide a substantially fieldfree space within the bulb portion 10 of the tube. The conductive coating 34 extends from a point within the neck 12 into the bulb portion 10 to a point closely adjacent to the phosphor screen 16, as shown in FIGURE 1.

The field of focusing coil 18 causes the electrons flowing through electrode 28 to be converged and focused to a small spot on the phosphor screen 16. Furthermore, the beam may be scanned in a rectangular raster formation by a plurality of scanning coils formed into the neck yoke 20. The structures and operation of coil 18 and neck yoke 20 are well known and need not be further described.

FIGURES 3 and 3a disclose a novel cathode-control grid arrangement, in accordance with the invention, and in greater detail than shown in FIGURE 2. This structure includes a cathode electrode 36 consisting of a hollow metal tube closed by a solid wall portion 38 at one end. The cathode 36 houses a filament 40, which is twisted into a double helix, the turns of which are appropriately insulated from each other and from the walls of the cathode tube 36. The outer surface of cathode end wall 38 is coated with a thermionic electron emitting material 42 of any appropriate type such as a mixture of barium and strontium oxides, for example. This coating 42 provides an electron emission when heated to an elevated temperature by the cathode filament 40.

Cathode tube 36 is supported within a ceramic disk 44, which in turn is fixed within a tubular electrode structure 46. Electrode 46 is closed at one end by a wall 48 having a single central aperture 50 of substantially 0.035 inch overlying the cathode coating 42. The cathode end wall 38 is positioned within the supporting tubular member 46 so that the cathode coating 42 is closely spaced in the order of five mils from the adjacent surface of end wall 48.

Fixed to the end wall 48 of the electrode 46 are a plurality of insulating and conducting rings or apertured disks in a sandwich arrangement. Mounted on the end wall 48 is a first metallic washer 52 having a thickness of 15 mils and a single central aperture 54 of 60 mil diameter coaxially aligned with aperture 50 of end wall 4%. An insulating Washer 56 of 2 mil thick mica, for example, is positioned between electrode 52 and end wall 48. A second insulating washer 58 is fixed between electrode 52 and a second metallic washer 60 of mil thickness having a central aperture 62 with a diameter of 35 mils aligned with apertures 54 and 50 respectively. A much thicker metallic washer 64 having an aperture 65 with a diameter of 100 mils is fixed in contact with washer 60. Insulating washer 58 may be of any appropriate material, such as 2 mil thick mica for example, to insulate electrodes 60 and 64 from electrode 52. These several insulating and conducting washers 56, 52, 58, 60 and 64 are fixed together tightly by any appropriate means, such as for example by a bolt 66, which is threaded into an insulating ring 68 positioned within the tubular electrode 46.

Across the aperture 54 of electrode 52 and in accordance with the invention there is fixed to electrode 52 a first fine mesh screen 70, and across aperture 62 of the electrode washer 60 there is fixed to electrode 60 a second fine mesh screen 72. These mesh screens are those having 500 openings per inch, for example. As shown more specifically in FIGURE 3, the electrode cup 26 is positioned with its central aperture 24 axially aligned with the apertures 50, 54 and 62 of the electrode structures 48, 52 and 60, respectively.

The electrode 46, with end wall 48, is operated at or below the potential of the cathode electrode 36, which is maintained at ground, during tube operation. Electrode washer 52 is operated at a constant positive potential between 25 and volts, for example, while the combined electrode washers and 64 are operated at a potential between 4 volts and +4 volts. Incoming video signals are appropriately amplified by means indicated by an amplifier tube 74, the plate of which is capacitively coupled to the combined electrodes 60 and 64, as shown in FIGURE 3. The second grid screen 72 functions as a control grid, as described below. The accelerating electrode cup 26 is operated in the order of +250 volts.

During tube operation, the first grid screen 70, operated at its postitive potential is a space charge grid, which accelerates the electrons emitted from the cathode surface 42 through the aperture 50 of end plate 48. The portion of end plate 48 surrounding aperture 50 is maintained at a small negative potential and masks peripheral portions of the cathode emitting surface 42 from the accelerating field of the space charge grid screen 70 and thus tends to form the electrons passing through aperture 50 into a collimated beam. Some of the electrons of the beam are collected by the positive first grid screen 70, but most of the electrons pass through the first screen and enter the retarding field between the grid screens 70 and 72, since the control grid screen 72 will be negative in the absence of any signal applied to the electrodes 58 and 60. However, as the control grid screen 72 is driven positively, most of the electrons between the grid screens 70 and 72 will pass through the mesh openings of the second screen 72, although some will be intercepted by the second screen 72.

Electrons passing through control grid scren 72 are converged by an electron lens field formed between electrode Washer 64 and the more positive accelerating electrode 26. As shown in FIGURE 3, washer 64 has substantial thickness axially in the order of 30 mils, so that the field between it and electrode 26 dips into aperture to form a strong converging lens field. This field tends to focus the electrons passing through the control grid screen 72 into a cross-over region, where the electron beam is of greatest density and its cross-sectional area is minimized. This cross-over region is adjacent to aperture 24 of electrode 26 and is imaged on the phosphor screen 16 by the lens system consisting of a prefocusing lens formed between electrodes 26 and 28 and the magnetic lens formed by coil 18.

A typical operation of the novel gun structure of FIG- URES 3 and 3A is shown schematically in FIGURE 4, where the control voltage V of control grid screen 72 is plotted against the beam current values in microamperes. FIGURE 4 represents the operating conditions with voltage V- of the space charge or first grid screen at 50 volts positive and the voltage V of the collimating electrode 48 at ground, or zero voltage. Curve 89 represents the beam current I striking the phosphor screen 16 of the tube. This current is cut ofi when the control grid screen 72 is held at a 3 volts and reaches a maximum when the voltage of control grid screen 72 is around a +3 volts. The curve shows that the maximum drive range for control grid 72 is about 6 volts.

1 kinescope guns of conventional type.

Curve 82 shows the space current flow I to the space charge grid screen 70. This current flow increases slowly as the control grid voltage V of the second grid screen 72 increases in the region below beam cutoff. However, as the control grid voltage V passes zero bias, the current 1- to the space charge screen 70 decreases abruptly. The reason for this is that, below beam cutoff, electrons passing through the space charge grid screen 70 are reflected by the control grid screen 72 and either return to the cathode surface or are collected by the space charge grid screen 70. The abrupt drop in the current to the space charge grid screen 70 is accompanied by the beginning of electron transmission through the control grid screen 72, as its voltage is raised above cut-off bias.

At a few volts positive bias on the control grid screen 72 no electrons return to the cathode surface nor to the space charge grid screen 70. As shown by curve 80, the beam current I to screen 16 saturates at a small positive voltage bias on the control grid screen 72. Curve 84 shows the changes in cathode current 1;; during the increase of control bias on control grid screen 72.

When the control grid screen 72 is positive, it also intercepts electrons from the electron beam. This effect is shown by curve 86 and indicates that the current I to the control grid screen 72 saturates rapidly. This is because the accelerating field of electrode 26 prevents excessive collection of the beam electrons by the grid screen 72. The portion of the beam current lost to the control grid screen 72 is usually less than would be expected from the percentage of open area of screen '72. This is due to a slight mesh focusing action of grid screen 72, which tends to crowd the beam electrons into the mesh openings.

Each mesh opening in the control grid screen 72 serves as a small lens. The strength of the lens depends upon the field strength on both sides of the screen. The square of the transverse velocity imparted to an electron passing close to the edge of a mesh opening depends proportionately upon the field strength and the diameter of the mesh opening. The effect of mesh induced transverse velocities is equivalent to the effect of transverse thermal velocity components at the cathode of a conventional gun. They increase the size of the crossover and consequently limit the tube resolution. Electron guns of the type described above and shown in the accompanying drawings, which use two grid screens of which the control grid screen 72 employs 500 openings per inch, exhibit resolution equivalent to that found in commercial The resolution of guns employing a control grid mesh 72 having 200 mesh per inch, however, is degraded a moderate amount.

In contrast to conventionally operated guns the control grid 72 may be operated with a positive potential relative to ground without any adverse effects in beam focus.

, In conventional guns, operating the control grid close to zero potential and beyond in a positive direction causes the disappearance of the first crossover region and in the manner such that the beam focus is lost entirely and a blooming eifect occurs resulting in loss of picture at the screen. However, the deep grid aperture 65 of electrode 64 retains the convergence field between the control grid 72 and electrode 26 so that electrons passing through the control grid 72 remain converged to a crossover point which can always be imaged on the screen in the manner described.

The cathode loading conditions are somewhat different in the new gun than those usually encountered in conventional electron guns. In conventional kinescope guns, the peak current density, which occurs at the center of the cathode is approximately 2.5 times larger than the av- V erage density over the area from which the emission is drawn. Due to the interception of electrons by the grid screens in the novel gun, the cathode current I is about twice as large for the same maximum beam current. The

emission density, however, is more uniform and in some designs may be spread over a considerably larger area. In the novel gun described, the net cathode current I varies less than 2:1 over the modulation voltage range applied to control grid screen 72. This is in contrast to the infinite ratio existing for conventional guns. This feature has some advantage when the gun electrodes are used as an amplifying stage by using either electrode meshes 70 or 72 as the plate of an amplifying diode.

There are a large number of geometrical parameters that effect double mesh grid gun performance. Among these are the size and thickness of the various apertures and their relative spacings, the mesh sizes, locations and their relative alignments. A complete treatment of the eifect of each variable will not be given here. Reference will be made, however, to some of the more interesting parameters.

An increase in the spacing between the cathode surface 42 and end wall 48 reduces the electric field at the cathode and consequently reduces the beam current. It also tends to make the break point in the beam current I less abrupt and to increase the ratio of the space charge electrode screen current I to beam current I Similar effects are observed in poorly activated tubes, or in tubes purposely operated with reduced cathode temperature.

The size of the aperture 50 is a particularly interesting parameter. If a large opening is employed, emission takes place from a larger cathode area. However, much of the increased current does not enter the beam when electrode 46 is operated at cathode potential. Instead, the currents I and I72 are increased. If a negative potential is applied to electrode 46, it is possible to decrease the I and I currents, while only slightly decreasing the beam current I Further increase in the negative bias applied to electrode 46 ultimately reduces the beam current I as well. Comparing the operating characteristics of a gun of the novel type described and constructed with an aperture 50 of 0.050 inch diameter reveals a sizeable increase in maximum beam current at the same accelerating potential applied to the space charge electrode screen 70.

The operation of electrode 46 with negative bias and an enlarged aperture 50 may be explained as follows: An increase in the size of aperture 50 tends to allow emission from a larger cathode area. This produces a stream of electrons which is too large to pass freely through the opening 62 of control grid electrode 60. Instead, much of the stream is collected by the solid periphery of electrode 60 or is deflected to various parts of the structure of control grid electrode 60. If anegative potential is applied to electrode 46, however, a focusing field is created in the region between the cathode surface 42 and electrode 46. The stream of electrons is focused down so that relatively fewer electrons are collected by the edges of and by the space charge grid aperture 52 and the control grid aperture 62. The cathode current density is also reduced by the application of negative bias to elec trode 46. In addition, some reduction in cathode emission area is effected. At sufficiently negative bias on electrode 46, the later two effects can significantly reduce the current I emitted by the cathode and cancel the gain in beam 'current I Increases in the thickness of aperture 54 of electrode 52 move all of the control characteristics (electrode currents vs. bias of control grid screen 72) toward the positive region of the graph. This is illustrated by operating a gun employing a space charge grid 54 having a thickness of .020 inch instead of .015 inch. This shifts cutoff about two volts in the positive direction.

Reduction of the space charge grid voltage V reduces the accelerating field at the cathode and results in lower cathode emission. All of the tube currents are attenuated in approximately the same proportion when the cathode current 1,; is reduced in this manner. A small shift in cutaoaaera J off and break point voltage accompany the reduction in electrode currents.

What is claimed is:

1. An electron gun for a cathode ray tube, said gun comprising an electron source, an accelerating electrode having an aperture therethrough spaced from said electron source, a plurality of multi-apertured electrodes spaced in alignment between said electron source and said accelerating electrode aperture, and means between said accelerating electrode and the one of said multi-apertured electrodes adjacent to said accelerating electrode for forming a convergent lens between said accelerating electrode and said adjacent multi-apertured electrode for forming electrons passing through said adjacent multi-apertured electrode into a convergent beam.

2. An electron gun for a cathode ray tube, said gun comprising an electron source, an accelerating electrode having an aperture therethrough spaced from said electron source, a plurality of multi-apertured electrodes spaced in alignment between said electron source and said accelerating electrode, a tubular electrode portion extending between the one of said multi-apertured electrode adjacent to said accelerating electrode and said accelerating electrode to form a convergent lens for forming said electrons into a first cross-over, and means for focusing to a small spot electrons passing through said multi-apertured electrodes and said accelerating electrode aperture.

3. An electron gun for a cathode ray tube, said gun comprising an electron source, an accelerating electrode having an aperture therethrough spaced from said electron source, a plurality of multi-apertured electrodes spaced in alignment between said electron source and said accelerating electrode, a tubular electrode portion extending between the one of said multi-apertured electrode adjacent to said accelerating electrode and said accelerating electrode to form a convergent lens for forming electrons passing through said adjacent multi-apertured electrode into a first cross-over, and means for focusing to a small spot electrons passing through said multi-apertured electrodes and said accelerating electrode aperture.

4. An electron gun for a cathode ray tube, said gun comprising an electron source, an accelerating electrode having an aperture therethrough spaced from said electron source, a plurality of multi-apertured electrodes spaced in alignment between said electron source and said accelerating electrode, the one of said multi-apertured electrodes adjacent to said accelerating electrode including a tubular electrode portion extending toward said accel erating electrode to form a convergent lens for forming said electrons into a first cross-over region.

5. An electron gun for a cathode ray tube, said gun comprising an electron source including an electron emitting surface, a multi-apertured control electrode spaced from and overlying said electron emitting surface, an apertured collimating electrode positioned adjacent to said electron emitting surface between said electron source and said multi-apertured control electrode, and a multi-apertured accelerating electrode between said collimating electrode and said control electrode for accelerating electrons from said electron emitting surface through said collimating electrode to a region adjacent to said control electrode.

6. An electron gun for a cathode ray tube, said gun comprising an electron source including an electron emitting surface, a multi-apertured control electrode spaced from and overlying said electron emitting surface, an apertured collimating electrode positioned adjacent to said electron emitting surface between said electron source and said multi-apertured control electrode, and a multiapertured accelerating electrode between said collimating electrode and said multi-apertured control electrode for accelerating electrons from said electron emitting surface through said collimating electrode to a region adjacent to said control electrode, and an apertured accelerating convergence field-forming means on the side of said multiapertured control electrode away from said electron emitting surface to accelerate electrons through said control grid and form said last mentioned electrons into a converging beam.

7. An electron discharge device including an electron gun and a fluorescent screen toward which electrons are directed, said gun including a cathode, a first apertured electron collimating electrode, a multi-apertured second electrode adapted to have a positive voltage applied thereto relative to said cathode, a multi-apertured third electrode adapted to have a negative voltage applied thereto for providing a space charge between said second and third electrodes, an apertured accelerating electrode, and electrode means positioned between said third electrode and said accelerating apertured electrode and including a short tubular member having a passage therethrough whereby a focusing field is produced between said apertured accelerating electrode and said electrode means.

8. An electron gun for a cathode ray tube, said gun comprising an electron source and an accelerating electrode having an aperture therethrough spaced from said electron source, a plurality of mutli-apertured electrodes in alignment between said electron source and said accelerating electrode aperture, a single apertured collirnating grid aligned between said multi-apertured electrodes and said electron source, and means for focusing to a small spot electrons passing through said multiapertured grids and said accelerating electrode apertures.

9. A cathode ray tube comprising an electron gun and a fluorescent screen toward which electrons are directed by said gun, said gun including a cathode electrode, a multi-apertured first accelerating electrode spaced from said cathode electrode, a collimating electrode adapted to be operated near cathode potential spaced adjacent to said cathode electrode and between said cathode electrode and saidfirst accelerating electrode, an apertured second accelerating electrode spaced from said first accelerating electrode on the side thereof away from said cathode electrode, and a multi-aperture control elec-. trode positioned between and aligned with said first and said second accelerating electrodes.

10. A cathode ray tube comprising an electron gun, said gun including a cathode electrode, a multi-apertured first accelerating electrode spaced from said cathode electrode, a collimating electrode adapted to be operated near cathode potential spaced adjacent to said cathode electrode and between said cathode electrode and said first accelerating electrode, an apertured second accelerating electrode spaced from said first accelerating electrode on the side thereof away from said cathode electrode, a multi-aperture control electrode positioned between and aligned with said first and said second accelerating electrode, and means between said control electrode and said second accelerating electrode for forming electrons passing through said control electrode into a converging beam.

11. A cathode ray tube comprising an electron gun, said gun including a cathode electrode, a multi-apertured first accelerating electrode spaced from said cathode electrode, a collimating electrode adapted to be operated near cathode potential spaced adjacent to said cathode electrode and between said cathode electrode and said accelerating electrode, an apertured second accelerating electrode spaced from said first accelerating electrode on the side thereof away from said cathode electrode, a multi-aperture control electrode positioned between and aligned with said first and said second accelerating electrodes, and means including said second accelerating elcc trode and a tubular portion extending from said control electrode toward said second accelerating electrode for converging electrons passing through said second accelerating electrode.

10 References Cited in the file of this patent 2,852,716 Lafierty Sept. 16, 1958 UNITED STATES PATENTS 21,877,369 NiCOll Q. Mar. 10, 1959 2,226,439 Nicoll Dec. 24, 1940 FOREIGN PATENTS 2,318,423 Samuel May 4, 1943 5 107,093 Sweden Apr. 13, 1943 2 1 Skellett p 10, 1946 637,375 Great Britain May 17, 1950 2,426,626 Llewellyn Sept. 2, 1947 2,441,254 Stromeyer May 11, 1948 OTHER REFERENCES 2,469,843 Pierce May 10, 1949 Termain; Radio Engineers Handbook, McGraW-Hill 2,690,517 Nicoll Sept. 28, 1954 10 Book Co., Inc., 1943, page 317.

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