CA1138519A - High potential, low magnification electron gun - Google Patents

Abstract

RCA 72,353 HIGH POTENTIAL, LOW MAGNIFICATION ELECTRON GUN
Abstract The electron gun, which is especially adapted for use in color picture tubes, comprises in the order named:
a cathode, an apertured-plate control grid, an apertured-plate screen grid, and at least two tubular focusing electrodes. The quality of the gun's beam spot is improved by: 1. Establishing an operating electric field between the screen grid and first focusing electrode which is between about 100 and 400 volts/mil (3937-15748 volts/mm), thereby reducing aberration effects in the beam-forming region of the gun; 2. Making the screen grid thick so as to prevent the high first focusing electrode voltage from penetrating the region between the control grid and screen grid, thereby allowing the control grid-screen grid field to provide a divergent effect on the electron beam prior to beam crossover and thus give a reduced crossover angle; 3. Elongating the first focusing electrode to provide an optimum filling of the main focus lens with the beam to maximize the object distance of the focusing system; and 4. Structuring the screen grid and first focusing grid to provide a flat electrostatic field therebetween to avoid prefocusing action in that region, so as not to cause an effective reduction of the object distance of the focusing system.

Description

113851~

~ RCA 72,353 - High Potential, Low Magnification Electron Gun This invention relates to cathode ray tubes, 5 particularly to color picture tubes of the type useful in hom_ television receivers, and to electron guns therefor.
Electron guns typically used in color picture tubes comprise a plurality of aligned electrodes including a i cathode, control grid, screen grid, and two or more focusing 10 electrodes. That portion of the gun up to the screen grid constitutes the beam forming region, and that portion beyond the screen grid constitutes the focusing region. In the operation of these guns, electrons are emitted from the ' cathode and converged to a crossover in the vicinity of the 1~ screen grid. This crossover is then imaged at an image plane ,, on a screen as a small spot by a main focus lens established 't between the focusing electrodes in the focusing region of the gun. The convergence angle at which the electrons approach the crossover is herein termed the crossover entrance angle,

2~ and the divergence angle at which the electrons leave the crossover is herein termed the crossover exit angle. The crossover entrance and ~xit angles would be substantially equal to each other in the absence of any deflection field at the crossover. However, in actual practice the presence 25 of electric fields in this region causes a constant bending of the electron rays as they enter and exit from the cross-over, thus producing a complex crossover and a difference in the entrance and exit angles.
Most workers in the art have generally believed that ~there is little interplay between the beam forming region and the focusing region of the gun; and when attention has been given to one of these two regions for improving the electron gun, usually little note has been given to the other. Notwithstanding this belief in the prior art, we have 35 found that the first crossover, which is imaged on the screen : by the focusing system of the gun, is much further forward in the gun than where it was heretofore believed to be. ~his has in turn led us to realize the interdependence hetween this beam-forming func~ion of the gun and the subsequen~
focusing function of the gun. As a result, we have disco~er~
" ~
, 1-13~9 . .

1 -2- RCA 72,353 that a judicious choice and combination of design parameters of the gun can produce an unexpected improvement in beam-spot performance of the gun.
In accordance with the present invention, the principal characteristics of the novel electron gun, relative to the same class of prior art guns, are a thick screen grid electrode, a strong electric field between the screen grid and first focusing electrode, and/or an increased object distance lOof the main focusing system. To obtain optimum results from these design concepts, it is preferable that prefocusing of ! the electron beam subsequent to the crossover ~e eliminated or at least significantly reduced.
In the drawings, 15 FIGURE 1 is a schematic illustration of a typical electron gun and the general nature of the electron beam-forming and focusing functions thereof.
FIGURE 2 is a schematic elevation view of a cathode ray tube embodying the novel electron gun.
20 FIGURE 3 is a longitudinal elevation, partially in section, of one embodiment of the novel electron gun of FIGURE 2.
FIGURE 4 is an enlarged section of the beam-forming region of the novel electron gun of FIGUR~ 3.
25 FIGURE 5 is an enlarged section similar to that of FIGURE 4, but illustrating for comparison a beam-forming region of a typical prior art gun.
? FIGURE 6 is a view similar to that of FIGURE 5, ~ illustrating another prior art type of electron gun.
30 FIGURE 7 is a graph illustrating the relationship between beam size at the crossover and electric field strength between G2 and G3.
FIGURE 8 is a graph showing the relationship between G2 thickness and G3 length in the novel electron gun.
35 FIGURES 9a-9d are schematic illustrations comparing the beam-forming and focusing action of the novel electron gun with that of the prior art guns.
FIGURES 10 and 11 are section views of alternative embodiments of thick G2 electrodes usable in the novel electrcn gun.

1~3~S19 - 1 -3- ~CA 72,353 FIGURE 1 shows a typical electron gun as discussed J above, including a cathode 2, control grid 3, screen grid 4, and focusing electrodes 5 and 5. The beam forming region is sdesignated 7; the focusing region,8; the cathode-emitted electrons,9; the crossover,10; the screen,ll; the crossover entrance angle, a; and the crossover exit angle,~.
; FIGURE 2 illustrates a rectangular color picture - tube 10 having a glass envelope comprising a rectangular faceplate panel 12 and a tubular neck 14 connected by a rectangular funnel 16. The panel 12 comprises a viewing face-plate 18 and a peripheral side wall 20 which is joined to the funnel 16 with a frit seal 21. A mosaic three-color phosphor screen 22 is disposed on the inner surface of the faceplate 1518. The screen is preferably a line screen with the phosphor ' lines extending perpendicular to the intended direction of high frequency scanning. A multiapertured color selection shadow mask electrode 24 is removably mounted by conventional means in predeter~.ined spaced relation to the screen 22. A
20novel in-line electron gun 26, shown schematically by dotted lines, is centrally mounted within the neck 14 to generate and direct three electron beams 28 along coplanar convergent paths through the mask 24 to the screen 22.
The tube of FIGURE 2 is designed to be used with an 2sexternal magnetic deflection yoke 30 disposed around the neck 14 and funnel 12 in the neighborhood of their junction, for scanning the three electron beams 28 horizontally and vertically in a rectangular raster over the screen 22.
Except for the novel modifications as hereinafter 30described, the electron gun 26 may be of the 3-beam in-line type similar to that described in U.S. Patent No. 3,772,554, issued to R.H. ~ughes on November 13, 1973.
FIGURE 3 is an eleva~ion in partial central long-itudinal section of the 3-beam bipotential gun 26, in a plane 3sperpendicular to the plane of the coplanar beams 28. As such, structure pertaining to but a single one of the three beams is illustrated in the drawing. The electron gun 26 comprises two glass support rods 32 on which the various electro~es are mounted, These electrodes include three equally spaced coplanar cathodes 34 (one for .~ ~

113~S~9 1 -4- RCA 72,353 each beam, only one of which is shown), a control srid ~Gl) electrode 36, a screen grid (G2) electrode 38, a first lens or focusing (G3) electrode 40, and a second lens or focusing (G4) electrode 42. The G4 electrode inciudes an electrical shield cup 44. All of these electrodes are aligned on a centxal beam axis A-A and mounted in spaced relation along the glass rods 32 in the order named. The focusing electrodes G3 and G4 also serve as accelerating electrodes in the bipotential electron gun 26.
Also shown in the electron gun 26 are a plurality of magnetic members 46 mounted on the floor of the shield cup 44 for the purpose of coma correction of the raster produced by the electron beams as they are scanned over the screen 22. The coma correction magnetic members 46 may, for example, be as those described in the above-referenced U.S. Patent No. 3,772,554.
The tubular cathode 34 of the electron gun 26 includes a planar emitting surface 48 on an end wall thereof.
20 The Gl and G2 electrodes include transverse plate portions 50 and 52, respectively, which have aligned central apertures 54 and 56, respectively, therein. The G3 comprises an elongated tubular mem~er having a transverse wall 58 adjacent to the G2, which has a central aperture 60 25 thexein. The G4 r like the G3, comprises a tubular member;
and these two electrodes, at their facing ends, have inturned tubular lips 62 and 64 between which the main focusing lens of the electron gun is established.
In a bipotential form as described above, the 30 novel electron gun 26 may b~ characterized by the following:
l. A strong operating electric field between the G2 and G3 of 100-400 volts/mil (3937-15748 volts/mm), and preferably of 150-250 volts/mil (5906-9843 volts/mm~ to extract a beam of minimum diameter from the crossover.
2. A thick, flat G2 plate portion 52 whose thickness is from 0.4-1.0 times the diameter of the G2 aperture 56 to reduce the crossover angles of the electron beam.

3. An unusually long G3 having a length of 40 2.5-5.0 times the G3 main focus lens diameter to ma~imize 113~51g 1 -5- RCA 72,353 object distance and reduce magnification in the electron gun.
In most cases this will be about 40-60 times the thickness of the G 2 .

4. A G2 which has surrounding its aperture a flat portion whose diameter is equal to or greater than about twice the G2-G3 spacing, to avoid prefocusing of the electron beam.
FIGURE 4 is a greatly enlarged section of the beam-forming region of the novel electron gun 26 . This figure illustrates the nature of the equipotential field lines which are developed between the cathode, Gl, G2, and G3 during operation of the gun; and also the nature o~ the electron paths as they leave ~he cathode, converge to a crossover, and 16 diverge therefrom on their way toward the main focus lens.
Typical of electron guns which operate with a crossover of the beam is the strongly convergent field in the vicinity of th~ cathode and the G1 represented by the field lines 66. These serve to strongly converge the 20 electron rays 68 as they leave the cathode 34 and form them into a crossover 70 from which they then diverge as they proceed toward the main focus lens.
The gun 26 is constxucted with a relatively close G2-G3 spacing and/or operated with a relatively high G3 a5 voltage so as to produce a strong field between the G2 and G3. Such hi~h voltage field from the G3 dips into the aperture of the G2 as illustrated by the equipotential lines 72. However, unlike prior art electron guns in which the G2 electrode may be of substantially the same thickness as that 30 of the Gl, and wherein the high voltage from the G3 penetrates completely through the aperture of the G2, the thick G2 of the present gun is so thick relative to the diameter of the G2 aperture 56, that the field 72 penetrates only part wa~ through the aperture. This in turn allows the 3~ field formed by the Gl voltage, as represented by the field lines 74, to ~ip into the G2 aperture 56 from the Gl side of the G2 and exert a divergent force on the electron rays 68.
This serves to reduce the crossover entrance angle ~ (see FIGURE l) from that which it would otherwise be and to move 40 the crossover 70 farther forward toward the screen than it ~13851~
1 -6- RCA 72,353 would otherwise be. This in turn produces a smaller crossover exit angle ~ and hence a tighter beam bundle as the electron rays 76 diverge from the crossover and proceed toward the main focus lens. At an arbitrarily predetermined distance from the cathode 34, the electron rays 76 are shown to have a relatively small, or tight, bundle 78.
Also characteristic of the novel electron gun 26 is the relatively flat transverse plate portion 52 of the G2.
Such a flat electrode structure results in field lines 82 being established ~etween the G2 and G3 which themselves are relatively flat and void of any significant prefocusing action. The avoidance of a prefocusing action in this region of the electron gun results in a reduced magnification as is hereinafter explained in greater detail.
FIGURE 5 is a greatly enlarged section view, similar to that of FIGURE 4, but of a prior art gun 84 having a conventional thin-walled G2 rather than the thick G2 of the novel electron gun 26. In FIGURE 5 the electxon gun 84 20 comprises a cathode 86, a Gl 88, a G2 90, and a G3 92. The prior art electron gun 84 has the identical electrode spacings and dimensions of the electron gun 26, except that its G2 90 is of a thin plate conventional type as opposed to the thick plate G2 38 of the electron gun 26.
The electron gun 84, like the novel gun 26 of FIGURE 4, exhibits the strongly converging field represented by equipotential lines 94 in the Gl aperture adjacent to the cathode. As with the novel gun 26, this field converges the electron rays 98 leaving the cathode to a crossover 36.
30 However, with the electron gun 84, by virtue o~ the thinner nature of the G2 electrode, the field lines from the high G3 voltage penetrate completely through the aperture of the G2, creating additional convergent action in the region between the Gl and the G2, as illustrated by the field lines lO0.
35 This is in contrast to the field 74 produced in the novel gun 26. The result of this added convergence action is to create a larger crossover entrance angle ~ (see FIGURE 1) and to move the crossover 96 closer to th~ cathode than was the case with the novel electron gun 26. A consequence of this 40 is that the crossover exit angle ~ of the elec~ron rays 102 113~5~9 1 -7- RCA 72,353 emerging from the crossover ~6 is grPater, thus producing at the same predetermined distance from the cathode a less tightly grouped beam bundle 1~4 than the beam bundle 78 of 6 the electron gun 26. The shape of the equipotential field lines 106 between the G2 and G3 in the electron gun 8~ are essentially equivalent to the field lines 82 in the novel electron gun 26. However, the strength of the field may be significantly less than with the novel gun 26.
FIGUR~ 6 illustrates a prior art electron gun 108 which is identical to the prior art gun 84 except for the G2 electrode. The gun 108 includes a cathode 110, a Gl 111, a G2 112, and a G3 113. The G2 is of cup-shaped nature including an upstanding peripheral wall 114. The effect of 1~ the wall 114 is to shape the equipotential lines 115 in the region between the G2 and G3 to produce a prefocusing convergent action on the electron rays 116 as they depart from the crossover 118 of the beam. The result is to convergently bend the rays 116 after they leave the 20 crossover, to produce a tighter beam bundle 120 somewhat similar in size to that of the beam bundle 78 of the novel electron gun 26. However, as will be explained in greater detail hereinafter, achievement of the tight beam bundle 120 in the electron gun 108 does not allow the achievement also a6 of a reduction of magnification equivalent to that achieved by the novel electron gun 26.
It is the prefocusing action produced by the convergent field lines 115 in the region between the G2 and the G3 that the novel gun 26 is designed to avoid. This is 30 accomplished in the novel gun 26 by the avoidance of any structure, such as the upturned lip 114 of the G2 electrode, which curves the field lines 115 from an otherwise relatively flat character in the vicinity of the electron beam rays 116.
FIGUP~ 7 illustrates the relationship between beam 36 spot size and the strength of the electric field between a G2 and G3 of a gun of the general class discussed herein. In FIGURE 7,field strength is plotted against the ratio of the actual beam spot size Scr at the crossover to the theoretical beam spot size 5th at the crossover. The theoretical 40 minimum beam spot size St~ at the crossover is that determin~d 113&S19 1 -8- RCA 72,353 by thermal emission contribution to the crossover spot size.
As illustrated, the spot size ratio drops sharply as the field strength increases from about 150 to 250 volts~mil (5906-9843 volts/mm) EG2_G3,and levels off on either end of this range.
In a typical prior art bipotential gun having a simple single main ~ocus lens such as disclosed in the above-cite~ ~T, S.
Patent No. 3,772,554 there might be provided a G2-G3 spacing Of about 55 mils (1.397 mm), a G3 voltage of about 6000 volts and a G2 voltage of about 600 volts. Such construction and operational parameters results in the gun operating with an EG2 G3 field of about 98 volts~mil (385~ volts/mm). By comparison,typical preferred embodiments of the novel electron gun 26 are preferably provided with G2-G3 spacings of from about 33 to 48 mils (0.838-1.219 mm), a G3 voltage of about 8500 volts and a G2 voltage of about 625 volts, sulting in EG2_G3 fields of from about 239 to 164 volts/mil (9409 - 6457 volts/mm). As shown in FIGU~E 7, the plotted spot size ratio (which is a ~uality measurement of the spot size,with unity being optimum) is about 2.5 for the prior art ~un as compared with about 1.6 for ~he novel electron gun 26 operated with an EG2 G3 field of 239 volts/mil (9409 volts/mm).
26 The spot size ratio improvement from 2.5 to 1.6 would suggest that higher EG2 G3 fields are desirable.
However, in the absence of some compensating changes in the electron gun, the mere increase of the EG2 G3 field results in an accompanying increase in the crossover exit angle ~ of the electron beam,due to a greatly increased convergent field being established in the G2 aperture prior to the crossover and a greatly increased divergent field being established in the G3 aperture subsequent to the crossover.
~ne standard prior art technique for compensatin~ for the 36 increased crossover exit angle has been the establishing of a prefocusing lens between the G2 and G3. However, as hereinafter explained in detail, such a prefocusing field cannot possibly provide an optimum compensation for the increased crossover exit angle.
Another prior art approach for dealing with such an li3~S19 1 -9- RCA 72,353 increase in the crossover exit angle is suggested in U. S. Patent No. 3,995,194 issued to Blacker et al.on November 30, 1976,wnere, in contrast to a simple single lens

5 focusing system, a complex three-lens main focusing system is employed. Such a complex focusing system is, however, costly both from the standpoint of gun construction and provision of the additional operating potentials.
FIGURE 8 is a graph showing crossover exit angles ~
10 and optimized G3 lengths as functions of various G2 thicknesses in an embodiment of the novel electron g~n 26 having a G2 aperture diameter of 25 mils (0.635 mm) and a G3 lens diameter of 214 mils (5.436 mm). The curve shows that as G2 thickness is varied from 10 mils (0.254 mm) or 0.4 times the G2 aperture, 15 to 25 mils (0.635 mm), or 1.00 times the G2 aperture, the crossover exit angle ~ decreases from 0.0675 radian to 0.042 radian. As the crossover ~ngle ~ decreases, the beam diameter is reduce~ and increasingly longerG3 electrodes can be utilized without over-filling the lens with the beam, thus obtaining an 20 increase in the object distance of the focusing system and a corresponding decrease in magnification. The curve also shows that for a G2 thickness of 10 mils (0.254 mm), an optimized G3 length of 550 mils (13.970 mm) is required, and that for a G2 thickness of 25 mils (0.635 mm) an optimized G3 length of 1060 26mils (26.924 mm) is required. The G2 thickness can thus be stated in terms of the ratio of G3 length/G3 lens diameter.
This ratio is seen to vary from 2.57 to 4.95 as the G2 thick-ness varies from 10 to 25 mils (0.254-0.635 mm). A range of suitable G3 lengths thus varies from about 2.5 to 5.0 for the 30suitable variation of G2 thickness of 0.4 to 1.0 times the G2 aperture diameter. From these figures it can also be noted that for this particular em~odiment of the novel gun 26, the optimized G3 lengths vary from about 4~ to 60 times the G2 thickness over the preferred operating range of dimensional 35variations as described hereinO
FIGURFS 9a through 9d schematically illustrate the effects of prior art electron gun design relative to that of the present novel electron gun, with respect to the achievement of a reduced magnification. As is well known in 40the art~ the magnification of an electron yun is expressed ~13~S~9 1 -10- RCA 72,353 by the formula M = p- ~ (1) 5 wherein: a M is the magnification of the beam spot;
Q is the image distance, i.e., the distance between the main focus lens and the image plane on which the beam spot is to be imaged;
P is the object distance, i.e., the distance between the beam crossover and the main focus lens;
Vc is the voltage at the crossover; and 16 Va is the voltage at the anode or image plane.
FIGURE 9a illustrates the nature of the electron beam formation in the novel electron gun 26 wherein electrons are converged from the cathode 34 to a first crossover 70 at a relatively long distance from the cathode and with a rPlatively small crossover entrance angle ~. The electr~ns then diverge from the crossover to a main focus lens MF where they are focused to image the crossover on the anode A. By virtue of a relatively small crossover exit angle ~, the a~ expansion of the beam bundle when it reaches the main focus lens is still relatively small, thus allowing it to operate in the low spherical aberration central region of the lens and produce a relatively unaberxated beam spot on the screen.
Also, because of this relatively small crossover exit angle ~ of the beam, the object distance Pl is relatively large.
Accordingly, relative to prior art guns, a favorable, or reduced, magnification is achieved by virtue of the reduced ratio of Ql/Pl FIGURE 9b illustrates the elfect of attempting to 36 achieve the same magnification with the prior art electron gun 84 by making P2 equal to Pl. Since the gun 84 operates with a larger crosscver exit angle ~, its electron rays diverge rapidly from the crossover 96~ and by the time they reach the main focus lens MF they have expanded to such a 4~ large size that they suffer severe spherical aberrations in RCA 72,353 passing through the lens aperture.
FIGURE 9c illustrates, for the electron gun 84, one attempted solution to the problem described with respect to FIGURE 9b. Here the cathode 86 of the gun is moved closer to the main focus lens MF such that the object distance P3 is reduced, so that the expansion of the beam bundle will not be excessive by the time it reaches the main focus lens.
This, of course, avoids severe spherical aberrations, but results in increased magnification due to a reduced object distance P3 and consequently an increased Q3/P3 ratio.
FIGURE 9d illustrates the attempts of the prior art to solve the problems described with respect to FIGURES
9b and 9c by the use of a prefocus lens in the electron gun 108. Because the electrons leave the crossover 118 with a relatively large crossover exit angle ~, they are prefocused in the region between the G2 and the G3 with the prefocusing lens PF as described with reference to FIGURE 6. The electrons then leave the prefocusing lens PF with a smaller 20 divergence such that, when they reach the main focus lens MF, they are in a relatively tight beam bundle similar in size to that achieved with the novel electron gun 26 (FIGURE 9a~. This would appear to achieve an equivalent magnification since Q4/P4 is equal to Ql/Pl. However, this 2~ achievement i5 fictitous since in the electron gun 108 of FIGURE 9d, focusing is achieved by a pair of lenses, viz., the prefocusing lens PF and ff~ main focus lens MF. These two lenses produce an e~uivalent focusin~ lens EF located between the prefocusing and main focusing lens, thus 30 producing an effective object distance P5 and an effective image distance Q5. The result is a magnification proportional to Q5/P~ which is greater than that achieved by the novel electron gun 26 having a magnification proportional to Ql/Pl as illustrated in FIGURE 9a.
The comparisons discussed with reference to FIGU~ES 9a-9b illustrate the advantage to be achieved in obtaining a tight beam bundle~ not as a ocusing function provided by a prefocusing lens following the G2, hut as a beam-forming function provided in the region of the Gl and 4~ G2~ This advantage is achieved through the use of a high 1 -12- RCA 72,353 EG2 G3 field and a thick G2 relative to the G2 aperture.
In a preferred bipotential embodiment of the invention as incorporated in the novel electron gun 26, the following dimensions, spacings and operating potentials are used:
mils mm Cathode - Gl spacing "a" 3 0.076 Gl thickness "b" 50.127 Gl aperture diameter "c"25 0.635 Gl - G2 spacing "d" 110.279 G2 thickness "e" 200.503 G2 aperture diameter 1fll 25 0.635 G2 - G3 spacing "g" 330.838 G3 aperture diameter "h"60 1.524 G3 length "i" 92523.495 G3 lens diameter "j"2145.436 G4 lens diameter "k"2275.766 G3 - G4 spacing "1" 501.270 volts Cathode cutoff potential 150 Gl potential 0 G2 potential 625 G3 potential 8500 G4 potential 30000 The thick G2 of the novel gun 26 has heretofore been descrihed as comprising a single thick apertured plate 52. However, the apertured plate of the thick G2 may be provided by a stack or lamination of a plurality of thinner 30 apertured plates having their apertures aligned.
For example, FIGURE 10 shows an alternative thick G2 130 comprising a pair of relatively thin apertured plates 132 separated by a spacer 134. The effective thickness of the G2 130 is the distance between the outwardly facing 3~ surface of one of the apertured plates 132 to the oppositely outwardly facing surface of the other plate 132.
FIGURh' 11 illustrates another alternative embodiment of a thick apertured G2 140. The G2 140 comprises a pair of medium thick apertured plates 142 which are 40 abutted flush with one another and wnich ha~e the apertures 113851~

1 -13- RCA 72,353 aligned. The effective thickness of the thick G2 140 is the distance from the outwardly facing surface of one of the plates 42 to the oppositely outwardly facing surface of the other plate 142.

Generally speaking, for a given G3 voltage, the smaller the G2-G3 spacing, the more desirable the electron optical characteristics of the electron gun. As the G2-G3 field is increased toward 400 volts/mil (15748 volts/mm), an increasingly smaller spot size is produced on the screen, all other factors being fixed. For example, a novel gun 26 made with a 33 mil (0.838 mm) G2-G3 spacing, operated at 239 volts/mil (9409 volts/mm) ~G2 G3~ provided a spot size at a given beam current of 2.7S mm, whereas the same gun with a 48 mil (1.219 mm) G2-G3 spacing and at the same EG2 G3 and beam current provided a spot size of 2.95 mm. If the G2-G3 spacing is made so small as to obtain an EG2 G3 greater than 400 volts/mil (15748 volts/mm), a problem of severe voltage instability results, with arc~overs occurring between the G2 and G3 electrodes. An EG2 G3 of 150-250 volts/mil (5906-9~43 volts/mm) has proved to be a pre~erred working range. This range covers the steepest portion of the curve where the most significant adjustment of the beam character 25 is obtained for a given change of field strength. The lower end of this preferred range provides a significant improvement over prior art guns which operate at about 100 volts/mil EG2 G3~ while the upper end of the preferred range stays well clear of any severe voltage breakdown 30 problem.
The diameters of the Gl and G2 apertures are chosen ~ollowing conventional electron gun design criteria.
~onsideration is given to maximum beam current desired, spot size, and drive sensitivity. The thickness of the G2 is 35 then determined in accordance with design criteria of the present teach~ng. A ~2 thickness o~ 0.4-1.0 times the diameter of the G2 aperture has proved to provide the desired divergent action at the entrance to the G2. If the G2 thi~kness is made less than 0.4 times the diameter of the 40 G2 aperture, too little or no divergent action is o~tained.

113~S~5 1 -14- RCA 72,353 As the G2 thickness begins to exceed the size of the G2 aperture, aberration effects become pronounced and the outer electron rays of the beam begin to be directed inwardly to a premature crossover resulting in a defocused beam spot which appears as a dense core having a halo therearound.
Furthermore, as the ratio of G2 thickness to G2 aperture diameter begins to exceed unity, a useless drift region is created through the G2, and the aperture becomes increasingly difficult to fabricate from a grid blank by conventional punching techniques. Thus, the range of 0.4 to 1.0 constitutes a practical range, not only from the standpoint of electron optics, but also from the standpoint of mechanical fabrication procedures.
The length of the G3 is selected so that the electron beam has a diameter in the main focus lens at the far end of the G3 of approximately half or slightly less than half the diameter of the lens-forming opening in the G3 when the gun is operated at an arbitrarily chosen standard 20 highlight drive current of 3.5 milliamps. In a gun having the preferred structural dimensions and operating volta~es set forth above, the electron beam diameter in the main focus lens was about 87.74 mils (2.229 mm), or 0.41 times the diameter of the G3 at the lens when driven at 3.5 milliamps 25 beam current. If the G3 is made longer, the object distance is increased and the magnification thereby further reduced. However, in so doing the beam diameter becomes larger in the lens, and spherical aberration of the lens becomes a greater problem. If the G3 is made shorter, 30 spherical aberration is reduced, but at the sacrifice of an increase in magnification. Designing the ~un to provide the maximum acceptable beam diameter in the main focus lens also obtains the advantage of a less dense beam which suffers less from space charge effects. As the G2 thickness is 35 varied ~rom about 0.4 to 1.0 times the G2 aperture diameter, the crossover exit angle ~ of the beam varies from a~out 0.0675-0.042 radian, so that the G3 length is optimized from a~out 2.5-5.0 times the diameter of the ~,3 lens opening.
Experiments have shown that the 2.5-5.0 relationship 40 ~etween G3 length and G3 lens diameter holds not only for 1138S~9 1 -15- RCA 72,353 25-mil (0.635-mm) G2 apertures (FIGURE 7), but for other suitable aperture sizes as well.
In addition to spherical aberration being a limitin~
factor in allowable beam diameter, so also are distortions which the yoke field produces on the beam cross section if the beam diameter is allowed to become excessively large in the yoke field. This is especially true of the recently developed self-converging, precision-inline type of tube-yoke combinations The reduced crossover angles, as taught herein,require a weaker main focusing lens to image the crossover on the screen. Since the main focus lens is established between the G3 and G4, and since the G4 has the ultor screen potential applied thereto, the G3 voltage must be higher than that of a conventional gun in order to provide the desired weak lens. This has the effect of providing greater penetration of the G3 voltage into the G2 aperture, which theoretically conflicts with the desire to avoid complete 20 penetration to allow creation of the desired divergent field action at the entrance of the G2 aperture. However, this apparent conflict can be compensated for by simply increasing the ratio of G2 thickness/G2 aperture diameter beyond that which would otherwise be required. An advantage of the weak 26 main lens is inherently lower spherical aberration.
Experiments have shown that a Gl-G2 spacing of from 9-15 mils (0.229-0.381 mm) provides an optimum workable range. If the spacing is made greater than 15 mils (0.381 mm), the divergent field at the entrance of the G2 moves into 30 or beyond the crossover~ ~hus failing to obtain the desired effect of a reduction in the crossover entrance angle ~. If this spacing i~ made less than about 9 mils, mechanical tolerance problems resulting in Gl-G2 shorts begin to prevail~ Furthermore, if the spacing is made significantly 35 less than 9 mils, the resultant divergent field at the entrance of the G2 can be strengthened such that the electron beam is so compressed that space charge effects take over and destroy the benefits of the desired small crossover angle. A similar result of too strong a diverging field at 40 the entrance to the G2 occurs if the voltage difference il3~

1 -16- RCA 72,353 between the Gl and the G2 is made too great.
Variations in the strength of the divergent field at the entrance to the G2 aperture, in addition to affecting the size of the crossover entrance angle ~, also have the effect of moving the crossover forward or rearward. However, this movement of the crossover is by a relatively small amount and thus does not become a significant design criterion.
Although the curve of FIGURE 8 calls for a G3 length of slightly less than 900 mils (22.86 mm) for a 25-mil (0.889-mm) G2 aperture, in the specific dimensional data set forth as an example of the novel electron gun 26, a G3 length of 925 mils (23.495 mm) was provided. This additional length 15 was added to the G3 for the p~rpose of achieving an overall structure which would operate properly with a G3 voltage of 8500 volts and with 30,000 volts on the G4~ The departure from optimum G3 length is insignificant considering the trade off of spherical aberration versus magnification.
The novel electron gun structure has been described as comprising a part of a 3-beam inline gun. However, the novel structure may also be embodied in a 3-beam delta gun or in a single-beam gun . Similarly, although described as embodied in a bipotential type gun, the novel structure may 25 be embodied in other types of guns such as those using tripotential or unipotential focusing systems.
For other than bipotential focusing systems, the data given herein for G3 length may not be applicable.
H~wever, appropriate lengths of the focusing ~lectrodes 30 employed may be determined simply by determining the location of the focus lens or lenses such that optimum fllling of the lens or lenses by the electron beam is established.

Claims (11)

RCA 72,353 Claims

1. An electron gun comprising in spaced relation, in the order named, a cathode, an apertured plate control grid, an apertured plate screen grid, an apertured tubular first lens electrode, and an apertured second lens electrode, wherein said screen grid has a thickness of 0.4-1.0 times the diameter of the aperture of said screen grid, and said first lens electrode has a length of 2.5-5.0 times the lens diameter of said first lens electrode.

2. The electron gun of claim 1, wherein said screen grid is structured to establish between said screen grid and first lens electrode a substantially flat electrostatic field which is substantially void of prefocusing action.

3. The electron gun of claim 1, having approximately the following dimensions and spacings:
mils mm Cathode - control grid spacing 3 0.076 Control grid thickness 5 0.127 Control grid aperture diameter 25 0.635 Control grid-screen grid spacing 11 0.279 Screen grid thickness 20 0.508 Screen grid aperture diameter 25 0.635 Screen grid-first lens electrode spacing 33 0.838 First lens electrode aperture diameter 60 1.524 First lens electrode length 925 23.495 First lens electrode lens diameter 214 5.436 Second lens electrode lens diameter 227 5.766 First lens electrode-second lens electrode spacing 50 1.270 RCA 72,353

4. The electron gun of claim 3, adapted for operation with the following electrical potentials:
volts Control grid potential 0 Screen grid potential 625 First lens electrode potential 8500 Second lens electrode potential 30000

5. The electron gun of claim 1, comprising means for establishing an electric field between the screen grid and first lens electrode of 100-400 volts/mil (3937-15748 volts/mm).

6. The electron gun of claim 5, wherein said electric field is about 150-250 volts/mil (5906-9843 volts/mm).

7. The electron gun of claim 6, wherein the screen grid-first lens electrode spacing is from about 33 to 48 mils (0.838-1.219 mm).

8. The electron gun of claim 6, wherein said electric field is about 239 volts/mil (9409 volts/mm) and said screen grid thickness-aperture diameter ratio is about 0.8.

9. The electron gun of claim 8, wherein the screen grid-first lens electrode spacing is about 33 mils (0.838 mm), said screen grid thickness is about 20 mils (0.508 mm), and said screen grid aperture diameter is about 25 mils (0.635 mm).

RCA 72,353

10. The electron gun of claim 1, wherein said grids and electrodes generate a beam of electrons which is converged to a crossover that is imaged by an electron lens at an image plane, and said grids and electrodes are dimensioned and spaced to provide means for reducing the penetration through the screen grid aperture of a high voltage screen grid-first lens electrode field and for establishing a divergent shape to a control grid-screen grid field at the entrance to the screen grid aperture, to reduce the beam crossover entrance angle and thus the spherical aberration experienced by said beam in said electron lens;
means for trading off the reduced spherical aberration in said electron lens for an increased object distance in the focusing system of said gun; and means for establishing a substantially flat electrostatic field between the screen grid and first lens electrode which is substantially void of prefocusing action, whereby maximum object distance is obtained.

11. The electron gun of claim 10, further comprising means for increasing the screen grid-first lens electrode field so as to extract said beam from said crossover with reduced space charge and aberration effects thereon.