Note: Descriptions are shown in the official language in which they were submitted.
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
1 138519
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
1138519
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.
