Multiple beam electron discharge tube having bipotential acceleration and convergence electrode structure

A multiple beam cathode-ray tube employs a bipotential electrode structure to provide acceleration and convergence of the electron beams without the use of a resistive helix coil. In a preferred embodiment, the bipotential electrode structure (10) is employed in a cathode-ray tube (14) in which a cathode (28) and a grid electrode structure (30) cooperate to form plural beams of high velocity electrons. The bipotential electrode structure includes an immersion lens cylinder (16) that is positioned upstream of a tubular electrode element (18). The outer diameter (226) of the immersion lens cylinder is less than the inner diameter (228) of the tubular electrode element, thereby allowing the downstream end of the immersion lens cylinder to extend into the upstream end of the tubular electrode element. A potential difference applied between the immersion lens cylinder and the tubular electrode element accelerates the electrons in the multiple beams and converges them to form an array of crossovers at a plane (64). Magnetic focus coils (54) image the array of crossovers on a display surface (36). Magnetic deflection coils ( 44) scan the electron beams in a raster pattern across the display surface to form a video display image thereon.

TECHNICAL FIELD 
The present invention relates to electron beam discharge tubes and, in 
particular, to a multiple beam cathode-ray tube that employs a pair of 
adjacent tubular electrode elements to form a bipotential electrode 
structure for accelerating and converging the multiple electron beams. 
BACKGROUND OF THE INVENTION 
Multiple beam cathode-ray tubes generate, scan, and focus a plurality of 
electron beams as a group. Cathode-ray tubes of this type are capable of 
displaying pixel image data of high brightness at relatively high pixel 
data rates. 
Multiple beam cathode-ray tubes typically include a resistive linear helix 
coil that is wound on the inner surface of the tube. A potential 
difference applied to the two ends of the helix coil generates a linearly 
varying potential along the length of the coil. This potential accelerates 
the electron beams and converges them onto an image surface that is 
positioned upstream of a display screen. Focus coils and deflection coils 
generate magnetic fields that, respectively, focus the image surface on 
and scan the electron beams across the display screen. 
Cathode-ray tubes employing helix coils for acceleration and convergence of 
the electron beams typically include a drift tube section that receives a 
relatively low potential. As a consequence, such cathode-ray tubes suffer 
from unacceptable beam-to-beam compression. For increasing amounts of 
electron beam current, beam-to-beam compression is observed on the display 
screen as a narrowing of the vertical distance separating adjacent 
horizontal lines formed by the scan of the electron beams. Moreover, helix 
coils are expensive to manufacture and accumulate electrical charge during 
the operation of the cathode-ray tube. Charge accumulation on a helix coil 
generates spurious electric fields, which degrade the convergence 
performance of the helix coil and, thereby, cause a suboptimal focusing of 
images on the display screen. 
SUMMARY OF THE INVENTION 
An object of this invention is, therefore, to provide a multiple beam 
cathode-ray tube that is relatively inexpensive to manufacture. 
Another object of this invention is to provide such a cathode-ray tube that 
does not employ a helix coil for acceleration and convergence of the 
multiple electron beams. 
A further object of this invention is to provide in such a cathode-ray tube 
compensation for beam-to-beam compression. 
The present invention is a bipotential acceleration and convergence 
electrode structure for use in a multiple beam cathode-ray tube. In a 
preferred embodiment, the cathode-ray tube includes a grid electrode 
structure that forms multiple electron beams an directs them along a drift 
tube section toward the bipotential electrode structure. The bipotential 
electrode structure includes an immersion lens cylinder that is positioned 
adjacent to and upstream of a tubular electrode element. The bipotential 
electrode structure does not employ a resistive helix coil, thereby making 
the cathode-ray tube relatively inexpensive to manufacture. A potential 
difference applied between the immersion lens cylinder and the tubular 
electrode element forms electric fields that accelerate and converge the 
electrons in the multiple beams. 
The bipotential electrode structure of this invention provides stronger 
lensing action than that provided by the combination of an immersion lens 
cylinder and a helix coil. Moreover, the bipotential electrode structure 
allows the application of a relatively large potential to the drift tube 
section, which cooperates with the bipotential electrode structure and the 
grid electrode structure to compensate for beam-to-beam compression 
between adjacent ones of the multiple electron beams. Such a potential on 
the drift tube section requires, however, correspondingly large potentials 
between the electrodes comprising the grid electrode structure. A 
relatively large spacing between adjacent electrodes prevents arcing 
between them. 
Additional objects and advantages of the present invention will be apparent 
from the following detailed description of a preferred embodiment thereof, 
which proceeds with reference to the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENT 
With reference to FIG. 1, a bipotential acceleration and convergence 
electrode structure or means 10 of the present invention is contained 
within an evacuated envelope 12 of a multiple beam electron discharge tube 
14. Bipotential electrode structure 10 comprises an immersion lens 
cylinder 16 and a tubular electrode element 18 that is formed by a 
conductive film 22 on the inner surface of a tubular glass neck 24 of 
envelope 12. 
In a preferred embodiment, tube 14 is a cathode-ray tube with a relatively 
large screen (e.g., 48 cm diagonal) for a television-type monitor. 
Envelope 14 includes a tubular glass neck 24 and a glass funnel 26. A 
cathode 28 positioned within glass neck 24 at one end of envelope 12 
cooperates with a grid electrode structure 30 to form plural narrow 
writing beams of high velocity electrons. 
Grid electrode structure 30 includes four spaced-apart, disk-shaped 
electrodes. The beams of electrons propagate along a central longitudinal 
axis 34 toward a display screen or surface 36 positioned on the end of 
envelope 12 opposite to cathode 28. A layer 38 of phosphorescent material 
is coated on the inner side of display surface 36 to form a fluorescent 
screen for cathode-ray tube 14. Conductive film 22, which is 
electron-transparent, is deposited by evaporation on the inner surface of 
layer 38 of the phosphorescent material to provide a high-voltage 
electrode for display surface 36. Film 22 is also deposited on and extends 
along the inner surfaces of neck 24 and funnel 26 as will be described in 
greater detail below. 
The beams of electrons strike film 22 on display surface 36 to form a video 
image in layer 38 of phosphorescent material . Cathode-ray tube 14 is 
preferably of the magnetically deflected type having a deflection yoke 44 
that includes a horizontal deflection coil and a vertical deflection coil 
that deflect the electron beams in, respectively, the horizontal direction 
and the vertical direction in a conventional raster-scan pattern. 
In a preferred embodiment, grid electrode structure 30 generates a bundle 
of eight individually modulated parallel beams of electrons that propagate 
along central longitudinal axis 34 in neck 24 to display surface 36. The 
eight electron beams exit grid electrode structure 30 in a generally 
circular off-axis array positioned around central longitudinal axis 34 and 
propagate through convergence electrode structure or means 46, which 
directs their propagation paths toward central longitudinal axis 34. 
The electron beams propagate through a drift tube section 48 and converge 
toward the center of a limiting aperture electrode 50. The length of and 
the magnitude of a potential applied to drift tube section 48 affect the 
magnification of the size of the array. The converged bundle of electron 
beams exit limiting aperture electrode 50 and propagate through 
bipotential electrode structure 10. Immersion lens cylinder 16 and tubular 
electrode element 18 of bipotential electrode structure 10 are formed from 
electrically conductive materials and, in the presence of a potential 
difference applied between them, carry different potentials of 
substantially constant magnitude, as described in greater detail below. 
This characterizes electrode structure 10 as having a bipotential. (In 
contradistinction, a potential difference applied to opposite ends of a 
resistive helix coil generates a plurality of voltages along the length of 
the coil.) 
As the beam current increases, bipotential electrode structure 10 
cooperates with grid electrode structure 30 and drift tube structure 48 to 
maintain a uniform vertical distance between adjacent horizontal lines 
formed on display surface 36 by the raster-scanned electron beams. The 
electron beams are accelerated by an accelerating voltage of between 18 
and 25 kV applied to conductive film 22, which extends from display 
surface 36, through funnel 26, and part way into neck 24. The accelerating 
voltage is delivered from the anode (not shown) of cathode-ray tube 14. 
The bundle of beams propagating along the length of neck 24 is subjected 
to conventional electromagnetic correction fields developed by rotation 
coils 54, astigmatism coils 56, and magnetic focus coils 58. 
Immersion lens cylinder 16 and tubular electrode element 18 of bipotential 
electrode structure 10 are axially aligned with central longitudinal axis 
34. The outer diameter of immersion lens cylinder 16 is less than the 
inner diameter of tubular electrode element 18 so that one end of 
immersion lens cylinder 16 extends into tubular electrode element 18. 
Immersion lens cylinder 16 is electrically connected to a DC voltage 
supply 59 of between 600 and 2,000 volts. 
Bipotential electrode structure 10 employs tubular electrode structure 18, 
rather than a helix coil, to provide electron beam acceleration and 
convergence. It is believed that this electrode configuration forms 
between immersion lens cylinder 16 and tubular electrode element 18 
equipotential lines that are more compressed than the equipotential lines 
that would be formed between a similar immersion lens and a helix coil. 
Bipotential electrode structure 10 generates, therefore, lensing action of 
greater strength than that which would be generated by an immersion lens 
cylinder and a helix coil. Certain components of cathode-ray tube 14 are 
designed to offset or weaken this lensing action to provide cathode-ray 
tube 14 with the appropriate amounts of electron beam acceleration and 
convergence. 
The lensing action of bipotential electrode structure 10 is weakened 
primarily by applying a relatively large potential to drift tube section 
48. The relatively large potential applied to drift tube section 48 
requires that relatively large potentials be applied between the 
electrodes forming grid electrode structure 30. The spacing between 
electrodes in grid electrode structure 30 is made relatively large to 
prevent arcing between the electrodes. It is believed that, in addition to 
weakening the lensing action of bipotential electrode structure 10, the 
relatively large potential on drift tube section 48 allows less time for 
space charge interaction between adjacent electron beams. The result is an 
array of electron beams with increased brightness but without noticeable 
beam-to-beam compression. Moreover, in cathode-ray tubes having a 48 cm 
diagonal display screen, bipotential electrode structure 10 provides a 
cathode-ray tube having an overall length that is about 5 cm shorter than 
the length of a cathode-ray tube that employs a helix coil. 
Grid electrode structure 30 includes an exit electrode 60 that has an array 
of apertures through which the eight electron beams propagate toward 
electrode structure 46. The electron beams emitted by cathode 28 propagate 
initially through a first planar grid electrode 62, which forms a first 
array of electron beam crossovers at exit electrode 60. The first array of 
crossovers is made as small as practicable to minimize the amount of 
demagnification that is required to produce on display surface 36 the 
desired vertical distance between adjacent horizontal lines formed by a 
raster scan of the electron beams in the array. 
The voltage applied to drift tube section 48 controls the size of the array 
(i.e., the spacing between adjacent beams in the array) on display surface 
36 by controlling the axial position of a second array of crossovers. For 
example, in a 2000 line, 25.4 cm high display, a first array of crossovers 
of 2.13 mm diameter can be reduced to a diameter of 0.889 mm. The electron 
lens formed by drift tube 48, electrode structure 46, and conductive film 
18 accomplishes this size reduction by causing the array to be demagnified 
at the entrance of the accelerating field of conductive film 18. The 
second array of crossovers is then formed in an image surface on plane 64 
that is located about 2.5 cm into the portion of neck 24 coated with 
conductive film 18. Magnetic focus coils 58, which are positioned 
downstream of image plane 64, image the second array of crossovers onto 
display surface 36. The production of the second array of crossovers 
facilitates a dynamic change in the array size in accordance with the scan 
position of the array. 
The magnitude of the potential difference applied between immersion lens 
cylinder 16 and tubular electrode structure 18, which is substantially 
constant, affects the amount of magnification of the size of the array and 
the axial position of plane 64. Changes in the magnitude of the potential 
difference do occur, however, during the raster scan of the electron beams 
across the display screen. In particular, deflection yoke 44 typically 
produces less magnification of the array at the edges of the display 
screen than at its center. To maintain a constant array size across the 
display screen, a voltage compensating circuit 66 is electrically 
connected to DC voltage supply 59 and adjusts the potential difference 
applied between immersion lens cylinder 16 and tubular electrode element 
18 in accordance with the magnitude of the deflection signal, thereby to 
compensate for magnification variations generated by deflection yoke 44. 
The changes in the magnitude of the potential difference are typically in 
the range of .+-.15% of the total potential difference. The magnitude of 
the potential difference is considered, therefore, to be substantially 
constant. 
An image appearing on display surface 36 is rendered in a conventional 
raster-scan pattern and comprises, therefore, a series of parallel 
stripes. Each stripe includes plural sets of pixels spaced apart by equal 
distances in linear arrays along the length of the stripe. The number of 
linear arrays included in each stripe corresponds to the number of 
electron beams. Each of the linear arrays in a stripe is formed by a 
separate scan of one of the electron beams across display surface 36. Each 
stripe is formed by concurrently scanning the eight electron beams 
horizontally across display surface 36. The stripes in a series are, 
therefore, vertically stacked in raster-scan fashion on display surface 36 
to synthesize an image that comprises a two-dimensional array of pixels. 
FIG. 2 shows the preferred array geometry of eight grid apertures 70, 72, 
74, 76, 78, 80, 82, and 84 that produce on display surface 36 pixel 
elements that are separated by integer multiples of unit pixel spacing in 
both the horizontal and vertical directions. The eight pixel elements in 
the array represent apertures in exit electrode 60 and the other 
electrodes included in grid electrode structure 30. Corresponding 
apertures in the electrodes are axially aligned so that the electrons 
emitted from cathode 28 propagate through the electrodes as a bundle of 
eight electron beams. 
More specifically, the pixel array of grid electrode structure 30 comprises 
eight circular apertures 70, 72, 74, 76, 78, 80, 82, and 84 that are 
arranged in a generally circular off-axis pattern about a center point 88, 
which is coincident with central longitudinal axis 34. Adjacent apertures 
are spaced apart in both the horizontal and vertical directions by 
distances that differ by an integer multiple of a predetermined amount, 
"d," which in a preferred embodiment equals 0.1524 mm. The radius of each 
aperture is the same and equals 0.1524 mm. The horizontal and vertical 
distances between the apertures are shown in FIG. 2. 
Scanning the electron beams horizontally produces eight horizontal lines 
90, 92, 94, 96, 98, 100, 102, and 104 that are vertically spaced apart by 
a distance "2d" at exit electrode 60. The horizontal lines are vertically 
spaced apart on display surface 36 by a distance that is determined by the 
magnification required to provide the desired screen resolution. As was 
described above, the eight lines form a stripe. Whenever the pixel array 
is properly rotated, demagnified, scanned, focused, and astigmatized, 
there is vertically uniform line-to-line pixel spacing. Whenever the eight 
electron beams are individually modulated during a scan of the electron 
beams in accordance with appropriately timed video signals, video images 
are formed on display surface 36. 
FIG. 3 is a cross-sectional view of grid electrode structure 30, which 
produces the eight individually modulated electron beams. With reference 
to FIG. 3, grid electrode structure 30 includes coaxially aligned first 
and second ceramic upper support cylinders 110 and 112, respectively, and 
ceramic lower support cylinder 114, which is separated from cylinders 110 
and 112 by first planar grid electrode 62 and a second planar grid 
electrode 116. A ceramic annular insulator 118 electrically isolates and 
mechanically separates electrodes 62 and 116 so that different voltages 
can be applied to them. 
Lower cylinder 114 supports a cathode support assembly 120 that positions 
cathode 28 proximally adjacent to electrode element 62. Upper cylinder 110 
supports a third planar grid electrode 124, which is separated from grid 
electrode 116 by a relatively large distance so that a relatively large 
voltage difference can be applied between them. Upper cylinder 112 
supports electrode 60 which, as was stated above, constitutes the exit 
electrode of grid electrode structure 30. 
With reference to FIGS. 1-3, electrons emitted from cathode 28 propagate 
through the axially aligned apertures 70, 72, 74, 76, 78, 80, 82, and 84 
in electrodes 62, 116, 124, and 60 to form the eight electron beams. The 
electron beams exit the apertures in electrode 60 and propagate through a 
convergence electrode structure 46, which converges the eight electron 
beams in the manner described below. 
Each of the electrodes 116, 124, and 60 is of a disk shape whose apertures 
70, 72, 74, 76, 78, 80, 82, and 84 are electrically common to one another. 
Electrode 62, which is called the "control grid electrode," is of a disk 
shape but is designed with radial slots so that a different electrical 
voltage can be applied to each of the eight apertures in it. 
Each of electrodes 62, 116, 124, and 60 is preferably formed from a metal 
foil circular disk. Each of electrodes 62, 116, and 124 is of 
approximately 0.0762 mm thickness and 13.284 mm diameter. Electrode 60 is 
of approximately 0.127 mm thickness and 13.284 mm diameter. Each of 
electrodes 62, 116, 124 and 60 is brazed to the ones of ceramic annular 
insulator 118, cylinder 110, and cylinder 112 that separate it from the 
next adjacent electrodes. Annular insulator 118 is approximately of 0.254 
mm thickness and has a 4.572 mm inner diameter. Upper cylinders 110 and 
112 are approximately 1.27 mm in length and have a 10.16 mm inner 
diameter. Lower cylinder 114 is approximately 5.08 mm in length and has a 
5.842 mm inner diameter. 
FIG. 4 shows the details of control grid electrode 62. With reference to 
FIG. 4, control grid electrode 62 is of circular shape and is divided into 
eight wedge-shaped segments 130, 132, 134, 136, 138, 140, 142, and 144 
that have extending outwardly from their outer edges conducting tabs 150, 
152, 154, 156, 158, 160, 162, and 164, respectively. The wedge segments 
are formed by cutting radial slots from the periphery to points near the 
center point 88 of the electrode. The slots bisect the linear distance 
between adjacent apertures but do not extend all the way to center point 
88. Cutting the slots in this manner provides electrical isolation of the 
electron beams passing through the apertures of adjacent wedge segments. 
The terminal points of the slots 170, 172, 174, 176, 178, 180, 182, and 184 
that form segments 130, 132 , 134, 136, 138, 140, 142, and 144 are cut to 
form a generally circular center tab 190 that is connected only to segment 
130. Center tab 190 blocks the flow of electrons emitted from cathode 28 
along central longitudinal axis 34 and prevents them from striking 
electrode 116. The blocking of electron flow by center tab 190 prevents 
unnecessary heating of electrode 116, which would cause secondary electron 
emission from electrode 116 or cause defamation of electrode 116 and a 
consequent misalignment of its apertures with the apertures of adjacent 
electrodes 62 and 124. The impact of electrons on electrode 116 could also 
cause secondary electron emission. 
Slots 170, 172, 176, and 180 define straight lines, and slots 174, 178, 
182, and 184 define dogleg profiles. The reason for the dogleg profiles is 
that lower cylinder 114 has eight slots 192 (FIG. 3--only two shown in 
phantom) positioned in equally spaced angular intervals around its 
periphery. The regions between adjacent slots 192 in lower cylinder 114 
provide individual support surfaces for the wedge segments. The slots cut 
between adjacent apertures near the center of control grid electrode 62 do 
not, however, define wedge segments of equal angular extent because the 
aperture array does not define a true circle. As was stated above, an 
aperture array geometry of this character was required to create the 
horizontally and vertically uniform pixel spacing on display surface 36. 
The dogleg profiles of slots 176, 178, 182, and 184 facilitate, therefore, 
the formation of wedge segments of a size that align with the support 
surfaces of lower cylinder 114. 
Since the wedge segments are electrically isolated from one another, the 
number of electrons propagating through any one of them can be separately 
controlled. This is accomplished by applying a voltage on the conductive 
tab of the desired wedge segment. Each one of the wedge segments of 
control grid 62 is biased at a negative potential relative to the ground 
potential on cathode 28, thereby to provide a standard triode operation. 
Each one of electrodes 116, 124, and 60 receives an applied voltage that 
is common to the apertures in it. Electrode 116 is used to adjust the 
electron beam cutoff voltage. The lowest cutoff voltage of any segment of 
control grid electrode 62 is -20 volts. To accomplish this, a voltage of 
between 200 volts and 500 volts is applied to electrode 116. 
Electrode 124 cooperates with electrode 116 in collimating and accelerating 
the electron beams. The voltage applied to electrode 124 controls the 
divergence of each of the electron beams and thereby affects the 
brightness of the resulting image. The voltage applied to electrode 124 is 
between the voltages applied to electrodes 116 and 60, and typically 
ranges from 100 volts to 500 volts. Varying the voltage on electrode 124 
from 600 volts to 140 volts varies the brightness of the image on display 
surface 36 from minimum brightness to maximum brightness, respectively. 
FIG. 5 shows convergence electrode structure 46 and drift tube section 48, 
which are positioned downstream of and receive the parallel electron beams 
emerging from grid electrode structure 30. Convergence electrode structure 
46 and drift tube section 48 are of cylindrical shape and have their axes 
coincident to central longitudinal axis 34. Convergence cylinder 46 
converges the bundle of eight electron beams toward central longitudinal 
axis 34 as they propagate through limiting aperture electrode 50. 
Convergence is necessary because of the generally circular, off-axis pixel 
array geometry defined by the apertures in electrodes 62, 116, 124, and 60 
of grid electrode structure 30. In the absence of compensation of some 
type, this array geometry would cause a substantial number of the 
electrons in each beam to strike the periphery of the aperture 194 of 
limiting aperture electrode 50. 
A preferred form of compensation entails positioning convergence cylinder 
46 immediately adjacent and downstream of exit electrode 60 and biasing 
convergence cylinder 46 negative relative to electrode 60 and drift tube 
section 48. The resulting electric field developed within convergence 
cylinder 46 can be characterized by equipotential surfaces that develop 
force lines which direct the eight beams toward central longitudinal axis 
34. As a consequence, a substantial number of the electrons in the eight 
beams shift their propagation directions and pass through aperture 194 of 
limiting aperture electrode 50. Passing a substantial number of the 
electrons through limiting aperture electrode 50 results in a reduction in 
beam current loss and thereby provides a brighter display. 
In a preferred embodiment, a potential of between 400 volts and 1500 volts 
is applied to convergence cylinder 46, and a potential of between 600 and 
2000 volts is applied to drift tube section 48, which is electrically 
connected to electrode 60 of grid electrode structure 30. The magnitude of 
the potentials applied to, and the combined length 196 of, convergence 
cylinder 46 and drift tube section 48 affect the magnification of the size 
of the array of electron beams. The combined length 196 of convergence 
cylinder 96 and drift tube section 48 is about 76.96 mm. Convergence 
cylinder 46 and drift tube section 48 are of a length 198 of 7.239 mm, and 
a length 200 of 66.29 mm, respectively. Convergence cylinder 46 is spaced 
apart from electrode 60 by a distance 204 of 1.27 mm and from drift tube 
section 48 by a distance 206 of 1.27 mm. Convergence cylinder 46 and drift 
tube section 48 have inner diameters 210 of about 12.7 mm, and the 
circular aperture 194 in limiting aperture electrode 50 has a diameter 212 
of between 3.175 and 6.35 mm. Slits 214 along the length of drift tube 
section 48 prevent the formation of eddy currents in the magnetic fields 
in the drift tube section. Four glass mounting rods 216 (only two shown) 
provide the support for the components contained in neck 24. 
Immersion lens cylinder 16 is comprised of two cylinder portions 218 and 
220 of different diameters. Cylinder portion 218 has an inner diameter 210 
of 12.7 mm and a length 222 of 7.62 mm. Cylinder portion 220 has an inner 
diameter 224 of 2.286 cm, an outer diameter 226 of 2.54 cm, and is of 
sufficient length to extend about 6.35 mm into tubular electrode element 
18 at its entrance end. Tubular electrode element 18 has an inner diameter 
228 (FIG. 1) of 31.877 cm. Cylinder portion 218 is spaced apart from 
aperture limiting electrode 50 by a distance 230 of 0.889 mm. 
It will be obvious to those having skill in the art that many changes may 
be made in the above-described details of the preferred embodiment of the 
present invention. The scope of the present invention should, therefore, 
be determined only by the following claims.