Cathode-ray tube with variable energy of beam electrons

A cathode-ray tube with variable energy of electrons its beam formed by an electron accelerating means and an electron focusing means which are arranged in tandem along the beam axis. The electron focusing means comprises a magnetic lens with a constant magnetic-field intensity and an electrostatic lens so arranged with respect to the magnetic lens that defocusing of the electron beam by the magnetic lens, due to variations in the energy of the beam electrons, is compensated for. The electrostatic lens comprises two axisymmetric electrodes, the first electrode being electrically associated with the electron accelerating means, along the beam axis, and the second electrode being electrically associated with the screen of the cathode-ray tube.

The present invention relates to electron-beam devices, and more 
particularly to cathode-ray tubes with variable energy of electrons of 
their beams, intended for use, generally, in information display systems. 
A new class of cathode-ray tubes (CRT) has recently come into being, that 
of cathode-ray tubes with variable energy (rate) of electrons in their 
beams, depending on which the color of glow, afterglow duration or other 
characteristics of the tube screen vary accordingly. These tubes operate 
on the principle of different depth of penetration of an electron beam 
into a solid body (in this case, the tube screen) depending on the energy 
of the beam electrons. They are generally known as "penetron" tubes. 
The screen of such a cathode-ray tube is composite, including a number of 
superimposed layers, separated by a barrier layer, of materials exhibiting 
different properties when exposed to an electron beam, e.g. different 
color of glow, different afterglow duration, (darkening) ability. When an 
electron beam impinges upon such a screen, depending on the energy of its 
electrons, i.e. the accelerating voltages, it will penetrate a respective 
layer, exciting it. According to an alternative embodiment of such tubes, 
the screen may be in the form of a single layer including particles coated 
with a number of layers exhibiting different properties. 
In the process of operation of these cathode-ray tubes, the energy of the 
beam electrons has to be varied rapidly with the video frequency , i.e. 
the accelerating voltage has to be varied from a lower level (normally 
within 4 to 8 kV) to a higher level (normally within 8 to 15 kV) or vice 
versa. This is usually followed by defocusing of the electron beam on the 
screen, which necessitates means for compensating for this defocusing. 
In most prior art cathode-ray tubes with variable energy of electrons in 
their beams, an electrostatic lens is used as the main focusing lens. In 
this case, dynamic focusing adjustment of the electron beam is introduced, 
as the energy of the beam electrons varies, consisting of applying a 
focusing signal from a special generator to one of the electrodes of the 
electrostatic lens each time a variation in the beam electron energy 
occurs. 
It is well known that the use of an electrostatic lens as the main focusing 
lens substantially limits the resolution of the cathode-ray tube in which 
it is used, particularly in the case of the electron beam's current 
intensity being 100 muA and over. 
Despite the theoretical possibility of improving the resolution of a 
cathode-ray tube by using a magnetic lens as its main focusing lens, where 
the magnetic lens includes an electromagnetic coil with an inductance of 
about 200 mH, it was not widely used until quite recently because the 
electromagnetic coil varies simultaneously with beam electron energy 
variations. 
A cathode-ray tube with variable beam electron energy is also known, 
comprising a source of electrons which are focused into a beam with the 
aid of an electron accelerating and an electron focusing means, the latter 
including a magnetic lens, as well as an electron beam deflecting means 
arranged downstream of the magnetic lens along the beam axis. 
In this prior art cathode-ray tube, the magnetic lens is built around an 
electromagnetic coil of a special structure. In this case as well as in 
the case of using electrostatic focusing lenses, to effect focusing 
adjustment of the electron beam as the energy of its electrons varies, a 
focusing signal is applied to the electromagnetic coil from a separate 
generator, which complicates the cathode-ray tube control circuit. 
The magnetic lens formed by said electromagnetic coil is not adequately 
short to provide for optimal focusing of the electron beam, this being due 
to the airgap in its magnetic core being made substantially greater than 
in other prior art electromagnetic focusing coils with a view to 
decreasing its inductance. Moreover, despite the inductance of the coil 
being decreased, the limitations imposed on the rate of change of the 
magnetic flux in the coil, hence, on the quick action of the cathode-ray 
tube, cannot be fully eliminated. 
It is an object of the present invention to provide a cathode-ray tube with 
variable energy of electrons in its beam, in which the electron beam can 
be automatically focused on the tube screen each time the energy of 
electrons of this beam varies. 
Another object of the invention is to automatically maintain the invariable 
size of the characters appearing on the screen. 
The principal object of the present invention is to provide a cathode-ray 
tube with variable energy of electrons in its beam, in which the electron 
beam can be automatically focused on the tube screen and the size of the 
characters appearing on the screen can be automatically maintained 
invariable each time the energy of electrons of this beam varies. 
SUMMARY OF THE INVENTION 
This object is attained because in a cathode-ray tube with variable energy 
of electrons in its beam formed by an electron accelerating means and an 
electron focusing means, arranged in tandem along the axis of this beam, 
whih is made to scan a screen with the aid of a beam deflecting means 
arranged downstream of the electron focusing means along the beam axis. 
According to a major feature of the invention, the electron focusing means 
is made up of a magnetic lens with a constant magnetic field intensity and 
an electrostatic lens arranged as follows: The electrostatic lens is 
placed with respect to the magnetic lens so that defocusing of the 
electron beam by the magnetic lens, due to variation in the energy of the 
beam electrons, is compensated for. The electrostatic lens comprises two 
axisymmetric electrodes, the first electrode being electrically associated 
with the electron accelerating means and the second electrode with the 
screen. 
It is expedient that the ends of the electrostatic lens electrodes, facing 
each other, be arranged upstream of the median plane of the magnetic lens, 
along the beam axis. 
It is also expedient that the ends of the electrostatic lens electrodes, 
facing each other, be arranged on either side of the median plane of the 
magnetic lens, the distance therebetween exceeding the diameter of the end 
of the smaller electrode, and that the electrodes are and electrically 
associated with each other through a means for setting up a distributed 
electric field, disposed between said electrodes. 
The means for setting up a distributed electric field should preferably be 
made in the form of a strip of a resistive material, bent along a helical 
line. 
Preferably, the means for setting up a distributed electric field can also 
be made in the form of a number of diaphragms electrically interconnected 
by means of members made of a resistive material. 
It is also advisable that the cathode-ray tube be provided with an 
additional electron beam deflecting means arranged between the electron 
accelerating means and the first electrostatic lens electrode, along the 
beam axis. 
Such a structure of the proposed cathode-ray tube with variable energy of 
electrons in its beam offers a high resolution of about 2,500 lines per 
screen, substantially simplifies the control circuit and improves its 
quick response.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, the cathode-ray tube with variable energy of 
electrons in its beam comprises an axisymmetric for shaping an electron 
beam 1 (FIG. 1), including, enclosed in a glass bulb 2, an electron source 
3 in a rear portion, made up of a cathode heater 4 and a modulator 5. The 
electron beam shaping system also includes an electron accelerating means 
arranged downstream of the modulator 5, that is toward a front portion of 
the bulb along the beam axis, and made up of an accelerating electrode 6 
in the form of a cylinder with holes being made in the sealed ends thereof 
for electrons to pass therethrough 
The electron beam shaping system furthermore includes an electron focusing 
means, incorporating an electrostatic bipotential lens made up of a first 
electrode 7 and a second electrode 8, both being arranged downstream of 
the accelerating electrode 6, along the beam axis, so as to form a gap 
therebetween, and shaped as cylinders. The electron focusing means also 
includes a magnetic lens built around a conventional electromagnetic coil 
9 arranged externally of the bulb 2, with its magnetic core 10 having an 
airgap 11 and accommodating a winding 12. 
The electrostatic lens (7,8) is so arranged with respect to the magnetic 
lens (9-12) as to ensure compensation for defocusing of the electron beam 
by the magnetic lens, due to variations in the energy of the beam 
electrons. In the present embodiment of the proposed cathode-ray tube, the 
electrostatic lens (7,8) has a median plane 13 passing through the middle 
of the gap between the first and second electrodes 7 and 8, facing each 
other with their ends, and arranged intermediate of a median plane 14 of 
the magnetic lens (9-12), passing through the middle of the airgap 11, and 
the accelerating electrode 6. 
The accelerating electrode 6 is electrically associated with the first 
electrode 7 of the electrostatic lens through a jumper 15. The second 
electrode 8 of the electrostatic lens is associated, through metal springs 
16, with a conductive graphite coating 17 known as aquadag applied to the 
inner walls of the bulb 2 and passing into an aluminium coating applied to 
a screen 18 accommodated in the flaring portion of the bulb 2. The screen 
18 is a single-layer structure incorporating particles of a luminescent 
material Y.sub.2 O.sub.2 S:Eu which exhibit red luminescence as the 
electron beam 1 impinges upon the screen 18, the voltage across the screen 
18 being equal to about 6 kV. The layer also incorporates particles of a 
luminescent material (ZnCd)SiCu, each particle being enveloped by a 
barrier layer and exhibiting green luminescence when the voltage across 
the screen 18 becomes equal to 12 kV. 
Arranged externally of the bulb 2 downstream of the electromagnet coil 9, 
along the beam axis, is a primary means for deflecting the electron beam 
1, in the front portion of the bulb 2, built around an electromagnetic 
coil 19 and enabling the screen 18 to be scanned by the electron beam 1. 
Accommodaed in the bulb 2, between the accelerating electrode 6 and the 
first electrode 7 of the electrostatic lens is an additional means for 
deflecting the electron beam 1, made up of two pairs of electrostatic 
deflection plates: vertical deflection plates 20 and horizontal deflection 
plates 21. This additional means is intended to make the electron beam 1 
scan a restricted area of the screen 18, the position whereof is 
determined by the deflecting coil 19 and whose size corresponds to that of 
the character displayed on the screen 18 of the cathode-ray tube. 
The operation of the proposed cathode-ray tube is controlled from a source 
22 of control signals which is coupled, via a beam current control unit 
23, to the modulator 5 and, via a character generator 24, to the 
electrostatic deflection plates 20 and 21. The source 22 is also 
connected, through an orthogonal deflection unit 25, to the deflecting 
coil 19 and, through a high-voltage switch 26, to the screen 13. The 
high-voltage switch 26, in turn, has a high-voltage power supply 27 
connected thereto. 
Independent power supplies 28 and 29 are connected to the accelerating 
electrode 6 and to the electromagnetic coil 9 forming the magnetic lens. 
The cathode heater 4 of the proposed cathode-ray tube is grounded (this 
detail being omitted from the smaller-scale FIGS. 2 and 3). 
The character generator 24 uses a conventional circuit (cf. "Display System 
Technique", Mir Publishers, Moscow, 1970, pp. 326-337 /in Russian); the 
high-voltage switch 26 also uses a conventional circuit (cf. "Electron 
Industr.", France, 1968, No. 112, pp. 293-325). 
Another embodiment of the proposed cathode-ray tube with variable energy of 
electrons of its beam is possible, and this is shown in FIG. 2. The set-up 
is basically similar to the one described above with the difference that 
the bipotential electrostatic lens (in FIG. 1, parts 7,8) has electrodes 
30 and 31 made preferably in the form of a conductive coating applied to 
the inner wall portions of the bulb 2 and in the present spaced by a 
distance which exceeds embodiment, spaced apart by a distance which 
exceeds the internal diameter of the coated portion of the bulb 2. In 
addition, the ends of the electrodes 30 and 31, facing each other, are 
arranged on either side of the median plane 14 of the magnetic lens and 
electrically associated with each other through a means for setting up a 
distributed electric field, inserted between these electrodes, e.g. a 
helically bent strip 32, as shown. 
In this embodiment of the cathode-ray tube, the electrode 30 is again 
electrically associated with the accelerating electrode 6, but through 
spring members 33, while the electrode 31 is an extension of the 
conductive coating 17, and the additional electron beam deflecting means 
is in the form of an electromagnetic coil 34 fitted onto the bulb 2. 
The proposed cathode-ray tube with variable energy of electrons of its beam 
may have a third embodiment basically similar to the second one, the 
difference being that, there are used as the means for setting up a 
distributed electric field, a number of diaphragms 35 (FIG. 3) 
electrically interconnected through spacers 36 made from a resistive 
material. In addition, the first electrode (35,36) of the electrostatic 
lens is made integral with the accelerating electrode 6, while a second 
electrode 37 is shaped as a cylinder with holes made in the sealed ends 
thereof, for the electron beam 1 to pass therethrough and be electrically 
associated with the conductive coating 17 on the inner walls of the bulb 
2, through the springs 16. 
The proposed cathode-ray tube according to the first embodiment (FIG. 1) 
operates as follows. 
As a signal is applied from the electron beam current control unit 23 (FIG. 
1) to the modulator 5 of the electron source 3 and an accelerating voltage 
is fed from the power supply 28 to the accelerating electrode 6, an 
intensity-modulated divegent electron beam 1 is formed between the 
electron source 3 and the accelerating electrode 6. This beam 1 passes 
between two pairs of deflection plates 20 and 21 of the additional 
electron beam deflecting means, whereby it is deflected through an angle 
proportional to the signal applied to the plates 20 and 21 from the 
character generator 24. Then, the deflected but not yet focused divergent 
electron beam 1 passes through the electrostatic lens made up of the 
electrodes 7 and 8. 
Assume now that the screen 18, hence the electrode 8, receives the lower of 
the two rated voltages, the one equal to 6 kV, the ratio between the 
voltages across the electrodes 7 and 8 making up the electrostatic lens 
and the intensity of the magnetic field inside the electromagnetic coil 9 
making up the magnetic lens in such a way that each of the two lenses when 
taken separately, is too weak to independently provide for a sharp image 
of the beam cross-over to appear on the screen 18. A sharp image of the 
cross-over in the form of a spot of minimal size can only be obtained on 
the screen 18 when the electron beam is focused by both the electrostatic 
and the magnetic lenses. 
As is well known, the magnification ratio of a combination of two lenses is 
equal to the product of their individual magnification ratios. Once the 
voltage across the screen 18, hence across the electrode 8 of the 
electrostatic lens, starts to be increased, with a given constant 
distribution of the magnetic-field intensity along the axis of the 
cathode-ray tube, attained by means of the power supply 29, the spot is 
defocused, in which case the focal power of the bipotential electrostatic 
lens increases due to a higher ratio between the voltages across the 
electrodes 7 and 8, while the focal power of the magnetic lens diminishes 
due to a higher rate of electrons of the beam 1 through this lens. The 
voltage across the electrode 8 reaching a certain level, the cross-over 
reappears on the screen 18. Thus, a spot of minimal size can be obtained 
on the screen at two preset voltages. 
Since the required voltages across the screen 18, hence across the 
electrode 8, are usually determined by the luminescent material used, the 
focal power of the combination of lenses is selected accordingly by 
varying the distance between the median planes 13 and 14 of the 
electrostatic and magnetic lenses, respectively. The voltage across the 
electrode 7, as well as the intensity of the magnetic field inside the 
electromagnetic coil 9, make up the magnetic lens. 
By varying the above parameters within a small interval, it is possible to 
regulate the two fixed voltages across the second electrode 8, hence 
across the screen 18, wherewith the electron beam can be automatically 
focused on the screen 18 within a wide range depending on the luminescent 
materials used in the screen. 
As the linear magnification ratio of a combination of electrostatic and 
magnetic lenses practically remains invariable in the case of transition 
from one fixed voltage across the second electrode 8 to the other, this 
combination of lenses will realign the electron beam 1, deflected by the 
additional deflecting means e.g. 20, 21 or 34), with the axis of the 
cathode-ray tube with different voltages across the second electrode 8, 
i.e. when the electron beam 1 is refocused on the screen 18. The 
additional deflecting means deflects the electron beam 1 at a constant 
rate of electrons, with any one of the two fixed voltages across the 
screen 18 owing to this additional deflecting means being arranged 
upstream of the electrode 7, the voltage across which is maintained 
constant in the course of operation of the cathode-ray tube. 
Thus, owing to the use of a combination of appropriately and mutually 
arranged bipotential electrostatic and magnetic lenses, not only stable 
focusing of the electron beam 1 on the screen 18 is attained, unaffected 
by variations in the energy of its electrons between two fixed limits 
(i.e. by varying accelerating voltage), but there is attained also a 
practically invariable sensitivity to deflection of the electron beam 1 by 
the additional deflecting means, which enables characters equal in size to 
be obtained on the screen 18 regardless of the extent to which the voltage 
across the screen 18 varies. 
The cathode-ray tube in accordance with the second embodient (FIG. 2) 
operates in a similar manner. 
The only difference is due to the use of a "geometrically weak" extended 
bipotential electrostatic lens, instead of a "geometrically strong" 
bipotential electrostatic lens as well as to a different mutual 
arrangement of the electrostatic and magnetic lenses. In the first 
embodiment of the proposed cathode-ray tube, the operation of the 
electrostatic and magnetic lenses is spatially separated, while in the 
second embodiment it is spatially brought closer together. 
When one of the rated voltages (U.sub.1), the one higher than that across 
the accelerating electrode 6 and the electrode 30 electrically associated 
therewith, is applied to the screen 18, hence to the electrode 31, the 
electrostatic lens (30) becomes an accelerating one. 
For the sake of simplicity, in the following description of the operation 
of the second embodiment of the proposed cathode-ray tube, the left, 
central and right portions of the electrostatic lens, along the beam axis, 
are designated in the drawing as I, II and III, respectively. 
Since the area corresponding to the portion I of the accelerating 
electrostatic lens is collecting, the electrons of the divergent beam 1 
are acted upon by a radial force directed towards the tube axis. From the 
area corresponding to the portion I, the beam enters that area 
corresponding to the portion II in which a practically uniform electric 
field is set up. Along this portion II, too, the beam 1 is acted upon by a 
radial force determined by the action of the magnetic field of the 
electromagnetic coil 9, making up the magnetic lens, and directed towards 
the tube axis. The area corresponding to the portion III is scattering. 
As a constant voltage U.sub.2 which is lower than the voltage U.sub.1 
applied to the screen 18, hence to the electrode 31, is fed to the 
electrodes 6 and 30, the focal power of the magnetic lens is selected such 
as to enable the beam 1 to be focused on the screen 18 by the combined 
action of the electrostatic and the magnetic lenses. 
An increase in the voltage U.sub.1 is followed by that in the total focal 
power of the collecting and scattering portions I and III of the 
bipotential lens. If the focal power of the magnetic lens were constant, 
the beam 1 would be focused at a point upstream of the screen 18. However, 
the focal power of this lens decreases as the voltage U.sub.1 increases 
since the rate of the electrons passing through the lens varies in 
proportion to the voltage U.sub.1. 
The focal power of the electrostatic lens does not increase proportionally 
with k/U.sub.1 (k is a constant determined by the geometry of the 
electrodes of this lens), but is a more complex function since the rate of 
electrons of the beam 1 steadily increases as the beam 1 passes through 
this electrostatic lens. 
The combined action of the electrostatic lens which gradually becomes 
stronger and the magnetic lens which gradually becomes weaker as the 
voltage U.sub.1 increases, permits practically stable focusing of the spot 
to be obtained on the screen 18 at any value of U.sub.1 provided it is 
within a preset range determined by the luminescent material used in the 
screen 18. This rules out the necessity of employing additional units for 
dynamic focusing adjustment of the electron beam 1 as its energy varies, 
which units would otherwise complicate the control circuit of the 
cathode-ray tube. 
The operation of the cathode-ray tube with variable beam electron energy 
according to the third embodiment (FIG. 3) is in perfect analogy with that 
of the just previously described one. 
The proposed cathode-ray tube with variable energy of electrons of its beam 
permits the electron beam to the automatically focused on the screen at 
two fixed values of the beam electron energy or within a continuous broad 
range (6 to 20 kV and sometimes above) of electron energy variation. 
In all of the above-described embodiments of the proposed cathode-ray tube, 
variations in the beam electron energy may occur both in the case of a 
constant voltage being maintained across the screen and in the case of 
varying the voltage across the cathode heater and accelerating electrode. 
The use in the proposed cathode-ray tube of a magnetic and an electrostatic 
lens makes it possible to obtain a higher resolution, the beam current 
intensity being equal to about 100 muA. Experimentally, a resolution of 
2,500 lines per screen has been obtained with the beam current intensity 
being 100 muA. 
Since in the proposed cathode-ray tube the intensity of the magnetic field 
of the magnetic lens is constant, the latter can be made up of any 
electromagnetic coil or permanent magnet. In the latter case, the power 
supply can be dispensed with. Another advantage offered by the use of a 
magnetic lens with a constant magnetic-field intensity resides in that it 
imposes no additional limitations on the quick response of the cathode-ray 
tube. 
When the proposed cathode-ray tube is used in digital, alphabetic or 
symbolic (also called alphanumeric) information display units, the size of 
a character is maintained constant on the screen as the energy of the beam 
electrons varies.