US2266773A

US2266773A - Electron device

Info

Publication number: US2266773A
Application number: US141910A
Authority: US
Inventors: Russell R Law
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1937-05-11
Filing date: 1937-05-11
Publication date: 1941-12-23
Anticipated expiration: 1958-12-23
Also published as: DE757643C; BE428026A; FR837811A

Description

5 Sheets-Sheet l R. R. LAW

ELEGTRON DEVICE Filed May 11, 1937 Dec. 23, 1941.

INVENTOR RUSSE? ATTORNEY Dec. 23, 1941. R. R. LAW

ELECTRON DEVICE Filed Mayu,4 1957 5 Sheets-Sheet 2 INVENTOR Dec. 23, 1941. R, R. LAW f 2,266,773

ELEGTRON DEVICE l Filed May ll, 1957 5 Sheets-Sheet 5 0mm/n50 mamar/0N affzfcmo/v oms/ry /N ,4 mass-avm RUSSE/.L l?. LW

ATTORNEY Dec. 23, i941. R. R. LAW

ELEC/TRON DEVI/CE Filed May 1l, 1957 5 Sheets-Sheet 4 lNVENTOR RUSSELL R. LW BY ATTORNEY Dec. 23, 1941. R. R. LAW

ELECTRON DEVI'GE Filed May 1l, 1937 5 Sheets-Sheet 5 mgm INVENTOR RUSSELL A. LA W W/WW ATTORNEY Patented mec., 23, 1941 FFICE ELECTRON DEVICE Russell R. Law, West Orange, N. J., assigner to Radio Corporation oi' America, a corporation of Delaware AApplication May 11, 1937, Serial No. 141,910

16 Claims.

This invention relates to electronic devices and, in particular, to a cathode ray electron gun and focusing system for cathode ray projection tubes for use in television. To reproduce a picture by television of adequate size, it has been proposed to first produce a small primary image which is in turn projected onto a viewing screen of suitable size'by an appropriate optical system. In such a system, the primary image must be very bright to provide sufficient screen illumination and where the primary image is derived from the energy in an electron beam, a beam of high power is required.

Conventional electron guns of the type commonly used in present day cathode ray tubes are not altogether satisfactory due to the inability of such electron guns to give a suiciently large beam current in the small spot required by the conditions of a small primary image of high line definition.

The design of conventional electron guns for use in cathode ray tubes, such as described in the paper entitled Theory of Electron Gun by Maloi and Epstein in the Proceedings of the Institute of Radio Engineers, vol. 22, No. 12, page 1386 et seq. (Dec. 1934), provides a cathode region lens which produces a rst cross-over near the cathode at relatively low-voltage and a nal focusing electron lens which serves to reimage the first cross-over on the distant fluorescent screen. The use of the term cross-over is in accordance with the terminology used by those skilled in the art and defined on page 1399 of the above referred to article by Maloff and Epstein and locates the points on the axis of an 3 electron beam at which minimum areas of the beam takes place.

In such electron guns, the spot size is dependent upon the control electrode potential, generally the modulation potential superimposed upon a xed potential, due to changes in the size of the first cross-over which is inherent in such designs. Furthermore, the cross-over is inherently large due to the very pronounced eiects of initial velocity of the emitted electrons from the cathode which is present in the formation of a cross-over at low voltage. As a consequence, the` first cross-over increases in size with increasing beam current, thereby producing the objectionable defocusing eifect known as blooming, in

stroys the detail of the picture, but also reduces the contrast range of the reproduced picture. Other distortions arise from the imperfections of the cathode and focusing electrodes alignment and the spherical aberrations present in the electron lenses.

Accordingly, one of the main objects of my invention is to provide a new and improved method and means for producing large images of electrooptically transmitted scenes.

An important object of my invention is t0 provide an improved cathode ray projection tube having a new and improved electron gun.

Another important object of my invention is to provide an electron gun which will give a high beam current in a small spot whose size is substantially independent of the beams modulation for use in cathode ray tubes.

Still another important object of my invention is to provide a cathode ray tube with a. much reduced spherical aberration by the use of magnetic lenses of special design.

Another object of my invention is to provide an electron gun with an improved first cross-over.

A further object of my invention is to provide an electron gun having a rst cross-over electrostatic lens for accurately focusing a large current from a large area cathode into a small crossover.

A still further object of my invention is to provide an electron gun and focusing system in which the full available second anode voltage is used for rst cross-over formation and in which, a small defining aperture ls located at the first crossover for fixing the size and locationof the rst cross-over.

Another object of my invention is to provide an improved way of mounting and aligning the cathode and the modulating and focusing electrodes and anodes.

Yet another object of my invention is a new method of modulating the beam intensity of a high beam current cathode ray tube.

Other objects of my invention will be readily ascertained by reading the following detailed description together withthe drawings.

In the drawings,

Fig. 1 shows schematically in block diagram, a television reproducing system embodying my improved projection tube and method;

Fig. 2 shows diagrammatically one form of my projection tube and focusing system;

characteristics of an electron gun embodying my invention;

Figs.;` 6 and 7 show in detail two embodiments and aligning the elements of one form of my electron gun.

In Fig. 1 is shown a cathode ray tube I in register with an optical system 3, shown conventionally as a single .lens although a more complex system may be used, and a viewing screen 5, which may be ofeither the opaque or translucent type in accordance with viewing the projected image from the projection side or the rear side of the screen. f

'A high current beam of electrons 63 is formed by the electron gun comprising the cathode 25, the electrodes 21, 29, 3I and 33 and the anodes 35 and 31, and projected upon the luminescent screen 39. Under-the impact of the focused beam of electrons light is emitted by the screen 39 which, in turn, is collected by the optical system 3 and focused upon the viewing screen.

'I'he electrodes 29, 3l and 33 are energized by the electrical energy source 9 and maintained at positive potential with respect to the cathode 25. The rst and second anodes 35 and 31 are energized by the potential source I3. The specially shaped magnetic focusing coil I9 is energized by the power supply II and shielded from the de iecting coils 2l and 23 by a magnetic shield 53. The dei'lecting coils are energized by their respective deector circuits I and I1 for causing the beam to trace a path across the screen 39 in mutually perpendicular directions simultaneously and in synchronism with the transmitting scanning apparatus. {The intensity of beam current is modulated bythe video signals by the amplifier 1 which are simultaneously applied to both electrodes 29 and 3l through the coupling condensers 41 'and 49 respectively. Direct coupling can also be used to provide background control or a separate background control tube can be connected to the

resistors

43 and 45 at the junction of these resistors and the condensers 41 and 49, so that the potential of the electrodes 29 and 3l may be varied in accordance with the average integrated light intensity of the entire scene in a similar manner disclosed by the pending Kell application Serial No. 565,226 filed September 26, 1931.

The deflecting and synchronizing circuits, video 'amplifier and power supplies may take the form shown in the complete television receiver shown in U. S. Patent No. 1,975,056, issued to W. L. Carlson on September 25, 1934, and are not described here in detail since any conventional television receiver may be used with my new and improved projection tube.

The electrodes 29, 3I and 33 are all energized with positive voltage in a predetermined increasing order by the source 9 as will be pointed out in more detail below. In the

leads

51 and 59 are provided

resistors

43 and 45 respectively to maintain the proper bias on electrodes 29 and 3| as the beam intensity is varied by the video signals, as describedin the Shoenberg et al. application, Serial No. 745,838, filed September 28, 1934.

The electron gun structure together with my improved magnetic lens will now be described in detail. Before describing the electron gun in detail. however, it is necessary to digress for an yanalysis of the cross-over formation in order to more properly understand how I achieved the aims and objects of my invention.

For a generalized analysis of first cross-over formation we may consider any non-uniform potential field having axial symmetry wherein E=f(r, z). Let the cathode K, Fig. 3, conform to one of the equipo'tential surfaces defined by Ei=f(1', e). In the absence of space charge we may so choose this potential function that all electrons leaving the cathode 'with zero velocity of emission will unite in a common point p at the cross-over. Let us designate the paths of these electrons as principal trajectories. Other electrons leaving the cathode surface with initial velocities of emission will deviate from these principal trajectories by amounts depending upon the magnitude and direction oi. their initial velocities.

Inasmuch as the non-uniform potential func' tion E=f(r, z) is symmetrical about the axis, it constitutes an electron lens. The action of this electron lens is best illustrated by observing its eifect upon a few representative electrons. For example, electrons originating at point o which do not deviate greatly from the principal trajectory will be brought to a common focus at point f. Similarly, electrons originating at adjacent points o' and o" will be focused at points f and f respectively. For small deviations from the principal trajectory the force tending to restore an electron to its particular principal trajectory is everywhere proportional to its displacel ment. Furthermore, the displacement is in turn proportional to the initial radial velocity. Neglecting the effects of initial longitudinal velocity, the deviation of the kth electron from its principal trajectory may be expressed by k=deviation of kth electron from its principal trajectory Ek=initial radial velocity of the kth electron in equivalent volts E=voltage applied to cross-over-forming system f(Z)k=function of Z describing the deviation of the kth electron from its principal trajectory.

all electrons, consequently the radial position of Il the thermally emitted electrons leaving the cathode surface have a Maxwellian velocity distribution, the current contributedby electrons with initial radial velocity components lying between ro and ro+Aro is:

ci d1 a =i Mada 6) The ratio of the current due to electrons with initial radial velocities lying between ra= a'nd ro=ra to the total space current is i'odio I Which yields If ro be expressed in equivalent volts Ero =1 "T (9) Substituting Equation 5 in 9, the current in the cross-over inside the radius 1 is cross-over-forming For purposes of subsequent analysis it is convement to abbreviata Equation 10 as These last two equations are suicient to describe many features of the cross-over. For purposes of illustration let us suppose that a cathode temperature T, a cathode-to-cross-over distance Z, and a potential distribution F are selected such that the coeiiicient In addition, let the-voltage across the first-crossover-forming system assume the values.`1, Erand 4 kilovolts. The parameter aE then assumes the

values

1, 2, and'4 per square centimeter respectively. Let us now see what happens at the cross-over under these conditipns`Fig. 4 shows the variation of current densi-wiper unit totalv space current with radius for these three values of applied voltage computed from Equation 12.

The current density is seen to be greatest in the center and has the maximum values amperes per cm?. At the lower value of the parameter aE, in this case when the applied potential is 1 kilovoltpthe maximum current density is seen'to be only 0.32 In amperes/cm, and

it is observed to drop oi very slowly with radial distance. At the higher value of the parameter aE, in this case when the applied potential is 4 kilovolts, the maximum current density is 1.27 Is amperes/cm?, or four times as great, and drops of! very rapidly with radial distance away from the center. Thus at the higher voltage, the current density at the center of the cross-ever per unit space current is increased because the beam is concentrated into a smaller cross-over.

To deiine sharply the edge of the iirst crossover and prevent radical changes in its size due to defocusing by modulation, it is desirable to use a small deiining aperture located at the cross-over. In contemplation of. this we become interested in determining how the current through a rst-cross-over-dening aperture depends upon the size of the aperture andthe parameter aE. Figure 5 illustrates how the current through a cross-over-deiining aperture varies with the radius of the defining aperture. These plots are computed from Equation 11 for the same values of the parameter aE.

In Fig. 5 it will be observed that larger and larger fractional parts of the total space current may be concentrated into a cross-over of given size as the parameter aE is increased. For example, if as before a=0.001 per square om., 50% of the total space current may be concentrated into a cross-over-defining-aperture 1.68 mm. in diameter with a potential of 1 kilovolt. At 2 kilovolts, 50% of the total space current may be concentrated into a 0.84 mm. aperture. Thus at higher and higher values of voltage, a given fractional part of the total space current can be concentrated into a smaller and smaller crossover-demng-aperture.

In addition to the eiects of different voltages l applied to the cross-over-forming system, it is evldent'that any alteration in cathode temperature, cross-over-forming system geometry, or potential distribution factor which may alter the coeilicient a will have an effect analogous toa change in voltage insofar as concentration of the beam at the cross-over is concerned. For

I -the voltage E applied to the rst-cross-overexample, the curves of Figs. 4 and 5 might be taken to represent a case wherein the applied voltage was constant at one kilovolt and the coeiiicient a assumed the values 0.001, 0.002, and 0.004 per square cm'., respectively. In the light of these observations we would conclude that the cathode temperature T should be kept as low as possible consistent with satisfactory emission,

forming system should be as high as possible must recall that the electrons issuing from the first cross-over are to be reimaged on the .distant screen by a final focusing lens. The usable vaperture of the final focusing lens is limited by'v lts aberrations. As a consequence, the spread :ify the beam emerging from the first cross-over must be kept within the limitsv imposed by the available aperture ofthe final focusing. lens. f

SecondVthe available voltage may be apportioned to the two functions of first-cross-over formation and final' focusingin any desired manner. vThat is, we may use only a-part of the available voltage for first-cross-over formation, reserving the remainder forffinal focusing; or,

ythe entire available potential may be used for lirst-cross-overy formation and final yfocusing may be accomplished by ya'magnetic lens or an f.

, electrostatic lens of the retarding-electrode type.

The significance of rthese two considerations may be evaluatedy in the following manner. useful beam current in a cross-over of radius r ls given by Equation 1'1. For purposes of analysis we may suppose the vcathode current to. be.,30

space-charge limited according to the conventional three-halves-power law. In this event Cathode diametery Cathode-to-cross-over-distan ce) 2 inasmuch as the spread'of the 'beam is directly proportional tothe ratio of the-cathode diameter y tothe cathcde-to-cross-over distance,` this ratio Ls limited by the permissiblev 'beam spread for any particular ,potential distribution. In practice, therefore, IsEm.

Tilev v were formed at some lower voltage, say E1=2 lcilovolts,A the magnification wouldy be and the first-cross-over-deflning aperture would be Hlln'.

in diameter for thev same final spot size. The ratio of the current through the-final aperture to the total space current in the two cases is; however, seem to be ythe same for both cases. That is 4 1'2E=(2.22.)2(v2):(1)2(10) :constant That is, the ratio of beam current to total space rvcurrent is theoretically the rsame. for either a high or a low voltage first cross-over. f

If we assume the total cathode current to be space ycharge limited and'to vary approximately as the three-halvespower of the voltage, We immediately see the benefit to be derived from using highvoltage for first-cross-over formation for any given value of. the coefficient a.r Forl since the ratio of beam current to total space current for a given final spot size is independent of voltage, vthe totalspacecurrent and likewise the beam current vary approximately as the threehalves-power of the voltage applied to the firstcross-over formingr system.'v To return to our preceding example Where 10 kilovolts are avail.

able, we would expect an increase in beam current of A(5)3/2 or approximately 10 fold when we changed from a 2-kilovolt flrst-cross-over formying voltage to one ofthe 10 kilovolts. Provided the permissiblecathode emissiondensity is not If we use full second-anodevoltage for first- :ross-over formation the object and image spaces )f the final focusing lens will have the same iniex of refraction and the magnification will de- Jend simply upon the ratio of object to image iistance. On the other hand, if the first cross- Jver if formed at some voltage E1 which is a frac- ;ional part of the total voltage E2, final imaging 'nay give a demagnication due to the differing .ndices of refraction in the object and image ;paces. Because of this demagniiication, We ;hould be willing to accept a larger first cross- )ver at low voltage. This characteristic may be feadily analyzed if we neglect the shift in posizion of the equivalent thin lens and consider the magnification to be Image distance EE m Object distance E2 exceeded, it is desirable to use all available voltage for first cross-over formation.

vIn Fig.'6 I have shown one electron gun built in accordance with my invention based on the theory I have evolved above.

This electron gun uses full available voltage for first cross-over formation and has a firstcross-over-dening aperture located at the first cross-over. This first-cross-over-defining aperture serves to fix the size of the electron object imaged on the screen by the final focusing lens. The relative voltages applied to the

intermediate electrodes

85, 8l, and 89 determine the potential distribution in the rst-cross-over-forming system. Modulation of the beam current is accomplished by varying the potentials on electrodes and 81. Inasmuch as full second-anode voltage is used for first-cross-over formation, the final focusing-lens object and image space have the same index of refraction and the nal spot size is given by Final spot size:

First cross-over-defning aperture size (Image-distance) (Object distance) The minimum image distance is fixed by the available defiecting power. The maximum object distance is determined by the available aperture of the final focusing lens and the spread of the beam. It is, therefore, desirable to keep the spread of the beam low. It has been pointed out already that the spread of the beam emerging from the rst-cross-over increases with cathode diameter. Consequently, the cathode should be as small as is consistent with the total desired space current at a practical emission density. If we assume 0.5 ampere per square centimeter to be the maximum permissible emission density, then for an electron gun, from which a total space current of 4 milliamperes is desired, the minimum permissible cathode diameter is about l mm. i

The spherical indentation on the cathode improves the performance of the 'rst-cross-over forming system. Such a curved surface, limitedarea cathode also possessesl advantages in assembly in that a suitable spacer may be interposed between the cathode and the first control grid element for accurately positioning the cathode without contaminating the active emitting surface.

Although iinal focusing may be accomplished by either magnetic or retarding type electrostatic lenses, the electron gun illustrated in Fig. 2 Uses a magnetic lens. This choice was based on experimental study which showed that larger aberration-free apertures could be obtained with magnetic lenses than with conventional concentric-cylinder electrostatic lenses. The magnetic ,final-focusing lens illustrated in Fig. 2 is wound vfon a spool of special shape in order to provide a 'more advantageous flux distribution. The shape is such to provide an annular magnetic coil whose inside diameter varies parabolically with the vortex of the parabola toward the axis of the tube. This new and unconventional design of a magnetic focusing lens has made it possible to obtain very small spherical aberration even with relatively large beam diameter. An iron-end plate 53 serves to shield the magnetic lens from the deflecting coils and to prevent interaction between the focusing and deecting fields. Inasmuch asthe spread of the beam emerging from therst-cross-over forming system illustrated in Fig. 6 is about 6 degrees for a beam diameter of 6 mm., the eiective object distance should not ex# ceed 60 mm. since the minimum image distance must be about 160 mm. to give adequate deection sensitivity, the first-cross-over-dening aperture must be about 0.1 mm. in diameter to give a 0.25 mm. spot on the screen. The choice of this final-spot size is based on a consideration of the picture size and number of scanning lines. The picture size is, in turn, iniluenced by the optical system used for projection.

In this gun the indirectly heated cathode 8| is supported from the Wehnelt cylinder 33 by three nichrome uprights 97 spaced 120 around the axis of the cathode 8| and tabs 99. The indirectly heated cathode has a spherical depression which is coated with electron emitting material. In register and in spaced relation to the cathode are four accelerating

electrodes

85, 81, 89 and 9|. Each of the electrodes is apertured and the aperture size decreases continually from electrode to electrode with electrode 9| having the smallest aperture ||3. The electrode 9| is in the form of an annular cylinder closed at .the end nearest the cathode. Three tungsten wires 93 fastened to the other end of the electrode 9| may contact to the coating 31 of the tube I, as shown in Fig. 1. The coating of the bulb which serves as a second anode is a conducting layer and may be, for example, aquadag or a thin layer of silver, or a thin layer of silver which is coated with aquadag, to reduce internal illumination and stray light. The electrodes are held rigidly in alignment and coaxial with one another by the heavy studs IDI, |03, |05, |07 and |09 imbedded in glass heads 95. It will be understood that Fig. 6 is a cross-section, only one of the three beads and series of studs being shown. The other two beads and series of studs are spaced around the periphery of the electrodes at an angle of degrees with each other. In one such gun the apertures had the following sizes and voltages applied:

` Diameter of Electrode aperture Voltage The voltages are all measured with respect to the cathode and all voltages are positive. rIhe high voltage applied to the electrode 9| is through the contact cap 5|, the conducting layer 3l, and the tungstenwire 3d of the tube l, as shown in Fig. 1. The entire electrode assembly of the gun shown in Fig. 6 is supported by three heavy rigid wires |2| spaced approximately 120 apart as shown in Fig. 8 by element 22|. These heavy wires are imbedded in the press |25 of the 'tube wall structure |23.

It will thus be seen that my electron gun structure has a rigid unitary assembly supported from the glass press |25 and prevented from moving under mechanical agitation by the rigid construction and the spring leads 93, which further help to center and position the structure. Accordingly, a projection tube constructed as I have described, may be subjected to heavy mechanical shock without in any way aiecting the alignment of the elements with respect to one another or with respect to the iluorescent screen within the tube. The aperture ||3 is positioned right at the cross-over point in accordance with the theory which I have outlined above.

An alternative form of electron gun is shown in Fig. 7 in which, however, an additional electrode is provided. In this type of gun the apertures are no longer uniformly reduced in size as the order of the aperture position is increased from the cathode. The rst-cross-over-defining aperture, however, is of the same size as that shown in Fig. 6. In the structure shown in Fig. 7, however, the spacing between the electrodes is no longer uniform and the combination of the nonuniform spacing and change in voltage distribution produces the same potential proportional factor so that the cross-over area is substantially the same as that for the system shown in Fig. 6. One such form of gun shown in Fig. 'l had the following aperture sizes, spacing distances and potentials:

Spacing Aperturcs Electrodes diameter frgclfilige' Voltage in mches aperture Inche The electrode |63 has a cylindrical extension |65 coaxial with the apertureA for collecting any secondaryv electrons which might be emitted by` are interposed in the region, of the focusing fields produced between the electrodes. To this end, a V-block has a slot 2|3 cut at the apex of the V perpendicular to the base. trodes 203 and spacers 205 are alternately stacked and maintained adjacent to each other on a spacer support rod 2|9 which passes through the The elecapertures provided in the spacers and the electrodes. The stacked spacers and electrodes are then laid in the V 2|| of the block end and blocks 201 and 2|5 placed one at each end ofv the stacked array. The faces of the end blocks engaging the stacked elements have centrally located holes so that the spacer rod 2|9 may be placed therein for preliminary alignment. The

C clamp 209 is then placed about the assembled and when this has been done, the C clamp is then. .tightened up with considerable pressure.

faces of the C clamp with which the end blockv The 2|5 is in contact, has a copper face 2|1, the purpose of which is to provide a certain amount of elasticity to take up expansion, as will be described hereinafter. The electrodes 203 may have the short stud members 22| -spot welded thereto prior to the assembly or .after the array has been firmly clamped. When the array has been firmly clamped, the C clamp together with the array is removed and glass beads made molten and affixed around the heavy studs, as shown in Fig. 6.

Suitable means may be provided for holding a C clamp to leave the ends of the separators to manipulate the beads and the gas flame. During the process of aftixing the beads to the studs 22| the electrodes and end blocks absorb considerable heat from the flame used to make molten the glass and to heat the studs 22|.. Considerable expansion, therefore, of the assembly takes place, but by the use of the copper disk 2|1, this expansion is prevented from causing warping or distortion of the electrodes, since the copper disk which is soft, yields before the electrodes, which are of solid metal. The use of the copper disk to take up the expansion has the further advantage of preventing excessive pressures .building up between the electrodes and the insulating spacers, and thus, avoids crushing of the spacers, which, if not prevented, would result in misalignment of the electrodes.

When the assembly has been cooled, the C clamp is released and the support rod 2|9 removed, which permits the spacers to drop out, leaving a unitary electrode structure with the only insulating material in the form of the glass beads outside of the region of the electrostatic fields used for focusing.

In place of a V block, parallel cylindrical rods appropriately spaced may be used for aligning the elements, the spacing between the rods being less than the diameter of the electrodes. This method of assembly has the further advantage of avoiding distortions which arise from the usual jig method of assembling electrodes. Where the jig method is used, the jig itself must be heated by the gas flame in order to appropriatelyv fix the electrodes with respect to one another. Such heating, due to unequal expansions of the jig structure, warps the jig structure and introduces undesirable misalignments. The use of my method, however, avoids in the first place, the use of costly jigs and such misalignments.

While by suitable adjustment of the potentials of the electrodes 29, 3| and 33, a linear modulation characteristic may be provided, I prefer to adjust the potentials of these electrodes to provide an approximately square-law modulation characteristic. By providing such a characteristic so that the beam current varies as the square of the modulation potential, an improved contrast ratio of light to dark portions of the picture is obtained. This is 'desirable since it results in a picture having better viewing qualities. The stray light from the room which limits the density of the darkest portions is rendered ineffective by the increased intensity values of the lighter portions of the picture. A further advantage results, likewise, whenever there is a tendency for the material comprising the fluorescent screen, to

- ent.

depart from linear conversion of electronic energy to light energy, or where saturation effects of the screen at high beam current values are pres- Under such circumstances, the non-linear modulation characteristic of the beam tends to compensate for the non-linear conversion characteristic of the screen which has generally a curvature of opposite sign to that of the modulation characteristic. Thus,` the overall effect is one calculated to reproduce the transmitted image with identical density values of the original image.

Having described my invention, claim is:

1. A cathode ray tube comprising an envelope, a cathode, a shield electrode concentric with and surrounding said cathode, and displaced longitudinally from said cathode and said shield and in the order named, a plurality of apertured disk electrodes of uniform diameter with decreasing effective apertures in register with said cathode, each of said disk electrodes being adapted to be maintained at positive potential, an anode supported from the wall of the envelope, and a iluorescent screen supported by the envelope normal to the axis of the cathode.

2. A cathode ray tube comprising an envelope, a cathode, a shield electrode concentric with and what I surrounding and supporting said cathode, and displaced longitudinally from said cathode and said shield and in the order named, a plurality of modulating disk electrodes adjacent to and in register with said cathode, a plurality of focusing disk electrodes in register with said modulating electrodes, said focusing electrodes having decreasing eiiective apertures, an anode supported on the wall of the envelope, and a fluorescent screen supported by the envelope normal to the axis of the cathode.

3. A cathode structure for electron devices comprising an apertured cup-shaped electrode, a plurality of supporting members equi-spaced within the cup-member and ailixed to the planar wall of the cup, a metallic cylinder positioned coaxial with the cup-member and projecting through the aperture, and a plurality of support members engaging the metallic cylinder and each of the supporting members.

4. A cathode structure for electron devices comprising an aperture'd cup-shaped electrode, a plurality of supporting members equi-spaced within the cup-member and amxed to the planar emissive material deposited in the recess of the metallic cylinder.

5. An electrode structure for a cathode ray tube comprising a cylindrical electrode planarly closed at one end, said end being'apertured, a cathode supported from within -the cylindrical electrode and projecting through the aperture, a plurality of apertured disk electrodes in register with the cathode, an apertured cup-shaped electrode in register with the cathode and the apertured disk electrodes, stud members afiixed to each of the electrodes, and supporting means engaging the studs and holding the electrodes in predetermined spaced relation.

6. The method of-assembling disk electrodes wherein spacers are used, which comprises the steps of alternating stacking spacers and the electrodes, lightly compressing the stacked spacers and electrodes, aligning the spacers and electrodes between guides, compressing under great pres-- sure the aligned electrodes and spacers, aiixing radial support arms to each of the electrodes, said radial arms being equally spaced around the periphery of the electrodes, molding a vitreous body about the radial arms to maintain the electrodes'in predetermined spaced relationship, to-

tally releasing the compression and subsequently .removing the spacers.

7. The method of reducing spherical aberration in electron optical systems, which comprises the steps of producing electrons from a source. directing the electrons through progressively increasing accelerating elds to produce cross-over,

' terminating the acceleration at the point of crossover, directing the electrons passing'the crossover point toward a fluorescent screen, and producing intermediate the cross-over point and the iiuorescent screen an electromagnetic ileld Whose intensity varies parabolically as a function of the distance between the cross-over point and screen. 8. The method of reducing spherical aberration in electron optical systems, which comprises the steps of producing electrons from a source, directing the electrons through progressively increasing accelerating fields to produce cross-over, terminating the acceleration at the point of crossover, directing the electrons passing the cross- .over point toward a fluorescent screen, producing intermediate the cross-over point and the uorescent screen an electromagnetic iield Whose intensity varies parabolically as a function of the distance between the cross-over point and the lscreen, producing two mutually perpendicular deiiecting elds for moving the electrons directed toward the fluorescent screen over predetermined scanned areas, and electromagnetically shielding fields from the produced deecting iields.

9. An electrode structure for a cathode ray tube comprising a cylindrical electrode planarly closed at one end, said end being apertured, a cathode supported from within the cylindrical electrode and projecting through the aperture, a plurality of apertured disk electrodes in register with the cathode, said disk electrodes being equispac'ed from each other and having progressively decreasing apertures, an. apertured cup-shaped electrode in register with the cathode and the apertured disk electrodes, stud members aixed to each of the electrodes, and supporting means engaging the studs and holding the electrodes in predetermined spaced relation.

10. An electrode structure for a cathode ray tube comprising a cylindrical electrodeplanarly closed at one end, said end being apertured, a cathode supported from within the cylindrical electrode and projecting through the aperture, a plurality of apertured disk electrodes in register with the cathode, said disk electrodes having equal apertures of the same size and being spaced from each other with progressively increasing distances, an apertured cup-shaped electrode in register with the cathode and the apertured disk electrodes, stud members aflixed to each of the electrodes, and supporting means engaging the studs and holding the electrodes in predeterminedv spaced relation.

11. The method of assembling and uniting the parts of an electron gun having at least two electrodes which comprises supporting said electrodes on an arbor in substantial axial alignment and longitudinally spaced, welding metal anchoring tabs at the adjacent ends of the electrodes with parts of an electron gun having at least two cylindrical electrodes, which comprises supporting said electrodes bearing metal anchoring tabs welded to their adjacent ends on an arbor in axial alignment and longitudinally spaced, and then forming rigid insulator links between said adjacent tabs by applying insulator beads in a viscous state to said tabs to bridge the same, and allowing said beads to harden and then removing the arbor.

13. The method of assembling and uniting the parts of an electron gun having at least two electrodes which comprises welding metal anchoring tabs at the adjacent ends of the electrodes, supporting said electrodes on an arbor in substantial axial alignment and longitudinally spaced, and then forming insulator links by applying insulator beads in a viscous state to bridge said tabs and allowing said beads to harden, and then removing the arbor.

14. The method of assembling and uniting the parts of an electron gun having at least two cylindrical electrodes which comprises welding metal anchoring tabs to the ends ofthe electrodes, supporting said electrodes substantially coaxially on a common rigid support with the tabs on one electrode spaced from but substantially aligned with and adjacent to the tabs on the other electrode, forming rigid insulator links by applying insulator beads in a viscous state to adjacent tabs to bridge the same, allowing said beads to harden, and then removing said electrodes from said common support.

15. The method of supporting and insulating electrodes in exact spaced relationship such that a eld can be established between them which has desired directing properties upon electrons traversing said eld, wherein said electrodes, provided with tabs at their adjacent ends, are rst mounted on jigs in the desired spaced relationship and then adjacent pairs of tabs are bridged by applying viscous beads which are a1- lowed to solidify before said jigs are removed.

16. The method of assembling electrodes having parallel disc portions with aligned passages riphery of the electrodes, molding a vitreous body for an electron stream which comprises the steps about the radial arms to maintain the electrodes of alternately stacking spacers and the elecin predetermined spaced relationship, totally retrodes, aligning the electrodes, clamping the elecleasing the clamping force on the electrodes, and trodes tomaintainvsaid alignment, alxing radial 5 subsequently removing the spacers.

supporting arms to each of the electrodes, said radial arms being equally spaced around the pe- RUSSELL R. LAW.