US2533073A - Cathode beam tube - Google Patents

Description

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CATHODE BEAM TUBE 2 Sheets-Sheet 2 Filed NOV. 28, 1945 53m- .EAT 3 Patented Dec. 5, 1950 CATHODE BEAM TUBE Paul K. Weimer, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 28, 1945, Serial No. 631,440

(Cl. Z50-156) 2 Claims.

This invention relates to television pick-up tubes in which the electrons of the beam are decelerated substantially to zero electron velocity at the target, upon which part of the beam electrons land in proportion to the image charge and the remainder return towards the gun and land on a multiplier dynode. The invention is particularly suitable for television pick-up tubes having two-sided targets, one side of which is scanned by the beam and the other side bombarded by photo-electrons from a photocathode to produce the charge image. This type of tube is disclosed in my application filed September 16, 1944, Serial No. 554,494, now U. S. Patent 2,433,- 941 issu-ed January 6, 1948. The pick-up tube Adisclosed in that application is called an image orthicon and that name will be applied to the tube in this disclosure.

The image orthicon has customarily been operated with a wall coating supplied with about 100 volts and with an electromagnetic focusing eld of about 8() or more gauss, which causes the tube to operate with six or more orders of focus between the first stage of the multiplier and the target. To obtain improved resolution it is desirable to operate the pick-up tulbe with a higher wall coating Voltage, which would give a lower order of focus. Also, in this same image orthicon, it is desirable to use fewer ampere turns for the electromagnetic deflection coils, so as to reduce the power cost in operating the tube. This can be done if the electromagnetic focusing field is reduced in strength, which likewise gives a lower order of focus. When these two changes were made in the past, it was found the lower order of focus caused non-uniform landing of the beam electrons over the target area, with complete failure of the beam to land at the edges of the picture area, or raster.

It is an object of this invention to construct a tube so that there will be uniform landing of electrons over the whole target area.

Another object of the invention is to construct a television pick-up tube in which the beam electrons land on the outer portion of the target with the same velocity as on the inner portions.

Other objects will appear in the following description, reference being had to the drawings, in which:

Fig. 1 is a diagram illustrating the magnetic field of the deflection coil alone of a cathode ray tube.

Fig. 2 is a diagram illustrating the resultant of the deflection field and the focusing iield 'in a cathode ray tube.

Fig. 3 is a diagram illustrating the phase relations at the target of the helical motion of the electrons in a cathode ray tube.

Fig. 4 is an illustration of a television pick-up tube embodying the invention.

Fig. 5 is an end view of the tube of Fig. 1.

Fig. 6 is a modification showing a different form of decelerating lens.

The effect of non-uniform landing of the beam on the target is to produce a dark shading signal obscuring certain sections of the picture. This shading, which tends to occur most prominently near the edges of the picture and diminishes substantially to zero at the center, is due to the failure of the beam to approach suiciently close to the target to deposit any electrons. The nonuniform landing of the beam on the target is associated with the helical motion of the beam.

In its travel toward the target, the electron usually acquires a small transverse velocity v1. perpendicular to the magnetic focusing eld. Its path down the tube is then a helix whose radius is given by the equation where Vt is the transverse velocity component m expressed in Volts and H is the magnetic field in gausses.

The total kinetic energy of the electron at any point is determined by the space potential at that point. Any helical energy must, therefore, be acquired at the expense of the longitudinal energy. If the helical energy is retained as the electron approaches the target, the electron will fail to strike the target by the voltage equivalent of the energy of helical motion. The result is the partial or complete loss of signal in the regions where the beam fails to discharge the target. For a uniformly shaded picture with tubes of the image orthicon type, the beam must land uniformly over the entire target to within a tenth of a volt. At the same time, the helical motion producing non-uniform landing may amount to as much as 5 or 10 volts.

The helical motion contributing to non-uniform landing is of two types; (A) that motion introduced independently of deection, and (B) that introduced by or as a result of deflection. In type A the chief source of helical motion is mechanical misalignment of the gun or multiplier with respect to the tube or to the magnetic field. Other sources may be extraneous electric or magnetic fields whose effects are not symheli@ al motion ser-,erst

v`motie in the other parts of the picture.

metric about the axis of the turbe. In type B two equally important sources of helical motion are the deflection coil and the decelerating lens. In either case the effect is a maximum at the edges of the picture and negligible in the center.

The transverse motion arises in the deflection coil due to the curvature in the magnetic field at the ends of the coils, as shown in Fig. 2. In general, lrhe helical motion is greater for higher voltage on the wall, lower magnetic elds, shorter deflection coils, and greater angles of scan. As shown in Fig. 2, the fringe field of the deflection coil combines with the focusing yfield to give vva resultant nel-d in which thecurvature-,of the bend at the exit end of the coil is greater than at the entrance end, as shown at c, b and c, respectively, for an electron entering along the axis of the tube, although the total vangle of each bend is the saine. The result of this fact is ,thatIaithoughbothbends contribute to the total cal @notion produced by `the `,deflection coil, the second bend produces the `greater eiect and so has more induence in controlling the phase of the resultant helical motion.

The helical .motion produced by the deceleraticn vlens at the edge of the picture is due to .the

components of the electric field perpendicular to the magnetic field. rllhis transverse compojnent produces on the electron a force which is iiadial outward throughout kmost of the deceleration pa i and which rises from zero to a masifails to zero laga-in within the decelera- 'ihe phase and magnitude of the 'i yby the .deceleration lens ,aredependent on the wall coating voltage, election near the tar-get, the magnetic eter of the lens ring, that is, its r the axis of the tube.

u u motions of types A and B discussed z ibove have ldii rent phase relations over the target. ,if the :been ,pdssesses helical motion before deflection (type A), the phase after deflection will appear as indicated by the arrows at A in Fig. 3, assuming that the deection introduces no additional helical motion. The type B motion, illustrated at B in Fig. 3, is such that the V.electron at `all parts of the picture moves in the `same `dirt-ctien (same angle) relative to a line connecting the asis of the tube to that point. At the right ef 3 the resultant of the two eiiecfs .A and Bis shown with elimination of helical motion kOcc 'f ring in the lower right-hand cor-ner,

thearrows indicating the direction of the helical The Yrecognition of the difference of these two types of V*helical motion Vis necessary in order to obtain good however, is that the sources to be balanced be of the same type; that is, in type B the eect of the deection coil may be balanced by effect of the deceleration lens and in type A the effect may be balanced by means of a special alignment coil.

In tubes of the image orthion type, thegun is axially centered in the tube, with a multiplier kedge of the target.

which introduces no transverse electric field on the axis. An alignment coil is necessary only for those tubes in which a m chanical misalignment of the gun or persuader electrode has occurred, or for tubes which are not correctly aligned in the focusing field. It has been found possible to make an electrical correction for these misalig-nments by introducing over the envelope near the gun a weak magnetic field perpendicular .to the axial field and extending about an inch in length, as disclosed in the application of Albert Rose, filed April l1, 1942, Serial No. 438,562, now U. S. Patent 2,407,905, issued September 17, 1946. This results in a bend or kink in the field lines just in front of the gun, which would of itself introduce helical motion of type A. By rotational adjustment of the alignment coil and by variation of current within the coil, an operating condition may be found in which the coil will ycancel the initial misalignment.

lnpractice, the vadjustment of the Valignment coil lis made while .observing `the picture to ascertain when the most .uniform landing in the .center -of the picture is obtained. When the vcorrect compensation has been made with the alignment coil, the remaining non-.uniformity of landing will appear-as ya darkfshading at the edges. This -is due to helical motion of type B introduced by the `deliection .coil `or the deceleration lens, -or both. The dark edges of the picture due .to type B helical motion are vnot objectionable when the wall coating voltage is low and the magnetic field high, giving vmany orders .of focus between `the sun and the target. Such a mode of operation V as above indicated, however, is not ldesirable from the standpoint of resolution and power required for deflection. Under normal voperating condi-- .tions with ve focal loops between gun and target (e. g., with a wall voltage of 127 volts `and magnetic field of 56 gauss), the helical motion introduced by either the deflection coil `or the .deceleration lens .is of theorder of 5 volts at the If uni-form landing towithin 0.1i volt is to ,beachieved at the-target, it is apparent that means are requiredto cancel this motion before lthe beam reaches the target.

it `has been found possible :to obtain uniform land-ing over the kentire picture; by balancing the Aelect of the vdeceleration lens .against lthe deection coil in such a way that the lens transforms of the tube and deiiectioncoil for Aa specified wall voltage and magnetic eldintensity. The correct phase for cance'lation is obtainedl by adjusting manually for best landing the distance of vthe deflection Vcoil from the deceleration lens. This .adjustment may be easily made without` disturbing the focus of the picture and, 'evenV though the magnitudes of the two sources Avmay be slightly mismatched, there is always` a best position of the deflection coil. There may, howevergrbe` more than one position of the deflection .coil which will give the proper phase at vthe deceleration lens for cancelation. These Occur at a ydistance apart equal to A, the distance between the nodesin the beam. This is given by magnetic field in gausses.

It has been found that the balance between the deceleration lens and the deection coil is unchanged if the wall coating voltage and magnetic field are varied together so that remains constant. In general, the balance of these two sources of helical motion is a function of the following factors:

(1) Structural dimensions of the tube between gun and target, with the decelerating ring structure being most critical;

(2) Length, diameter and winding arrangement of the deflection coil;

(3) Uniformity and intensity of the axial magnetic field, especially over the target;

(4) Voltage on the wall coating, decelerating ring and persuader;

(5) Position of deflection coil relative to the target.

The last three factors may be varied while observing the picture, but the positioning of the deflection coil relative to the target oiiers the most satisfactory final adjustment for attainment of good landing. This is due to the fact that this adjustment does not change the focus of the picture, or affect the size of the return pattern to any great extent.

The manner in which the phase balancing is best obtained in cathode ray tubes of the type disclosed in my said application will appear from the description of the drawing.

Referring to Fig. 4, the evacuated envelope I contains a gun 2 at one end thereof with a target 3 and photocathode 4 at the other end.r

The gun 2 consists of the usual indirectly heated cathode 5 (the heater not being shown) surrounded by the control electrode or grid I5. Around the grid is the anode i, the end 8 of which constitutes the rst dynode of the multiplier. In the drawing, the cathode, grid and rst anode are partially sectioned to illustrate their spaced relation and tubular construction. The grid and first anode have the usual minute orifices, as indicated, through which the beam 9 is projected to reach the target with approximately zero velocity. Tubular electrode I3 is axially placed around the front part of the gun adjacent thereto. This is usually referred to as a persuader, because its function is to direct the secondary electrons emitted by the iirst dynode 8 into the succeeding multiplier dynode, as will be hereinafter referred to. Wall coating II is applied'to the inner wall of envelope i in known manner.

Outside the envelope I is placed the defiecting coil unit I2, consisting of a frame I3 having two electro-magnetic coils with their eld axes perpendicular to each other and to the longitudinal axis of the tube. One of these coils deflects the beam in a vertical direction in Fig. 1 and the other defiects it at right angles to the plane of the drawing. These coils are of well known construction and hence have not been individually shown. It will be understood that the deiiection coils will have periodically varying voltages applied thereto, say by saw-tooth generators (not shown) of suitable frequencies, to produce line and frame scansion.

Outside of the envelope I is the compensating coil I4 having a field perpendicular to the axis of the tube, as disclosed in said application of Albert Rose. As shown in Figure 4 coil lli comprises two separate coils connected in series and indicated by the cross-hatched and dotted lines. These two coils are similar to those used in the deilecting unit I2 and are of well known construction. The current passing through the two coils at I4 will provide a magnetic field essentially in a plane perpendicular to the axis of the tube I. By adjusting this coil circumferentially around the tube axis, helical motion of type B can be eliminated. Also, outside the coils I2 and I4 is placed coil I5, which produces a strong magnetic focusing field parallel to the axis of the tube on both sides of the target.

Around the gun is placed a plurality of additional multiplier dynodes I6, I'I, I8 and I 9 and a collecting electrode 20, which is a screen of suitable mesh. This collector electrode is connected to the desired utilization, such as an ainpliiier tube (not shown). The multiplier dynodes are particularly described in my said application and it will be suicient to say respecting the dynodes I6, I1 and I8 that each consists of a disc of metal having angularity displaced radial blades 22, somewhat like an electrical fan, held in a suitable annular frame and having a sufficient axial opening to pass in non-conducting relation over the first anode 'I'.

In front of the blades of the dynodes shown are screens 23 secured to the frames so as to be in electrical Contact with the associated multiplier blades 22, from which they are suitably spaced. The multiplier dynodes I6, Il and' I 3 are partially broken away to show the blades and screens.

The multiplier dynode I9 is the final multiplier stage and consists of a fiat annulus spaced from and surrounding the first anode, similarly to the other dynodes. The multiplier dynodes 8, i6, Il, I8 and I9 may be made of any metal having good electron-emitting properties, or may have a coating of active material to produce suitable emission of secondary electrons upon bombardment by primary electrons. The gun cathode 5 is connected to ground. By way of example, the voltages of the various electrodes are indicated on the drawing, the voltages being in relation to ground.

At the front end of the tube, the photocathode 4 may be a semi-transparent coating of lightsensitive emissive material on the inside of the end of the envelope I. A metal frame 24 supports the thin glass target 3, such as disclosed in the application of Albert Rose, filed September 20, 1940, Serial No. 357,543, abandoned in favor of continuation application Serial No. 631,- 441 filed November 28, 1945, which now is U. S. Patent 2,506,741 issued May 9, 1950, collecting screen 25 spaced from the target, as disclosed in the application of Harold B. Law, filed April 19, 1945, Serial No. 589,241 now U. S. Patent 2,460,093, but it may be spaced and supported in any other desired way. The various electrodes would be supported in the tube envelope in well-known ways, not illustrated.

Adjacent the target 3 is arranged the decelerating ring 26, which is grounded to gun cathode potential or thereabouts. Between the frame 24 and the photocathode 4 is electrode 2l, which is connected to a potential more negative than ground, as disclosed in my said application. The photoelectrons bombarded by the light image from photocathode 4 are accelerated at high electron velocity to the target 3 along the magnetic lines of the focusing coil I5 and bombard secondaries therefrom, which are collected by screen 25. This sets up a charge image on the target 3.

.The'parts thus far described are disclosed in my said 'application and are not, per se, claimed herein.

The .means by which the helical motion Yof type B is balanced out is shown in Figs. 4 and 5. This consists of a rack 28 attached to the frame I3 of the deflection coil. Pinion 2-9 on shaft 30 meshes with `this rack. -Knurled head 3l on shaft 326 permits manual adjustment of the deflection 'coil I2 toward and from the decelerating :lens 126 for balancing out the helical 'motion of type B. These parts are mounted -on 'a cylinder 32 .fitting over the envelope I. A flange 33 having an opening for passage vof rack 28 is Welded to, yorv is .otherwise .fastened to, the end of this cylinder.. The compensating coil I4 is rotatably mounted -o'n a second cylinder 313 Welded or otherwise fastened to flange 33 'outside .rack 2S. The cylinder 32 .is cut at 35 partially around its periphery at 1.thefgu'n end and bent rinto ears 35', through which bolt 3E .is passed, so that the cylinder 432 may be rigidly clamped -into .position fon the yenvelope I.. rFiber gaskets or 'the like (not shown) may be placed under the .clamping ring .to prevent breakage of the envelope. Ears 35a, .El Welded to the cylinder 32, together with the opening .in .ilange 33, act as a guide .for the movable rack 28. Shaft 39 is journaled Ain these years, which also hold .the pinion 129 in position on the rack. .A bracket 33 .may have one end Welded to cylinder .32 .and the tother end secured, as by screws 35, 'to .the .frame of the focusing coil I5.

In the .operation of the tube, the light image .of v.the object to .be transmitted .is .focused .on the pho'tocathode 4 and this -ejects .electrons froni'the photocathode. The electrons .are accelerated `un- -der action `of relatively positive electrode 2l and screen 2.5 along the lines of the electro-.magnetic focusing ield and strike glass target. .Secondary electrons are bombarded therefrom and are collected -by screen 25. "I'hus, a .charge vimage .is .set up fon the side of the target lreinote rfrom the :gun 'l2 representative 7of the light image. This .sets up a potential pattern at the other side .of the target and electrons from beam 9 .land thereon `in sufficient number to discharge the charges on 'the etherside -adjacentscreen 215. This discharge is laccomplished by passage through 'the glass target which .is .suicienitly .conductive for '.-tran'sverse currents, but or" .sufc'ient resistance :to prevent passage of the electrons or spreading of the .potentials transversely .over the target, as explained in said .Rose and Lavv applications. The beam =9, modulated by .loss of the electrons, returns toward the gun 2 and lands on the ilrst dynode 8.. The

secondary electrons bombarded therefrom pass through the screen 2.3 under influence of the potential thereof, as Well as the potential `of persuader electrode .I-Sl, and bombard from the vanes 22 secondary .electrons inmultiplied number, which are attracted into the third multiplier stage. VThis is repeated in the remaining stages until secondary electrons from the dynode 1.9 ofthe ilnal stage in greatly increased number .are collected by vscreen electrode .2D and passed to lthe .ampliiers (not shown) and thence to a local `kineoscope in which the .picture canbe observed. The amplified signals, 'of course, are then trans- L-mitted toa distance for reception as desired.

As already indicated, .'the -coil if: will be :manuvally rotated until the best picture `at the middle or" the target is obtained .in the local kineoscope. This eliminates fthe typeAh'elical motion. Then, vknurled head 3l is turned back :and .forth `until the .picture becomes y'clear all the Way to the outer 'edges by adjustment .of the vdeflection -coil I2 toward orfrom the decelerating lens 26. This eliminates or greatly reduces the type B helical motion.

It will be apparent that the deflection coil 'may be moved by various .mechanisms Without departing from the scope of the invention.

In the modification shown in Fig. 6, a decelervating electrode de of screen form operated at or slightly above the Wall coating 'voltage is shown in place of the decelerating ring 26. This modi'- fication is fully disclosed in U. S. Patent 2,462,619, issued November 20, 1948, 'to P. K. Weimer. The ylens effect O'f 'the de'cele'rating member is .greatly reduced by the uniform held produced thereby in front of the target. This greatly reduces the helical motion due to the deceleratinlg electrode and 'there is usually 'insufficient helical motion introduced in -.this modiiication to balance out that of the 'deflecting coil with the same coil design. The deflecting coil can then be constructed to balance the eilect of .the bend at the emerging end of the coil against that o the bend at the entering end, as previously referred .to herein. The small residual helical `motion .introduced by the deilecton coil can be .balanced against that of the screen lie, as decribed in connection with Fig. 4. The screen should be tar enough Afrom the target to be 'out of focus. Its best out-of-focus .position 'is when its .distance from the target `in centimeters equals i/Screen voltage In this type of decelerating electrode a spurious signal may be produced by interference between the outgoing beam and the returnbeam, depending upon the transit time of the electrons between their i'lrst and second passage through the screen. Any spurious .signal may be reduced to ,negligible value by experimental adjustment `o the distance between screen 40 and target 3 for best performance. Use of extra ne mesh screen of from 500 .to ..1000 -mesh ,per inch makes Ythe spurious .signals of much less significance.

rThe use of a decelerating screen, as in Fig. .5, has the important yadvantage that the size or the scan of the return beam over the first dynode 8 is much reduced. This is vdue to the small .lens effect of vthe screen as compared with that of a decelerating ring.

Having 4described my invention. what I claim 1. A cathode .ray beam tube of the orthicon type comprising, an envelope containing a target, a gun having a cathode, grid and anode for projecting a beam of electrons toward said target, a Wall coating anode and a decelerat-ing electrode adjacent said target, means for produping .an axial magnetic .focusing field between the .rst .mentioned .anode and the target, -a deilecting unit .for producing two magnetic fields Aat right angles to each other Vand to said vfocusing eld, and an adjusting device 4connected to said unit lfor moving it laxially of .the envelope towards and from said decelerating electrode to `balance the helical :motion produced by said unit.

2. An electron discharge device comprising, elect-ron gun means for forming vand projecting an electron beam along a path, a targetelectrode pos"V -ioned transversely .to said beam .path and spaced therefrom, a decelerating electrode ad jacent said target electrode Aand between .said electron igun means and said target, means :for producing an axial magnetic focusing eld be- 10 tween said electron gun means and said target REFERENCES CITED electrode, a deecting unit for producing between said gun means and said decelerating electrode magnetic beam deflecting elds at right angles The following references are of record in the file of this patent:

to said focusing eld, and an adjusting device 5 l UNITED STATES PATENTS connected to said unit for moving said unit Number Name Date axially of said beam path toward and from Said 2,102,421 Kuehni Dec. 14, 1937 decelerating electrode to balance helical motion 2,356,535 Ruska Aug. 22, 1944 produced by interaction of said deflecting and 2,377,972 Schade June 12, 1945 focusing fields. 10 2,407,905 Rose Sept. 17, 1946 PAUL K. WEIMER. 2,418,487 Sproul Apr. 8, 1947

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