US2217197A - Cathode ray device - Google Patents

Description

oct. 8, 1940. v C J. DAVISSQN 2,217,197

CATHODE RAY DEVICE Filed neo. 5o, 193s v 2 sheets-sheet 1 AAAAA AAAAAA I *YEAH IMA MAA RE C TIF/E R ,IdF/Ing s e e DEFLEC/ON PLATES /A/ VEN TOR C. J. 0A V/SSOA/ AT ORN/5y Oct. 8, 1940.

c. J. DAvlssoN CATHODE RAY DEVICE Filed Dec. 30, 1936 2 Sheets-Sheet 2 vm IN voLTs c. J. 0,4 l//sso/v Patented Oct. 8, 1940 UNITE CATHODE RAY DEVICE Application December 30, 1936-, Serial No. 118,277

2.4 Claims.

This invention relates to cathode ray devices and more specifically to methods .of and apparatus for generating, focussing, accelerating and modulating they intensity of electron beams in cathode ray devices.

An object of this invention is to provide improved cathode ray generating and controlling means for cathode ray tubes.

Another object of this invention is to provide a novel electron gun system for a cathode ray device.

A further object of this invention is to provide an electron lens system in Awhich apertured plates are used throughout for focussing the beam.

A still further object of this invention is to provide a novel modulating system for electron beams in cathode ray devices.

A feature of this invention is that within the electron lens system a field free space is defined, in which region the cathode ray beam is modified.

Another feature .of this invention is that a condensing electron lens system is provided for concentrating an intense electron beam upon the plane of a diaphragm having a square aperture therein and a projecting electron lens system is provided for forming an electron image of this aperture uponva screen or target.

For illustrating a practical application .of the invention, there is hereinafter described in dedetail a tube which may be briefly characterized as a high vacuum cathode ray device electron optically designed for use as a receiver for a 72- line television image. It is to be understood that the invention in its various aspects is not limited in its application to any specific type or form of cathode ray device but is of quite general application.

Investigation has disclosed that ideally such a cathode ray receiver as that just mentioned would employ a uniformly bright square spot, modulated in brightness but not in area. It would have a linear brightness versus modulating voltage relationship over a range at least a` factor of thirty below maximum brightness, and likewise a linear deflection versus deflecting voltage relationship over the range determined by the field of the image. Further, there would be no distortion of the spot due to deflection nor deflection of the spot by modulation. The size of the image would be n times the size of the spot, 'n being the number of lines, so that when the image field is illuminated without modulation, a uniform field would be obtained without evidence of line structure. Moreover, the image would be approximately as large as the fluorescent screen permits so that the size of S of the spot is given by the equation:

where D is the effective screen diameter. Finally, provision would be made for adjusting brightness in operation without changing the linear modulation characteristics, maximum brightness being as great as possible. The present invention sufficiently well meets these ideal requirements'.

Briefly stated, the electrode structure of the television receiving tube mentioned above is as follows:

A cross-shaped filament serving as a cathode is located between and parallel toa back electrode and an accelerating focussing electrode. A ne g ative potential with respect to that of the cathode is applied to the back electrode and a positive potential is applied to the first accelerating electrode, the effect of which is to produce a uniform field between the two members to cause the electrons emitted from the four arms of the cross-shaped filament to traverse 'paths which are substantially parallel to the optical axis of the tube. Three diaphragm members are located in a metallic cylinder forming a second accelerating `anode. #The potential of this' second accelerating anode is placed at a value which is positive with respect to that of the first accelerating vanode. A third accelerating anode comprising lan apertured diaphragm is placed at a potential which is positive With respect to that of the second accelerating anode and is electrically connected `to a conducting coating on the inside walls of the tube. The distances of the first accelerating member and the first and second diaphragms of the second accelerating member from the cathode and the relative potentials applied thereto are calculated so as to cause electrons from the cathode tov be focussed in the plane of the middle diaphragm of the second accelerating member.. The final diaphragm of the second` accelerating member (with respect to the position of the filament) cooperates with the third accelerating electrode to form a projection lens system to focus an electron image of the aperture in the middle diaphragm upon a screen or target. The beam is modulated by a pair of modulating plates which vary the number yof electrons incident upon the aperture in the middle'diaphragm of the second accelerating anode in accordance with the amplitudes of signals received from a transmitting station. The beam Fig. 1 is a schematic diagram of a televisionv receiving tube and its associated circuits which embody this invention;

Fig. 2 is a schematic diagram showing the relative spacing of the various elements of the electrode system in the tube shown in Fig. '1;

and

Figs. 3 to 6, inclusive, are graphical representations used to explain the operation .of this in vention.

Referring more specically to the drawings, Fig. 1 shows a cathode ray tube and its associated circuits for use as a television receiving device. This tube comprises a gas-tight envelope I containing an electron gun assembly for generating, focussing and accelerating a beam .of electrons, means for modulating this beam, and means for deecting the beam so that it traverses every elemental area of a field of view on a uorescent screen II located at one end of the tube.

The electron gun arrangement comprises a cathode I2, a back plate or electrode I3, a first accelerating anode I4, a second accelerating anode I5, and a third accelerating anode IE which is electrically connected to a conducting coating I7 located on the inside walls of the tube. Broadly speaking, the electron gun arrangement comprises two electron focussing or lens systems of the electrostatic type, .one a condensing lens system for concentrating a beam of electrons generated by the cathode I2 upon the aperture in the metallic diaphragm S which is located inthe metallic anode cylinder I5, and a projection lens system for projecting an image of this electron illuminated aperture upon the screen or target lII. Preferably the shape of the aperture in the diaphragm S is made square or rectangular.

'I'he cathode I2 is preferably formed in the shape of a cross from a single sheet of tungsten of the order of .001 inch thick. The opposite ends of this cross are electrically connected together and terminals I8 and I9 connected to a suitable source of heating current 20. A resistance 52 is also connected across terminalsl and I9, the mid-point of which is connected to ground. By this method of connection the electrostatic and electromagnetic elds due to filament current and potential are reduced to substantially zero. For a more complete description of `a crossshaped element similar to the one described above, reference may be made to Patent 2,117,- '709, issued May 17, 1938 to C. J. Davisson.

The back electrode I3 comprises a circular plate spaced a short distance from the cathode I2 and parallel thereto. The first accelerating electrode I4 comprises an apertured circular plate which is located on the side of the cross-shaped cathode element remote from the back electrode I3. A positive potential with respect to that of the cathode is applied to the first accelerating anode I4, while.a negative potential with respect to that of the cathode is applied to the back-electrode I3. In order to produce a uniform eld between the back electrode I3 and the rst accelerating anode I4, the spacings of these members I3 and I4 from the cathode I2 are so chosen with respect to the respective potentials applied thereto that the ratio of these potentials is equal 'to the ratio of their respective distances from plied thereto a positive potential with respect to that of the first accelerating anode I4. This ,l potential and the spacing and aperture sizes of the diaphragms of the members I5 and I4 are so chosen that the electron beam is brought to a focus in the plane of the apertured diaphragm S. The relative spacings and potentials applied yto the various electrode members will be considered more fully below.

The third accelerating anode I6, which preferably comprises a metallic circular plate having an aperture therein, is placed at a positive potential with respect to that of the second accelerating anode I5. members I5 and I6 and the distance between them is so chosen that an electron image of the aperture in the plate S is formed on the fluorescent screen II, which maybe of any suitable fluorescent material such as, for example, willemite. The third accelerating anode is electrically connected inside the tube to the conducting coating I'I which serves to make a eld free space betweenthe anode I6 and the iluorescent screen II and at the same time to serve as a return path for the electrons impinging upon the fluorescent screen II.

In order to cause the electron beam generated by the electron gun apparatus described above to .scan every elemental area of the field of view on the screen or target II in turn, suitable deflecting means, such as, for example, two pairs of deecting plates 23 and 24, the axes of which are located at right angles to each other, are provided. To the deilecting plates 23 are applied dei'lecting voltages ofy framing frequency cycles) and of saw-tooth wave form to produce the vertical deection, while deflecting voltages of line scanning frequency (1440 cycles) and of saw-tooth wave form are applied to the deecting plates 24 to produce horizontal deection of the beam. Any suitable sweep circuits (not shown) may be used to generate these horizontal and vertical deection voltages. For example, reference may be made to Patent 2,178,464, issued October 31, 1939 to M. W. Baldwin, Jr., which discloses a suitable balanced sweep circuit for this purpose. Connections may be made from the balanced sweep circuits to the pairs of plates 23 and 24 by means of coupling condensers 25 and 26 and 21 and 28, respectively, of about 1 microfarad capacity each. Coupling resistances 29 and 39 of the order lof 20 megohms each are respectively connected across the-pairs of plates. The mid-points of the resistances 29 and 30 are connected to the third accelerating anode I6 in order that the average potentials of the deflecting plates do not deviate from the potential of the third accelerating anode i6 and thus introduce distortion of the beam with the consequent distortion of the image which would be caused by a change in the velocity of the beam. For a full description of the advantages of balanced sweep circuits in cathode ray television devices, reference may be made to the above-mentioned Baldwin patent and also to Patent 2,209,199 is- The potentials applied to the sued July 23, 1940 to Frank Gray, Serial No. 65,606.

The direct current potentials for biasing the vdifferent elements of the electron gun are pref- 1 erably derived from an alternating current oscil- 35 of about 1 microfarad each which are connected between vthese terminals and ground.V

Modulation of the electron beam generated by the apparatus described -above is achieved by applying signals between a pair of modulating plates M1 and M2 which are located in a eld free space determined by lthe cylinder I and the diaphragms 2I and S, all of these elements being rplaced at the same potential. The potentials applied to these modulating plates are balanced with respect to the potential of the cylinder I5 by means of a resistance 36 of the order of 200,000 ohms, the mid-p oint of which is connected to the cylinder I5, and a balanced modulating circuit is connected to the terminals of the resistance 36. An input signal is applied (after demodulation by a suitable circuit or circuits) to the primary winding 31 of a transformer 38, the secondary winding 39 of which is connected to the input circuit of two tubes 40 and 4I which fills ing plates M1.

are connected in push-pull manner to produce a balanced output. The cathodes 42 and 43 of the tubes 40 and 4I are connected to ground. A negative bias is applied to the grids 44 and 45 by means of a connection 46 from the midpoint of the transformer secondary winding 39 tov a point on the potentiometer resistance 33 Which is negative with respect to the tap representing ground potential. The output circuits of the tubes 40 and 4I include resistances 4l and 48 of about 5,000 ohms each, the common terminal of which is connected to the mid-point of the resistance 35. The output of the tubes 40 and 4I is applied to the modulating plates M1 and M2 through'coupling condensers 49 and 50 of about 1 microfarad capacity each.

By means of the arrangement described in the preceding paragraph, the average of the potentials applied to the modulating plates M1 and M2 is at all times equal to the potential applied to the accelerating anode I5 and hence tothat of the diaphragms 2I and S which are connected to the metallic cylinder. Modulationis thus achieved in a field free space except for the variations introduced by the changes in the potentials applied to the modulating plates; that is, if the average oi the potentials -applied to the plates is always equal to the potential of the diaphragms at both ends of 'the plates there is no acceleration imparted to the beam while it is being modulated.

Modulation is achieved by deflecting the electron beam more or less upon the square hole in the apertured diaphragm S. Electrons are deected towards the more positive of the modulat- By thus controlling the number of electrons incident upon the square hole in the apertured diaphragm S, modulation of the brightness of the beam can be achieved. The cross section of the beam at the square aperture in the plate S has, of course, an unequal distributio-n, as explained eleswhere herein, the distribution being a maximum atthe center and theoretically falling off to zero at infinity. Strictly speaking, therefore, the beam is never fully deiiected off the aperture and the distribution within the aperture is never exactly uniform. This unequal distribution, however, is rarely, if ever, discernible to the observer viewing the image screen.

Specific details of the tube generally described above will now be given in order toI fully describe a tube which has been used in practice. A square hole in the plate S was made equal to .006 inch by punching. So far as a square spot on the fluorescent screen in concerned, its source is this .006 inch hole and what happens to the electrons before they reach the hole has no effect upon the size and shape of the image produced on the fluorescent screen by the projector lenses in the diaphragms 22 and I6. The optical analogue is the projection lantern. The inside wall of the Pyrex glass cylinder is coated with aquadag and this coating I1 is shown connected to member I6 by a spring 5I which is released by burning out a lament during pum-ping. During the lsealing-in process this spring should not be permitted to scrape against the aquadag.

As an aid to visualizing the electrode assembly, it should be mentioned that between the cylinder I5 and the element I6 and between pairs of elements I3 and I4 and I4 and I5 are placed cylindrical glass insulators (not shown) of the `salme diameter, which diameter is somewhat greater than that of the element I5 and less than that of the other elements. The electrode structure is supported on three metal rods passing through peripheral apertures in elements I3, I4, and I6 and a flange or flanges (not shown) on element I5. These rods are bolted to a metal band which is clamped to the base of the tube. to Fig. 2 it will be seen that the electrically independent parts are the back electrode I3 (designated in Fig. 2 as P1), the filament F, the first accelerating anode P1, the second -accelerating anode comprising .the apertured diaphragms P2, Sand P2 (which are all placed at the same potenti-al) M1 and M2, the third accelerating anode P3 (which is electrically connected to the aquadag conducting layer I'I) and each ofthe four deilecting plates. Thus a total of twelve leads is required. All metal parts are aluminum.,

except P1 which is of nickel and the filament F which is of tungsten. Metal ilanges protect the cylindrical insulators which prevent or render harmless the accumulation of charges.

The tube is thoroughly pumped. practical to degas the metal parts by high frequency, but the tube is baked for several hours at 460 C.; par-ts adjacent the filament are heavily bombarded; and the tube is iinally sealed off from the pumps at a temperature of about 100-C. and a pressure of 7 107 millimeters of mercury. p

The proper `values of the potentials applied to the various electrode members an-d the distances of these members from each other will now be considered.

The principal physical consideration made use of in arriving at the design of the television receiver tube described `above is that in an axially symmetrical electric lfield the convergence of a paraxial beam of electrons satises the diierential equation Referring It is im- Before defining the symbols in this equation it will be well to state that beam in the foregoing sentence has a special restricted meaning and to explain what this meaning is.

Let it be imagined that electrons are streaming along the axis of an axially symmetrical fieldnot necessarily all on the axis but near it. If they have all come from the same, emitting surface their speeds as they cross any given plane normal to the axis will be sensibly the same. 'I'heir trajectories will not, in general, be straight lines. If we 'draw tangents to their trajectories at the points at which they cut a given plane these will not, in general, meet in a point. If they do meet in a point the stream of electrons constitutes a beam in the sense in which the word is here used. The point of meeting may be in front of the plane (in the direction of motion of the electrons or back of it; it may be on the axis or oi .it.` The only requirement is that the tangents meet in a point on or near the axis. If this condition is satisfied the stream of electrons is a cal considerations that if the tangents drawn at one cross-section meet in a point then the tangente drawn at all other cross-sections also i tive or negative.

meet in a point. The convergence of the beam at any given cross-section is kdefined as the recprocal of the distance from the cross-section to the point at which the tangente meet, and this is the quantity which is represented by the above equation symbol c.r

The convergence of a beam may be either posi- If at a given cross-section the tangents are parallel, the convergence of the beam Vat this cross-section is zero. lf the trajectories are intersecting one another at a given cross-section, the distance from the plane to the point of intersection of the tangents is zero and the convergence of the beam is infinite.

No actual stream of electrons is strictly speaking a single beam. If the stream originates at a themionic cathode the electrons leave the cathode with small initial speeds variously directed. The

tangents to their trajectories at the emitting surface do not meet in a point and it follows from theoretical considerations already alluded to that at no cross-section will they meet in a point.

If, however, not all of the electrons in the stream are considered but only those which leave a given very small element of the cathode, it is clear that-these constitute a beam at their point of origin, since here the tangents intersect. This being true they constitute a beam at all crosssections further along their paths. The vwhole stream of electrons which travels along the axis of the held is an assemblage of such elementary beams originating at the cathode, or may so be regarded. v

There are, however, other Ways of regarding the stream. The electrons which leave a ilat cathode in some particular directionr may be thought of as constituting a beam. They form a beam of definite convergence at the cathodenamely, zero-and so have a definite convergence elsewhere along their path. The whole stream may be regarded as an assemblage of beams of this kind if desired.

What the differential equation does is to show how the convergence or" a beam-any beamvaries along the axis of an axially symmetrical electric l'ield.

Let c stand for the convergence of the beam at thecross-section` throughthe point a onthe axis, e be the distance along the axis measured on the axis relative to the potential of the cathode which is ordinarily assigned the value zero. Then V'=dV/dz, (the first derivative of V with respect to e) and V"=d2V/dz2, (the second derivative of V withrespect to a). What the differential equation says is that at every point along the axis the rst derivative of the convergence of Va beam (dc/dz) is equal to c2-cV/2V|V/4V, where the quantities in this expression are those for the particular point or particular cross-section (particular z) under consideration. The equation is derived from the fundamental laws of electrodynamics. It applies to every beam no matter how constituted and whether on the axis or somewhat off.

lf the value of V at al1 points along the axis is' known, that is, if V is known as a function of z, (V=f(z) this function can be written into the general equation and thus a particular differential equation in terms of c and e can be obtained. The solution of this equation will be of the form F(c,2) :a constant. It will show the convergence of any beam at any value of a in this particular field, provided only its convergence at some one value of e' is known. A beam may, of course, have any convergence at any value of e, but once its 4convergence at one value of z is fixed, its convergence at every other value of z is iixed. Mathematically this amounts to xing the value of the constant in the requation F(c,z) =a con stant. If it is known that at .ai the convergence of the beam is c1, then the constant is F(C1,e1). F(c .z)'=F(ci,21) then describes a particular beam in the particular field.

Thus in the cases considered, the beams starting from elements of the cathode are beams in4 which the convergence is OO at 2:05 thus c1=-, 21:0. The beams made up of .electrons leaving along parallel lines are beams of Zero convergence at 2:0.

Axially symmetrical fields are produced by ap` plying potentials to electrodes which are coaxial gures of revolution-such as plates containing circular holes, cylinders, etc., strung along a common axis. When the configuration of the electrodes is fixed and the potentials applied to them are. known, the potentials at points along the axis V=f(z) can (in principle) be calculated.

ReplacingV in the general differential equation by a) the appropriate diiferentia1 equation in c and e is Iobtained. Solving this and evaluating the constant of integration, a description of the beam is obtained. Y

Let there now be considered any set of parallel plates' containing not too large circular holes. It can'be shown that when a beam passes through the field about a hole in an arrangement of this kind, its convergence vis increased by an amount Ac=(V2'-T/T1)/4V, Where V is the potential of the plate and V2 and V1 are the values of :iV/dz on the emergence and incidence sides of the plate respectively-more strictly V2 and V1 are the values CIV/dz would have on the two sides of the plate if the diameter of the holes were reduced to zero.

When a beam of light passes through a lens of focal length f its convergence is increased by an amount l/f. It is natural therefore to regard the field about a hole in a plate as a lens for electrons,L of focal-length f=4V/(V2'-V1')..

Thus if adjacent plates in the arrangement being considered are at 2:22 and 2:21, and if their potentials are V2 and V1, V between the plates will have` ythe value V2-V1)/ (azi-e1). If a beam-entering the region through a hole in plate P1 has a convergence c1, then, by the law just stated, it will have at the second plate convergence c2, such that l 2V21/2 1 2V11/2 Lcm1/2+ vf Wt v' These two laws, the lens law and the law just stated are all that are required to calculate the convergence of a beam at any pointalong its path through any number of plates at any assigned potentials-provided, of course, its convergence at some one value of z is known.

Thus let it be supposed that the cathode is a flat surface normal to the 'axis of the field. All electrons which leave this surfacenormally (with zero lateral components of initial velocities) might be considered as a beam. These constitute a beam of convergence zero at 2:0. In the eld to the rst plate the convergence will change in such a way as to keep constant When` the beam passesY through a hole in the first plate, its convergence is increased by an amount Ac: (V2'-V1) /4V1 as already explained. The convergence of the beam is increased vfrom :0 to c=Ac. In the second region the convergence again satisfies the law 2V1/2 Vl The Value of the constant is obtained by using the known value of the convergence of the beam as it enters the region. Applying the law, the convergence of the beam at the second plate is now calculated and thenits convergence after passing through the second plate and so on through as many plates as is desired.

l The beam being considered is that composed of electrons which leave the flat cathode normally (Without initial lateral velocity). The beam composed of all the electrons leaving a small surface element might have been considered instead. In this case the convergence of the beam at 2:0 is instead of Zero. The `calculations are carried through as in the pre-vious case, but the convergences found for the beam are, of course, everywhere different.

- It is important to understand how these two kindsof beams are related, the beam of normally emitted electrons on the one hand, and the beams emitted fromusmall surface elements w+ constant on1the. other. Itis' clear that a beam of the latter type shares some electrons With the normally emitted beam; all of the electrons emitted normally from a given small surface element belongy .naturally to both-beams. Not only is this true, but it is also true that of all of the electrons emitted from the small element, those emitted normally are in a Way the most important. The reason isthat the initial energies of the emitted electrons are small compared to the energy they acquire in the field only a short distance from the cathode. Because the initial energies vare comparatively small, the paths or trajectories traversed by the electrons from a small element are none of them so very different from what they would be if the initial velocities were Zero.. The actual trajectories V,all lie close to the trajectories of an electron which leaves the elementfrom rest. This isparticularlytrue of the trajectories of electrons which leave the element with normalinitial velocity, but no lateral velocity, for

these start off from the surface in the same direction as that taken by an electron initially at rest;

It is because of these considerations thatthe electrons emitted from a surface element, although they leave the element in all possible directions, are, at a short distance from the cathode,formed into a narrowbeam. At the center of this elementary beam are the electrons which left the surface normally. The elementary beam is not, however, sharply bounded. The situation is rather this, that the electrons emitted normally from the element follow a certain trajectory through the field, beingnear the axis of the field in some places, further from it in others, and perhaps at certain points crossing it. The trajectories of the other electrons emitted fromv the same element cluster about this principal trajectory. YThe density of these other tra.-

.jectories lis greatest at the center of the' cluster, declines and approaches zero asymptotically with distance from the center.A It is a probability distribution, like bullet marks about the bulls-eye of a target and for this reason not sharply bounded.

Itis important to realize that these trajectories which cluster about the principal trajectory are thetrajectories of the beam of electrons whichrleave the small element. Where the convergence of this beam goes to infinity, the cluster contracts to a point, `all trajectories of the -cluste-rpass through a point and this point is, of course, on the principal or central trajectory. g

- The trajectories of normally emitted electrons are all principal trajectories, each `is the principal trajectory of all trajectories starting from some surface element. Thus, one Way of describing the whole stream ofelectrons is to give the convergence of the beam of normally emitted electrons for every value of z and in addition to giverthe convergence of the elementary beams also for everyvalue of a. All elementary beams have the same convergence at a given Value of e butnever the same convergence as the beam of normally emitted electrons, the beam of electrons pursuing principal trajectories. It is impossible, for example, to make both convergences infinite at the same value of e. It is impossible, for this reason, to bring al1 of the electrons from an extended cathode to a point focus. The principal trajectories can be made to pass through a point, but where this happens the convergence of the elementary beams is not infinite. The convergence of the elementary beams can be made infinite, but

Where this happens the principal trajectories are spread out, and do not come to a focus;

` This latter condition is that in which a real image of the emittingsurface is formed. The elementary beams .are brought to a focus (convergence made infinite) on a fluorescent. screen; the

electrons which started from asmall element of the cathode are brought together on a small element of the screen, but Where this happens the beam of principal trajectories is necessarily not ltube described above at the square aperture in the diaphragm S. The arrangement of ribbon filament, backing plate and apertured lens plates is designed to produce an intense focal spot in the plane of the square aperture.

'Ihe primary consideration inthe design of this part of the tube is to arrange for a crossing of the principal trajectories in this plane. Further back, it was explained how the convergence of a beam at any pointV or cross-section along the axis of a eld determined by any set of arbitrarily disposed plates containing circular holes can be calculated, the plates being at any potentials. While this is possible, it is'not actually done. A person really starts with the idea of bringing the principal trajectories to focus somewhere out along the axis, and seeks as general expressions as possible for the positions and potentialsof lens plates which will produce this result. It is found that the suitable conditions are not at all unique. It is found that there must be at least two lens plates if the focus is to be produced in a eld free space. But it has been discovered that the plates may be anywhere at all between the filament and the point of focussing. No matter where they are placed the principal trajectories will cross at the chosen point when appropriate potentials are applied to the plates, or rather when the potentials applied to the plates have an appropriate ratio.

For best' results, certain vother factors need to be taken into consideration. One of these is that V13/2/z12) must not have too low a value or trouble will be encountered with space charge (V1 is the potential of the rst plate and e1 Vits distance from the filament). Another is the maximum current density in the focal spot. The choice of geometry must be such as to have this as high as possible. Still another is the matter of the solid angle occupied by the electrons which pass through the square aperture. point in getting a great lot of electrons through the square hole if they are going to be spread over a. solid angle much greaterv than that subtended by the'lens which is designed to form them into an image on the fluorescent screen. There is also the question of the convergence of the elementary beams. The arrangement should be such, if possible, that the elementary beamsenter the eld free region with a positive convergence. This is needed to make the focal spot small and also to avoid enlarging the solid angle occupied by the electrons beyond the square hole. There is also the requirement that suflicient room be left between the second lens plate and the square hole There is no.

to accommodate the'modulator plates. The eld between these movesthe focal spot oi and on the square hole. Besides the geometry of the condenser system there is also the question of its scale.

Considering now the beam from the plane of the square hole to the screen, this hole is effectively a new source just as in light optics. An entirely new set of beams is set up. The elementary beam from this point onward is made up of the electrons which stream through a given small element of the aperture. At the aperture it is a certain distance 01T the axis of the system and has a convergence of There is now no principal trajectory. The electrons in these new elementary beams are distributed more or less uniformly in a solid angle only slightly larger than that subtended by the projector lens, toward which they are directed.

The design of the projector lens is based on the same considerations as were previously used in the design of the condenser lens system. Quite a large lens aperture is used in order to gather in all of the electrons `which come through the square hole, but if the aperture is too large a poor image' of the square hole is produced on the screen because of lens aberration of one sort or another.

The projector lens is really a combination of two lenses, one rather weak lens (P3) and one strong positive lens (Pz). The apertures are adjusted to make the F-number of each lens as great at least as '7 or 8. This is about the F-number (ratio of focal length to diameter) of the positive lens. The F-number of the negative lens is greater. From previous experience it is known that a lens of the type here used of F-number '7 or 8 produced sharp images.

Regarding the magnification of the projector system what is wanted on the screen is a spot 30 mils square. This means that the square aperture of which the spot is an image must be 30/M mils square, where M is the magnification of the projector lens system.

The really important quantitatively expressed relationships which have been exactly applied in the-making of tubes in accordance with this invention are:

(I) 'Ihat the eld about a circular hole in a plate is, for'electrons, a lens of focal length (II) That in a uniform eld the convergence of a beamof electrons traveling parallel (or antiparallel) to the direction of the field, or nearly so, varies in such a way that the expression remains constant.

These laws are deduced from the general differential equation l where Z refers to the distance from the filament.

'I'hen when V2:Vi Z2:Zi, the hole in plate P1 constitutes a positive electron lens and that in plate P2 a negative electron lens. As previously discussed, it is possible to calculate the electron distribution on the plane S for any voltage ratio and geometry. More usefully, it is possible to calculate the geometry required to give a maximum intensity-at-peak of the electron distribution on S, subject to the restriction that all of the electrons incident on the square hole shall be limited to a certain angular spread about the axis, such that they will also pass through the circular hole in plate Pz', which constitutes the rst lens of the projector system. There is the further restriction that the magnification of the projector lens system shall be about 5 which, in conjunction with the overall length of the tube helps to determine the distance between S and Pz; and also a restriction is imposed on the F-numbers of the projector lenses. Finally, there are the restrictions of space charge limitation of emission from the filament, and, perhaps, of power dissipation by plate Pi which, however, is not a limit in this tube. The geometry of Figs. 1 and 2 is an optimum design, the only more or less arbitrary restriction being that on the F-numbers of the electron lenses. The recommended spacing be- .tween the elements of the electron gun, as shown in Fig..2, is as follows; between the back electrodes P1 and the filament F, 2 millimeters; between the filament F and the first accelerating anode P1, 6 millimeters; between the first accelerating anode Pi and the diaphragm P2 ofthe second accelerating anode, 4 millimeters; between the diaphragm P2 and the diaphragm S, 10 millimeters; between the diaphragm S and the diaphragm P2', 25 millimeters; between the 4diaphragm P2' and the apertured diaphragm P3, comprising the third accelerating anode, 15 millimeters; and between the third accelerating anode Ps and the fluorescent screen II,`30 centimeters. With these spacings and with voltage ratios which will be given below, a very ne intense spot is produced on the fluorescent screen Il.

The remaining design problems are those of the modulating plates and of the deflecting plates. The former are made as large as geometry will permit and are adequately spaced; their sensitiv'- ity will be discussed below. The latter nd their sensitivity fixed by the distance to and the diameter of the iiuorescent screen. t

'Ihe diameters of the circular plates have not been mentioned for the reason that these are not critical. v

In operation, the potentials V1, V1', V2 and V3 are supplied from the alternating current `rectifier 3l which is connected to the Voltage divider circuit 33 of about 1 megohm total resistance. Changes in output voltage do not change voltage ratios, except temporarily due to condensers in the circuit. Electron optical solutions are always in terms of voltage ratios; changestin absolute value over wide ranges of voltage do not affect the' electron focussing. n

The focussing ratio l@ V2 for the projector lens system is 4 2.- This was determined by visual observation of the spot at low intensity, that is, with filament current low. This ratio is not dependent upon any other factor, either number of electrons in the beam, absolute value of voltage, or other voltages, such as V1, Vi', or modulating and deecting voltages, within the limits of operation of these factors.

The correct Voltage ratio V 1' ZL" Vl Z1 is similarly practically independent of all other factors. By geometry l Z1 3 approximately, but a slight filament displacement can make relatively large changes in the ratio, and it appeared thatthe correct ratio was about 1/4. At about this ratio the current i1 to plate P1 is but half of the total filament emission (temperature-limited), a condition usually satislied. The shape of the ii versus Vi curve conrms'the value 1A, and visual observation of short images of the filament, ywhich may be formed on the screen if desired, also leads to the value of 1A. But in any event, for the spot, what happens between the filament and S is of no importance. beam current should be obtained and the secondary consideration is that i1 should be as small as practicable to reduce the power dissipated on the plate P1. Fig. 3 shows beam current as a' function of for (1) temperature limited and (2) `space charge limited emission. It will be seen from these curves that a value of :Yi v Vi of approximately 0.25 or 0.26 is the optimum value.

It seems advisable to specify the method employed in measuring the beam current. Ideally, a Faraday box which could be moved to intercept the beam would be used. It is observed, however, that plate Pa and the aquadag coated tube itself comprise a good Faraday box, and the current measured in the lead to P3 is the beam current provided (A) that all electrons passing through S and lens P2' also pass through lens P3, (B) that the current measured is not effectively reduced by current to the deecting plates and (C) the leakage currents are taken into account. All of these conditions can be fulfilled so that the beam current i3 is therefore the current to plate P3 and the aquadag coated bulb.

The next set of curves; as shown in Fig. 4 shows beam current as a function of the condensing voltage ratio E t V1 when V1 is held constant. Curve I is taken for avalue of V1=v152 Yvolts vand a value of Is` (the The criterion is that maximum CII filament current) :4.40 amperes. Under these conditions the current to the plate P1 is limited by space charge. Curve 2 is taken for a Value of V1=24O Volts and a value of Ia=3.60. Under these conditions the current to plate P1 is limited by the temperature of the lament. 'Ihe maximum of these curves is attained at a value of V1 for constant V2 might be considerably different but Fig. 5 shows the optimum to be nearly the same. In taking the curve shown in Fig. 5, V2 was held constant at 1,000 volts, lament current being 4.40 amperes and the ratio of -Vi' to V1 being held constant at 0.25.

Thus far, the data have shown that there exist optimum values of the voltage ratios -Vl' V2 17T-0.26, Tf1-7.1 and that these ratios are essentially independent of filament emission and of absolute values of voltage. Thus, the condensing lens system is focussed so that maximum intensity occurs at S. Also when V3 V2 is equal to 4.2, the projector lens system focusses this spot on the fluorescent screen.

The data on deflection and on modulation will now be considered. The two pairs of deecting plates are distinguished by subscripts h and o; the latter are nearest to the screen and produce vertical deflection whilethe former produce horizontal deflection, the tube being mounted horizontally. If all plates, both deflecting and modulating, are insulated, it will be necessary to specify the average potential of each pair and the potential difference between them. The average potentials are designated The potential differences are designated Vh, Vv, Vm, the subscript m, of course, applying to the modulating plates. Thus deecting and modulating voltages refer to potential differences, and not to potential as measured from the zero potential, namely, the filament. The spot is distorted when either Vn or Vv is different from Va; consequently, in all tests Vm is made equal to V2. In some eases V2 may be made greater than Vm as for example in an arrangement shown in Patent 2,168,760, issued August 8, 1939 to C. J. Calbick.

The deflection sensitivities Sh and Sv may be dened by the equations where dn and dv are the deflections in centimeters` on the fluorescent screen produced by deecting voltages Vh, Vv, respectively. lIdeally these sensitivities are constant over the deflec- More genwill define the sensitivities over the screen` range. Conceivably Sv may also depend on dh, since the electrons pass between the horizontal deflecting plates before passing between the Vertical deflecting plates. Since Vb: Vu: V3:

the question of what happens vwhen these conditions are not satisfied does not arise. The sensitivities Were vdetermined by photographing the spot in several deflected positions. From this photograph Sh=68.9, Sv=64.0, and 6=015. Here 0 is the angular departure from mutual orthogonality. The horizontal sensitivity is constant to within the accuracy of measurement, about` one-half of 1 per cent; the vertical sensitivity changes from 63 on one edge of the screen to 65 at the other, i. e., it shows a measurable variation across the screen. Over the television image field, its extreme variation is less than 2 per cent and is quite undetectable in the image field. By making Vv an alternating potential, the spot sweeps out a vertical line, which is straight for all values of Vn; conse quently Sv does not depend on Vh.

Fg. 6 shows a modulation curve taken with V2=1000 volts and the filament current Ia=`4.0 amperes this curve being plotted with beam current in microamperes as ordinates versus Vm in volts as abscissae. It will be observed that the peak of the modulation curve does not occur at Vm=0 but at a slightly positive value and also that a longer linear region is available on the right than on the left side of the maximum. This region is therefore selected for operation.

The above described tube made in accordance with this invention produces a Well defined high intensity square `spot on the screen when the beam is stationary. Television images produced by this tube are relatively Vfree from distortions commonly present. The tube is especially adapted for monitoring or in other places Where especially high quality results are desired.

Various modifica-tions may obviously be made without departing from the spirit of the invention, the scope Vof this invention being defined in the appended claims.

What is claimed is:

1. In a cathode ray device, means for generating a beam of electrons, a cylindrical electrode located in such a position that its axis co incides with the longitudinal axis of said device Which also passes through the center of said beam generating means, said cylindrical electrode having three apertured diaphragms located transversely of the axis thereof and each being placed at the potential of the cylinder, the aperture in each diaphragm being placed to surround the axis of said cylinder, and a pair of plates between said first and second apertured diaphragms to modulate said beam by varying the portion thereof Which passes through the aperture of the second of said apertured diaphragms, the plates being located on opposite sides of said beam.

2. In a cathode ray device, means for generating a stream of electrons, an apertured diaphragm, a screen, Vmeans for concentrating said stream upon the aperture of said apertured diaphragm, means for deflecting said stream `in accordance .withavariable modulating voltage to v cause varying-portions of said stream to pass throughsaid aperture in accordance with the variations of said modulating voltage, and means foriorming `an electron image of said aperture uponwsaid screen whereby the shape of said image varies in accordance With said'modulating voltage. 1

3. In a cathode ray tube, a cathode for emitting electrons, an anodevcomprising a metallic cylinder having threel diaphragms, each diaphragm arranged in a plane which is transverse ofthe axis of the cylinder and each vhaving an aperture located so asA to surround said axis, means for vimparting a positive potential to said cylinder'vwith respect to that of said cathode, means including the apertured diaphragm nearest said cathode for focusing electrons upon the plane of the middle diaphragm, a screen, and means. including the remaining apertured diaphraginfor focusing an image of said middle diaphragm on said screen.

4. In a cathode ray device, a pair of electrodes, a; cathode placed ,between said pair of electrodes, and means for applying potentials to said electrode members and to said cathode of such value that said cathode is in a substantially uniform field between said electrode members.

5. -I n a cathode ray device, an electron emitting` electrode, a second electrode, an apertured plate on' theside of said electron emitting electrode remote from said second electrode, means for applying-,a negative potential with respect to the' potential o f said electron emitting electrode to said lsecondelectrode, and means for applying a positive potential with respect to that of said electron." emitting electrode to said apertured plate-the ratio of the magnitudes of the potentials-applied to ythe second electrode and to the aperturedV plate being made substantially equal to the ratio of the distances of these members from theelectron emitting electrode.

(i. Anf electron gun arrangement comprising a back electrode, a cathode, a first accelerating ano de, a secondgaccelerating anode, and a third accelerating anode. 1

'7. An electron gunsystem comprising a planar cathode, a back electrode parallel to said cathode, a. rst acceleratingl anode comprising an apertured diaphragm, a second accelerating anode comprising Va vmetallic cylinder having three apertured' ,diaphragma each diaphragm located transversely of the agis of said gun system and spaced-'along it, means including said first accelerating. anode and the diaphragm of said second accelerating anode nearest said irst accelerating anode for' concentrating the beam of electrons :upon .the plane of the middle diaphragm ofsaid' second accelerating anode, a third accelerating ranode comprising a metallic lapertured diaphragm; and means including said third accelerating anode and the apertured diaphragm in g saidy second accelerating anode farthest removed from said first accelerating anode for projecting an electron image of the aperture in said middle diaphragm of said second accelerating anode upon said screen.

8. In a cathode ray device, a cathode, a back electrode, an accelerating electrode, means for applying apositive potential to said accelerating electrode with respect to the potential of said cathode, and means for imparting a negative potential with respect to the potential of said cath g ode to said back electrode, the ratio of the voltage applied to the back electrode to .that applied to the accelerating electrode being approximately 1 to 4.

9. In a cathode ray device, a ribbon iilament, an anode, and a plate located on the side of said filament remote from said anode, the plane of said plate being parallel to the plane of said lament.

10. In a cathode ray device, a tungsten ribbon filament, ankanode, and a plate located on the side of said filament remote from said anode, the plane of said plate being parallel to the plane of said filament.

11. In combination, an apertured member located in a space free fromr fields caused by potentials applied to elements Voutside said space, an electron emitting element, a back electrode, and two apertured elements between the apertured member and the electron emitting element, vone of the latter defining aboundary of said space.

l2. In a cathode ray device, a ribbon lament, an anode, and a plate located on the side of said filament .remote fro'msaid anode, the plane of said plate beingy parallel to the plane of said lament, and means for applying potentials to the lament, the plate vand the anode, said potentials being of such value thata substantially uniform eld exists between the plate and the anode.

13. In a cathode ray device, a pair of electrodes, a cathode inthe form of perpendicularly crossed elements placed between said pair ofelectrodes, and means ,for placing said electrode members and said cathode atsuch potentials that said cathode is in a substantially uniform field between said electrode members.

14. An electron gun arrangement comprising a back electrode, a cathode,v a iirst accelerating anode, a second accelerating anode comprising a metallic cylinder-havingjthree apertured diaphragms therein each diaphragm having an aperture surrounding the axis of said cylinder, and a third accelerating anode.

15. In an electron optical system comprising means includinga pair of electrodes on opposite sides of a` flat cathode Vfor forming a beam of parallel electron rays, a rst apertured diaphragm, means Vforfforming a eld free space around said firstY apertured diaphragm, a screen or target, electrostatic means. for converging said beam so that ,in the absence of deflecting fields substantially all` of said electrons pass through the aperture insaid diaphragm, means for deflecting saidy beam within'said field free space in accordance with signals so as to vary the number of electrons passing through said aperture, and means including a pair of apertured diaphragms for causing said electrons to form a diverging beam to form an enlarged electron image of the aperture in said first apertured diaphragm upon said screen or target.

16. In an electron optical system comprising means including a pair of electrodes on opposite sides of a fiatcathode for forming a beam of parallel electron rays, an apertured diaphragm, a screen, means for converging said beam so that in the absence of` deflecting fields substantially all of said electrons pass through the aperture in said diaphragm, means for moving said converging beamin accordance with signals so as to Vary .the numberofelectrons passing through said aperture, and 'means' for causing said electrons to form a diverging beam to form an enlarged electron image of said aperture upon said screen.

,17. In an electron optical system comprising means including a pair of. electrode members on opposite sides of a at cathode for forming a beam of parallel electron rays, an apertured diaphragm, means for forming a field free space around said apertured diaphragm, a screen or target, electrostatic means for converging said beam so that in the absence of delecting fields substantially all of said electrons pass throughl the aperture in said diaphragm, means for defleeting said beam within said field freespace in accordance with signals so as to vary the number of electrons passing through said' aperture, and means for causing said electrons to form a diverging beam to form an enlarged electron image of said aperture upon said screen.

18. In an electron optical system comprising means including a pair of 'electrode members on opposite sides of a fiat cathode for forming a beam of parallel electron rays, an apertured diaphragm, means for forming a eld free space around said apertured diaphragm, a screen or target, electrostatic means for converging said beam so that in the absence of deflecting elds substantially all `ef saidV electrons pass through the aperture in said diaphragm, means for deflecting said beam within said eld' free space in accordance with signals so as to vary the number of electrons passing through said apertures, and electrostatic means for causing said electrons to form a diverging beam toform an enlarged electron image of said aperture upon said screen.

19. An electrode arrangement of an electron be-am device comprising a cathode member, an

I anode member and a backing member for said cathode, said members being spaced apart with said cathode between the others of said members, and means for applying potentials to said members respectively having the relationship that the ratio of the potential applied to said backing member to that applied to said anode is substantially equal to the ratio of the distance between said cathode and said backing member to the distance between said cathode and said anode.

20. An electrode arrangement of an electron beam device comprising a cathode member, an anode member and a backing member for said cathode, said members being spaced apart with said cathode between the others of said members, and means for applying potentials tol said members respectively having the relationship that the ratio of the potential applied to said backing member to that applied to said anode is substantially equal to the ratio of the distance between said cathode and said backing member to the distance between said cathode and said anode, said backing member and said anode being substantially parallel to each other throughout their extents.

21. A cathode ray device comprising means for generating a beam of cathode rays, an apertured element through the aperture of which said beam passes, a pair of deilecting plates, one on each side of the beam emerging from said aperture and equidistant therefrom, means for applying modulating potential across said plates to deflect said beam a small amount in accordance with variations in said modulating potential, a second apertured element adjacent said plates with its aperture positioned to receive said beam as it emerges from the space between said plates and to pass a portion only of said beam for certain Values of said modulating potential so that the number of electrons passing is under control' of" the modulatingpotential, and means for causing the average of the potentials applied to said able modulatingvoltage to cause varying por-V tions ofrsaid stream to pass through said aperture in accordance with the variations ofv said modulating voltage, said stream having a substantially rectangular cross-sectional area immediately after passing through said aperture with one dimension only 'of said area varying in accordance with said modulating voltage, and means for forming an electron image of said aperture upon said screen, whereby the shapeof said image varies in accordance withsaidf modulating voltage. f

23. In a cathode ray device, means for generating a stream of electrons, an apertured diaphragm, the aperture in said diaphragm having at least one straight boundary across which said stream is swept by deflecting means, a screen, means for concentrating said stream upon 'the aperture of said apertured diaphragm, means for deilecting said stream in accordance with a variable modulating voltage to cause varying portions of said stream tofpass through said aperture in accordance with the variations of said modulating voltage, said stream having a substantially rectangular cross-sectional area immediately after passing through said aperture with one dimension only of said area varying in accordance withy said modulating voltage, means for forming an electron" image of said aperture upon said screen whereby the vshape of said image varies in accordance with said modulating voltage, and additional deflecting means for deflecting said stream after it is modulated to cause it to scan said screen inparallel elemental strips extending in the direction of said varying dimension of the cross-sectional area of said stream.

24. A cathode ray device comprising means for generating a beam of cathode rays, a first apertured element through the apertureof which said beam passes, a pair of deecting plates, one on each side of the beam emerging from said aperture and equidistant therefrom, means for applyingrmodulating potentialacross said plates to deflect said beam a small amount in accordance with Variations in said modulating potential, a second apertured element adjacent said plates with its aperture positioned to receive said beam as it emerges from the space between said plates and to pass a portion only of said beam for certain values of said modulating potential so that the number of electrons passing is under control of the modulating potentiahmeans for placing the rst apertured element at the same potential as the second apertured element, and means for causing the average of the potentials applied to said plates to be at all times substantially equal to the potential of said apertured elements.

CLINTON J. DAvIssoN.

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