US2217860A - Split cathode multiplier - Google Patents

Description

Oct. 15, 1940. 11 FARNSWQRTH 2,217,860

SPLIT GATHODE- IULTIPLIER Filed March 22, 1937 3-Sheets-Shet '2 SIGNAL INVENTORY PH/LO r. FAPNSWORTH.

ATTORNE Y8.

06h P. 1'. FARNSWORTH' SPLIT CATHODE HULTIPLIER 3 Sheets-Sheet 3 Filed March 22. .1937

A TTORNE YS.

Patented Oct. 15, 1940 SPLIT CATHODE MULTIPLIER Philo T. Farnsworth, Springfield Township, Montgomery County, Pa, assignor, by mesne assignments, to Farnsworth Television & Radio 001'- poration, Dover, DeL, acorporation of Delaware Application March 22, 1937, Serial No. 132,325

12 Claims. (Cl. 250-27) My invention relates to electron multipliers, and more particularly to that type of electron multiplier where electrons are directed to successively impact surface elements to produce a current augmented by secondary emission at each impact. This application is a continuation in part of my prior application, Serial No. 80,194, filed May 16, 1936, for a Means and method for producing electron multiplication."

Hitherto electron multipliers have been roughly divided into two general types. First, the socalled direct-current multiplier, where the generation of secondaries takes place by successive impact of an electron stream with a series of emitting elements energized to successively increasing steady potentials; and second, that type of multiplier where repeated impacts are made between a pair of operatively opposed surface elements energized by alternating potentials.

These two types of multipliers have required totally different physical constructions, but I have found that there are constructions which will separately or simultaneously produce multiplication as in either one or both of the types outlined above. I a

It is the main object of the present invention to provide a multiplier structure and circuits therefor which will allow the multiplier to be used for practically all amplifier or oscfllator purposes, and which may be energized either with direct current, with alternating current, or with combinations of both sources.

Among the other objects of my invention are: To provide a means and method whereby electron multiplication may take place with high efficiency, and in particular, to provide a multiplier structure which may be used in various circult." as a tool to produce practically any of the phenomena and results which can be obtained by the use of a single or a plurality of gridcontrolled electron relay tubes. I also provide a device having the added advantage that because of the extremely high electron multiplication which takes place between input and output of the device, it is possible to greatly reduce the input energy required to produce a given output.

It is, therefore, a still further object of my invention to produce, within a single envelope, amplifications hitherto considered only to be obtainable by the use of multistage circuits. Logically it follows, therefore, that the fundamental structure described in the present invention may be used as a direct current multiplier, as a detector-amplifier, as a modulator-amplifier, as a.

radio-frequency amplifier, and as an oscillator and oscillator-amplifier.

My invention possesses numerous other objects and features of advantage, some of which, to-

gether with the foregoing, will be set forth in 5 the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawings:

Figure l is a longitudinal sectional view of one preferred embodiment of my improved multiplier tube. Figure 2 is a graph delineating input characteristics of a tube such as shown in Figure 1.

Figure 3 is a circuit diagram showing the device of Figure 1 connected as a radio-frequency amplifier.

Figure 4 is a circuit diagram showing the tube illustrated in Figure 1 connected as an oscillation amplifier.

Figure 5 is a circuit diagram showing the tube illustrated in Figure 1 connected as a directcurrent multiplier.

Figure 6 is a circuit diagram showing the tube illustrated in Figure 1 connected as a detectoramplifier.

Figure '7 is a circuit diagram showing the tube illustrated in Figure 1 connected as a modulatoramplifier.

Figure 8 is a circuit diagram showing the tube of Figure .1 connected for alternating-current energization; and

Figure 9 is a circuit diagram showing the tube of Figure 1 connected to have the first cathode energized by alternating current, and the second cathode energized by direct current.

The broad objects of my invention may be more easily understood by direct-reference to the drawings.

Referring first to Figure 1, an envelope I is provided at one end with a re-entrant stem 2. Passing through the stem 2 are a pair of heater leads 4 supporting a heater coil 5, which is of high-resistance material, such as tungsten,v for example, and which may be raised to incandes cence by the passage of current therethrough; I prefer to form this coil in the form of a conical spiral. Surrounding the heater coil 5 is a conical electron input source 6 supported by a lead I and closed at the open end by an insu- 55 lator 9 through which heater leads 4 may pass. The source 6 is preferably of metal, and is preferably covered on the outside with a good electron emitter, such as barium and strontium ox-- ides. I prefer, in the embodiment shown, that the longitudinal section of the cone exhibit a cone angle of approximately twenty-three degrees.

Surrounding the conical electron source, I position a conical grid I0, having the same sectional angle as the cathode, coaxial with and spaced slightly from the source surface. This grid may be in the usual form of a spiral refractory wire wound on supports, or it may be any other apertured control electrode, as' is well known in the art.

The side walls of the envelope I are utilized to support a pair of secondarily emissive ring cathodes II and I 2, these two cathodes being placed one above the other and shaped to describe a truncated cone, the small end of which surrounds an apical portion of the grid structure just described. The lower cathode II is supported from the side wall by leads l4 and I5, and the upper cathode I2 is supported in like manner by leads I6 and Il. Leads I5 and I! are brought entirely through the side wall to afford electrical connection to the two cathodes. The large end or base of the truncated cone described by the cathodes is surrounded by a cup 20 provided with a. central aperture 2|, the entire cup being supported on leads 22 and 24, the latter passing through the side wall of the tube for electrical connection. The aperture 2| is closed by a relatively coarse-meshed output screen 25, and extending downwardly, attached to the cup 20, is an accelerating anode 26 formed of relatively fine-meshed screening shaped into a truncated cone coaxial with cathodes II and I2, and positioned relatively close to the inner surface of the cathodes. The accelerating anode extends the full length of both cathodes, and I prefer to form the body of the screen of small-diameter wire so that the total aperture area is greater than the total wire area.

Just above the output screen 25, I position a collecting anode 21, supported from the end of the tube by lead 29 passing through the wall of the envelope. I prefer to shape electrode 21 to have a concave surface presented to the aperture screen, and to stiffen its edge by rolling a flange 30 thereon. The concave surface is preferably treated to prevent secondary emission, and for this purpose may be carbonized. I prefer, when the angle of the input source is approximately twenty-three degrees, to make the angle of the secondarily emissive cathodes approximately twenty-eight degrees, and, as can be readily seen in the drawings, the angles are opposite.

The final step in the processing of the tube I have just described is the sensitization of cathodes II and I2 to be secondarily emissive when impacted by an electron traveling at the proper speed. As it is advantageous to obtain as many secondaries as possible from one electron impact, I prefer to sensitize the inner surface of the cathodes II and I2 with materials of extremely low work function, such as, for example, caesium or caesium on silver oxide, processed to have as high a secondary-emission function as possible. I have found that a caesium-silver oxide surface may be processed to give as high as ten and twelve secondary electrons per impacting primary electron, and to produce secondary emission at a ratio greater than unity at electron velocities in the vicinity of twenty volts.

Considering the operation of my device, I will describe it first operating as a straight directcurrent energized multiplier. This circuit is shown in Figure 5. In this diagram, as in the other diagrams in this application, the heating source for heater coil 5 has been omitted for purposes of clarity. Input source 6 is grounded. Cathode II, immediately surrounding the input structure, is placed at a potential higher than cathode 6 by source 35. All sources herein represented by battery symbols shall be deemed to include any well known means for producing a potential difference. Cathode I2 is made still higher in potential by source 36, accelerating anode 26 is placed at a still higher potential by source 31, and the collecting anode 21 is still higher in potential by connection to source 38 through output resistor 39. Input energy to grid I0 is supplied through input condenser 40, and a bias is placed on grid It by bias assembly 4I through choke coil 42.

Electrons emitted from source 6 in an amount depending upon the potential on grid I0 are accelerated outwardly along radial paths by the potential on accelerating anode 26. Most of them pass through the meshes of accelerating anode 26 and impact the first cathode H, sensitized for secondary emission. secondaries are emitted having a low initial velocity, and these secondaries are accelerated back through the apertures in anode 26, passing through the space encompassed by anode 26' and through the opposite apertures of the anode to impact the second cathode I2, which is also secondarily emissive. The secondaries emitted from the second cathode I2 are accelerated back into the space inside of anode 26, and pass out of the space through output screen 25 and are then drawn to collecting electrode 21. Inasmuch as all anode and secondary cathode potentials remain constant, the number of electrons reaching collecting electrode 21 is a function of the number which pass through grid III at the input end of the device. As I have been able to obtain an emission of ten or twelve secondaries at each impact, and as the aperture areas in anode 26 are relatively large, I have been able to obtain a net secondary generation of six electrons per impact. It should be noted that the angles involved are such that when electrons leave cathode 6, they take an upward direction to impact first secondary cathode II, and that when they leave this cathode the angle is such that they will be certain to impact the second cathode I2, as they follow radii perpendicular to the emitting surface. There will thus be but one impact per cathode, and therefore, when used in this manner, the number of secondary generating impacts is equal to the number of cathodes. It can readily be understood that while I have only shown two cathodes, namely, II and I2, in case greater multiplication is desired additional cathodes may be utilized, giving additional secondary generating impacts, and cone angles may be altered to insure arrival of secondaries on the next larger cathode ring, The rings will also increase in axial length along the progression.

If reference be made to the graph shown in Figure 2, it will be seen that in one operating structure variations of one volt on the input grid will cause a variation of electron emission of about one and one-half milliamperes. If, in a.

' quency amplifier.

two-stage multiplier, an over-all multiplication of thirty-six be obtained, it will be seen that the input variation of one volt will then cause a variation in output current of fifty-four milliamperes, making the device an extremely sens'itiv and powerful tool for all electrical work.

It is also obvious, by reference to the graph in Figure 2, that the device may also be used as a detector simply by varying the grid bias, for example, or by handling the input grid H) in exactly the same manner as that of the ordinary triode tube would be handled. The detector circuit is shown in Figure 6 and is identical with that shown in Figure 5, with the exception that the input is a radio-frequency signal, the choke coil used to connect the bias to the grid [0 is a radio-frequency choke coil 44, and the device has a by-pass condenser 45 from the output resistor to ground.

Figure 3 shows the device used as a radio-fre- Radio-frequency current is fed through input condenser 40, the grid being connected to ground through input resonant circuit 45, and the output from the collecting electrode being provided with a parallel resonant circuti 46, tuned to the same frequency as the input circuit. Under these conditions the device operates as a straight radio-frequency amplifier. In-

asmuch as the device is an amplifier, it can readily be seen that it is an extremely eificient selfoscillator, and, referring to Figure 4, if the input grid Ill be connected to the input source by a tuned circuit 45, and the collecting electrode 2'! be grounded through an output resonant circuit 46, then it is possible for the device to self-oscillate. In this case I prefer to parallel-feed the collecting anode 21 through feed choke coil 41.

Inasmuch as this circuit is much like that described in conjunction with-the diagram shown in. Figure 3, utilized as a radio-frequency amplifier,

it is obvious that precaution should be taken that the radio-frequency amplifier circuit, as shown in Figure 3, shall not oscillate. This may be done in a number of diiferent manners, such as detuning either the input or output circuit, neutralizing as with a triode, or by other expedients readily understandable to those skilled in the art.

The tube of my invention is shown connected as a modulator-amplifier in Figure 7. Here, ra-

die-frequency current is supplied to grid l0 through input condenser 40, and the grid is through a modulation resistor 5|.

biased for proper amplification by bias source 4 I. The usual energizing sources 35, 36, 3! and 38 are applied as described for other circuits, but,

in this case, modulation energy from a modulator, as symbolized by connected modulator tube 50, is supplied to the first secondary cathode l I A resonant output circuit 46 is used in this case as a work circuit, lead 54 being utilized to draw energy therefrom.

In operation, this tube simply acts as a radiofrequency amplifier of the energy received by grid l0. Grid [0 regulates the number of electrons reaching secondary cathode ll. However, the potential on secondary cathode l I is not constant, due to the fact that it is being changed by modulator tube 50. Thus, within a'selected range on the secondary-emission curve, there will be more or less secondaries produced in strict accordance with the changes of potential on cathode l I. The

secondary stream emitted from cathode ll'will therefore vary in accordance with the r-f input and with the modulation, and this stream will be amplified upon impact with secondary cathode l2, where the stream will generate new secondaries to be picked up by collector 21.

All of the circuits heretofore described have been circuits in which the secondary-emission cathodes have been energized to a steady poten- In Figure 8, both cathodes II and I2 are supplied with alternating-current from oscillator 60 through radio-frequency transformer BI, and in addition, the secondary cathodes II and I2 are usually additionally supplied with a steady positive bias from sources 35 and 36 respectively, source 36 being by-passed with a condenser 62 as it is in thealternating-current path. The,

midpoint of the secondary winding of the transformer 6| is connected to the negative end of source 31 and to the positive end of source 35 through a protective choke coil64. Grid I 0 is energized in the usual manner through input condenser 40, the grid being properly biased by bias source 4|. The output is taken across output resistor 39. In operation, electrons are emitted from input source 6, controlled by grid l0, and are accelerated to impact first secondary cathode ll. Here, secondaries are emitted, and I prefer to so adjust the frequency that several secondary generating impacts are made per cycle. This mode of operation is described in my copending application, Serial No. 96,614, filed August 18,. 1936, now United States Patent No. 2,128,580 issued August 30, 1938.

-At the end of the impacting portion of the cycle, electrons leave the vicinity of first cathode H and are directed-toward second cathode l2. The fact that the two cathodes are out ofphase is immaterial, as electrons will arrive at cathode l2 either at the proper time to make further sec.-v

ondary generating impacts, or will oscillate in the space within accelerating anode 26 until they are pulled through the screen 25 to be collected. However, I have found that with proper adjustment of accelerating potentials on the accelerating anode 26, electrons will arrive at second cathode l2 with the voltage thereon in proper phase to give secondary emission. Here, on this sec- 0nd cathode, multiple impacts are also produced during the'impacting time, and during the remaining period the electrons oscillate in the space within anode 26 until they lose velocity and are collected by collecting anode 21after passing througliscreen 25.

The circuit shown in Figure 9 is a modulator circuit similar to that shown in Figure 7, with the exception that in this case the modulation potentials are supplied through input condenser 40, and the radio-frequency current to be moduinput potential may be given to grid l0, and generator 60 and transformer 6| utilized to give the lower cathode ll an alternating-current energization, whereas the upper cathode l2 will have a direct-current energization. Utilized in this manner the tube can be used as a straight amplifier wherein several secondary generating impacts may be obtained on the lower cathode by the use of alternating current thereon, whereas only a single secondary generating impact will be obtained on the upper cathode l2.

So far, in this discussion, I have described only the current amplification of the device which occurs below aperture screen 25. It will be obvious to those skilled in the art that when the multiplied electron stream appears above the aperture screen 25, a virtual cathode is there formed in space. Inasmuch as multiplication, modulation, or other variation of the stream has already taken place, it is obvious that exceptionally high power output may be obtained by making the potential difference between screen 25 and collecting anode 21 exceptionally high, as, for example, 10,000 volts. Thus, my device is adapted not only for exceptionally large current amplifications, but also for exceptionally large power amplifications.

Furthermore, it will be apparent to all those skilled in the art that the electrode assembly, without including collecting electrode 21, can be made to self-oscillate. As a matter of fact, a diode comprising accelerating anode 26 and only one cathode, H or l2, will self-oscillate, provided they are connected by a resonant circuit as described in my prior and copending application, Serial No. 61,042, filed January 27, 1936, now United States Patent No. 2,137,528 issued November 22, 1938. It is therefore obvious that when used as an oscillation generator, and high potentials utilized between collecting electrode 21 and screen 25, an extremely high power oscillating output may also be obtained. In other words, whether used as a direct-current amplifier, alternating-current amplifier or self-oscillator,

- the output may be controlled, varied or modulated at the beginning of the multiplication cycle where control energy may be small, whereas high power may be obtained from the device by the use of high potentials at the output end.

When used in conjunction with alternating currents, it is also obvious that multipliers of the character herein described may be readily stabilized as to frequency. For example,-if, in the circuit shown in Figure 8, oscillator 60 be stabilized as, for example, by crystal control thereof, the multiplied output will be stable. Again, a relatively unstable source 50 may be used and a stiff high-Q circuit may be used for the transfer circuit 6|, as exemplified, for instance, by a transmission line. In all these latter cases the stabilizing elements are required to carry only a small fraction of the power dissipated in the device, and the tube may be stabilized independently of the input grid.

Referring again to Figure 1, while I have shown input source 6 as being a thermionic cathode controlled by a grid II], it is obvious that source 6 may be, for example, a photoelectric cathode subject to the influence of light. It is also possible to liberate electrons from other apparatus so that they will travel in directions suitable to land them on the first cathode I I. I therefore use the term input source in this specification in its broadest meaning, and do not desire to be limited to any particular method for generating or obtaining the input electrons, since the fundamental operation of [the tube is entirely independent of the type of input source. I also wish to point out that I do not consider myself limited, in any way, to the exact construction of the tube herewith described, it being used as an example only, as there are many variations in size, shape and position of the electrodes which will perform the method involved and give the results required. For example, while I have shown the cones involved as being of circular section, it is obvious that the contours may be varied within wide limits without departing from the spirit of my invention.

It will be seen from the above description that the tube I have described here in one preferred physical embodiment is an extremely efllcient electronic tool, and one which is capable of being used in practically any circuit where heretofore a standard grid-controlled electron relay has been used, with the additional feature that extremely high outputs are obtainable because of the amplification which takes place after the first control of primaries, and that combinations of function can be obtained which heretofore have required a plurality of tube structures. The angular relationships shown and described are also important, because the paths of the electrons become longer as the electron density increases, thus reducing space charge limitations, as has heretofore been described in my application Serial No. 80,194, of which the present application is a continuation in part, and which is referred to at the beginning of this specification.

I claim:

1. In combination, an envelope containing a hollow apertured accelerating anode having a small input end and a larger output end, a plurality of coaxial unheated cathodes surrounding said anode and serially positioned along said anode from input to output end, means for generating electrons adjacent the inner surface of the small+ end, and means for collecting electrons adjacent the large end after a plurality of cathodic impacts between generation and collection.

2. In combination, an envelope containing a plurality of orbital unheated. cahodes positioned serially to describe a truncated cone and energized to successively increasing potentials in accordance with size, means for generating and controlling electrons positioned adjacent the inner surface of the smallest cathode, means for directing generated electrons against inner surfaces of said cathodes in order, beginning with the smallest, and means for collecting secondary electrons emitted from the largest cathode.v

3. In combination, an envelope containing a plurality of orbital unheated-cathodes positioned serially to describe a truncated cone and energized to successively increasing potentials in accordance with size, means for liberating electrons adjacent the smallest cathode, a single accelerating electrode positioned to accelerate electrons emitted from the inner surface of one cathode ring in a direction insuring impact with an area on the inner surface of the next largest ring substantially diametrically opposite the point of origin of the secondary electrons, said accelerating electrode being at a higher potential than any of said cathodes, and means for collecting secondary electrons emitted from the largest cathode.

.tal cathodes of increasing size, means for liberin the space bounded by one cathode into the v aameao cathodes for directing electrons within the space enclosed by one cathode to the inner surface of the next, and means for collecting electrons emitted from the largest cathode.

5. In combination, an envelope containing a plurality of orbital unheated cathodes positioned serially to describe a truncated cone, means for energizing said cathodes to potentials increasing in relation to size, means for directing electrons in the space bounded by one cathode into the space bounded by the next larger cathode, a conical electron source having at least its apex entering the space bounded by the smallest cathode, and electron-collecting means adjacent the largest cathode. l

6. In'combination, an envelope containing a plurality of orbital unheated cathodes positioned serially to describe a truncated cone, means for energizing said cathodes to potentials increasing in relation to size, means for directing electrons space bounded by the next larger cathod, a conical electron source having at least its apex entering the space bounded by the smallest cathode, a conical control electrode surrounding said source, and electron-collecting means adjacent the largest cathode.

7. An electron multiplier comprising an envelope containing a plurality of cathode rings describing a truncated cone, a coaxial apertured accelerating electrode enclosed by said rings, means for energizing said rings 'to potentials increasing with respect to the size thereof, means for energizing said accelerating electrode to a potential higher than any of said cathodes, means for emitting electrons within the bounds of the smallest cathode, and means for collecting electrons emitted from thelargest cathode.

8. An electron multiplier comprising an envelope containing a plurality of cathode rings describing a truncated cone, a coaxial apertured accelerating electrode enclosed by said rings,

.means for energizing said rings to potentials increasing in respect to the size thereof, means for energizing said accelerating electrode to a potential higher than any of said cathodes, means for emitting electrons within the bounds of the smallest cathode, means for removing electrons from the influence of the potential on the largest .cathode, means-for accelerating the removed electrons, and means for collecting-the accelerated electrons.

9. Anelectron multiplier as set forth in claim 7, with additional means for applying an alternating potential on one of the rings 01' a frescribing a truncated cone, a coaxial apertured accelerating electrode enclosed by said rings,

means for energizing two adjacent rings to the same steady potential, means for energizing said two rings with an alternating potential, means for energizing said accelerating electrode to a potential higher than any of said cathodes, means for emitting electrons within the bounds of the smallest cathode, and means for collecting electrons emitted from the largest cathode.

12. An electron multiplier comprising an envelope containing a plurality of cathode rings describing a truncated cone, a coaxial apertured accelerating electrode enclosed by said rings, means for energizing two adjacent rings to the same steady potential, means for energizing said two rings with an alternating potential in opposite phase, means for energizing said accelerating electrode to a potential higher than any of said cathodes, means for emitting electrons within the bounds of the smallest cathode, and means for collecting electrons emitted from the largest cathode.

PHJLO 'I'. FARNSWORTH.

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