US2501095A - Electron tube of the velocity modulation type - Google Patents

Description

March 21,, 1950 x A, RECORD 2,50LQ95 ELECTRON TuE OF THE VELOCITY MODULATION TYPE Filed Sept. 21, 1945 INVENTOR. FRANK A. RECORD A TTOR/VEV Patented Mar. 21, 1950 ELECTRON TUBE OF THE VELOCITY MODULATION TYPE Frank A. Record, Potsdam, N. Y., assignor to the United States of America as represented by the Secretary of. War

Application September 21, 1945, Serial No. 617,874

This invention relates to electronic tubes for operation at high radio frequencies. More particularly it relates to radially constructed velocity-modulated tubes for the production of: electronic oscillations and for the: amplification of electronic energy.

In conventional radio tubes the presence of the control grid in the stream of electrons directly affects the electrons emitted. from the cathode. This results in the production of a charge-density modulation. It may be proven that this variation in space current, caused by the action of the grid, induces a similar variation of current in the grid circuit. When operating at low frequencies, these induced current variations which result in an instantaneous difference in the number of electrons approaching and leaving the grid are relatively unimportant because they arev small and are approximately 90 out of phase with. the grid voltage fluctuations and therefore cause no appreciable power loss. However, when. tubes are operated at frequencies where the transit time of the electrons over the space current path. becomes an. appreciable portion. of the time of one cycle, the induced grid current increases in magnitude and. approaches the same phase as that of the grid. voltage. This phenomenon. causesthe apparent shunt resistance of the. grid circuit to vary inversely as the square of. the operating frequency- Therefore, at high frequencies the use of convention types of control. grids results in such low shunt impedance and such high. power losses that conventionally designed tubes become almost completely inoperative and useless.

' A partial solution to this problem has been found in av velocity-modulated tube in which. the electrode structures are so designed and. constructed that the discharge current is velocitymodulated without the appearance of any appleciable charge-density modulation near the control electrode. In the conventional velocitymodulated tube. electrons are emitted from the.

cathode, accelerated in a given direction, and are reacted. upon by control electrodes associated with a resonant cavity or circuit, in such a manner as to alternately accelerate and decelerate the electrons. After the electrons pass the control electrodes andtraverse a predetermined distance they become bunched in accordance with a particular waveform resulting from their difference in velocity. A second resonant cavity or circuit is provided and energized in proper phase to catch the bunched electrons: and absorb energy from them.

Conventional. velocity-modulated tubes may be 7 Claims. (01. 315-4) divided into two types; the reflex type and the direct electron beam type. In general, the former is used as an oscillator and the latter as an amplifier. Conventional reflex type tubes utilizing the same grids as bunchers and catchers, have the same buncher and catcher voltage and that voltage is such that the tube operates normally with an overbunched electron beam. Any reduction in this voltage would mean a sacrifice oiv output. The output in such a tube increases rapidly as the beam current is increased beyond the value of the starting current until overbunching occurs. The efiiciency then follows a curve approximately proportional to the reciprm cal of the beam current. Thus the efiiciency is limited in such. tubes since the optimum buncher voltage does not occur at the point of maximum output. Furthermore, in a longitudinally constructed tube the beam current is limited because of the relatively small. emissive area which is due to the necessity of emitting electrons in a single direction. It is thus seen that the usual longitudinally constructed velocity-modulated tube is limited in its eiiiciency and output by the overbunching of electrons and by its limited beam current.-

In tubes employing the emission of electrons:

from the cathode in a single direction, the means for altering the velocity of some of these electrons with respect to the others, the means for extracting energy from the bunched electrons.

and the periods of the groupings are all interdependent. Also, all velocity-modulated tubes relying upon internal cavities for extracting. the output from the tube are subject to critical ad-- justment and narrow operating frequency ranges since the tuning and spacing of. the cavities are interdependent. The use of bellows for tuning these tubes causes a variation in the spacings of the elements and has proven to be impractical since the narrow variations they afford and. their been limited in thei use because they contain,

resonant circuits in the form of annular chambers designed-to resonate at a particular frequency. As a result, these tubes also possess a fixed. Q value (ratio of the amount of energy stored to the amount of energy lost per cycle) which makes their tuning a delicate and difii W cultproblem. The advantages of the increase in beam current obtained in such tubes is nullifled by their complicated and fragile structure and by the necessity for delicate adjustments without affording a marked increase in the narrow operating frequency range of longitudinally constructed velocity-modulated tubes. This has caused such tubes to be infrequently used by the art.

It is an object of the present invention to provide a velocity-modulated tube which has a much larger output than the velocity-modulated tubes of the prior art.

It is also an object of the present invention to provide a velocity-modulated tube which is capable of operation over a wider range of frequencies than tubes of the prior art.

It is another object of the present invention to provide a velocity-modulated tube which, while capable of operation over a wider band of frequencies than tubes of the prior art, will also operate over a predetermined narrow range of frequencies at an increased efficiency as compared to the velocity-modulated tubes of the prior art.

It is another object of the present invention to construct a velocity-modulated tube which will produce a greatly increased beam current over tubes of the prior art.

It is a further object of the present invention to provide a velocity-modulated tube which is capable of being diverted from operation over a wide range of frequencies to operation over a desired narrow range of frequencies or vice verse. by a substitution of the external cavity used without affecting any change in the actual or relative position of the electrodes within the tube.

Generally, this invention comprises a velocitymodulated tube whose electrodes are arranged as concentric cylindrical structures and which provides a radial stream of electrons which are velocity-modulated and are caused to deliver radio frequency energy to a resonant cavity which is external to the tube structure but electrically connected to the electrode structure of the tube.

Other objects, features, and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description of the invention taken in connection with the accompanying drawings in which:

Fig. 1 is a cross-sectional view of a reflex type velocity-modulated tube employing the principles of the invention with the grids shown diagrammatically; and

Fig. 2 is a diagrammatic representation of the external part of a cavity suitable for use in conjunction with the tube shown in Fig. 1.

Referring to Fig. 1, the reflex type tube is generally designated as I0. The general concentric cylindrical construction of the tube I may be properly appreciated by visualizing it as a figure of revolution about its longitudinal axis AA.

The cylindrical cathode structure II has a cylindrical emitting surface I2 of strontium oxide, barium oxide, or other highly emissive material sprayed on a sleeve of nickel or other suitable material. sleeve I2, disposed along longitudinal axis A--A of tube I0, and connected at one end to sleeve I2 and brought out to terminal I3a. External connection'to cathodeII is made by wire I4 brought outto terminal I4a. Heating current is supplied as by transformer 52 through leads connected to;

terminals I31; and Ma. Placed at either end of the sleeve I2 are the circular discs I5, I5, the diameters of which are equal and larger than that Heater element I3 is contained within of the sleeve I2 by an amount sufficient to enable them to serve collectively as beam forming electrodes. The buncher grid I6 is of slightly larger diameter than cathode structure II and is concentrically situated about it and is rigidly supported by the buncher grid structure I'I. Likewise, concentrically situated t about buncher grid I6 and of slightly larger diameter than that grid is the catcher grid I8 which is rigidly supported by'the catcher grid structure I9.

Grids I6 and I8 are of similar construction. They are made of small wires of suitable material and arranged parallel to longitudinal axis A-A of tube ID and spaced at discrete intervals around a circumference. Their respective supporting structures I1 and I9 are metallic cylindrical elements described hereinbelow in more detail. The wires of grids I6 and I8 are mechanically and electrically attached at their ends to their respective supporting structures. It is important that each individual grid wire of grid I6 be radially aligned with a corresponding grid wire of grid I8, that is, there must be an equal angular separation between each grid wire along the circumference of the respective supporting structures I1 and I9.

Outside of and concentric'with grid I8 is a repeller plate 20, annular in shape and for maximum efficiency having a length parallel to axis A-A usually somewhat greater than that of member 22 which serves the dual purpose of posi-' tioning the repeller plate 20 and of acting as an" electrical conductor between the repeller plate 20 and the outer shell 2|. Member 22 may be a series of small wire supports fastened to repeller plate 20 and shell 2| or may be a continuouswasher or flange-like part connecting repeller plate 20 to shell 2I all along its circumference.

Buncher'grid supporting structure I! is sealed near one of its ends by glass to metal seal 23 through which heater element lead I3 and cath-' ode lead I4 pass. At its other end it is reduced in diameter as at 24 and sealed by glass to metal seal 25.' Seal 23 also mechanically secures the position of cathode connection I4, heater element I3, and cathode structure I I. At the other end the diameter of II is reduced thereby providing a smaller portion 24 which lends itself to a more easy sealing and facilitates the mechanical construction of the tube. This portion is of no electrical or other functional significance.

The buncher grid supporting structure IT is sealed in a similar way to the catcher grid supporting structure I 9 by the glass'to metal seals 26 and 21. ing structure I9 is sealed to the"='0uter shell 2| by glass to metal seals 28 and 29. It is noted that, in addition to their usual function of sealing the tube for evacuation,'the glass to metal seals serve the important function of giving mechanical strength and rigidity to the entire structure of the tube. They also afford a convenient and per manent method of maintaining the proper spac ing between the electrodes of the tube.

' The electrode supporting structures I I, I9, and 2| should be of such material that theyare good; electrical conductors and have a coefficient of eX- pansion equal to that of the glass used for making the seals. There are a wide variety of materials Ina like manner, catcher grid support-- possessing such a quality; an alloy of nickel, cobalt, and iron is one example of a material which has been widely used and found to give satisfactory operation.

The cavity till used in connection with the tube ml is divided into two equal portions as shown in Fig. 2 and is connected between the buncher grid [6 and catcher grid H by a friction connection having electrical conductivity to the external p01"- tions and 3| at one end of tube It and 32 and 33 at the otherend of the structures H and l9 and friction fingers 4i and 42 on the cavity structure.

The action and significance of the cavity which should be used in conjunction with the present invention will be more fully described hereinbelow.

- The actual and relative values of the potentials applied to the various electrodes of the tube ill will difier in each application of the invention by an amount which will be more fully understood from the operating principles of the invention as explained in this specification. It may be stated, however", that equal potentials are applied to the buncher grid l6 and catcher grid it as by battery 50 and that these grids are positive with respect to the cathode structure i l and that the repeller plate 26 is usually negative with respect to the cathode and the grids as by battery 5|.

The cylindrical electron emitting surface i2 is much greater in area than that employed in conventional longitudinally constructed velocitymodulated tubes, and, therefore, it aiiords a much higher beam current which results in an equally increased output. This means that for a given output the tube it] may be physically smaller than a longitudinally constructed tube and that the application of given potentials will result in a larger beam current.

In its'more simple form, this invention is a device for converting direct current energy into radio frequency energy by alternately accelerating and decelerating a stream of electrons. The transit time between fixed points is utilized to produce an alternating electron stream which delivers power to a cavity resonator.

The radial electron stream emitted from the cylindrical cathode sleeve i2 is of uniform average velocity and proceeds toward the buncher grid [-6 due to the accelerating voltage supplied by the cathode and the presence of discs l5, l5. As these electrons pass into the interaction gap St between the buncher grid I6 and the catcher grid 18, they encounter the strong alternatin radio frequency field present in the interaction gap. This field tends to accelerate or decelerate the electrons dependent upon the potential of the field at the instant electrons enter it. Thus the radio frequency field causes the electron stream to suffer velocity-modulation. The electrons then proceed past the catcher grid i8 and enter into the drift space between catcher grid I8 and repeller plate 20. In this drift space the electrons which are accelerated at one instant overtake electrons decelerated during the previous instant which causes a bunching of electrons to occur. and approach repeller plate 20, they encounter a negative or retarding direct current field caused by the potential on that plate from battery 5! and the electrons reverse their direction and achieve a more discrete bunching in the drift space 35 before they again reach the catcher grid IS. The phase of these reflected electrons is such that they reinforce the oscillations of the resonant cavity.

The bunches of electrons are grouped around- A the electrons continue their transit electrons which pass buncher grid I'G at the in stant' the radio frequency field iszero and de creasing. Since cavity 43 is provided to deliver radio frequency energy, these same electrons which made a first transit of the interactiongap 34 when the radio frequency field was zero and decreasing must make their second transit when this voltage is at a maximum negative value. The minimum beam current required to maintain oscillations is determined by the configuration or the cavity used, the cavity losses,. the power required for bunching, and the circuit constantswhich determine the phase of the reflected. electrons. Also, the impedance of the electron beam must be less than the shunt impedance of the.

cavity.

A decrease in the transit time of the electrons may be accomplished by increasing the repeller voltage which in turn increases the starting. current.

After being reflected by the repeller plate 28-, the electrons give up a certain amount of their: energy to the catcher grid l8. Any electrons which pass the catcher grid during their second transit are collected by buncher grid 1 5 and their It is pointed out that the results obtained from this relation should not be confused with the number of times the beam is bunched in the drift,

space.

Generally speaking, the beam may be bunched several times in the drift space of an amplifier if sufficient buncher voltage is available, but it isusually bunched only once in an oscillator. To maintain oscillations at a given frequency, it is required only that either the accelerator or repeller voltages be varied. A series of acceleration or cathode voltages exists which satisfy thephase relations for oscillations when the repeller" voltage is fixed. Also several modes of oscillation are possible if the repeller voltage is varied and the acceleration voltage is fixed. It is, therefore,

obvious that these effects may be combined and a change in acceleration voltage compensated for However, the" by a change in repeller voltage. repeller voltage is usually the one varied since it requires no power when it is sufiiciently negative to reflect the electrons that may have been accelerated to greater than the average velocity;

For oscillations to occur it is also necessarythat the following equation be approximately satisfiedt where" O is the average electron transit time across the? drift space in cycles, and f3 N is any positive integer or zero.

. A reduction in the acceleration voltage increases the transit time and an increase of the acceleration voltage decreases the transit time. For a linear potential variation in the drift space the transit time may be computed from the following equation:

0 500X AW.

where 0 is the average electron transit time across the drift space in cycle,

X is the drift distance in centimeters,

A is the wavelength in centimeters, and

Vc is the potential diiference between the cathode and resonant cavity in volts.

The potential difference between the cathode and the resonant cavity is often referred to as the cathode voltages since the two are numerically equal. A simple relation between the cathode voltage and the operating frequency is obtained by combining the above two equations which yields the result that:

For any given tube the distance X is a fixed characteristic of that tube. Hence, for any given frequency, i, there are a number of values of V: which correspond to different values of N. The resulting oscillations for these different values of N are referred to as modes of oscillation. Also, it is clear that for any given mode of oscillation Vc varies inversely as the square of the frequency.

The two separate sections 43, 43 of the external cavit used in conjunction with the present invention are made equal in length and the total length between their extremities is adjusted to be equal to one-half wavelength of the operating frequency less the length equivalent to the capacitative reactance within the tube. These two sections 43 of variable electrical length when combined with the tube structure act as a single one-half wavelength cavity having a maximum voltage at or near the center of the tube structure and a minimum voltage at either of its ends. Annular plungers 44 are provided to adjust the length of cavity 43.

This external cavity is excited by the interaction of the fringing radio frequency field with the electron stream bunched by initial passage through the fringing field and then caught by reflected bunches of electrons returning from the negative repeller or, if the repeller voltage should be less than that of the catcher grid, caught by secondaries from the repeller.

The proper choice of cavity design permits the application of the proper electrode voltages without any interference to the radio frequency field of the cavity or its mechanical tuning mechanism. In choosing this coaxial cavity its inner diameter is such that it slides over and makes good electrical contact with portion 3t of cylindrical grid structure I! connected to buncher grid I 6, and the inner diameter of its outer structure is such that it slides over and makes good electrical contact with portion 3! of cylindrical grid structure l9 connected to catcher grid [8.

It is emphasized that the Q of the resonant system is dependent upon the Q of its associated quency range, and the frequency stability of tubes employing this invention are primarily determined by the proper application by the user of the known principles pertaining to the characteristics of the loaded circuit employed. It should be mentioned that for operation over a wide range of frequencies the Q value of the loaded circuits should be low and that for operation over a narrow frequency range with high frequency stability, large output, and high efficiency, the Q value of the loaded circuits should be large.

The loaded shunt impedance is approximately proportional to the loaded Q value and it does not decrease appreciably as the load decreases; also, if the load is decreased until the output power approaches zero, the electrical tuning range does not decrease appreciably, but, as the load is decreased, the range of voltage over which thetube will oscillate increases. The starting current is inversely proportional to the shunt impedance of the cavity; therefore, any change in the configuration of the cavity which decreases the shunt impedance increases the minimum current required to maintain oscillations.

It is pointed out that the usual method of applying the potentials to the buncher grid l6 and the catcher grid I8 is by applying these potentials to the cavity in a conventional manner. In designing a cavity to operate with this invention it is a convenient and common practice to make an opening 45 approximately in the center of the portion of the cavity which connects to the external portions 32 and 33 so that the electrical connections may be made to the cathode connection I401. and heater element i3a. Electrical connection to the repeller plate 20 is made by any conventional type adapter located anywhere on the surface of the outer shell 2!, or it may be made by direct connection of the conductonto the shell. The radio frequency output is extracted from the cavity 43 by means of a conventional single turn coupling loop 46 which is physically supported by the plunger 44.

Means for cooling the tube l0 may be provided by forcing air through appropriate pipes into each section of the resonant cavity and allowing it to escape through small slits 41 provided for this purpose and spaced around the circumference of the cavity and between the fingers 4| and 42.

"These slits are longitudinal with respect to the cavity and should start at the end of the cavity where that end makes contact with the structure of the grids and extend a little beyond this contact surface. This type slit permits easy circulation of air and further serves to insure a good electrical contact between the cavity and the tube structure. If the output of the tube is especially large due to an increase in the demands upon it, the additional cooling necessary may be obtained by using external fans.

While there has been presented a detailed description of the construction and operating principles of a reflex type velocity-modulated tube employing the teachings of this invention, it is understood that the same type of construction and principles of operation equally apply to a direct electron beam type tube.

In the construction and operation of a directelectron beam type tube employing this invention there are no unusual or unexpected changes from that of a reflex type tube made in accordance with this invention. In this construction three concentric cylindrical grids will be used in place of the buncher and catcher grids of the reflex type tube and the repeller plate will be replaced sign to the 'one used with the reflex type tube and concentrically arranged is used for operation with the direct electron beam tube.

v One cavity should be common to the inner and middle grids and the other connected to the outer grid. As in the case of the reflex type tube, the electrical length of each cavity is approximately equal to one-half a wavelength of the operating frequency and plungers are provided for adjusting its physical length.

In the direct beam type tube, the electrons which are radially emitted from the cathode pass into the interaction gap between the inner and middle grids where the presence of the strong electromagnetic field causes the electron stream to suffer velocity modulation. After the electrons pass the middle grid they enter into the drift space between the middle and outer grids where a more discrete bunching of the electrons is accomplished. Finally, the electrons pass the outer grid in such phase relation with the electromag-' netic field of the cavity connected to that grid that they give up energy to this field and, hence, to the cavity. The electrons which succeed in getting past the outer grid are collected by the collector electrode and their energy is dissipated as heat.

While there has been here described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention.

What is claimed is:

1. An ultra-high frequency tube of the reflex velocity modulation type comprising, a generally cylindrical vacuum-tight evacuated enclosure having glass end walls, two concentric cylindrical grids within said enclosure having conductive tubular extensions passing through said glass end walls, a cylindrical cathode within the inner grid, a cylindrical epeller electrode external to the outer grid, means for projecting a stream of electrons radially through said grids, and separate annular cavity resonator chambers external to the vacuum space and comprising a major part of the effective resonating space in the tube telescopically slidable over the outer surfaces of said tubular extensions and in electrical contact therewith.

2. An ultra-high frequency tube of the reflex velocity modulation type comprising a cylindrical cathode, first and second cylindrical grids, a cylindrical repeller electrode and a vacuumtight evacuated cylindrical enclosure having glass end walls all assembled on a common axis, means for projecting a stream of electrons radially through said grids, conductive tubular extensions passing through said glass walls and connected to the ends of first and second grids respectively, and separate annular cavity resonator chambers external to the vacuum space and comprising a major part of the effective resonating space in the tube telescopically slidable over the outer surfaces of said tubular extensions and having a plurality of longitudinal slits forming resilient contact fingers adapted to engage said outer surfaces.

3. An ultra-high frequency tube of the reflex velocity modulation type comprising, a generally cylindrical vacuum-tight enclosure formed by a conductive tubular member having glass end walls sealed thereto, two concentric cylindrical grids within said enclosure, conductive tubular extensions connected to each grid passing through said glass walls, a cylindrical cathode within the inner grid, a cylindrical repeller electrode external to the outer grid, means for projecting a stream of electrons radially through said grids, and a separate annular cavity resonator chamber at each end of the structure comprising a major part of the effective resonating space in the tube telescopically slidable over theouter surfaces of said tubula grid extensions, said resonator chambers having a plurality of longitudinal slits forming resilient contact fingers adapted to engage said outer surfaces and having adjustable annular tuning rings forming an electrical short circuit between said grid tube extensions.

4. An ultra-high frequency tube of the reflex velocity modulation type comprising, a cylindrical cathode, a pair of cylindrical grids external to said cathode, a cylindrical repeller electrode external to the grids and an evacuated metal cylinder having glass end walls external to and supporting the repeller electrode all assembled on a common axis, tubular extensions connected to and supporting the ends of each grid and passing through vacuum-tight seals in said glass end walls, means for projecting a stream of electrons radially through the grids, and annular cavity resonator chambers comprising a major part of the effective resonating space in the tube having resilient contact fingers telescopically slidable on the external surfaces of said tubular grid extensions, said resonator chambers including adjustable tuning rings forming an electrical short circuit between said grid tube extensions.

5. An ultra-high frequency tube of the reflex velocity modulation type comprising, a cylindrical cathode, a first cylindrical grid external to the cathode, a second cylindrical grid external to first grid, a cylindrical repeller electrode external to second grid and an evacuated cylindrical vacuum-tight enclosure having glass end walls, means for projecting a stream of electrons radially through said grids, conductive tubular extensions for first grid passing through vacuum-tightseals in said glass end walls, conductive tubular extensions for second grid external to the first extensions passing through said glass walls, and annular cavity resonator chambers comprising a major part of the effective resonating space in the tube external to the vacuum space and having resilient contact fingers telescopically slidable on the external surfaces of said tubular extensions, said resonator chambers including adjustable annular tuning rings forming an electrical short circuit between said first and second tubular grid extensions.

6. In an electronic oscillator for generating very high radio frequencies of the reflex velocity modulation type which includes means for establishing a radial flow of electrons and modulating said electron flow with a pair of coaxial grids supported by the inner tubular parts of two annular cavity resonators and having a cylindrical repeller electrode external to said grids; that improvement wherein the outer parts of said cavity resonators consist of enlarged annular chambers closed at their outer tubular ends and telescopically fitted to the external surfaces of said tubular inner parts of the cavity resonators.

7. The structure of claim 6 wherein said annular chambers are of substantially larger diameter than the walls of said inner parts of the cavity resonators whereby the overall length of 11 the tube is reduced and the cavity resonators made more nearly toroidal with consequent reduction in cavity losses.

FRANK A. RECORD.

REFERENCES CITED The following references are of record in the file of this patent:

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