US2486398A - Velocity modulation device and method - Google Patents

Description

Nov. 1, 1949.

E- F EENBERG VELOCITY MODULATION DEVICE AND METHOD Filed May 29, 1943' INVENTOR EUGENE FEENBERG- TTORNEY Patented Nov. 1, 1949 VELOCITY MODULATION DEVICE AND METHOD Eugene Feenberg, New York, N. Y., assignor to The Sperry Corporation, a corporation of Delaware Application May 29, 1943, Serial No. 488,967

17 Claims.

This invention relates to electron velocity modulation apparatus and methods and is particularly concerned with apparatus and methods for increasing operating efficiency of electron velocity modulation devices of the cavity resonator type.

Cavity resonator type electron velocity modulation devices of the general kind disclosed in United States Letters Patent No. 2,242,275, are becoming widely used in the ultra high frequency field. One difficulty experienced with such devices is that of relatively low efficiency. Although it has been calculated by mathematical physicists that the highest obtainable efficiency from known devices Of this type is about 58 per cent, this theoretical efficiency is difiicult to approach in practice because of losses in the electron beam and in the resonators and lack of control over certain dimensional factors which' determine the transit time of the electrons in the regions where they interact with the electromagnetic fields of the resonators.

It is a major object of the present invention to provide novel cavity resonator apparatus capable of operating at relatively high efficiency as compared to known apparatus of this type.

A further object of the invention is to provide cavity resonator apparatus embodying novel arrangements for passing an electron beam through such apparatus in such manner as to most efficiently extract ultra high frequency energy from the beam.

It is a further object of the invention to provide a novel method of passing an electron beam through cavity resonator apparatus for efficient ultra high frequency energy extraction.

A further object of the invention is to provide novel ultra high frequency apparatus of the cavity resonator type wherein a pre-focussed electron beam is passed successively through interacting electromagnetic field portions and energy is extracted from the beam at a predetermined region of maximum energy exchange.

Further objects of the invention will presently appear as the description proceeds in connection with the appended claims and the annexed drawings wherein,

Fig. 1 is a partly sectional and diagrammatic view illustrating a preferred manner of practicing the invention; and

Fig. 2 is an explanatory diagrammatic view of part of Fig. 1.

emission surface IA of cathode H is located to project a beam or stream of electrons into a cavity resonator device designated at l5.

Device l5 comprises a cylindrical metal barrel l6 separated into input and output resonator chambers IT and I8 respectively by an internal annular wall IS, a hollow cylindrical and concentric drift tube 2! extending through wall [9, a further hollow cylindrical and concentric internal tubular wall 22 beyond the drift tube, and annular end walls 23 and 24 on barrel It. Any of the above walls or tubes may be made integral with each other or suitably secured together without departing from the spirit of the invention.

Input resonator I1 is provided with a pair of grid electrodes 25 and 26 concentrically located on the axis of the device l5 in wall 23 and the adjacent end of drift tube 2i respectively, Output resonator I8 is provided with an internal annular aperture 2! defined by the adjacent ends of tubes 2! and 22. This aperture is preferably unobstructed, with no grids thereacross.

A concentric conical collector electrode 30 designed to prevent secondary emission electrons from entering tube 22 is arranged to receive the electron beam emerging from device [5. Electrode 30 and the glass closure 20 extending over the cathode are sealed to the resonator structure, the interior of the whole device being evacuated.

The electron beam from emitting surface I4 is coupled to the oscillatory alternating electromagnetic field within input resonator Il while passing between grids 25 and 26, and. that alternating field so coacts with the beam as to impart alternate acceleration and retardation forces to the passing electrons. As a result, the electrons in the beam become separated into spaced groups along the beam as they proceed along drift tube 2 I, the separation and grouping depending on the field characteristics. Where device [5 is an amplifier, the field within resonator ll is excited by ultra high frequency energy introduced by input coaxial line 28. Where device 15 is an oscillator. line 28 is'connected to a suitable coaxial feedback line, such as at 29, coupled to the field Within output resonator 18. Where device I5 is an oscillator, wall l9 may be substantially omitted except for supports for tube 2!, so that the oscillator is of the type disclosed in U. S. Letters Patent No. 2,259,690.

By the time the electron beam has reached aperture 21, the electron grouping along the beam is preferably optimum. While traversing aperture 27, the electron grouped beam excites in output resonator IS an ultra high frequency alternating electromagnetic field. This field in turn reacts with the beam to extract ener y therefrom to maintain the field in oscillation and to provide such energy as may be drawn off through transmission line 2!! orany other output terminal.

In general the above described velocity modulation operations such as the electron accelerates. 103ting and decelerating function of ,inputresonator ll, which subsequently results in electron groupie ing or bunching, and the energy extracting or catching function of output tresonator literature. Further detailed disclosure thereof than the above is deemed unnecessary to .understanding the actual invention herein.

I have discovered that, by critically controllingdistribution of the electrons in the beam and correlating the so-controlled beam with the cavity resonator device, unexpectedly high efficiency in operation may be obtained. This is especially true where the output resonator is of the gridless type'as illustrated at l8.

With especial reference to Fig. 2, lines of force E represent the electric fieldcomponent of the alternating electromagnetic field within resonator l8, which extend through aperture 21 into the tubular electron beam passage as illustrated. This electric field is very strong adjacent aperture 2'! and relatively weak adjacent the axis of device I5.

With no excitation of resonator I! and no focusing electrode 3!, the electrons from surface M would tend to pass through the drift tube in a substantially parallel beam, space charge effects being prevented by the positive ions normally present in the drift tube. When resonator I? is excited so as to produce the above described electron grouping, however, the excess negative ion regions in the beam resulting from such grouping experience space charge-effects due to mutual repulsion of the negative charges, which radially disperse the beam at those regions. dispersion results in some of the bunched electrons being lost to the walls of tube 2|.

Theelectron beam is so associated with resonator |8-that the direction of the resonator field at the gap is opposite to that of the beam when the electron groups are traversing aperture 21., the field during this period deriving energy from the beam; and the field direction is the same as the beam direction when those regions of the beam intermediate the electron groups or bunches are traversing aperture 21, the field during this period serving to accelerate the available electrons'in the beam. It is clear therefore that the above described radial dispersion results in loss in efiiciency of the device, because part of the beam is lost to the wall of the drift tube when the phase relations are correct for the resonator I8 to extract energy from the beam, while no electronsare lost from the beam when the phase relations are such as to drain energy from the resonator l8 into the beam-(accelerating the electrons as they pass the opening 21).

According to my invention this dissipation of energy from the electron beam to thedrift tube Walls is prevented, and this is accomplished in the following manner.

The electron beam, immediately after leaving surface i4, is passed through a suitable focusing electrode 3!, which is preferably of hollow cylindrical form as illustrated so as to act uniformly This radial on the electrons in the beam. Electrode 3| is energized by a suitable tap from battery l3 and is designed to relatively sharply focus the beam electrons at a point F adjacent aperture 21. Resistor 35 provides a variable focus control. Preferably point F is a small distance axially beyond aperture 21, so that the beam is slightly underfocused while traversing aperture 21, for best practical operation.

The focused electron beam is, therefore, of substantially,,frusto-conical form since it uniformly reduces in "diameter while bunching is taking place along tube 2|. However, as soon as the bunchingassumes :such proportions that space charge effects-assert themselves, the above mentioned radial dispersion due to mutual repulsion of .the electrons takes place at the electron bunches along the beam. The location where such radial dispersion begins may be arbitrarily indicated at 32, and lines 33, 34 indicate the approximate envelope of that portion of the beam which comprises the electron groups or bunches. Radial. dispersion of the beam sufficient to reach the drift tube walls is therefore delayed until the beam has passed aperture 21. The beam electrons between the bunches, not being subject to space charge effects, are focused substantially at F, thus remaining within the focused beamenvelope as illustrated.

As is well-known, inductive energy extraction from the electron beam by the resonator l8 oocurs by interaction between the electrons of the beam and the field E of the resonator (Fig. 2). When the field E opposes or decelerates the electrons, as indicated by the arrows in Fig. 2, energy is transferred from the electrons to the resonator, as desired. When the field aids or acceleratesthe electrons, the electrons take energy from theresonator, reducing the output and efiiciency. Tubes of the present type are designed to operate so that the numerous electrons of the electron bunches give up energy to the resonator, while only few electrons between bunches take energy from the resonator. In this way, many more electrons give up energy than take away energy, and a net increase in useful output is obtained.

According to the present invention, the output and efficiency are further increased over prior art tubessuch as discussedin the preceding paragraph. A first increase in efiiciency results from the fact that, by the present arrangement, the electrons inthe bunches are not decreased in number by being collected by the wall of drift tube 2|, thereby permitting more electrons of the bunches to traverse gap 2'! and give up more energy to the resonator.

A second increase in efiiciency follows from the use of the converging focussing of the beam and ofthe annular aperture 27, without any grids such as 25, -26, of resonator H, for extracting energy from the stream of electrons. This increase in output is caused by two factors. First, the efiiciency of energy transfer between an electron and. an alternating field such as E of Fig. 2 is increased by having the electron acted on by the field for a very short time. Within the limits of the structure of the present inventions, the shorter this time interval, the greater will be the energy transfer. By the present arrangement, the electrons between bunches are focussed at the center of tube 2| and are acted upon for .a relatively long time by the field E, sothat these electrons are inefiicient in taking energy from the resonator. On the other hand,

the majority of the electrons of the bunches, being somewhat radially dispersed, pass through gap 21 near the wall of tubes 2| and 22. During the first and last portions of theirjlight through the field, the paths of these electrons are substantially perpendicular to the field lines, as

shown in Fig. 2, so that substantially no internator and electrons is a function of the field strength. For higher field strengths, more energy is given up by or added to the electrons, depending on the field direction. In the present arrangement, the field intensity is greatest at the aperture 21 and least at the center of tubes 21 and 22. Accordingly, the bunches of electrons, being radially dispersed, pass through the highest field intensities, giving large amounts of energy to resonator l8, while the electrons between bunches pass through a weak field, and can take only very small amounts of energy away from the resonator. Thus conditions for energy extraction from the beam are best, while conditions for energy extraction by the beam are poorest,

resulting in maximum output and optimum operation.

I have therefore provided electron beam focusing and energy extracting arrangements which materially increase efficiency of operation in cavity resonator devices. The above described arrangement may vary in specific embodiment as desired without departing from the spirit of the invention. Instead of being provided with grids as at 25, 28 the input resonator may be coupled to the beam through gridless aperture means similarly to the output resonator at 21. The invention may equally well be applied to high power cavity resonator devices defined by figures of revolution of the above described structures about various axes, similarly for example to Fig. 5 of said Patent No. 2,242,275 and Figs. 7, 12 and 15 in said Patent No. 2,259,690.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Cavity resonator apparatus comprising means defining a cavity resonator adapted to contain an alternating electromagnetic field, means for projecting through said resonator an electron beam having longitudinally spaced electron groups therealong, said resonator having an apertured Wall through which the resonator field is coupled with said electron grouped beam, and means along the path of said beam for causing said beam to assume a substantially conical form within said resonator, the apex of said conical form being located beyond said aper- 6. tured wall, whereby more eflicient interaction between said beam and said resonator is effected.

2. The apparatus defined in claim 1, wherein said aperture in said resonator wall is entirely unobstructed.

3. The apparatus defined in claim 1, wherein said means for causing said beam to assume a substantially conical form includes an electrode positioned adjacent said beam projecting means. 4. Ultra high frequency apparatus comprising hollow resonator means adapted to contain an alternating electromagnetic field, means in said apparatus for projecting an electron beam through said resonator means so that said electrons are subjected to velocity modulation by said field, further hollow resonator means along the path of said beam and through which said velocity modulated electron beams is adaptedto be projected, said furtherresonator means being adapted to contain an alternating electromagnetic field and being formed with an aper-,

ture through which its field is adapted to be coupled with said beam, and means along the path of said beam for causing said electrons to assume converging paths during passage between said resonator means, said paths converging at a point beyond said further hollow resonator means, whereby said beam is prevented from losing electrons to the resonator walls during said passage.

5. The apparatus defined in claim 4, wherein said means for causing electrons to assume converging paths includes an electrode coaxial with said beam path and positioned before said hollow resonator means.

6. The apparatus defined in claim 4, wherein said hollow resonator means is adapted to impress velocity variations upon the electrons of said beam, whereby said electrons are subsequently formed into longitudinally spaced electron groups With substantially fewer electrons intermediate said groups, said further resonator means is adapted to contain an electromagnetic field having a relatively high intensity region and a relatively low intensity region, and said means for causing electrons to assume converging paths includes means for passing said electron groups through said relatively high field intensity region and. means for passing said fewer electrons through said relatively low field intensity region.

7'. High frequency apparatus comprising hollow resonator means adapted to contain spaced electromagnetic field portions, means defining a drift passage between said portions, means in said apparatus for projecting an electron beam through said hollow resonator means and said drift passage, whereby said beam is adapted to be coupled in energy exchanging relation with said field portions, and means for controlling radial dispersion of said beam for preventing loss of electrons to the walls of said drift passage and obtaining optimum interaction of the beam with said field, said controlling means being disposed along the path of said beam for converging said beam to a point located in said drift passage beyond said hollow resonator means.

8. High frequency apparatus comprising means for producing an electron stream having periodic groups of electrons, a cavity resonator having an annular gap surrounding the path of said stream and adapted to extract energy from said stream by interaction of said stream and the alternating electromagnetic field of said resonator, and means along the path of said stream for causing the electrons thereof to assume paths converging. toward a :point slightly beyond: said aperture, wherebythe electrons of said groups are dispersed by'inter-electron-forces and-passed through relatively strong electric field portions of said resonator field, while electrons-between said electron groups'are converged and passed through relatively weak portions of said resonatorfield.

9. Ultra high frequency apparatus wherein a beam comprising a longitudinal series of electrons grouped into portions of relatively high electron density separated .by portions of relatively W electron density is maintained in energy-exchanging relation with .an oscillatory electromagnetic field for .eificient extraction of r energy from said beam, said apparatus comprise ing means defining a hollow resonator-adapted to contain an oscillatory electromagnetic field and having an annular aperture therewithin surrounding the path of said beam for coupling said beam with said field and also having a tubular portion through which said electron beam is introduced to said resonator, and means along the path of said beam forcausing a first part thereof to assume a radually decreasing cross section during its travel along said tubular portion and for causing a second part of said beam to assume a gradually increasing cross section, said decreasing cross section part and said increasing cross section part being in mutually partially overlapping relation.

10. The apparatus defined in claim 9 wherein the end of said tubular portion and a wall of said resonator define said annular aperture.

11. Eflicient ultra-high-frequency energy generating apparatus comprising first and second alternatingelectromagnetic field defining means, and means for producing an electron beam and directing it through said first electromagnetic field. defining means and thence through said second electromagnetic field defining means, said beam producing means including means for tapering said beam toward a focal point beyond said second field means, said tapering means being adapted to converge said beam over a length greater than the distance from said first field defining means to said focal point.

12. The method of efficiently deriving high frequency energy from an electron beam having focusing and accelerating potentials associated therewith which comprises the steps of adjusting said focusing and accelerating potentials to concentrate said beam into a tapered beam converging on a predetermined focal point, segregating electrons in said concentrated beam into groups longitudinally spaced along said beam, and generating an alternating electromagnetic field of annular configuration, said last-named step including passing said electron grouped beam in energy exchanging relation with said alternating electromagnetic field in advance of saidpoint in such phase that energy is extracted by said field from said beam.

13. The method of efficiently deriving high frequency energy from an electron beam of normally undefined cross section, said beam having adjustable focusing and accelerating potentials associated therewith, which comprises the steps of adjusting said focusing potential to provide tapering of said beam, adjusting said accelerating potential to cause electrons in said tapered beam to become suitably periodically grouped, said grouped electrons being caused to disperse by the inter-electron forces while electrons between said: groups are sharplyconverged, passing said grouped electrons .through a relatively strong portion of an alternating electromagnetic field having a non-uniform cross-sectional'field strength forextracting energy from said grouped electrons and passing said electrons between groups through a relatively weak portion of said field, whereby relatively little energy is extracted from said field by said latter electrons.

14. The method of operating on an electron beam of normally undefined cross section to effi ciently derive high frequency energy therefrom, said beam having focusing, accelerating and high frequency alternating potentials associated therewith, which comprises the steps of adjusting said focusing potential-to concentrate said beam into a beam of gradually diminishing cross-section, adjusting the relative strengths of said accelerating andhigh frequency alternating potentials to cause electrons in said concentrated beamto become periodically segregated primarily into suitable groups, whereby the relatively high interelectron forces within said groups to cause electrons of said groups to become dispersed while the electrons between groups remain converged, passing said dispersed electron in energy-exchanging relation with a portion of an alternating electromagnetic field having a non-uniform cross-sectional field strength in such phase that energy is transferred from the electron groups to the field and with a relatively short efiective transit time in saidfield portion, whereby efficient energy-exchange occurs, and passing said electrons between groups through another portion of said field with a relatively long effective transit time, whereby only a small amount of energy is extracted from said field by said latter electrons.

15. The method of efficiently deriving high frequency energy from an electron stream which comprises the steps of providing a first energy field distribution in the space through which said stream passes to concentrate said stream into a tapered stream converging toward a predetermined focal point, providing a second energy field distribution in the space through which said tapered stream passes to cause the electrons in said tapered stream to become grouped, causing the electrons of said groups to yield up energy to a relatively intense component of an alternating electromagnetic field having a nonuniform cross-sectional field strength, and causing electrons of said stream between groups to extract energy from a relatively weak component of said field.

16. The method of eflic'iently generating ultrahigh-frequency energy consisting in the steps of producing an electron beam. adjusting the focusing and accelerating potentials thereof to provide tapering of said beam toward a given focal point, and passing of said. tapered beam through an alternating electromagnetic field portion in advance of said focal point, andto provide grouping of the electrons of said beam in accordance with the field existing in said alternating electromagnetic field portion, and to provide passing of said electron grouped beam through a second a1- ternating electromagnetic field portion also in alvance of said focal point.

17. The method of eificiently generating ultrahigh frequency energy consisting in the steps of producing an electron beam, subjecting said beam to adjustable focusing and accelerating potentials, adjustingsaid focusing potential to provide tapering of said beam toward a given focal point, adjusting said accelerating potential to provide suitable grouping of the electrons of said tapered beam into portions of relatively high electron density separated by portions of relatively low electron density, generating an alternating electromagnetic field by passing said tapered, grouped electron beam therethrough in advance of said focal point.

EUGENE FEENBERG.

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

Number McNally Jan. 7, 1947

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