US2961571A - Injected beam axiotron - Google Patents

Description

Nov. 22, 1960 E. J. cooK INJECTED BEAM AXIOTRON Filed April 1e, 195e ite f INJECTED BEAM AXIOTRON Filed Apr. 16, 195s, ser. No. 728,843

7 claims. (Cl. als-3.5)

The present invention relates to ya traveling wave anipliier having crossed electric and magnetic iields.

As is generally known, a traveling Wave ampliiier includes a slow wave structure such as a helix or folded strip line for propagating an electromagnetic wave with a phase velocity much less than the speed of light, which is in contrast to most circuits in which the phase velocity is greater than the speed of light. An electron gun injects an electron beam along the slow wave structure in close association thereto at a velocity approximately equal to that of the phase velocity of the electromagnetic Wave. The electrons in this beam form intobunches that interact with the electromagnetic wave in a manner such that energy is transferred to the electromagnetic waive.

In many traveling wave ampliliers the electron beam, when interacting with the electromagnetic Wave, becomes velocity modulated to such an extent that the desired phase relationship between the electromagnetic wave and the electron bunches in the beam is destroyed, which, of course, decreases the operating eliiciency.

Accordingly, an object of the present invention is to provide a more eflicient traveling wave amplifier.

Another object is to provide a traveling 'Wave amplilier in which the velocity of the electrons in the electron beam in the direction of propagation ofthe slow wave is maintained substantially constant.

In some traveling wave amplifiers in which the electron beam is hollow, the shape o'f the beam is maintained by rotating the electrons around cylindrical shells in vth'e beam at high velocities. The energy in vthe form lof this rotational movement is inaccessible to an axial interaction with the slow wave and thus does not contribute to the gain of the amplifier.

Thus, a further object of the present invention is to provide a hollow-beam type traveling wave amplifier in which only a small amount of energy is utilized for rotating the electrons.

Still another Vobject is to provide a cross-field type traveling wave amplifier having high gain.

These and other objects are achieved in `one traveling wave amplilier embodiment of my invention in which an equilibrium beam is injected coaxially into an interaction region including -a slow wave structure along which an electromagnetic wave is propagated. By equilibrium beam is meant a beam in the shape of a right circular cylinder, the electrons of whichV do not mofve radially. A direct magnetic lield in the shape of circles coaxial with the electron beam and a radial electric lield `along with a slight rotation of the inner electrons of the beam maintain the shape of the electron beam as it progresses along the slow wave structure. This magnetic `eld also maintains the axial velocity of the velectron lbeain substantially constant.

The novel features #believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference tofthe following rates Patent 'rice description, taken in connection with the accompanying vdrawing in which:

Fig. l is la cross-sectional side View of a continuous Wave traveling wave amplier embodiment rof my invention, and

Fig. 2 is a diagrammatic illustration of a pulsed traveling wave amplifier embodiment.

Referring now to Fig. 1, I have illustrated an evacuated cylindrical shell 11 with ends 13 and 15 constructed yfrom a conducting material such as stainless steel that will maintain a vacuum. A conducting rod 17 extending along the axis of shell 11 is insulated at one end therefrom by an insulator 19 to form a series circuit so that when a source of direct potential 20 is applied between a conducting band 21 mounted around an end of shell 11 and a conducting plug 23 inserted in an end of rod 17, a 'direct current iiows along rod 17 and back along shell 11. This current Vthrough rod 17 produces a magnetic field yin the form of circles concentric with rod 17, or in other Words that are in the theta direction, that decrease in' magnitude in a direction radially away from rod 17. Shell 11 is utilized as the return current path since, due to the symmetry of current ow through it, there is no flux within shell 11 resulting from this current ow. Alternatively, the -return current path could be a separate wire from which the interior of Vshell 11 is magnetically shielded. But the illustrated arrangement is much simpler. y

The electromagnetic wave to be amplified isrconducted to the interior of shell 11 `by means illustrated as a c'oaxial line with an outer conductor`27 and a inner conductor 29 between which `an insulator 31 extends for providing a vacuum seal. Inner conductor 29 is connected to rod 17 while outer conductor 27 is one arm of a T connection that has another arm that is a quarter wave length helix section 33, and a third arm Vthat is a coaxial line section 35. Helix `section 33 is made a quarter Wave length long at the midfrequency of the Ainput electromagnetic wave, to present an open circuit so that substantially all o-f the input electromagnetic wave propagates along coaxial line section 35. Of course, a helix does not have to be used, but it has the advantage of Vproviding a quarter wave length component in a shorter space than, for example, 'a coaxial line section.

From coaxial line section 35 the electromagnetic wave goes into an interaction region 37 wherein it propagates along a Vslow wavestructure 39 shown to be a helix but which could be any slow wave structure having a radius great enough to prevent shorting by rod 17 and along which the electromagnetic wave propagates with an electric field Vhaving a `component in the axial direction but none in the theta direction. The only other component of interaction region 37 is an outer drift tube '41 which, with slow wave structure 39, acts as a means for producing an electric field in they radial direction when `an electric potential, Vdescribed below, is applied across them. This lield can ybe, made substantially uniform along slow wave structure 31 if this structure comprises `several helices connected in parallel. Y i

Slow wave structure 39 has suflicient length -such Vthat a hollow electron beam A43 injected into interaction region 37 delivers maximum power to the electromagnetic wave, but it is not so long as to intercept beam V 43. That is, beam 43, in delivering energy tothe electromagnetic wave decreases in radius. Consequently, if slow wave 'structure 39 were too long, it would intercept beam 43;

The radius of drift tube 41 should be great enough `so that it does not adversely affect the coupling between beam 43 and the electromagnetic wave propagated along slow wavestructure 39.

Slow wave structure 39 terminates in a T connection, one larm 'of which comprises a coaxial line section 45,

another arm of which is a quarter wave length helix 47 which presents an open circuit, and the third arm of which is a means for abstracting the ampliiied electromagnetic Wave from the amplifier. This last means is illustrated as a coaxial line with an outer conductor 49 and an inner conductor 51 between which an insulator 53 provides a vacuum seal.

Electrodes in an electron gun region 54 produce an equilibrium beam 43 and inject it into interaction region 37. The means for producing beam 43 comprises a cathode 55 heated by a heater 57 wound non-inductively so that when it is energized by an alternating current source 59 through leads extending through some insulators 63, the current in heater 57 does not produce a magnetic field which, if present, would interfere with the movement of electrons from cathode 55. Beam current 43, the magnitude of which is controlled by a potential applied to a control grid electrode 65, is accelerated toward the axis of the amplifier by means shown as an anode electrode 67 having a potential applied to it that is positive with respect to the cathode electrode potential. While being accelerated, beam 43 is injected by action of the theta magnetic field and the dellection electrode 69 into the space between slow wave structure 39 and drift tube 41 on an equi-potential corresponding to the velocity of the outer electrons in beam 43.

The inner electrons in beam 43 are given an angular momentum as they cross magnetic ux lines produced by a magnet comprising a group of coils 71 energized by a source of potential 73 and Wound within a magnetic yoke 75. This magnet is positioned with respect to cathode 55 such that an electron emerging from cathode 55 near the interior of beam 43 crosses more flux lines than an electron emerging from the exterior of beam 43. In fact, the electrons on the exterior of beam 43 preferably do not cross any liux lines. Of course, when an electron crosses ux lines, it is acted upon by a force directed at right angles to the direction of electron movement and to the direction of the ilux lines. In the present system, this force causes the electrons to rotate in cylindrical shells within beam 43.

The amplier has a collector region 81 comprising a depressor electrode S3 made suthciently negative in potential to deflect electron beam 43 into a collector electrode 85. A potential on collector electrode 85 opposes the movement of beam 43 such that most of the kinetic energy of the electrons in beam 43 is abstracted before they strike electrode 85. Consequently, electrode 85 does not heat to a significant degree.

The sources of potential for the electrodes include a source of direct potential 87 for maintaining cathode 55 at a potential of V1 volts below ground potential, wherein the potential of shell 11 is ground potential. Another direct potential source 89 maintains the deflection electrode 69 at cathode potential or slightly below, depending upon the desired point of injection of beam 43 into interaction region 37. An adjustable direct potential source 91 controls the potential on grid electrode 65 and thus the magnitude of beam current 43.

Anode electrode 67 is maintained negative with respect to ground by a source of direct potential 93, but not nearly so negative as cathode 55 so that it is positive with respect to cathode 55 and thus can accelerate beam 43. Source 93 and another source 95 maintain the outer drift tube 41 negative with respect to ground thereby creating a radial field between drift tube 41 and slow wave structure 39 such that the electrons in beam 43 experience a force directed towards slow wave structure 39. These sources also place a negative potential on depressor electrode 83 to enable it to deect electron beam 43 into collector electrode 85. By means of a variable source of direct potential 97, the potential on collector electrode 85 can be adjusted so that the electrons from beam 43 strike electrode 85 with substantially zero velocity.

veo

4 The energy abstracted from electron beam 43 is converted into electrical energy which is stored in sources 97 and 93.

While many features of the amplifier can be obtained from conventional considerations, some are not apparent. For example, it can be shown that the direct voltage V1 on slow wave structure 39 with respect to cathode 55 should be:

wherein e0 is the permeativity of free space, RA and RB are the inner and outer radii, respectively, of beam 43, IB is the magnitude of beam 43, V is the axial velocity of the electrons in beam 43, R1 and R2 are the radii of slow Wave structure 39 and outer drift tube 41, respectively, land e and m are the charge and mass, respectively, of an electron. Thus, with slow wave structure 39 at ground potential, source 87 should apply a potential V1 to cathode 55. Also, it can be shown that the direct voltage on the outer drift tube 41 with respect to cathode 55 should be:

1 1 I B R2 l R132 V2 VPE.; im vlRn-R-{Qi "e 2.. m

Since cathode 55 is maintained below ground by a potential of V1, the potential produced by sources 93 and 95 on outer drift tube 41 with respect to ground should be V2V1 The magnetic ield produced by the current ow through rod 17 maintains the axial velocity of the electrons in beam 43 substantially constant as they progress through interaction region 37 and also contributes to the focus of beam 43 within interaction region 37. The direction of current flow through rod 17 is such that the resulting magnetic iield produces a radially outward force on the electrons of beam 43 as they progress along interaction region 37. As the axial velocity of these electrons decreases in interaction region 37, due to the loss of energy to the electromagnetic wave propagated on slow wave structure 39, this outward force decreases with the result that the radial electric field between slow Wave structure 39 and outer drift tube 41 causes these electrons to move toward slow wave structure 39. In so moving the electrons cross magnetic ilux lines produced by the current flow through rod 17, which impart a forward axial force on the electrons that counteracts, to a large extent, the retardation of the electrons. Consequently, the axial velocity of the electrons is maintained substantially constant, and the loss of energy by the electrons corresponds to an inward movement and not a retardation of the axial velocity. It can be shown that the magnitude of current IR required in rod 17 is:

I L R1 In R linfa REL-RA2 V2 wherein no is the permeability of free space.

Preferably, some of the electrons in beam 43 rotate because, due to the finite thickness of this beam, the electrons on the inner portion of beam 43 pass through a greater acceleration potential from anode electrode 67 than do the electrons near the exterior and, as a consequence, will have a greater axial velocity than the outer electrons unless some means are inserted for either abstracting this extra energy or for converting it into a form other than that represented by axial velocity. The magnetic ield from coils 71 provide this means by establishing a flux linkage which, in electron gun region 54, causes the inner electrons to rotate thereby converting this surplus axial velocity into rotational energy.

. Y Y Thus, all of the electrons .in beam 43 have approximately the same axial velocity veven .though the total energy `of the inner electrons is vgreater than ,that of the outerlelectrons. Preferably, coils 7.1 produce amagnetic flux linkage having zero magnitude atfthe end o f ycathodeSS from which the `outer electrons emerge and a value equalttoz at the end from which the inner electrons emerge. Between these two limits, -the flux linkage of a given electron at the cathode is related to its final equilibrium position R by the following:

1fit-Bil The shape of electron beam 43 is maintained in interaction region 37 by the balancing of several forces. First of all, the mutual repulsion between electrons in -beam 43 produces a forceradially outward tending to cause these electrons to spread towards outer drift tube 41. There are two other outward forces: the force resulting from the electrons crossing the magnetic field due to the current flow through rod 17, and also the centrifugal force of the rotating electrons. Only one inward force balances these outward forces, which force results from the electrostatic field established between outer drift tube 41 and slow wave structure 39. Of course the outward forces vary throughout the radial thickness of beam 43, but then so does the inward force so that there is a substantial balancing of forces everywhere in beam 43. Due to the outward force on the electrons produced by the magnetic field from the current in rod 17, less centrifugal force is required to maintain the beamfshape than in prior hollow beam traveling wave amplifiers. Consequently, the angular velocity and thus the energy in the form of this velocity is much less in the present system. In fact, for operation,ithe electrons need not have angular velocity. That is, coils 71 and core 75 may be removed. However, then beam 43 will not then be an equilibrium beam and the operation will'not be as satisfactory. Y

In the operation of the amplifier, the input electromagnetic wave is guided by the input coaxial line comprising conductors 27 and 29 to the slow wave structure 39 which propagates this wave at the axial velocity of electron beam 43, thereby establishing a magnetron action causing the electrons in beam 43 to form in bunches having a phase relationship with the electromagnetic wave such that energy is transferred from the electron beam 43 to the electromagnetic wave. .Since the axial `velocity of these electron bunches is maintained substantially constant along the length of interaction region 37 there is maximum energy exchange and thus maximum gain. After amplification, the electromagnetic wave is guided from the amplifier by the output coaxial line comprising conductors 49 and 51. i

Although this amplifier operates similarly to a magnetron, it has higher gain because the coupling coefficient between the electron beam 43 and the electromagnetic wave in interaction region 37 has a finite, rather than zero, value at the start of the interaction. Also, in the present system, the average and instantaneous velocities of the electrons are very nearly the same, which makes it feasible to use a depressed collector and thus recover the energy that would otherwise be lost in heat. In the linear magnetron the depressed collector is not feasible Ibecause the average and instantaneous velocities of the electrons may vary widely due to cycloidal paths of the electrons. Thus, the present device provides a decided improvement in efficiency.

21X' RAZRBZ 1/2 [e HB2-RAZ V ...e0 m

Although rthe continuous wave amplifier ofFig. l may be required `in many applications, `,there are, Ano doubt, many other applications in which a pulsed amplifier is suitable. A pulsed amplifier embodying the-,principles of my invention is illustrated in Fig. 2.

In the embodiment of Fig. '2 the structure-is the same as that illustrated for Fig. 1 with the exceptions ofthe means for supplying thecurrent IR to rod.17 and the means for placing a potential V1 on cathode 55. Consequently, the structure within shell 11 hasnot been illustrated again. The Fig.` 2 embodiment has an input terminal 101 to which ,a trigger voltage pulse is applied at a time t1 before it is v,desired to operate the amplifier. For example, in a radar application, this trigger pulse may be a pulse coinciding with Vthe transmission of a radar pulse, in which case, of course,` the amplifier Vis used to amplify the receivedecho pulse. 'Ihe kpulse applied to terminal 101 triggers an kaxial current pulser 103 which then energizes rod 17 with a current pulse 105 having a maximum amplitude of IB, previously identilied, and a duration` of the order of `40 microseconds. A very convenient circuit for pulser 103 is a Ychargeable delay line having a discharging ignitron that, is triggered by the pulse applied at terminal 101. Pulse 105 also energizes a delay circuit 107 which may be a phantastron that can be adjustedto produce a pulse 109 a time t1 after being energized Vby pulse 105, wherein l1 is preferably 1A the duration of pulse 105. The forward edge of pulse 109 causes acathode pulser 111 tol produce a pulse 113 that isapproximately equal to half the duration of pulse 105 sothat it occurs during the flattest portion of pulse 105. Pulser 11 may comprise a monostable multivibrator and an amplifier that amplifies the output pulse from the multivibrator. Pulse 113, which is applied to cathode 55 by a lead 115, is of sufficient vmagnitude to raise cathode 55 from ground potential to a potential V1- which then causes beam 43 to be produced. Since the cathode 55 is energized and thus beam current 43 flows for only the .center portion of pulse 105, during which the current through rod 17 is substantially constant, the conditions of operation for the pulse intervals are the same for the amplifier in Fig. 2 as for the operation of the amplifier of Fig. 1.

While the invention has been described with respect to certain specific embodiments, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. I intend, therefore, by the appended claims, to cover allsuch modifications and changes as fall within the true spirit andV scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United` States is:

1. In a system for amplifying an electromagnetic signal, a slow wave structure for propagating said electromagnetic signal with a substantially constant phase velocity less than the speed of light, means for injecting a hollow beam of electrons around said slow wave structure, means providing an electric field for urging said beam toward said slow wave structure and means for providinga magnetic eld for applying a forward propelling force to electrons in said beam in response to inward radial movement of electrons in said beam resulting from transfer of energy to said beam for maintaining the velocity of the electrons in said electron beam substantially equal to the phase velocity of said electromagnetic signal as they progress along said slow wave structure, said propelling force being proportional to the radial component of velocity of said electrons.

2. In a systemforamplifyng an electromagnetic signal, an interaction region including a slow wave structure for propagating said electromagnetic signal with a substantially constant phase velocity less than the velocity of light, an electron gun region including means for injecting a hollow beam of electrons into said interaction region and around said slow wave structure at a velocity substantially equal to the phase velocity of said electromagnetic signal, means establishing a radial electric field around said slow wave structure for attracting said electrons radially inwardly and means for producing a magnetic field in the theta direction around said slow wave structure, which iield in interacting with said electron beam is effective to impart an axial velocity thereto in response to radial inward movement of said beam resulting from loss of energy to said signal to cause the velocity of the electrons in said beam to be maintained substantially constant as they progress along said slow wave structure.

3. A system for amplifying an electromagnetic signal, comprising an electrical signal interaction region including a linearly arranged slow wave structure for propagating said electromagnetic signal with a .phase velocity less than the velocity of light, wherein said slow wave structure is constructed such that said electromagnetic signal when propagating along said slow wave structure has an electric field with a component parallel to said slow wavestructure but with no component in the theta direction, means for producing an electric field in a radial direction along the length of said interaction region that increases in magnitude towards said slow wave structure, means for producing a direct magnetic eld in the theta direction around said slow wave structure that increases in magnitude in a direction towards said slow wave structure, wherein the direction of said magnetic field is such that an electron passing through said interaction region in the direction of Vand parallel to the propagation of said electromagnetic signal is acted upon by a force that is radially outwards; an electron gun region comprising means for injecting a hollow beam of electrons into said interaction region substantially coaxial with said slow wave structure and at a velocity substantially equal to the phase velocity of said electromagnetic signal when it is propagating along said slow wave structure, means for rotating some of the electrons in said beam such that the outward forces on said electrons in said interaction region produced by said direct magnetic eld, the space charge effects, and the centrifugal forces, are approximately balanced by the inward force caused by said electric field; and a collector 4region comprising means for abstracting some of the kinetic energy of the electrons in said beam after they have passed through said interaction region, and means for collecting said electrons after this kinetic energy has been abstracted.

4. In a system for amplifying an electromagnetic signal, a signal interaction region, means for producing a magnetic field inthe theta direction in said interaction region, means for producing a radially inward directed electric ield in said interaction region, and means for injecting a hollow beam into said interaction region, means imparting an orbital movement to some of the electrons in said beam, the electric field force applied to each electron being substantially equal and opposite to the combination of forces of the magnetic field, the centrifugal effects and space charge repulsion applied thereto.

5. A system for maintaining the shape of a hollow equilibrium beam of electrons comprising, means for producing a magnetic ield in the theta direction around the axis of said beam so that said electrons are acted lupon by a force directed radially outward, means for producing an electric. ield that is directed inward toward the axis of said beam, and means for producing a rotation of some of the electrons of said beam whereby the outward forces produced my space charge effects, said magnetic field and the centrifugal forces from rotation are substantially balanced by the inward force from said electric field. 4

6. A system for amplifyingV an electromagnetic signal comprising a cylindrically-arranged interaction region including a rod of conductingV material positioned along the center of said interaction region, a slow'wave structure arranged coaxially'with said rod for propagating said electromagnetic signal with an electric field having a component parallel to said slow wave structure but with no component in the theta direction, said` slow wave structure having a radius R1, a hollow cylindrical outer drift tube having a radius R2 much greater than R1 and arranged substantially coaxially with said slow wave structure; an electron gun region including a cathode for producing an equilibrium beam of magnitude IB and having an inner radius RA and an outer radius RB, means for accelerating said electron beam to a velocity V which is approximately equal to the phase velocity of said electromagnetic signal when propagating along said slow wave structure, means for producing a magnetic flux linkage at the cathode for the electrons on the interior of the beam approximately equal to:

wherein e0 is the permeativity of free space, and e and m are the charge and mass, respectively, of an electron, and for the electrons on the exterior of the beam is approximately zero and for each of the other electrons varies according to from the interior to the exterior of said beam, wherein R is the radius of the electron in the equilibrium beam, mean for injecting said beam into said interaction region substantially coaxially with said slow wave structure; a collector region comprising means for abstracting some of the kinetic energy of the electrons in said equilibrium beam after they have passed through said interaction region, and means for collecting said electrons after this kinetic energy has been abstracted; means for producing a current ow in said rod of magnitude:

L Re t .Mofo Razr" Raz V2 wherein o is the permeability of free space, in a direction for which the magnetic field produced thereby produces a radially outward force on the electrons of said electron beam, means for applying a voltage on said slow wave structure with respect to said cathode approximately equal to and, means for applying a voltage on said outer drift tube with respect to said cathode approximately equal to R2 l R32 V2 2 7. In a system for amplifying an electromagnetic signal, a slow wave structure for propagating said electromagnetic signal with a substantially constant phase velocity less than the speed of light, means for. producing an electric field about said slow wave structure and convergent radially in the direction thereof, means for injecting a hollow beam of electrons around said slow wave structure and perpendicular to said electric eld, means for producing a magnetic field about said slow wave structure in the region of said electric field and in the theta direction and means for imparting an orbital motion to some of the electrons in said beam, said electric field, magnetic field and beam velocity being interrelated to maintain the velocity of the electrons in said beam 9 10 substantially equal to the phase velocity of said electro- 2,643,353 Dewey June 23, 1953 magnetic signal as they progress along said slow Wave 2,645,737 Field July 14, 1953 structure and impart energy to said signal. 2,760,101 Reverdin Aug. 21, 1956 2,761,088 Warnecke et al. Aug. 28, 1956 References Clted 1n the le of this patent 5 2,812,467 Kompfner Nov 5, 1957 UNITED STATES PATENTS 2,300,052 Lindenblad Oct. 27, 1942

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