US2888610A

US2888610A - Traveling wave tubes

Info

Publication number: US2888610A
Application number: US398525A
Authority: US
Inventors: Edward C Dench
Original assignee: Raytheon Manufacturing Co
Current assignee: Raytheon Co
Priority date: 1953-12-16
Filing date: 1953-12-16
Publication date: 1959-05-26
Anticipated expiration: 1976-05-26

Description

May 26, 1959 E. c. DENCH TRAVELING WAVE TUBES Filed Dec. 16. 1953 VOLTAGE MODULAT/NG VOLTAGE MODULA TING VOL TA GE VOLTAGE ORNEY H NC w D c eva ji United States Patent Ofiice 2,888,610 teau y 6, .9 9

' TRAVELING WAVE TUB'ES Edward -C. Dench, Needh'am, Mass., assignor to Raytheon Manufacturing Company, Waltham, Mass., a corporation of Delaware Application December 16, 1953, Serial No. 398,525

14 Claims. (Cl. 315-39.3)

This invention relates to a traveling wave oscillatorand more particularly to means for producing amplitude modulation of a traveling wave oscillator.

Traveling wave oscillator tubes are known which make use of the interaction between an electron beam and one of the space harmonics of the rf wave traversing a periodic delay line. One embodiment of a cylindrical traveling wave oscillator is shown and described in an application by E. C. Dench for US. Letters Patent, Serial No. 391,628, filed November 12, 1953. Such an oscillator may also be of linear configuration.

Modulated traveling wave oscillators, whether linear or cylindrical, comprise essentially an electron gun including a cathode, a grid, closely associated with the cathode, and an accelerating electrode spaced from, and maintained at a positive potential with respect to, said cathode and grid. Adjacent the electron gun is the main portion of the tube which comprises the periodic anode delay line previously referred to and a negative electrode or sole arranged adjacent to said periodic delay line. The electron beam produced by the electron gun is caused to traverse the interaction space between said periodic delay line and sole under the combined influence of a radial electric field established between said periodic structure and sole and a magnetic field transverse to said electric field.

Because of the low amplification factor inherent in a modulating grid of the type just described, said grid must operate at a potential of the order of several hundred volts. Owing to the high operating potential of such a modulating grid, the field through which the electron beam progresses prior to its entrance into the interaction space is considerably distorted, thereby causing the beam to pursue an irregular trajectory and impairing the efiiciency of the tube. Furthermore, the modulating grid just described presents considerable capacitance, particularly at very high operating frequencies. Grid modulation at microwave frequencies, therefore, requires the use of a modulator of high power rating.

Pursuant to this invention, the crossed fields in the region of the electron gun are so proportioned that the electrons emanating from the cathode travel along cycloidal paths. The length of each cycloidal orbit is dependent upon the relative strengths of the transverse fields. Positioned a distance of one cycloidal orbit from the cathode is a control electrode which is at substantially the same level of potential as said cathode so that the electrons emitted from the cathode return to the vicinity of said control electrode after describing one cycloidal orbit. The electrons next leave the vicinity of said control electrode and may describe any number of halfcycloidal orbits before entering the interaction space between the periodic anode delay structure and the sole. By proper adjustment of the field between anode and sole, the beam may be made to progress with a linear trajectory through the aforesaid interaction space.

By connecting a low voltage modulating source between the cathode and the control electrode, that portion of the electron beam emitted by the cathode which is collected by said control electrode is varied. Consequently, the beam current in the interaction space may be madulated and amplitude modultion of the traveling wave tube may be effected. Since this control electrode is a small, low capacity electrode, satisfactory modulation may be obtained at high frequencies of the order of several megacycles with .a small amount of driving power and a minimum of thermal dissipation. Furthermore, owing to the low operating voltage of the control electrode, the crossed fields in the tube undergo very little, if any, distortion and hence a proper electron beam trajectory is obtained.

To further enhance the modulation efiiciency, that is, to increase the percentage modulation for a given value of modulating voltage, several control electrodes may be employed, each spaced from the other by one or more cycloidal orbits and each connected to the modulating source.

Other and further objects of this invention will be apparent as the description thereof progresses, reference being had to the accompanying drawing wherein:

Fig.1 is a diagrammatic representation of a linear traveling wave oscillator according to the invention;

Figs. 2 to 4 are diagrammatic views illustrating various modifications. in the arrangement of the electrodes of the tube of Fig. 1;

Fig. 5 is a view illustrating the use of a plurality of control electrodes for modulating the electron beam; and

Fig. 6 is a diagrammatic representation of a cylindrical traveling wave oscillator according to the invention.

Referring to Fig. 1, the electron gun comprises a cathode 10, a control electrode 12 coextensive therewith, and an accelerating electrode 14 maintained positive with re spect to said cathode and control electrode and spaced transversely therefrom. For purposes of simplicity, the cathode heater has been omitted in the drawing. Control electrode 12 is preferably a cold electrode that is it is not adapted to emit electrons thermionically. The cathode may be substantially circular or may consistvof an elongated emitting element such as a tungsten ribbon of the type shown in the aforesaid copending application.

The electron gun is arranged adjacent to the energy-interacting portion of the tube comprising the periodic anode structure 18 and the sole or principal electrode 20. In addition to the electric field E established between the accelerating electrode and cathode of the electron gun, a magnetic field B normal to said electric field is produced and is indicated in the drawing by crosses. I

In order to present a substantially continuous surface opposite the accelerating electrode, thereby permitting a uniform electric field to be maintained between the opposed surfaces, an auxiliary member or members 11 are provided adjacentto and coextensive with both cathode 10 and control electrode 12, as shown in Fig. 1. The configuration of these auxiliary members is, of course, dependent upon the shape and size of the cathode and control electrode, as well as on the spacing therebetween. For example, if the cathode and control electrode are circular, member 11 may be a single continuous curved member containing apertures slightly larger than the aforesaid electrodes, so that the latter are sufficiently separated electrically from the member and from one another, and at the same time close enough to the member to present a substantially continuous surface parallel to accelerating electrode 12. It may be desirable to use a plurality of members 11 cooperating with the cathode and control electrode to form a substantially continuous surface. For example, these members may be in the form of strips disposed parallel to the longitudinal axis of the tube. The

3 cathode and control electrode are preferably of the same order of magnitude and area. Cathode 10 and auxiliary member 11 are electrically connected to form an equipotential surface. This potential is indicated on the drawing as zero volts and is the reference potential of the traveling wave tube.

The electrons leaving the cathode under the influence of combined electric and magnetic fields travel in cycloidal paths. The length I (see Fig. 2) of the cycloidal orbits is a function of the magnitude of both the electric and the magnetic fields.

The velocity of an undefiected electron in uniform mutually perpendicular electric and magnetic fields of strength E and B, respectively, in the direction normal to both of said fields is given by This is a velocity of the electron when the force exerted thereon by the electric field just balances the force on the electron in the magnetic field.

An electron whose initial velocity is zero or between zero and the velocity for an undefiected electron will, after leaving the cathode, start to move toward the positive electrode 14. As it moves it gains kinetic energy from the electric field and hence gains velocity. The magnetic force on the electron thus increases, and eventually becomes large enough to bend the electron trajectory back until the electron again has its original velocity and is at substantially the same potential in the electric field as that at which it started. This action is repetitive and the electron starts out on another cycloidal orbit.

Positioned at or adjacent the cusp of the first cycloidal orbit of the electron beam 15 is the control electrode 12 which, in the absence of any modulating voltage, is at substantially the same potential as said cathode. After leaving the vicinity of this control electrode, the electrons move in another cycloidal path. It is desirable that the electron beam enter the interaction space 19 between the periodic anode and the sole along a rectilinear path tangential to the top of a cycloidal are so as not to enter a retarding or accelerating field and thereby move in an irregular manner in said interaction space.

The velocity for an undeflected electron (one moving in a rectilinear path) is equal to where E is the strength of the electric field between anode and sole and B is the strength of the associated magnetic field. It may be shown that the velocity parallel to the anode 18 and sole 20 of the electron beam at the top of the cycloidal arc is twice the average velocity, or velocity for an undefiected beam, or

Where E is the strength of the electric field between both the cathode and the accelerating anode. Equating the two velocities The electric field in the electron gun should be half that in the main portion of the tube.

The electron velocity corresponds to V electron volts, where V is the voltage 4 relative to the cathode, and is synchronous with the wave to be generated in the interaction space. The electron should enter the interaction space along an equipotential line of potential V Now the voltage at the point located a distance y from the cathode (top of cycloidal arc) is given by The voltage level along the path of the beam in the interaction space should also be equal to V That is, if the distance between the beam 15 and the sole is y;, the potential of the point of entrance of the beam into the interaction space 19 is given by Equating Equations 2 and 3 E'y =V =Ey From Equation 1 That is to say, the distance between the electron beam in the interaction space and the sole should be half the distance between the cathode and the top of the cycloidal are for applications in which the cathode and sole are at the same potential. From inspection it is obvious that the distance y is equal to y If desired, the sole may be moved away from the beam, that is, from the position shown in Fig. 1, by a distance z and simultaneously biased with a negative voltage V with respect to the cathode where V =EZ (7) For example, if the electric field E is 600 volts per millimeter and the sole is moved from the position shown in Fig. l by, say, one millimeter (that is, z=one millimeter) in order to lie coextensive with the cathode, the sole would have to be biased 600 1 or 600 volts negative with respect to the cathode. Likewise, if the sole were brought nearer to the anode than shown in Fig. 1, the sole would have to be biased positively, in accordance with Equation 7. As shown in Fig. 3, the same effect may be produced by leaving sole 20 fixed in the position shown in Fig. 1 and moving anode 18 away from the beam by a distance z and simultaneously increasing the potential V of the anode by an amount 2 where E=Ez' For example, assume that the anode is moved one millimeter further from the beam than in the position shown in Fig. 1; that is, z' in Fig. 3 is one millimeter. If the field E is 600 volts per millimeter, the anode potential must be increased by an amount 600 1=600 volts over and above the anode potential of Fig. 1.

Both anode and sole may be moved away from the beam simultaneously provided the potential on both electrodes is increased.

Although the two systems-the electron gun and the main portion of the tube-have been shown as insulated from one another, this need not be the case. As shown in Fig. 4, a separate accelerating electrode is dispensed with and a portion 18' of the periodic anode extends opposite the cathode and control electrode, including the associated equipotential structure. In this event, the spacing between anode and sole must be reduced to half the distance between the accelerator and cathode in order that the relationship shown in Equation 4 shall obtain.

An accelerating electrode is usually desirable in order to maintain the field in the electron gun constant in spite of variations in anode voltage incident to tuning of the traveling wave oscillator. A separate accelerating e ectrode may, of course, be included in the system shown in Fig. 4. In this event, the accelerating electrode would be arranged substantially parallel to and adjacent the extension 18 of anode 18.

Referring again to Fig. l, a source 22 of modulating voltage of the order of fifty volts is connected between the cathode and control electrode 12. If the source is adjusted so that the potential on the control electrode is a few volts positive with respect to the cathode, all of the electrons leaving the cathode will be collected by this control electrode and the beam will be cut oif. If the control electrode is at the same potential as the cathode or slightly negative with respect thereto, substantially all of the electrons emitted by the cathode will be allowed to depart from the vicinity of the control electrode and continue into the interaction space 19 of the tube. In between these values of modulating potential, a portion of the electron beam will be collected by the control electrode and the remainder will be directed through the interaction space. The proportion of the total number of electrons which enter the interaction space will be a function of the magnitude of the modulating potential. Since the electrons in beam impinge upon the control electrode with negligible velocities of the order of less than ten volts (corresponding to the cutoff voltage) the likelihood of secondary emission is negligible. Positive insurance against secondary emission may be provided, however, by coating the auxiliary electrode with a material such as graphite which does not emit secondarily at relatively low voltages.

Although only one control electrode is suflicient in most applications, it is desirable, in applications where greater efliciency of modulation is required, to use several control electrodes spaced one cycloidal orbit I from one another, as shown in Fig. 5. With two or more of such control electrodes, each contributes to the control of the electron beam with consequently greater control for a given modulating voltage.

The principles of the subject invention are not limited to linear traveling wave tubes but are equally applicable to cylindrical traveling wave tubes, as shown in Fig. 6, and described in detail in the aforesaid copending application. For purposes of clarity the various connections to the tube electrode have been omitted. The modulator is connected between cathode 10 and control electrode 12, as in the tube of Fig. l. The tube of Fig. 6 includes an arcuate periodic anode structure 18 spaced from a concentrically disposed sole or principal electrode 20 to form the usual interaction space 19. As shown in Fig. 6, the periodic anode structure is of the interdigital type including a plurality of interdigital fingers 24 spaced from a circumferential back wall 25. This invention, however, is not limited to interdigital periodic structures. For example, the anode may be a periodically loaded waveguide or strapped vane structure.

The cathode 10, control electrode or electrodes 12, and associated member or members 11 are mounted within an elongated slot 30 in the sole. An arcuate accelerating electrode 14 is arranged coaxial with and opposite to said cathode, control electrode, and associated members. The beam, after traversing one-and-a-half cycloidal orbits, is projected through the interaction space along an arcuate path substantially concentric with said anode structure. The usual collector electrode 26 may be provided so that the tube structure is non-reentrant. Although not shown in Fig. 6, for reasons of clarity, a radial electric field is established between the periodic anode structure 18 and the sole 20, while a transverse magnetic field is established normal thereto, and in the plane of the paper. One of the pole pieces 28 of the magnetic field-producing means is shown in Fig. 6.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A traveling wave electron discharge device comprising an electron-emitting electrode and an electron-collecting control electrode, means for directing electrons from said electron-emitting electrode toward said control electrode along a cycloidal orbit, said control electrode being a substantially continuous solid element whereby none of said electrons pass therethrough, a periodic radiofrequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure, means for controlling the number of electrons moving from said control electrode into the region between said structure and said electrode element, said means for controlling including means for causing certain of said electrons to be collected by said control electrode.

2. A traveling wave electron discharge device comprising an electron-emitting electrode and an electron-collecting control electrode, said control electrode comprising means for suppressing secondary electron emission, means for directing electrons from said electron-emitting electrode toward said control electrode along a cycloidal orbit, said control electrode being a substantially continuous solid element whereby none of said electrons pass therethrough, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure, means for controlling the number of electrons moving from said control electrode into the region between said structure and said electrode element, said means for controlling including means for causing certain of said electrons to be collected by said control electrode.

3. A traveling wave electron discharge device comprising an electron-emitting electrode and an electroncollecting control electrode spaced from said electronemitting electrode, said spaced electrodes being of the same order of magnitude and area, means for directing electrons from said electron-emitting electrode toward said control electrode along a cycloidal orbit, said control electrode being a substanitally continuous solid element whereby none of said electrons pass therethrough, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure, means for controlling the number of electrons moving from said control electrode into the region between said structure and said electrode element, said means for controlling including means for causing certain of said electrons to be collected by said control electrode.

4. A traveling wave electron discharge device comprising an electron-emitting electrode andan electroncollecting control electrode, at least one auxiliary electrode maintained at the potential of said emitting electrode and cooperating with said emitting electrode and said control electrode to present a substantially continuous surface, means for directing electrons from said electron-emitting electrode toward said control electrode along a cycloidal orbit, said control electrode being a substantially continuous solid element whereby none of said electrons pass therethrough, a periodic radio-frequency wave energy tranmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure, means for controlling the number of electrons moving from said control electrode into the region between said structure and said electrode element, said means for controlling including means for causing certain of said electrons to be collected by said control electrode.

5. A traveling wave electron discharge device comprising an electron-emitting electrode and an electroncollecting control electrode, means for directing electrons from said electron-emitting electrode toward said control electrode along a cycloidal orbit, said control electrode being located at a cusp of said cycloidal orbit, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure, means for causing certain of said electrons to be collected by said control electrode, certain of said remaining electrons moving from said control electrode along a cycloidal path and being introduced into the region between said structure and said principal electrode element at a point tangential to the peak of said cycloidal path, said electrons in said region being in energy-exchanging relation with the fields of said wave energy.

6. A traveling wave electron discharge device comprising an electron-emitting electrode and an electroncollecting control electrode, said control electrode comprising means for suppressing secondary electron emission, means for directing electrons from said electronemitting electrode toward said control electrode along a cycloidal orbit, said control electrode being located at a cusp of said cycloidal orbit, a periodic radio-frequency Wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure, means for causing certain of said electrons to be collected by said control electrode, certain of said remaining electrons moving from said control electrode along a cycloidal path and being introduced into the region between said structure and said principal electrode element at a point tangential to the peak of said cycloidal path, said electrons in said region being in energy-exchanging relation with the fields of said wave energy.

7. A traveling wave electron discharge device comprising a plurality of spaced electrodes certain ones of which are electron-emitting electrodes and certain ones of which are electron-collecting control electrodes, means for directing electrons from each of said electron-emitting electrodes toward an adjacent control electrode along a cycloidal orbit, said control electrodes being substantially continuous solid elements whereby none of said electrons pass therethrough, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure whereby an interaction space is formed between said structure and said principal electrode element, and means for controlling the number of electrons moving from said control electrodes into said interaction space, said means for controlling including means for causing certain of said electrons to be collected by said corresponding control electrodes.

8. A traveling wave electron discharge device comprising a plurality of spaced electrodes one of which is an electron-emitting electrode and certain ones of which are electron-collecting control electrodes, means for directing electrons from each of said electron-emitting electrodes toward an adjacent control electrode along a cycloidal orbit, said control electrodes being substantially continuous solid elements whereby none of said electrons pass therethrough, a periodic radio-frequency Wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure whereby an interaction space is formed between said structure and said principal electrode element, and means for controlling the number of electrons moving from said control electrodes into said interaction space, said means for controlling including means for causing 8 certain of said electrons to be collected by said corresponding control electrodes.

9. A traveling wave electron discharge device comprising an electron-emitting electrode and an electroncollecting control electrode, means for directing electrons from said electron-emitting electrode toward said control electrode along a cycloidal orbit, said control electrode being located at a cusp of said cycloidal orbit, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure, means for providing a potential difierence between said control electrode and said emitting electrode for causing a certain number of said electrons to be collected by said control electrode, the number of said electrons collected being a function of said potential difierence, certain of said remaining electrons moving from said control electrode along a cycloidal path and being introduced into the region between said structure and said principal electrode element at a point tangential to the peak of said cycloidal path, said electrons in said region being in energy-exchanging relation with the fields of said wave energy.

10. A traveling wave electron discharge device comprising a plurality of spaced electrodes certain ones of which are electron-emitting electrodes and certain ones of which are electron-collecting control electrodes, means for directing electrons from each of said electron-emitting electrodes toward said control electrode along a cycloidal orbit, said control electrode being located at a cusp of said cycloidal orbit, a periodic radio-frequency Wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure whereby an interaction space is formed between said structure and said principal electrode element, and means for providing a potential difference between each of said control electrodes and said emitting electrodes for causing a certain number of said electrons to be collected by said control electrodes, the number of said electrons to be collected by said control electrodes being a function of said potential difference, certain of said remaining electrons moving from said control electrode adjacent said interaction space along a cycloidal path and being introduced into said interaction space at a point tangential to the peak of said cycloidal path, said electrons in said interaction space being in energy-exchanging relation with the fields of said wave energy.

11. A traveling wave electron discharge device comprising an accelerating member, an electron-emitting electrode and an electron-collecting control electrode spaced from and aligned with said emitting electrode, means for producing an electric field between said accelerating electrode and said spaced electrodes which is substantially uniform, means for producing a magnetic field transverse to said electric field, said transverse fields being of such strength as to cause electrons emitted from one of said electrodes to move toward said control electrode along a cycloidal orbit, said control electrodes being a substantially continuous solid element whereby none of said electrons pass therethrough, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the Wave energy being transmitted, a principal electrode element spaced from said structure whereby an interaction space is formed between said structure and said principal electrode element, means for controlling the number of electrons moving from said control electrode in said interaction space, said means for controlling including means for causing certain of said electrons to be collected by said control electrode, said electric and magnetic fields between said accelerating member and said spaced electrodes further being of such strength as to direct certain of said electrons from said collector electrode along a portion of a cycloidal path prior to introduction into said interaction space, said electrons within said interaction space being in energy-exchanging relation with the fields of said wave energy.

12. A traveling wave electron discharge device ,comprising an accelerating member, an electron-emitting electrode and an electron-collecting control electrode spaced from and aligned with said emitting electrode, means for producing an electric field between said accelerating electrode and said spaced electrodes which is substantially uniform, means for producing a magnetic field transverse to said electric field, said transverse fields being of such strength as to cause electrons emitted from one of said electrodes to move toward said control electrode along a cycloidal orbit, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure whereby an interaction space is formed between said structure and said principal electrode element, means for controlling the number of electrons introduced into said interaction space, said means for controlling including means for causing certain of said electrons to be collected by said control electrode, means for producing transverse electric and magnetic fields in said interaction space, the strength of the electric field in said interaction space being substantially twice that of the electric field existing between said accelerating member and said spaced electrodes, said electric and magnetic fields between said accelerating member and said spaced electrodes further being of such strength as to direct certain of said electrons from said collector electrode along a portion of a cycloidal path prior to introduction into said interaction space, said electrons within said interaction space being in energy-exchanging relation with the fields of said wave energy.

13. A traveling wave electron discharge device comprising an accelerating member and a plurality of spaced aligned electrodes, one of said electrodes being an electron-emitting electrode and at least one of said electrodes being a control electrode, means for producing an electric field between said accelerating electrode and said spaced electrodes which is substantially uniform, means for producing a magnetic field transverse to said electric field, said transverse fields being of such strength as to cause electrons emitted from one of said electrodes to move toward said control electrodes along cycloidal orbits, said control electrode being located at a cusp of said cycloidal orbits, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radiofrequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure whereby an interaction space is formed between said structure and said principal electrode element, means for controlling the number of electrons introduced into said interaction space, said means for controlling including means for causing certain of said electrons to be collected by said control electrode, said electric and magnetic fields between said accelerating member and said spaced electrodes further being of such strength as to direct certain of said electrons from the collector electrode adjacent said interaction space along a cycloidal path, said electrons introduced into said interaction space being in energy-exchanging relation with the fields of said wave energy.

14. A traveling wave electron discharge device comprising an accelerating member and a plurality of spaced aligned electrodes, one of said electrodes being an electron-emitting electrode and at least one of said electrodes being a control electrode, means for producing an electric field between said accelerating electrode and said spaced electrodes which is substantially uniform, means for producing a magnetic field transverse to said electric field, said transverse fields being of such strength as to cause electrons emitted from one of said electrodes to move toward said control electrodes along cycloidal orbits, the remaining ones of said spaced electrodes being maintained at the potential of said emitting electrode and cooperating with said emitting electrode and said control electrodes to present a substantially continuous surface parallel to said accelerating electrode, said control electrode being located at a cusp of said cycloidal orbits, a periodic radio-frequency wave energy transmission structure for producing adjacent thereto radio-frequency fields of the wave energy being transmitted, a principal electrode element spaced from said structure whereby an interaction space is formed between said structure and said principal electrode element, means for controlling the number of electrons introduced into said interaction space, said means for controlling including means for causing certain of said electrons to be collected by said control electrode, said electric and magnetic fields between said accelerating member and said spaced electrodes further being of such strength as to direct certain of said electrons from the collector electrode adjacent said interaction space along a cycloidal path, said electrons introduced into said interaction space being in energy-exchanging relation with the fields of said wave energy.

References Cited in the file of this patent UNITED STATES PATENTS