US3249792A

US3249792A - Traveling wave tube with fast wave interaction means

Info

Publication number: US3249792A
Application number: US101739A
Authority: US
Inventors: Richard H Pantell
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1961-04-10
Filing date: 1961-04-10
Publication date: 1966-05-03
Anticipated expiration: 1983-05-03

Description

y 3, 1966 RQH. PANTELL 3,249,792

TRAVELING WAVE TUBE WITH FAST WAVE INTERACTION MEANS I FIGJ Filed April 10' 1961- FIG. 5 42 79;

INVENTOR. RICHARD H. PANTELL ATTORNEY United States Patent s 249 792 TRAVELING WAVETUBE WITH FAST WAVE INTERACTION MEANS Richard H. Pantell, Palo Alto, Calif., assignor to Varian The present invention relates in general to microwave tubes cap able of operating in the millimeter wave band and in particular to fast wave tubes employing electrons which traverse a periodic path.

Cumulative interaction between an electron beam and an electromagnetic circuit field can be obtained by having the phase velocity of the circuit wave approximately equal to the beam velocity. Such a circuit wave is usually termed a slow wave, since the phase velocity is less than the free space velocity of light whereas in the ordinary waveguide the phase velocity is greater than or equal to the velocity of light. A slow wave circuit may be obrtained by means of a conducting helix or by periodic loading of a waveguide. The characteristics of a slow wave circuit make it difficult to achieve interaction with the slow wave circuit in the millimeter wavelength region, because the electromagnetic field strength is greatest at the conducting surface and decays in magnitude approximately in an exponential form away from the surface and the periodicity of the slow wave structure is less than the freespace wavelength. The decay of electromagnetic field strength away from the conducting surface means a decreased beam coupling impedance unless the beam is compressed into an area extremely close to the metallic walls of the waveguide. This results in problems of beam interception and heat dissipation. The necessity for having periodic loading which is less than the free-space wavelength introduces difficult slow wave circuit fabrication problems at millimeter wavelengths.

One approach which eliminates the above objections to the use of slow wave circuits for generating millimeter wavelengths is to inject electrons into a longitudinal magnetic field with some initial transverse motion. The electrons will rotate in a helical-beam path at the cyclotron frequency in the transverse plane. If an electromagnetic wave which is polarized in the transverse plane and which oscillates at the cyclotron frequency is present, a cumulative energy exchange between the electron and the wave will occur. There are two aspects to this type interaction: the synchronism condition between the electromagnetic wave and the electron beam and the bunching or discrimination phenomenon of the electron beam. The synchronism condition requires some relationship among the parameters of the system so that a cumulative interaction can occur between an electron beam and a fast electromagnetic wave. The bunching or discrimination phenomenon involves the manner in which an initially unbunched beam of electrons may transfer energy to a circuit wave; in other words, the means by which electrons which enter in an accelerating field do not absorb as much energy as the electrons in the decelerating field deliver. A tube structure utilizing interaction between a periodic beam and a fast wave circuit is taught in my co-pending application U.S. Serial No. 32,762 entitled Traveling Wave Interaction Device filed May 31, 1960, now U.S. Patent 3,183,- 399, granted May 11, 1965. The present application is directed to an improvement over the structure taught in the aforementioned co-pending application.

It is therefore the object of this invention to provide an improved microwave tube incorporating electron beam interaction with fast waves.

The main feature of this invention is to produce a traveling wave tube type electron tube having the necessary focusing structure to produce a periodic beam tra- 10 of the tube.

"ice

jectory whereby the electron beam and a two wire electromagnetic wave circuit may cumulatively interact.

Another feature of the present invention is the use of a parallel plane magnetron using cross fields to provide a cumulative interaction between an electron beam pursuing a cycloidal path and an electromagnetic wave.

Other features and advantages of the present invention will become apparent upon a perusal of the specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a sectional view, in part diagrammatic, of a fast wave electron tube apparatus employing a feature of the present invention,

FIG. 2 is a cross-sectional view of FIG. 1 taken along line 22 in the direction of the arrows,

FIG. 3 shows an w-B diagram showing interaction between the electron beam and an electromagnetic wave,

FIG. 4 shows in schematic form how the bunching of electrons occurs in a rectangular waveguide, and

FIG. 5 is a sectional view in part diagrammatic, of a linear magnetron employing the features of the present invention.

Referring now to FIGS. 1 and 2 there is shown a high frequency, traveling Wave type tube in which an electron beam traversing a helical path in a DC. longitudinal magnetic field interacts with a suitable wave. More specifically, a canted cathode emitter 1 is disposed at one end of a cylindrical glass casing 12 and is heated to an operating temperature by heater elements and supplied with the necessary operating potentials from a source (not shown) via heater leads generally shown at 2.

An anode is defined by apertured ring 3 provided for drawing the electrons from the canted cathode emitter 1 at an angle, and forming them into a beam. The beam emerging from the anode passes into an axial magnetic field at the aperture in anode 3 at an angle where the beam is given an angular velocity. The beam entering the traveling wave tube portion of the structure is matched to magnetic focusing structure 4 for producing and maintaining a helical path at the cyclotron frequency. After passing through the focusing structure 4 the beam is collected by a collector 5, hermetically sealed at its periphery to the glass casing 12.

Wave energy which it is desired to amplify is fed into the input section of the tube via a parallel transmission line 6 operating in the dominant mode. The input wave energy is propagated through the waveguide section 6 of the tube forming a beam field interaction space 10 of the tube apparatus. In this beam interaction space 10 the transverse electric fields of the circuit mode interact with the spiraling electron beams to produce amplification of the wave energy by cumulative interaction. The amplified wave energy on parallel line 6 is then extracted from the tube at 7 and conducted to any suitable load (not shown).

Suitable operating potentials are applied to the anode member 3 with respect to the cathode emitter 1. A power supply (not shown) is provided to insure suitable operating potential. The high cathode to anode D.C. potential is held off via a glass insulator 9 of the tube envelope 12. The power supply is preferably grounded at the positive end thereof such that the tube body may be operated at ground potential thereby minimizing the risk to operating personnel.

The particular magnetic focusing utilized in the structure of FIGS. 1 and 2 is a solenoid 4 which provides a longitudinal magnetic field within the interaction area An adjustable power supply is provided (not shown) for the solenoid 4 so that the ratio of transverse to longitudinal motion of the electron beam can be adjusted.

In operation of the present structure, a condition of synchronism between the beam and electromagnetic field is obtained so that the electron beam 6 sees the same phase of the electromagnetic field E all along its trajectory. Further, bunching of the electrons occurs along the beam path so that the initially unbunched beam of electrons may transfer energy to the circuit wave.

For a circuit wave propagating as elwFW/m the synchronism condition is for and z=ia t, Where =D.C. angular velocity of the beam and n: an integer, a' constant D.C. longitudinal velocity of the beam and c=free space velocity of light. Therefore, the synchronous frequency w is:

The minus sign corresponds to a beam velocity and circuit waves propagating in the same direction. The plus sign is for a phase velocity opposite to longitudinal direction of the beam motion.

Before bunching occurs, electrons are distributed uniformly on the surface of a cylinder rotating at the cyclotron frequency. Under such a condition there will be no net energy exchange between the electron beam 6 and the electromagnetic fields E. There are as many electrons in an accelerating field as there are in a retarding field, therefore, it is necessary to accomplish R.F. bunching. In a strong D.C. longitudinal magnetic field, electron motion in the transverse plane is inhibited by the presence of the RR field. It is only along the longitudinal direction that the electrons can move with relative ease.

For an explanation of the bunching phenomenon see FIG. 4. For a TB wave the only component of RF. field producing a longitudinally directed force is H -H in the transverse plane interacts with the DC. azimuthal electron velocity to provide longitudinal bunching. Electrons which are 180 apart in the azimuthal position in a transverse plane are moved in opposite longitudinal directions as shown by the dotted arrows. Eventually electrons which are separated by 180 are bunched A /2 apart. After bunching has occurred the electrons are in the form of a helix with a pitch equal to the guide wavelength A and rotating in a helical-beam path at the cyclotron frequency. It is then possible to have the majority of the electrons in a position and moving in a direction so as to deliver a net energy to the circuit. Energy interchange occurs between the transverse motion of the electrons and the transverse R.F. electric field. It is important for the helical-beam tube to have most of the beam energy in the transverse motion as the bunching force is proportional to the tranverse velocity, v,, and the number of interaction intervals is inversely proportional to the longitudinal velocity, Z

It is desirable to have the pitch of the beam pursuing its helical path small, to allow more interaction between the electrons and the electromagnetic circuit.

Bunching analogous to that described will occur between the electromagnetic field of FIGS. 1 and 2 and the helical-beam path of electrons.

An appropriate w-B diagram is illustrated at FIG. 3 to show how interaction occurs between the helical electron beam 5 and the electromagnetic wave H The solid line in FIG. 3 is the usual dispersion curve for a transmission line. The dash lines are obtained by letting /z' )z, which expresses the DC. relationship between angular and longitudinal motion. Thus, an effective propagation constant of the form (ipqi /z' ifi) results, where fi==w/v p=an integer corresponding to the number of angular variations of the RF. field. Curve I corresponds to a field propagating in the negative 2 and positive directions and curve II corresponds to a field propagating in the positive z and positive directions.

A tube of this type will support amplification in a fast wave tube not only in both the forward and backward directions but if the tube current is high enough, backward oscillation will occur since the feedback mechanism is inherently built in. It is herein noted that the term fast wave tube defines an electron tube in which an electron beam interacts with a circuit wave that has a phase velocity greater than or equal to the free-space velocity of light. The circuit waves are termed fast waves and the circuit may be, for example, an unloaded cylindrical waveguide as shown in FIGS. 1 and 2 or a parallel plane conductor as shown in FIG. 5.

Referring now to FIG. 5 there is shown another em bodiment of the present invention. In this instance, a periodic trajectory of the electron beam is accomplished by using crossed electric and magnetic fields. Specifical- 1y, FIG. 5 shows a linear magnetron consisting of a cathode plate 31 including an emitter portion 31' located on the lower half near one end of a parallel wire transmission structure 32 formed by spaced anode conductor 34 and cathode conductor 31 and straddling a beam wave interaction area 33 of the linear magnetron. The upper wall 34, opposite from the cathode emitter 3-1, is provided with a positive DC potential from power supply 40. A collector 36 is provided at the end of the magnetron opposite from cathode emitter 31 and insulated therefrom by insulators 41. It is noted that the upper and lower walls of parallel wire structure 32 are insulated from each other by side wall portions of a glass envelope 42. An output loop 37 for supplying the output R.F. energy to the desired load is provided.

During operation, electrons are emitted from cathode emitter 31 and enter into the magnetic field B, into the paper, provided by magnet 38, partially shown, where they .are perturbed into a cycloidal path to the right by the crossed electric and magnetic field provided by magnet 38. Assuming the correct parameters have been obtained between the potential difference of the anode, the cathode and magnetic field strength, a portion of the electrons will proceed to the collector in a cycloidal path.

Considering the R.F. field within the interaction area 33 to propagate as :z the synchronous condition between the beam e and the circuit wave is where E is the DC. potential between the cathode and anode, B is the magnetic lines of force across the DC. potential E and where E is the charge on the particle and m=the mass of the particle. Therefore,

For low-energy electrons,

so that wE'flB. For pulsed operation, this flux density is not extraordinarily difiicult to obtain. The energy delivered to the RE. circuit field is derived from the transverse kinetic energy of the beam.

The means for phase discrimination for the parallelplate magnetron is as fol-lows. Electrons entering an accelerating R.F. field experience an increase in transverse energy and return to the lower plate 31 upon completing a revolution. Electrons entering a decelerating field experience a decrease in transverse energy and so they remain in the RF. field of the interaction region 33.

What has been shown then are microwave tubes capable of operating in a millimeter waveband employing cumulative interaction between fast circuit waves and electrons which traverse a periodic path.

Since 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. In a high frequency electron discharge apparatus including, a wave beam interaction region, means for producing a substantially unidirectional beam focusing magnetic field within said interaction region, means for forming and projecting a stream of electrons into said magnetic field with a certain velocity taken in the mean direction of the stream and with an initial component of velocity directed transversely of said unidirectional magnetic field to produce a resonant periodic trajectory of said electron stream superimposed upon the mean direction of the stream, a two Wire transmission means disposed adjacent said electron stream for propagating electromagnetic wave energy at a velocity substantially greator than said certain electron velocity taken in the mean direction of the stream, means for synchronizing the resonant periodic trajectory of said electron stream with said electromagnetic wave energy on said transmission means such that a cumulative energy transfer from said electron stream to said faster wave occurs in said WEIIVG- beam interaction region.

2. The high frequency electron apparatus according to claim 1 where said resonant periodic stream trajectory is a substantially helical trajectory.

3. The high frequency electron apparatus according to claim 1 where said resonant periodic stream trajectory is a substantially cycloidal trajectory.

References Cited by the Examiner UNITED STATES PATENTS 2,730,648 1/1956 Lerbs 313-156 X 2,840,757 6/1958 Dench 315-393 GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner.

G. R. OFELT, VINCENT LAFRANCHI,

Assistant Examiners.

Claims

1. IN A HIGH FREQUENCY ELECTRON DISCHARGE APPARATUS INCLUDING, A WAVE BEAM INTERACTION REGION, MEANS FOR PRODUCING A SUBSTANTIALLY UNIDIRECTIONAL BEAM FOCUSING MAGNETIC FIELD WITHIN SAID INTERACTION REGION, MEANS FOR FORMING AND PROJECTING A STEAM OF ELECTONS INTO SAID MAGNETIC FIELD WITH A CERTAIN VELOCITY TAKEN IN THE MEAN DIRECTION OF THE STREAM AND WITH AN INITIAL COMPONENT OF VELOCITY DIRECTED TRANSVERSELY OF SAID UNIDIRECTIONAL MAGNETIC FIELD TO PRODUCE A RESONANT PERIODIC TRAJECTORY OF SAID ELECTRONS STREAM SUPERIMPOSED UPON THE MEAN DIRECTION OF THE STREAM, A TWO WIRE TRANSMISSION MEANS DISPOSED ADJACENT SAID ELECTRON STREAM FOR PROPAGATING ELECTROMAGNETIC WAVE ENERGY AT A VELOCITY SUBSTANTIALLY GREATER THAN SAID CERTAIN ELECTRON VELOCITY TAKEN IN THE MEAN DIRECTION OF THE STREAM, MEANS FOR SYNCHRONIZING THE RESONANT PERIODIC TRAJECTORY OF SAID ELECTRON STREAM WITH SAID ELECTROMAGNETIC WAVE ENERGY ON SAID TRANSMISSION MEANS SUCH THAT A CUMULATIVE ENERGY TRANSFER FROM SAID ELECTRON STREAM TO SAID FASTER WAVE ACCURS IN SAID WAVEBEAM INTERACTION REGION.