US2707758A - Travelling wave tube - Google Patents

Description

May 3, 1955 CHAO' c. WANG TRAVELLING WAVE TUBE 2 Sheets-Sheet 1 Filed Dec; 19, 1950 IN INVENTOR its? 614/70 6. WANG BY May 3, 1955 g o c, WANG 2,707,758

TRAVELLING WAVE TUBE Filed Dec. 19, 1950 2 Sheets-Sheet 2 INVENTOR 67/40 6. WZI/VG /gj M ATTORNEY United States Patent 0 M TRAVELLING WAVE TUBE Chao C. Wang, Bayside, N. Y., assignor to The Sperry Corporation, a corporation of Delaware Application December 19, 1950, Serial No. 201,602

9 Claims. (Cl. 315-3) This application is a continuation in part of application Serial No. 186,730, filed by me September 26, 1950, and entitled High Frequency Apparatus.

The present invention relates to improvements in travelling wave tubes, in which an electromagnetic wave and an electron stream travel together throughout an extended interaction space and interchange energy in such manner as to convert some of the kinetic energy of the electrons to wave energy for augmenting the original wave.

One of the principal objects of this invention is to a provide travelling wave tubes having higher efficiency and considerably greater power handling capability, with a given physical size, than has been attainable hitherto.

More specifically, it is an object of this invention to provide travelling Wave tubes with improved beam f forming and beam maintaining means, whereby a beam of high electron density may be directed through a relatively long interaction space, such as along the interior of a helix, with a minimum amount of loss by interception.

A further object is to provide travelling wave tubes including improved magnetic focussing means arranged to control an electron beam as required with practically attainable magnetic flux densities.

Prior are techniques in the design and operation of travelling wave tubes have involved the use of magnetic fields to direct electron beams through and parallel to the axis of a slow wave propagating structure such as a helix. The theory of such prior art devices is based on the fact that an electron in motion through a magnetic field tends to follow the magnetic lines of force. Thus, by arranging a magnetic field so that its lines of flux go the way it is desired to make the electrons go, the beam may be held together, against the forces of mutual repulsion between the electrons (i. e. space charge), as it passes through the helix.

The electrons do not follow the magnetic flux lines exactly, because a magnetic field exerts zero force on an electron moving along a line of force. However, as

an electron moves across the magnetic flux, the field de- 'tlects it toward a path coinciding with a line of force. Thus each electron in the beam follows a path which undulates about a line of magnetic force. If it were po's sible to increase the strength of the magnetic field indefinitely, the undulations could be made as small as desired, and as a practical matter, the electrons could be considered to follow the lines of force. With the relatively small space charge forces occurring in low-density electron beams, this condition can be attained.

In a travelling wave tube, the diameter of the helix or equivalent slow wave propagating structure is determined to a considerable extent by the Wavelength of the electromagnetic energy with which it is to operate. Thus the frequency places a limit on the cross sectional area of the beam that can be used, and tubes for high frequency and/or high power operation necessarily require high density electron beams, with high space charge re- 2,707,758 Patented May 3, 1955 pulsion forces. With the above described prior art type of magnetic focussing, it is found that impracticably high magnetic flux densities would be necessary to hold a beam of the desired electron density together with tolerable undulations.

According to the. present invention, the electrons do not follow the magnetic flux lines, but are deliberately made to cross the lines of force as they enter the magnetic held, in such manner that they are directed into helical paths. In these helical paths, they continuously cut across the field at such a rate that they are subjected to a radial inward force which is equal to the sum of the outward space charge repulsion and centrifugal forces. Thus, if the electrons are made to enter the field Without any radial velocity, and as long as they remain in a linear uniform intensity part of the field, the radii of their helical paths remain constant, and the beam diameter is constant, without undulations.

The invention Will be described with reference to the accompanying drawings, wherein:

Fig. l is a longitudinal section of a travelling wave tube, showing the relationship between the structural elements and the electron beam.

Fig. 2 is a perspective view of a portion of Fig. 1, showing the path followed by an electron therein under the influence of the magnetic field, space charge repulsion and centrifugal forces.

Fig. 3 is a graph showing the relationship between the radial forces and the rotational velocity of an electron in a magnetic field.

Fig. 4 is a graph showing the magnetic field strength as a function of axial position in the vicinity of the beam entrance region in the tube of Fig. l, and

Fig. 5 is a side elevation of the tube mounted on a base.

Referring to Fig. l, the principal elements of a travelling wave tube include a slow Wave propagating structure such as a helix 1 of conductive wire, a cathode 3 for emitting electrons, focussing and accelerating electrodes 5 and 7 for directing the electrons to form a beam flowing axially through the helix 1, and a collector electrode 9. In the operation or the device, the electrodes 5, 7 and 9 and the wave propagating element 1 are maintained at a relatively high potential, for example several thousand volts, positive with respect to the cathode 3. Energy to be amplified is applied, as by means of an input wave guide 11, to the end of the element 1 nearer the cathode. This sets up an electromagnetic wave which travels axially along the helix at a velocity which is relatively slow with respect to the velocity of light.

The velocity of the electron stream passing through the helix depends upon the accelerating potential, i. e. the voltage between the cathode 3 and the accelerating electrode 7. This voltage is adjusted to make the electron velocity substantially equal to the velocity of wave propagation along the helix. The travelling field produced by the wave on the helix interacts with the electron stream in such manner that some of the kinetic energy of the electrons is given up to the wave, so that the wave increases in amplitude as it travels along the helix. The amplified wave energy may be taken'oii the helix at its end near the collector electrode, by means of an output wave guide 13.

In order to obtain high efficiency and also to prevent undue heating of the helix, it is necesary to make the beam go all the way through the helix to the collector, with as few as possible of the electrons being intercepted by striking the helix conductor. The dash lines 15 in Fig. 1 represent the outline of a beam of the desired type, with the radii of the electron paths substantially constant throughout the length of the helix, and the diave res ametcr of the beam slightly less than the inside diameter of the helix.

The beam may be convergent between the cathode 3 and the accelerator 7 as shown, to obtain a greater electron density than would be practical otherwise with presently available emitting materials. The focussing electrode 5 and the accelerator '7 are designed to provide an electric field pattern for forming the electrons into a beam of the required cross sectional area at the orifice in the electrode 7. A suitable beam-forming arrangement for this purpose is described in U. S. Patent 2,564,743, issued August 21, 1951, to Chao C. Wang, and entitled Changed Particle Eeam Forming Apparatus.

Apertured magnetic pole pieces 17 and 19 are provided near the respective ends of the helix 1, and are adapted to be magnetically polarized in opposite senses (north and south) by an external magnet, not shown in Fig. 1. Preferably these pole pieces are arranged so that substantially none of the flux between them goes through the cathode surface, or into the region between the cathode 7 and the accelerator electrode. For this purpose, the pole piece 17 may be provided with a tubular skirt 21 magnetically shielding the cathode region.

Fig. 5 shows the tube of Fig. l with the pole pieces 17 and 19 mounted on a base 20 permeable to magnetic flux. The base 20 may be a permanent magnet of the type commonly employed to provide a magnetic field for electronic discharge devices requiring such a field for their operation. Alternatively, if the base 20 is not a permanent magnet, an electric coil 22 may surround the helix 1 outside the tube envelope and between the wave guides 11 and 12, to drive a magnetic flux between the pole pieces 1 19. In this latter case, the provision of the base 20 reduces the magnetomotive force required in the coil 22 to supply the necessary magnetic flux. Fig. 5 also shows the end closure 6, which may be sealed to the pole piece 17 enclosing cathode 3.

The magnetic field distribution between the pole pieces 17 and 19 in the vicinity of the helix and the electron beam is indicated approximately in Fig. 1 by the thin solid lines 23, which may be considered to represent lines of force. In the region between the pole pieces 17 and 19 but remote from the pole piece apertures 25 and 27, the flux is of uniform density and parallel to the helix axis. At the aperture 25, the flux fringes as shown, with the lines of force crowding together to terminate around the edge and on the interior surface of the aperture 25. In the practice of the present invention, it is preferable to make all of the magnetic finx which is contained within the boundary or outline 15 of the beam ,1

in the uniform density part of the field, fringe out to cross the boundary 15 in the neighborhood of the pole piece 17. When this is done, each electron, before it enters the uniform field region, must cross the same total flux as there is between its path and the central axis, in the uniform field region.

Where a constant radius beam like that shown in Fig. l is desired, it is necessary that the electrons enter the magnetic field with zero radial velocity. With a converging beam, this requirement can be met by placing the beam forming assembly at such distance from the pole piece that the radially inward components of momentum of the electrons in the converging stream are just neutralized by the radially outward space charge forces as the electrons enter the magnetic field in the orifice 25.

In Fig. 2, the heavy solid line 29 represents the path of one electron, at the edge or boundary of the beam. As in Fig. 1, the thin solid lines 23 represent magnetic lines of force, but only a few typical lines are shown in Fig. 2. It is assumed that the pole piece 17 in Fig. 2 is polarized as a north pole. The lines 23 in Fig. 2 may be considered to indicate approximately average or median lines in the field.

As the electron enters the field, in the vincinity of the point 31, it is moving originally with a velocity v in a straight line parallel to the longitudinal axis of the tubes.

Cit

Owing to the outward fringing of the field in the aperture 25, there is a radially inward magnetic field component Br at this point. The electron, being a charged particle, is acted upon as it crosses the field Br by the so-called Lorentz force Ft. which is at right angles to both the velocity v and the field component Br. This force accelerates the electron in the direction of tthe arrow Ft in Fig. 2, giving it an angular velocity of rotation about the axis. As the electron crosses all of the fringing flux, this velocity is increased to a certain value w which depends upon the initial velocity v and the total amount of flux which is crossed.

The rotational component of velocity of the electron causes it to continue to cross the magnetic flux lines in the uniform portion of the magnetic field. At the point 33, for example, the electron is moving at the velocity v, and crossing the field B with a transverse component of velocity Iw, where r is the radial distance of the electron from its axis of rotation. This produces a radially inward Lorentz force F erwB, where c is the charge on the electron.

The electron is subject also to a radially outward centrifugal force of mrwg where m is the mass of the electron. Thus the net inward force available to overcome the space charge repulsion force is the difference between the centrifugal and Lorentz forces, and is a function both of angular velocity and magnetic field strength B. Fig. 3 shows how the radial force, considered positive in the outward direction, varies as a function of velocity with a given field strength B. It is apparent that this force is negative (i. e. inward) between the limits of w=0 and e w=B and has a maximum negative value when Thus, the magnetic field is most effective in producing inward force for overcoming space charge repulsion when the angular velocity is It can be shown that the angular velocity w is obtained when the electron crosses, in the region of fringing flux, all of the flux which lies between its path and the axis in the uniform field region. Since this requires the least magnetic field to maintain the electron in a path of constant radius, it is the mode of operation which is preferred. It is possible, however, to maintain the desired electron paths with angular velocities other than w providing a stronger field is used.

The foregoing discussion relates to an edge electron, i. e. one at the boundary of the beam. Other electrons, nearer the axis of the beam are subject to similar forces. They cross less flux in the vicinity of the aperture 25 and have a lower transverse velocity no in the uniform field, but are subject to less total space charge force and less centrifugal force, so they follow paths similar to those of the edge electrons, but at smaller radii.

While a beam of uniform radius may be preferable in travelling wave tubes, it is possible to produce a beam having different radii at different points along its length, by making the electrons enter the magnetic field with a radial component of velocity. In this case, the electrons oscillate in and out about the equilibrium radius as they travel along the beam, in a manner analogous to the oscillation of a pendulum about its equilibrium position when it is released from some other position.

In designing a tube like that of Fig. l to operate as described, one of the principal problems is to determine the proper axial distance between the beam-forming means and the pole piece 17 to make the electron beam enter the magnetic field in the required manner. The

"mum value Bm at a distance beam forming means, including the cathode 3, focussing electrode 5, and accelerator electrode 7 may be designed as described in the above-mentioned Patent 2,564,743 to produce a beam of a predetermined radius a, with the electrons moving in substantially parallel rectilinear paths, at a point A which is an axial distance 2 from some reference point such as the surface 8 of the accelerator electrode 7, in the absence of magnetic field. The point A will be inside the aperture 25, a certain distance S from the face 18 of the pole piece 17. When this distance S is found, the axial spacing Z+S of the surface 8 from the pole face is known.

The required distance may be found experimentally by building a tube with adjustable spacing and measuring the current intercepted by the helix, changing the spacing until the current is a minimum. Another method is to build a potential model, with surfaces curved to represent the fields existing in a proposed structure, and roll steel balls along these surfaces. The paths followed by the balls will be similar to the paths followed by electrons in gaggespondhngfieldgandthus the proper arrangement of the elements can be deduced.

The distance S can also be found empirically from a curve of magnetic flux density as a function of axial position in the vicinity of the aperture 25. Fig. 4 shows the axial flux density B as a function of axial distance z from the plane of the pole face 18 of the tube of Fig. 1. Bo represents the flux density in the uniform field region remote from the pole face. Near the aperture 25, the

lines of force crowd together to terminate on the edge and and the flux density has a maxi- F from the pole face. Inside the aperture (where z is negative) the flux density falls off rapidly with increase in z. Curves like that of Fig. 4 may be obtained or may be analytically determined from the pole piece configuration.

As shown in Fig. 4, A1 is the distance to the left of the point of maximum flux Bm, where the flux is /2Bm. A2 is the distance to the right of the point of maximum flux where the flux is Vz(Bm+Bo). The point A should be placed a distance Az to the left of the point of maximum flux Bm:

interior of the aperture,

where Since Bm is at the distance F to the right of the pole face 18, the distance S in Fig. 1 is:

and the distance from the accelerator surface 8 to the plane of the pole face 18 should be made:

In one travelling wave tube which has been built and operated, embodying the present invention, the dimensions and locations of the various parts are as follows:

The cathode and beam-forming elements were designed to provide a beam having a diameter 2a of .090 inch. The

magnetic field strength is not critical, a value of Bo=1000 by actual measurement of the field, dd

Cathode diameter, .500 inch Radius of curvature of cathode, .454 inch Diameter of inner ring 4 (see Fig. l), .516 inch Axial thickness of ring 4, .010 inch Inside diameter of cylinder 6, 0.577 inch Axial length of electrode 5, 0.212 inch Distance of ring 4 from plane of the edge of the cathode,

.0175 inch Distance of accelerator surface cylinder 6, .076 inch The above described tube has been operated with a power output (pulsed) of six kilowatts, at a frequency of 5600 megacycles per second. The beam current was 2 amperes, at a voltage of 1400 volts. Over 98 percent of the beam current went to the collector electrode.

The various dimensions given are by way of example only. Other dimensions, determined as described, may be used. Moreover, any particular set of dimensions may be scaled, i. e. increased or decreased maintaining the same ratios throughout. If this is done, the magnetic field intensity must be increased in the same ratio as the dimensions are decreased, and vice versa.

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 8 from the end of the and not in a limiting sense.

What is claimed is:

1. A travelling wave amplifier including a helical wave conductor, a cathode adjacent one end of said wave conductor, electrostatic focussing means adjacent said cath ode for directing electrons emitted by said cathode toward said wave conductor, and magnetic focussing means for directing said electrons through and along the axis of said wave conductor comprising a substantially flat plate of magnetically permeable material between said cathode and said wave conductor and perpendicular to said axis,said plate having an orifice for the passage of electrons ginto said wave conductor and being adapted to act as anztccelerator electrode, and a tubular magnetic shield extending from said plate toward said cathode.

2. A travelling wave amplifier including a.helica1 wave conductor, a cathode adjacent one end of said wave conductor, a collector electrode adjacent the other end of said wave conductor, electrostatic focussing means for directing electrons emitted by said cathode toward said wave conductor, and an accelerator electrode between said cathode and said wave conductor having an orifice for the passage of electrons into said wave conductor, magnetic focussing means for directing said electrons through said wave conductor comprising a pole piece surrounding the space between said accelerator electrode and said helix, and extending over and magnetically shielding said cathode, a second pole piece in the vicinity of said collector electrode, and a magnet extend ing between said pole pieces.

3. A travelling wave amplifier including a conductive helix for carrying electromagnetic wave energy, a cathode near one end of said helix, a collector electrode near the other end of said helix, electrostatic focussing means adjacent said cathode for directing electrons emitted by said cathode toward said helix, and magnetic focussing means for directing said electrons through and along the axis of said helix comprising a pole piece surrounding the space between said electrostatic focusing means and said helix, and provided with a skirt extending over said cathode for magnetically shielding said cathode, said pole piece having an orifice for the passage of electrons into said helix and being adapted to act as an accelerator electrode, a second pole piece suri-a'gating structure of substantially tubular for ing a longitudinal .pole piece for producing a magnetic field which has a the length of said prising a wave guide structure for propagating microwave electromagnetic energy along an axis from one end toward the other end thereof, at a speed much slower than the velocity of light, means for producing an electron stream directed along said axis in the direction from said one end toward the other end, said stream producing means including a. focussing electrode for directing the electrons in said stream along parallel paths in a region adjacent said one end of said wave guide structure, and means for producing a magnetic field aligned substantially parallel to said axis substantially throughout the length of said wave guide structure, said magnetic field producing means including an apertured pole piece surrounding said region and a magnetic shield comprising a tubular extension from said pole piece, surrounding said stream .s aid'boundary in said wave guide.

- '5. A travelling wave tube includin a slow wave ro and havaxis, means including an apertured component radial to said axis in a region odtside said structure and adjacent one end thereof and is substan- -tially uniform and parallel to said axis throughout the space enclosed by said propagating structure, an elecj. tron gun for producing a beam of electrons whose paths are substantially parallel and rectilinear at a point which is a predetermined distance from said electron gun, said "electron gun being spaced from said pole pieces to position said point substannally at the median of the region where said magnetic field has a radially directed component, whereby the electrons in said stream are de- -f lected to enter said uniform field substantially without radial velocity but with a helical motion about said axis.

6. Microwave energy vacuum tube apparatus comprising awave guide structure for propagating microwave electromagnetic energy along an axis from one end -.toward the other end thereof at a speed much slower than; the velocity of light, means including a cathode for producing an electron 1 axis in the direction from said one end toward the other stream directed along said end, said stream producing means including a focussing electrode for directing the electrons in said stream along parallel paths in a predetermined compact cylindrical region adjacent said one end of said wave guide structure, means for producing a magnetic field aligned substantially parallel to said axis substantially throughout wave guide structure, said magnetic field producing means including an apertured pole piece surrounding said region for directing the magnetic lines of force through said compact cylindrical region with radial components of direction therethrough, a magnetic shield surrounding said cathode and focussing electrode and for substantially shielding the magnetic lines of force from extension into any region of radial components of electron velocity, and vacuum envelope means enclosing said cathode and said axis and the electron stream space therealong throughout the length of said wave guide structure.

7. A travelling wave tube including a slow wave propagating structure of substantially tubular form and having a longitudinal axis, means including a magnetic pole piece adjacent one end of said structure and having a portion with an aperture substantially coaxial with said structure for producing a magnetic field which has a component radial to said axis in said aperture and is substantially uniform and parallel to said axis throughproducing means, for directing 51 -the magnetic lines of force radially through said region out the space enclosed by said propagating structure, and means including a cathode and beam forming electrodes for producing a stream of electrons whose paths become substantially parallel and rectilinear at a point which is a predetermined distance from said cathode, said apertured portion of said pole piece being between said cathode and beam forming electrodes on one side and said one end of said slow wave propagating structure on the other side, said last mentioned means being spaced from said pole piece to position said point in said aperture substantially at the median of the region where said magnetic field has a radially directed component.

8. A travelling wave tube including a slow wave propagating structure of substantially tubular form and having a longitudinal axis, means including a magnetic pole piece having a face adiacent one end of said structure and having a portion with an aperture substantially coaxial with said structure for producing a magnetic field which is substantially parallel togs aicl axis and has a uniform density Bo throughthe space enclosed by said-- propagating structure and a maximum axial density Bm at a distance F from said face in the direction of said propagating structure, means including a cathode and beam forming electrodes for producing a stream of electrons whose paths are substantially parallel and rectilinear at a point which is a predetermined distance from said cathode, said apertured portion of said pole piece being between said cathode and beam forming electrodes on one side and said one end of said slow wave propagating structure on the other sidefsai d last mentioned 7.

means being spaced from said pole piece to position said point substantially a distance AZ inside said aperture from said pole face, where lector electrode, and a helix extending between said cathode and said collector; a body of magnetic materialhavr-,- ing an aperture between said cathode and said helix and 1 a tubular skirt extending over and surrounding said cathode, and a tubular envelope surrounding said helix and sealed to said body to cooperate therewith as a vacuum enclosure, with at least one surface of said body outside said enclosure.

References Cited in the file of this patent UN TED STATES PATENTS 2,225,447 Haeff Dec. 17, 1940 2,300,052 Lindenblad Oct. 27, 1942 2,305,884 itton Dec. 22, 1942 2,431,077 Poch Nov. 18, 1947 2,524,252 Brown Oct. 3, 1950 2,567,674 Linder Sept. 11, 1951 2,591,350 Gorn Apr. 1, 1952 2,608,668 Hines Aug. 26, 1952 2,632,130 Hull Mar. 17, 1953

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