US3207946A - Electron gun for generating laminar electron flow - Google Patents

Description

sept. 21, 1965 H.A.HAUS ETAL ELEGTRON GUN FOR GENERATING LAMINAR ELECTRON FLOW Filed Dec. 27, 1960 2 Sheets-Sheet 1 /NMENTURS @y 6mm ATTQRMEY Sept. 21, 1965 H. A. HAUS ETAI. 3,207,945

ELECTRON GUN FOR GENERATING LAMINAR ELECTRON FLOW Filed nec. 27. leso 2 sheets-sheet 2 l N VEN TORS HERMAN H. HAUS DA VID H. WH/ TE ROY A PA A NA NEN By w@ PWM ATTORNEY United States Patent O 3,207,946 ELECTRON GUN FOR GENERATING LAMINAR ELECTRO'N FLOW Hermann A. Haus, Milton, David H. White, Medford, and Roy A. Paananen, Lexington, Mass., assignors to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Dec. 27, 1960, Ser. No. 78,599 7 Claims. (Cl. S15-39.3)

This invention relates to electron d-ischarge devices and more particularly to an improved traveling wave tube wherein noise produced by a slipping stream of electrons, sometimes called diocotron noise, is reduced.

One of the principal causes of high level radio frequency (RF) noise in crossed field beam type microwave tubes is the diocotron amplification of cathode noise in the electron beam. It is generally believed that this noise originates from a growing space charge wave propagated by a slipping stream of electrons. An analysis of the origin and buildup of this noise with a view to explaining and predicting the noise is set forth beginning on page 99 of the Journal of Applied Physics vol. 28, May 1957. Other theories attempting to explain the origin of this noise assume that it originates as shot noise from the cathode. However, these theories indicate an amount of noise several magnitudes too small to explain the observed noise, and so the shot effect and electron ion collision are generally eliminated as a probable cause of the noise. The slipping stream theory, on the other hand, predicts a substantial level of noise of the same order as noise levels observed in the operation of crossed field traveling wave tubes and magnetrons.

The slipping stream theory generally arrives at an exponential growth of diocotron noise wave amplitude of the form Exp (az), where a is the growth constant of the noise wave, and z is distance along the beam. This theory assumes a relatively thin electron beam focused by crossed electric and magnetic fields in an interaction space approximately midway between a slow wave propagating structure and an electrode coeXtensive therewith. It can be shown that the growth constant a of a perturbation arising in the beam is expressed substantially as follows:

In the above equation w is the angular frequency of the noise component arising from the particular perturbation, u is the direct current (D.C.) velocity of the beam through the interaction space, t is the thickness of the beam, wp is the electron plasma angular frequency and we is the is the cyclotron angular frequency inherent to a crossed field tube. The electron plasma angular frequency is generally a function of electron density and merely expresses the tendency of a concentration of electrons to expand or contract to a given density, the expansions and contractions occurring at a resonant frequency. The cyclotron angular frequency of electrons is, of course, proportional to the magnetic field strength.

When reasonable values such as exist in the interaction space are assumed for parameters in the above equation and an input noise perturbation corresponding to normal shot noise occurring at the cathode is assumed, the noise predicted by the above equation in the output of a tube is smaller than observed noise by many orders of magnitude. On the other hand, it will be noted, the rate of growth predicted by the equation is faster, the higher wp and the smaller u. Consequently, a beam drifting at low velocity (which normally does not occur in the interaction space) should exhibit more diocotron noise gain. It is therefore reasonable to assume that strong diocotron gain exists in the region of the tube where beam velocity 3,207,946 Patented Sept. 2l, 1965 is low, such as in the vicinity of 'the cathode. Upon making this assumption, an analysis indicates that noise is much higher than shot noise when the beam enters the interaction region adjacent the slow wave propagating structure in typical crossed field traveling wave tubes and magnetrons.

In the present invention it is assumed that diocotron noise arises and gains in strength in the low velocity region of the beam, and, consequently, the noise can be short-circuited in the region of low beam velocity so that total diocotron noise gain throughout the tube is significantly less than it would be otherwise. It is preferred to short-circuit this noise in the immediate vicinity of the cathode where beam velocity is normally low and where the slipping stream phenomena is most likely to commence giving rise and gain to the diocotron noise.

In the present invention electrons rising from the cathode are stratified in the immediate vicinity above the cathode into layers or streams of lam-inar flowing electrons. In any event, there is a stratification of electrons in the immediate vicinity of the cathode into a plurality of layers to thereby reduce electron turbulence or motion transverse to the streams just before the electron streams merge forming a thin beam at the entrance to the interaction space. In one embodiment of the invention the beam is traversed with thin wires or finsvor other objects of high electr-ical conductivity in the immediate vicinity of the cathode where electron velocity is low. These wires or fins may all be placed at the same potential, or they may be placed at different potentials. When properly located and energized the wires or fins will short out perturbations which would give rise to diocotron noise and which would be greatly amplified in the low electron velocity region between the cathode and the tube interaction space.

In other embodiments of the invention the above-mentioned stratication is accomplished in a different manner. The cathode is composed of a plurality of different electron emissive bodies disposed in close relationship and insulated from each other. In operation, the same or different potentials may be applied to the separate electron emissive bodies so that the abovebmentioned stratification of electrons will occur in the immediate vicinity of the cathode substantially reducing the transverse perturbations which give rise to diocotron noise.

Other features and objects of the present invention will be more apparent from the following specific description taken in conjunction with the drawings in which:

FIGS. la and 1b illustrate the prior art showing a crossed field traveling wave tube and the manner in which slipping stream amplification or diocotron noise occurs;

FIG. 2 illustrates one embodiment of the present invention incorporated in a typical crossed field type traveling wave tube;

FIG. 3 illustrates a traveling wave tube cathode having a plurality of thin wires traversing the electron beam in the immediate vicinity of the cathode causing stratification in the beam, reducing diocotron noise;

FIG. 4 illustrates a structure similar to that shown in FIG. 3 wherein individual wires may be placed at different potentials providing a more complete elimination of diocotron noise; and

FIG. 5 illustrates a cathode composed of segmented electron emissive portions insulated from each other producing the stratification or substantially laminar electron flow in the immediate vicinity thereof so as to substantially eliminate diocotron noise.

FIGS. 1a and 1b illustrate a typical crossed field or M-type traveling wave tube included here to illustrate the source of diocotron noise in accordance with the slipping stream theory. As shown in FIG. la an envelope 1 encloses a slow wave structure 2, a sole electrode 3, a cathode 4, an accelerating electrode 5 and a collector electrode 6. Potentials are applied to the electrodes from power supply 74 producing an electric field in the interaction space 8 and a magnet (not shown) produces a transverse magnetic` field B in the same space. A microwave signal is applied to one end of delay line 2 via coaxial connector 9 and amplified microwave signal is obtained from the other end of structure 2 via coaxial connector 10. In operation electrons emitted from cathode 4 are compelled to travel arcuate paths at a relatively low velocity in the cathode space 11 between cathode 4 and accelerating electrode 5. These arcuate paths, shown by dotted lines, converge at the entrance to the interaction space 8 between the sole electrode and slow wave structure. It is herein proposed in accordance with the slipping stream or diocotron effect, that the velocity of electrons varies across the stream because of potential changes caused by space charge. These potential changes are most pronounced in the area where the electron Velocity is low as in cathode space 11 between cathode 4 and accelerating electrode 5 and becomes somewhat less pronounced in the interaction space 8 Where electron velocity is greatly increased. Once the space charge builds up in space 11 causing the electrons to bunch, there results a modulation of the beam which is a noise modulation and which is amplified further as the beam progresses through the interaction space 8. Obviously, this noise bunching or modulation of the beam will induce noise in slow wave structure 2 thus adding noise to the microwave output.

One explanation of the growth mechanism whereby the slipping beam causes electrons to bunch, thus imposing 1b. For example, FIG. 1b illustrates an infinitesimal section of the electron path 12. Assume that a transverse perturbation of electrons over this path occurs, and that perturbation is represented by line 13. Now, if the perturbation is viewed in a coordinate system moving with the electrons at the general velocity of the electrons, the D.C. electric field disappears, and the magnetic field is unaltered so that electrons at a phase indicated by point 14 experience an upward force caused by all other electrons and similarly electrons at a phase indicated by point 15 experience a downward force. Were it not for the strong magnetic field, these forces would immediately augment the original perturbation. However, because of the magnetic field the electric ield causes the electrons at perturbation phases indicated by points 14 and 15 to move in the direction indicated by arrows a and b respectively. As a result, the density of electrons in the area of a plane perpendicular to the figure indicated by line 16 decreases, whereas the density of electrons in a plane parallel thereto indicated by line 17 increases causing periodic bunching of electrons in a reference frame moving in the direction of the beam at a velocity generally equal to the electron velocity. This bunching causes a longitudinal electric field which because of the strong magnetic field causes the original perturbation to be augmented. Thus, the perturbation grows, producing a noise modulation of the beam inducing noise in the slow wave structure 2 which will appear in the output of the tube.

FIG. 2 illustrates one embodiment of the present invention including an interdigital type slow wave propagating structure 20, an elongated sole electrode 21 coextensive with the structure forming an interaction space 22, a cathode structure 23 at one end of the space and an accelerating electrode 24 opposite the cathode structure bounding cathode space 25 all enclosed by a conductive envelope 26. Coaxial terminal 27 provides a support and lead to sole electrode 21; coaxial terminals 28 and 29 provide leads to each end of slow wave structure 20; terminal 30 provides a support and lead to accelerating electrode 24; terminal 31 provides support, voltage and heater current to electron emissive body 32, and another terminal (not shown) provides support and leads to grid Y tron emitting part of the cathode.

structure 33. A power supply couples potentials to cathode structure 23 and electrodes 21 and 24 producing electric fields in spaces 22 and 25 and a magnet, not shown, produces transverse magnetic field B in the spaces.

The electrodes shown in FIG. 2 and the general structure obviously correspond to electrodes and structures shown in FIG. 1a, and, similarly, in operation, electrons emitted from emissive body 32 proceed over generally arcuate paths through cathode space 25 and enter the interaction space 22 between the sole and delay structure as a thin beam 34 in energy exchanging relationship with waves propagated on structure 20. This relationship obviously may be as in forward wave interaction or as in backward wave interaction, and so the device may operate as an amplifier or as an oscillator (carcinotron).

FIGS. 3, 4, and 5 illustrate various structures of cathode 23 in accordance with the present invention whereby diocotron noise is substantially eliminated. FIGS. 3 and 4 illustrate structures having a plurality of thin wires traversing the electron beam immediately above the elec- The wires are preferably orientated substantially perpendicular to the general motion of the electrons which are emitted from the emissive surface, and the length of the wires is preferably made a predetermined relationship to the wave length of the noise which is ordinarily most dominant in the tube. For example, if the length of the wires is approximately one half wave length of the predominant diocotron noise frequencies at the velocity of the electrons in the immediate vicinity of the cathode, it has been found that such diocotron frequencies are substantially short-circuited by the wires and thus attenuated in the beam.

The cathode structures shown in FIGS. 3 and 4 each include a plurality of thin wires traversing the beam; however, it should be clearly understood that the invention is not limited to the use of thin wires as shown, since many other body shapes of suitable dimension may be disposed in the stream in the immediate vicinity of the cathode o r disposed in the stream where electron velocity is relatively low to achieve the same result. For example, fins could be employed instead of the thin wires, or spiral wires could be employed, or numerous other shapes of conductive structure generally resembling antennas could be employed to short-circuit the diocotron noise frequencies.

In FIGS. 2 and 3 an elongated body of electron emissive material 32, preferably hollow to permit the insertion of a heating coil 35 is enclosed in a grid structure 33. An opening 36 in the structure permits electrons emitted from body 32 to ow into the cathode space 25 where the electrons 37 are compelled by crossed electric and magnetic fields in that space to travel over arcuate paths forming a narrow beam upon entry into the interaction space 22 between the sole and delay structure. A plurality of wires 38 inserted into the stream of electrons so as to substantially cover the opening 36 are supported in place by a cylindrical body 39. Body 39 may be conductive and attached to grid structure 33 so as to be at the same potential as the structure, or it may be composed of non-conductive material 40 where it is supported by structure 33 and have a lead 41 extending from one end thereof so that a different potential may be applied to wires 38 than is applied to structure 33.

It is hypothesized that the wires 38 cause electron flow from emissive body 31 to be laminar, thus eliminating a great deal of electron turbulence which would occur were it not for the wires inserted in the electron stream. This electron turbulence is equivalent to the perturbations which gives rise to the diocotron noise as already described above with reference to FIG. 1b, and it follows that by eliminating the perturbations the source of diocotron noise is substantially eliminated. The resulting laminar flow of electrons in the space between the cathode and accelerating electrode is represented by broken lines 42.

FIG. 4 illustrates another embodiment of the invention including a cathode structure similar to the structure in FIG. 3 and in which cylinder 43 supporting the Wires 38 is composed entirely of insulating material so that separate leads 44 may be connected to each of the wires and dierent potentials applied thereto from source 45. As a result, laminar electron flow, denoted by lines 46, is obtained across the entire width of the beam in cases where electric field irregularities exist in the vicinity of the cathode or in which electron velocity irregularities across the beam introduced by the edges of the opening 36 would not permit completely laminar electron ilow if all wires were at the same potential.

FIG. 5 illustrates another cathode structure whereby generally laminar ow of electrons is insured by the use of a plurality of segments of electron emissive bodies disposed in close relationship and insulated from each other. In FIG. 5 three electron emissive bodies 47, 48 and 49 are disposed in close relationship and separated by layers of insulating material 50 and 51. Each of these bodies is hollow to permit insertion of a heating coil which also provides a potential to the body. Heating coils 52, 53 and 54 are provided for this purpose, and each are shown coupled to different potentials of source 55. As a result, electron ow from the cathode can be made substantially laminar in at least the planes represented by broken lines 56.

The present invention anticipates various structures for insuring a laminar electron flow or a stratication of electron llow in a beam of electrons, particularly in areas of a traveling wave tube where electron velocity is relatively low, such as in the immediate vicinity of the cathode. While the structures illustrating specic embodiments of the invention include, for example, an array of wires placed in the electron stream to shortcircuit diocotron noise or to substantially attenuate electron perturbations transverse to the stream, thereby eliminating the source of diocotron noise, it is to be clearly understood that these are all made only by way of example and do not limit the spirit or scope of the invention as set forth in the accompanying claims.

What is claimed is:

1. An electron discharge device comprising a slow wave propagating structure coextensive with an interaction space, means emitting electrons, means for producing transverse electric and magnetic elds in said space for compelling said electrons to enter said space along arcuate paths and travel in energy exchanging relationship with waves propagated in said structure and a plurality of electrically conductive members disposed in said stream of electrons across said arcuate paths immediately adjacent to said emitting means to cause substantially laminar ilow of electrons.

2. An electron discharge device comprising a slow wave propagating structure, means emitting electrons, means for producing transverse electric and magnetic elds in said space for compelling said electrons to enter said space along arcuate paths and travel in energy exchanging relationship with waves propagated in said structure and a plurality of biased electrically conductive members disposed in said stream of electrons across said arcuate paths immediately adjacent to said emitting means whereby laminar kflow of electrons from said emitting means is insured to thereby substantially eliminate electron perturbations transverse to the general direction of motion of electrons in the stream.

3. An electron discharge device comprising a slow wave propagating structure, means emitting electrons, means for producing transverse electric and magnetic fields in said space for compelling said electrons to enter said space along arcuate paths and travel in energy exchanging relationship with waves propagated in said structure and a plurality of biased electrically conductive members disposed in said stream of electrons across said arcuate paths immediately adjacent to said emitting means substantially eliminating electron perturbations which give rise to diocotron noise.

4. An electron discharge device comprising a slow wave propagating structure, means emitting electrons and means for producing transverse electric and magnetic fields in said space for compelling said electrons to move along arcuate paths in energy exchanging relationship with Waves propagated in said structure, said electron emitting means including biased short-circuiting members disposed across said arcuate paths immediately adjacent thereto for attenuating electron perturbations insuring laminar electron ow therealong.

5. A cathode structure comprising a body composed of electron emitting material, means for heating said material, a conductive surface enclosing said body, an opening in said surface from which electrons emerge, means producing transverse electric and magnetic tields in the area about said opening for compelling said electrons to move along arcuate paths and a plurality of electrically conductive members spaced in said opening and being electrically connected to said conductive surface.

6. A cathode structure according to claim 5 and means for energizing said conductive members at a dilferent potential with respect to said body enclosing conductive surface.

7. A cathode structure according to claim S and means for energizing each of said conductive members at a diierent potential.

References Cited by the Examiner UNITED STATES PATENTS 2,223,908 12/40 Bull 313-86 2,768,328 10/56 Pierce 315-393 2,817,040 12/57 Hull S15-3.5 2,914,700 11/59 Paananen 315-393 2,938,139 5/60 Lerbs S15- 5.29 X 3,107,313 10/63 Hechtel S15-5.29 X

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, ROBERT SEGAL, Examiners.

Claims (1)

1. 5. A CATHODE STRUCTURE COMPRISING A BODY COMPOSED OF ELECTRON EMITTING MATERIAL, MEANS FOR HEATING SAID MATERIAL, A CONDUCTIVE SURFACE ENCLOSING SAID BODY, AN OPENING IN SAID SURFACE FROM WHICH ELECTRONS EMERGE, MEANS PRODUCTING TRANSVERSE ELECTRIC AND MAGNETIC FIELDS IN THE AREA ABOUT SAID OPENING FOR COMPELLIG SAID ELECTRONS TO MOVE ALONG ARCUATE PATHS AND A PLURALITY OF ELECTRICALLY CONDUCTIVE MEMBERS SPACED IN SAID OPENING AND BEING ELECTRICALLY CONNECTED TO SAID CONDUCTIVE SURFACE.

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