US2936393A

US2936393A - Low noise traveling-wave tube

Info

Publication number: US2936393A
Application number: US631129A
Authority: US
Inventors: Malcolm R Currie; Donald C Forster
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1956-12-28
Filing date: 1956-12-28
Publication date: 1960-05-10
Anticipated expiration: 1977-05-10

Description

M. R. CURRIE ET AL 2,936,393

LOW NOISE TRAVELING-WAVE TUBE May 10, 1960 2 Sheets-Sheet 1 Filed Dec. '28. 1956 INVENTORS Malcolm RCurrie, Donald C.Fors1er,

BY {A (99m.

AGENT 5 -l l I I I I I M. R. CURRIE ETAL LOW NOISE TRAVELINGWAVE TUBE May 10, 1960 2 Sheets-Sheet 2 Filed Dec. 28, 1956 Fig. 6.

I i l l CATHODE Fig. 5.

CATHODE i/ /W r K P Fig. 8.

iNVENTORS, Malcolm R.Currie,

Donald C.Forsfer,

AGFN

the electron gun;

United States Patent LOW NOISE TRAVELING-WAVE TUBE Malcolm R. Currie, Beverly Hills, and Donald C. Forster,

Torrance, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware 1 Application December 28, 1956 Serial No. 631,129 12 Claims. (31. 315-3) 'The subject invention relates generally to microwave tubes, .and-more particularly to a traveling-wave tube havng an extremely low noise figure.

In electron discharge devices, and especially in amfplifiers and oscillators, the usefulness of the device is frequently limited by its own inherent noise generation which the tube amplifies to a level which is detrimental to the other functions of the tube; or the noise may'cause spurious modes of oscillation; or such noise may cause a multitude of other deleterious eifects on the operation -of the deivce. This is particularly true in traveling- .wave tube devices, and especially in backward-wave amplifiers.

, In the past, there have been developed two principal types of low noise electron guns: one utilizing the veloc- 'ity jump principle, in which the beam undergoes acceleration or de acceleration beyond the electron gun itself to effect'noise reduction, and the other incorporating regions of more gradual potential changeto simulate an 'efie'ctive impedance change along the beam analogous to that of a transmission line. This latter type also uses ,acathode extending beyond and surrounded by a beamforming electrode to obtain variable perveancc, and in this type of gun the beam-formingelectrode is held at thercathodejpotential, or at some negative potential with :respect thereto. Both of these types of electron guns are used to form solid cylindrical electron beams. Their cathode-anode regions are of conventional theoretical :design "to give a uniform surface emission with minimum beam spreading, although it is believed by some that in :the" conventional gun some beam spreading does occur and that this may be beneficial in achieving some slight :noise reduction. xqiln the co-pending application entitled Low Noise Traveling-Wave Tube, by Malcolm R. Currie, filed concurrently herewith and assigned tothe assignee of this application, there is disclosed an electron gun for back- .w'ard-w'ave devices or other hollow beam devices in which center discs forming part of the various electron The present invention bases its primary object to provide a low noise traveling-wave tube in which noise reduction is achieved within the cathode-anoderegion of V Another object of the present invention is to provide a low noise traveling-wave tube which has a noise figure even below the so 'called' theoretical minimum noise figure i j -It is a further'obje'ct to provide'such an electron gun in which the noise reduction is achieved in the region 'betweenthe cathode and anode; n 4 I "It 'is still another object to provide this noise reduction 7 providing an arrangement of potentials which cause electron beam formation in 'a manner to obtain an exceedingly low noise figure'in theele'ctron tube;-

Briefly, these and other objects areachievedin the present invention by providing in the region of the emissive surface of thecathode a beam-forming electrode which is at a positive potential higher than that of the first accelerating anode which is placed at some distance in front of the cathode. This configuration provides in the cathode-anode region a potential distribution with a cross-over or saddle point and an axial potential profile in which the electron beam is subjected to sudden acceleration followed by a relatively long region of virtual drift. As a result, a slight adjustmentof the potential distribution results in a relatively large manipulationof the noise quanities. An unconventionally large axial magnetic field is also provided which nullifies any dis persive eifect on the electrons from the radial gradient caused by the positive beam-forming electrode, and which additionally decreases the noise figure of the electron tube.

These and other novel features which are uniquely characteristic of this invention, both as to its structure and operation, together with further objects and advantages, will become apparent to one skilled inthe traveling-wave tube art from the following detailed description and drawings.

In the drawings:

Fig. 1 is a schematic and partly sectional view of a traveling-wave tube utilizing the low noise electron gun of the present invention;

Fig. 2 is a detailed sectional view in perspective of a typical low noise electron gun according to the present invention; a

Figs. 3 and 4--are electric field plots for electron guns of the present invention; 2

Fig. 5 is a plot of axial potential of the gun of the present invention;

Fig. 6 is a plot of axial potential in a conventional electron gun; and

Figs. 7, 8, 9 and 10 are sectional views illustrating alternative embodiments of the present invention.

Referring now to the drawings for purposes of description of a typical structure involved in various embodiments of the present invention, and more particularly to Fig. 1, there is shown a backward-wave travelingwavetube 10 having an outer envelope 12 containing and supporting an appropriate slow-wave structure 14, which may be a helix as shown. Envelope 12 is enlarged at its left end to house and support electron gun 16. Terminating the opposite end of envelope 12' is a collector electrode'18 designed to intercept and dissipate the kinetic energy of the electron stream produced by electron gun 16. Electromagnetically coupled to the electron gun end of slow-wave structure 14 is an output waveguide 20 which achieves coupling to slow-wave structure .14

'member, is located in the region of the emissive surface "of cathode 26. A second accelerating anode 32 is axially aligned with electrode 30 and placed in front thereof. A third accelerating anode 34 is positioned "in front of anode 32. Dielectric rods 35, axially disposed about the periphery of the electrodes, support the outer portions of the several electrodes of electron'gun Cathode 26' is maintained at a low direct currentpo tent'ial byabattcry' 37 having a negative terminal indi- A 2,936,393 Patented May 10, recoand parallel thereto.

trode P.

cated as V a suitable tap on battery 37, indicated at V, indicates the heating voltage applied to filament 28; another tap indicated at V .more positive than V is the potential ofthe second anode. 32, while V even-more positive, is the potential at which profile-shaping electrode 30 is maintained. Ground potential, the positive terminal of battery 37, is even more positive and is the potential of the final accelerating anode 34 and slowwave structure 14.

A positive voltage source 36, of the order of a few hundred volts, is connected between collector electrode 18 and ground for purposes of minimizing the effects of secondarily emitted electrons therefrom. An enveloping outer solenoid 38, energized by a voltage source 40, produces an axial constraining or confining magnetic field of the order of 2000 gauss for purposes of constraining or confining the electron stream emitted by cathode 26.

Referring to Fig. 2, a practical embodiment of an electron gun of the present invention is shown in some detail. A dielectric base plate 42 supports a flanged left-hand end of a hollow multi-layered metal heat shield 44 to the right-hand extremity of which is attached an annular cathode 46 which is indirectly heated by a filament 48. A set of axial outer dielectric rods 50 arranged about the periphery of base plate 42 supports a profile-shaping electrode 52 and a series of accelerating

anodes

53, 54, 55 and 56, each of which consists of an outer annular member indicated by these reference numbers and an inner circular member indicated by corresponding primed reference numbers, viz., 52, 53', 54', 55 and 56'. Profile-shaping electrode 52 (and 52') is positioned in a plane near the cathode emissive surface The inner circular members are axially aligned and supported on an inner dielectric rod 58, which is secured to base plate 42 and which may be made of sapphire for strength as well as insulation. Each of the inner circular members is electrically connected to its respective outer member by respective conductors, indicated typically by wires 60 and 62 extending along inner dielectric rod 58, and which pass through dielectric capillary tubes, which may be made of quartz, through base plate 42 and thence to the respective outer annular member. Again, cathode 46 is maintained at the lowest, that is, the most negative, direct current potential; anode 53 is at potential V which is next in order of positiveness; profile-shaping electrode 52 is next highest, and then, in order of increasing positiveness,

anodes

54, 55 and 56 at potentials V V and V respectively. An aquadag coating 63, as well as matching ferrule 24, may also be maintained at potential V which in this example may be ground potential.

To be particularly noted is the fact that profile-shaping electrode 52, including its inner member 52, is at a higher direct current potential than is the next anode electrode 53 with its inner member 53 Fig. 3 for simplification illustrates, by way of example, a solid beam electron gun, while Fig. 4 illustrates principally a hollow beam electron gun. Referring to Fig. 3, the cathode-anode potential field is schematically and simply illustrated by lines of equal potential distributed between a cathode K at potential V 21 profileshaping electrode P at positive potential V and an accelerating anode A at an intermediate potential V It is important to note in both Fig. 3 and Fig. 4 the crossover or saddle region, and the unusual radial gradient occurring between cathode K and profile-shaping elec- It is believed that this gradient causes an increased emission from the edges of cathode K, and that this, at least in part, gives rise to the extraordinary low ,noise'figure found empirically to exist when such a po- .region of an electron gun constructed in accordance with the present invention.

At the cathode, potential V of course exists. The potential then increases very rapidly to an elbow or bend 64, and then rather gradually increases to the potential V existing at the anode. Fig. 6 iullstrates by contrast the axial potential profile of a conventional electron gun.

Figs. 7, 8, 9 and 10 illustrate alternative embodiments of the present invention. In each figure the potential of the cathode K is V the potential of the profileshaping electrode P is most positive at V While the anode A is at an intermediate potential V In Fig. 7 the profile-shaping electrode P has a circular cross-section and is placed somewhat in front of the plane of the emissive surface of the cathode K. In Fig. 8 the profileshaping electrode P is placed substantially coplanar with the emissive surface of the cathode K and has a very sharp edge projecting toward the cathode in a manner to achieve a very high and nonhomogeneous potential gradient in the region between the electrodes. Fig. 9 shows a profile-shaping electrode P which is somewhat behind the emissive surface of the cathode and provides a slight variation in the shape of the potential profile in the region of the electrodes. Fig. 10 shows the profile electrode P even further behind the emissive surface of the cathode, which orientation may vary the shape of the potential profile even more to thus illustrate the versatility of the electron gun of the present invention.

In operation, typical values for the potentials on the various electrodes illustrated in Fig. 2 with respect to the cathode are as follows: +13 volts for profile-shaping electrode 52, 52', +5 volts for anode 53, 53', +45 volts for

anode

54, 54, volts for anode 55, 55', and 500 volts for anode 56, 56', as well as aquadag coating 62, matching ferrule 24 and slow-wave structure 14. With a set of voltages such as these and a magnetic confining field of 1600' gauss, a noise figure well below the 6 db ordinarily considered to be the theoretical minimum due to shot noise and velocity variation has been achieved.

There has thus been disclosed a traveling-wave tube, particularly of the backward-wave type, embodying a low noise electron gun which achieves an extraordinarily low noise figure in the tube by reduction of noise in the electron gun cathode-anode region itself. Prior art methods of minimizing noise have not dealt principally with this region in the electron gun. This new electron gun utilizes a unique method of noise reduction in the cathode-anode region. Due to the profile-shaping electrode near the cathode having a relatively high positive potential, a high emission density over a small radial thickness near the edge of the cathode surface is achieved and contributes to the noise reduction on the beam. A further advantage of the present invention is that, as illustrated by the graph of Fig. 5, the electrons initially undergo rapid acceleration followed by what is effectively a relatively long drift region. In this region the electron velocity is low and manipulation ofthe noise quantities over a wide range of operating parameters may be readily achieved, as bome out by results obtained from experiments utilizing, for example, the gun configurations of Fig. 2, as well as the alternative configurations illustrated by Figs. 7 through 10.

What is claimed is:

1. 'In a traveling-wave tube having a low noise figure, a low noise electron gun comprising: a cathode having an emissive surface in a plane perpendicular to the axis of said tube; a profile-shaping electrode disposed about said emissive surface substantially in a plane including said surface; and an accelerating anode disposed along the axis of said traveling-wave tube away from said cathode in a plane parallel to that of said profile-shaping electrode, said anode being at'a potential algebraically intermediate the potentials of said cathode and said profileshaping electrode, the latter being the more positive, said potentials providing a relatively electric field-free drift region between said cathode and said anode.

2. Anaemia-re ate mat a low noise figure comprising: slow-wave structure means for propagating t awl -wa e ternalt nsmi in means, a e

tog-said slow-wave structure means; magnetic means sub- Y tihn a .en mp ssins aid trav n w tube for P aimi a lcnsitad nal .v castra ni ;m fie along traveling-wavetube; View noise electron gun means for producingand projectinga stream of electrons along slow-wave structure in amanner to achieve electromagnetic energy exchange relation between said streams oftelectrons and said traveling waves, said electron'gun means including; an emissive cathodeyan electrical posaid slow-wave structure, the potential of said first accelerating anode being algebraically intermediate those of said cathode and said profile-shaping electrode, the latter being the more positive.

v 3. A low noise traveling-wave tube comprising: slowwave structure means for propagating traveling waves; external transmission line means coupled to said slowwave structure means; magnet means substantially encompassing said traveling-wave tube for producing a longitudinal constraining magnetic field along said traveling-wave tube; low noise electron gun means for producing and projecting a stream of electrons along said slowwave structure in a manner to achieve electromagnetic energy exchange between said stream of electrons and said traveling waves, said electron gun means including: a thermionic cathode having an emissive surface which is substantially in apl ane perpendicular to the axis of said traveling-wave tube; an electrical potential profile-shaping electrode being disposed substantially in the plane of said emissive surface and spaced therefrom at a distance which is small compared to the size of said electron gun means;

' a first accelerating anode being in a plane parallel to the plane of said electrode and spaced therefrom towards its slow-wave structure; and at least one other accelerating anode parallel to said first accelerating anode and disposed further toward said slow-wave structure; said cathode being adapted to be maintained at zero potential, said first accelerating anode being adapted to be maintainedat a next more positive potential with respect'to said cathode, said profile-shaping electrode being adapted to be maintained at a potential which is next more positive with respect to said first accelerating anode and said at least one other accelerating anode being all more positive electrically than said profile-shaping electrode.

4. A low noise traveling-wave tube comprising: slowwave structure means for propagating traveling waves; external transmission line means coupled to said slowwave structure means; magnet means substantially encompassing said traveling-wave tube for producing a longitudinal constraining magnetic field along said travelin'g-wave tube; low noise electron gun means for producing and projecting a stream of electrons along said slowwave structure in a manner to achieve electromagnetic energy exchange between said stream of electrons and said travelingwaves, said electron gun means including: a thermionic cathode having an emissive surface which is substantially in a plane perpendicular to the axis of said being in a plane which is parallel to the plane of said electrode and spaced therefrom towards said slow-wave structime; and at least one other accelerating anode parallel to said first accelerating anode and :disposedfurther toward that slow-wavestructure; said cathode beingmaintained at zero potential, said first accelerating anode'being maintained at a next more positive potential with respect to said cathode, said profile-shaping electrode being maintained at a potential which is next more positive with respect to said first acceleratinganode and'said at least 'one other accelerating anode being more positive electrically than said profile-shaping electrode. V o

5. In a traveling-Wave tube having a low' noise figure, a hollow beam electron gun comprising; an annular thermionic cathode having an emissive surface which is substantially in a plane perpendicular to the axis of said tube; a profile-shaping electrode lying substantially perpendicularly to said axis and comprising an outer portion having a circular opening slightly larger than said cathode and an inner coplanar circular member smaller in diameter than said cathode; a first accelerating-anode disposed substantially in a plane parallel to that of said profile-shaping electrode further along the axis of said traveling-wave tube from said cathode and comprising an outer member having a circular opening of a diameter slightly larger than the outside diameter of said hollow beam and an inner coplanar circular member havin a diameter which is slightly less than that of the inside diameter of said hollow beam; one or more other accelerating anodes similarly shaped with respect to said first accelerating anode and disposed further along said traveling-wave tube; supporting means extending along the axis of said traveling-wave tube for supporting said inner circular members coplanarly with their outer members; and a plurality of conducting means extending along said supporting means electrically connecting each of said inner circular members to respective ones of said outer members whereby said hollow electron beam may pass uninterrupted through said accelerating anodes without interference from supporting and connecting wires,

said cathode being at zero potential, said first accelerating anode being next more positive, said profile-shaping electrode being next more positive, and said one or more other anodes being successively more positive.

6. A low noise electron gun according toclaim 5 in which the plane of said profile-shaping electrode is substantially coincident with the plane of said emissive surface.

7. A low noise electron gun according to claim 5 in which the plane of said profile-shaping electrode is disposed away from said emissive surface toward said traveling-wave tube. V

8. A low noise electron gun accordingtodaim 5 in which the plane of said profile-shaping electrode is disposed behind said emissive surface at a distance away from said traveling-wave tube. V

9. A low noise traveling-wave tube electron gun comprising: a thermionic cathode having an emissive surface which is substantially in a plane perpendicular to the axis of the traveling-wave tube; a profile-shaping electrode lying substantially perpendicular to said axis and disposed axially relatively near said emissive surface and having an opening disposed therein slightly larger than said cathode and co-axial with said axis; a first accelerating anode disposed substantially in a plane parallel to that of said profile-shaping electrode and being disposed further along said axis of said traveling-wave tube from said cathode and having a parallel opening of a diameter slightly larger than said emissive surface; and at least one other accelerating anode similarly shaped with respect to said first accelerating anode and'disposed in co-axial alignment therewith further along said travelingwave tube from said emissive surface, said cathode being at zero potential, said first accelerating anode being positive with respect thereto, said profile-shaping electrode being next more positive, and said at least one other anode being successively more positive.

10. A low noise electron gun according to claim 5 in which said profile-shaping electrode is substantially in a plane coincident with the plane of said emissive surface.

11. A low noise electron gun according to claim; 9 in which the said profile-shaping electrode is disposed substantially in a plane displaced from said emissive sur- 5 .face toward said traveling-wave tube. 1

12. A low noise electron gun according to claim 9 in which said profile-shaping electrode is disposed substantially in a plane behind said emissive surface by an axial distance which is small compared to the diameter of said 10 emissive surface.

18 References Cited in'the file of this atent; 3

UNITED STATES PATENTS Hartley Aug. 7,, 194's Varian Sept; 21,1948 Hull ,Mar. 17, 195 3 Heising Sept. 1,1953 Wang Apr; 10, 1956 Field Sept. 11, 1956 Quate May'14, 1957 Field et a1. July 23, 1957 Bryant et a1. July 23, 1957 Birdsall Dec. 17, 19 57