US2992354A - Travelling wave tubes - Google Patents

Description

July 11, 1961 A. LERBS El'AL 2,992,354

TRAVELLING WAVE TUBES Filed Feb. 17, 1955 b b L 1 4s 6 & \w

' I FIG.6 2 1 2 41 10 w \fi\ 3 Sheets-Sheet 1 July 11, 1961 A. LERBS EIAL 2,992,354

TRAVELLING WAVE TUBES Filed Feb. 17, 1955 S Sheets-Sheet 2 .F 10 I FIG. 9

FIG-H men to another.

United States Patent v TRAVELLING WAVE TUBES Alfred Lerbs and Daniel Revel-din, Paris, France, as-

s1 gnors to Compagnie' Generale de Telegraphic Sans F11, a-corporation of France b Filed Feb.'17, 1955, Sen-No. 488,751 "Claims priority, application France Mar. 4, 1954 26 Claims. (Cl. 315--3.5}

The present invention relates to travelling wave tubes of the magnetrontype, and more particularly to a system of electrodes for highpower tubes of this type.

It is diflicult to construct travelling wave magnetron tubes whose peak power orcontinuous operating power may attain 100 kw. or more. In order to obtain such powers with tubes of this type, it is necessary to have electron optical systems capable of emitting very concentrated beams, having high perveance. The 'perveance is given by the expression J/ V where J expresses the beam current in-amperes and V the accelerating voltage of the beam in volts. This perveance must be capable of attaining in respect of 100 kw. tubes, values of the order of 10- I being of the order of several amperes.

Now, in thepr'esent stated the art, it-is difficult to obtain cathodes having not only asurface which is sufiiciently small to emit concentrated beams, but also a sutficient density of emission to emit such intense currents.

might materially diminish the -H.-'F. energy output of the tube.

Two cases may be encountered. 'In the first "case, the

impact of the electrons on the negative electrode causes no secondary electron emission. The electrons inquestion are lost and nolonger participate in t-the 'interaction mechanism between the -H.-F. wave and electrons.

In the second case, the impact of these electrons causes a secondary emission. rejoin the beam and may eventually intensify the energy exchange. But it is clear that such a secondary emission which takes place over the entire surface of the electrode, depends to a large extent on the condition of the surface of the negative electrode. Experience has shown that its main eifect is to render the characteristics er tubes of the same series very variable from one speci- Theincrease in power that it might atford owing to the increase in the beam current is very variable. Moreover, such a secondary emission increases the beam current mainly'in the vicinity of the collector electrode. The secondary electrons do not have time to impart energy to the wave before being'captured by the collector. The current ofthe latter, which causes aloss of energy, is therefore increased without any real benefit.

The present invention has for its main object to provide high-power travelling wave magnetron'tubes having characteristics which may be maintained in the mass production of these tubes.

According to the invention, the negativeelectrode or these tubes comprises two parts. The surface *of "the first 1 part adjacent the cathode may include -a covering capable of causing a high "secondary emission while'the surface -of *"the second part adjacent "the collector is pro- The secondary electrons 2,992,354 Patented July 11, 1961 vided with means which prevents any secondary emisstem.

The present invention will be better understood from the ensuing description with reference to the accompanying drawings in which:

FIG. 1 is a longitudinal sectional view of an amplifying tube of rectilinear structure embodying the invention;

FIG. 2 is an end view of this tube;

FIGS. 3 and 4 are cross-sectional views of the negative electrode taken on lines AB and CD, respectively, of FIG. 1.;

FIG. 5 is a plan view of the negative electrode;

FIGS. 6 and 7 are cross-sectional views of other em- 15 bodiments of negativeelec'trodes of tubes embodying the invention;

FIG. 8 is across-sectional view of a backward travelling wave oscillator tube of circular structure embodying the invention;

FIGS. 9 and 10 are views of variants of the terminal parts of the tube shown in FIG. 1, and

FIGS. 11 to 15 are diagrammatic views of other examples of -tubes embodying the invention.

'The tube shown in FIG. 1 comprises, in the known manner, a metallic envelope 4, a delay line 5 with its input 6, its output 7 and its attenuation 8. A source of current 26 brings this delay line, the envelope 4 andthe collector 9 to a positive potential relative to the cathode 1. Thelatter'is supported by thenegative electrode "27 which is parallelto the-delay line 5. The cathode is heated by the circuit 28 I supplied from a source of energy, not shownin the drawing, connected for "exampie to the electrode 27. The interaction space bounded -by theelectrodes 5 and 27 is traversed by'a magnetic held, the lines of force of which are perpendicular to the plane of the figure; this field is generated by magnets 29 seen in FIG. 2. According to the invention, the

negative electrode 27 consists of twoelements 2 and 3,

cross-sectional views of which are shown in FIGS. 3 40 .and 4 respectively.

The element -2 of the negative electrode adjacent the cathode comprises a fiat surface disposed between two longitudinal raisededges or flanges 12 (FIG. 3). This surface may be-covered by a layer 10 of material pos- .sessing ahigh coefiicient of secondary emission. It is obvious that the element 2 may be composed entirely of a homogeneous material having a high coefi'icient of secondary emission; in this case, the layer 10 is no longer necessary.

The-element'3,-'shown in FIG. 4, is provided with longitudinal traps 13, known per so (see U.S. application Serial No. 429,346, filed by Reverdin on May 12, 1954); these traps are preferably disposed in a direction parallel with the-direction of the beam. FIG. 5, which is aplan view of the electrode 27, clearlyshows the respective positions of the elements 2and 3.

A'tube of this kind operates in the following manner: The electrons from theca'thode 1 which. impinge upon thesurfacelO of theelement 2, cause a high secondary emission. When the beam interacts with the travelling wave propagated along the delay line 5, a part of the electrons -is absorbed" by the delay line. The electrons thereby eliminated are replaced in the beam by the sec- .ondary electrons emitted by the surface 10. The num- -ber of-electrons interacting with the wave therefore remains substantially constant between the cathode and .the collector andthe power output of the tube is im- .proved. Anysecondary emission caused by the impact -of electrons on the element 6 of the negativeelectrode, however would produce a high current between the nega- "tive electrode and the collector, whereas the length of the path ofthe secondary electrons would be too short to a1- similarly circular structure, are known per se.

3 low their interaction with the wave to contribute to amplification. This harmful emission is avoided owing to the presence of the traps 13. The electrons impinging on the element 3 are captured by these traps and the secondary electrons are incapable of escaping therefrom.

Good results have been obtained by giving the element 2 a length which is five times that of the element 3. Indeed, it is fairly advantageous to make the absorbing element 3 fairly long so that it is capable of absorbing, without causing secondary emission, as many as possible of the electrons that have not imparted sufficient energy to the wave propagated in the line. On the other hand, the element 2 must also be long enough to cause all the useful secondary electrons to participate in the interaction. The aforementioned proportion of 5:1 as between the length of elements 2 and 3- satisfies these contradictory conditions.

The start of the absorbent element 3 must not be too near the end of the delay line and preferably the absorbent element should be prolonged beyond this end. In this way the current of the collector is diminished. Indeed, a large part of the electronic current is absorbed by the element 3; the losses due to the current of the collector are thus reduced.

The layer may be obtained in various ways, for example a thin layer of metal such as platinum deposited electrolytically, or of a certain alloy for example copperberyllium. Certain oxides, and in particular alkaline oxides, are evenmore efiective than metals. But these oxides are generally difficult to deposit in the form of a thin layer capable of strongly adhering to a metallic surface. They require a rough or porous metallic surface. For obtaining this rough or porous surface, one process consists in mechanically or chemically treating the metallic surface of the electrode 2; for example, a suitable molten metal may be sprayed on this surface by means of, for example, a Schoop gun.

According to the variant shown in FIG. 6, the layer 10 having a high coefiicient of secondary emission does not extend over the entire width of the electrode 2, shown in FIG. 2, but terminates at a certain distance b from the flanges 12 thereby defining with the latter two bands 16. The width b of these bands 16 is chosen to be about equal to the height a of these flanges. In order to eliminate any secondary emission on the bands 16, a slit 17 may be machined into each of the latter (FIG. 7) along the length of electrode 2; several similar parallel slits could of course be provided in each band. These slits absorb the primary electrons which impinge on the negative electrode in the same way as the traps 13 of the element 3 and thus traps are provided along at least the side portions of the negative electrode over its entire length. Thus, the secondary emission occurs in a zone whose width is precisely determined; furthermore, the beam does not extend into the lateral regions of the interaction space, where the high frequency fields are often too weak, to cause an appreciable interaction.

There is shown in FIG. 8 a backward travelling wave oscillator of circular form embodying the invention. The differences between this oscillator and an amplifier of These differences lie in the disposition of the output 107 adjacent the cathode 101 emitting the electron beam and in the disposition of the attenuation 108 on the delay line 105 adjacent the collector 109. The interaction space is traversed by a magnetic field the lines of force of which are perpendicular to the plane of the figure, as in the case of the tube shown in FIG. 1. The same two elements 2 and 3, referenced 102 and 103, are also in the negativeelectrode, together with the layer 110.

It is possible in this embodiment not to prolong the absorbent element 103 beyond the end of the delay line 105. Indeed, the last part of this delay line is covered by the attenuation 108 and does not participate in the interaction mechanism. The element 103 is thus, in cffeet, already prolonged beyond the effective end of the line, i.e. from the start of the part covered with the attenuation 108. l

It is obvious that it is of interest to make the element 3 as long as possible so as to reduce the losses of the collector. However, this should not be done to the detriment of the element 2 for the reasons explained above. It would be possible, at the cost of lengthening the tube, to prolong the element 3 beyond the output end of the delay line 5. To avoid so increasing the length of the tube the element 3 may be given an incurved form as shown at 203 in the tube illustrated in FIG. 9; it is disposed round the element 215 and is integral with the electrode 227. The electron paths incurve under the effect of the magnetic field and follow the annular passageway 228 between the element 204 and the element 203. The electrons, which are not captured by the electrode 203, rejoin the collector 209 which is brought to a high positive potential.

Similarly, it may be advantageous, as shown in FIG. 10, to increase the length of the part of the element 202 situated between the cathode 201 and the point facing the entrance 6 of the delay line 205. In FIG. 10, the element 202, which has a high coefiicient of secondary emission on account of the layer 210*, permits utilization of a cathode 201 having a weak emission. The electronic current at the entrance of the interaction space may be rendered sufiiciently intense by the addition of secondary electrons. If the negative electrode is rectilinear, the necessity to lengthen the tube once more arises. To avoid this, the tube may be constructed as shown in FIG. 10 with the part 229 of the electrode 202 wound round an element 214 integral with the electrode 227. Under the effect of the electric and magnetic fields, the electrons in the space 230 between the electrode 204 and the element 214 move in a circular path which conducts them to the entrance of the interaction space.

FIGS. 11 to 15 illustrate some variants of tube embodying the invention. In these figures the electrodes in question are illustrated diagrammatically together with their supply sources.

1 The tube diagrammatically illustrated in FIG. 11 comprises three negative electrodes 320, 302 and 303 facing the anode 304 and the delay line 305 both of which latter are brought to the same potential. Electrode 320 supports the cathode 301. A layer 310 having a high secondary emission is disposed on the surface of the electrode 320 between the cathode 301 and the entrance of the interaction space at 306. A second electrode 302 follows the electrode 320. The electrode 302 faces the delay line 305 and it is brought to a negative potential relative to that of the cathode 301.

In order to maintain the same electric field in both the space 304301 and the space 305-302, the electrode 302 is situated at a distance from the electrode 305 greater than the distance between the electrode 320 and the electrode 304. The electrode 302 also comprises, over the part of its surface between the attenuation 308 and the output 307 of the delay line 305, a layer 310' having a high secondary emission.

The secondary electrons emitted by the layer 310 are added to the primary electrons emitted by the cathode 301. The totality of these primary and secondary electrons forms the beam that enters in the interaction space.

As the exchange of energy from the wave to the electrons is particularly intense in the vicinity of the portion of the line comprised between the attenuation 308 and the output 307, it is advantageous to provide, in this region, a layer 310', having a high emission coefiicient, whose secondary emission augments the intensity of the beam. This emission is favored by the low'potential of the layer 310' which permits an energetic acceleration of the secondary electrons, the potential difference between the layer 310' and the collector being high.

The electrode 303 is in alignment with the electrode "302 and is'situated at the right of the output 307 ro'fthe delay line 305, as viewed in FIG. 11. This electrode is -brought to a potential higher than that of the electrode "302. Consequently, the potential diifere'nce between the source of potential with connections thereto is shown In the tube shown in FIG. 12, the negative electrode 27 of the FIG. 1 is replaced by a system of three electrodes 420, 421 and 422. The electrode 420 supports the cathode40l1 and a layer 410 having a high secondary emission. The electrode420 is situated facing the anode 404 at the left of the interaction space, as viewed in FIG. 12. The electrode 421 faces the line 405 in the region comprised betweenthe entrance 406 and the first end of the attenuation 408.

The electrode 422 extends along the last portion of the interaction space between attenuation 408 and the end 407 of the interaction space. The electrode 422 supports a second layer 410 having a high emission coefficient and the absorbent element 403. The electrode 421 is at the highest negative potential. After this, in the order of increasing potentials, are the electrode -422 and the electrode 420 both of which are at substantially equal potentials. In order to maintain the field substantially constant in the spaces 420-404 and 421405, the electrode 421 is placed at a distance from the delay-line 405 which isgreater than that of the electrode 420 therefrom. The electrodes 420 and 422 are substantially in alignment. The electrode 422 is a little more negative than electrode '420 and the electric field existing in the spaces 420-404 and 421405 is a little weaker than that in the space 422- 405. Thus the secondary electrons in the space 422-405 are subjected to a greateraccelerating voltage, which to a certain extent favors the secondary emission. Such an arrangement permits defining with precision the portion of the interaction space where the beam is partially supplied by the secondary emission.

In the following description of embodiments shown in FIGURES 13, 14 and 15, there are many similarities to the apparatus shown in FIGURE 11 and like reference numerals are used to denote like parts in each of these figures.

In the tubeshowvn in FIG. 13, the'absorbent electrode 303 is Wholly inside the interaction space. The electrode 303 has been moved nearer the delay line 305. The distance of the electrodes 320, 302 and 303 and their respective potentials have been so selected that the field between "303 and 305 is substantially equal to the field between 302 and 305.

In the tube shown in FIG. 14, the'electrode 303 has been split upinto two parts, the split being situated facing the end 307 of the delay line. The first part 303a is united with the electrode 302 and the'second part 3031) is at the 'same level but is brought to a less negative potential than'uhe electrode 302.

The potentials of the various electrodes are the same as those of the tube shown in FIG. 13. Hence, the elec trons that are capable of attaining the electrode 303a are absorbed'there by an electrode brought to a very negative potential. The electrons which attain the elec- "trode 303b are absorbed there by an electrode brought to a negative potentialwhichisless negative orlower in absolute value.

In thetube shown-in FIG. -15, the electrode 303b of FIG.- 14 is replaced by a series of absorbent sections 303b, 3031) and 303b', brought respectively to differnt fixed ngative' potentials, relatively to the cathode 301. Said potentials are all the higher as these electrodes are nearer to the collector. In this way, the electrons are always captured by an electrode 3113b brought to a potential which is only slightly higher than the potential corresponding to their kinetic energy. It is obvious that the electrons 'c'aptured'by the electrode 303 b have a kinetic Theresultant secondary emission is always weak, and the output energy of the tube is improved.

1. In a travelling-wave magnetron tube of the type comprising a delay line having two extremities and an electrode system electrically negative with respect to said means for producinga magnetic'field having its lines of force directed through said space perpendicular tothe lines of force of said electrostatic field and to thedirection of said primary beam, and electron collecting means disposed at said outlet: the surface of said negative electrode systemfacing said delay line having at least one first portion near said beam producing means with a high secondary omission factor, and at least one second portion disposed near said electron collecting means, and provided with means for absorbing electrons impinging thereupon without releasing secondary electrons therefrom.

2. Tube as claimed in claim 1, wherein said first portion extends over the entire length between said beam producing means and said second portion.

3. Tube as claimed in claim 1, wherein said first portion extends over the length between said beam producing means and the extremity of said delay line defining said inlet.

4. Tube as claimed in claim 1, wherein said delay line is provided with attenuating means in a region intermediate the extremities thereof, said attenuated region having two extremities, said first portion extending from the point of said negative electrode system facing the extremity-of said attenuated region remote from said beam producing means to said second portion.

5. Tube as claimed in claim 1, wherein the portion of said negative electrode system between said beam producing means and the point defining said inlet is curved.

6. Tube as claimed in claim 1, wherein the part of said second portion extending beyond the pointdefining said outlet is curved.

7. Tube as claimed in claim 1, wherein said secondportion is between two bands, said bands extending in the direction of said beam and having a smaller secondary emission factor than said first portion.

8. Tube as claimed in claim 7, wherein longitudinal grooves are provided in said bands.

9. Tube as claimed in claim 1, wherein said negative electrode system comprises two longitudinal edge portions extending in the direction of said beam perpendicular to the surface of said electrode system and bounding said negative electrode system on both sides thereof.

10. Tube as claimed in claim 1, wherein said negative electrode system comprises a single electrode.

11. Tube as claimed in claim 1, wherein said negative electrode system comprises several separate electrodes.

12. Tube as claimed in claim l1,wherein said separate electrodes are disposed "at different distances from'said delay line.

13. Tubeas claimed in claim 11, wherein means are provided for biasing said separate electrodeswith difierent negative potentials with respect to said delay line.

"14."Ina travelling wave tube of the type comprising a delay line and an electrode system parallel to and facing said delay line and defining therewith a wave and electron interaction space having inlet and outlet ends, means for producing and directing a beam of electrons through said space from said inlet to said outlet in a direction parallel to said delay line and electrode system including means for applying a potential difference between said delay line and system to establish a transverse electric field in said space and means for establishing in said space a magnetic field having its lines of force perpendicular to the lines of force of said electric field: said electrode system including electrode means along a first-portion of said inner action space for producing secondary emission in response to impinging thereon of electrons from said beam and a negative electrode means comprising at least a portion adjacent said outlet provided with means for absorbing electrons impinging thereon without releasing secondary electrons therefrom.

15. In a travelling wave tube of the type comprising a delay line and an electrode system parallel to and facing said delay line and defining therewith a wave and electron interaction space, means for producing and directing a beam of electrons through said space from one end thereof to the other end in a direction generally parallel to said delay line and electrode system including means for applying a potential diiference between said delay line and said system to establish an electric field in said space transversely with respect to said direction and means for establishing in said space a magnetic field having its lines of force perpendicular to the lines of force of said electric field and to the direction of said beam, a negative electrode included in said system comprising at least a portion adjacent said outlet provided with elongated traps with their lengths extending generally in the direction of the beam for absorbing beam electrons en- 'tering said traps without releasing secondary electrons therefrom.

16. In an electron tube having a source of primary electrons, a pair of spaced electrodes between which said electrons are propagated in the form of a beam, means for guiding said electrons to move past one of said electrodes, said guiding means including means for establishing crossed electric and magnetic fields between said electrodes, said one electrode being located so that it may be struck by some of the beam electrons and including elongated traps with their lengths extending generally in the direction of the beam for receiving electrons impinging on said one electrode and entrapping secondary electrons to reduce secondary emission from said one electrode.

17. An electron tube according to claim 16, wherein said traps include a plurality of grooves extending gen- .erally in the direction of electron beam propagation.

18. An electron tube according to claim 16, wherein said one electrode includes elongated traps along substantially its entire length, said traps being elongated generally in the direction of beam propagation.

19. An electron tube according to claim 18, wherein said traps are a plurality of parallel slits in said one electrode.

20. In a travelling wave tube of the type comprising a delay line and an electrode system parallel to and facing said delay line, means for producing and directing a beam of electrons through the space defined by said delay line and electrode system and in the direction generally parallel thereto, said beam directing means including means for establishing a transverse electric field in said space and means for establishing in said space a magnetic field having its lines of force perpendicular to both the lines of force of said electric field and to the direction of said beam, said electrode system including an electrode at a negative potential relative to said delay line and comprising at least a portion provided with elongated traps with their lengths extending generally in the direction of the beam for absorbing electrons impinging thereon and minimizing the release of secondary electrons therefrom. v

21. A travelling wave tube according to claim 20, wherein said traps include a plurality of grooves in said negative electrode, said grooves extending generally in the direction of said electron beam.

22. In an electron tube having a source of primary electrons, a pair of spaced electrodes, means for propagating said electrons generally in the form of a beam in said space and generally in a direction along the surface of one of said electrodes, means for establishing crossed electric and magnetic fields between said electrodes, each field being generally perpendicular to the path of said beam between said electrodes, for guiding said electrons to move generally in said direction, said one electrode being so located that it may be struck by some of the beam electrons and including traps for reducing the secondary emission from said electrode caused by beam electrons impinging thereon, said traps including grooves in said one electrode having a component of their length extending in the direction of said beam.

23. In a travelling wave tube of the type comprising a delay line and an electrode system parallel to and facing said delay line, means for producing and directing a beam of electrons through the space between said delay line and said electrode system, said means including means for establishing crossed electric and magnetic fields each generally perpendicular to the path of the electron beam, said electrode system including an electrode in a position where it may be impinged by some beam electrons and comprising at least a port-ion provided with traps for reducing the secondary emission therefrom due to impingement of beam electrons thereon, said traps being defined by elongated electrically conducting structures on said one electrode, said structures each having a component of their length extending generally in the direction of said electron beam in said space.

24. Electron tube apparatus according to claim 16, wherein said means for establishing crossed electric and magnetic fields between said electrodes includes means for maintaining said one electrode electrically negative with respect to the other of said pair of spaced electrodes.

25. Electron tube apparatus according to claim 24, wherein said electrodes have circularly arcuate form defining an arcuate space for propagation of the electron beam.

26. Electron tube apparatus according to claim 25, wherein said other electrode of said pair of spaced electrodes is a delay line structure.

References Cited in the file of this patent UNITED STATES PATENTS 2,312,723 Llewellyn Mar. 2, 1943 2,401,777 Shepherd July 11, 1946 2,520,603 Linder Aug. 29, 1950 2,547,142 Shepherd Apr. 3, 1951 2,582,185 Willshaw Jan. 8, 1952 2,607,904 Lerbs i Aug. 19, 1952 2,613,335 Fremlin et a] Oct. 7, 1952 2,680,209 Veronda June 1, 1954 2,694,783 Charles Nov. 16, 1954 2,695,929 Reverdin Nov. 30, 1954 2,704,350 Lerbs Mar. .15, 1955 2,741,718 Wang Apr. 10, 1956 2,853,641 Webber Sept. 23, 1958 2,861,212 Lerbs Nov. 18, 1958 FOREIGN PATENTS 1,081,937 France June 16, 1954

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