US2853644A - Traveling-wave tube - Google Patents

Description

Sep 23, 1958 L. M. FIELD 3 6 TRAVELING-WAVE TUBE Filed July 30, 1956 III I ll INVENTOR. 4557-59 M 774-20 firrolewz-ys United States Patent M TRAVELING-WAVE TUBE Lester M. Field, Los Angeles, Calif., assignor to California Institute Research Foundation, Pasadena, Calif., a corporation of California Application July 30, 1956, Serial No. 600,840

16 Claims. (Cl. 315-335) This invention relates to microwave tubes and more particularly to means for dissipating the heat developed in the slow-wave structure of a traveling-wave tube.

The traveling-wave tube consists, in essence, of an electrically conducting structure so arranged as to cause electrical signals of very high frequency which are placed upon the structure to propagate along the principal axis of the structure at a velocity which is a small fraction of the velocity of light. An electron stream from an electron gun is simultaneously sent along the axis of the propogating structure at a velocity almost equal to that of the signal propagation. Under these conditions an interaction occurs between the wave on the structure appropriate to thesignal and the electron stream which is injected at the velocity of the wave which causes the wave amplitude to increase the same number of times its starting value in each increment of distance. This type of growth is commonly referred to as an exponential increase in amplitude with distance.

Unfortunately one of the limitations in operating such a structure as has just been described, is that the electron stream cannot be shot through an appropriate long circuit without at least some fraction of the electrons striking the circuit. A limitation on the power which can be amplified in this way is the power which can be sent into the electron stream without causing the metallic circuit to become so hot that it gives off gas or vaporizes and makes the tube inoperative. Of course, since an electron stream is used, the circuit and stream are usually placed in a vacuum container and the whole device operated at high vacuum.

An object of the present invention is to provide for a high degree of cooling of the helical wire which forms the circuit on which the waves propagate at low velocity. The present invention provides for the removal of heat from each turn of the principal helix or helical wire by attaching to each turn a metallic connection from this turn to an outside metallic pipe which is more easily cooled. This outside metallic pipe is the actual vacuum envelope and hence on its outside surface is in direct contact with cooling water or cooling air. In other cases the large outside metallic surface may be just inside of the vacuum envelope but may be so large that both by conduction through a glass wall, for example, and by radiation of heat at a relatively low temperature, it remains at a sufliciently low temperature that no vaporization or degassing occurs.

In order to prevent the metallic connection from the propagating helical structure to the outside metallic wall from disturbing the waves propagating on the structure, it has proven useful to make the connecting member onequarter of a wavelength long at the middle of the frequency band which it is intended should be amplified. The middle of the band is specified because, since the wavelength of the waves changes as one changes the frequency, it is impossible that the supporting member or supporting stub be exactly one-quarter of a wavelength long for a'wide range of frequencies or wavelengths. If

2,353,64i4 Patented Sept. 23, 1958 ICC it is chosen to be exactly right in the middle of the operating band it will be nearly the right length for adjacent wavelengths not too far removed from the mid-band wavelengths. The length of one-quarter wavelength is used because at this length no effect upon the propagating waves of the main helix occurs. Such a one-quarter wavelength stub supported helix has been the subject of a previous patent application, notably that of L. M. Field and C. K- Birdsall, Serial No. 490,088, filed February 23, 1955; and Serial No. 503,590, filed April 25, 1955.

The present invention describes means and techniques for making such stub supports such as required for tubes to work at a few thousand volts at frequencies of 10,000 me. and higher. It utilizes instead of a simple radial stub, a pair of curved semi-circular stubs leading over to the side wall. Such stubs are produced with the great accuracy of length and spacing required by making them as a wound helix and then simultaneously brazing them to the side wall and to the helix which they are to support. By winding the supporing helix to precisely the same pitch as the actual propagating helix and not re moving either one from the mandril on which it is wound until after they are brazed together, a pair of supporting stubs connecting to each turn of the propagating helix is produced with extreme accuracy and ease. At the same time, the supporting helix is brazed to a ridge along the side wall. This ridge shorts the supporting helix from turn to turn in such a way that it no longer acts as a helix on its own, but rather as simply a series of quarter wave stub supports to the main helix. One-half of a circumference of the supporting helix is almost exactly a quarter wavelength of the free space wavelength for the frequency at mid-band.

An object of the invention is therefore to provide im proved means for conducting heat from a traveling-wave tube slow-wave structure without producing a substan tial' change in the impedance of that structure over the operating frequency band of the tube.

Another object of the present invention is to provide improved means for supporting the active helix slow wave transmission line of helix traveling wave tubes.

Another object of the present invention is to provide a helix supporting structure for very small diameter helices such as are operated in very high frequency traveling-wave amplifiers, which supporting structure is arranged to provide good heat conduction from the helix, so that relatively high beam current and power characterize the electron beam with which the radio frequency currents in the helix are to interact.

Another object of the present invention is to provide a helix supporting structure which is fabricated by the use of conventional precision helix winding processes.

Another object of the present invention is to provide a supported helix structure suitable for wide band, continuous wave operation in a very high frequency traveling wave-tube.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages of the invention, may be better understood when taken in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of a traveling-wave amplifier embodying the present invention;

Figs. 3, 4, 5 and 6 are views of three different alternative embodiments of the heat conducting and slow-wave structure of the amplifier of Fig. 1.

Referring to Fig. 1, a traveling-wave-tube amplifier 10 is illustrated having a cylindrical, conductive, non-magnetic envelope 12 which may be made of copper. An electron gun 14 is sealed in the leftextrernity'of the envelope, as viewed in Fig. l. Electrorr'gun 14 is employed to produce a stream of electrons and to direct it along the longitudinal axis of envelope 12.

A solenoid 16 is disposed concentrically about envelope 12 to provide an axial magnetic field along the electron stream path whereby the stream may be constrained along the complete length of the envelope. Such a field may be of the order of 600 to 1200 gauss. In order to produce the magnetic field, a direct current is maintained in sole noid 16 by means of a potential source 18.

A non-magnetic conductive'cooling tank 19 having'the shape of a double-walled hollow cylinder and which may also be made of copper, is disposed concentrically between solenoid 16 and envelope 12. A liquid or gaseous coolant may thus be circulated through the tank to conduct heat away from both solenoid 16 and envelope 12.

Electron gun 14 essentially comprises a cathode cylinder 20, a heater 22, a focusing electrode 24, and an accelerating anode 26. Heater 22 is connected across a suitable source of potential 28, the negative side of the heater being connected to cathode 20. The negative side of source 28 is then connected to the negative terminal of a potential source 30, the positive side of which is connected to ground in order to maintain cathode at a potential of about 30,000 to 35,000 volts negative with respect to ground. Focusing electrode 24 has a frusto-conical shape with an internal surface of revolution forming an angle of 67 /2 degrees with its axis of symmetry. Focusing electrode 24 is maintained at the same potential as cathode 20 by an appropriate connection thereto. Accelerating anode 26 is maintained at about 200 volts positive with respect to ground by a connection to a tap 32 on potential source 30.

A disc-shaped'magnetic pole piece 33 and dielectric spacer 34 are disposed contiguously to a cylindrical appendage 27 of anode 26 to maintain the gun 14 in alignment with envelope 12. A glass stem 36 with integral leads 37 Welded to the gun 14 and an anode lead 38 comprise the remaining supports of gun 14.

Within envelope 12, adjacent to and to the right of gun 14, as viewed in Fig. 1, slow-wave structure 40 in the form of a helix is provided in electrical contact with and supported in part by a rectangular internal input waveguide segment 42 and a rectangular internal output waveguide segment 44. The slow-wave structure 40 and Waveguide segments 42 and 44, which may consist of copl per, are all maintained at ground potential by an electrical connection 45 from envelope 12 to ground. Waveguide segments 42 and 44 are positioned in contact with envelope 12 and sleeve 35. This will be better understood as hereinafter explained in connection with Figure 2.

Slow-wave structure 40'comprises a helical wire. Waveguide segments 42 and 44 have transverse end portions 43 and 49, respectively, which have'fa'cingapertures 53 and vided at their outer ends with mica windows 62and 63 respectively, which provide vacuumseals. A vacuum is thus maintained from the windows 62 and 63 to the opposite end of envelope 12 at'the'extreme left of glass stem 36.

The stream electrons are intercepted by a collector electrode 52 at the opposite extremity of envelope 12 with respect to electron gun 14. Collector 52, which protrudes outside of the envelope 12, is also supported by a suitable aperture in dielectric spacer 54 so as to have a large surface external to the evacuated chamber for heat dissipation purposes and may include fins as shown to aid in conducting away the heat that is generated by the stream electrons when they are collected. Accordingly, collector 52 is preferably fabricated of a metal having good electrical and heat conducting properties, as in the case of structure 40, such as, for example, copper. A potential of the order of 200 volts positive with respect to structure 40 is applied to collector 52 in order to prevent secondary electrons which may be produced by the stream electrons impinging upon its surface from reaching slow-wave structure 40. This potential is applied by means of a connection from collector 52 to the positive terminal of a source 56, the negative terminal of which is grounded.

The helical wire 40 is supported in a novel manner, using another helical wire 51. These two Wires have the same pitch. The supporting helix 51 is brazed to the active helix 4%) at every single turn, and at a point diametrically opposite to where it touches the helix, it is brazed to the cooling fin or pedestal 35A extending from the envelope or sleeve 35. Each turn of the active helix then has two quarter wave length long metallic connections over to the cooling fin as indicated in Figure 2, the two greater wave length sections of each supporting coil turn being designated by reference numerals 51A, 5113. Each of the sections 51A, 513 extends in an arcuate path from the slow-wave structure 40 a distance equal to where n is any positive odd integer and A is the free space wave-length corresponding to mid-frequency of the operating band for which the tube 10 is designed. This requirement is made in order to avoid changing the impedance or" the slow-wave structure in the center of the operating band, and very little about the center. Wave propagation at the band center will thus be essentially the same with or without the use of the sections 51A,

In the modified arrangement as shown in Figure 3, the active helix 140, support helix 51 and the grounded envelope 35 are inserted as a unit into the tubular glass vacuum envelope Coupling of high frequency energy in this case is obtained by inserting the ends of the active helix into a wave guide structure 92. The ends of the helix 140 which define the input and output ends of the active helix serve as probes 140A that are sealed in the glass envelope 90.

In the modified arrangement shown in Figure 4, a ceramic rod 92 is positioned within the supporting helix 51. While the supporting helix 51 tends to support no radio frequency fields of the normal helix mode, nonetheless when the active helix 40 is exited, strong radio frequency fields of a different type may exist in certain portions of the supporting helix 51. It is thus possible to provide the required localized attenuation while supporting within one or more of the supporting helices a ceramic rod 92 or other element with a carbon coating or other impregnation. The rod may be treated also with other lossy materials.

Figure 5 illustrates another alternative arrangement in which three supporting helices have corresponding turns raised on the one end to. the active helix 140 and the metallic envelope 12 in the manner as illustrated in connection with Figures 1 and 2. 1

Inasmuch as no current flows onto these supporting arcuate one-quarter wavelength stubs 51A, 51B at the frequency at which they are'exactly one-quarter wavelength long (provided they are perfect conductorsv or decrease rapidly in amplitude as they travel.

lossle'ss material it is possible to provide not only a V I small effect upon the propagation of the waves for frequencies which differ from this mid-band frequency but also it is possible to make the structure show appreciable loss or attenuation to waves propagating on it at frequencies to either side of the center frequency. This is done by making the stubs somewhat lossy, as indicated above, or by making them have a moderately high surface resistivity. Then, at frequencies off the center frequency where the stubs are not one-quarter wavelength long, appreciable current flows into the stubs and they have moderately high surface resistivity, as taught in accordance with other aspects of the present invention, they cause any waves propagating on the structure to This is a very useful characteristic inasmuch as such attenuation may not be necessary at or near mid-band to prevent oscillation (because near mid-band the circuit ends may be extremely well matched to external circuits and hence no reflections at these ends will occur of sufiicient magnirude to cause self-oscillation), but it may be very necessary well away from this mid-band frequency where the matches tend to become badly reflected and hence start to produce oscillation, unless the circuit shows appreciable attenuation. The matches can, in general, only be made extremely good over a limited frequency range,

:and this limiting range could then be placed around the :mid-band frequency. Since it is generally true that a 'circuit made lossy to radio frequencies by making it have high resistance also becomes a poor heat conductor, one :might first consider that the heat removal properties are seriously impaired by making the circuit somewhat lossy. This, however, need not be the case when the supporting stubs are poor heat and electrical conductors, but are covered with a thin film which has a high surface resistivity. When this film is sufficiently thick, in the order of one-thousandths of an inch for 10,000 megacycles, almost all of the radio frequency current at this frequency fiows in the surface skin and hence the material appears quite lossy. At the same time, the heat is conducted throughout the closed section of the supporting wires and the majority of it is conducted by the high heat conducting inner portions of the stubs.

It is thus seen that according to the present invention a new method of supporting helix propagation lines is devise-d which is relatively easy to use in manufacturing and which produces a support in helix having excellent microwave properties. While the stub supported structure described and claimed in the aforementioned co-pending application is useful for relatively high power, long wavelength helix tubes, difiiculties are encountered to such an extent to render the stubs shown therein inappropriate as a support for a small diameter, fine pitch helix as used for instance at X band frequencies in a traveling-wave tube designed to produce ten or twenty watts of continuous wave power. The mechanical problem of attaching these quarter-wave long stubs to the helix then becomes difficult, as the successive turns are very close together. A typical example of such a structure is one requiring a helix having sixty turns per inch in which the stubs would then be spaced .002 inch apart. Such a stub supported structure would evidently be very difficult to make it exactly uniform; and the requirements for uniformity makes the design practically impossible to realize in a useful tube. At frequencies in the K band, or at even shorter wavelengths, the situation becomes correspondingly worse.

Thus, according to the present invention, a new method of supporting helix propagation lines is devised which is relatively easy to use in manufacturing and which produces a supported helix having excellent microwave properties, essentially the same as those of the stub supported helix.

In the manufacturing process, a first helix is wound using the normal helix winding technique, which helix is to serve as the slow wave propagating helix and through which the electron beam is projected. The first helix is wound on an oxidized stainless steel mandrel and is then retained'on the mandrel. A second helix, usually of different diameter, is then wound on a second similar mandrel with precisely the same pitch as the first one and is retained on its mandrel. The ratios of the diameter of the two helices may be in the order of two or three to one, or thereabouts, the diameter of the active helix being determined by the desired properties of it as a wave propagation element, and the diameter of the second helix being determined by the need to make each half turn of a helix effectively one-quarter wave in length.

.The two mandrels are then aligned so that their axes are parallel and are placed in a jig so that the first turn of the first helix touches the first turn of the second helix and so that each successive turn of the first helix touches a succeeding turn of the second helix. The structure is held in this manner in the jig and is placed in a plating solution so that a copper or gold or other plating may be applied to both helices. After plating and appropriate cleaning, the structure, while still being held in position by the jig, is placed in a brazing finish, and the contacts between the turns of the helices are brazed. The mandrels, after the structure has cooled, are moved directly, or if they cannot be removed from the helices easily, are slightly etched chemically and are then removed from the helices. If properly oxidized or otherwise coated, the mandrels slip out of the coils easily.

After brazing the support helix to the active helix,

' portions of the outside of the supporting helix may be coated with any well known carbon or metallic material which is lossy in the frequency range being used. If desired, simple resistance wire may be used in forming the support coil but it is considered that the thermo loss problem limits the use of such a structure to a traveling-wave tube of a particular design.

By the use of any of the embodiments of the device of the present invention stop bands may be provided or avoided as desired and heat may be conducted from traveling-wave tube slow-wave structures without reducing their forward wave impedances or their impedances to Waves within their respective operating frequency bands.-

What is claimed is: v

1. In a wave-type amplifier having a conductive slowwave structure-in the form of a helical coil for propagating electromagnetic waves, a helical supporting coil, means providing axially periodic discontinuities along the slow-Wave structure comprising consecutive half turns of said supporting coil extending radially from and being connected to the slow-wave structure at predetermined intervals therealong, and conductive means connecting the extremities of said half turns remote from said slowwave structure conductors.

2. The invention as defined in claim 2, wherein said conductive means are spaced from the slow-wave structure, where n is any positive odd integer, and A is the free space wavelength of the mid-frequency of the operating band of said amplifier.

3. The invention as defined in claim 2, wherein said predetermined intervals are equal to where m is any positive integer, and a is the unloaded waveguide wavelength of the mid-frequency of the backward wave self-oscillation band of the slow-wave struc ture.

4. A wave-type amplifier comprising an evacuated envelope, an electron gun disposed at one end of said envelope for producing an electron stream, a collector electrode disposed 'atlthe opposite'end of said envelope to in tercept the stream -e1e'ctrons,'means for directing said stream along a predetermined path from-said electron gun to said collector electrode, a slow-Wave structure disposed contiguously about said path forpropagating electromagnetic waves, 'and a helical supporting coil having a plurality of consecutive arcuate conductors having a thickness small in comparison to the circumference of said slow-Wave structure, and conductive means connecting the outer extremities of said conductors to conduct heat away from said slow-wave structure.

5. A wave-type amplifier comprising a conductive slow-wave structure for propagating electromagnetic Waves within a predetermined frequency band having a mid-frequency corresponding to a free space wavelength of k said slow-wave structure including a coil disposed concentrically about said path,a helical supporting coil having a plurality of consecutive conductive arcuate fins having an arcuate thickness small in comparison to the circumference of said slow-wave structure, each of said conductive fins extending arcuately away from a convolution on said coil a distance substantially equal to I where n is a positive odd integer, and a conductive cylinder disposed about said conductive fins, the outer extremities of said conductive fins being in physical and electrical contact with said conductive cylinder, whereby heat may be'conducted away from said slow-wave structure without materially reducing the impedance of said slow-wave structure to waves of frequencies within said predetermined band.

6. A wave-type amplifier comprising an evacuated envelope; an electron gun disposed at one end of said envelope for producing an electron stream; a collector electrode disposed at the opposite "end of said envelope to intercept the stream electrons; means for directing said stream along a predetermined path from said electron gun to said collector electrode; a slow-wave structure disposed contiguously about said path for propagating waves Within a predetermined frequency band having a mid-frequency corresponding to a free space Wavelength of A said slow-wave structure comprising a coil, concentrically about said path, a supporting coil of the same pitch as the first mentioned coil having convolutions thereof on contact with corresponding convolutions on the first mentioned coil, each of said convolutions of said supporting coil comprising a pair of arcuate conductive fins, said conductive fins extending arcuately from said first coil a distance substantially equal to where n is any positive odd integer; and conductive means connecting the outer extremities of said conductive fins, whereby heat may be conducted away from said slowwave structure without materially reducing the impedance of said slow-wave structure to waves of frequencies within said predetermined band.

7. In a wave-type amplifier having a conductive helix for propagating electromagnetic waves, means providing axially periodic discontinuities along the conductive helix comprising a supporting coil having the same pitchas said helix-withadjacent convolutions of said helix and coil being connected.

8. In a wave-type amplifier having a conductive helix for propagating electromagnetic waves, means comprising a supporting coil having convolutions thereof, connected to'the helix at predetermined intervals therealong, and conductive means interconnecting the convolutions of said supporting coil at regions spaced substantially from the conductive helix-where n is any positive odd integer and A is the freespace wavelength ofthe midfrequency of the operating band of the amplifier, said predetermined intervalsbeing equal to helix for propagating electromagnetic waves; a heat dissipating envelope surrounding'said active helix and extending longitudinally thereof; and at least one supporting helix having convolutions joined to said active helix and to said envelope, said supporting helix being formed of heat conductive material to transfer heat through said convolutions from said active helix to said envelope.

12. A Wave-type-amplifier as set forth in claim 11 wherein: a plurality of equally spaced supporting helices are disposed between'said active helix and said envelope.

13. A wave-type amplifier as set forth in claim 11 wherein: a rod having a surface of lossy material extends through said supporting coil to effect localized attenuation.

14. A wave-type amplifier as set forth in claim 11 wherein: said supporting coil is coated with a lossy material.

15. A wave-type amplifier as set forth in claim 11 wherein: the ends of said active helix define radially directed probes extending through and insulated from said envelope; and a Wave guide receives each of said probes.

16. A wave-type amplifier as set forth in claim 11 wherein: Wave-guides receive the axial ends of said active helix; and an electron gun including a focusing electrode and accelerating anode is disposed coaxially of said active helix at'one end thereof.

References (Iitetl in' the file of this patent UNITED STATES PATENTS 2,757,310 Robinson et al July 31, 1956 2,768,322 Fletcher Oct. 23, 1956

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