US2806171A - Helix support for traveling-wave tube - Google Patents

Description

Sept. 10, 1957 A. H. IVERSEN HELIX SUPPORT FOR TRAVELING-WAVE TUBE 2 Sheets-Sheet 1 Filed June 7, 1954 Na \w NV Sept. 10, 1957 A. H. IVERSEN HELIX SUPPORT FOR TRAVELING-WAVE TUBE 2 Sheets-Sheet 2 Filed June 7, 1954 dean me 072/ IV. flame/4 Patented Sept. 10, 1957 nice assay/'1 HELIX SUPPORT FOR TRAVELING-WAVE TUBE Arthur H. Iversen, Santa Monica, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a car poration of Delaware Application June 7, 1954, Serial No. 434,853

2 Claims. (31. sis-3.5

This invention relates to helix supporting devices adaptable for use in traveling-wave type amplifier tubes, and more particularly to an improved method and means of supporting a helical conductor with a synthetic crystalline rod.

By way of further explanation, a traveling-wave tube amplifier employs a wave transmission path constituted of a conductive helix to propagate an electromagnetic wave at a velocity susbtantially less than the velocity of light, the wave propagation velocity being purposely reduced to a practical electron velocity whereby an electron stream may be directed through the electric field of the wave at a velocity substantially equal to that of the wave. High frequency energy is, in such a case, transferred from the stream to the wave to cause the wave to grow or to be amplified as it progresses along the helix.

It is desirable that the interaction of the electron stream and the traveling-wave be as complete as possible to attain maximum gain. It is, therefore, necessary to accurately align the helix and the electron gun from which the stream is projected in order to maintain collimation of the paths of the stream electrons with the helix so that a maximum number of them will travel. through the helix without being intercepted, thus enabling the stream electrons to interact with the field of the wave over the complete efiective length of the helix.

It is at present the practice to mount a conductive helix in a traveling-wave tube envelope with three or four ceramic rods which are attached externally to the helix turns. The rods are rigidly attached lengthwise of the helix to produce a support which is actuallyintegrated with the slow-wave structure. These rods, which are normally constituted of a material called zircon, are glazed or attached to the helix with a thin glass coating. In the construction ofv this type of helix support it is necessary to thoroughly fuse the glass over the rods before they are attached to the helix. This requirement necessitates a controlled atmosphere heating of the rods, as distinguished from flame heating, because zircon has poor thermal shock properties. Stresses set up within the zircon rods by internal transient temperature dilferentials consequently prohibit the use of a flame to produce the required glass fusion in air at room temperatures.

A disadvantage exists in the assembly process of a typical present day traveling-wave tube in that the alignment of its associated electron gun and helix is an unusually time-consuming operation. The radial movement of the rods and helix are generally constrained only by the enclosing envelope which is usually made as a shrink fit. If the envelope shrinks too much,'the helix inside diameter is varied and the helix must be ground out. When the envelope inside-diameter is too large, a permanent or fixed alignment is not produced between the electron gun and the helix because the envelope, which is normally constituted of glass, does not bearagainst the zircon rods. .This procedure is necessary because of the unusually low compressive and tensile strength of the zircon rods.

Thus in the assembly of a traveling-wave tube, both the rods and the helix must be precision ground so as to avoid breakage due to misalignment. It is a further disadvantage of typical present day traveling-Wave tubes that the low relative strength of zircon limits the type of application to which such a tube may be put.

In practicing the disclosed invention wherein crystalline rods of synthetic saphire are employed, the construction process of mounting a conductive helix is substantially accelerated by the use of an open flame to eifect the heating of the rods in the prefusing step. This is made possible by the unusually good thermal shock properties of synthetic sapphire.

A solution to the electron gun and helix alignment problems is also obtained by practicing the present invention in regard to the new method of traveling-wave tube assembly disclosed herewith wherein assembly time is reduced without any sacrifice in alignment accuracy. The method of assembly involves a step of rigidly mounting the sapphire rods and attached helix by making indentations in the surrounding glass envelope near the rods whereby the rods are subjected to compressive stresses and bending moments. This procedure is not possible with zircon rods because of their relatively low physical strength.

Another advantage attendant upon the use of the new assembly method is the facility with which the electron gun may also be rigidly mounted within the tube envelope. An alignment assembly rod, ground to a very close tolerance, may be projected through the mounted helix. The anode of the assembled electron gun, which may have an inside diameter equal to that of the helix, may then be disposed on the end of the rod and the gun may be quickly and easily sealed to the tube envelope by centering the envelope in a chuck of a glass sealing lathe and sealing the gun directly to the envelope.

The method of assembly peculiar to the present invention also obviously eliminates the necessity of accurately ground mounting rods and obviates the necessity of a precision shrunk enclosing envelope. It is also not necessary that the outside diameter of the helix be precision ground when the present invention is employed.

It is an advantage of the present invention that the scope of traveling-wave type applications may be broadened by the employment of sapphire rods. Synthetic sapphire is considerably stronger than ceramics such as, for example, zircon, that are usually employed for this purpose. It is thus apparent that sapphire rods may be used to replace zircon rods in instances where the strength of zircon has formely been a limiting factor, such as, for example, in aircraft installations where vibration is always a susbtantial consideration.

An advantage concomitant with the use of sapphire rods for a helix support is their low cost. For example, their cost is several times less than zircon. In some applications such as, for example, in guided missile telemetering applications, transmitting and receiving apparatus are considered expendable and are often destroyed with a missile at the termination of one flight. It is evident that the efliciency of such experimental development may be increased by the use of inexpensive expendable equipment.

It is, therefore, an object of the invention to provide ceramic supporting rods having a high thermal shock resistance for a traveling-wave tube helix adaptable to a construction process for providing a helix support wherein a glass may be fused about the rods with an open flame in air at room temperatures.

Another object of the invention is to provide an economical helix support of unusually high strength for mounting the conductive helices of traveling-wave type amplifier tubes.

, A further object of the invention is to provide an improved method of'aligning helix with the electron gun structure in a traveling-wave tube to facilitate efficient operation of the tube.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in whichan embodiment of the invention is 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 arenot intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of a traveling-wave tube amplifier wherein an embodiment of the present invention is employed;

Fig. 2 is a section of 22' of Fig. l; and

Figs. 3, 4, 5 and 6 illustrate the steps in the method of construction of the disclosed helix support.

Fig. 7 illustrates the method of assembly pertaining to the invention.

Referring now to the drawings, there is shown in Fig. 1 an embodiment of a microwave amplifier which comprises a traveling-wave tube 10 including input and output matching cavities 12 and 14 with input and output coaxial cables 16 and 18, respectively connected to the separate cavities. Traveling-wave tube 10 is provided with an evacuated chamber formed by an elongated cylindrical envelope 20 which has an enlarged portion at the left extremity, as viewed in the drawing. This enlarged portion houses an electron gun 22 for propagating a stream flow of electrons along a predetermined path that lies along the longitudinal axis of elongated envelope 20.

In order to maintain the electron stream along the predetermined path, tube 10 is provided with a solenoid 24 symmetrically disposed about the complete length of envelope 20. Solenoid 24 is connected to an appropriate direct-current source, such as a battery 26 which produces in solenoid 24 an axial magnetic field sufiicient to keep the electron stream focused or constrained along the active length of the tube.

Electron gun 22 comprises a cathode 28 with a heating element 30, a focusing electrode 32, and an accelerating anode 34. Heater 30 is connected across a source of potential, such as battery 36, the negative terminal of which is connected to cathode 28 by a connection to an appropriate terminal of heater 30. Focusing electrode 32 is of the Pierce type and, accordingly, is maintained near the potential of cathode 28 by a connection thereto. Anode 34 is maintained at a potential of the order of from 200 to 600 volts positive with respect to ground and of the order of 1500 volts positive with respect to the potential of cathode 28. This is accomplished by means of a battery 38 interconnected between cathode 28 and anode 34 with a suitable intermediate tap connected to ground.

Proceeding along and concentrically about the electron streampath in the direction of electron flow there are a matching ferrule 42 connected over a lead 44 to one end of a conductive helix 46, whach has its other end connected over a lead 48 to a matching ferrule 50. Helix 46 and ferrules 42 and 50 are all maintained at ground potential by a suitable connection from ferrule 50 to ground, as shown. After passing through matching ferrule 50 the stream electrons are intercepted and collected by a collector electrode 52 which; is maintained at a potential of the order-of 200 volts positive with respect to that of matching ferrule 50 in order to prevent secondary electrons from being emitted from collector electrode 52. This potential is applied by means of a connection from collector electrode 52 to the positive terminal of a battery 54, the negative terminal of which is referenced to ground.

Helix 46 is maintained in position with respect to the predetermined path of the electron stream by a plurality of round rods 70, as shown in Fig. 2, to which it is attached in a manner hereinafter described. Movement of rods 70, only one of which is shown in Fig. l, is axially, radially, and azimuthally constrained by indentations, represented by a plurality of dark circles 72 in envelope 20.

As shown in Fig. 2, helix 46 normally defines a crosssectional area which is substantially smaller than that defined by the outside diameter of its turns. It is also general practice to space successive turns of helix 46 as close together as possible in order to eliminate any electron lcakage to the tube envelope. This leakage results in the charging of the inner surface of the tube envelope in a negative direction which has an extremely deleterious efiect upon the production of gain. The outside diameter of the turns of helix 46 or the diameter of rods are not limited to any optimum magnitude or ratio, it is generally preferred that the former be substantially larger than the latter. Representative values of the outside diameters of a helix and a rod are 0.38 inch and 0.06 inch, respectively.

Materials such as tungsten and moybdenum which are suitable for making helix 46 are employed universally because of the requirement that the helix retain its shape during severe heating, under a high vacuum, and under physical stress. It is particularly necessary that the helix retain its form especially with respect to its pitch and inside diameter.

According to the present invention, rods 70 are employed to retain helix 46 in longitudinal and coaxial position within envelope 20. The material presently used for such supporting rods is known as zircon. This material, actually a ceramic containing zirconium oxide, is known chemically as ZrO2.SiO2. Zircon, evaluated according to physical strength is relatively weak, e. g. it has ultimate strength in tension, compression and shear which are much too low for some aircraft applications where physical shock and vibration is a controlling design factor. However, employment of the material of the present invention simplifies assembly of the tube and substantially increases thetube resistance to physical shock thus broadening the scope and number of applications of traveling-wave tubes.

In accordance with the present invention, rods 70 are constituted of synthetic sapphire, chemically known as alpha'alumina, OC-AlzOs, and metallurgically known as corundum. Rods 70 can be crystal grown to size and centerless ground to very close tolerances if desired. A method sometimes called. the Verneuil process for producing synthetic sapphire was first developed in France. In this process, a synthetic sapphire crystal having a shape resembling that of a ball is made to grow by means of a flame. This growing process is therefore called the flame process or the boule process, boule being the French'for'ball. Synthetic sapphire is commercially available in practically any reasonable desired shape or form. Its ultimate strength under any type of stress is normally three to four times that of zircon. Sapphire has a hardness next to diamond and a coefiiecient of thermal expansion which practically coincides with 'molybdenum, the latter physical property being a construction advantage which will be better understood in connection with the process of tube construction hereinafter described. Unlike .zircon, synthetic sapphire can withstand a high degree oftherrnal shock. .This .isan advantage which will also be recognized in the glazing process used in connection with the construction .of the tube. Good thermal shock properties are usually defined as the capacity to withstand internal stresses which result from nonuniform heating-or cooling. All the remaining physical properties of sapphire and particularly its electrical and and contiguous to the helix.

ated by means of a carbon jig 150 such as shown in chemical properties are as good as zircon andmost of them are normally better, especially for use as helix supporting rods in traveling-wave tubes. '1

It has previously been found to be desirable to glaze rods 70 to helix 46 and mount to the whole rigid combined structure in envelope 20 at once in order to speed up assembly. The process of glazing comprises simply suspending a powdered glass in a liquid such as Water; applying the suspension lightly to the rods; heating the rods until a layer of, glass or glaze is formed on the rods; afiixing the rods symmetrically about the helix; heating the whole structure to the vicinity of the working temperature of the glass, i. e. the fusing temperature of the glass; and allowing the structure to cool. Rods 70 may thereby be securely affixed to helix 46 without misaligning the latter, the rods 70 having a fairly low modulus of elasticity. Any hard glass may be used to accomplish this result. Such glasses should have a coeflicient of thermal expansion near that of the helix metal and that of the sapphire. Since the latter coefficients are nearly the same, no substantial problem exists as to matching a glass with a suitable coefiicient. The designation hard glass often includes a borosilicate glass, i. e. a glass having more than five percent boric oxide. Several typical compositions of such glasses are: (1) 1.67% SiOz, 22% B203, 6.5% NazO and K20; and 2% A1203; (2) 72% Si02, B203, 10% N320 and K20, and 5% A1203; and (3) 73% SiOz, 16.5% B20, 4.5% NazO and K20, and 6% PhD.

The Coming Glass Company designates a borosilicate glass by the expression 7052, which is kept as closely as possible at all times to a standard chemical composition. If a batch of such a glass departs only a few percent from a thermal expansion coefficient of 46 1'0-' centimeters per centimeter per'degree centigrade, the batch is discarded and not sold under the designation 7052. This standard glass has been found to be particularly useful in glazing sapphire rods to a molybdenum helix. Although the ranges of the thermal expansion coeflicients of the sapphire and the molybdenum are respectively 59 to 67 10' and 53 to 57 10-' over a range of about 300 C. even harder and definitely softer glasses may be used. Glasses suitable for glazing sapphire rods to a tungsten helix are Coming 3320 or 7720.

Representative values and the steps employed in a specific process of glazing sapphire rods to a molybdenum helix follow:

1. Coat each rod with one stroke of a brush filled with a liquid suspension of Corning 7052 powdered glass, as shown in Fig. 3 wherein the powdered glass application is suggested by a brush 130 filled with a liquid suspension of powdered glass and rods 70 shown below the brush.

2. Heat each rod in air with a flame until the glass fuses and partially flows, i. e. to a temperature of about 800 C. This is illustrated in Fig. 4 wherein the powdered glass is being prefused to the rods 70 by means of a torch 140.

3. Allow the glass on the rods to cool. Repeat step 3 about two times to get adequately thick coating, e. g. .005 inch.

4. Aflix the rods longitudinally and symmetrically about This is preferably effectu- Fig. 5. This jig 150 comprises a base block 152 having a length equal to or greater than that of helix 46. Lengthwise across the top of base block 152 is a groove 154 having the cross sectional area of an equilateral triangle of such a size that it is slightly smaller than a circumscribing equilateral triangle about the helix 46. A top block 156 having a notch type hinge 157 and a weight 158 for compressing the helix is disposed on top of the base block 152 thus forming a triangularly shaped hole through the block. Two smaller grooves 160 and 162 are disposed along the midpoint of the triangular groove '154 in base block 152 and a third smaller groove 164 is disposed over the center of groove 154 in the block 156. In its operation, the rods 70 are automatically held in the desired position relative to helix 46 when inserted in the smaller grooves 160, 162 and 164 about the helix 46 as shown in the figure. Carbon is employed for the blocks 152 and 156 as it will not adhere to the glass after it has been heated to its softening point.

5. Heat the rods 70 aflixed to the helix 46 by the carbon jig in an oven to from 950 to 1050" C. in an atmosphere of forming gas which may be constituted of percent nitrogen and 15 percent hydrogen.

6. Allow the entire structure to cool.

The finished product of the glazing operation is shown in Fig. 6 wherein the rods 70 are shown glazed to the helix 46. 7

It is pertinent to note that step 2 specifies heating with a flame. This is not necessary, but oven heating, which wouldbe the usual substitute, is considerably more cumbersome and time consuming. The thermal shock resistance of sapphire makes it particularly adaptable to heating with an open flame whereas the thermal shock resistance of zircon is normally too low to withstand the internal stresses set up by the application of an open flame such as that specified above.

In the past it has been necessary to exercise extreme care to form the slender portion of envelope 20 so that its interior is of uniform diameter, accurately dimensioned, and very straight axially. It is also normally desirable to accurately dimension the outside diameters of helix 46 and rods 70. By practicing the method of assembly of the present invention, it, nevertheless, becomes unnecessary to require such precision. The strength of the helix support may also be increased and, at the same time, assembly time may be shortened.

In the efficient assembly of the internal elements of envelope 20 ferrules 42 and 50 and leads 44 and 48, as shown in Fig. 1, may be easily joined to helix 46 either before or after helix 46 is mounted in envelope 20. The highresistance to thermal shock of the sapphire rods enable the indentations 72 in envelope 20 to be made with facility by simply flame heating the envelope and deforming the proper portion with a pointed tool. The high strength of the sapphire rods enables the movement of the rods 70 within envelope 20 to be constrained by indentations 72. This function aids in actual tube assembly and also obviates the precision normally required in making the helix 46, the rods 70, and the envelope 20 in that the tolerance between rods '70 and envelope 20 is no longer critical. The envelope dimples on the sides of rods 70 are made very close to the rods so as to sustain a radial force thereon and thereby to eliminate any azimuthal motion and to partially eliminate axial motion. The dimples at the ends of the rods 70 are in fact made so close to the rods that the rods 70 are actually bent radially inward and subjected to a permanent bending moment. Any axial movement of rod 70 is thereby efficiently prevented. The bending of rods 70 does not, however, effect any appreciable bending of helix 46 because of the relatively high modulus of elasticity of molybdenum or tungsten in comparison to that of synthetic sapphire.

Electron gun 22 will normally be completely assembled in a sub-assembly process and will be ready to be sealed to envelope 20. In this case, the minimum inside diameter of anode 34 may be precision made with the same diameter as that of helix 46. A metal rod made to fit within helix 46 and anode 34 to very close tolerances,

'say 0.001 inch may be employed to align the electron alignment rod with anode 34, anode 34 being a single element of the completely assembled electron gun 22',

sealgun 22 to envelope 20 with the lathe.

The method of assembly of tube illustrated in Fig.

-7 wherein a more practical anode 74 having a tubular appendage 76 is employed. In accordance with the present invention, a metal alignment rod 78 is thus shown projected through helix 46 with anode 7 4 cappe on the end. Anode 74 is intended to be attached to the remaining elements of the electron gun; however, a plurality of lead in wires 80 and a supporting glass base 82 are the only gun elements visible because of the type of orthogonal projection shown. Helix 46 and rods 70 are positioned within envelope and rigidly held while envelope 20 is heated with torch 140. Tool 170 is then employed to make indentations 72. Rod 78 is pro jected through helix 46 and anode 74, being an integrated part of the assembled electron gun 22, is mounted onto the end of rod 78. Envelope 20 is mounted in a chuck of a glass sealing lathe and the end 174 of envelope 20 is heated. The end 174 then deforms and surrounds base 82 whereby a seal is made. Envelope 20 then is allowed to cool while rod 78 is maintained in position. A precision alignment is thereby perfected and helix'46 is prevented from intercepting a substantial number of stream electrons when the tube is operated although the stream is directed contiguous to helix by virtue of the alignment produced.

The alignment of collector 52 is not at all critical. The assembly of this tube element then becomes only a matter of centering envelope 20 and collector 52 in a glass sealing lathe with the unaided eye and sealing the envelope to the collector.

Thus it is seen that the use of sapphire rods 70 and the construction and assembly processes herein disclosed do not detract from the efiicient operation of the traveling-wave tube in that the projection of the electron stream along the; length of helix 46 is facilitated without 'agreat lossfof electrons thereto. Furthermore, itis apparent from the foregoing description that the scope of;the applications of traveling-wave tubes embodying sapphire rods of the present invention is extendedto environments requiring tubes havinga high resistance to physical shock.

' What is claimed is:

1. A wave-type amplifying device comprising a molybdenum helix for propagating an electromagnetic signal wave, a plurality of synthetic sapphire rods disposed uniformly about and extending lengthwise of said helix, a glass bond for afiixing said rods to each turn of said helix to hold each of said turns in a predetermined relation with respect to each other, an evacuated envelope including an elongated cylindrical chamber disposed concentrically about said rods, means for producing an electron stream, means for directing said electron stream along a predetermined path at a velocity to produce interaction between said wave and the electrons of said stream, and a plurality of indentations disposed in said envelope contiguous to said rods to constrain axial, radial and azimuthal movement of said rods and thereby to retain said helix in a position that is accurately concentric with and parallel with said path.

2. The wave-type amplifying device as defined in claim 1 wherein said plurality of synthetic sapphire rods are primarily constituted of crystalline alumina.

References Cited in the file of this patent UNITED STATES PATENTS 2,602,148 Pierce July 1, 1952 2,653,270 Kompfner Sept. 22, 1953 FOREIGN PATENTS 984,595 France Feb. 28, 1951

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