GB2050047A - Travelling-wave tube with variable-geometry delay-line supports - Google Patents

Abstract

The delay line of a travelling- wave tube is held in position by means of insulating support rods (7-9) having a geometrical shape which varies along the axis of the tube in order to eliminate certain parasitic oscillations. <IMAGE>

Description

SPECIFICATION Travelling-wave tube with variable-geometry delay-line supports This invention relates to a traveling-wave tube in which the delay line is held in position by means of supports having a geometrical shape which varies along the axis of the tube.

As is now well known, a traveling-wave tube is constituted by the association of a long and narrow electron beam with a delay line having a nonresonant periodic structure.

The electrons of the beam transfer energy to the microwave as this latter travels along the line when certain conditions of synchronism of the wave with the beam are satisfied. In practice, the delay line is usually constituted by a helix or a circuit derived from a helix; the electrons propagate along the axis of the helix which is also the axis of the tube. Among the circuits derived from a helix can be mentioned the helix having multiple conductors, comprising two interlaced strands, a counter-helix or its topological equivalents, or else the ring and bar circuit. However, for the sake of enhanced simplicity, the delay line will be assimilated with a simple helix throughout the following description.

In all traveling-wave tubes, in particular at high power levels, parasitic oscillations appear at frequencies at which the phase shift of the microwave is close to 77 between two consecutive turns of the helix. In fact, in this mode of operation known as the 77 mode, there exists a cutoff frequency at which the microwave energy is not propagated along the delay line.

This results in a strong interaction of the wave with the electron beam, thus causing instabilities and oscillations. In order to prevent these oscillations, it is a known practice to vary the length 1of the turns of the helix along the axis of the tube and to produce a correlative variation in the pitch p of the helix in order to comply with the conditions of synchronism mentioned above. In particular, delay lines of conical or quasi-conical shape have been constructed as described, for example, in French patent Application No 76 28394 (publication No 2 365 218) and in the patent of Addition No 77 28741 (publication no 2 422 265) in the name of THOMSON-CSF.

However, these different solutions are attended by drawbacks of a technological order such as the difficulty involved in forming conical surfaces having perfectly identical slopes, namely on the one hand the surface which carries the helix and on the other hand either the surfaces of the longitudinal insulating bars which maintain the helix in position within the tube or the internal surface of said tube.

The present invention is directed to a structure which makes it possible to overcome this disadvantage by varying not the geometrical length of the turns of the delay line along the axis of the tube, but the electrical length of said turns by producing a variation in the dielectric charge of the line, this being achieved by a variation in geometry of the insulating bars which maintain said line in position within the tube.

Other aims, features and results of the invention will become apparent from the following description which is given by way of example and not in any limiting sense, reference being made to the accompanying drawings in which: Figure 1 is a diagram of a traveling-wave tube comprising a helical delay line; Figure 2 illustrates a first embodiment of the structure according to the invention; Figures 3a to 3c are transverse sectional views showing three variants of the geometrical shape of the insulating supports employed in the structure according to the invention; Figures 4a, 4b and 5 illustrate two alternative embodiments of the structure according to the invention; Figures 6a and 6b illustrate another alternative embodiment of the structure according to the invention, provision being made in this embodiment for two additional support bars.

In these different figures, the same elements are designated by the same references.

In the diagram of Fig. 1, there are shown: an electron gun designated by the general reference G and constituted by a cathode K which emits an electron beam 3 in a direction Z-Z, a control electrode W of the Wehnelt type and an anode A; a delay line 4 which may, for example, by of the helix type of generally cylindrical shape having an axis Z-Z and adapted to surround the electron beam 3; a device 5 for focusing the electron beam 3 during its travel within the line 4, and finally a collector C for the electrons of the beam. The device further comprises an input E and an output S for the microwave energy which is propagated along the line 4. These different elements are contained within a sealed enclosure or sleeve (not shown in the diagram) having a generally cylindrical shape and an axis Z-Z.

The principle of operation of a device of this type will be briefly recalled: the velocity of the electrons of the beam 3 is modulated periodically by the field associated with the wave which propagates along the delay line 4.

Under the influence of this velocity modulation, the electrons are grouped together in clusters and an energy transfer takes place from the clusters of electrons to the wave which propagates along the line provided that a certain condition of synchronism is satisfied between the velocity of the electrons and one of the phase velocities of the wave which travels along the line. As is already known, this condition of synchronism is defined by calculation and differs according to whether the objective to be achieved is either maximum gain or maximum efficiency. In the case of a cylidrical helix, it is a known practice to produce this synchronism by varying the pitch of the helix.

As explained in the foregoing, in order to prevent the occurrence of parasitic oscillations within the tube which operates in the 7r mode, a known expedient consists in varying the geometrical length of one turn of the helix 4.

The aim of the present invention is to overcome the technological problems posed by the abovementioned solution by carrying out a variation, not in geometrical length, but in electrical length of the turns along the axis of the tube, this variation being produced by varying the dielectric charge of the line. In fact, calculations and experiments performed by the present Applicants have shown that, contrary to a principle which had won general acceptance in the prior art, the coupling impedance between the microwave frequency field and the electrons of the beam does not need to be of maximum value along the entire length of the tube and that, in particular, the tube efficiency can be appreciably enhanced in the presence of a coupling impedance of decreasing value towards the end of the line, that is, towards the output S.In the structure according to the invention, the decrease in coupling impedance is achieved by increasing the dielectric charge of the line, thus securing a number of advantages: an increase in the electrical length of the turns along the axis of the tube, thereby eliminating certain parasitic oscillations; an improvement in heat removal: it is in fact known that the thermal power to be dissipated rises sharply at the end of the line; an increase in dielectric mass at the end of the line therefore permits better dissipation.

Fig. 2 illustrates a first embodiment of the structure according to the invention.

In this figure, there is shown a cylindrical envelope 6 constituting the sleeve of the tube having an axis Z-Z, a helix 4 having a generally cylindrical shape and the same axis Z-Z being placed within said sleeve and maintained in position by means of three supports in the form of insulating bars. By way of example, provision can be made for three insulating bars as designated in the figure by the reference numerals 7, 8 and 9. These supporting and insulating bars can be made of beryllium oxide, for example, and brazed to the helix 4 and to the sleeve 6.

This structure is illustrated in transverse cross-section in Fig. 3a, this view being taken along an axis A-A at right angles to X-X. In this figure, there is again shown the sleeve 6 of circular cross-section containing the helix 4 and the three supports 7, 8 and 9.

Said supports have four faces: the face which is in tangential contact with the helix 4, for example the face 74 in the case of the supporting bar 7, is flat; the opposite face (namely the face 76 in the case of the bar 7) is a cylinder having substantially the same radius as the sleeve 6 so as to be in contact with this latter at all points; the lateral faces of the supporting bars (namely the faces 70 and 71 in the case of the bar 7) are not normal to the face 74 in this embodiment but are inclined with respect to this latter at an angle which is greater than 90 and preferably greater by 15 to 35 .

It is further apparent from the figures that the faces 71 and 70 are flat but are not parallel to each other. Thus said faces are placed in divergent relation in the direction of propagation of electrons within the tube, thereby increasing the dielectric zone in the vicinity of the helix along this axis. It is in fact known that the electrical length of a turn is equal to its geometrical length as corrected by the dielectric charge applied thereto. More specifically, the geometrical length is to be corrected by a coefficient which is substantially equal to the square root of the dielectric constant of the material constituting the insulating supports (7, 8, 9) in respect of the entire portion of the turn which is located in the vicinity of the face 74.In consequence, the divergence of the faces 70 and 71 in the direction of propagation of the electrons results in widening of the face 74, thereby producing an increase in electrical length of the turns along the entire axis Z-Z as stated earlier.

The increase in dielectric charge of the line along the axis is illustrated in Fig. 3a in the case of a particular cross-sectional shape of the insulating bars 7, 8 and 9. Similarly, the transverse sectional views of Figs. 3b and 3c show the same variation in dielectric charge but in the case of different cross-sectional shapes of the bars.

Fig. 3b differs from Fig. 3a only in the shape of that face of each bar (7b, 8b and 9b) which is in contact with the helix 4. Said face which is now designated by the reference 74b in the case of the bar 7b, by the reference 84b in the case of the bar 8b and by the reference 94b in the case of the bar 9b is no longer flat but cylindrical and has the same radius as the external radius of the helix 4 in order to improve the contact between bar and helix. A structure of this type is preferred at low operating frequencies since it is known that the dielectric bars employed are of relatively large size in such a case and brazing would be insufficient to ensure a contact between the helix and bars having flat sur faces such as the surface 74.

Fig. 3c illustrates the case in which the insulating and supporting bars designated in this figure by the references 7c, 8c, 9c have a rectangular cross-section. This means on the one hand that the faces 74c and 76c corre sponding to the faces 74 and 76 of Fig. 3a are now flat and on the other hand that the lateral faces 70c and 71 c corresponding respectively to the faces 70 and 71 of Fig. 3a are now normal to the aforesaid faces 74c and 76c. This type of configuration is preferred at the higher operating frequencies at which the bars employed are of relatively small size and therefore at which brazing is sufficient to ensure a contact between helix, bars and sleeve 6. The advantage of this structure clearly lies in the ease of technological execution which is thus made possible.

Figs. 4 illustrate another embodiment of the structure according to the invention in which the variation in electrical length of the turns of the helix is not uniform along the axis of the tube.

Fig. 4a is a transverse sectional view of the traveling-wave tube in which the concentric sleeve 6 and helix 4 are again shown. By way of example, said helix 4 is held in position by means of three insulating supports designated in this case by the references 7d, 8d and 9d.

The cross-sectional shape of these supports may be identical with the supports of Fig. 3a but are distinguished from these latter by the fact that their lateral faces (70 and 71 in the case of the bar 7 in Fig. 3a) are each formed in this case by two flat surfaces.

Fig. 4b illustrates one of these insulating bars considered alone such as the bar 7d, for example. Each lateral face is constituted by two flat facets, namely the facets 72 and 73 on one side, the facets 77 and 78 on the other side. The opposite faces 72 and 77 can be parallel to each other or slightly divergent as shown in the figure; the facets 73 and 78 which follow said faces are inclined to each other at an angle which is greater than the preceding.

This embodiment has the advantage of substantially increasing the dielectric charge only at the end of the line, thus achieving better optimization of the bar dimensions than in the previous embodiment. It is accordingly possible to choose a small cross-section at the beginning of the line, thus increasing the coupling impedance in this zone and consequently increasing the gain per unit length, and a cross-section of very considerably higher value at the end of the line, which makes it possible in particular to ensure more efficient heat removal. However, this embodiment is slightly more complex than in Fig. 3a from a technological standpoint.

Fig. 5 illustrates a variant of the preceding embodiment and is similar to Fig. 4b but shows an insulating and supporting bar 7e, the lateral faces of which are each constituted by three flat facets designated successively by the references 10, 11 and 1 2 on one side and by the references 13, 1 4 and 1 5 on the other side, thus defining three regions of variable width in the bar.

The advantage of this embodiment lies in the fact that the dielectric charge of the line can be varied with a higher degree of fineness, thus making it possible to improve heat dissipation and efficiency but the technological execution of this form of construction is of course more difficult.

Figs. 6a and 6b illustrate another alternative embodiment of the structure according to the invention in which provision is made for additional supporting bars.

Fig. 6a is a transverse sectional view of the tube in which there are again shown the sleeve 6 and the helix 4 which have a common axis Z-Z, and six supporting bars: three bars 7f, 8f and 9f extend as in the previous instance over the entire length of the tube, the shape of which is similar, for example, to the shape described in Fig. 3a but the lateral faces of which (namely the faces 70f and 71f in the case of the bar 7f) are parallel to each other in this case; and three additional bars 7g, 8g and 9g which are smaller in length and extend for example over the last third of the delay line, the design function of said additional bars being to increase the dielectric charge.

Fig. 6b illustrates one of the additional bars such as the bar 7g, for example. The face 74g of said bar which corresponds to the face 74 of Fig. 3a is no longer in contact with the helix 4 over its entire length: in fact, the bar 7g has a tapered portion 75 at one end so as to ensure that the variation of impedance is not too abrupt along the axis Z-Z.

Moreover, the lateral faces of each additional bar (709 and 719) as shown in the figures are parallel to each other but can also be inclined to each other at a nonzero angle as shown for example in Fig. 2 or Fig. 3 in order to carry out a progressive increase in the dielectric charge of the line. Furthermore, the bars 7f, 8f and 9f of Fig. 6a can adopt one of the configurations described earlier in order that the dielectric charge of the line may also be varied.

In all the alternative embodiments described in the foregoing, the pitch of the helix constituting the delay line varies along the axis of the tube in order to maintain the abovementioned conditions of synchronism between electron beam and microwaves.

The foregoing description has been given in the case of a delay line having a helical and cylindrical shape. However, consideration can clearly be given to other high-frequency delayline structures supported by dielectric bars such as the ring and bar line or the ring and loop line or else the pseudo-conical helix described in the patent Application and certificate of Addition cited earlier.

Claims (9)

1. A traveling-wave tube comprising within a vacuum enclosure an electron gun for producing an electron beam, and a periodical delay line with variable pitch, for propagating a microwave which interacts with the electrons of said beam, electrically insulating bars for maintaining said delay line within said vacuum enclosure, said bars being placed substantially parallel to the axis of said enclosure and having a cross-section at right angles to said axis which varies along said axis.

2. A traveling-wave tube according to claim 1, wherein the insulating-bar zone in the vicinity of said delay line increases in width along said axis of said enclosure in the direction of propagation of said microwave.

3. A traveling-wave tube according to claim 1, wherein the cross-section of said insulating bars is of rectangular shape.

4. A traveling-wave tube according to claim 1, wherein the cross-section of said insulating bars has a curvilinear trapezoidal shape, that face of each bar which is in contact with the enclosure being substantially of the same shape as said enclosure.

5. A traveling-wave tube according to claim 1, wherein the cross-section of said insulating bars has a curvilinear trapezoidal shape, that face of each bar which is in contact with the enclosure and with the delay line being respectively of the same shape as these latter.

6. A traveling-wave tube according to claim 3, wherein said electron beam, said vacum enclosure and said delay line have substantially the same axis, and wherein the lateral faces of the insulating bars which are not in contact either with said delay line or with said enclosure are inclined at an angle to said axis.

7. A traveling-wave tube according to claim 6, wherein said angle is constant.

8. A traveling-wave tube according to claim 6, wherein said angle is variable in a discrete manner.

9. A traveling-wave tube according to claim 1, wherein said tube comprises additional electrically insulating and supporting bars at the end of the line in the direction of propagation of the electrons.

1 0. A travelling-wave tube substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.