EP0022016B1 - Slow-wave structure with varying pitch for a travelling-wave tube, and travelling-wave tube using such a slow-wave structure - Google Patents

Description

The present invention relates to a delay line for traveling wave tube of cylindrical structure, more particularly a delay line of the type with ceiling (s) and rings.

We know in particular from French patent 1,162,425 a delay line of the above type consisting of a cylindrical metal guide for example and, in the straight sections of this guide, coaxial metal rings each fixed to the wall of the guide by at least one metal rod; it further comprises at least one ceiling constituted by a metallic wedge fixed to the inner wall of the guide and extending over the entire length of the latter any section of this ceiling by a plane perpendicular to the axis of the guide having the shape of a circular crown sector.

On the other hand, the delay lines of the above type, used in traveling wave tubes appear as formed by a succession of elementary sections or cells made up of the elements described above, all identical, repeating periodically along of the electron beam propagation axis. However, traveling wave tubes using this type of constant pitch line have an HF power available at the tube outlet, limited to about a third of that applied by the power supplies.

Accordingly, the object of the present invention is to increase the interaction efficiency of traveling wave tubes using lines of the ceiling (s) and ring type by providing a new line structure.

• - un tronçon de guide d'ondes métallique comportant des plafonds de même épaisseur que le tronçon, occupant, chacun, une partie de sa section, et s'étendant d'une fraction de son pourtour à une certaine distance de son centre ;
• - un anneau métallique coaxial fixé à la paroi du tronçon de guide par des tiges-supports disposées dans les espaces libres entre les plafonds ; l'ensemble des cellules constituant un guide d'ondes de section uniforme, dans lequel les plafonds d'une part, et les tiges supports d'autre part, sont alignés, s'étendant autour du faisceau d'électrons le long de son trajet dans le tube, caractérisée en ce que la ligne se subdivise en trois groupes de cellules à l'intérieur de chacun desquels le pas est constant, les pas des second et troisième groupes de cellules étant respectivement plus petits et plus grands que celui du premier groupe de cellules lorsqu'on se déplace le long de la ligne en partant du canon à électrons et la variation du pas étant obtenue par dilatation ou contraction d'au moins une cote des cellules. Selon un mode de réalisation préférentiel la variation du pas est réalisée en modifiant à l'intérieur de chaque cellule les cotes des éléments constitutifs de la cellule, à savoir l'épaisseur de l'ensemble anneaux et tiges supports.

The present invention therefore relates to a delay line for a traveling wave tube consisting of contiguous cells each composed of the following elements:

• - A metallic waveguide section having ceilings of the same thickness as the section, each occupying a part of its section, and extending a fraction of its periphery at a certain distance from its center;
• - a coaxial metal ring fixed to the wall of the guide section by support rods arranged in the free spaces between the ceilings; the set of cells constituting a waveguide of uniform section, in which the ceilings on the one hand, and the support rods on the other hand, are aligned, extending around the electron beam along its path in the tube, characterized in that the line is subdivided into three groups of cells inside each of which the pitch is constant, the steps of the second and third groups of cells being respectively smaller and larger than that of the first group of cells when moving along the line starting from the electron gun and the variation of the pitch being obtained by dilation or contraction of at least one dimension of the cells. According to a preferred embodiment, the variation of the pitch is achieved by modifying the dimensions of the constituent elements of the cell inside each cell, namely the thickness of the assembly of rings and support rods.

The present invention can also be applied to delay lines of the above type comprising a metallic conductor in contact with the ceilings on one of the faces of the section and forming a short circuit between them, as described in the first addition to the patent. number 82.236. This feature eliminates certain parasitic operating modes. In this case, the variation of the pitch can be achieved by modifying the distance between rings and short-circuits.

• Figure 1 Une vue d'ensemble en perspective de la cellule constitutive de la ligne à retard de l'invention ;
• Figure 2 Une vue schématique en coupe d'un tube à onde progressive incorporant une ligne à retard de l'invention.

The invention will be better understood by referring to the description which follows and to the attached figures which represent:

• Figure 1 An overall perspective view of the cell constituting the delay line of the invention;
• Figure 2 A schematic sectional view of a traveling wave tube incorporating a delay line of the invention.

FIG. 1 represents, in perspective, an elementary cell 10 of a delay line of the invention. Each of these cells consists, in accordance with the art of the cited patents, in a metal guide section with circular section 11, comprising two metal ceilings 1 extending symmetrically from the wall of the guide section up to a certain distance from the center from the right section of it. In a plane parallel to that of the ceilings, is disposed a circular metal ring 3 supported by two diametrically opposite rods 4 fixed to the wall of the guide section.

The two ceilings 1 are interconnected by a loop 2 made of an electrically conductive material. In the example, this loop, or short circuit, consists of a linear conductor applied to the two ceilings and entirely located in their plane; the figure shows, without markers, the four holes used in the example for cooling the line by circulation of fluid; the different guide sections similar to that described are mounted and clamped against each other, to form the delay line of the invention, the rods 4 all parallel to each other, as well as the ceilings 1; the assembly is made vacuum tight by brazing.

The result of an example of this assembly is shown in section in Figure 2, where we find, with the same references, cells having the same general structure as that of the previous figure. The figure comprises along the axis Oz of propagation of the beam, from left to right, three sections designated by I, II, and III. in which the step, L of the line, constant in each of them, has different values. This step L inside a section is equal to the thickness of the cell.

The variation of the step L is carried out, according to the invention, by modifying, inside each cell, or the dimensions of the constituent elements of the cell, thickness of the ring and support rod assembly, or the distance between ring and short circuit, in particular.

It is thus possible to vary slowly and possibly continuously, according to the desired law, the pitch of the delay line. The delay rate imparted to the electromagnetic wave propagating along the line varies in opposite direction to the pitch.

In the example of Figure 2, there is shown a line with three main sections of different pitches, this pitch being constant inside each of the sections; the latter are interconnected by other line portions, not shown, where the pitch varies progressively from its value in the straight section. The differences in dimension have been deliberately exaggerated in the figure to show the variation in pitch from one section to another. It will be noted that in band S the delay line has a length of a few tens of centimeters, while the thickness of a cell is only a few millimeters.

As seen in Figure 2 and in accordance with the present invention the pitch of the structure decreases first (section II) compared to that in the first section, I, near the electron gun, then increases in section III , beyond its value in section I.

The thickness of the assembly formed by the ring 3 and its support rods 4, roughly uniform in the example of Figure 1, is designated by d in this figure; the distance between ring 3 and short circuit 2 by d 'in Figure 2.

It will be noted that in this last figure, which corresponds to one of the possible variants of the invention, the thickness of the support rods is very substantially greater than that of the ring itself.

The Applicant's experimental work has in fact demonstrated that, in order to obtain the desired increase in yield, all other things being equal, the pitch of the delay line moving away from the electron gun must first decrease, then increase in the last part of the line, contrary to the conception of the prior art according to which the pitch of the delay structure had to decrease in the last part of it, opposite to the electron gun. In addition, it appears that the dispersion of the velocities of the electrons within the beam does not increase prohibitively in the last section, despite the significant increase in efficiency obtained with the delay lines of the invention.

This efficiency reached values of 50% during the tests of the applicant, with extreme variations of pitch of + 9% compared to the initial pitch, that is to say the pitch of section I. The efficiency of which this is the ratio of the high frequency power collected at the outlet of the tube to the continuous power applied to the tube. In the context of the invention, these variations can range from a few centimeters of the step to several times this step.

As has been said, the variation of the pitch of the line of the invention can be obtained in two ways: either by modifying the thickness of the assembly, 3-4, formed by the ring and its supports, or by modifying the distance from the ring 3 to the short circuit 2. The first way of proceeding proves to be the most effective for varying the pitch; it also has the advantage of causing a slight disturbance in the coupling impedance between the electron beam and the delay line.

The improvement in efficiency obtained with the delay line of the invention is further increased precisely because of this coupling impedance. With the arrangement of the invention, in fact, the increase in pitch in the last part of the line is not incompatible with a reduction in the coupling impedance in this part, and a lesser dispersion of the electronic speeds, confirmed by calculation.

The focusing of the electron beam is thereby facilitated and the transmission coefficient of the beam improved: the losses of electrons along the line are less, which allows a better beam utilization factor and an average power. higher frequency available at the outlet of the stronger tube, all other things being equal.

In addition, by “enlarging” the elementary cells to increase their pitch, the thermal impedance of the line at the location of these cells is reduced, while increasing its possibilities of dissipation, which is doubly advantageous.

Finally, it should be noted that a low coupling impedance generally corresponds to a poorly dispersive structure, that is to say a large passband. The arrangements provided by the invention to increase the efficiency of the tube therefore go in the direction of a greater bandwidth, which, as is known, is one of the main characteristics of traveling wave tubes.

• - l'épaisseur des anneaux 3 - cote a - pouvant notamment aller du simple au double ;
• - la largeur des tiges supports d'anneau 4 - cote b ;
• - la largeur des plafonds 1 - cote c.

Other modifications inside the cell allow, at the same time as the variation of the pitch, to lower the coupling impedance, in particular the increase of:

• - The thickness of the rings 3 - dimension a - which can in particular range from single to double;
• - the width of the ring support rods 4 - dimension b;
• - the width of the ceilings 1 - dimension c.

You can add grooves to the ceilings, that is to say provide for them stepped edges. By these means, it is easily possible to reduce this impedance to a third or a quarter of its value.

The structure of the variable-pitch delay line of the invention has many advantages, which result from the above, without causing technological complications of implementation.

A number of the provisions provided by the invention have been listed in the foregoing for varying the characteristics of the cell constituting the line. It goes without saying that it is possible, in the context of the invention, to combine these provisions.

The invention is not limited to the examples described and shown but applies, in general, to any delay line with ceiling (s) and rings, in particular in the case of lines which would include a number of ceilings, different from that of the example shown, arranged in the intervals between the support rods of the rings. It applies, inter alia, also to the case of waveguides having a section other than the circular section shown and, in general, it includes all the variants accessible to those skilled in the art from that shown .

FIG. 2 to which reference has been made for the description of the delay line of the invention shows in section the whole of a tube constructed with this line, the latter, formed of three sections l, II and III, bears the reference 20. The tube comprises a metal envelope 21 closed at its two ends by an electron collector 22 and by an insulating foot 23 in which is housed an electron gun whose cathode has been shown at 24 and its heating filament at 25, a focusing electrode 26, as well as an acceleration electrode 27. The cathode emits, under the action of the power source 28, an electron beam 29 whose outline is shown in dotted lines. The delay line is coupled to the input and output circuits of the tube, amplifier in the example, by the antennas 30 and 31. In the figure, the reference 32 designates the solder ring ensuring the vacuum-tight assembly of the cells 10 between them. For clarity, finally, we have limited, in each section, the cells to a small number, when in reality each of these sections can comprise of the order of several tens.

A tube equipped with a delay line according to the invention supplied on the frequency of 3 gigahertz for high frequency power of 300 kilowatts peak, for a total applied peak power of 600 kilowatts, in the form of pulses of 25 _fJS with a form factor of 60 and a beam intensity of 17 amps. The line included a hundred cells.

The invention is generally applicable to the production of high power levels with a large bandwidth and with a high efficiency in the microwave field, particularly in the centimeter range.

Claims (7)

1. Delay line for a travelling wave tube formed of joined structures (10) each of which is composed of the following members :

- a section of a metallic waveguide (11) comprising roof members (1) of the same thickness as the section, each roof member occupying a portion of its cross-section and extending from a fraction of its periphery to a determined distance from its center ;

- a coaxial metallic ring (3) secured to the wall of the guide section by mounting rods (4) located within the free spaces between the roof members ; the structures together forming a waveguide of uniform section in which the roof members,. on the one hand, and the mounting rods, on the other hand, are aligned and extend about the electron beam along its path within the tube, characterized in that the line is subdivided into three sets of structures (I, II, III), the pitch within each set being constant and the pitch of the second and third sets of structures being smaller and larger than that of the first set of structures, respectively, when considered along the line starting from the electron gun, and the pitch variation being obtained by an increase or reduction of at least one dimension of the structures.

2. Delay line according to claim 1, characterized in that the variation of dimension consists in a variation of the thickness of the rings (3) and of their mounting rods (4), all having the same thickness d.

3. Delay line according to claim 1, characterized in that, in case the delay line comprises, in addition, a metallic conductor (2) contacting the roof members (1) on one of the faces of the section to form a short-circuit therebetween, the variation of dimension consisting in a variation of the distance between the rings (3) and the short- circuits (2).

4. Delay line according to any of claims 1 to 3, characterized in that the coupling impedance is simultaneously reduced.

5. Delay line according to claim 4, characterized in that the reduction of the coupling impedance is obtained by increasing the thickness (a) of the rings (3).

6. Delay line according to claim 4, characterized in that the reduction of the coupling impedance is obtained by increasing the width (b) of the ring mounting rods (4).

7. A travelling wave tube, and more especially an amplifying tube, comprising means for generating an electron beam and for propagating it to a collector which captures it, and a delay line which is positioned along the beam path and along which the electromagnetic waves interacting, in operation, with the beam propagate, characterized in that the delay line is a line according to claim 7.

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