EP0048648B1 - Coupled cavities delay line for a travelling-wave tube, and travelling-wave tube comprising such a line - Google Patents

Description

The present invention relates to delay lines with coupled cavities for traveling wave tubes as well as microwave tubes comprising such a line.

Delayed lines with coupled cavities are more particularly used in traveling wave tubes where they provide the interaction between an electron beam moving along the line axis and an electromagnetic wave traversing the line. When conditions of synchronism of the wave and the beam are fulfilled, the electrons yield energy to the electromagnetic wave.

The delay lines with coupled cavities consist of a series of resonant cavities, separated from each other by pierced walls, by at least one coupling opening between cavities and by an opening for the passage of the electron beam, focused according to the axis of the line.

It is advantageous to obtain a high energy conversion efficiency with this type of line, more particularly in the case of traveling wave tubes with high average power. A high conversion efficiency is usually obtained with a high coupling impedance between the beam and the line.

• (Vr^p = vitesse de phase de l'harmonique d'espace avec lequel interagit le faisceau;
• Vg = vitesse de groupe de l'onde électromagnétique)
• par le quotient R/Q de la cavité.
• R = résistance shut de la cavité,
• Q = son coefficient de surtension.

In the case of periodic lines with coupled cavities, this coupling impedance is proportional to the product of the quotient

• (Vr ^p = phase velocity of the space harmonic with which the beam interacts;
• Vg = group speed of the electromagnetic wave)
• by the R / Q quotient of the cavity.
• R = shut resistance of the cavity,
• Q = its overvoltage coefficient.

ce qui entraîne une ligne dispersive et une faible bande passante, et/ou à une valeur élevée du quotient R/Q, ce qui entraîne une robustesse thermique médiocre.A high coupling impedance leads to a high value of the quotient

which results in a dispersive line and low bandwidth, and / or a high value of the R / Q quotient, which results in poor thermal robustness.

However, in the case of a traveling wave tube with high interaction efficiency, the electron beam is highly disturbed, particularly in the final part of the delay line where the amplitude of the microwave voltage between the mouthpieces of cavities becomes a high fraction of the acceleration voltage of the electrons which gave them their initial speed.

This disturbance results in a defocusing of the beam and in an intense electronic bombardment of the mouthpieces of cavities which raises their temperature, all the more that the thermal resistance of these zones is important, which is the case of the hyphothèse considered .

This rise in temperature limits the average power that the traveling wave tube can provide.

We can consider pushing this average power limit by passing a coolant through channels drilled in the partition wall of the cavities; this solution leads to a technological complication in the production of the structure.

By document EP-A-0 037 309, which is part of the state of the art in accordance with article 54-3 of the European Patent Convention, there is known a delay line with coupled cavities in which - see Figure 2 - the diameter of the flanges bordering the opening for the passage of the electron beam increases as we approach the end of the traveling wave tube located on the side of the collector. However, there is a limitation in the way of increasing the diameter of the collars by the need to maintain good coupling between the wave and the beam.

Thus, a traveling wave tube provided with a delay line with coupled cavities designed according to the prior art, will only have a strong interaction efficiency at the expense of either the amplification band which will be weak vis-à-vis -vis what is possible to achieve with this type of line, that is to say the average power that can provide the traveling wave tube.

The present invention relates to a delay line with coupled cavities of increased thermal robustness and having as good a conversion efficiency as the lines with coupled cavities of the prior art, without reduction of the amplification band.

This improvement is obtained thanks to a progressive decrease in the coupling impedance in the final part of the delay line of the traveling wave tube, unlike the prior art.

This reduction in the coupling impedance is mainly obtained by gradually decreasing the R / Q ratio of the cavities. To keep such a high value of the conversion efficiency despite the decrease in the coupling impedance, it is necessary to use, in conjunction with the progressive decrease in the coupling impedance, a computer-calculated variation of the delay rate C / Vr ^p of the line as a function of the abscissa z of the line.

• C: speed of light,
• V _l ^p : phase velocity of the space harmonic with which the beam interacts.

The present invention relates to a delay line, for traveling wave tube, with cavities re-entrant complexes, externally limited by a cylindrical wall whose axis coincides with that along which the electron beam of the tube propagates, and laterally limited by walls perpendicular to this axis, each wall being common to two cavities , said walls being pierced by at least one coupling opening between cavities and, in their center, by an opening bordered by a flange of the same axis forming the re-entrant part of the cavity, the internal diameter of the flanges increasing in the direction of propagation of the electron beam, characterized in that this delay line has in its final part a progressive reduction in its coupling impedance as well as a variation in its delay rate, this progressive reduction in the coupling impedance being obtained in particular by a progressive increase in the direction of propagation of the electron beam of the width (D ₂ -D,) / 2 of the mouthpieces of cavities, where D _i is the diameter be inside the flanges and D ₂ their outside diameter.

• - Figure 1a: une vue en perspective d'une ligne à retard à cavités couplées, selon l'art antérieur.
• - Figure 1b: une coupe transversale d'une ligne à retard à cavités couplées, selon l'art antérieur.
• - Figure 2: une coupe transversale d'une ligne à retard à cavités couplées selon l'invention.

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

• - Figure 1a: a perspective view of a delay line with coupled cavities, according to the prior art.
• - Figure 1b: a cross section of a delay line with coupled cavities, according to the prior art.
• - Figure 2: a cross section of a delay line with coupled cavities according to the invention.

Figure la represents a perspective view of a delay line with coupled cavities, according to the prior art.

The delay line, shown in FIG. 1 a, comprises disks 1, aligned parallel to each other along the same axis 00 'coinciding with the axis of propagation of the electron beam, disks forming the wall common to two neighboring cavities 10.

Each disc is pierced on the one hand by two coupling openings between cavities 2, symmetrical with respect to the axis of the line 00 ′, on the other hand by the opening 3 for the passage of the electron beam 6, generally circular, located in the center of the disc. This opening is bordered by a flange 4 of the same axis called the cavity spout.

The cavities 10, three in number in the figure, are limited by a cylindrical wall 5.

• D₁ le diamètre intérieur des collerettes 4,
• D₂ leur diamètre extérieur, au niveau de leur jonction avec les disques 1,
• D₃ et D₄ les diamètres intérieur et extérieur de la paroi cylindrique entourant les cavités,
• E, l'épaisseur des disques 1,
• H, la largeur des cavités la somme E + H étant égale au pas P de la ligne à retard.

Figure 1b shows a cross section of the delay line with coupled cavities of Figure 1a. This figure shows the geometric parameters of the delay line, namely:

• D ₁ the internal diameter of the flanges 4,
• D ₂ their outside diameter, at their junction with the discs 1,
• D ₃ and D ₄ the inside and outside diameters of the cylindrical wall surrounding the cavities,
• E, the thickness of the discs 1,
• H, the width of the cavities the sum E + H being equal to the pitch P of the delay line.

FIG. 2 represents a cross section of a delay line with coupled cavities, according to the invention.

• - le diamètre D₁ de l'ouverture pour le passage du faisceau d'électrons, ce qui permet d'assurer une meilleure transmission du faisceau et par là même d'assurer un meilleur rendement, avec un échauffement moins important de la ligne.
• - la largeurD₂-D_{1 2} des becs de cavités, 2
• - l'épaisseur E des parois communes à deux cavités.

The delay line with coupled cavities according to the invention has in its last section called the output line a progressive increase in the direction of propagation of the electron beam, of some of its parameters, namely:

• - The diameter D ₁ of the opening for the passage of the electron beam, which ensures better transmission of the beam and thereby ensures better performance, with less heating of the line.
• - the width D ₂ -D _{1 2} of the cavity nozzles, 2
• - the thickness E of the walls common to two cavities.

An additional contribution of the present invention resides in the fact that the extraction of microwave energy from the beam towards the payload being carried out with an output section whose coupling impedance decreases, the availability of electronic speeds is less important and focusing the beam is made easier. This adds an additional factor to the average power capacity that the traveling wave tube can provide.

In addition, by making the structure less dispersive, not only does it weaken the coupling impedance, but it also increases the bandwidth; the arrangements provided by the present invention therefore go in the direction of an increase in the width of the amplification band, which constitutes one of the most advantageous characteristics of traveling wave tubes.

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 (2)

1. Delay line for progressive waves tube of coupled cavities of a concave shape, which are externally limited by a cylindrical wall the axis of which coincides with it along the length of which is propagated the electrons beam of the tube, and laterally limited by walls perpendicular to this axis, each wall being common to two cavities, the said walls being bored by at least one coupling opening between cavities, and at their centre, by an opening edged by a flange of the same axis forming the concave part of the cavity, the internal diameter (D_i) of the flanges increasing in the direction of propagation of the elec- tions beam, characterized in that this delay line presents in its final part a progressive decrease of its coupling impedance as well as a variation of its delay rate, this progressive decrease of the coupling impedance being expecially obtained by a progressive increase, along the direction of propagation of the electrons beam, of width (D₂- D₂)/2 of the mouths of the cavities, D₁ being the internal diameter of the flanges and D₂ their external diameter.

2. Delay line according to claim 1, characterized in that the progressive decrease of the coupling impedance is also obtained by a progressive increase, along the direction of propagation of the electrons beam, of thickness (E) of the walls common to the two cavities.

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