EP3489987B1 - Internal load for travelling-wave tube using a delay line in a folded waveguide - Google Patents

Description

The present invention relates to a delay line or folded slow wave guide for traveling wave tube acronym TOP.

• une première étape : obtenir un groupement des électrons en paquets, c'est-à-dire réaliser une modulation de densité du courant du faisceau au rythme du signal hyperfréquence ; et
• une deuxième étape : placer les paquets d'électrons ainsi obtenus dans une phase où ils sont ralentis par le champ afin de céder leur énergie à l'onde.

In most microwave tubes, the interaction between the wave and the beam is broken down into two stages:

• a first step: obtaining a grouping of electrons in packets, that is to say carrying out density modulation of the beam current at the rate of the microwave signal; and
• a second step: placing the electron packets thus obtained in a phase where they are slowed down by the field in order to give up their energy to the wave.

In the case of PTs, the grouping of electrons in a packet is obtained by placing the beam in the field of a traveling wave whose phase speed is equal to the speed of the electrons. In a moving frame the electrons see the field of a standing wave. The electrons are slowed down on one cycle and accelerated on the next. A bundle of electrons is formed around the phase for which we pass from an accelerating field to a decelerating field.

A conventional waveguide, of rectangular or cylindrical section is not suitable for the interaction because the phase speed of the wave which propagates in this guide is greater than the speed of light while the speed of the electrons is lower at the speed of light. In addition, an electric field parallel to the displacement of the electrons is required, while the fundamental mode of rectilinear guides of rectangular or cylindrical section is perpendicular to the axis of the guide. To obtain a phase velocity lower than that of light, you need a special guide called a slow wave guide or delay line. Most often the delay line is a periodic line obtained by repetitively translating a basic cell, to obtain a succession of identical cells. This is the case of the helix, the line with coupled cavities, the interdigital line ...

In the field of PTs operating at millimeter wavelengths, a so-called folded guide delay line is often used. This line is obtained by periodically positioning rectangular waveguide sections perpendicular to the beam axis, and alternately connecting the straight guide sections by 180 ° E-plane elbows. The side view of the folded guide is shaped like a coil. The beam slip hole is located in the middle of the rectangular guide straight section. The electric field in the guide is perpendicular to the long side of the guide, and therefore parallel to the movement of the electrons, which makes it possible to modulate the beam. The electron therefore moves in the sliding hole, emerges in the right guide section where it is subjected to the action of the electric field (interaction space), crosses the sliding hole again and emerges in the following interaction space . The electron therefore sees the successive interaction spaces with a period equal to the pitch of the line while the geometric period of the line is equal to twice the pitch. The pitch corresponds to the distance between two straight guides separated by an elbow.

The length of the folded waveguide (straight part and elbows) is determined so that the phase shift of the wave in the guide corresponds to the phase variation linked to the displacement of electrons from one interaction space to the next.

Traveling wave tubes use a delay line with a number of sections greater than or equal to 2. The input section is terminated with a load and the output section begins with a load. Intermediate sections begin and end with a load. A load is called a volume containing a material absorbing the RF wave connected to the delay line, such that, in the plane of the connection, the impedance presented by the volume is as close as possible to the characteristic impedance of the line to delay in order to achieve a good adaptation (i.e. to minimize the wave reflected by the load).

The figure 1 schematically represents a delay line or slow waveguide for traveling wave tube comprising three sections 1, 2, 3. The delay line shown comprises an input 4 and an output 5.

The loads 6 at the output of the first section 1, at the input of the second section 2, at the output of the second section 2, and at the input of the third section 3 are called breaking loads or "sever loads" in English. Between the end of one line and the start of the next, the electron beam passes through a sliding hole without an RF field, which contributes to unbundling the beam (it is therefore a loss of modulation).

If the reflection coefficients at both ends of a section and the gain of the section are too high, it is possible to observe oscillation in the delay line section. This is why the length of the different sections is determined so as to limit the gain, taking into account the reflection coefficient of the loads at breaking.

The most common TOPs, an example of which is illustrated on figure 2 , use a delay line comprising a propeller 7 held in a sleeve 8 by three dielectric uprights 9.

On a delay line of the figure 2 , the load is generally carried out by depositing, on the uprights 9 supporting the propeller 7, a layer of material having high microwave losses, such as graphite. A material exhibiting microwave losses is characterized by a finite electrical conductivity σ (unlike a perfect conductor whose electrical conductivity is infinite), which results in a conduction current σE (E being the electric field) and losses resistive σE ² . In a medium exhibiting microwave losses, the wave undergoes exponential attenuation as a function of the distance. By varying the thickness of the deposit, a load is produced, the attenuation (microwave loss) and the reflection coefficient of which gradually increase, thus making it possible to obtain good adaptation over a wide frequency band.

In such a helical delay line, the length of the load leads to a significant loss of modulation, therefore to a loss of PT gain, which must be compensated by an increase in the gain of the other sections, therefore to an increase in the total length of the TOP.

The figure 3 schematically represents the attenuation on an upright 9, as a function of the thickness z of the deposit of the material having high microwave losses, such as graphite. The higher the attenuation, the darker the gray color that represents it.

In the case of PT using a delay line with a folded guide, it is known practice to interrupt the corrugations to pass from a folded guide 10 to a straight guide 11, in which a load absorbing electromagnetic energy is placed. Such a straight guide can be either parallel, as shown in the figures 4 and 5 , or perpendicular, as shown in the figure 6 , to the sliding hole 12 of the beam.

For such embodiments, although using the same guide section, the periodic line in folded guide and the straight guide containing the load do not have the same impedance and it is necessary to make an adaptation between the two, which is not broadband, and limits the PT band.

Alternatively, as shown in figure 7 , it is known practice to interrupt the delay line with a folded guide in order to place a block of lossy dielectric 13 with a determined geometry in order to minimize the reflections of the load.

This variant comprises an abrupt transition between the periodic folded guide 10 and the microwave loss dielectric block 13 which is equivalent to charging the periodic folded guide 10 by a microwave loss resonator having many resonances, which limits the frequency band in which the load is well suited.

An aim of the invention is to overcome the problems mentioned above.

• une plaque centrale comprenant un trou de glissement de faisceau, rectiligne de même direction que l'axe longitudinal de la plaque centrale, et une fente repliée en forme de serpentin ayant ses replis dans le sens de la largeur du guide ;
• une plaque inférieure et une plaque supérieure fermant le guide d'onde, respectivement disposées sur et sous la plaque centrale ;
• au moins une rainure de section pouvant être variable, réalisée selon l'axe longitudinal du guide, sur au moins une face interne au guide de la plaque inférieure, la plaque supérieure ou la plaque centrale, comprenant au moins partiellement un matériau présentant des pertes hyperfréquences ;

pour former un guide à onde lente fermé dans lequel se propage une onde lente hybride dont l'amplitude est atténuée d'au moins 20 dB entre le début et la fin de la partie de la ou les rainures contenant un matériau à pertes.It is proposed, according to one aspect of the invention, a slow wave line in folded guide provided with an internal load comprising:

• a central plate comprising a beam sliding hole, rectilinear in the same direction as the longitudinal axis of the central plate, and a bent serpentine-shaped slot having its folds in the direction of the width of the guide;
• a lower plate and an upper plate closing the waveguide, respectively disposed on and under the central plate;
• at least one groove of section which can be variable, produced along the longitudinal axis of the guide, on at least one face inside the guide of the lower plate, the upper plate or the central plate, comprising at least partially a material exhibiting microwave losses ;

to form a closed slow waveguide in which propagates a hybrid slow wave whose amplitude is attenuated by at least 20 dB between the beginning and the end of the part of the groove or grooves containing lossy material.

Thus the reflections of the load are minimized, and the attenuation of the electromagnetic energy is not abrupt.

The losses are gradually introduced into the line as a folded guide, which presents an analogy with the progressive increase in the deposition of graphite on the propeller supports.

In one embodiment, the material exhibiting microwave losses is a microwave loss dielectric.

Thus, the wave undergoes exponential attenuation with a maximum of power dissipated at the start of the attenuated zone if the distribution of the lossy material is uniform.

According to one embodiment, at least one groove has a constant section and comprises an increasing quantity of the same microwave-loss dielectric when the abscissa increases along the axis of the guide oriented in the direction of wave propagation.

Thus, a small proportion of the power can be absorbed at the start of the charge and a higher proportion thereafter, with the advantage of better distributing the power dissipated over the length of the load.

In one embodiment, at least one groove has a constant or increasing section when said abscissa increases and is filled with microwave loss dielectric whose level of microwave losses increases when said abscissa increases.

Thus, a small proportion of the power can be absorbed at the start of the charge and a higher proportion thereafter, with the advantage of better distributing the power dissipated over the length of the load.

As a variant, the microwave loss material is a layer of a mixture of metals selected from iron, nickel, molybdenum, and titanium, at least partially covering the internal surface of a groove.

Thus, it is not necessary to machine a dielectric block, then solder this block or crimp it into the lower or upper plates in order to ensure the flow of heat between the dielectric block where the power is dissipated and the cold source placed around the delay line.

For example, at least one groove has a section of which the edge length is constant and comprises an increasing amount of a layer of the same said mixture of metals as said abscissa increases.

As a variant, at least one groove has a section whose ridge length is constant or increasing as said abscissa increases and comprises a layer of the same said mixture of metals.

• percer dans une plaque centrale un trou de glissement de faisceau, rectiligne de même direction que l'axe longitudinal d'une plaque centrale, et une fente repliée en forme de serpentin ayant ses replis dans le sens de la largeur du guide ;
• réaliser au moins une rainure de section pouvant être variable, selon l'axe longitudinal du guide, sur au moins une face interne au guide d'une plaque inférieure, une plaque supérieure ou la plaque centrale, comprenant au moins partiellement un matériau présentant des pertes hyperfréquences ;
• disposer la plaque inférieure et la plaque supérieure fermant le guide d'onde, respectivement sous et sur la plaque centrale ;

de manière à former un guide à onde lente fermé dans lequel se propage une onde lente hybride dont l'amplitude est atténuée d'au moins 20 dB entre le début et la fin de la partie de la ou les rainures contenant un matériau à pertes.It is also proposed, according to another aspect of the invention, a method of manufacturing a slow wave line in folded guide provided with an internal load comprising steps consisting in:

• drilling in a central plate a beam sliding hole, rectilinear in the same direction as the longitudinal axis of a central plate, and a folded slot in the form of a serpentine having its folds in the direction of the width of the guide;
• produce at least one section groove that can be variable, along the longitudinal axis of the guide, on at least one face inside the guide of a lower plate, an upper plate or the central plate, comprising at least partially a material exhibiting losses microwave frequencies;
• arranging the lower plate and the upper plate closing the waveguide, respectively under and on the central plate;

so as to form a closed slow waveguide in which propagates a hybrid slow wave whose amplitude is attenuated by at least 20 dB between the beginning and the end of the part of the groove or grooves containing a lossy material.

• la figure 1 illustre schématiquement une ligne à retard ou guide à onde lente pour tube à ondes progressives comprenant trois sections, selon l'état de la technique ;
• la figure 2 illustre schématiquement une ligne à retard comprenant une hélice maintenue dans un fourreau par trois montants diélectriques, selon l'état de la technique ;
• la figure 3 illustre schématiquement l'atténuation sur un montant d'une ligne à retard de la figure 2, en fonction de l'épaisseur du dépôt du matériau ayant des pertes hyperfréquences élevée, selon l'état de la technique ;
• la figure 4 illustre schématiquement une ligne à retard en guide replié, comprenant une charge absorbant de l'énergie électromagnétique dans un guide droit parallèle au trou de glissement du faisceau, selon l'état de la technique ;
• la figure 5 illustre schématiquement une ligne à retard en guide replié, comprenant une charge absorbant de l'énergie électromagnétique dans un guide droit parallèle au trou de glissement du faisceau et replié vers les cellules de la ligne , selon l'état de la technique ;
• la figure 6 illustre schématiquement une ligne à retard en guide replié, comprenant une charge absorbant de l'énergie électromagnétique dans un guide droit perpendiculaire au trou de glissement du faisceau, selon l'état de la technique ;
• la figure 7 illustre schématiquement une ligne à retard en guide replié interrompue par un bloc de diélectrique à pertes d'une géométrie déterminée pour minimiser les réflexions de la charge, selon l'état de la technique ; et
• les figures 8, 9 et 10 illustrent schématiquement une vue d'ensemble et des vues en coupe d'un guide à onde lente replié pour tube à ondes progressives, selon un aspect de l'invention.

The invention will be better understood by studying a few embodiments described by way of non-limiting examples and illustrated by the appended drawings in which:

• the figure 1 schematically illustrates a delay line or slow wave guide for traveling wave tube comprising three sections, according to the state of the art;
• the figure 2 schematically illustrates a delay line comprising a helix held in a sheath by three dielectric uprights, according to the state of the art;
• the figure 3 schematically illustrates the attenuation on an amount of a delay line of the figure 2 , depending on the thickness of the deposit of the material having high microwave losses, according to the state of the art;
• the figure 4 schematically illustrates a delay line in folded guide, comprising a load absorbing electromagnetic energy in a straight guide parallel to the beam sliding hole, according to the state of the art;
• the figure 5 schematically illustrates a delay line in folded guide, comprising a load absorbing electromagnetic energy in a straight guide parallel to the beam sliding hole and folded towards the cells of the line, according to the state of the art;
• the figure 6 schematically illustrates a delay line in folded guide, comprising a load absorbing electromagnetic energy in a straight guide perpendicular to the beam sliding hole, according to the state of the art;
• the figure 7 schematically illustrates a folded-guide delay line interrupted by a lossy dielectric block of a determined geometry to minimize the reflections of the load, according to the state of the art; and
• the figures 8 , 9 and 10 schematically illustrate an overall view and sectional views of a folded slow wave guide for traveling wave tube, according to one aspect of the invention.

In all of the figures, the elements having identical references are similar.

In the present description, the embodiments described are in no way limiting, and the characteristics and functions well known to those skilled in the art are not described in detail.

• une plaque centrale 20 comprenant un trou 21 de glissement de faisceau, rectiligne de même direction que l'axe longitudinal z de la plaque centrale 20, et une fente 22 repliée en forme de serpentin ayant ses replis dans le sens de la largeur du guide ;
• une plaque inférieure 23 et une plaque supérieure 24 fermant le guide d'onde, respectivement disposées sur et sous la plaque centrale 20 ;
• au moins une rainure 25 de section pouvant être variable, réalisée selon l'axe longitudinal z du guide, sur au moins une face interne au guide de la plaque inférieure 23, la plaque supérieure 24 ou la plaque centrale 20, comprenant au moins partiellement un matériau présentant des pertes hyperfréquences ;

pour former un guide à onde lente hybride tel que l'amplitude d'une onde soit atténuée d'au moins 20 dB entre le début et la fin de la partie de la ou les rainures contenant un matériau à pertes.On the figure 8 is schematically represented a slow wave line in folded guide for traveling wave tube, provided with an internal load comprising:

• a central plate 20 comprising a beam sliding hole 21, rectilinear in the same direction as the longitudinal axis z of the central plate 20, and a slit 22 folded in the form of a serpentine having its folds in the direction of the width of the guide;
• a lower plate 23 and an upper plate 24 closing the waveguide, respectively arranged on and under the central plate 20;
• at least one groove 25 of variable section, produced along the longitudinal axis z of the guide, on at least one face internal to the guide of the lower plate 23, the upper plate 24 or the central plate 20, comprising at least partially a material exhibiting microwave losses;

to form a hybrid slow waveguide such that the amplitude of a wave is attenuated by at least 20 dB between the start and the end of the part of the groove (s) containing lossy material.

In other words, the present invention consists in gradually introducing electromagnetic losses into the delay line in folded guide to avoid a sudden transition between the periodic line and a rectangular guide, or between the periodic line and a dielectric block. , in a manner equivalent to the increase in the deposit of graphite on the uprights of a delay line with a helix known from the state of the art.

To do this, the folded-guide delay line is coupled to another transmission line which has significant losses, and the coupling between the two lines increases in the direction of wave propagation. If we define a cell as the volume delimited by two planes perpendicular to the axis of the beam distant by one step (that is to say the distance between two straight guides separated by an elbow), the amplitude of the wave decreases from one cell to the next.

On the example of figure 8 , two grooves 25 of variable section, increasing with the abscissa of the axis z of the guide, in this case symmetrical with respect to the mean plane of the central plate 20, are made on the internal face of the guide of the lower plate 23 and on the inner face of the guide of the top plate 24, and are filled with microwave loss dielectric material, such as a ceramic material (alumina, beryllium oxide, aluminum nitride) sintered with elements adding microwave losses ( carbon, iron, titanium, ...).

The transmission line with high electromagnetic losses can be machined in the lower 23 and / or upper 24 plates which are brazed with the central plate 20 in which the coil 22 is machined in order to constitute the delay line as a folded guide. It is therefore a waveguide machined hollow in the lower 23 and / or upper 24 plates. It can also be partially or totally machined in the central plate 20.

On the example of figure 8 , the variable section groove made on the internal face of the guide of the upper plate 24 is not visible.

The figure 9 shows a sectional view of a folded slow wave guide for traveling wave tube, according to one aspect of the invention.

The figure 10 shows different cross sections of the example of the figure 9 .

As a variant, any embodiment comprising at least one groove 25 of section which may be variable (variable or constant), produced along the longitudinal axis z of the guide, on at least one face internal to the guide of the lower plate, the upper plate or the central plate, comprising at least partially a material exhibiting microwave losses, is possible.

To achieve these electromagnetic losses, it is possible to partially or completely fill the grooves 25 with one or more dielectrics with microwave losses, or deposit on the walls one or different materials with microwave losses, so that along said longitudinal axis oriented in the direction of propagation of the wave, when the abscissa increases, the amplitude of the wave is attenuated by 20 dB between the start and the end of the load.

The most explicit cases are as follows.

At least one groove 25 may have a constant section and comprise an increasing quantity of the same microwave-loss dielectric as said abscissa increases.

As a variant, at least one groove 25 may have a constant or increasing section when said abscissa increases and be full of microwave loss dielectric whose level of microwave losses increases when the abscissa increases.

As a variant, at least one groove 25 may have a section whose edge length is constant and comprise an increasing amount of a layer of the same mixture of metals chosen from: iron, nickel, molybdenum, titanium, at least partially covering the surface. internal surface of a groove as the abscissa increases.

As a variant, at least one groove 25 may have a cross section whose stopping length is constant or increasing when said abscissa increases and comprise a layer of the same mixture of metals chosen from: iron, nickel, molybdenum, titanium.

The large side of the guide machined in the lower and upper plates determines the opening in the folded guide line, and therefore the coupling between the two transmission lines. A low height of the lossy guide may correspond to a guide operating below the cut-off frequency and therefore to an energy not propagating in the lossy guide. In this case, the guide behaves more like a damped resonant cavity coupled to the folded guide.

• percer dans une plaque centrale (20) un trou (21) de glissement de faisceau, rectiligne de même direction que l'axe longitudinal (z) d'une plaque centrale (20), et une fente (22) repliée en forme de serpentin ayant ses replis dans le sens de la largeur du guide ;
• réaliser au moins une rainure (25) de section pouvant être variable, selon l'axe longitudinal (z) du guide, sur au moins une face interne au guide d'une plaque inférieure (23), d'une plaque supérieure (24) ou de la plaque centrale (20), comprenant au moins partiellement un matériau présentant des pertes hyperfréquences ;
• disposer la plaque inférieure (23) et la plaque supérieure (24) fermant le guide d'onde, respectivement sous et sur la plaque centrale (20) ;

de manière à former un guide à onde lente fermé dans lequel se propage une onde lente hybride dont l'amplitude est atténuée d'au moins 20 dB entre le début et la fin de la partie de la ou les rainures contenant un matériau à pertes.The method of manufacturing such a slow wave folded guide line provided with an internal load comprises steps consisting of:

• drill in a central plate (20) a beam sliding hole (21), rectilinear in the same direction as the longitudinal axis (z) of a central plate (20), and a slit (22) bent in the form of a serpentine having its folds in the direction of the width of the guide;
• making at least one groove (25) of section which can be variable, along the longitudinal axis (z) of the guide, on at least one face inside the guide of a lower plate (23), of an upper plate (24) or the central plate (20), comprising at least partially a material exhibiting microwave losses;
• arranging the lower plate (23) and the upper plate (24) closing the waveguide, respectively under and on the central plate (20);

so as to form a closed slow waveguide in which propagates a hybrid slow wave whose amplitude is attenuated by at least 20 dB between the beginning and the end of the part of the groove or grooves containing a lossy material.

The plates, generally parallelepipedal, can be produced by conventional rolling or milling processes.

The beam sliding hole 21 can be made by EDM, and the slot 22 in the central plate can be made by wire EDM.

The grooves 25 can be produced by micro-milling or by electroerosion.

Claims (8)

1. A folded-waveguide slow-wave structure equipped with an internal load, comprising:

   - a central plate (20) comprising a rectilinear beam slip hole (21) of same direction as the longitudinal axis (z) of the central plate (20), and a serpentine- shaped folded slit (22) having its folds in the direction of the width of the waveguide;

   - a lower plate (23) and an upper plate (24) closing the waveguide, placed on and under the central plate (20), respectively;

   - at least one groove (25) with a cross section that may be variable or constant along the longitudinal axis (z) of the waveguide, in at least one face internal to the waveguide of the lower plate (23), the upper plate (24) or the central plate (20), at least partially comprising a material with hyperfrequency losses;

   in order to form a closed slow-wave structure in which a hybrid slow wave propagates, the amplitude of which is attenuated by at least 20 dB between the start and the end of the portion of the groove(s) containing a lossy material
2. The folded-waveguide slow-wave structure according to claim 1, wherein the material with hyperfrequency losses is a dielectric with hyperfrequency losses.
3. The folded-waveguide slow-wave structure according to claim 2, wherein at least one groove (25) has a constant cross-section and comprises an amount of a given dielectric with hyperfrequency losses that increases as the abscissa increases along the axis of the waveguide, which is oriented in the direction of wave propagation.
4. The folded-waveguide slow-wave structure according to claim 2, wherein at least one groove (25) has a cross section that remains constant or increases as said abscissa increases and is filled with dielectric with hyperfrequency losses, the level of microwave losses of which increases as said abscissa increases.
5. The folded-waveguide slow-wave structure according to claim 1, wherein the material with hyperfrequency losses is a layer of a mixture of metals chosen from iron, nickel, molybdenum and titanium, at least partially covering the internal surface of a groove (25).
6. The folded-waveguide slow-wave structure according to claim 5, wherein at least one groove (25) has a cross section the edge length of which remains constant and comprises an amount of a layer of said same mixture of metals that increases as said abscissa increases.
7. The folded-waveguide slow-wave structure according to claim 5, wherein at least one groove (25) has a cross section the edge length of which remains constant or increases as said abscissa increases and comprises a layer of said same mixture of metals.
8. A process for manufacturing a folded-waveguide slow-wave structure equipped with an internal load, comprising:

   - drilling in a central plate (20) a rectilinear beam slip hole (21) extending in the same direction as the longitudinal axis (z) of a central plate (20), and a serpentine-shaped folded slit (22) having its folds in the direction of the width of the waveguide;

   - producing at least one groove (25) with a cross section that may be variable or constant, along the longitudinal axis (z) of the waveguide, in at least one face internal to the waveguide of a lower plate (23), of an upper plate (24) or of the central plate (20), and at least partially comprising a material with hyperfrequency losses;

   - placing the lower plate (23) and the upper plate (24) closing the waveguide, under and on the central plate (20), respectively; so as to form a closed slow-wave structure through which propagates a hybrid slow wave the amplitude of which is attenuated by at least 20 dB between the start and the end of the portion of groove(s) containing a lossy material.

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