EP0401065B1 - Method of construction of a helical delay line - Google Patents

Description

The present invention relates to a particular construction method which makes it possible to manufacture very wide band traveling wave tubes having a very low dispersion; this method of construction consists in supporting the helix of the delay line of a broadband traveling wave tube by means of dielectric supports ensuring the isolation arranged between the helix and fins or other metallic supports projecting towards the center from a metallic envelope which surrounds the whole.

The invention also relates to a traveling wave tube manufactured according to this method of construction.

Progressive wave tubes (TOP) are well known in the prior art and are preferred to other microwave tubes for applications which require a very large intrinsic passband in amplification. The wide bandwidth allowed by the helix construction of the delay line results from the low dispersion of the electromagnetic waves which propagate along the helix delay line as a function of frequency; in other words, the speed v of the wave which propagates along the helical line depends very little on the frequency of the wave in a wide range of frequencies centered on the nominal operating frequency of the tube traveling waves.

The coupling between the high frequency (HF) signal applied to the input of the tube and from there to the helical delay line and the electron beam depends on the synchronism of the propagation of the two along the longitudinal direction traveling wave tubes. The speed of the electron beam depends on the acceleration voltages created inside the tube, as a first approximation, and it is modified by the energy exchange that occurs with the electromagnetic field, in second approximation; if, as a first approximation, the speed of the high frequency wave which propagates along the propeller depends only on the geometry of the propeller, in second approximation, it also depends slightly on the frequency, which ultimately limits the bandwidth of the traveling wave tube.

For practical reasons, the use of traveling wave tubes in amplifier equipment does not allow the adjustment of operating voltages to modify the speed of the beam electrons when the frequency of the signal to be amplified varies, and it is therefore desirable to '' have as little variation as possible in the speed of the electromagnetic wave as a function of frequency. However, in all practical modes of construction of a delay line, the phase speed of the electromagnetic wave depends on the frequency, which gives rise to a dispersion d = Vp / Vg, where Vp is the phase speed along the delay line at the operating frequency and Vg is the group speed.

Figure 1 shows a typical curve which represents the variation of the c / v ratio as a function of the wavelength, where c is the speed of light. A value of d approximately equal to unity means that the phase velocity along the delay line is practically constant when the frequency varies, which is the condition which must be fulfilled for broadband operation, if possible in full operating bandwidth. A very large value of d corresponds either to an infinite value of Vp (wave guided at the cutoff frequency), or to a value of Vg close to zero, which means that the energy does not propagate along the line at delay.

In practice, for any wave propagation circuit (here, the delay line) having periodic geometric or electrical characteristics, the circuit stops transmitting energy in a propagation mode given at frequencies such that the half wavelength in this mode is equal to the period of the geometric characteristics of the delay line. These frequencies are called "cutoff frequencies in mode ¶". There are also cutoff frequencies in zero mode when the phase difference over a period of the slow wave structure is equal to zero or a multiple of 2¶.

At these frequencies, the dispersion as defined above tends towards infinity. These cutoff frequencies cannot be avoided in a physical circuit because Vg cannot grow indefinitely as Vp grows indefinitely, any more than Vg cannot be prevented from becoming infinitesimally small for a finite value of Vp.

In the prior art, it is known that these cut-off frequencies can be displaced, but at the cost of a reduction in the efficiency of the traveling wave tube in operation: the intensity of the electric field is reduced for a given level of power. propagating in the delay line at a given phase velocity, which reduces interaction with the electron beam in the tube. In the case where the cut-off frequencies are not displaced by any means, the cut-off frequencies observed experimentally can be called "natural cut-off frequencies".

A method known in the prior art for reducing this drawback consists in adding an anisotropic charge to the basic helix delay line, which gives a very low dispersion which can even become zero or negative.

The best known of these methods of applying a load, illustrated in FIG. 3, consists in having metal fins in the shape of a U, with a capacitive effect, between the dielectric bars supporting the propeller, the ends of which are positioned very near the propeller (a few tenths of a millimeter). It is difficult to obtain economically reproducible results in an industrial manufacturing process when employing this process. This process generally requires the use of a difficult technique of soldering.

Another method known in the prior art consists in disposing the capacitive charges between the dielectric bars supporting the helix, as shown in FIG. 2, but this solution reduces the coupling impedance of the circuit and the efficiency of the tube.

Another known method consists in replacing the fins formed in the form of the U mentioned above and represented in FIG. 3, by localized metallizations of the dielectric bars supporting the propeller, as shown in FIG. 4 and is described in the patent. US-A-4 264 842. This process is also difficult to implement industrially if it is desired to obtain reproducible results economically.

Patent US-A-4 689 276 recommends brazing by the "diffusion bonding" method under pressure the dielectric supports on metal elements which project from the interior surface of the envelope.

The object of the invention is therefore to obtain a higher cut-off frequency without the drawbacks of the methods of the prior art. The fundamental physical principles known in the prior art can be explained in a new construction method according to the invention, which gives a very low dispersion and, consequently, an enlarged useful bandwidth while lowering the industrial cost price and the complexity of the assembly and improving the reproducibility of the characteristics of the tube. The present invention relates to a method of constructing a traveling wave tube described in claim 1.

According to a preferred embodiment of the invention, said support elements are made of metallic material.

According to one embodiment of the invention, the dielectric supports have the form of continuous bars which are at their tower supported by metal supports. The dimensions of the dielectric supports are thus smaller than those of the previous embodiment, which leads to an improvement in the thermal conductivity of the propeller to the envelope which surrounds the assembly.

According to another embodiment of the invention, the dielectric supports have the form of discontinuous pads which are arranged between each turn of the propeller and a continuous metal support. These studs can be assembled on the metal support before introducing this sub-assembly into the envelope which surrounds the assembly, which improves the precision of the assembly and facilitates the manufacture of the tube. In addition, the reduced dimensions of the studs have the advantage of allowing the use of expensive materials such as diamond and boron nitride with a cubic network with centered faces, for example.

According to another embodiment of the invention, the continuous metallic support is in the form of a vacuum-tight enveloping structure which, when it is placed inside the jacket surrounding the assembly, leaves a space between this outer jacket and said support structure, space in which a liquid or a cooling gas can be circulated.

• la figure 1, déjà décrite plus haut, reproduit une courbe typique représentant la variation du rapport de la vitesse de la lumière c à la vitesse v de propagation de l'énergie le long de la longueur de la structure à onde lente en hélice en fonction de la longueur d'onde lambda, en unités arbitraires ;
• la figure 2, déjà décrite plus haut, est une vue en coupe transversale d'un mode de construction d'une ligne à retard à hélice de l'art antérieur dans lequel l'hélice est supportée par des barreaux diélectriques formant supports à l'intérieur d'une enveloppe cylindrique étanche au vide entourant l'ensemble, et dans lequel les barreaux diélectriques formant supports sont en contact tant avec l'hélice qu'avec l'enveloppe, et ces "ailettes" métalliques massives à charge capacitive sont disposées entre les barreaux diélectriques formant supports dans l'espace compris entre l'enveloppe et l'hélice ;
• la figure 3, déjà décrite plus haut, représente une vue en coupe transversale d'un autre mode de construction d'une ligne à retard à hélice de l'art antérieur, dans lequel l'hélice est supportée par des barreaux diélectriques formant supports en contact avec l'hélice et avec une enveloppe cylindrique étanche au vide entourant l'ensemble comme sur la figure 2, avec des ailettes en forme de U disposées entre les barreaux diélectriques formant supports fixées à la paroi intérieure de l'enveloppe étanche au vide et en saillie vers l'hélice jusqu'à une faible distance de celle-ci ;
• la figure 4, déjà décrite plus haute, est une vue en coupe transversale d'un autre mode de construction d'une ligne à retard à hélice de l'art antérieur, dans lequel l'hélice est supportée par des barreaux diélectriques formant supports en contact avec l'hélice et avec une enveloppe cylindrique étanche au vide entourant l'ensemble comme sur la figure 2, avec des métallisations localisées déposées sur les faces des barreaux diélectriquées qui font face à l'espace situé entre l'hélice et l'enveloppe ;
• la figure 5 est une vue en coupe transversale d'une forme de réalisation d'un mode de construction d'une ligne à retard à hélice selon l'invention, dans lequel des barreaux diélectriquées en forme de U formant supports sont disposés entre l'hélice et des supports métalliques qui sont en saillie par rapport à la paroi intérieure de l'enveloppe cylindrique à vide entourant l'ensemble, vers l'hélice ;
• la figure 6 est une vue en coupe transversale d'une autre forme de réalisation d'un mode de construction d'une ligne à retard à hélice selon l'invention, dans lequel des barreaux diélectriques en forme de T formant supports sont disposés entre l'hélice et des supports métalliques rainurés qui sont en saillie par rapport à la paroi intérieure de l'enveloppe cylindrique étanche au vide entourant l'ensemble; vers l'hélice ;
• la figure 7 est une vue en perspective d'un détail d'une autre forme de réalisation d'un mode de construction d'une ligne à retard à hélice selon l'invention, dans lequel l'hélice est supportée par des plots diélectriques qui sont disposés entre l'hélice et des supports métalliques qui sont en saillie par rapport à la paroi intérieure de l'enveloppe cylindrique étanche au vide entourant l'ensemble, vers l'hélice ; et,
• la figure 8 est une vue en coupe transversale d'une autre forme de réalisation d'un mode de construction d'une ligne à retard en hélice selon l'invention, dans lequel des barreaux diélectriques formant supports ou des plots sont disposés entre l'hélice et une structure métallique étanche au vide entourant l'ensemble qui est à son tour disposée à l'intérieur d'une enveloppe cylindrique entourant l'ensemble et créant des espaces entre elle-même et la structure formant support, dans lesquels un liquide ou un gaz de refroidissement peut être mis en circulation.

Other characteristics and advantages of the invention will appear on reading the detailed description of nonlimiting exemplary embodiments given below in relation to the appended drawings, in which:

• FIG. 1, already described above, reproduces a typical curve representing the variation of the ratio of the speed of light c to the speed v of energy propagation along the length of the slow wave structure in helix as a function lambda wavelength, in arbitrary units;
• Figure 2, already described above, is a cross-sectional view of a construction method of a propeller delay line of the prior art in which the propeller is supported by dielectric bars forming supports at the inside a vacuum-tight cylindrical envelope surrounding the assembly, and in which the dielectric bars forming supports are in contact with both the helix and the envelope, and these massive metal "fins" with capacitive load are arranged between the dielectric bars forming supports in the space between the casing and the propeller;
• Figure 3, already described above, shows a cross-sectional view of another embodiment of a propeller delay line of the prior art, in which the propeller is supported by dielectric bars forming supports in contact with the propeller and with a vacuum-tight cylindrical envelope surrounding the assembly as in FIG. 2, with U-shaped fins arranged between the dielectric bars forming supports fixed to the interior wall of the vacuum-tight envelope and projecting towards the propeller to a small distance from it;
• Figure 4, already described above, is a cross-sectional view of another embodiment of a propeller delay line of the prior art, in which the propeller is supported by dielectric bars forming supports in contact with the propeller and with a vacuum-tight cylindrical envelope surrounding the assembly as in Figure 2, with localized metallizations deposited on the faces of the dielectric bars which face the space between the propeller and the envelope ;
• Figure 5 is a cross-sectional view of a shape of an embodiment of a propeller delay line according to the invention, in which U-shaped dielectric bars forming supports are arranged between the propeller and metal supports which project from the inner wall of the cylindrical vacuum envelope surrounding the assembly, towards the propeller;
• Figure 6 is a cross-sectional view of another embodiment of an embodiment of a propeller delay line according to the invention, in which T-shaped dielectric bars forming supports are arranged between the propeller and grooved metal supports which project from the inner wall of the vacuum tight cylindrical envelope surrounding the assembly; towards the propeller;
• FIG. 7 is a perspective view of a detail of another embodiment of an embodiment of a propeller delay line according to the invention, in which the propeller is supported by dielectric pads which are arranged between the propeller and metal supports which project from the inner wall of the vacuum-tight cylindrical envelope surrounding the assembly, towards the propeller; and,
• Figure 8 is a cross-sectional view of another embodiment of an embodiment of a helical delay line according to the invention, in which dielectric bars forming supports or studs are arranged between the helix and a vacuum-tight metallic structure surrounding the assembly which is in turn arranged inside a cylindrical envelope surrounding the assembly and creating spaces between itself and the support structure, in which a liquid or cooling gas can be circulated.

With reference to the drawings, in which the same numerical references designate the same elements in the various embodiments, FIGS. 2, 3 and 4 included for explanatory purposes represent known embodiments of the prior art in which the propeller is supported in its vacuum-tight envelope 2 surrounding the assembly by means of dielectric bars 3, and metallic elements 4 are arranged in the space between the propeller and the envelope and between the dielectric supports in a symmetrical manner.

FIG. 5 represents an example of a mode of construction of a traveling wave tube with a propeller according to the invention in which the propeller 1 is supported in its vacuum-tight envelope 2 surrounding the assembly by dielectric supports 3 ensuring the insulation which are in turn supported by metal elements 4 which project from the inner wall of the vacuum-tight envelope 2 surrounding the assembly, towards the helix and on which the dielectric supports are arranged. In the embodiment of the invention shown in Figure 5, a groove is formed along the length of the dielectric support 3 so as to receive the edge of the metal element 4, which ensures the precision of the assembly and greatly simplifies the manufacturing process compared to the prior art.

FIG. 6 represents another example of a construction of a traveling wave tube with a propeller according to the invention in which the propeller 1 is supported in its vacuum-tight envelope 2 surrounding the assembly, by dielectric supports 3 providing isolation, characterized in that the dielectric supports 3 are in turn supported by metal elements 4 which project from the inner wall of the vacuum-tight envelope 2 surrounding the assembly, towards the helix and on which the dielectric supports 3 are arranged. In this exemplary embodiment of the invention shown in FIG. 6, a groove is formed along the length of the metal element 4 so as to receive the edge of the T-shaped dielectric bar forming a support 3, which ensures assembly precision and greatly simplifies the manufacturing process compared to the prior art.

The metal elements of Figures 5 and 6 may advantageously have the shape of a wedge whose thick end rests on the inner face of the vacuum-tight envelope 2 surrounding the assembly.

FIG. 7 shows another example of a method of constructing a traveling wave tube with a propeller according to the invention in which the propeller is supported in its vacuum-tight envelope 2 surrounding the assembly by dielectric supports 3 ensuring insulation, characterized in that the dielectric supports 3 are in turn supported by metal elements 4 which project from the inner wall of the vacuum-tight envelope 2 surrounding the assembly, towards the helix and on which the dielectric supports 3 are arranged. In the embodiment of the invention shown in FIG. 7, the dielectric supports 3 are no longer continuous bars as in the two preceding examples of embodiment of the invention shown in FIGS. 5 and 6, and are on the contrary disconstinus and formed by dielectric pads providing isolation and positioned along the length of the continuous metallic element 4 in such a way that they support each turn of the individual helix. As in the embodiment of the invention shown in Figure 5 and described above, a groove is shaped in the dielectric material to receive the edge of the metal element 4, which ensures the accuracy of the assembly. Other embodiments can easily be imagined with the use of dielectric pads. For example, a groove can be formed along the length of the metal element as in Figure 6 or local holes can be distributed along the length of the metal element to receive dielectric pads having a suitable protruding part to position and hold the studs in the groove or in the holes.

FIG. 8 represents another example of a method of constructing a propeller traveling wave tube according to the invention in which the propeller 1 is supported in its vacuum-tight envelope 2 surrounding the assembly for dielectric supports ensuring the insulation 3, characterized in that the dielectric supports 3 are in turn supported by a metal element 4 which protrude relative to the inner wall of the vacuum-tight envelope 2 surrounding the assembly, towards the helix and on which the dielectric supports 3 are arranged. In this exemplary embodiment of the invention shown in Figure 8, a groove is formed along the length of the dielectric supports 3 so as to receive the ribs forming supports of the metal element 4, which ensures the precision of the 'assembly and greatly simplifies the manufacturing process compared to the prior art. The exemplary embodiment according to the invention shown in FIG. 8 is further characterized in that the metal elements 4 form a vacuum-tight envelope surrounding the propeller 1 and disposed inside the cylindrical outer envelope 2, defining and delimiting spaces 5 formed between the two envelopes, in which a gas or a liquid can be circulated to cool the delay line.

In several preferred embodiments of the invention as shown by the nonlimiting examples described above and the corresponding figures 5, 6, 7 and 8, metal elements 4 are used to support the dielectric elements 3 forming propeller supports ensuring isolation, but the invention also relates to a construction method applicable to the manufacture of propeller delay lines in which the metallic elements 4 are replaced by any other material having a finite electrical length, positioned in the immediate vicinity of the dielectric supports 3 so as to partially compensate for the capacitive charge effect of the dielectric at the frequencies close to the natural cutoff frequency. Likewise, the invention also relates to embodiments in which additional metallic elements can be arranged between the elements 4 which support the dielectric supports 3 of the helix 1.

In all cases, it is understood that the arrangements described above illustrate only a small number of the many possible specific embodiments which may represent applications of the principles of the invention. Other arrangements, numerous and varied, can easily be imagined in accordance with these principles by those skilled in the art without departing from the scope of the invention as defined in the claims.

Claims (9)

1. Method of construction of travelling-wave tubes with a helix (1), the helix (1) being contained in a vacuum-tight envelope (2) and being supported by dielectric supports (3), these dielectric supports themselves being supported by support-forming elements (4) projecting with respect to the inner surface of the envelope (2), these elements (4) having a finite electrical length in immediate proximity to the dielectric supports (3) such that the capacitive load of these dielectric supports (3) is partially compensated for by the presence of the support-forming elements (4) at frequencies close to the natural cut-off frequency, characterized in that the dielectric supports and the support-forming elements are fashioned in such a way as to fit into one another in order to achieve a compact assembly.
2. Method of construction of travelling-wave tubes with a helix according to Claim 1, characterized in that the support-forming elements (4) with finite electrical length are of metallic material.
3. Method of construction of travelling-wave tubes according to one of Claims 1 or 2, characterized in that the support-forming elements (4) with finite electrical length and the dielectric supports (3) are fashioned in such a way as to produce a mutual locking of these components along the length of the helical structure.
4. Method of construction of travelling-wave tubes according to one of Claims 1 to 3, characterized in that the supplementary metallic elements are arranged between the support-forming elements (4) with finite electrical length.
5. Method of construction of travelling-wave tubes with a helix according to one of Claims 1 to 4, characterized in that the dielectric supports have the form of continuous bars.
6. Method of construction of travelling-wave tubes with a helix according to one of Claims 1 to 4, characterized in that the dielectric supports (3) have the form of discontinuous pads.
7. Method of construction of travelling-wave tubes with a helix according to one of Claims 1 to 6, characterized in that the support-forming elements (4) with finite electrical length have the form of continuous bars.
8. Method of construction of travelling-wave tubes with a helix according to one of Claims 2 to 6, characterized in that the support-forming metallic elements (4) have the form of a continuous, vacuum-tight envelope.
9. Method of construction of travelling-wave tubes with a helix according to Claim 8, characterized in that the continuous, vacuum-tight metallic envelope, when it is arranged within the outer envelope (2) surrounding the assembly, creates spaces in which a gas or a liquid can be made to flow.

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