EP1251544B1 - Microwave amplifier electron tube with miniature entry plug and method of fabrication - Google Patents

Description

The invention relates to amplifiers electronic tubes operating at microwave. It applies more particularly to traveling wave tubes (TWTs) also called TWT (Traveling Wave Tube), and it is therefore about such a tube that it will be described. The invention is particularly useful for making small radial size TOPs operating at high frequencies (typically 30 GHz). Such tubes are used for example for the transmission of telecommunication signals between the earth and satellites.

It is briefly recalled that a TOP is a vacuum tube using the principle of the interaction between an electron beam and a microwave electromagnetic wave, to transmit a portion of the energy contained in the beam of the microwave beam to the microwave wave. electrons, so as to obtain at the outlet of the tube a microwave wave of energy greater than that of the wave injected at the inlet of the tube.

Figure 1 recalls the general principle of a TOP. The represented TOP is a helical TOP, but other types of TOPs such as coupled cavity TOPs, meandered folded TOPs, etc., are equally concerned with the invention.

The TOPs comprise an elongate tubular sleeve 10 in which the vacuum is made, with at one end an electron gun 11 emitting an electron beam 12 and, at a second end, a collector 14; the collector collects the electrons that have given up some of their initial energy to the electromagnetic wave that we want to amplify. The electron beam is substantially cylindrical over almost the entire length of the tube between the barrel 11 and the collector 14. This cylindrical beam shape is obtained on the one hand thanks to the shape of the cathode 16 of the barrel (cathode convergent in form of bowl), and secondly by means of magnetic focusing means provided over the entire length of the sheath between the output of the electron gun 11 and the inlet of the collector 14. These focusing means are annular permanent magnets 18, magnetized axially and alternately magnetizing from one magnet to the next; these magnets surround the sleeve and are separated from each other by pole pieces 20 with high magnetic permeability.

In the case of a helical TOP, the electron beam passes inside a conductive helical structure 22 along which the microwave electromagnetic wave to be amplified circulates; the amplification of microwave energy occurs by interaction between this wave and the electron beam passing in the center of it. The propeller serves to slow down the microwave wave, in that if the velocity of the wave that propagates along the helix is substantially the speed of light, the velocity of the wave along the axis of the helix will be only a fraction of the speed of light; this fraction is determined by the angular pitch of the helix. A meandering guide or coupled cavities can play the same role of retarder as the propeller.

A microwave signal input, 24, is connected to a first end of the helix, on the side of the electron gun 11, and a microwave signal output 26 is provided at a second end of the helix, on the collector 14 side. .

The input 24 of the TOP is most often constituted by a coaxial plug whose central core is electrically connected to the end of the propeller. The plug is brazed or soldered to the tube, and a ceramic window, brazed or welded in the body of the plug, is provided to seal the vacuum between the inside of the tube (under vacuum) and the outside (at free air) while allowing the passage of the microwave from the outside to the propeller.

In the same way the output 26 of the TOP may be constituted by a coaxial plug provided with vacuum sealing means. But it can also be constituted by a section waveguide for example rectangular. In the output plug as in the input plug, a vacuum-tight structure, with a piece of ceramic allowing the microwave waves to pass, must in all cases be provided.

The microwave signal input and output parts are bulky and difficult to put in place. Magnets also pose problems of setting up. These difficulties result in particular from the fact that the vacuum tubes require soldering operations to ensure sealing at different points, and high temperature steaming operations with pumping, for evacuation. Steaming operations are performed at temperatures up to 500 ° C for example and for periods of up to several tens of hours. The brazing operations include soldering between ceramic pieces and metal pieces, and these brazing are done by passing through an oven at even higher temperatures (780 ° C typically, or more). Magnets can not normally support these operations and the placement of magnets must take this into account.

In practice, the mounting of such a TOP is in principle in the order of the following steps: installation of the helical structure 22 in the sleeve of the tube, positioning of the pole pieces along the sheath, set up and welding on the tube of the input and output plugs, welding connections between the ends of the propeller and the input and output plugs; welding the electron gun on one side, the collector on the other side; then pumping / steaming the tube to ensure the degassing of internal parts, closing or sealing the tube when the vacuum is made; then placing permanent magnets in the periodic intervals between pole pieces: each magnet is cylindrical and consists of two half-rings placed around the sheath between two consecutive pole pieces; the half-rings are tightened against each other to reconstitute a complete cylinder, for example by means of an elastic strapping around each magnet.

Given that the good functioning of the TOP is based on the quality of the axial alternating magnetic field inside the propeller, and that this quality depends on that of the permanent magnets, it is often necessary to proceed then to manual external adjustments in sticking here and there additional magnetic pieces that empirically correct the irregularities of distribution of the magnetic field.

Overall, it should be remembered from this description that the manufacture of such tubes is a difficult and expensive operation.

The present invention aims at providing a manufacturing method and a tube structure which make the realization less expensive without deteriorating the quality of the tube produced, and even improving it from the point of view of the regularity of the distribution of the magnetic fields obtained on along the sheath.

To achieve this, the invention proposes a method for manufacturing a vacuum electron tube, in which the tube comprises on the one hand an electron gun capable of emitting an electron beam in the axis of a sheath. cylindrical and secondly a series of permanent focusing magnets of the beam, distributed around the sleeve between a signal input to be amplified and a signal output, this method being characterized in that annular magnets each made of in one piece, we put these magnets in one piece on the sleeve together with polar parts between magnets, and we proceed to a pumping operation / baking of the tube at high temperature in the presence of magnets, for the purpose of degassing of the internal parts of the tube, the magnets being made of a material capable of recovering most of its magnetization properties after a thermal cycle at the high temperature reached during the pumping / steaming operation.

The high temperature is several hundred degrees, up to about 500 ° C.

The electron gun is preferably placed in the sheath by welding or local brazing. There is no need to pass the tube through a high temperature brazing furnace after placement of the magnets and prior to the stoving operation (i.e., the high temperature pumping operation).

This use of magnets baking at several hundred degrees Celsius avoids having to cut in half the annular magnets to then assemble around the sheath; in the prior art, the assembly of two annular half-magnets deteriorated the quality of the magnetic focusing field present inside the helix and made it necessary to make empirical and sometimes tedious corrections after manufacture, during the tests of the tube.

The magnets having this resistance to high temperatures may be based on samarium-cobalt including transition metals (copper, iron, zirconium in particular). They are typically in a material of the type Sm ₂ X _{17 in} particular, where X is a combination of several transition metals, including cobalt, copper, iron and zirconium; they are sold in particular by the company Electron Energy Corporation in Landisville USA and can withstand temperatures up to 550 ° C at least.

To best use this manufacturing method, the invention provides an original tube input / output structure; this structure allows the introduction of monobloc ring magnets after the realization brazing which is necessary to ensure both the vacuum tightness and the passage of microwave waves at the place of the input structure and the output structure. With the input / output structure according to the invention, it is possible to realize typically all soldering at 780 ° C or more before the placement of the magnets, and this although the magnets are made of a single annular piece . By input / output structure is meant here a structure for the passage of microwave signal between the inside and outside of the tube, either at the tube inlet or at the outlet. The invention applies either to the entry, the exit or both.

Thus, the invention proposes a vacuum microwave tube comprising an electron gun capable of emitting an electron beam in the axis of a metallic cylindrical sheath which extends between a microwave signal input to be amplified and a amplified signal output, this tube being characterized in that at least one of the input and the signal output comprises a dielectric plate forming a microwave energy transmission window, this plate being embedded and soldered in a closed opening in the wall of the sheath, the plate does not protrude beyond the outer perimeter of the cylindrical sheath and sealing in a vacuum-tight manner said opening. In this way, magnets can be threaded on the sleeve passing over the inlet window (or exit) after soldering operations of this window. The magnets will undergo the bake temperature, but not the soldering temperature (s).

The dielectric plate is preferably constituted by a tri-plate structure with two dielectric layers and three levels of metallization separated by these two layers; the metallizations comprise a first level of metallization comprising (in the direction of the propagation of energy) an input conductor, a second intermediate level comprising a metallization connected to an electrical mass and a coupling slot formed in this metallization, and a third metallization level comprising an output conductor. The input conductor and the output conductor each have a portion facing the coupling slot and the energy flow is through the coupling slot, between the input conductor and the output conductor.

If the structure according to the invention is located at the inlet of the tube, which will be the most frequent case, the inlet metallization is outside the tube, and receives a microwave signal to be amplified. The exit metallization is located inside the tube and is connected for example to the input of a propeller of the TOP or other wave propagation slowing structure.

With this input or output structure, the magnets can be slid onto the sleeve after having brazed thereto the dielectric plates constituting vacuum-tight transmission windows, after which, before or after the tube is heated, a solder is welded. access sheet to the window (for example a laser-welded coaxial plug).

The realization of the window and the connection between the inside of the tube and the outside is particularly easy. In addition, it allows a reduction of dimensions of the whole tube, minimizing the place occupied by the connections, so that this structure can be used advantageously only because of this reduction of dimensions and not only because of the more great ease of manufacture of the tube.

In the general case, it is especially the microwave signal input that can be useful to achieve in this way. The access sheet to the window will in principle be a coaxial plug, the core of which will be connected to a metallized part of the plate on the side located outside the tube. But an external connection to a waveguide is also possible.

• la figure 1, déjà décrite, représente la structure générale d'un TOP;
• la figure 2 représente une réalisation pratique de l'entrée d'un TOP de l'art antérieur ;
• la figure 3 représente en perspective la structure générale d'entrée du tube selon l'invention ;
• la figure 4 représente une vue en coupe de la structure d'entrée ;
• la figure 5 représente une vue de dessus ;
• la figure 6 représente en coupe la structure d'entrée à laquelle est raccordée une fiche coaxiale.

Other features and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

• Figure 1, already described, represents the general structure of a TOP;
• FIG. 2 represents a practical embodiment of the input of a TOP of the prior art;
• FIG. 3 is a perspective view of the general inlet structure of the tube according to the invention;
• Figure 4 shows a sectional view of the input structure;
• Figure 5 shows a view from above;
• Figure 6 shows in section the input structure to which is connected a coaxial plug.

Figure 2 shows an example of a propeller TOP construction in the prior art. The same references as in Figure 1 are used for the same elements. Only the upstream part of the tube is represented. The electron gun is not represented. The barrel is fixed, on the left of the figure, on a metal pole piece 30 which terminates the sheath 10. The conductive helix 22, carried by an insulating structure 23, extends all along the sheath, and is centered by compared to the sheath. The electron gun emits an electron beam centered in the axis of the helix.

The annular magnets 18, each consisting of two half-rings, are evenly distributed throughout the tube and separated by pole pieces 20 of high magnetic permeability material. These pole pieces 20, fixed prior to mounting the magnets, are all shown, but only a few magnets 18 have been shown to show how they fit, after steaming the tube, in the spaces between two consecutive pole pieces 20. The two half-rings constituting a magnet 18 are held in place around the sheath by elastic clips or straps, not shown so as not to weigh down the figure.

The microwave is brought to the input of the tube by an input plug 24, here a coaxial plug. This input plug is located at one end of the sleeve, on the side of the electron gun, facing the end of the propeller 22.

The coaxial input plug has a conductive core 25 inside an envelope 27. The end of the core is generally welded to the upstream end of the helix to establish a conductive connection for the wave microwave between the core of the plug and the propeller. In all cases, a microwave window must be provided somewhere in the form. The window is a cylindrical or rectangular ceramic dielectric piece 32 brazed in a vacuum-tight manner inside the plug. The construction may be similar for the microwave output. In Figure 2 there is not shown the coaxial cable which brings the energy to the input plug. This cable is screwed onto the end of the plug in such a way that the cable core comes into contact with the core of the plug and the cable casing comes into contact with the casing 27 of the cable. plug completely surrounding the dielectric window 32.

• le fourreau 10 avec une fenêtre de transmission hyperfréquence 40, étanche au vide, intégrée à l'intérieur du périmètre du cylindre constituant le fourreau de sorte qu'aucun élément de la fenêtre ne dépasse de ce périmètre ; un aimant 18 ou une pièce polaire 20 peuvent ainsi être enfilés sur le fourreau en passant par-dessus la fenêtre ;
• un corps 34 de réception de câble coaxial, destiné à être soudé à la paroi du fourreau au niveau de la fenêtre d'entrée ;
• et une extrémité de câble coaxial 36, destinée à être vissée dans le corps de réception 34.

FIG. 3 represents a general view of the input assembly of the tube according to the invention. The view is represented partially broken down into three elements:

• the sheath 10 with a microwave vacuum transmission window 40, integrated inside the perimeter of the cylinder constituting the sheath so that no element of the window exceeds this perimeter; a magnet 18 or a pole piece 20 can thus be slipped on the sleeve passing over the window;
• a coaxial cable receiving body 34, intended to be welded to the wall of the sleeve at the level of the entrance window;
• and a coaxial cable end 36, intended to be screwed into the receiving body 34.

The window 40 is constituted by a dielectric plate with several levels of metallization, of which we see an upper metal track 42 intended to be electrically connected to the end of the core 38 of the coaxial cable 36. The window sealingly closes an opening breakthrough in the wall of the sleeve to the interior space of the tube. The window is brazed against the edges of this opening. Notches and / or flats 46 are machined on the outer wall of the sleeve to form planar surfaces on which fit corresponding flat surfaces of the body 34.

The receiving body 34 is for example welded by laser welding on the wall of the sheath 10, but this welding occurs only in a late stage of manufacture and more specifically, it intervenes only after steaming of the tube.

The dielectric plate 40 constituting the microwave window is actually preferably constituted by a superposition of two or more dielectric plates as will be explained later, with the interposition of metal layers between the dielectric plates.

FIG. 3 shows, at the end of the sheath, a smaller diameter sheath section on which an end pole piece, not shown, can be inserted serving as a support for the electron gun (also not shown) and serving as a magnetic shielding to prevent any influence of the magnets on the barrel. The end pole piece will be welded for example to the laser on the end of the sheath (after threading the magnets 18 and the pole pieces 20). Similarly, the electron gun will be welded for example laser on this support pole piece before emptying and steaming the tube, the magnets being in place. High temperature solders not acceptable for magnets will be avoided as well to fix the support part as to fix the barrel, and this in order not to damage the magnets in place.

Figure 4 and Figure 5, respectively in section and in plan view, show in more detail the constitution of the microwave window 40 recessed within the outer perimeter of the sheath. Figure 4 is a section along line A-A 'of Figure 5.

An opening 50 has been formed in the wall of the sheath 10 at the upstream end of the propeller 22; this opening communicates the inside 52 of the sheath (inner vacuum) with the outside. This opening 50 is sealingly closed by the microwave window 40. This comprises a metallized dielectric plate, with three metallization levels which are respectively an external metallization 42 for the arrival of the input signal, an internal metallization 58 for the signal transmission to the helix, and a central metallization 64 constituting a ground plane provided with a narrow slot-shaped local demetallization 62. The slot is a microwave coupling slot between the outer metallization and the inner metallization. In practice, the window consists of two dielectric plates 54 and 56 superimposed, both metallized, one of which can be metallized at least partially on both sides (the upper plate 56 preferably).

The external metallization, on the upper dielectric plate 56, constitutes the conductive track 42 already mentioned, intended for the arrival of the input signal. It preferably has an L shape, one branch (the one which is perpendicular to the longitudinal axis of the sleeve, will be connected to the core 38 of the coaxial cable 36 (Figure 3), and the other branch (the one which extends parallel to the sheath) crosses the demetallized slot 62 of the central metallization 64. The free branch length of the L remaining beyond the crossing with the slot is preferably equal to about a quarter of the wavelength of the central operating frequency of the tube, to maximize the coupling of energy through the slot 62.

The internal metallization 58, on the lower dielectric plate 54 and located on the inside of the tube, is connected to the conductive helix 22 by a conductor 60. This metallization 58 consists of a small zone located below the intersection between the upper track 42 and the central slot 62. The internal metallization 58 and the external metallization (42) therefore comprise parts located opposite one another on either side of the coupling slot 62. The microwave energy can Pass from the outer conductor to the inner conductor through the slot.

The set of two plates rests on a peripheral rim 66 of the wall of the sheath, this flange surrounding the opening 50; the dielectric plate is soldered to this rim in a vacuum-tight manner. Preferably one of the two plates protrudes with respect to the other, the wall of the peripheral rim having a corresponding recess, so that the central metallization 64 comes directly in contact with the wall of the sheath. This makes it possible to connect the central metallization to the ground potential represented by the sleeve, the shielding of the coaxial cable being also connected to ground via the receiving body 34.

It is clearly seen in Figure 4 that the window thus formed and soldered in the opening 50 does not extend beyond the circular perimeter of the sleeve and that magnets and pole pieces can be threaded onto the sleeve after brazing of the window.

• soit par une superposition de deux plaques diélectriques métallisées ; la métallisation centrale peut être déposée sur la surface inférieure de la plaque supérieure ; elle peut également être formée par une plaquette de métal découpée indépendante des plaques diélectriques et situées entre ces plaques.
• soit par une structure de céramique à couches métallisées sérigraphiées, superposées et cocuites.

The metallized dielectric plate constituting the entrance window may be constituted

• either by a superposition of two metallized dielectric plates; the central metallization can be deposited on the lower surface of the upper plate; it can also be formed by a cut metal plate independent of the dielectric plates and located between these plates.
• or by a metallized ceramic structure screen printed, superimposed and coked.

Figure 6 shows the complete connection between the helix and the coaxial cable to form the input structure of the tube.

The flat faces of the receiving body 34, are applied against the sleeve 10, and more precisely against the notches and flats machined in the wall of the sleeve (46 in Figure 3). The reception body 34 is welded, preferably by laser, on the sheath 10 (weld point 70 in FIG. 6). The coaxial cable 36 is screwed into the receiving body 34. The core 38 of the coaxial cable passes through the receiving body and faces the conductive pad 42; it is welded either directly to this beach or connected by a conductor 68 to this beach. The receiving body 34 is in electrical contact with both the outer casing of the coaxial cable 36, with the sheath 10, and with the central metallization 64 of the microwave window.

This input microwave tube structure is compact and allows the realization of very small tubes, useful for example for TOP working at very high frequencies (30 GHz for example). For example, the outer diameter of the sheath 10 is about 5 millimeters, and the inner diameter about 2 millimeters.

• brasure de la fenêtre dans l'ouverture 50 du fourreau (température d'au moins 780°C par exemple) ;
• mise en place de l'hélice dans le fourreau, et soudure de l'extrémité de l'hélice à la plage conductrice intérieure 58 ;
• enfilage des aimants monoblocs cylindriques sur le fourreau, alternés avec des pièces polaires également enfilées sur le fourreau ; cette opération peut intervenir avant mise en place de l'hélice ;
• soudure au laser du collecteur et du canon à électrons (ou de l'un des deux seulement si l'autre avait déjà été soudé ou brasé avant l'enfilage des aimants) ;
• étuvage du tube pendant plusieurs heures à une température de 500° C environ, puis fermeture de l'ouverture de pompage ;
• soudure au laser du corps de réception 34 du câble coaxial ; toutefois, cette soudure peut être faite avant l'étuvage, et plus précisément entre l'enfilage des aimants et l'étuvage ;
• soudure de l'âme du câble coaxial sur le conducteur d'entrée 42 de la fenêtre d'entrée.

The manufacturing method of the tube may be as follows, the subassemblies such as the sleeve with its opening 50, the propeller, the electron gun, the collector, the pole pieces, etc., being considered as manufactured before the operations. that follow :

• brazing the window in the opening 50 of the sheath (temperature of at least 780 ° C for example);
• placing the helix in the sheath, and welding the end of the helix to the inner conductive pad 58;
• threading the cylindrical monoblock magnets on the sleeve, alternated with polar parts also threaded on the sheath; this operation can take place before setting up the propeller;
• laser welding of the collector and the electron gun (or only one of them if the other had already been soldered or soldered before the magnets were threaded);
• baking the tube for several hours at a temperature of about 500 ° C, then closing the pumping opening;
• laser welding of the receiving body 34 of the coaxial cable; however, this welding can be done before the parboiling, and more precisely between the threading of the magnets and the parboiling;
• welding the core of the coaxial cable to the input conductor 42 of the input window.

Claims (8)

1. Method for producing a vacuum electron tube, wherein the tube comprises on the one hand, an electron gun capable of emitting an electron beam in the axis of a cylindrical sleeve (10) and on the other hand, a series of permanent magnets (18) for focusing the beam, distributed around the sleeve between a signal input (24) to be amplified and a signal output (26), the said method being characterized in that annular magnets each made of a single piece are used, the said one-piece magnets are placed around the sleeve at the same time as the polar parts (20) between magnets, and the tube is pumped/stoved at high temperature in the presence of the magnets, in order to degasify the internal parts of the tube, the magnets being made from a material capable of recovering most of its magnetizing properties after a thermal cycle of pumping/stoving at high temperature.
2. Method according to Claim 1, characterized in that the high temperature is several hundred degrees, up to about 500°-C.
3. Method according to either of Claims 1 and 2, characterized in that the electron gun is secured by a weld at a temperature not exceeding the stoving temperature.
4. Vacuum hyperfrequency tube, comprising an electron gun capable of emitting an electron beam in the axis of a cylindrical metal sleeve (10), which extends between a hyperfrequency signal input (24) to be amplified and an amplified signal output (26), the said tube being characterized in that either the signal input or output or both comprise(s) a dielectric plate (40) forming a hyperfrequency energy transmission window, the said plate being embedded and soldered in an opening (50) arranged in the sleeve wall, the plate not extending beyond the outer perimeter of the cylindrical sleeve and closing the said opening in a vacuum-tight manner.
5. Tube according to Claim 4, characterized in that the one-piece cylindrical magnets (18) surround the sleeve.
6. Tube according to either of Claims 4 and 5, characterized in that the dielectric plate is a three-plate structure with two dielectric films (54, 56) and three metallization levels (42, 58, 64) separated by these two dielectric films.
7. Tube according to Claim 6, characterized in that the metallizations comprise a first metallization level comprising an input conductor, a second intermediate level comprising a metallization connected to an electrical earth and a coupling slit (62) arranged in this metallization, and a third metallization level comprising an output conductor, the input conductor and the output conductor each comprising a part facing the coupling slit and the energy passing via the coupling slit, between the input conductor and the output conductor.
8. Tube according to Claim 7, characterized in that the window is a signal input window of the tube, the first metallization level being an external metallization of the tube and the third level being an internal metallization of the tube, connected to a structure for slowing down wave propagation, such as a propeller.

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