EP1680799B1 - Low spurious radiation microwave tube - Google Patents

Description

The invention relates to microwave tubes including klystrons or TOP (traveling wave tubes).

The document US 4,233,539 describes such a tube.

The figure 1 represents a simplified diagram of a microwave electron tube essentially comprising three main subassemblies an electron gun 12, a microwave structure 14 and a collector 16.

The electron gun 12 comprises a cathode 18 generating an electron beam 20 in the microwave structure 14 where the interaction occurs between the electron beam 20 and an electromagnetic wave created in the microwave structure. More precisely, the electron beam transfers part of its energy to the electromagnetic wave.

The collector 16 thermally dissipates the kinetic energy of the electrons of the beam 20 remaining after interaction with the electromagnetic wave.

The electrons emitted by the cathode are accelerated under a voltage Vo applied between cathode and the anode of the tube and are characterized by a current la.

The microwave structure is composed of resonant cavities and sliding tubes in the case of klystrons and a helix or coupled cavities in the case of a TOP.

The microwave structure of the TOP comprises an input window 22, on the side of the barrel of the tube, for injecting the power to be amplified Pe into the structure and an output window 24, on the side of the collector, to extract the amplified output power Ps. .

The gains G = 10 log10 (Ps / Pe) are of the order of 40 to 50 dB and the interaction efficiencies ηi = Ps / Vo lo generally between 30 and 60%. These input and output windows are dielectric devices, often alumina, which transmit, almost without losses, in the operating frequency band of the tube, the input microwave power Pe, inward of structure, and the output power Ps, to the outside of the structure, as appropriate, while the inside of the vacuum tube insulator (residual pressure ≤ 10 ^-7 Torr) to the outside atmosphere.

1 Torr equals 133 Pa.

Another essential subassembly of the tube is a magnetic circuit 40 (see figure 1 ) surrounding the microwave structure 14 comprising an electromagnet or permanent magnets associated with polar parts for driving the magnetic flux at the electron beam 20 which, thus, is focused that is to say maintained at a small diameter and substantially constant. This magnetic circuit is external to the vacuum chamber of the tube except sometimes some polar parts.

An ion pump 42, indicated on the figure 1 , serves to maintain the vacuum inside the tube; it is not always necessary.

The collector 16 is a hollow cylinder, as indicated in figure 1 . The electrons of the beam bombard the inner walls 44 of the collector 16, which heat up. The heat is then discharged through the outer walls of the collector which are cooled according to the power densities considered by forced air, by circulation of water, by radiation.

The collector is at the potential of the body of the structure 14 of the tube, that is to say of the mass, the cathode being at potential-Vo.

The collector 16 can be directly attached to the body 14 as indicated in FIG. figure 1 . The collector can also be electrically isolated from the body, but connected to the body by an external electrical connection.

The figure 2 shows a partial view of a TOP having a microwave structure 50 having coupled cavities 52 and a manifold 58 attached to the microwave structure 50 electrically isolated from the body of the tube, and in particular an upper pole piece 60, by a directory insulator 62 The electron beam 20, at the output of the microwave structure, enters the collector 58 through an opening 64. Electrons along different paths 66 are collected by the internal walls 68 of the collector.

It is often necessary to separately measure the current lb of the electrons that are intercepted by the microwave structure and the Icoll current of the electrons going to the collector. These two currents are of very different amplitudes, often an Ib / Icoll ratio of some% or even 1% or less.

To do this, the collector is isolated from the body by the insulator 62 for example ceramic, often alumina (see figure 2 ). The figures 3a and 3b schematically represent the electrical connections of different tube elements of the figure 1 , with the Al 70 power supplies. It is the body of the tube which, generally, is directly connected to the mass M or to the ground, and this for practical reasons, because it is connected naturally to the external installation by the guides. of input and output waves, often by the armature of the electromagnet, and sometimes by the tuning systems of the cavities, thermal probes. The hydraulic connections of the collector, when they exist, must then be sufficiently insulating to force the current Icoll not to borrow them as a return path to the + pole of the power supply, via the mass.

• isolement électrique entre corps (ou pièce polaire) 60 et collecteur 58 ;
• étanchéité et maintien du vide à l'intérieur du tube ;
• solidité mécanique pour maintenir fermement le collecteur sur le corps, malgré parfois certaines vibrations provenant du système de refroidissement et malgré les chocs pouvant provenir du transport et de l'installation.

The collector is isolated from the body by means of an annular ceramic 62 ( figure 2 ), or any other insulator, in general, which plays several important roles:

• electrical isolation between body (or pole piece) 60 and manifold 58;
• sealing and maintaining the vacuum inside the tube;
• mechanical strength to firmly hold the manifold on the body, despite some vibrations from the cooling system and despite shocks that may come from transport and installation.

But this 60-collector body isolation 58 appears, from the microwave point of view, as a true radial line, itself composed of several lines of different impedances Z1, Z2, ... Zi in series.

The figure 4 shows a detail view of the space Gd of connection between a body 80 and the collector 82 of a microwave tube. This space is presented as a series of impedance lines Z1, Z2, Z3 in series between the inside and the outside of the tube. The value of these impedances is related to the geometrical characteristics (h, d ...) of the lines and the presence or absence of ceramic insulator (εo, σ). We can refer to the book " Ramon's Field and Waves in Communications Electronics, Whinnery et al (Ed: John Wiley & Sons ) ".

As a result, if electromagnetic energy is present at the input Ecl of the collector, it can couple to this radial guide and radiate outwards (Pr).

The presence of electromagnetic energy at the manifold inlet may be due to leakage from the exit cavity (or propeller), although the sliding tube connecting it to the collector is at the cutoff at the operating frequency F and, generally, at 2.F. But this tube is often too short thus allowing transmission by evanescent mode.

This electromagnetic energy can also come from one of the many resonances of the collector excited at F, 2.F ... by the electron beam, still a little modulated.

In other words, the radial guide can bring back to the level of the electron beam a Zed impedance sufficient for the beam, still a little modulated, to yield microwave energy, at a low level, not negligible, which is then radiated outwards via the radial guide between body and collector.

However, the specifications often impose a very low level of microwave leakage, for example Pr <0.1 mw / cm ² at 10 cm from any outside surface of the tube.

The problem is therefore to minimize the parasitic radiated power Pr, which comes from the inlet of the collector via the body / collector isolation, similar to a radial guide.

In order to attenuate the parasitic radiation of the microwave tubes of the state of the art, the invention proposes a microwave tube comprising an electron gun generating an electron beam in a cylindrical microwave structure of the tube, the microwave structure providing at an output a microwave wave, an electron collector of the beam comprising at least one electrode being mechanically connected to the microwave structure by a dielectric, the mechanical connection forming a radial wave propagation guide parasitic microwave radiation of the tube, characterized in that that, in order to attenuate the spurious radiation of the tube, the radial guide comprises at least one quarter-wave microwave trap having, at least the operating frequency F of the tube, an open circuit for the microwave wave propagating in said guide radial propagation of parasitic radiation.

The idea is to implement "λ / 4 traps, at the radial guide appearing in the mechanical connection between the body of the tube containing the microwave structure and the collector. These guides are those used, for example, on the connecting flanges of the waveguides or in the mounting of antennas or detector crystals.

In a first embodiment of the microwave tube according to the invention, the radial guide comprises a microwave trap at the operating frequency F of the tube having a collinear cylindrical groove with the axis of revolution ZZ 'of the tube opening into said radial connecting guide of the tube. body with the manifold of the tube.

In a variant of this first embodiment of the microwave tube according to the invention, the radial guide comprises another microwave trap at the frequency 2.F having another collinear cylindrical groove with the axis of revolution ZZ 'of the tube opening into said radial guide. connecting the body with the tube collector.

There is another type of collector which is not only isolated from the body but also composed of several electrodes, each being brought to an intermediate potential between -Vo and mass. The potentials are then chosen so that the electrons are braked before their impact on the internal walls and that the heat dissipated power is as small as possible. After interaction, the dispersion of the speeds at the inlet of the collector is important: that is why there are several electrodes, each slowing the electrons occupying this or that part of the speed spectrum. This technique called "collectors depressed" (depressed collectors in English) is mainly applied to the TOP cooled by air or radiation. It allows a significant increase in efficiency by reducing the power dissipated, equal to Vo.lo without depressed collector, as we have seen previously.

The proposed invention applies to all types of collectors, in particular between the different electrodes of the "depressed" type collectors, comprising a plurality of mechanically connected electrodes, each connection between two consecutive electrodes forming a radial guide for propagating microwave radiation. parasites (Pr) of the tube, in addition to the microwave trap between the body and a first electrode, and to attenuate the spurious radiation of the tube, the radial guide between two consecutive electrodes comprises at least one microwave quarter-wave trap having, at least the operating frequency F of the tube, an open circuit for the microwave wave is propagating in said radial guide propagation parasitic radiation. But the following presentation will refer to a collector "not depressed", that is to say, standard, for the sake of simplification of the presentation.

• la figure 1, déjà décrite représente un schéma simplifié d'un tube électronique hyperfréquences ;
• la figure 2, déjà décrite, montre une vue partielle d'un TOP ;
• les figures 3a et 3b, déjà décrites, représentent les connexions d'alimentation électrique des différents éléments du tube de la figure 1 ;
• la figure 4, déjà décrite, montre une vue de détail de la zone de raccordement d'un tube hyperfréquences ;
• la figure 5a représente, une vue partielle simplifiée en coupe, de la zone de raccordement entre un corps et un collecteur d'un tube hyperfréquences ;
• la figure 5b montre une première réalisation du piège hyperfréquence d'un tube hyperfréquence selon l'invention ;
• la figure 5c montre une variante du tube hyperfréquences selon l'invention ;
• la figure 5d montre une autre variante du tube hyperfréquences selon l'invention ;
• les figures 6 et 7 montrent respectivement des vues partielles de la zone de raccordement entre le corps et le collecteur d'un tube de l'état de l'art sans piège, et d'un tube avec piège suivant l'invention ;
• la figure 8a, montre un montage de mesure de la puissance parasite rayonnée dans la zone de couplage entre le corps et le collecteur d'un tube selon l'invention ;
• la figure 8b montre une première mesure dans le cas d'un collecteur comportant deux rainures ;
• la figure 8c montre les mêmes mesures mais avec collecteur comportant une seule rainure.

The invention will be better understood using exemplary embodiments according to the invention, with reference to the indexed drawings in which:

• the figure 1 , already described, represents a simplified diagram of a microwave electron tube;
• the figure 2 , already described, shows a partial view of a TOP;
• the figures 3a and 3b , already described, represent the power supply connections of the various elements of the tube of the figure 1 ;
• the figure 4 , already described, shows a detailed view of the connection zone of a microwave tube;
• the figure 5a represents a simplified partial sectional view of the connection zone between a body and a collector of a microwave tube;
• the figure 5b shows a first embodiment of the microwave trap of a microwave tube according to the invention;
• the figure 5c shows a variant of the microwave tube according to the invention;
• the figure 5d shows another variant of the microwave tube according to the invention;
• the figures 6 and 7 show respectively partial views of the connection zone between the body and the collector of a tube of the state of the art without trap, and a trap tube according to the invention;
• the figure 8a shows a mounting for measuring the parasitic power radiated in the coupling zone between the body and the collector of a tube according to the invention;
• the figure 8b shows a first measurement in the case of a manifold having two grooves;
• the figure 8c shows the same measurements but with manifold with a single groove.

The figure 5a represents a simplified partial sectional view, along a plane passing through the axis ZZ 'of revolution of the microwave structure of the tube, of the connection zone between a body 90 and a collector 92 of a microwave tube.

The collector 92 is mechanically connected to the body of the tube containing the microwave structure by an insulator 94. The electron beam 20 at the output of the microwave structure penetrates, along the axis ZZ ', through an opening 95 in the collector and then dissipates thermally by striking the inner walls 96 of the collector (lines el).

The space Gd between the body 90 and the collector 92 behaves, as has been said previously, as a line or radial radial guide. This space is presented in the figure 5a as a toric shape volume of very small thickness between a face 100 of the body and a face 102 of the collector spaced apart by the insulator 94.

The figure 5b shows a first realization of a microwave trap of a microwave tube according to the invention.

These traps are machined or reported at the base, or better, machined in the base of the collector cylinder, the thickness of which, at this point, is often sufficient to receive one or more coaxial grooves.

• λg étant la longueur d'onde dans le guide radial (espace Gd),
• k étant un nombre nul ou entier ;
• c étant la vitesse de la lumière dans le milieu considéré, ici, le vide, de façon à créer une impédance infinie, au niveau de la rainure 104, donc, une violente désadaptation qui réfléchit, en grande partie, la puissance hyperfréquence venant de la ligne radiale à la fréquence F.

The collector 92 has a groove 104 of circular shape around the axis ZZ 'of rectangular section and depth equal to λ / 4, the groove opening on one side, in the radial guide (Gd space of the figure 5a ), λ = c / F being the wavelength in the groove coaxial with the operating frequency F of the tube, the groove being at a distance d1 from the point where the radial guide opens on the side of the internal opening 95 of the collector 92, such that: $$\mathrm{d}\mathrm{1}\mathrm{=}\left(\mathrm{.lamda.g}\mathrm{/}\mathrm{4}\mathrm{+}\mathrm{k\; \lambda g}\mathrm{/}\mathrm{2}\right)$$

• λg being the wavelength in the radial guide (space Gd),
• k being a zero or integer number;
• c is the speed of light in the medium considered, here, the vacuum, so as to create an infinite impedance, at the groove 104, therefore, a violent mismatch that reflects, in large part, the microwave power from the radial line at frequency F.

The transmitted power, thus radiated Pr towards the outside of the tube, through the insulator 94 then becomes very small.

The wavelength λg in the radial guide depends on the portion of the guide considered, and in particular, the radial abscissa r with respect to the axis ZZ 'of the tube.

But note that the widths of the guides represented respectively by the width Ed of the groove, (distance ab on the figure 5b ) and the thickness Eg of the radial guide (distance bc) are infinitely small in front of the lengths of these same guides: the position of the open circuit "brought back" (infinite impedance) is then poorly defined, and the electromagnetic waves can then cross partially the trap thanks to the local presence of higher order modes. Therefore, the Ed and Eg widths should be as small as possible to have the best possible blocking of the parasitic radiated power

The electron beam is modulated not only at the operating frequency F of the tube but also, to a lesser extent, at 2.F and beyond, it being understood that at 3.F, 4.F ... this modulation is completely negligible.

avec k' entier,

• λ'g étant la longueur d'onde dans le guide radial (espace Gd) à la fréquence 2.F (voir figure 5c).

The figure 5c shows a variant of the tube according to the invention. In this variant of the tube, the collector 92 has a second groove 108, like the first 104, of circular shape around the axis ZZ 'of rectangular section and of depth equal to λ / 8 opening in the same way a side of the groove in the radial guide (space Gd of the figure 5a ), the second groove 108 being at a distance d2 from the point where the radial guide opens out on the side of the internal opening 95 of the collector 92 such that: $$\mathrm{d}2\mathrm{=}\left(\mathrm{\lambda \text{'}g}\mathrm{/}\mathrm{4}\mathrm{+}\mathrm{k\; \text{'}}\mathrm{.}\mathrm{\lambda \text{'}g}\mathrm{/}\mathrm{2}\right),$$

with k 'whole,

• λ'g being the wavelength in the radial guide (space Gd) at the frequency 2.F (see figure 5c ).

Thus, any power at 2.F frequency will also be blocked and can not radiate outside the tube.

We can notice that the radial line between the "open circuit at the groove 104" bc "and its opening" of "at the entrance 95 of the collector 92 is the seat of standing waves, all the more intense as the Zed coupling impedance between the body and the collector (see figure 5a ) is close to the internal impedance of the microwave generator equivalent to the modulated beam at the input of the collector.

• M représentant le couplage faisceau/guide radial,
• If (F) la composante du courant faisceau à la fréquence F ;
• Zed, l'impédance à l'entrée de la ligne radiale est induite à l'entrée de la ligne radiale et du fait de la réflexion presque totale par le circuit ouvert en « bc ». Cette portion de la ligne radiale est le siège d'ondes stationnaires.

In other words, a voltage Ved = Zed.M.if (F), with:

• M representing the coupling beam / radial guide,
• If (F) the component of the beam current at the frequency F;
• Zed, the impedance at the input of the radial line is induced at the entrance of the radial line and because of the almost total reflection by the open circuit in "bc". This portion of the radial line is the seat of standing waves.

In some places important fields may appear, with risk of breakdown or multifactor phenomenon always very noisy.

In addition, the voltage Ved may be such that it reflects electrons to the microwave structure then producing parasitic modulations and oscillations.

The solution giving rise to the embodiments, according to the invention, described above is then that the guide, present at its input in "ed" a zero impedance or very low value (Ved # o).

This justifies the value of the distance d1, already indicated above, between the first groove 104 of the trap and the inlet "e" of the guide at the opening 95 of the manifold. This length d1 or "ce" in the figure 5b is such that the open circuit at the groove 104 in "cb" is reduced to the level of the inlet of the guide, in "de" in a short circuit.

• λg/4 (ou λg/4 + k.λg/2), k étant nul ou entier) avec λg, la longueur d'onde dans le guide radial, qui varie suivant le rayon r considéré, λg (r). Les calculs analytiques de λg sont très complexes et les ajustements de longueur et de façon générale les dimensions du piège se font par simulation expérimentale et par ordinateur.

Remember that the length "this" is therefore:

• λg / 4 (or λg / 4 + k.λg / 2), where k is zero or integer) with λg, the wavelength in the radial guide, which varies according to the radius r considered, λg (r). The analytical calculations of λg are very complex and the adjustments of length and in general the dimensions of the trap are done by experimental simulation and by computer.

avec k' entier et λ'g longueur d'onde dans le guide radial à la fréquence 2.F.Following the same reasoning transposed to the frequency 2.F, the second groove 108 will be placed at a location "c" of the guide, such as the distance "c'e" (ie d2, see figure 5c ) between the position "c" of this second groove 108 in the radial guide and the entry "e" of the guide is: $$\mathrm{Length}=\mathrm{\lambda \text{'}g}/4\mathrm{or\; \lambda \text{'}g}/4+\mathrm{k\; \text{'}}\mathrm{.}\mathrm{\lambda \text{'}g}/2,$$

with k 'integer and λ'g wavelength in the radial guide at frequency 2.F.

In summary, the base of the collector 92 is machined, so as to create one or more "quarter-wave" grooves or traps that bring fictitious open circuits across the radial guide formed by the isolation body 90 collector 92. These circuits fictional openers prevent much of the power to pass from the inside of the tube to the outside and therefore blocks any parasitic radiation.

In addition, the positions of these traps are chosen so that the impedance reduced to "ed", at the entrance of the radial guide, is zero at the frequencies considered, generally the operating frequency F of the tube and 2.F , (distance ce = λg / 4 or λg / 4 + k.λg / 2 with k integer and λg wavelength of radial guide at frequency F and similarly at 2.F with 7λ'g length of wave in the radial guide at the frequency 2.F).

The figures 6 and 7 show respectively partial views of the connection zone between the body 110 and the collector 112 of a tube without microwave traps and the same tube connection zone made according to the invention comprising two traps having two grooves 114, 116 respectively for the frequencies F and 2.F.

The length of the grooves is λ / 4 with λ = c / F, or λ / 8, c being the speed of light in the medium in question, that is to say that of the groove. This is usually vacuum, but the grooves can also be filled with reduced dielectric constant dielectric, εr (> 1). In this case, λ, as well as the length of the grooves, is reduced in the ratio of the square root of εr with respect to the case where the grooves are under vacuum. It is then possible to envisage a reduction in the length of the grooves in a ratio of about three, if one fills the cis-length of alumina (εr = 9).

On the other hand, in another variant of the microwave tube according to the invention, shown in FIG. figure 5d , we can place the insulation 62 of the figure 2 or the insulation 94 of the figure 5b , ie the insulation connecting the body to the collector (or connecting two electrodes of an isolated collector), closer to the axis ZZ ', so that one or more grooves are no longer under empty, as in the case of figure 5b but in the air.
However, the dielectric constant of the air being practically that of the vacuum, this arrangement does not change the invention, but is a technological variant.

The figure 8a , shows a mounting for measuring the radiated parasitic power in the connection zone between the body and the collector of a tube according to the invention. The assembly comprises a body 120 and a collector 122 separated by an insulator 124. The collector comprises a first groove 126 for the operating frequency F of the tube and a second groove 128 for the frequency 2.F, the grooves being coaxial with the axis ZZ 'of the tube.

In the measuring assembly of the figure 8a , the operating frequency is F = 4900 MHz, the inner diameters of the body 120 and the collector 122 have a diameter D of 33 mm. The distance Dcc separating the body from the collector is 5 mm.

The positions and dimensions of the grooves are as follows: First groove 116: diameter D1 = 105 mm depth P1 = 15.3 mm. second groove 116: diameter D2 = 63.7 mm. depth P2 = 7.65 mm. D1 and D2 around the axis ZZ '.

A microwave signal Pe is injected by an emitter 130 at the axis ZZ 'of the tube, into the collector body coupling zone, a probe 132 is placed outside the tube at the connection zone to measure the radiated parasitic power Pr.

The Figures 8b and 8c show attenuation curves Att. according to the measurement frequency Fm, between the signal injected Ue by the transmitter in the measuring circuit of the figure 8a and the parasitized signal Pr radiated by the tube picked up by a probe 132. Let Att = Pr / Pe.

• 35 dB à la fréquence F
• 25 dB à la fréquence 2.F

The figure 8b shows a first curve in the case of a tube having a manifold having two grooves 126, 128, one for the frequency F and the other for the frequency 2.F. There is an attenuation between the power injected by the transmitter 130 and the parasitic power sensed by the probe 132 of approximately:

• 35 dB at frequency F
• 25 dB at frequency 2.F

The figure 8c shows the same measurements with the same tube the figure 8a tube, the manifold having a single groove 126 for trapping the frequency F.

An attenuation of approximately -35dB is still observed at frequency F but no attenuation at frequency 2.F.

The invention, in addition to the significant attenuation of parasitic radiation, has the advantage of easy disassembly of the body collector of the tube, which is not the case of the embodiments of the tubes of the state of the art using insulating resins to mechanically fasten the collector to the body of the tube at the output of the microwave structure.

Claims (9)

1. Microwave tube comprising an electron gun (12) generating an electron beam (20) in a cylindrical microwave structure (14, 50) of the tube, the microwave structure delivering a microwave at one output, a collector (16, 58, 82, 92) for collecting electrons from the beam comprising at least one electrode that is mechanically coupled to the microwave structure via a dielectric (62, 94), the mechanical coupling forming a radial waveguide for propagating spurious microwave radiation (Pr) from the tube, characterized in that, in order to attenuate the spurious radiation from the tube, the radial waveguide includes at least one quarter-wave microwave trap having, at least at the operating frequency F of the tube, an open circuit for the microwave propagating in said radial waveguide for propagating spurious radiation.
2. Microwave tube according to Claim 1, characterized in that it includes a microwave trap at the operating frequency F of the tube, having a cylindrical slot (104, 114) collinear with the axis of revolution ZZ' of the tube and emerging in said radial waveguide for coupling the body (90) to the collector (92) of the tube.
3. Microwave tube according to Claim 2, characterized in that it includes another microwave trap at a frequency 2F, having another cylindrical slot (108, 116) collinear with the axis of revolution ZZ' of the tube and emerging in the radial waveguide for coupling the body to the collector of the tube.
4. Microwave tube according to either of Claims 1 or 2, characterized in that the collector (96) includes a circular slot (104, 114) around the ZZ' axis with a rectangular cross section and a depth equal to λ/4, the slot emerging via one side in the radial waveguide (Wg), λ = c/F being the wavelength at the operating frequency F of the tube, the slot being at a distance d1 from the point where the radial waveguide emerges on the same side as the internal opening (95) of the collector (92), such that: $$\mathrm{d}\mathrm{1}\mathrm{=}\left(\mathrm{\lambda g}\mathrm{/}\mathrm{4}\mathrm{+}\mathrm{k\lambda g}\mathrm{/}\mathrm{2}\right)$$

   

   - λg being the wavelength in the radial waveguide;

   - k being zero or an integer; and

   - c being the velocity of light in the medium in question.
5. Microwave tube according to Claim 2, characterized in that the collector (92) includes a second slot (108, 116) of circular shape around the ZZ' axis, having a rectangular cross section and a depth equal to λ/8, said second slot emerging alongside the slot in the radial waveguide, the second slot being at a distance d2 from the point where the radial waveguide emerges on the same side as the internal opening (95) of the collector (92) such that: $$\mathrm{d}2\mathrm{=}\left(\mathrm{\lambda ʹg}\mathrm{/}\mathrm{4}\mathrm{+}\mathrm{kʹ\lambda ʹg}\mathrm{/}\mathrm{2}\right),$$

   

   with k' being an integer and λ'g being the wavelength in the radial waveguide (Wg) at the frequency 2F.
6. Microwave tube according to one of Claims 2 to 5, characterized in that the waveguide has, at its input at "ed", a zero impedance or an impedance of very low value (Ved ≈ 0).
7. Microwave tube according to one of Claims 1 to 6, characterized in that the collector is of the "depressed collector" type comprising several mechanically coupled electrodes, each coupling between two consecutive electrodes forming a radial waveguide for propagating spurious microwave radiation (Pr) from the tube, characterized in that, in order to attenuate the spurious radiation from the tube, the radial waveguide between two consecutive electrodes includes at least one quarter-wave microwave trap having, at least at the operating frequency F of the tube, an open circuit for the microwave propagating in said radial waveguide for propagating spurious radiation.
8. Microwave tube according to one of Claims 2 to 7, characterized in that the slots are filled with dielectric, of low dielectric constant, ε_r(> 1), λ, as well as the length of the slots, being reduced in the ratio of the square root of ε_r relative to the case in which the slots are in a vacuum.
9. Microwave tube according to one of Claims 2 to 6, characterized in that one or more slots are in air.

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