EP0413018B1 - Hyperfrequency wave generator device with virtual cathode - Google Patents

Abstract

The invention relates to a hyperfrequency wave generator which uses an electron beam and the phenomenon of oscillating virtual cathode, but which allows to obtain a spectral quality energy and a conversion yield which are improved with respect to conventional vircator generators. This is achieved by using separately the electrons issued from the virtual cathode (80), that is to say transmitted (82) or reflected (81), in order to transform their kinetic energy into hyperfrequency energy (4).

Description

The present invention relates to a microwave generator device using the virtual cathode phenomenon.

To generate microwave waves, it is known in particular to use devices called vircators, as described for example in document US-A-4,730,170, which take advantage of the space charge effects existing in beams of electrons produced by the barrel of an electron tube. Indeed, as is known, it is these effects which fix, for given voltages, a maximum value of the current which can be produced by an electron gun, or which can be transported in a given space for a set electrodes of given geometry. In a vircator, a current of electrons is injected into a defined space, most often several times the maximum current that could actually cross this space. There is then an accumulation of electrons which form a potential well, called a virtual cathode, and which causes the reflection of a more or less large fraction of the electrons in the beam. This virtual cathode is unstable, that is to say that the amplitude of its potential trough and its position oscillate, causing a periodic variation in the number of electrons reflected or transmitted. Such a device makes it possible to create electromagnetic fields with high microwave powers and under a reduced volume. However, it can be seen that the signal transmitted is of poor quality, that is to say that the power is transmitted in numerous modes in a series of simultaneous or successive frequencies. The document 1983 IEEE International Conference on Plasma Science, 23-25 May 1983, San Diego, California, IEEE Conference Record - Abstracts, IEEE, (New-York, US), TJT Kwan et al. : "microwave generation by virtual cathodes and reflexing systems", page 40, 2D6 summary, reports a study by simulation of the emitted signals which shows that they are emitted on two distinct frequencies, one which corresponds to the oscillation of the cathode the other, to the electrons reflected between the real cathode and the virtual cathode. The applications of this type of signal are therefore quite reduced. Furthermore, the conversion efficiency is poor (of the order of 2 to 3% at best) compared to the efficiency which it is possible to obtain with other generators, such as conventional speed modulation electronic tubes. .

The present invention relates to a microwave generator which uses the phenomenon of oscillating virtual cathode but which makes it possible to obtain microwave energy of better spectral quality and with a better conversion efficiency than conventional vircators.

More specifically, the subject of the invention is a device generating microwave waves as defined by claim 1.

• la figure 1, un mode de réalisation d'un dispositif générateur selon l'art antérieur, dans lequel le circuit hyperfréquence de sortie utilise les électrons transmis par la cathode virtuelle ;
• la figure 2, le dispositif de la figure précédente, dans lequel le circuit hyperfréquence de sortie assure en outre une post-accélération des électrons utilisés ;
• la figure 3, un autre mode de réalisation du dispositif selon l'invention, dans lequel le circuit hyperfréquence de sortie utilise d'une part les électrons transmis par la cathode virtuelle et, d'autre part, les électrons réfléchis par cette cathode virtuelle mais convenablement déphasés ;
• les figures 4, 5 et 6 représentent des variantes des modes de réalisation précédents dans lesquelles le faisceau électronique présente une section différente.

Other objects, features and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures, which represent:

• Figure 1, an embodiment of a generator device according to the prior art, in which the circuit output microwave uses the electrons transmitted by the virtual cathode;
• Figure 2, the device of the previous figure, wherein the output microwave circuit further provides post-acceleration of the electrons used;
• FIG. 3, another embodiment of the device according to the invention, in which the output microwave circuit uses on the one hand the electrons transmitted by the virtual cathode and, on the other hand, the electrons reflected by this virtual cathode but suitably out of phase;
• Figures 4, 5 and 6 show variants of the previous embodiments in which the electron beam has a different section.

In these different figures, the same references relate to the same elements.

FIG. 1 therefore represents a first embodiment of a device according to the prior art, seen in longitudinal schematic section.

This generator is a structure of revolution around a longitudinal axis ZZ.

It comprises an electron gun 1, formed by a cathode 11 and an anode, composed of a frame 20 and a screen 21. The cathode 11 is in the form of a conductive cylinder with axis ZZ the circumference of which projects 10, so that the electrons emitted by this cathode form an annular beam, represented by a dotted area 8 in the figure. The armature 20 of the anode consists of a hollow cylinder, of the same axis ZZ as the cathode; it is closed by an annular shoulder 23 and a screen 21 in the form of a disc, leaving an annular slot 22 for the passage of the electron beam. The screen 21 is for example fixed by three tabs (not shown) on the shoulder 23.

The known generator also includes an output microwave circuit 4 which is, in this embodiment, of the coaxial type; it is formed by an inner conductive cylinder 5 and an outer conductor constituted by the extension of the armature 20, between which an annular space 44 is defined. The output circuit is substantially symmetrical of the electron gun 1 with respect to a normal plane in the plane of the figure, that is to say that the external conductor has an annular shoulder 43 and a screen 41 bearing for example by legs on the shoulder 43 and defining with this shoulder a circular slot 42 for passage electrons from the beam 8. The latter is received by an annular projection 50 of the inner conductor 5. More generally, the drawings of the output circuit 4 and of the barrel 1 are such that the two impedances are close.

Between the elements 21, 23 on the one hand and 41, 43 on the other hand, there is a zone 3 called the injection region. This zone is bounded laterally by the wall 20.

The operation of this device is as follows.

The application to the cathode 11 of a negative voltage with respect to that of the anode causes the emission of the annular electron beam 8. For example, the armature 20, the screen 21 and the elements of the output circuit 4 are at ground potential and a voltage -V₀ is applied to the cathode 11. The parameters are chosen so that a virtual cathode 80 is formed in the injection region 3. The electrons transmitted by the virtual cathode 80 are represented by an arrow 82 and the electrons reflected by this cathode by arrows 81 Virtual. In addition, a longitudinal magnetic field (along the ZZ axis) is preferably applied to the structure, using means not shown, to focus the beam 8 thus produced.

The mechanism for forming a virtual cathode is recalled below. Inside an electron beam there is a charge of space: on the axis of the beam, the potential and the speed of the electrons are smaller than at the periphery of this beam. If the density of electrons and, consequently, the current transport increase, the potential and the speed of the electrons decrease until reaching zero: the electrons then form a negatively charged cluster, forming a well of potential called virtual cathode. This virtual cathode oscillates and the frequency of the oscillations depends in particular on the injection current; it is commonly measured in Gigahertz. Furthermore, the maximum current intensity beyond which the electrons form a virtual cathode is a function of the potential of the electron beam, as well as of the dimensions of the beam and of the injection region 3: the maximum current for a given electron beam is weaker when the injection area is of larger diameter.

According to the prior art, the dimensions of the device (electron gun and injection zone) and the current of the electron beam are chosen so that it is greater than the maximum current likely to flow through region 3, thus causing the formation of a virtual cathode. In this way, the electrons transmitted represent a current modulated at the frequency of oscillation of the virtual cathode. The electrons transmitted, and only they, see their kinetic energy converted into an electromagnetic field by the output circuit 4 and, more precisely, in the braking space between the conductor 5 and the screen 41. The energy produced is transmitted by the coaxial output circuit 4 to the outside.

It appears that the energy thus produced is with a yield much higher than that of conventional vircators. Indeed, the Applicant's research has shown that one of the reasons for the low efficiency of conventional vircators was the fact of using a coupling circuit which imposes an electromagnetic field of phase substantially equal to all the electrons, both transmitted and reflected. by the virtual cathode; Now these two kinds of electrons are substantially in phase opposition and the energies they create largely cancel each other out. According to the invention, the energy of the transmitted and reflected electrons is therefore used separately. In the prior art, only the transmitted electrons are used.

In addition, the fact of using, according to the invention, separately the transmitted and reflected electrons has the effect of allowing the realization of a closer coupling between electrons, and output circuit and, consequently, obtaining electromagnetic energy of better spectral quality.

An alternative embodiment (not shown) consists in placing the output circuit 4 so that only the electrons reflected by the virtual cathode are used.

It should also be noted that the dimensions of the barrel and of the injection region are preferably chosen so that the beam current is greater than, but close to the maximum current, so that the transmitted current is on average a fraction. significant of the total current injected into the injection region.

2 shows the device of Figure 1, which further comprises means for post-acceleration of the electrons used, also seen in longitudinal schematic section.

• à l'une de ses extrémités à la paroi 40 (potentiel de masse) ;
• à son autre extrémité à la cathode 11 (potentiel -V₁) ;
• en un point intermédiaire à l'anode 20, point tel que le potentiel y soit égal à -V₁ + V₀.

By way of example, the generator represented in FIG. 2 takes up the structure of that of FIG. 1, except that the output circuit 4 is electrically isolated from the electron gun 1. More precisely, the armature 20 forming the anode of the electron gun is without electrical contact with the external conductor, now marked 40, of the output circuit 4. By way of example, the conductor 40 extends around the armature 20 in the form of a hollow cylinder with the same axis ZZ as this frame. This embodiment further comprises means 7 for applying between the cathode 11 and the output circuit 4 a voltage V₁, greater than the cathode / anode voltage V₀. By way of example, the means 7 are constituted by a transformer whose primary 71 receives the supply voltage and the secondary 72 is connected:

• at one of its ends to the wall 40 (ground potential);
• at its other end to the cathode 11 (potential -V₁);
• at an intermediate point at anode 20, point such that the potential there is equal to -V₁ + V₀.

It should be noted that, as is known, so that the formation of a virtual cathode remains possible when the voltage V₁ used is greater than the voltage V₀ of the previous embodiment, it is necessary to increase the length of the injection region 3 and this all the more that the V₁ / V₀ ratio chosen is higher.

FIG. 3 represents a first embodiment of the generator according to the invention, in which both the transmitted electrons and the electrons reflected by the virtual cathode are used.

In this figure, we find the electron gun 1 formed by the cathode 11 and the anode 20, 21. The gun 1 produces, here too, an electron beam 8 under conditions such that there is formation of a virtual cathode 80 with reflection (arrows 81) of a part of the electrons and transmission (arrow 82) of another part of the electrons towards, for example, a metal wall 50 delimiting the injection region 3.

In this embodiment, the output microwave circuit 4 has two channels: one leads into a region marked 4A, between the anode 20 and the virtual cathode 80 and intended to recover the energy of the reflected electrons 81; the other opens into a region marked 4B, between the virtual cathode 80 and the wall 50 and it is intended to recover the energy of transmitted electrons 82. The electrons 81 reflected by the virtual cathode being with an average offset in time in the region of half a period oscillations of this virtual cathode with respect to the transmitted electrons 82, it is necessary, in order to combine their effects, to phase the energy produced by the ones by a value substantially equal to 180 ° with respect to the others; this is shown diagrammatically by a phase shifter 45, which can be produced by any known means and connected on one of the channels, 4A or 4B, before the energies existing in the two channels combine to form the output energy.

It should be noted that the wall 46, between the channels 4A and 4B, must be of sufficient thickness to prevent the fields present in the two channels from coupling before the virtual cathode 80, this thickness being of the order of magnitude of the distance from wall 46 to the virtual cathode.

FIG. 3 shows a particular embodiment of the circuit 4. Other variants are of course possible, which consist, for example, of producing, for each of the channels 4A and 4B, a structure of coaxial type as described Figure 1 for circuit 4.

FIG. 4 represents another embodiment of the device according to the invention, in which the beam produced by the electron gun is a solid cylinder, always seen in schematic longitudinal section.

In this figure, by way of example, there is a structure similar to that of FIG. 1, except that the emissive surface of the cathode, now marked 12, of the barrel 1 is in the form of a disc, so as to emit a solid cylindrical electron beam 88. In the same way, the internal conductor of the output circuit 4, now marked 51, is constituted by a flat surface in the form of a disc. The screens 21 and 41 of FIG. 1 have been replaced here by elements, marked 26 and 46, constituted by grids or metallic sheets sufficiently thin for their absorption of electrons to be very low.

The operation of this device is analogous to what has been described for FIG. 1, with the formation of a virtual cathode 83, reflected electrons 84 and transmitted electrons 85 whose kinetic energy is converted into microwave energy by the output circuit 4 .

It should be noted that, for satisfactory operation to be obtained, the diameter of the cathode 12 must be substantially less than the wavelength of the microwave energy obtained at the output, for example of the order of half -wave length. In practice, however, cathodes of larger diameter are usable, since the electrons tend to group around the periphery of the virtual cathode.

FIG. 5 represents another embodiment of the generator according to the invention, in which the electron beam used is a solid cylindrical beam and in which the generator further comprises post-acceleration means.

In this figure, there is a structure similar to that of Figure 2, except as regards the cathode 11 of the barrel 1, the central conductor 5 of the output circuit 4 and the screens 21 and 41, replaced respectively by the elements 12 , 51, 26 and 46 as described in Figure 4.

The same remarks as those made with respect to Figure 4 can be made here.

Likewise, FIG. 6 represents an embodiment similar to that of FIG. 3, but in which the annular electron beam is replaced by a solid cylindrical electron beam.

We therefore find a structure similar to that of FIG. 3, except as regards the structure of the cathode 11, now marked 12, and the electron beam 8 which becomes a full cylinder marked 88, as in the case of FIGS. 4 and 5.

Claims (6)

1. UHF-wave generator device, including:

   - an electron gun (1), capable of producing a beam of electrons (8; 88) in an injection region (3), the current transported being sufficient, having regard to the dimensions of the electron gun and of the injection region, to cause the formation of a virtual cathode (80; 83) in the injection region;

   - a UHF output circuit (4), performing the conversion of the kinetic energy of the electrons into UHF energy;

   the circuit being characterized in that it includes a first channel (4A), receiving the reflected electrons (81; 84) and a second channel (4B), receiving the transmitted electrons (82; 85), and a phase shifter (45) phase shifting the energy produced by one of the channels by substantially 180°.
2. Device according to Claim 1, characterized in that the output circuit (4) is of the coaxial type.
3. Device according to one of Claims 1 or 2, characterized in that the output circuit (4) is electrically isolated from the electron gun (1) and that an electron acceleration voltage (V₁) is applied between gun and output circuit.
4. Device according to one of the preceding claims, characterized in that the beam of electrons (8) is in the form of a hollow cylinder.
5. Device according to one of the preceding claims, characterized in that the beam of electrons (88) is in the form of a solid cylinder.
6. Device according to one of the preceding claims, characterized in that it further includes means for applying a magnetic field for focusing the beam of electrons.

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