EP3087580B1 - Microwave wave generator device with oscillating virtual cathode, with axial geometry, comprising at least one reflector and a magnetic ring, which is configured to be powered by a high-impedance generator - Google Patents

Description

The present invention relates to an oscillating virtual cathode microwave wave generating device (often referred to as VIRCATOR, derived from the English expression "VIRtual CAthode oscillaTOR"). The article of STEPHANIE CHAMPEAUX ET AL: "VACUUM ELECTRONICS CONFERENCE (IVEC), 2013 IEEE 14TH INTERNATIONAL, IEEE, May 21, 2013 (2013-05-21), pages 1-2, XP032445325, DOI: 10.1109 / IVEC.2013.6571105ISBN: 978-1-4673-5976-4 describes such a VIRCATOR.

An oscillating virtual cathode wave generator device of the prior art, or VIRCATOR, is shown schematically on the figure 1 .

The VIRCATOR comprises a diode consisting of a cathode 2 and an anode 3 + 4, emitting an electron beam 1, and a cylindrical waveguide 5. The anode consists of a thick armature 3 and a thin sheet 4 (often referred to hereafter as "thin anode 4" for simplification). The term "thin" is used here to mean that the sheet of the anode 4 has a thickness of the order of a micrometer, that is to say of a few microns or even a few tenths of a micrometer. The thin sheet 4 is coupled to the cylindrical waveguide 5. In other words, the thin anode 4 separates the cathode 2 from the cylindrical waveguide 5 by being located at an input of the waveguide 5, at an interface between the thick frame 3 and the waveguide 5; and the thick frame 3 generally surrounds the cathode 2.

This type of device is known to produce microwave pulses of high power.

For this purpose, a potential difference is applied across the terminals of the diode 2 + 3 + 4 creating an electronic emission at the cathode 2. When the emitted electronic current density exceeds the limit current density of Child-Langmuir, the electron beam 1 bursts under the effect of its own space charge. At the level of the thin sheet 4 of the anode, the transverse components of the electric field, with respect to an axis z representing a longitudinal axis of the cylindrical waveguide 5, cancel each other out. The electron beam 1 then begins to pinch under the effect of its magnetic field. When the current entering the cylindrical waveguide 5 exceeds the space charge limiting current (called the "critical" current, noted I _c ), the electron density becomes so strong that the beam can no longer propagate in the waveguide 5. A charge accumulation 6, commonly called "virtual cathode 6", is then formed beyond the thin sheet 4. The virtual cathode 6 then deviates many electrons until some return to the cathode 2, through the thin sheet 4.

avec γ = 1 + qV/mc², où q est la charge d'un électron, V la différence de potentiel appliquée entre les électrodes de la diode 2+3+4, m la masse d'un électron au repos, c la vitesse de la lumière, et ε₀ la permittivité du vide.In relativistic regime, an estimate of the critical current I _c is given by: $${\mathrm{I}}_{\mathrm{vs}}=\frac{4\pi {\epsilon }_0{\mathrm{mc}}^3}{\mathrm{<figure-callout\; id="2"\; label="q\; \gamma "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">q\; </figure-callout>}}\frac{{\left({\gamma }^{}\right)}^{}}{}\frac{{\left({}^{2/3}-1\right)}^{3/2}}{1+2\ln \left(\frac{{\mathrm{R}}_{\mathrm{BOY\; WUT}}}{\mathrm{r}}\right)}$$

with γ = 1 + qV / mc ² , where q is the charge of an electron, V the potential difference applied between the electrodes of diode 2 + 3 + 4, m the mass of an electron at rest, c speed of light, and ε ₀ the permittivity of the vacuum.

Given the bursting of the emission beam in the diode, the radius of the beam r entering the waveguide is of the order of the radius of the cylindrical waveguide R _G. An order of magnitude of the critical current I _c (in kilo-amperes) is then given by the following simplified expression: $${\mathrm{I}}_{\mathrm{vs}}\approx 17{\left({\mathrm{<figure-callout\; id="2"\; label="\gamma "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\gamma </figure-callout>}}^{2/3}-1\right)}^{3/2}$$

While approaching the thin anode 4, the virtual cathode 6 increases its charge density until it explodes under the effect of its own space charge and a new virtual cathode is reconstituted a little further in the waveguide 5. It is this principle of oscillation of the virtual cathode which is at the origin of an emission of a microwave wave 7.

The figure 1 represents a formation of an oscillating virtual cathode in a VIRCATOR-type device of the prior art when the beam current exceeds the critical current in the waveguide 5. The figure 2 represents the characteristic "diamond" signature of the oscillating virtual cathode 6 in the phase space with the acceleration and deceleration of the electrons at the passage of the thin anode 4 in their path of the cathode 2 to the virtual cathode 6 and vice versa, that is to say the amount of movement in the longitudinal direction and depending on the longitudinal position.

The virtual cathode 6 moves around an average position which is at a distance from the thin anode 4 approximately equal to that which separates the thin anode 4 from the emitting cathode 2 (the latter distance being denoted by d _AK ) . The electrons which are returned by the virtual cathode 6 to the cathode 2 while passing through the thin anode 4 are modulated at the frequency of the microwave wave 7 and interact with the electron beam 1 created in the space between the cathode 2 and the thin anode 4 by modulating it slightly. These backscattered electrons are braked between the thin anode 4 and the cathode 2. They are also diverted mainly towards the reinforcement of the anode 3.

At the same time, the electrons which cross the virtual cathode 6 take up energy from the microwave wave 7 which propagates in the waveguide 5, thus decreasing its intensity.

• La fréquence f de l'onde microonde 7 émise (exprimée en GHz) est fonction de la distance d_AK (exprimée en cm) qui sépare la cathode 2 de l'anode mince 4, et du facteur relativiste γ des électrons au niveau de l'anode mince 4 en relation avec la différence de potentiel appliquée à la diode 2+3+4. Cette fréquence peut être estimée par la formule suivante : $$\mathrm{f}=\frac{\mathrm{4,77}}{{\mathrm{d}}_{\mathrm{AK}}}\log \left(\gamma +\sqrt{{\mathrm{<figure-callout\; id="2"\; label="\gamma "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\gamma </figure-callout>}}^2-1}\right)$$
  

The dimensioning of an axial VIRCATOR according to the state of the prior art is as follows:

• The frequency f of the microwave transmitted wave 7 (expressed in GHz) is a function of the distance d _AK (expressed in cm) between the cathode 2 of the thin anode 4 and the relativistic factor γ electrons at the thin anode 4 in relation to the potential difference applied to the diode 2 + 3 + 4. This frequency can be estimated by the following formula: $$\mathrm{f}=\frac{4.77}{{\mathrm{d}}_{\mathrm{AK}}}\log \left(\gamma +\sqrt{{\gamma }^2-1}\right)$$
  

où k_0n représente la racine de l'équation de la fonction de Bessel J₀(k_0n) = 0, avec k₀₁ = 2,4048 et k₀₂ = 5,5201.The microwave wave 7, having an axial symmetry of revolution, evolves in so-called "magnetic transverse" modes, designated by "TM _0n ", the axial component of its magnetic field being zero. For it to propagate inside the cylindrical waveguide 5 in the single mode TM ₀₁ , it is necessary that the radius R _G of the cylindrical waveguide 5 is greater than the cutoff wavelength of the mode. following TM ₀₂ . The equation below (and not the opposite formula that appeared to be wrong) accounts for these propagation conditions: $$\frac{{\mathrm{k}}_{01}\mathrm{vs}}{2\pi \mathrm{f}}\le {\mathrm{R}}_{\mathrm{BOY\; WUT}}\le \frac{{\mathrm{k}}_{02}\mathrm{vs}}{2\pi \mathrm{f}}$$

where k _0n represents the root of the Bessel function equation J ₀ (k _0n ) = 0, with k ₀₁ = 2,4048 and k ₀₂ = 5,5201.

The length of the waveguide 5 is preferably equal to several times the wavelength λ of the electromagnetic wave 7 (λ = c / f).

A better operation of the coupling of the virtual cathode 6 with the electromagnetic wave 7 is obtained when the maximum density of the virtual cathode 6 at its mean position is situated in the vicinity of the maximum of the radial component of the electric field of the electromagnetic wave. . Considering that the electromagnetic wave 7 propagates in the single mode TM ₀₁ and also considering the bursting of the beam on emission, the radius R _c of the cathode 2 then preferably satisfies the following relation: $${\mathrm{R}}_{\mathrm{vs}}<1.8412\frac{{\mathrm{R}}_{\mathrm{BOY\; WUT}}}{{\mathrm{k}}_{01}}\approx 0.75\times {\mathrm{R}}_{\mathrm{BOY\; WUT}}$$

The device described above is of simple design. Its operation is robust and does not require recourse to an external magnetic field. On the other hand, its power output (ratio of the maximum power of the wave emitted to the maximum electrical power injected into the diode) is very low, of the order of approximately 1%. Moreover, the frequencies of the emitted wave directly follow the temporal variations of the applied voltage, which leads to obtaining an electromagnetic wave of poor spectral quality.

To overcome at least some of these disadvantages while maintaining an axial geometry, the implantation of one or more reflectors in the cylindrical waveguide 5 has been proposed.

This type of device has for example been the subject of the patent application WO2006 / 037918 . An example of a device as described in this application is represented on the figures 3 and 4 .

The reflectors are typically thin walls (that is to say of the order of one micrometer thick), transparent to the electrons and able to totally reflect the microwave wave 7 created by a virtual cathode. In addition, they have a circular cylindrical shape, that is to say disk. They are often made of aluminized mylar.

In the example shown on the figure 3 a first reflector 8 is positioned inside the waveguide 5 at a distance D1 from the thin anode 4. This distance D1 is equal to substantially twice the distance d _AK which separates the thin anode 4 from the cathode 2, such that a virtual cathode is created and positioned approximately midway between the thin anode 4 and the first reflector 8.

In this example, an additional reflector 9 is positioned in the cylindrical waveguide 5 beyond the first reflector 8, so that the distance separating two successive reflectors is substantially twice the distance d _AK that separates the reflector 8. thin anode 4 of the cathode 2, that is to say substantially the distance D1.

Reflectors can be "closed" or "open". As illustrated by figures 3 and 4 a reflector is said to be "closed" when it completely encloses a straight section of the cylindrical waveguide 5 (this is the case, for example, of the first reflector 8), and a reflector is said to be "open" when it is not obstructs a centered fraction of cross-section of the cylindrical waveguide 5, leaving a substantially annular opening 10 between the periphery of the reflector and the inner wall of the waveguide 5 (this is the case in the present example, additional reflector 9).

The reflector farthest from the thin anode 4 is preferably open to promote the propagation of the microwave wave towards the output of the cylindrical waveguide 5, the output being the end of the cylindrical waveguide 5 opposite to where the thin anode 4 is located.

Traditionally, an open reflector has a radius R greater than or equal to substantially 0.75 times the radius R _G of the cylindrical waveguide 5 to reflect the maximum of the radial component of the electric field of the wave.

The first reflector 8 has the function of reflecting the wave emitted by the virtual cathode, such as the thin anode 4. The wave reflected by the first reflector 8 interacts again with the electrons and the virtual cathode, amplifying the microwave wave 7. A first pseudo-cavity 11, cylindrical, formed between the thin anode 4, the first reflector 8 and an inner wall of the waveguide cylindrical 5 reinforces the power of the wave created by the virtual cathode. This enhancement of the wave contributes to improving the packetization of the electrons of the virtual cathode at the desired frequency.

By introducing a plurality of reflectors into the device (i.e., a number N), the microwave and packetization enhancement mechanism 7 in the first pseudo-cavity 11 is duplicated in subsequent pseudo-cavities formed by two successive reflectors (for example the first reflector 8 and the additional reflector 9 on the figure 3 ) and the cylindrical waveguide 5.

Thus the electrons which cross the reflector of rank (i) (1 ≤ i ≤ N-1, where N is the total number of reflectors present) create a virtual (i + 1) ^th cathode whose oscillation frequency is determined by the pseudo-cavity formed by the row reflectors (i) and (i + 1) and the inner wall of the waveguide 5. This pseudo-cavity contributes to reinforcing the electromagnetic wave 7 emitted by the (i + 1) ^th virtual cathode and packetization electrons.

If the reflector (i + 1) is open, the electromagnetic wave emitted by the (i + 1) ^th virtual cathode can flow in the waveguide 5 beyond the reflector (i + 1), in the direction the output of the guide, via the annular opening 10 present between the periphery of the reflector (i + 1) and the inner wall of the waveguide 5.

This type of device with reflectors makes it possible to obtain significantly improved performances compared with devices of the prior art without a reflector.

A device, emitting in band S at the output of the waveguide, that is to say in a frequency range from 2 GHz to 4 GHz, a single open reflector displays a performance improvement of the order of 4%. The addition of a second open reflector leads to an improvement of the order of 10%.

However, for such a device comprising reflectors, there is an optimal number of reflectors beyond which the power output decreases. For example, a device with three open reflectors displays an optimum efficiency of the order of 13%.

To further increase the efficiency of a VIRCATOR type device with reflectors as described above, the French patent application filed under number 12/62385, and not yet published, describes an oscillating virtual cathode microwave wave generator device comprising: a plurality of reflectors. All the reflectors are then open with the radius of each of the plurality of reflectors which is less than or equal to the radius of the previous reflector, the radius of the last reflector being smaller than the radius of the first reflector. Such a device is for example represented figure 5 according to an exemplary embodiment.

The device of the figure 5 here comprises a set of five reflectors (N = 5), commonly noted E _i and referenced here E ₁ to E ₅ , located in the waveguide 5, transparent to the electrons and configured to reflect the microwave wave created by a cathode Virtual. They are for example aluminized mylar.

All the reflectors E _i are "open" to facilitate the propagation of the wave emitted by the different virtual cathodes to the output of the waveguide 5.

The radius of the first reflector E ₁ located after the thin anode 4 in the waveguide 5 is preferably greater than or equal to 0.75 R _G. It thus reflects the maximum of the radial component of the electric field of the wave and thus strengthens the microwave wave emitted by the first virtual cathode, that is to say the virtual cathode formed just after the thin anode 4, between the thin anode 4 and the first reflector E ₁ .

• Le rayon du réflecteur de rang (i+1) est inférieur ou égal au rayon du réflecteur de rang i, c'est-à-dire du réflecteur directement précédent ;
• Le rayon du dernier réflecteur (ici E₅, ou noté de manière plus générale E_N, quel que soit N) est inférieur au rayon du premier réflecteur E₁.

The radius of the reflectors E _i following is gradually reduced without lower limit. The radius size of each reflector is possibly less than 0.75 R _G. The methods for reducing the size of the radius of the open reflectors are for example the following:

• The radius of the reflector of rank (i + 1) is less than or equal to the radius of the reflector of rank i, that is to say of the directly preceding reflector;
• The radius of the last reflector (here E ₅ , or noted more generally E _N , whatever N) is smaller than the radius of the first reflector E ₁ .

In the exemplary embodiment of the figure 5 , the reflectors E ₁ to E ₄ are of the same radius while the last reflector, E ₅ , is of smaller radius.

A device according to the invention described in the French patent application filed under number 12/62385, and not yet published, makes it possible to considerably increase the performance of a conventional axial VIRCATOR of the prior art, and in particular of an axial VIRCATOR with reflectors of the prior art as described in the application WO2006 / 037918 . For example, a device with five non-constant ray reflectors (with the radius of each reflector less than or equal to that of the immediately preceding reflector), transmitting in an S-band (i.e. in a frequency range from 2 GHz at 4 GHz), shows a return of 21%.

The operation of the VIRCATOR type devices of the prior art, described above, however, is limited to power generators whose impedance Z is less than a so-called "critical" impedance, noted Z _c . This critical impedance Z _c is defined as the ratio of the supply voltage V to the critical current I _c defined above, that is to say Z _c = V / I _c .

The figure 6 represents a propagation of an electron beam in the waveguide 5 in quasi-laminar mode when the impedance Z of the generator is greater than the critical impedance Z _c . This has the effect that no virtual cathode is formed. The figure 7 represents, for illustrative purposes, the absence of oscillating virtual cathode formation in the phase space. No electron can then be sent back towards the cathode 2 through the thin anode 4.

The object of the present application is to remedy at least in part the aforementioned drawbacks, and to further lead to other advantages.

The object of the present application is more particularly to enable an axial VIRCATOR type virtual cathode microwave generator device, with reflectors, to be able to operate while being coupled to a generator whose impedance Z exceeds the critical impedance. Z _c .

For this purpose, is proposed, in a first aspect, an oscillating virtual cathode microwave wave generating device, having an axial geometry, comprising a cathode, a thin anode and a cylindrical waveguide, of longitudinal axis z and of radius R _G , having a first end forming an inlet of the cylindrical waveguide and a second end forming an output of the cylindrical waveguide, the cathode being positioned upstream of the input of the cylindrical waveguide and configured to emitting electrons, and the thin anode being positioned at the entrance of the cylindrical waveguide, between the cathode and the cylindrical waveguide, and the device further comprising at least a first reflector located in the guide of wave, electron-transparent and configured to reflect a microwave wave created by at least one virtual cathode generated in the waveguide, the device being characterized in that it comprises in addition a narrow magnetic ring of width L _M along the longitudinal axis z, positioned externally around the cylindrical waveguide at a distance d _AM from the thin anode and with the first reflector positioned at a distance from the thin anode beyond the magnetic ring so that the magnetic ring is located between the thin anode and the first reflector, the magnetic ring being configured to generate a magnetic field capable of braking the electrons and to create a charge accumulation at the origin of a non-oscillating virtual cathode positioned between the thin anode and the first reflector.

Here, narrowly meant that the magnetic ring has a width L _M between about d _AK and about half of the radius of the waveguide R _G. It is for example equal to about d _AK .

The magnetic ring further has an inner radius R _M which is greater than R _G so that the magnetic ring surrounds the waveguide. The magnetic ring for example surrounds the waveguide at a distance from it. However, according to alternative embodiments, the magnetic ring is connected to the waveguide, or even in contact therewith.

Finally, the magnetic ring has a thickness, for example, chosen by a user according to the other sizing parameters of the device. The magnetic ring is for example a current coil or a permanent magnet so that it is then possible to dispense with power supply.

For example, the distance of _AM separating the magnetic ring from the thin anode along the z axis is equal to or greater than a distance d _AK separating the cathode from the thin anode.

In another example, the distance _AF1 separating the first reflector from the thin anode is equal to or greater than a sum of the distance d _AM , separating the magnetic ring from the thin anode, and the width L _M of the ring magnetic.

In yet another example, the distance of _AF1 separating the first reflector from the thin anode is equal to or greater than about twice the distance of _AK separating the cathode from the thin anode.

Advantageously, at least the first reflector located in the waveguide is an open reflector, that is to say that it obstructs only a centered fraction of cross section of the cylindrical waveguide, leaving an opening substantially annular between a periphery of the reflector and an inner wall of the waveguide.

According to a particular embodiment, the first reflector, open, possibly has a radius equal to or less than 0.75 R _G , the radius of the waveguide.

According to an advantageous embodiment, the device comprises a plurality of successive reflectors positioned in the cylindrical waveguide.

Two successive reflectors of the plurality of reflectors are for example separated from each other by a distance _dF-1Fi equal to or less than about twice a distance d _AK separating the cathode from the thin anode.

Or for example, two successive reflectors of the plurality of reflectors are separated from each other by a distance _dF-1Fi equal to or greater than about once the distance of _AK separating the cathode from the thin anode.

Each distance is for example between one to two times the distance d _AK .

In the context of the present application, whether the device comprises a reflector or a plurality of reflectors, the first reflector is the one positioned closest to the thin anode. That is to say, when the device comprises a plurality of reflectors, the first reflector remains the one positioned closer to the thin anode, so that the other reflectors of the plurality are positioned downstream of the first reflector.

In an exemplary embodiment in which the device comprises a plurality of successive reflectors, all the reflectors are then advantageously open.

And for example, the first reflector, open, possibly has a radius equal to or less than 0.75 R _G , the radius of the waveguide.

In addition, when the device comprises a plurality of reflectors, all the reflectors may have the same radius R _Fi .

However, according to an alternative embodiment, each reflector may have a radius equal to or less than that of the directly preceding reflector in the cylindrical waveguide so as to promote a guidance of the waves towards the output of the waveguide. The reflectors are thus successively decreasing with no lower limit, that is to say a last reflector in the waveguide, or even a second reflector (that is to say that positioned just after the first reflector), may have a radius smaller than that of the first reflector.

According to a preferred embodiment, the device comprises three reflectors positioned in the waveguide.

Such a ring makes it possible to operate a VIRCATOR in axial configuration, with at least one reflector, and a high impedance generator. The device also gains in compactness, since a generator with High impedance typically has less bulk than a low impedance generator.

The device according to the invention makes it possible to generate a monochromatic microwave emission.

The device according to the invention also makes it possible to transmit at a specific frequency a maximum of microwave power on the axis in a single mode.

The device according to the invention makes it possible to adapt a waveguide in axial configuration with reflectors to the impedance of the generator while retaining the emitted microwave frequency as well as the geometry of the waveguide.

The device according to the invention thus makes it possible to achieve efficiencies greater than 15% with high impedance generators in axial configuration with reflectors.

LIST OF FIGURES

• La figure 1 représente schématiquement un VIRCATOR axial classique de l'art antérieur selon un exemple de réalisation, selon une vue longitudinale, illustrant une création de cathode virtuelle oscillante ;
• La figure 2 présente un exemple de schéma instantané de la position des électrons dans l'espace des phases associé à la formation d'une cathode virtuelle oscillante ;
• La figure 3 représente schématiquement un VIRCATOR axial avec réflecteurs de l'art antérieur selon un exemple de réalisation tel que décrit dans le document WO2006/037918 , selon une vue longitudinale ;
• La figure 4 représente, en vue transverse du VIRCATOR de la figure 3, un réflecteur fermé et un réflecteur ouvert selon un exemple de réalisation ;
• La figure 5 représente un exemple de réalisation de VIRCATOR axial avec des réflecteurs ouverts tel que décrit dans la demande déposée sous le numéro 12/62385, et non encore publiée, selon une vue longitudinale ;
• La figure 6 illustre schématiquement la dynamique d'un faisceau d'électrons dans un VIRCATOR axial de l'art antérieur, par exemple sans réflecteurs, selon une vue longitudinale, quand l'impédance d'alimentation est supérieure à l'impédance critique, induisant un régime quasi laminaire et aucune formation de cathode virtuelle ;
• La figure 7 présente un exemple de schéma instantané de la position des électrons dans l'espace des phases en régime quasi laminaire, en l'absence de formation de cathode virtuelle ;
• La figure 8 présente, selon un vue longitudinale, un exemple de réalisation d'un VIRCATOR axial avec une bague magnétique selon l'invention, comportant ici des réflecteurs ouverts ;
• La figure 9 présente une vue transverse du VIRCATOR de la figure 8 ;
• La figure 10 présente un exemple de schéma instantané de la position des électrons dans l'espace des phases dans le VIRCATOR de la figure 8 ;
• La figure 11 présente schématiquement des iso-contours de l'intensité du champ magnétique selon une direction longitudinale du VIRCATOR de la figure 8 ;
• La figure 12 est un tableau récapitulatif de la distance entre l'anode et le premier réflecteur et des distances entre deux réflecteurs successifs pour des simulations numériques réalisées sur des dispositifs selon des modes de réalisation de la présente invention ; et
• La figure 13 est un tableau présentant un rendement en puissance (en pourcent) d'un dispositif selon des modes de réalisation de la présente invention en fonction du nombre de réflecteurs.

The invention according to an exemplary embodiment will be well understood and its advantages appear better on reading the detailed description which follows, given for information only and in no way limiting, and with reference to the accompanying drawings presented below.

• The figure 1 schematically represents a conventional axial VIRCATOR of the prior art according to an exemplary embodiment, according to a longitudinal view, illustrating an oscillating virtual cathode creation;
• The figure 2 presents an example of an instantaneous diagram of the position of the electrons in the phase space associated with the formation of an oscillating virtual cathode;
• The figure 3 schematically represents an axial VIRCATOR with reflectors of the prior art according to an exemplary embodiment as described in the document WO2006 / 037918 in a longitudinal view;
• The figure 4 represents, in transverse view of the VIRCATOR of the figure 3 , a closed reflector and an open reflector according to an exemplary embodiment;
• The figure 5 represents an embodiment of axial VIRCATOR with open reflectors as described in the application filed under number 12/62385, and not yet published, according to a longitudinal view;
• The figure 6 schematically illustrates the dynamics of an electron beam in an axial VIRCATOR of the prior art, for example without reflectors, in a longitudinal view, when the supply impedance is greater than the critical impedance, inducing a quasi-steady state laminar and no virtual cathode formation;
• The figure 7 presents an example of an instantaneous diagram of the position of the electrons in the phase space in quasi-laminar mode, in the absence of virtual cathode formation;
• The figure 8 presents, in a longitudinal view, an embodiment of an axial VIRCATOR with a magnetic ring according to the invention, here comprising open reflectors;
• The figure 9 presents a transverse view of the VIRCATOR of the figure 8 ;
• The figure 10 presents an example of an instantaneous diagram of the position of the electrons in the phase space in the VIRCATOR of the figure 8 ;
• The figure 11 schematically presents iso-contours of the intensity of the magnetic field in a longitudinal direction of the VIRCATOR of the figure 8 ;
• The figure 12 is a summary table of the distance between the anode and the first reflector and distances between two successive reflectors for numerical simulations performed on devices according to embodiments of the present invention; and
• The figure 13 is a table showing power efficiency (in percent) of a device according to embodiments of the present invention as a function of the number of reflectors.

A device according to one embodiment of the invention is represented for example here on the figure 8 .

As for a traditional device (see in particular the Figures 1 to 7 ), the device of the figure 8 comprises a diode composed of a cathode 102 and an anode, itself formed of a thin sheet called thin anode 104 and a thick frame 103. The cathode 102 has a radius R _c and the thin anode 104 typically has a thickness of the order of a micrometer, that is to say a few micrometers or even a few tenths of a micrometer.

The device further comprises a cylindrical waveguide 105 of internal radius R _G and length L _G. The cylindrical waveguide 105 has an axis z in a longitudinal direction, forming the longitudinal axis of the device.

The thick armature 103 surrounds the cathode 102, and the thick armature 103 and the cathode 102 are positioned at an inlet of the cylindrical waveguide 105 (left in the figure).

The thin anode 104 is here positioned at an inlet of the cylindrical waveguide 105, between the cylindrical waveguide 105 and the thick armature 103. The thin anode 104 and the cathode 102 are distant from each other. other from a distance denoted by _AK .

The cathode 102, the thin anode 104, the thickness armature 103 and the cylindrical waveguide 105 are positioned relative to each other aligned and centered on the z axis. They usually have circular sections.

où k'₁₁ représente la racine de l'équation de la fonction de Bessel J'₁(K'₁₁)=0 (k'₁₁=1,8412).To emit a microwave radiation on the axis, the radius R _G of the waveguide 105 is advantageously such that the microwave transmission frequency f is greater than the cutoff frequency of the fundamental mode TE ₁₁ and less than that of the following mode TM ₀₁ : $$\frac{{\mathrm{<figure-callout\; id="11"\; label="k"\; filenames="imgf0001.png,imgf0004.png"\; state="\{\{state\}\}">k</figure-callout>}}_{11}\mathrm{<figure-callout\; id="2"\; label="vs"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">vs</figure-callout>}}{2\pi \mathrm{f}}\le {\mathrm{R}}_{\mathrm{BOY\; WUT}}\le \frac{{\mathrm{k}}_{01}\mathrm{<figure-callout\; id="2"\; label="vs"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">vs</figure-callout>}}{2\pi \mathrm{f}}$$

where k _'11 represents the root of the equation of the Bessel function J' ₁ (K _'11) = 0 (k' ₁₁ = 1.8412).

The device according to the invention comprises a magnetic ring 112.

The magnetic ring 112 is advantageously narrow, of width L _M and of internal radius R _M , greater than R _G. In an exemplary implementation in which the ring is a coil, the ring then has for example a thickness which corresponds to a thickness of the conductive wire forming the coil. According to a particularly convenient embodiment, the width L _M is approximately equal to d _AK . In general, a ring is for example considered narrow if L _M is approximately equal to one half of the radius of the waveguide R _G.

It is positioned around the cylindrical waveguide 105, downstream of the anode 104, at a distance d _AM from the anode 104 along the z axis. Advantageously, the distance of _AM is approximately equal to the distance d _AK separating the cathode 102 and the anode 104.

The narrowness (along the longitudinal direction of the cylindrical waveguide 105 represented by the z axis) of the magnetic ring 112 thus ensures a magnetic field configuration dominated by the vanishing fields. In other words, because the magnetic ring 112 is narrow, it makes it possible to generate leakage fields configured to form a concentration of electrons between the thin anode 104 and a first reflector. The electrons, by winding along the lines of magnetic fields, are focused on the z axis and are, in fact, braked along the z axis. The beam current eventually exceeds the critical current I _c locally. This results in a local accumulation of charges, which is at the origin of the formation of a so-called "non-oscillating" virtual cathode. The virtual cathode is here "non-oscillating" in that few electrons are pushed back to the thin anode 104. The magnetic field produced by the ring 112 induces a stagnation of the electrons near the z axis.

The magnetic ring 112 is for example a current coil or a permanent magnet so that it is then possible to dispense with power supply.

According to a particularly advantageous embodiment of the present invention, the device comprises at least a first reflector F ₁ . The first reflector F ₁ is located at a distance d _AF1 from the thin anode 104 so that d _AF1 is equal to or greater than the sum of d _AM and L _M , and preferably equal.

In other words, the ring extends only to the first reflector and not beyond, as in devices using a magnetic field guide. The ring is positioned downstream of the anode, which differs from devices where the diode is immersed or semi-immersed, for example.

And according to a preferred embodiment, the device comprises a plurality of N reflectors F _i .

In this illustrated exemplary embodiment figure 8 , the device comprises a set of three reflectors F _i (that is to say with N = 3 and i ranging from 1 to N), which are here all open at their periphery. The reflectors F _i are located downstream of the thin anode 104 and the magnetic ring 112 in the cylindrical waveguide 105. The reflectors F _i are transparent to the electrons and are able to totally reflect the electromagnetic waves. The reflectors are for example made of aluminized mylar. In operation, all the reflectors are advantageously put at the same potential as the thin anode 104.

Each reflector has a radius R _Fi and two successive reflectors are spaced from each other by a distance d 1- _Fi .

The positioning of the reflectors F _i in the waveguide 105 is such that the microwave power is maximum at the output of the waveguide 105. In addition, the reflectors F _i are for example located at distances that are variable from one another, c that is, the distance of _AF1 and each distance of _Fi-1Fi can be all different from each other. In other words, all the reflectors of the device are fixed in the cylindrical waveguide 105, but the distances separating two successive reflectors may be different from each other and different from the distance _AF1 separating the first reflector F ₁ from the anode thin 104.

Advantageously, the distance _AF1 is equal to or greater than twice the distance d _AK , and each distance d _Fi-1Fi is for example between one to twice the distance d _AK . Indeed, as the electrons are rotated azimuthally in the cylindrical waveguide 105 by the field magnetic ring 112, the _AF1 distance d separating the first reflector F ₁ of the anode 104 is possibly substantially higher than that of vircator type devices of the prior art known and the distance between the rows of reflectors i and i + 1 is also possibly less than that of known prior art VIRCATOR devices.

If the current of the beam is sufficient at a reflector of rank i, an oscillating virtual cathode is initiated behind it, that is to say downstream of the reflector of rank i.

The rotation of the electrons by the magnetic field of the ring 112 conjugate to the effect of the centrifugal force leads to the bursting of the beam after the last reflector F _N (here F ₃ ). A large part of the electrons is absorbed by the inner wall of the cylindrical waveguide 105, the electrons remaining are remote from the center of the cylindrical waveguide 105, that is to say the z axis, which minimizes any possible interaction between the electrons and the magnetic waves at the center of the cylindrical waveguide 105 where the maximum of the microwave power of the TE mode _{11 is located} .

EXAMPLES DETAILED EMBODIMENTS

The behavior of an axial VIRCATOR emitting in the S-band and comprising N reflectors F _i and a magnetic ring 112 has been simulated numerically.

In the simulated devices, the cylindrical waveguide 105 is here of length L _G = 500 mm.

They comprise 1 to 3 reflectors, ie N = 1, 2 or 3, open at their periphery, of constant radius R _Fi less than R _G.

The distance separating the reflector F1 from the anode and the distances separating each reflector F _i from the preceding reflector, as a function of the number of reflectors F _i disposed in the waveguide, are summarized in the table of the figure 12 .

All the devices considered here make it possible to generate a mono-frequency microwave transmission in S-band on the z-axis according to the TE ₁₁ mode.

The generator considered here delivers a voltage of 500 kV.

The critical current I _c beyond which an electron beam no longer propagates in the cylindrical waveguide 105 is of the order 7.4 kA. The "critical" impedance Z _c for this device is thus 67.5 Ω (ohm).

The power generator considered here has an impedance of 70 Ω, that is to say greater than the "critical" impedance.

The flow of the beam in the guide is therefore almost laminar. The conventional process of forming the oscillating virtual cathode can not therefore be triggered in an axial VIRCATOR which would be devoid of a ring.

The formula which links the transmitted frequency to the distance d _AK and the applied voltage V indicates that the distance d _AK is advantageously chosen between approximately 15.6 mm and approximately 31 mm for the electromagnetic microwave radiation to be emitted in the S-band. _AK anode-cathode distance retained here is about 22 mm.

In order for the current emitted by the cathode to be adapted to an impedance of 70 Ω with a supply of 500 kV and an anode-cathode distance d _AK of approximately 22 mm, the radius of the cathode R _c is then about 22, 5 mm.

In order for the microwave transmission in the band S to be shaped according to the fundamental mode TE ₁₁ of the cylindrical waveguide 105, the cutoff frequency of the mode, f ₁₁ = 1.8412c / (2πR _G ), is advantageously less than or equal to 2 GHz. This induces a radius of the guide R _G greater than about 44 mm.

The radius R _G retained here is thus about 50 mm.

The configuration of the magnetic field leads locally to an increase of the beam current in the waveguide to exceed the critical current. Under the effect of the leakage fields, the electrons are focused on the axis and thus braked along the axis. This results in a local accumulation of charges at the origin of the formation of a virtual cathode. This virtual cathode is non-oscillating, few electrons are pushed towards the anode, the majority of the electrons are re-accelerated towards the exit of the guide. The magnetic field induces stagnation of the electrons in the vicinity of the axis.

The magnetic configuration is provided by the magnetic ring positioned here at a distance of _AM from the anode of about 29 mm.

In the exemplary embodiments considered here, the ring creating the magnetic field is here a current coil of 12750 A.tours (ampere-turns) with L _M = 25 mm and R _M = 60.5 mm.

The first open reflector of radius R _F1 = 35 mm is positioned at the rear face of the magnetic ring, at a distance from the anode of _AF1 = 54 mm, as indicated by _FIG. figure 12 . The first reflector, coupled to the magnetic ring, makes it possible to create the first oscillating virtual cathode behind the first reflector, that is to say downstream of the first reflector.

The positioning of the following reflectors, for implementation examples comprising two or three reflectors, radius R _Fi = 35 mm, optimizes the microwave power emitted in the S-band.

According to figure 12 in a configuration with two reflectors, the second reflector F ₂ is positioned at a distance d _F1-F2 of 25 mm from the first reflector F ₁ ; and in a configuration with three reflectors, the second reflector F ₂ is positioned at a distance d _F1-F2 of 29 mm from the first reflector F ₁ , and the third reflector is positioned at a distance d _F2-F3 of 25 mm from the second reflector F ₂ .

The figure 11 represents the iso-contours of the intensity of the magnetic field in longitudinal section of a device according to the invention comprising here a reflector. The maximum intensity of the magnetic field in the guide is of the order of 0.1 T (Tesla) in a section of the waveguide to the right of the magnetic ring 112, that is to say to a section positioned about half of the width L _M of the magnetic ring.

The figure 13 summarizes the performances obtained by the simulation of an axial VIRCATOR according to the invention comprising one, two or three reflectors.

The figure 13 shows that the power emitted increases with the number of reflectors. The yield achieved is of the order of 2.5% with a single reflector and 17.4% with three reflectors. An optimum of yield is obtained with three reflectors. The addition of a fourth reflector is of little use to improve the efficiency because the number of electrons decreases and becomes insufficient in the waveguide or near the z axis.

Thus, a device according to the invention powered by a high impedance generator makes it possible to emit microwave power in an S-band with a yield close to that obtained with a device in axial configuration with reflectors of the known prior art, powered with a low impedance generator.

The configuration with three reflectors ensures a minimum efficiency of 13.8% for a distance d _F2-F3 between a second reflector and a third reflector of between about 25 mm and about 31 mm, while maintaining the microwave emission frequency.

According to another example, a device according to the invention as described above, is coupled to a higher impedance generator, while emitting at the same microwave frequency according to the TE mode ₁₁ . For example, by maintaining a 500 kV supply voltage, an increase in the anode-cathode distance d _AK at 30 mm induces a decrease in the accelerating field in the diode and therefore a lower emitted current, of the order of about 4 kA. As a result, the diode is adapted to a higher power supply impedance, for example about 125 Ω. The density of the beam emitted being then less, slightly increasing the intensity of the current of the magnetic ring at 14250 A.tours, makes it possible to generate a single-frequency microwave emission at 2.31 GHz in the TE mode ₁₁ with a yield of 12 %. This performance can for example be improved by adjusting the positioning of the reflectors in the guide.

Of course, the present invention is not limited to the foregoing description, but extends to any variant within the scope of the claims below.

Claims (10)

1. A microwave wave generator with a virtual cathode oscillator, with axial geometry, comprising a cathode, a thin anode and a cylindrical wave guide (105), of longitudinal axis z and radius R_G, having a first end forming an entrance to the cylindrical wave guide (105) and a second end forming an exit from the cylindrical wave guide (105), the cathode (102) being positioned upstream of the entry to the cylindrical wave guide and configured to emit electrons, and the thin anode (104) being positioned at the entrance to the cylindrical wave guide (105), between the cathode (102) and the cylindrical wave guide (105), and the device further comprising at least a first reflector (F₁) that is located in the wave guide (105), transparent to electrons and configured to reflect a microwave wave created by at least one virtual cathode generated in the wave guide (105),
   the device being characterized in that it further comprises a narrow magnetic ring (112) of width (L_M) along the longitudinal axis z, positioned externally around the cylindrical wave guide (105) at a distance (d_AM) from the thin anode (104) and with the first reflector (F₁) positioned at a distance (d_AF1) from the thin anode (104) beyond the magnetic ring (112) such that the magnetic ring (112) is located between the thin anode (104) and the first reflector (F₁), the magnetic ring (112) being configured to generate a magnetic field adapted to brake the electrons and to create an accumulation of charge at the origin of a non-oscillating virtual cathode positioned between the thin anode (104) and the first reflector (F₁).
2. A device according to claim 1, characterized in that the distance (d_AM) separating the magnetic ring (112) from the thin anode (104) along the axis z is equal to or greater than a distance (d_AK) separating the cathode (102) from the thin anode (104).
3. A device according to any one of claims 1 or 2, characterized in that the distance (d_AF1) separating the first reflector (F₁) from the thin anode (104) is equal to or greater than the sum of the distance (d_AM), separating the magnetic ring (112) from the thin anode (104), and the width (L_M) of the magnetic ring (112).
4. A device according to any one of claims 1 to 3, characterized in that the distance (d_AF1) separating the first reflector (F₁) from the thin anode (104) is equal to or greater than approximately twice the distance (d_AK) separating the cathode (102) from the thin anode (104).
5. A device according to any one of claims 1 to 4, characterized in that at least the first reflector (F₁), located in the wave guide (105), is an open reflector.
6. A device according to any one of claims 1 to 5, characterized in that it comprises a plurality of successive reflectors (F_i) positioned in the cylindrical wave guide (105).
7. A device according to claim 6, characterized in that two successive reflectors of the plurality of reflectors (F_i) are separated from each other by a distance (d_Fi-1Fi) equal to or less than approximately twice a distance (d_AK) separating the cathode (102) from the thin anode (104).
8. A device according to any one of claims 6 or 7, characterized in that two successive reflectors of the plurality of reflectors (F_i) are separated from each other by a distance (d_Fi-1Fi) equal to or greater than a distance (d_AK) separating the cathode (102) from the thin anode (104).
9. A device according to any one of claims 6 to 8, characterized in that all the reflectors (F_i) are open and have a same radius R_Ri.
10. A device according to any one of claims 1 to 9, characterized in that it comprises three reflectors (F_i) positioned in the wave guide (105).

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