EP1444715A2

EP1444715A2 - Microwave generator with virtual cathode

Info

Publication number: EP1444715A2
Application number: EP02783221A
Authority: EP
Inventors: Alain-Joseph Thales Intellectual Property DURAND
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2001-09-28
Filing date: 2002-09-27
Publication date: 2004-08-11
Also published as: JP2005505112A; FR2830371A1; WO2003030204A2; WO2003030204A3; US20040245932A1; FR2830371B1

Abstract

The invention relates to a very high power microwave generator that employs the virtual cathode phenomenon. The inventive microwave generator (60) comprises an emission device (62) that can produce electrons in an output microwave circuit (64, 66), the quantity of electrons emitted being sufficient to cause a regular variation in the density of electrons in the output microwave circuit. Said circuit transforms the kinetic energy of the electrons into microwave energy according to a resonance mode. The electron emission device, which emits said electrons in several regions of the microwave circuit, comprises field maximas (Exa1, Exa2, Exa3) of the resonance mode. The invention is suitable for use in relation to high power microwave emitters and jammers.

Description

CATHODE MICROWAVE WAVE GENERATOR

VIRTUAL

The present invention relates to a very high power microwave generator using the phenomenon of 5 virtual cathode.

It is known, to generate instantaneous high powers in microwave, to use electronic tubes called vircators. Vircators are very high-power oscillator electronic tubes that use very intense electron beams capable of forming a virtual cathode.

In a beam of electrons, circulating in a tube or an essentially metallic enclosure, a hollow of potential is created, namely that the electrons do not have quite the speed which corresponds to their initial acceleration and particularly for the electrons from the center of the 5th beam. When the beam intensity becomes large, the potential trough in the center ends up being such that it no longer allows the circulation of electrons and the beam becomes hollow. For an even greater intensity I, greater than a certain critical current (ISIc), even the electrons on the edge no longer circulate, they turn back and the accumulation of electrons, called virtual cathode 0, is called virtual 0 of the cusp.

The virtual cathode is unstable, in fact, the amplitude of its potential trough and its position oscillate, which causes a periodic variation in the number of electrons reflected or transmitted. 5 A device such as the vircator makes it possible to create electromagnetic fields with high microwave powers and in a reduced volume.

Figure 1a shows a sectional drawing of a vircator 10 of the state of the art. In the vircator 10 a very short beam of electrons 12 is emitted in an enclosure 14 of cylindrical shape, generally, by the field emission of a cold cathode 16 (spikes, velvet, flat surface ...), the anode being a very thin metal sheet 18 or a metal grid. The electrons extracted from the cathode by the potential existing between the anode and the cathode largely cross this metal sheet 18, or this grid, and very quickly form behind, a virtual cathode 20, all the more easily as the enclosure 14 is a little wider here. A certain number of electrons operate back and forth between the real cathode 16 and the virtual cathode 20 in the form of microwave oscillations. This oscillation gives rise to electromagnetic radiation, in one of the modes defined by the geometry of all the elements constituting the vircator. Another source of radiation, but it seems, more modest resides in the displacement or the vibration of the virtual cathode 20 itself.

Figure 1b shows a sectional view of the vircator 10 in a plane perpendicular to the axis 721 of revolution of the cylindrical enclosure 14 and Figure 1c a representation of the variation of the field E in the enclosure in a passing plane by the ZZ 'axis of the enclosure. In this exemplary embodiment of the vircator of FIG. 1 a, the resonance mode is such that the field E in the enclosure passes through a first maximum m1, we will say thereafter that it has a first extrema m1, according to l 'axis ZZ' of the enclosure 14 and another extrema m2 of opposite direction to the first, of circular shape around the first extrema.

The orders of magnitude of the energies involved in a vircator are as follows:

Cathode voltage, V _k = 700 kV Cathode current, l _k = 30 kA

Output power, Psortie = 600 MW

Pulse width Pt emission, τ = 60 ns

Yield = 2.8%.

As certain needs (jamming of the airspace) require higher powers, the first idea is then to increase the beam power V xl - But any increase in voltage increases the probability of arc along the insulators and in the tube , unless operating with shorter τ pulses. It follows then that if the power increases, τ decreases, and the energy of the pulse Pt increases only very little. The second idea is to increase the efficiency of the vircator. We effectively manage, thanks to the feedback vircator (VCR), to double the yields and therefore the power.

FIG. 2 represents a basic drawing of a microwave generator with feedback and virtual cathode 30 of the prior art or feedback vircator 30 (VCR);

The feedback vircator 30 has a resonant cavity

32 of low height coupled to a waveguide 34. A cathode 36 of the electron gun 38 of the vircator, injects a beam of high intensity electrons through a first grid 40 into the resonant cavity 32 then through a second grid 42 in the waveguide 34. The height of the waveguide 34 is sufficient so that, when the cathode current lk is greater than a critical value lj _<c , a virtual cathode is created in the guide which repels the electrons incidents whose back and forth movements generate microwave waves. The signal generated in the guide 34 excites the resonant cavity 32, and the microwave fields in the cavity create an energy modulation of the beam, and therefore a grouping in bundles of the beam. The oscillator thus produced is a feedback vircator. There is a phase shift value between the fields in the resonant cavity and the fields in the guide which optimizes the yield.

However, in certain cases, the microwave powers thus obtained are not yet sufficient and the present invention proposes a means for further increasing them, while retaining the pulse durations τ, or even by enlarging them. Of course, to do this, there is no question of increasing the high voltage V, due to the unwanted arcs and breakdowns which would shorten τ and damage the tube. The scientific literature is relatively abundant on these phenomena of "puise shortening".

If V _k cannot be increased, it remains to increase lk. To do this: - we can bring the anode closer and extract more current; but, as the frequency varies schematically in a manner inversely proportional to the distance dKG between cathode and anode, the operating frequency is higher and, in any case, different. This solution does not respond to the problem posed, especially since, in general, the power decreases with frequency (more compact resonant volumes) and that the closing of the space between cathode and anode, distant from dkG, by the plasma emitted by both the anode and the cathode occurs earlier, causing a reduction in width τ of the pulse.

- one can also increase the surface of the cathode. But let us specify that the electrons performing the back and forth between the real cathode and the virtual cathode only radiate if they are in an extreme maximum), or in the vicinity of the maximum, of the electric component E of the electromagnetic field of a mode of resonance of the cathode / anode space. We cannot therefore increase this surface indefinitely and, generally, it is already, in this sense, optimized.

In order to increase the emission power of a vircator while retaining the pulse durations τ, or even by widening them, the invention proposes a microwave generator comprising an emission device capable of producing electrons in an output microwave circuit, the quantity of electrons emitted being sufficient to cause a regular variation of the electron density in the output microwave circuit, the circuit carrying out the transformation of the kinetic energy of the electrons into microwave energy according to a mode resonance, characterized in that the electron emission device emits electrons in several regions of the microwave circuit having field extremes of the resonance mode.

The emission device is an electron gun comprising several cathodes, so as to produce several electron beams and according to a main characteristic of the invention, each of the beams being emitted in a region of extremes of fields of the resonance mode of the microwave circuit.

A first object of this invention is to increase the pulse emitting power of the vircator without increasing the cathode currents or the anode voltages. A second object of this invention is to increase the efficiency of the transformation of the energy of the electrons into impulse electromagnetic energy necessary in certain applications.

A third object of this invention is to increase the duration of the electromagnetic pulse to bring it closer to the duration of the cathode / gate (or anode) voltage pulse. In a first embodiment of the generator, the microwave output circuit includes an enclosure having an electron input window emitted by the cathodes and a window for emitting the microwave waves produced by variations in the density of the electrons in the regions d 'extrema of the electromagnetic field in the enclosure. This structure is based on that of a "classic vircator".

In another embodiment of the wave generator ensuring high efficiency, the microwave circuit comprises, on the side of the emission device, an excitation guide followed by a resonant output cavity. The signal generated in the guide, exciting the resonant cavity, creates an energy modulation of the electron beam. This other structure is based on that of a “feedback vircator” (VCR).

The invention will be better understood with the aid of examples of embodiments of microwave cathode wave generators, with reference to the appended drawings, in which:

- Figures 1a, 1b, already described, show two sectional views of a microwave generator with virtual cathode (or vircator) of the prior art;

- Figure 1c already described shows the electromagnetic fields in a plane passing through the axis of revolution of the cavity of the wave generator of Figure 1a;

- Figure 2, already described, shows a principle drawing of a microwave generator against feedback and virtual cathode of the prior art (VCR); - Figure 3a shows a multibeam VCR, according to the invention;

- Figure 3b shows a front view of the vircator of Figure 3a according to the invention;

- Figure 3c shows the distribution of the electric field in the microwave circuit of the vircator of Figure 3b; - Figure 4a shows an embodiment of a conventional type vjrcator, according to the invention, with six electron beams;

- Figure 4b shows the magnetic H and electric field lines E of the vircator of Figure 4a;

- Figure 4c shows the variation in a plane of the electric field E in the enclosure of the vircator of Figure 4a; - Figure 4d shows a front view of the electron gun of the vircator of Figure 4a;

- Figures 4e and 4f show grids of the vircator of Figure 4a; - Figure 5a shows an embodiment of a conventional vircator according to the invention with five electron beams;

- Figure 5b shows the magnetic H and electric field lines E of the vircator of Figure 5a;

- Figure 5c shows the variation in a plane of the field E in the enclosure of the vircator of Figure 5a;

- Figure 5d shows a front view of the electron gun of the vircator of Figure 5a;

- Figure 6a shows an example of variation of the voltage pulse V _k as a function of time of a vircator; - Figures 6b, 6c and 6d show the respective microwave powers supplied over time by three cathodes of a vircator according to the invention;

- Figures 7a and 7b show two embodiments of the multibeam vircator according to the invention comprising different cathode / grid distances.

In the prior art vircator of FIG. 2, the tube comprises a resonant cavity 32 in a rectangular guide of small height (about 1/6 of the width) and of length equal to 3λ / 2 (λ being the length of oscillation wave in the vircator). The electron beam passes through the center of the cavity over an electric field belly. Only one third of the capacity of the cavity is therefore used to modulate the beam. The solution proposed according to a main characteristic of the invention consists in passing an electron beam through each belly of the electric field of the cavity. We can speak of a multibeam vircator (VMF). FIG. 3a shows a multibeam VCR 60, according to the idea developed above. The multibeam VCR 60 comprises a high-voltage gun 62 comprising three cathodes Ca1, Ca2, Ca3 of cylindrical shapes whose axes of revolution are located on the same plane P. As in the VCR 30 of the prior art of FIG. 2, the multibeam VCR according to the invention comprises an excitation guide 64 coupled to a resonant cavity 66 through a passage 68 between the guide and the cavity.

Each of the electron beams Fa1, Fa2 and Fa3 from the 5 cathodes Ca1, Ca2, Ca3 pass respectively into one of the extremes Exal, Exa2, Exa3 of the electric field existing in the guide and the resonant cavity.

The excitation guide 64 is delimited by a first grid 70 on the side of the high-voltage gun 62 and by a second grid 72 on the side of the cavity. The excitation guide 64 resonates in 5λ / 2, as does the output cavity (we could also operate with a resonance in 3λ / 2). The electric fields in the excitation guide Eg and in the resonant cavity Ec have, in this example of resonance mode, an extrema along an axis YY ′ of the barrel in the direction of emission of the electrons and a second 5 extrema of circular shape around this axis. In FIG. 3a are shown in dotted lines the variations of the fields Eg and Ec on the plane P of the cathodes Ca1, Ca2 and Ca3 passing through the axis YY 'of the barrel.

The central beam Fa2 excites the field in the axis YY 'of the tube in phase opposition with the two adjacent beams Fa1 and Fa3. However, this is the normal operation of a multibeam tube, which counts given the phase coherence of the set of two resonant circuits.

FIG. 3b shows a front view of the vircator of FIG. 3a according to the invention showing the position of the cathodes in the plane P passing through 5 the central beam axis F2 and FIG. 3c a distribution of the electric fields in the guide and in the cavity in front view.

FIG. 3a clearly shows several cathodes supplied in parallel from the rear, a single anode with several “screened” passages facing the cathodes and the extremes Exal, Exa2, Eax3 of electric fields E in the enclosure.

Note that the anode 70 can be "screened" over its entire surface, the main thing being that the grid thus formed does not allow the HF generated in the enclosure to pass.

But this idea of several electron beams in the field extremas can very well apply to a conventional vircator having an outlet enclosure. In this case, the notion of resonance is mainly applied to the space “Anode / or grid) - virtual cathode”, that is to say to the output enclosure.

FIG. 4a shows an exemplary embodiment of a conventional vircator 80 with six electron beams operating on a TM3lo type resonance mode.

The vircator 80 includes an electron gun 82 and an enclosure 84 separated from the gun by a mesh anode 86. The gun has six cathodes Cb1, Cb2, Cb3, Cb4, Cb5, Cb6 distributed regularly around an axis of revolution W of l 'enclosure 84 of cylindrical shape at an angular pitch of 60 degrees and equidistant from the axis W of the enclosure.

Figure 4b shows the lines of magnetic H and electric fields E for TM310 mode, in a plane perpendicular to the axis W. Fields E have extremes Exb1, Exb2, Exb3, Exb4, Exb5 and Exb6 changing sign at each angular offset of 60 degrees around the W axis. Note that the two directions (or signs) of the fields are represented respectively by a cross and by a point in a circle.

FIG. 4c shows the variation of the field E in the enclosure, in a plane Pb passing on the one hand by its axis of revolution W and, on the other hand, by the axes of two cathodes Cb1 and Cb4 located on the side and d other of this axis W of revolution. In this plane Pb appear two extremes Exb1 and Exb4 of opposite sign on either side of the axis of revolution W and this configuration of fields is repeated by loading with sign every 60 degrees corresponding to the angular offset α between the cathodes. Each electron beam of sufficient intensity, coming from each of the cathodes Cb1 to Cb6, produces a virtual cathode in the enclosure. FIG. 4a shows the two virtual cathodes Cvb1 and Cvb2 produced respectively by the beams coming from the cathodes Cb1 and Cb2.

FIG. 4d shows a front view of the electron gun 82 comprising the six cathodes around the axis W.

The mesh anode 86 can be constituted either, as shown in FIG. 4e, by a plate 88 comprising a mesh Gb1, Gb2, Gb3, Gb4, Gb5 and Gb6 circular, by cathode, each mesh facing its respective cathode either , as shown in Figure 4f, by a single circular grid 90 for all of the cathodes. FIG. 5a shows another embodiment of a vircator 100 of the conventional type operating on a resonance mode of the type

TM020.

The vircator 100 comprises an electron gun 102 and an enclosure 104 of cylindrical shape separated from the gun by a mesh anode

106. The barrel has five cathodes, a central cathode Cc1 in the axis

W of the enclosure and four cathodes Cc2 Cc3, Cc4 and Cc5 arranged at equal distance from the central cathode Cc1 according to an angular pitch α of 90 degrees.

FIG. 5b shows the lines of magnetic H and electric fields E for the mode TM020 in a plane perpendicular to the axis W.

The electric fields E have a central extremity Exc1 in the axis W of the enclosure and an annular extremum, constant along a circumference and of opposite sign.

FIG. 5c shows the variation of the field E in the enclosure, in a plane Pc passing on the one hand by its axis of revolution W and, on the other hand, by the axes of two cathodes Cc1 and Cc4 located on both sides other of the central cathode Cc1. In this plane Pc two extremes Exc2 and Exc4 of the same sign appear on either side of the central extremity Exc1 of opposite sign. As in the previous embodiments, each electron beam of sufficient intensity, coming from each of the cathodes Cc1 to Cc5, produces a virtual cathode in the enclosure. FIG. 5a shows three of the five virtual cathodes, a central virtual cathode Cvc1 and two virtual cathodes Cvc2 and Cvc4 produced respectively by the beams coming from the central cathode Cc1 and from two cathodes Cc2 and Cc4 located in the same plane Pc.

As for the embodiment of FIG. 4a, the screened anode 106 can be constituted either by a plate comprising a circular screen by cathode, each screen facing its respective cathode, or by a single circular screen for all the cathodes .

The fact remains that the cathode voltage variations V _k inherent in these large machines that are the sets: modulator (V _k .l _k ) + vircator (microwave oscillator) + antenna (microwave radiation), cause the frequency of oscillation of the electrons does not correspond to the resonance frequency FO of the mode enclosure wanted across the full voltage pulse width. As a result, electromagnetic radiation cannot occur during the entire voltage pulse. The microwave pulse is therefore singularly shorter than that of cathode voltage V _k . Indeed, the operating frequency, that is to say of the oscillations of the electrons or of those of the virtual cathode depends enormously on the high voltage V _k applied between cathode and grid (or anode). When the cathode voltage V _k increases, the oscillation frequency in the enclosure of the vircator increases along v with A ≤ α ≤%. FIG. 6a shows an example of variation of the voltage pulse V _k as a function of time t. The voltage pulse begins at time t0 and ends at time tf. The voltage V _k passes through respective values V i, V ₂ , V _k3 during successive time periods from tO to t1, from t1 to t2 and from t2 to tf. According to another characteristic of the multi-beam vircator according to the invention, the distances between the grid (or anode) and the cathodes of the electronic gun vary according to the cathode considered so as to compensate for the variations in the oscillation frequency of the vircator due to variations in the cathode voltage during the voltage pulse V _k . Thus a variation in one direction of the voltage V _k would result in an emission of electrons by at least one of the cathodes of the gun whose distance from the grid would be such that the oscillation would occur at the desired resonant frequency F0. For example, an increase in the voltage V _k would lead to an emission by a cathode closer to the grid at the desired frequency while a decrease in the voltage V would produce a mission at the same frequency by a cathode further from the grid.

If the voltage pulse V _k has n steps in time, if dki is the distance between a cathode Cki (or a group of cathodes) and the grid, with i = 1, 2, .. n, keeping the ratio v / dki constant for each cathode or group of cathodes, the oscillation frequency is kept constant during the voltage pulse.

The idea is to make an electron gun comprising cathodes whose distances to the grid are such that the V ° 7dki ratio remains constant when the plasma closes the space between cathode and anode for at least one cathode of the gun, during at least part of the voltage pulse.

FIGS. 7a and 7b show two embodiments of the multibeam vircator according to the invention comprising different cathode / grid distances.

Let us consider that the voltage pulse V is that of FIG. 6a comprising three voltages over time. In the first embodiment of FIG. 7a, a vircator 120 includes an electron gun 122 emitting three electron beams in a resonant enclosure 124 separated from the gun by a grid 126. The gun has three cathodes Cd1, Cd2, Cd3 whose distances respective d1, d2 and d3 at the grid 126 are such that the ratio V / dki for each of the cathodes remains constant. For this purpose, d1 will be adjusted to obtain the resonance frequency F0 at the voltage V i, d2 to obtain

F0 for Via and d3 to obtain F0 for V 3. Figures 6b, 6c and 6d show the respective microwave powers P1, P2 and P3 supplied over time by the three cathodes.

The first cathode supplies the power P1 at the frequency F0 during the time when the pulse is at V _k i, the second during the time when the pulse is at Vis and the third during the time when the pulse is at V _k 3. FIG. 6e shows the total microwave pulse supplied by the vircator at the resonance frequency F0 for a width much greater than that obtained by the vircators of the state of the art, substantially that of the voltage pulse. V.

In the second embodiment of a vircator 130 of FIG. 7b, the ends of the cathodes Cd1, Cd2 and Cd3 are on the same plane, a grid

132 of the enclosure has areas Pg1, Pg2 and Pg3 opposite the cathodes Cd1, Cd2 and Cd3 more or less distant from the cathodes so as to obtain different distances of 1, 2 and 3 grid / cathode.

In practice, we imagine not three beams, but more, for example five beams as in the barrel of Figure 5d. In this case, the cathodes are combined to form only three groups or rather as many groups as divisions of the voltage pulse V _k .

In these different embodiments, the emissive surfaces will be chosen to create the currents and the power required in each of the divisions of the pulse. The vircator according to the invention has many advantages compared to the vircator of the state of the art, among others we can cite:

- increased microwave power, at equal high voltage

- an identical microwave power, at lower high voltage, and therefore less limitation of the pulse width by

"Clicks".

- a lower impedance Z = V _k / l and, in some cases, better adaptation between the electric generator V _k and the vircator, resulting in a higher overall efficiency Generator more vircator, better stability and wider pulse,

- the excitation of the resonance mode on several of its electric field maxima, therefore less probability of causing resonance on other so-called “parasitic” modes. Therefore, faster start of the oscillation and better pulse stability.

Claims

1. Microwave generator (10, 30, 60, 80, 100, 120 130) comprising an emission device (38, 62, 82, 102, 122) capable of producing electrons in an output microwave circuit ( 14, 34, 66, 84, 104, 124), the quantity of electrons emitted being sufficient to cause a regular variation in the density of electrons in the output microwave circuit, the circuit carrying out the transformation of the kinetic energy of the electrons in microwave energy according to a resonance mode, characterized in that the electron emission device emits electrons in several regions of the microwave circuit having extremes (m1, m2, Exal, Exa2, Exa3, Exb1, Exb2, Exb3 , Exb4, Exb5, Exb6, Exc1, Exc2) of fields of the resonance mode.

2. Microwave generator according to claim 1, characterized in that the emission device is an electron gun (38, 62, 82, 102, 122) comprising several cathodes (Ca1, Ca2, Ca3, Cb1, Cb2 , Cb3, Cb4, Cb5, Cb6, Cc1, Cc2, Cc3, Cc4, Cc5, Cd1, Cd2, Cd3) so as to produce several electron beams (Fa1, Fa2, Fa3), each of the beams being emitted in a region of fields of the resonance mode of the microwave circuit.

3. Microwave wave generator according to claim 2, characterized in that the microwave circuit comprises, on the side of the emission device, an excitation guide (32, 64) followed by a resonant cavity (34, 66) output, the signal generated in the guide, exciting the resonant cavity, creating an energy modulation of the electron beam Fa1, Fa2, Fa3).

4. microwave generator according to claim 3, characterized in that the barrel (62) comprises three cathodes (Ca1, Ca2, Ca3) of cylindrical shapes whose axes of revolution are located on the same plane P, each of the beams electrons (Fa1, Fa2 and Fa3) from the cathodes (Ca1, Ca2, Ca3) passing respectively through one of the electric field extremes existing in the guide and the resonant cavity.

5. Microwave wave generator according to claim 2, characterized in that the output microwave circuit comprises an enclosure (14, 84) having an input window for the electrons emitted by the cathodes and an emission window for the microwave waves produced by variations in the density of electrons in the extrema regions (Exb1, Exb2, Exb3, Exb4, Exb5, Exb6) of the electromagnetic field in the enclosure.

6. microwave generator according to claim 5, characterized in that it comprises an electron gun (82) and an enclosure (84) separated from the barrel by a mesh anode (86), the barrel comprising six cathodes (Cb1 , Cb2, Cb3, Cb4, Cb5, Cb6) distributed regularly around an axis of revolution W of the enclosure (84) of cylindrical shape at an angular pitch of 60 degrees and equidistant from the axis W of the cavity .

7. Microwave wave generator according to claim 6, characterized in that it operates on a TM310 type resonance mode.

8. Microwave wave generator according to one of claims 6 or 7, characterized in that the mesh anode (86) can be constituted either by a plate (88) comprising a mesh (Gb1, Gb2, Gb3, Gb4, Gb5 and Gb5) circular by cathode, each grid facing its respective cathode or, by a single circular grid (90) for all the cathodes.

9. microwave generator according to claim 5, characterized in that it comprises an electron gun (102) and an enclosure (104) of circular shape separated from the barrel by a mesh anode (106), the barrel comprising five cathodes, a central cathode (Cc1) in the W axis of the enclosure and four cathodes (Cc2 Cc3, Cc4, Cc5) arranged at equal distance from the central cathode (Cc1) at an angular pitch α of 90 degrees.

10. Microwave wave generator according to claim 9, characterized in that it operates on a TM020 type resonance mode.

11. Microwave wave generator according to one of claims 2 to 10, characterized in that the distances between the grid (or anode) (70, 86, 106, 126, 132) and the cathodes (Cd1, Cd2 and Cd3 ) of the electronic gun (122) vary as a function of the cathode considered so as to compensate for the variations in the oscillation frequency of the generator due to the variations of a cathode voltage pulse V.

12. Microwave wave generator according to claim 11, characterized in that, the ends of the cathodes (Cd1, Cd2 and Cd3) are on the same plane, a grid (132) of the enclosure has areas (Pg1, Pg2 , Pg3) opposite the cathodes (Cd1, Cd2 and Cd3) more or less distant from the cathodes so as to obtain different distances (of 1, 2 and 3) grid / cathode.