Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J25/00—Transit-time tubes, e.g. klystrons, travelling-wave tubes, magnetrons
  • H01J25/02—Tubes with electron stream modulated in velocity or density in a modulator zone and thereafter giving up energy in an inducing zone, the zones being associated with one or more resonators
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J25/00—Transit-time tubes, e.g. klystrons, travelling-wave tubes, magnetrons
  • H01J25/74—Tubes specially designed to act as transit-time diode oscillators, e.g. monotrons

Definitions

• the present invention relates to a very high power microwave generator using the phenomenon of 5 virtual cathode.
• Vircators are very high-power oscillator electronic tubes that use very intense electron beams capable of forming a virtual cathode.
• 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.
• the potential trough in the center ends up being such that it no longer allows the circulation of electrons and the beam becomes hollow.
• I critical current
• 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.
• FIG. 1a shows a sectional drawing of a vircator 10 of the state of the art.
• 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.
• 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 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 ⁇ pulses. It follows then that if the power increases, ⁇ 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
• 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.
• 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 ⁇ , or even by enlarging them.
• the present invention proposes a means for further increasing them, while retaining the pulse durations ⁇ , or even by enlarging them.
• the high voltage V due to the unwanted arcs and breakdowns which would shorten ⁇ and damage the tube.
• the scientific literature is relatively abundant on these phenomena of "puise shortening".
• V k cannot be increased, it remains to increase lk.
• - 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 ⁇ of the pulse.
• 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.
• 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".
• 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).
• Figure 3b shows a front view of the vircator of Figure 3a according to the invention
• FIG. 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;
• FIG. 4c shows the variation in a plane of the electric field E in the enclosure of the vircator of Figure 4a;
• FIG. 4d shows a front view of the electron gun of the vircator of Figure 4a;
• FIG. 4e and 4f show grids of the vircator of Figure 4a;
• FIG. 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
• FIG. 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
• FIG. 7a and 7b show two embodiments of the multibeam vircator according to the invention comprising different cathode / grid distances.
• 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 3 ⁇ / 2 ( ⁇ 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.
• VMF multibeam vircator
• 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.
• 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 5 ⁇ / 2, as does the output cavity (we could also operate with a resonance in 3 ⁇ / 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.
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ 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
• the vircator 100 comprises an electron gun 102 and an enclosure 104 of cylindrical shape separated from the gun by a mesh anode
• the barrel has five cathodes, a central cathode Cc1 in the axis
• 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.
• this plane Pc two extremes Exc2 and Exc4 of the same sign appear on either side of the central extremity Exc1 of opposite sign.
• each electron beam of sufficient intensity, coming from each of the cathodes Cc1 to Cc5 produces a virtual cathode in the enclosure.
• 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.
• 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 .
• 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.
• 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 .
• 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.
• 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.
• 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.
• 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.
• d1 will be adjusted to obtain the resonance frequency F0 at the voltage V i, d2 to obtain
• FIG. 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.
• the ends of the cathodes Cd1, Cd2 and Cd3 are on the same plane, a grid
• 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.
• 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:

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• 2001
  • 2001-09-28 FR FR0112547A patent/FR2830371B1/fr not_active Expired - Fee Related
• 2002
  • 2002-09-27 WO PCT/FR2002/003310 patent/WO2003030204A2/fr not_active Application Discontinuation
  • 2002-09-27 EP EP02783221A patent/EP1444715A2/de not_active Withdrawn
  • 2002-09-27 JP JP2003533306A patent/JP2005505112A/ja active Pending
  • 2002-09-27 US US10/488,792 patent/US20040245932A1/en not_active Abandoned

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