A Microwave Wave Generator Device With A Virtual Cathode Oscillator And Axial Geometry, Comprising At Least One Reflector And A Magnetic Ring, Configured To Be Supplied By A High-Impedance Generator

Abstract

A microwave wave generator device with oscillating virtual cathode, with axial geometry, includes at least one first reflector positioned in a cylindrical waveguide downstream of a thin anode, positioned at the entrance of the cylindrical waveguide, between a cathode and the cylindrical waveguide. The device further includes a tight magnetic ring of width (L.sub.M) along the longitudinal axis z, positioned externally around the cylindrical waveguide, between the thin anode and the first reflector.

Claims

1. A microwave wave generator device with a virtual cathode oscillator and axial geometry, the device comprising a cathode, a thin anode and a cylindrical wave guide, of longitudinal axis z and radius R.sub.G, having a first end forming an entrance to the cylindrical wave guide and a second end forming an exit from the cylindrical wave guide, the cathode upstream of the entry to the cylindrical wave guide and configured to emit electrons, and the thin anode at the entrance to the cylindrical wave guide, between the cathode and the cylindrical wave guide, and further comprising at least a first reflector in the wave guide, transparent to electrons and configured to reflect a microwave wave created by at least one virtual cathode generated in the wave guide, the device further comprising a narrow magnetic ring of width (L.sub.M) along the longitudinal axis z, externally around the cylindrical wave guide at a distance (d.sub.AM) from the thin anode and with the first reflector at a distance (d.sub.AF1) from the thin anode beyond the magnetic ring, such that the magnetic ring is between the thin anode and the first reflector, wherein the magnetic ring is 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 between the thin anode and the first reflector.

2. The device according to claim 1, wherein the distance (d.sub.AM) separating the magnetic ring from the thin anode along the axis z is equal to or greater than a distance (d.sub.AK) separating the cathode from the thin anode.

3. The device according to claim 1, wherein the distance (d.sub.AF1) separating the first reflector from the thin anode is equal to or greater than the sum of the distance (d.sub.AM), separating the magnetic ring from the thin anode, and the width (L.sub.M) of the magnetic ring.

4. The device according to claim 1, wherin the distance (d.sub.AF1) separating the first reflector from the thin anode is equal to or greater than approximately twice the distance (d.sub.AK) separating the cathode from the thin anode.

5. The device according to claim 1, wherein at least the first reflector, in the wave guide, is an open reflector.

6. The device according to claim 1, further comprising a plurality of successive reflectors in the cylindrical wave guide.

7. The device according to claim 6, wherein two successive reflectors of the plurality of reflectors are separated from each other by a distance (d.sub.Fi-1Fi) equal to or less than approximately twice a distance (d.sub.AK) separating the cathode from the thin anode.

8. (canceled)

9. The device according to claim 6, wherin all the reflectors are open and have a same radius R.sub.Ri.

10. The device according to claim 1, wherein the device comprises three reflectors positioned in the wave guide.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0071] The invention according to an example embodiment will be well understood and its advantages will better appear on reading the following detailed description, given by way of indicative example that is in no way limiting, with reference to the accompanying drawings presented below.

[0072] FIG. 1 diagrammatically represents a conventional axial VIRCATOR of the prior art according to an example embodiment, in longitudinal view, illustrating virtual cathode oscillator creation;

[0073] FIG. 2 presents an example of an instantaneous diagram of the position of the electrons in the phase space associated with the formation of a virtual cathode oscillator;

[0074] FIG. 3 diagrammatically represents an axial VIRCATOR with reflectors of the prior art according to an example embodiment as described in document WO2006/037918, in longitudinal view;

[0075] FIG. 4 represents, in a transverse view of the VIRCATOR of FIG. 3, a closed reflector and an open reflector according to an example embodiment;

[0076] FIG. 5 represents an axial VIRCATOR example embodiment with open reflectors as described in the application filed under the Ser. No. 12/62385, and not yet published, in a longitudinal view;

[0077] FIG. 6 diagrammatically illustrates the dynamic 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 practically laminar regime and no virtual cathode formation;

[0078] FIG. 7 presents an example of an instantaneous diagram of the position of the electrons in the phase space in practically laminar regime, in the absence of virtual cathode formation;

[0079] FIG. 8 presents, in longitudinal view, an example embodiment of an axial VIRCATOR with a magnetic ring according to the invention, here comprising open reflectors;

[0080] FIG. 9 presents a transverse view of the VIRCATOR of FIG. 8

[0081] FIG. 10 presents an example of an instantaneous diagram of the position of the electrons in the phase space in the VIRCATOR of FIG. 8;

[0082] FIG. 11 diagrammatically presents iso-contours of the intensity of the magnetic field in a longitudinal direction of the VIRCATOR of FIG. 8;

[0083] FIG. 12 is a 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

[0084] FIG. 13 is a table presenting a power efficiency (in percent) of a device according to embodiments of the present invention as a function of the number of reflectors.

DETAILED DESCRIPTION

[0085] A device according to an embodiment of the invention is represented for example here in FIG. 8.

[0086] As for a conventional device (see in particular FIGS. 1 to 7), the device of FIG. 8 comprises a diode composed of a cathode 102 and an anode, itself formed from a thin sheet called thin anode 104 and from a thick frame 103. The cathode 102 has a radius R.sub.c and the thin anode 104 typically has a thickness of the order of the micrometer, that is to say of a few micrometers or even of a few tenths of micrometers.

[0087] The device further comprises a cylindrical wave guide 105 of inside radius R.sub.G and length L.sub.G. The cylindrical wave guide 105 comprises an axis z in a longitudinal direction, forming the longitudinal axis of the device.

[0088] The thick frame 103 surrounds the cathode 102, and the thick armature 103 and the cathode 102 are positioned at an entrance to the cylindrical wave guide 105 (on the left in the FIG.).

[0089] The thin anode 104 is positioned here at an entrance to the cylindrical wave guide 105, between the cylindrical wave guide 105 and the thick frame 103. The thin anode 104 and the cathode 102 are apart from each other by a distance denoted d.sub.Ak.

[0090] The cathode 102, the thin anode 104, the thick frame 103 and the cylindrical wave guide 105 are positioned in relation to each other aligned and centered on the axis z. They generally have circular sections.

[0091] To emit microwave radiation on the axis, the radius R.sub.G of the microwave guide 105 is advantageously such that the microwave emission frequency f is greater than the cut-off frequency of the fundamental mode TE.sub.11 and less than that of the following mode TM.sub.01:

[00005]$\frac{k_{11}^{}.Math.c}{2.Math..Math..Math..Math.f}{R}_G\frac{k_{01}.Math.c}{2.Math..Math..Math..Math.f}$

where k.sub.11 represents the root of the equation of the Bessel function J.sub.1(K.sub.11)=0 (k.sub.11=1,8412).

[0092] The device according to the invention comprises a magnetic ring 112.

[0093] The magnetic ring 112 is advantageously narrow, of width L.sub.M and inside radius R.sub.M, greater than R.sub.G. In an implementation example in which the ring is a coil, the ring then for example has a thickness which corresponds to a thickness of the conducting wire forming the coil. According to an embodiment that is particularly convenient, the width L.sub.M is approximately equal to d.sub.AK. Generally, a ring is for example considered to be narrow if L.sub.M is approximately equal to half the radius of the wave guide R.sub.G.

[0094] It is positioned around the cylindrical wave guide 105, downstream of the anode 104, at a distance d.sub.AM from the anode 104 along the axis z. Advantageously, the distance d.sub.AM is approximately equal to the distance d.sub.AK separating the cathode 102 from the anode 104.

[0095] The narrowness (in the longitudinal direction of the cylindrical wave guide 105 represented by the axis z) of the magnetic ring 112 thus provides a magnetic field configuration dominated by the stray fields. In other words, due to the fact that the magnetic ring 112 is narrow, it enables generation of stray 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 axis z and are, thereby, braked along the axis z. The current of the beam ends up locally exceeding the critical current I. This results in a local accumulation of charges, which is at the origin of the formation of what is referred to as a non-oscillating virtual cathode. The virtual cathode here is non-oscillating in that only a few electrons are repelled towards the thin anode 104. The magnetic field produced by the ring 112 induces stagnation of the electrons near the axis z.

[0096] The magnetic ring 112 is for example a current coil or a permanent magnet such that it is then possible to dispense with electrical supply.

[0097] According to a particularly advantageous embodiment of the present invention, the device comprises at least a first reflector F.sub.1. The first reflector F.sub.1 is situated at a distance d.sub.AF1 from the thin anode 104 such that d.sub.AF1 is equal to or greater than the sum of d.sub.AM and L.sub.M, and preferably equal thereto.

[0098] In other words, the ring only extends to the first reflector and not beyond, as in the devices having recourse to a guiding magnetic field. The ring is positioned downstream of the anode, which differs from the devices in which the diode is immersed or semi-immersed for example.

[0099] And according to a preferred embodiment, the device comprises a plurality of N reflectors F.

[0100] In the present example embodiment illustrated in FIG. 8, the device comprises a set of three reflectors F.sub.i (that is to say with N=3 with the value of i being from 1 to N), which are here all open at their periphery. The reflectors F.sub.i are located downstream of the thin anode 104 and of the magnetic ring 112 in the cylindrical wave guide. The reflectors F.sub.i are transparent to electrons and are adapted to reflect the electromagnetic waves totally. The reflectors are for example formed of aliminized mylar. In operation, all the reflectors are advantageously placed at the same potential as the thin anode 104.

[0101] Each reflector has a radius R.sub.Fi and two successive reflectors are apart from each other by a distance

[0102] The positioning of the reflectors F.sub.i in the wave guide 105 is such that the microwave power is maximum on exiting the wave guide 105. Furthermore, the reflectors F.sub.i are for example situated at variable distances from each other, that is to say the distance d.sub.AF1 and each distance d.sub.Fi-1Fi may be all different from each other. In other words, all the reflectors of the device are fixed in the cylindrical wave guide 105, but the distances separating two successive reflectors may be different from each other and different from the distance d.sub.AF1 separating the first reflector F.sub.1 from the thin anode 104.

[0103] Advantageously, the distance d.sub.AF1 is equal to or greater than twice the distance d.sub.AK, and each distance d.sub.Fi-1Fi is for example comprised between one to two times the distance d.sub.AK. As a matter of fact, as the electrons are set in azimuthal rotation in the cylindrical wave guide 105 by the magnetic field of the ring 112, the distance d.sub.AF1 separating the first reflector F.sub.1 from the anode 104 is possibly substantially greater than that of the known devices of VIRCATOR type of the prior art and the distance between the reflectors of ranks i and i+1 is also possibly less than that of the known devices of VIRCATOR type of the prior art.

[0104] If the current of the beam is sufficient at the location of a reflector of rank i, a virtual cathode oscillator is initiated behind that reflector, that is to say downstream of the reflector of rank i.

[0105] The setting in rotation of the electrons by the magnetic field of the ring 112 in conjunction with the effect of centrifugal force leads to the disintegration of the beam after the last reflector F.sub.N (here F.sub.3). A large proportion of the electrons is absorbed by the inside wall of the cylindrical wave guide 105, the electrons remaining are moved away from the center of the cylindrical wave guide 105, that is to say the axis z, which minimizes any possible interaction between the electrons and the magnetic waves at the center of the cylindrical wave guide 105 where the maximum microwave power of the mode TE.sub.11 is situated.

EXAMPLES

[0106] The behavior of an axial VIRCATOR emitting in the S band and comprising N reflectors F.sub.i and a magnetic ring 112 has been numerically simulated.

[0107] In the simulated devices, the cylindrical wave guide 105 is here of length L.sub.G=500 mm.

[0108] They comprise 1 to 3 reflectors, that is to say N=1, 2 or 3, open at their periphery, of uniform radius R.sub.Fi less than R.sub.G.

[0109] The distance between the reflector F1 of the anode and the distances separating each reflector F.sub.i from the preceding reflector, as a function of the number of reflectors F.sub.i disposed in the wave guide are given in the table of FIG. 12.

[0110] All the devices considered here make it possible to generate a single-frequency microwave emission in the S band along the axis z in the mode TE.sub.11.

[0111] The generator considered here delivers a voltage of 500 kV.

[0112] The critical current I.sub.c beyond which a beam of electrons no longer propagates in the cylindrical wave guide 105 is of the order of 7.4 kA. The critical impedance Z.sub.c for this device is thus 67.5 (ohm).

[0113] The supply generator considered here has an impedance of 70, that is to say greater than the critical impedance.

[0114] The flow of the beam in the guide is thus practically laminar. The conventional process for formation of the virtual cathode oscillator cannot thus be triggered in an axial VIRCATOR lacking a ring.

[0115] The formula which links the frequency emitted to the distance d.sub.AK and the applied voltage V indicates that the distance d.sub.AK is advantageously chosen between approximately 15.6 mm and approximately 31 mm in order for the microwave electromagnetic radiation to be emitted in the S band. The anode-cathode distance d.sub.AK chosen here is approximately 22 mm.

[0116] In order for the current emitted by the cathode to be adapted to an impedance of 70 with a supply of 500 kV and an anode-cathode distance d.sub.AK of approximately 22 mm, the radius of the cathode R.sub.c is then approximately 22.5 mm.

[0117] In order for the microwave emission in the S band to be put in the form of the fundamental mode TE.sub.11 of the cylindrical wave guide 105, the cut-off frequency of the mode, f.sub.11=1.8412c/(2R.sub.G), is advantageously less than or equal to 2 GHz. This leads to a radius R.sub.G of the guide greater than approximately 44 mm.

[0118] The radius R.sub.G chosen here is thus approximately 50 mm.

[0119] The configuration of the magnetic field leads locally to an increase in the current of the beam in the wave guide to exceed the critical current. Under the effect of the stray fields, the electrons are focused on the axis and thereby 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 from the guide. The magnetic field induces stagnation of the electrons in the neighborhood of the axis.

[0120] The magnetic configuration is provided by the magnetic ring positioned here at a distance d.sub.AM from the anode of approximately 29 mm.

[0121] In the example embodiments considered here, the ring creating the magnetic field is here a current coil of 12750 A.turns (Ampere-turns), with dimensions L.sub.M=25 mm and R.sub.M=60.5 mm.

[0122] The first open reflector of radius R.sub.F1=35 mm is positioned at the location of the back face of the magnetic ring, at a distance from the anode d.sub.AF1=54 mm, as indicated by FIG. 12. The first reflector, coupled to the magnetic ring, makes it possible to create the first virtual cathode oscillator behind the first reflector, that is to say downstream of the first reflector.

[0123] The positioning of the following reflectors, for the example embodiments comprising two or three reflectors, of radius R.sub.F1=35 mm, optimizes the microwave power emitted in the S band.

[0124] According to FIG. 12, in a configuration with two reflectors, the second reflector F.sub.2 is positioned at a distance d.sub.F1-F2 of 25 mm from the first reflector F.sub.1; and in a configuration with three reflectors, the second reflector F.sub.2 is positioned at a distance d.sub.F1-F2 of 29 mm from the first reflector F.sub.1, and the third reflector is positioned at a distance d.sub.F2-F3 of 25 mm from the second reflector F.sub.2.

[0125] FIG. 11 represents the iso-contours of the intensity of the magnetic field in a longitudinal cross-section of a device according to the invention here comprising one reflector. The maximum intensity of the magnetic field in the guide is of the order of 0.1 T (Tesla) in a cross-section of the wave guide level with magnetic ring 112, that is to say at a cross-section positioned approximately at half the width L.sub.M of the magnetic ring.

[0126] FIG. 13 summarizes the performance obtained by simulation of an axial VIRCATOR according to the invention comprising one, two or three reflectors.

[0127] FIG. 13 enables it to be noted that the power emitted increases with the number of reflectors. The efficiency attained is of the order of 2.5% with a single reflector and 17.4% with three reflectors. An optimum efficiency is obtained with three reflectors. The addition of a fourth reflector is of little utility for improving the efficiency since the number of electrons decreases and becomes insufficient in the wave guide or near the axis z.

[0128] Thus, a device according to the invention supplied by a high-impedance generator makes it possible to emit a microwave power in the S band with an efficiency close to that obtained with a device having axial configuration with reflectors of the known prior art, supplied with a low-impedance generator.

[0129] The configuration with three reflectors ensures a minimum efficiency of 13.8% for a distance d.sub.F2-F3 between a second reflector and a third reflector comprised between approximately 25 mm and approximately 31 mm, while maintaining the microwave emission frequency.

[0130] According to another example, a device according to the invention such as described above, is coupled to a generator with higher impedance, while emitting at the same microwave frequency in mode TE.sub.11. For example, while maintaining a supply voltage of 500 kV, an increase in the anode-cathode distance d.sub.AK to 30 mm induces a reduction in the accelerator field in the diode and thus a lower emitted current, of the order of approximately 4 kA. Therefore, the diode is adapted to a higher supply impedance, for example approximately 125. The density of the emitted beam then being less, slightly increasing the intensity of the current of the magnetic ring to 14250 A.turns makes it possible to generate a single-frequency microwave emission at 2.31 GHz in the mode TE.sub.11 with an efficiency of 12%. This efficiency may for example be improved by adjusting the positioning of the reflectors in the guide.

[0131] Naturally, the present invention is not limited to the preceding description, but extends to any variant within the scope of the following claims.