EP1982347B1 - Device for coupling between a plasma antenna and a power signal generator - Google Patents

Abstract

The present invention relates to a device for coupling between a plasma column serving as an antenna and an electric power signal generator, associated with a laser. This device comprises at least two conducting electrodes, each pierced with a hole, these holes being coaxial, the electrodes being connected on the one hand to a high voltage direct current source and on the other hand to power signal alternating current source, the laser(s) being placed do that its (their) beam arrives along the axis of said holes of the electrodes.

Description

The present invention relates to a coupling device between a plasma antenna and a power signal generator, and to a method of using a plasma antenna comprising such a coupling device.

Conventional (metallic) radio antennas generally operate in a narrow frequency band, and their dimensions are proportional to the operating wavelength. In the areas of low frequencies ("LF"), very low frequencies ("VLF") and extremely low frequencies ("ELF"), the height of "quarter-wave" antennas is expected to reach several hundred meters at several hundred kilometers (for example 750 m at 100 kHz), which makes their construction very difficult or even unachievable. Moreover, they can not in any case be easily moved. However, these frequency areas are used in particular for communications with submarines underwater.

To solve these problems, it is known to use antennas known as "plasma antennas", for example according to the patent. US 3404403 . This patent describes a plasma antenna comprising a pulsed laser source, means for focusing the laser beam at different points to ionize a column of air and means for coupling a signal at the base of the ionized air column, this column acting as a radiating element for transmitting and / or receiving a radio signal. Plasma antennas are also known from the patent US 6087993 and the patent FR-2 863 782 . In the first document, it is a question of making the antenna movable and of reducing the length of the ionized air column by modulating the excitation current of the ionization generator and concentrating the production of electrons in at least part of this column. In the second, a femtosecond laser is used to generate a filament in the ionized air column.

The plasma antennas described in these documents and operating by air ionization are stealthy and require no infrastructure, unlike conventional antennas. However, in all these known plasma antennas, the coupling between the plasma column and the electric power generator that generates the signal to be transmitted is not optimized. Indeed, for example the above-mentioned French patent FR-2,853,782 describes a capacitive coupling device (of the order of a few pF) or inductive coupling whose impedance is very low, which significantly degrades the power transfer between the electric generator and the antenna.

The subject of the present invention is a coupling device between a plasma column acting as an antenna and a power signal generator, which device allows a very good power transfer between the electric generator and the plasma column when the latter is formed. The present invention also relates to an antenna using such a device, antenna can operate at very low frequencies. Finally, the present invention relates to a method of forming a plasma column to form an antenna.

The coupling device according to the invention is associated with a laser and is characterized in that it comprises at least two conductive electrodes each pierced by a hole, these holes being coaxial, the electrodes being connected on the one hand to a continuous high voltage source and secondly a power signal generator, the laser being arranged so that its beam arrives along the axis of said electrode holes.

• la figure 1 est un schéma simplifié d'un dispositif conforme à l'invention pour la création d'une antenne à plasma;
• les figures 2 à 6 sont des schémas simplifiés du dispositif de la figure 1 montrant les différentes phases successives d'un exemple de mise en oeuvre de l'invention pour la création d'une antenne à plasma;
• la figure 7 est un chronogramme simplifié illustrant les phases de mise en oeuvre des figures 2 à 6;
• la figure 8 est un schéma simplifié d'une variante du dispositif de l'invention; et
• la figure 9 est un chronogramme d'une variante du procédé de l'invention, avec deux tirs laser.

The present invention will be better understood on reading the detailed description of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing, in which:

• the figure 1 is a simplified diagram of a device according to the invention for the creation of a plasma antenna;
• the Figures 2 to 6 are simplified diagrams of the device of the figure 1 showing the different successive phases of an exemplary implementation of the invention for the creation of a plasma antenna;
• the figure 7 is a simplified chronogram illustrating the phases of implementation of Figures 2 to 6 ;
• the figure 8 is a simplified diagram of a variant of the device of the invention; and
• the figure 9 is a chronogram of a variant of the method of the invention, with two laser shots.

The present invention is described below with reference to the creation of an ionized air column, and it is understood that the ionization of this column can be reduced to a filament ionization at the axis of symmetry, as described in the above-mentioned French patent, FR-2 863 782 when using a femtosecond type laser. It is also understood that the preferred embodiment of the device of the invention, as described below, comprises two electrodes pierced with coaxial holes, but the device of the invention may comprise a number more high electrodes. The device described below is shown in a position oriented so that the plasma column that it can create is vertical, but it is understood that this device can have any other orientation, so that the antenna for example horizontal. The plasma antenna obtained according to the invention is described here as an emission antenna, but it is understood that it can also be used in reception, provided, of course, to keep the low generator connected or connected. very low frequency described below.

The device represented in figure 1 comprises two metal plates 1, 2 forming electrodes and each pierced with a hole 3, 4 respectively, the two holes being coaxial, their common axis being referenced 5. The shape of these electrodes is not critical. They may for example be circular or polygonal. The holes 3 and 4 are preferably drilled in the center of these electrodes.

The electrodes 1 and 2 are connected on the one hand via ballast resistors 6 and 7 respectively, to a high voltage source 8, a resistor 9 being connected between the two electrodes, at their junction with the resistors 6 and 7. In the following figures, this resistor 9 is not shown, but it is understood that it may be present. The positive pole of the source 8 is preferably connected to the electrode 2 (in particular when these electrodes are arranged horizontally and at a short distance from the ground). On the other hand, the electrodes 1 and 2 are connected via continuous insulation capacitors 10, 11 respectively and a line 12, preferably coaxial, to a power transmitter 13 of low or very low frequency and high voltage peak which may be near or far away from the electrodes 1, 2 of the antenna. The shield of line 12 is grounded. The distance D between the electrodes 1 and 2 is a function of the value of the high voltage of the source 8. In general, this distance D must be greater than the breakdown distance between the electrodes in the environment in the absence of plasma column, and be smaller than the breakdown distance between the electrodes in the presence of the plasma column.

A priming laser 14 is disposed under the electrode 1, so that the axis of the beam that it produces coincides with the axis 5 at least just before reaching the electrode 1. Thus, if the it is desired to have the laser 14 so that its output axis is horizontal, then a mirror is provided which returns its output beam along the axis 5. It is also possible to have a semi-transparent mirror on the axis 5 if the we want to use two lasers. Two lasers can also be used, for example by dedicating one to the shots and the other to the maintenance of the ionized column forming an antenna.

According to some of the typical embodiments of the invention, in no way limiting, the electrodes 1 and 2 are circular and have a diameter of a few tens of centimeters to a few meters, their mutual distance D is about 50 cm to 1 m, the diameter of the holes 3 and 4 is about 1 cm. The voltage of the source 8 is about 10 to 20 kV, and the power provided by the transmitter 13 can be between a few hundred Watts and a few MW. The average power it delivers must be sufficient to maintain the plasma generated by the high voltage source 8.

We will first expose using the diagram of the figure 6 and the chronogram of the figure 7 the different successive phases of the creation of a plasma antenna using the device of the invention, in the case of a single shot of the laser 14. Then, with reference to the Figures 2 to 6 and the chronogram of the figure 9 the various steps of the plasma antenna formation will be explained in the case of two shots of the laser 14. For reasons of presentation of the explanations, these phases are exposed consecutively, but it is understood that in fact these phases can be simultaneous or almost simultaneous.

It is assumed that initially none of the elements 8, 13 and 14 is activated. To illustrate the chronology of the different phases, reference will be made to the time references T0 to T4 of the timing diagram of the figure 7 .

At time T0, the high voltage source 8 is activated.

At time T1, the laser 14 is fired, focused on the axis 5, beyond the electrode 2. This firing simultaneously produces a discharge between the electrodes 1 and 2 (ionized air column 17 between these electrodes) and the formation of an ionized column 18, thinner than the column 17, centered on the axis 5.

At time T2, the generator 13 is activated, which injects power into the "virtual" antenna constituted by the plasma columns 17 and 18 and which maintains the ionization of these columns, since, as illustrated in FIG. figure 7 , the instantaneous potential difference V _DC between the electrodes 1 and 2 is constant from the instant T1 (see relations below). It will be noted that it is necessary to respect between the instants T1 and T2 a minimum time (typically of the order of a few tens of nanoseconds) so that the plasma column is well established between the electrodes 1 and 2.

tandis que la tension appliquée par la source 8 aux électrodes 1 et 2 est de la forme -/+ V_DC.The signal delivered by the transmitter 13 can be written in the form: $${\mathrm{V}}_{\mathrm{AC}}\mathrm{=}\mathrm{A\; cos}\left(\mathrm{\omega t}\right)\mathrm{,}$$

while the voltage applied by the source 8 to the electrodes 1 and 2 is of the form - / + V _DC .

$${\mathrm{V}}_{\mathrm{E}\mathrm{2}}\mathrm{=}{\mathrm{V}}_{\mathrm{AC}}+{\mathrm{V}}_{\mathrm{DC}}$$

ce qui fait qu'il y a constamment la même différence de potentiel entre les électrodes 1 et 2.The instantaneous potentials of electrodes 1 and 2 are of the form: $${\mathrm{V}}_{\mathrm{E}\mathrm{1}}\mathrm{=}{\mathrm{V}}_{\mathrm{AC}}-{\mathrm{V}}_{\mathrm{DC}}$$

$${\mathrm{V}}_{\mathrm{E}\mathrm{2}}\mathrm{=}{\mathrm{V}}_{\mathrm{AC}}+{\mathrm{V}}_{\mathrm{DC}}$$

so that there is constantly the same potential difference between the electrodes 1 and 2.

In transmission mode, the electrodes 1 and 2 being brought to the same AC potential, there is no loss of alternative power, this power being injected almost completely into the plasma antenna and contributing to the maintenance of the plasma.

At the end of the transmission (T3), the signal of the transmitter 13 being suppressed, the ionized column 18 forming antenna disappears rapidly (between T3 and T4), and thereby the antenna disappears.

We have shown in figure 8 a variant of the device Figures 1 to 6 .

On this figure 8 , the same elements as those of Figures 1 to 6 are assigned the same numerical references. In this device the figure 8 in order to introduce a continuous potential asymmetry between the electrodes 1 and 2, instead of the resistor 9 of the figure 1 a potentiometric arrangement formed for example by a fixed resistor 19 in series with a variable resistor 20, these two resistors being connected between the electrodes 1 and 2, their common point being connected to the ground. The adjustment of the potentiometer thus formed allows a fine adjustment of the potentials applied to the electrodes 1 and 2, in order to compensate for the losses of direct current absorbed by the plasma conducting antenna. Indeed, the leak resistance on the side of the electrode 2 is lower.

In a variant of the invention (see chronogram of the figure 9 ), after the activation of the high voltage source (T0), a first laser firing (T1), located on the axis 5 between the two electrodes, is carried out, then a second laser firing (T2) focused on the same axis 5, but beyond the electrode 2, then the generator 13 (T3) is activated. The plasma antenna disappears (T5) shortly after the end of the activation of the generator 13 (T4). In detail the different stages of this process are as follows:

Figure 2 : after activating the high voltage source 8 (T0), the laser 14 is activated (T1) to perform a first "shot" focused on the axis 5, between the electrodes 1 and 2, to create by high voltage discharge a thin conductive plasma column 15 between these two electrodes.

Figure 3 laser firing causes the high voltage discharge 16 in the plasma column 15 between the electrodes 1 and 2.

Figure 4 The discharge 16 has the effect of widening the conductive plasma column between the electrodes 1 and 2, the enlarged column being noted 17. It will be noted that after the creation of the plasma antenna, it is possible to short-circuit capacitors 10 and 11 until the end of the use of the plasma antenna. The role of the high voltage generator 8 is then to maintain the ionized column 17 made conductive. It will be noted that the phenomena illustrated in Figures 2 to 4 are almost simultaneous, and have been decomposed for easy description.

Figure 5 A second firing of the laser 14 (T2), focused on the axis 5, is carried out beyond the electrode 2. This second firing causes the formation of a plasma column 18 in continuity of electrical conduction with the column. 17. Since the laser 14 is preferably of the femtosecond type, the column 18 is then reduced to plasma filaments, as described for example in the aforesaid French patent. FR-2 863 782 , and its length can reach several kilometers, which gives it the characteristics necessary for a low (or very low) frequency antenna.

Figure 6 : the transmitter 13 (T3), which injects power, is activated in the "virtual" antenna. Constituted by the plasma columns 17 and 18 and which maintains the ionization of these columns because, as illustrated in FIG. figure 9 , the instantaneous potential difference V _DC between the electrodes 1 and 2 is constant from the instant T1 (as explained above with reference to the figure 7 ).

In conclusion, thanks to the device of the invention, in addition to the advantages inherent to the actual plasma antenna, the conductive coupling between the electrodes and the antenna, a very good efficiency of the power transfer between the generator 13 and the generator is obtained. the antenna (these electrodes being brought to the same instantaneous AC potential, practically all the alternative power is injected into the antenna). In addition, this device is very economical, since it only requires a source of high voltage at low power.

Claims (6)

1. Device for coupling between a plasma column serving as an antenna and an electric power signal generator, associated with a laser (14), characterized in that it comprises at least two conducting electrodes (1, 2) each pierced with a hole (3, 4), these holes being coaxial, the electrodes being connected on the one hand to a high voltage direct current source (8) and on the other hand to a power signal alternating current source (13), the laser being placed so that its beam arrives along the axis (5) of said holes of the electrodes.
2. Device according to Claim 1, characterized in that the laser is a laser of the femtosecond type.
3. Device according to Claim 1 or 2, characterized in that it comprises, between the two electrodes, a potentiometric assembly (19, 20) in order to fine tune the potentials applied to these electrodes.
4. Method for using a plasma antenna comprising a coupling device according to one of Claims 1 to 3, characterized in that it comprises the following steps: activation of the high voltage source, firing of the laser, creating a plasma between the electrodes and beyond, on the common axis (5) of the holes of the electrodes, activation of the power signal generator up to the end of the transmission period.
5. Method according to Claim 4, characterized in that, between the moment of firing the laser and the activation of the power signal generator, a minimal time (Tm) of the order of a few tens of nanoseconds is observed.
6. Method for using a plasma antenna comprising a coupling device according to one of Claims 1 to 3, characterized in that it comprises the following steps: activation of the high voltage source, first firing of the laser, focused between the electrodes, on the common axis (5) of the holes of the electrodes, second firing of the laser, focused beyond the second electrode, on the same common axis, activation of the power signal generator up to the end of the transmission period.

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