EP2873307B1 - Surface-wave applicator and method for plasma production - Google Patents

Description

FIELD OF THE INVENTION

The present invention relates to a surface wave applicator for plasma production, and to a device and method for producing surface wave plasma.

BACKGROUND OF THE INVENTION

Surface wave plasmas are a type of high frequency plasma (HF), that is, at a frequency of 1 MHz or less to more than 10 GHz [1] in which the plasma is maintained by a electromagnetic wave (in particular radiofrequency or microwave) propagating along a dielectric tube in contact with the plasma.

The article by M. Moisan et al [2] provides a thorough review of the bibliography in this area.

Depending on the case, the plasma can be generated outside the dielectric tube or inside thereof, or both inside and outside the tube.

In this technology, the plasma and the dielectric tube constitute the propagation medium of the microwaves that generate the plasma along the propagation zone.

The microwave electromagnetic field is called surface because the intensity of the electric field is maximum at the interface between the dielectric tube and the plasma.

In general, surface wave plasmas are produced in the absence of a static magnetic field, except at low pressure where an axial magnetic field (i.e. in the tube direction) can be applied to improve the radial confinement of the plasma and / or produce plasma excitation at the electron cyclotron resonance.

Generally, the surface wave plasmas are produced in a dielectric tube by a surface electromagnetic wave generated from a gap gap field applicator (or "gap" according to the English terminology), as schematized on the figure 1 .

The figure 1 illustrates a sectional view of half of a dielectric tube 3 containing a plasma 4.

The X axis is the axis of revolution of the tube 3.

Around the tube are arranged electrically conductive elements 2a, 2b which have, in this configuration, main surfaces respectively parallel and perpendicular to the dielectric tube 3.

Moreover, the elements 2a and 2b are spaced apart from a gap G, whose width is typically of the order of a few mm.

An electromagnetic surface wave W is generated from the gap G.

At the surface of the conductive elements 2a and 2b, the electric field has only a radial component, that is to say in the case illustrated in FIG. figure 1 perpendicular to the surface of the conductive element 2a and the thickness of the conductive element 2b.

The electromagnetic wave W thus propagates in a direction perpendicular to the gap, and substantially symmetrically (waves W1 and W2) on either side of the axis of the gap G (which is perpendicular to the X axis dielectric tube 1).

Different devices, or applicators, for launching a surface electromagnetic wave in the dielectric tube have already been proposed.

In these devices, the dielectric tube passes through a box (the applicator is then called "surfatron") or a waveguide (the applicator is then called "surfaguide") which allows to apply to the tube, over a short length , the microwave electric field that will produce the plasma along which it can propagate.

The Figure 2A illustrates an example of a surfatron; the Figure 2B illustrates an example of a surfaguide. These two figures are extracted from [3].

The surfatron of the Figure 2A is a cylindrical box closed by a conductive partition 2b.

The dielectric tube 3, which is perpendicular to the partition 2b, has an axial conductive strip 2a along its entire length.

The tube 3 is arranged inside the cylindrical box, a gap G being formed between the end of the tube 3 and the conductive partition 2a.

The introduction of electromagnetic power is shown schematically by the reference P.

The surfaguide of the Figure 2B comprises a waveguide GO, which is traversed perpendicularly by the dielectric tube 3, the launch gap G being formed between the wall of the waveguide and the tube.

Most of these devices, which make it possible to launch a surface wave with azimuth symmetry (mode m = 0), have either impedance matching means of their own (as in the case of surfatron), or means independent (case of surfaguide).

Most often, as illustrated in figure 1 , the surface wave is symmetrical with respect to the gap.

The figure 3 illustrates the evolution of the radial and axial components of the electric field of the plasma 4 towards the outside of the dielectric tube 3 (medium A consisting for example of air or of a dielectric), as a function of the distance r in the radial direction to from the Z axis of the tube 3.

The ordinate axis indicates the intensity of the electrical component of the electromagnetic wave expressed in relative units.

We observe on the figure 3 that the axial component (dashed line) of the electrical component of the electromagnetic wave is continuous from the plasma 4 to the external medium A, while the radial component (solid line curve) of the electric field has a significant discontinuity in the dielectric tube 3.

Current devices, however, have a number of disadvantages.

First, most applicators (also called launchers) surface wave (or "surface wave launcher" in the English terminology) have a high complexity of design and manufacturing, resulting in a relatively high cost.

On the other hand, as can be seen at Figures 2A and 2B all these applicators have a large size compared to the diameter of the dielectric tubes generally used (the latter being usually of the order of cm).

This size is very detrimental especially in cases where a plurality of discharges is envisaged.

The impedance matching systems of these devices are also expensive and bulky.

Moreover, in most applicators, in the absence of an auxiliary device, such as a reflector, the plasma is produced on either side of the wave launch gap (by the upstream wave and the downstream wave).

However, in general, it is desired to produce plasma in one of these directions, resulting in lost power (up to a factor of 2) in many cases and therefore an unfavorable energy balance.

Finally, some devices are adapted to a given frequency (as in the case of surfaguide) and others can cover, with the same configuration, a limited range of frequencies.

However, the range of frequencies accessible to surface wave plasmas is much wider since it starts at less than 1 MHz (the beginning of the radio frequency domain (RF)) and covers the microwave domain up to more than 10 GHz.

An object of the invention is to provide a surface wave applicator that overcomes the aforementioned drawbacks.

The document WO03 / 103003 discloses a microwave energy applicator in an enclosure whose end is disposed in the wall of the enclosure.

BRIEF DESCRIPTION OF THE INVENTION

• un ensemble coaxial électriquement conducteur, formé d'une âme centrale et d'un conducteur tubulaire externe entourant l'âme centrale et séparé de celle-ci par un volume annulaire de propagation d'une onde électromagnétique, et
• un tube diélectrique inséré, à l'extrémité dudit ensemble coaxial, dans ledit volume annulaire de propagation de l'onde électromagnétique et s'étendant au-delà du plan de sortie de l'applicateur sur une longueur au moins égale au double du diamètre extérieur dudit tube, de sorte qu'une onde électromagnétique se propageant dans l'ensemble coaxial est introduite dans la section dudit tube diélectrique selon la direction longitudinale dudit tube afin de produire un plasma à onde de surface le long de la partie du tube diélectrique dont la paroi interne et/ou la paroi externe est en contact avec un gaz plasmagène.

According to the invention, there is provided a surface wave applicator for the production of plasma, comprising:

• an electrically conductive coaxial assembly formed of a central core and an outer tubular conductor surrounding the central core and separated therefrom by an annular volume of propagation of an electromagnetic wave, and
• a dielectric tube inserted at the end of said coaxial assembly into said annular volume of propagation of the electromagnetic wave and extending beyond the plane of leaving the applicator over a length at least equal to twice the outside diameter of said tube, so that an electromagnetic wave propagating in the coaxial assembly is introduced into the section of said dielectric tube in the longitudinal direction of said tube in order to produce a surface wave plasma along the portion of the dielectric tube whose inner wall and / or the outer wall is in contact with a plasma gas.

According to one embodiment, the ends of the central core and the outer conductor of the coaxial assembly are coplanar.

According to another embodiment, the outer conductor at least partially surrounds the dielectric tube beyond the plane of the end of the central core.

According to another embodiment, the central core occupies at least partially the interior volume of the dielectric tube beyond the plane of the end of the outer conductor.

Particularly advantageously, the coaxial assembly further comprises an impedance matching device.

According to an advantageous embodiment of the invention, the length of the dielectric tube inserted in the coaxial assembly is chosen to ensure impedance matching between the impedance of the plasma and the characteristic impedance of the coaxial assembly.

Furthermore, the coaxial assembly may comprise a circulation circuit of a cooling fluid arranged in the central core and / or in the outer conductor.

On the other hand, the dielectric tube may comprise a circulation circuit of a cooling dielectric fluid arranged in the interior volume and / or in the thickness of said tube.

According to one embodiment, the applicator further comprises a cylindrical permanent magnet whose direction of magnetization is parallel to the axis of the applicator, arranged at the end of the central core.

• un aimant permanent cylindrique dont la direction d'aimantation est parallèle à l'axe de l'applicateur, agencé à l'extrémité de l'âme centrale et,
• au moins un aimant permanent annulaire dont la direction d'aimantation est parallèle à l'axe de l'applicateur et de même sens que l'aimantation de l'aimant cylindrique central, agencé autour de l'extrémité du conducteur externe,

According to another embodiment, the applicator further comprises:

• a cylindrical permanent magnet whose direction of magnetization is parallel to the axis of the applicator, arranged at the end of the central core and,
• at least one annular permanent magnet whose direction of magnetization is parallel to the axis of the applicator and in the same direction as the magnetization of the central cylindrical magnet, arranged around the end of the outer conductor,

the magnetization of said magnets being chosen so as to form a magnetic field capable of providing, in a zone distant from the end of the applicator, an electron cyclotron resonance coupling with the microwave electric field generated by said applicator,

the outer radius and the magnetization of the annular magnet being further selected so that the magnetic field lines generated by said magnets pass through the electron cyclotron resonance coupling zone in a direction substantially parallel to the axis of the applicator .

According to one embodiment, the applicator comprises a confinement tube made of a dielectric material extending concentrically around the dielectric tube, said confinement tube being embedded in the external electrical conductor of the coaxial assembly.

Another object relates to a device for producing surface wave plasma, comprising an enclosure containing a plasmagenic gas and at least one applicator as described above, in which a part of the inner wall and / or the outer wall of the tube dielectric extending beyond the exit plane of the applicator is in contact with the plasma gas.

According to one embodiment, the dielectric tube is sealed and constitutes said chamber containing the plasma gas.

According to one variant, the dielectric tube is located inside the enclosure.

According to one embodiment of the invention, the enclosure comprises a confinement tube made of a dielectric material extending concentrically around the dielectric tube, said confinement tube being embedded in the external electrical conductor of the coaxial assembly of the applicator.

In a particularly advantageous manner, the embedment depth of said dielectric confinement tube is equal to (2k + 1) λ / 4, where k is an integer, λ is the wavelength of the electromagnetic wave propagating within a dielectric tube inserted in the coaxial assembly, said wavelength λ being given by the formula λ = λ ₀ / ε ^1/2 , where λ ₀ is the wavelength of the electromagnetic wave propagating in the vacuum or in the air and ε is the relative permittivity of the dielectric material of the confinement tube with respect to the permittivity of the vacuum

The dielectric tube may be open at its end opposite the coaxial assembly, the plasmagenic gas being in contact with the inner wall and the outer wall of the tube.

Alternatively, the dielectric tube may be closed at its end opposite the coaxial assembly, the plasmagenic gas being in contact only with the outer wall of the tube.

According to another variant, the dielectric tube may be closed at its end opposite the coaxial assembly, the inside of said tube being evacuated or filled with a material (solid or fluid) dielectric.

Furthermore, the chamber may comprise a device for introducing the plasma gas into the chamber and a device for pumping the plasma gas from inside to outside the chamber.

According to a particular embodiment, the central core comprises a conduit for introducing the plasma gas into the chamber.

The plasma gas pressure inside the chamber is preferably less than 133 Pa when a suitable magnetic field providing electronic cyclotron resonance is applied.

• la propagation d'une onde électromagnétique dans un ensemble coaxial électriquement conducteur, formé d'une âme centrale et d'un conducteur externe entourant l'âme centrale et séparé de celle-ci par un volume annulaire de propagation de l'onde électromagnétique, et
• l'introduction de ladite onde électromagnétique dans la section dudit tube diélectrique selon la direction longitudinale dudit tube, ledit tube diélectrique étant inséré, à l'extrémité dudit ensemble coaxial, dans le volume annulaire de propagation de l'onde électromagnétique et s'étendant au-delà du plan de sortie de l'ensemble coaxial sur une longueur au moins égale au double du diamètre extérieur dudit tube.

Finally, another object relates to a method of producing surface wave plasma along a dielectric tube whose inner wall and / or the outer wall is in contact with a plasma gas, characterized in that includes:

• propagating an electromagnetic wave in an electrically conductive coaxial assembly formed of a central core and an outer conductor surrounding the central core and separated therefrom by an annular volume of propagation of the electromagnetic wave, and
• introducing said electromagnetic wave into the section of said dielectric tube in the longitudinal direction of said tube, said dielectric tube being inserted at the end of said coaxial assembly into the annular volume of propagation of the electromagnetic wave and extending to the output plane of the coaxial assembly over a length at least equal to twice the outside diameter of said tube.

According to one form of implementation of the method, the electromagnetic wave is a microwave wave.

Optionally, the plasma gas pressure is less than 133 Pa and the plasma is produced by electron cyclotron resonance.

According to another embodiment of the method, the electromagnetic wave is a radiofrequency wave.

Advantageously, the coaxial assembly is cooled by a circulation of a cooling fluid inside said assembly.

Optionally, the dielectric tube is cooled by circulating a dielectric cooling fluid inside said dielectric tube.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est un schéma de principe d'un applicateur d'onde de surface conventionnel,
• les figures 2A et 2B présentent respectivement des illustrations d'un surfatron et d'un surfaguide appartenant à l'état de la technique,
• la figure 3 est un graphe illustrant l'évolution des composantes radiale et axiale du champ électrique du plasma vers l'extérieur du tube diélectrique,
• la figure 4 est un schéma de principe d'un applicateur d'onde de surface selon un premier mode de réalisation de l'invention,
• la figure 5 est un schéma de principe d'un applicateur d'onde de surface selon un deuxième mode de réalisation de l'invention (production de plasma à l'intérieur du tube diélectrique),
• la figure 6 est un schéma de principe d'un applicateur d'onde de surface selon un troisième mode de réalisation de l'invention (production de plasma à l'extérieur du tube diélectrique),
• la figure 7 présente un exemple de mode de réalisation permettant d'obtenir une adaptation d'impédance entre l'impédance du plasma et l'impédance caractéristique de la ligne coaxiale,
• la figure 8 est un schéma de principe d'un dispositif de production de plasma selon une forme d'exécution particulière de l'invention, correspondant à la production de plasma en régime dynamique, impliquant une introduction de gaz et un pompage,
• la figure 9 est un schéma de principe d'une variante d'un applicateur d'onde de surface selon l'invention, dans laquelle on applique en outre un champ magnétique au moyen d'un aimant permanent agencé à l'extrémité de l'âme centrale,
• la figure 10 est un schéma de principe d'une variante d'un applicateur d'onde de surface selon l'invention, dans laquelle on applique en outre un champ magnétique au moyen d'un premier aimant permanent agencé à l'extrémité de l'âme centrale et d'un second aimant permanent annulaire agencé à l'extrémité du conducteur externe,
• la figure 11A est un schéma de principe d'un applicateur d'onde de surface selon un autre mode de réalisation de l'invention (confinement du plasma produit à l'extérieur du tube diélectrique) ; la figure 11B est un schéma de principe d'une variante moins avantageuse de la figure 11 A.

Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings in which:

• the figure 1 is a block diagram of a conventional surface wave applicator,
• the Figures 2A and 2B respectively present illustrations of a surfatron and a surfaguide belonging to the state of the art,
• the figure 3 is a graph illustrating the evolution of the radial and axial components of the electric field of the plasma towards the outside of the dielectric tube,
• the figure 4 is a block diagram of a surface wave applicator according to a first embodiment of the invention,
• the figure 5 is a block diagram of a surface wave applicator according to a second embodiment of the invention (production of plasma inside the dielectric tube),
• the figure 6 is a block diagram of a surface wave applicator according to a third embodiment of the invention (production of plasma outside the dielectric tube),
• the figure 7 presents an exemplary embodiment for obtaining an impedance matching between the impedance of the plasma and the characteristic impedance of the coaxial line,
• the figure 8 is a block diagram of a plasma generating device according to a particular embodiment of the invention, corresponding to the plasma production in dynamic regime, involving a gas introduction and a pumping,
• the figure 9 is a block diagram of a variant of a surface wave applicator according to the invention, in which a magnetic field is also applied by means of a permanent magnet arranged at the end of the central core,
• the figure 10 is a block diagram of a variant of a surface wave applicator according to the invention, in which a magnetic field is also applied by means of a first permanent magnet arranged at the end of the central core and a second annular permanent magnet arranged at the end of the outer conductor,
• the figure 11A is a block diagram of a surface wave applicator according to another embodiment of the invention (confining the plasma produced outside the dielectric tube); the Figure 11B is a schematic diagram of a less advantageous variant of the figure 11 A .

DETAILED DESCRIPTION OF THE INVENTION

The figure 4 is a block diagram of a surface wave applicator 1 for plasma production according to the invention.

Said applicator comprises a coaxial assembly 2 electrically conductive, formed of a central core 20 and an outer tubular conductor 21 surrounding the central core 20 and separated therefrom by an annular volume 22 for propagating an electromagnetic wave W .

Such a coaxial assembly 2 is known in itself and its design is within the reach of the skilled person.

Furthermore, a dielectric tube 3 is inserted at the end of the coaxial assembly 2 in the annular volume 22 of propagation of the electromagnetic wave while extending beyond the exit plane of the applicator.

The applicator output plane is the interface between the coaxial assembly 2 and a volume containing a plasma gas, said output plane constituting a boundary between the applicator and the plasma generated by the electromagnetic wave from said gas plasma.

The dielectric tube thus comprises a first portion inserted into the annular volume 22 and a second portion protruding from the exit plane of the applicator, whose inner wall and / or the outer wall is likely to be in contact with a plasma gas.

For plasma generation, a plasmagene gas is brought into contact with the tube 3, the plasmagenic gas being able to be located inside and / or outside said tube or on either side of the tube, according to the applications, some examples of which will be described in detail below.

The tube 3 may be of any dielectric material, which is a medium adapted to the propagation of an electromagnetic wave without significant losses.

Preferably, the tube 3 may be silica (SiO ₂ ), alumina (Al ₂ O ₃ ) or aluminum nitride (AlN), without the invention being limited to these materials.

The tube 3 generally has a circular section and extends in a longitudinal direction X.

The radius of the tube 3 is typically of the order of one centimeter, that is to say between a few millimeters and a few centimeters depending on the application and the operating conditions.

Beyond the exit plane of the applicator, the dielectric tube 3 may have a gradual change in its diameter as is the case with some devices using surface wave plasmas.

The thickness of the tube 3 is generally of the order of one millimeter.

The thickness of the portion of the tube 3 inserted in the coaxial assembly is chosen so that the tube 3 occupies substantially the entire width of the annular volume 22.

Advantageously, sealing of the junction between the tube and the annular volume vis-à-vis the plasma gas by any suitable means.

The length of the tube depends on the intended application.

Typically, the length of the tube 3 is large in front of the diameter of the coaxial applicator (which is of the order of 1 cm) and may, depending on the application, have a length that may range from about 5 cm to 1 cm. meter order.

The length of the portion of the tube 3 extending beyond the exit plane of the applicator advantageously corresponds to the length on which it is desired to generate the plasma.

Preferably, the length of the portion of the extension extending beyond the outlet plane of the applicator greater than or equal to twice the outside diameter of the tube 3 is chosen, so as to produce the plasma essentially along said portion of the tube. tube.

In the opposite case, if the tube extends beyond the outlet plane of the applicator over a short length, that is to say typically smaller than the outside diameter of the tube, the plasma is generated directly at the outlet the applicator without creating a surface wave, which corresponds to a situation not covered by the present invention, wherein a plasma sheet is formed in the exit plane of the applicator.

As will be seen below, the tube 3 can be open at its end 33 opposite to the coaxial assembly 2; alternatively, the tube 3 can be closed at this end 33.

An electromagnetic wave W propagating in the annular volume 22 of the coaxial assembly 2 is introduced into the section of the dielectric tube 3 in the longitudinal direction X of said tube and propagates longitudinally in the thickness of said tube.

In the coaxial part of the device, the electromagnetic wave propagates in an electromagnetic transverse mode (TEM), that is to say a mode where the electric field is purely radial.

In the output Y plane of the applicator 1, the normal to the metal surface of the central core and the external conductor changes direction, passing from a radial direction to the axial direction, parallel to the X axis.

It therefore appears an axial electric field component (in addition to the radial component), which constitutes a very favorable situation for launching a surface wave (which comprises both an axial component and a radial component (cf. figure 3 ) along the dielectric tube beyond the exit plane of the applicator.

Depending on the configurations of the applicator and the plasma gas, the exit plane may consist of the plane defining the end of the central core 20 and / or of the outer conductor 21, the central core and / or the outer conductor 21 being in contact with the plasma gas.

In the embodiment illustrated in figure 4 the ends of the central core 20 and the outer conductor 21 are coplanar and form said output plane Y.

However, as will be seen below, the ends of the central core 20 and the outer conductor 21 are not necessarily coplanar.

In this case, the exit plane of the applicator is defined as the plane defining the end of the portion of the coaxial assembly which is in contact with the plasma gas, depending on whether the plasma gas is located inside and / or outside the dielectric tube 3.

Thus, in the embodiment illustrated in FIG. figure 5 , the plasma gas is confined inside the dielectric tube 3 and the outer conductor 21 protrudes from the central core 20.

In this case, the outlet plane Y of the applicator corresponds to the plane of the end of the central core 20, whatever the position of the end of the outer conductor 21.

Conversely, in the embodiment illustrated in FIG. figure 6 , the plasma gas is confined in an enclosure outside the dielectric tube 3, the outer conductor flush with the wall of said enclosure and the central core 20 protruding from the outer conductor 21.

In this case, the output plane Y of the applicator corresponds to the plane of the end of the outer conductor 21 and the wall of the enclosure, whatever the position of the end of the central core 20.

Due to the change of direction from the normal to the metal surface in the Y output plane of the applicator, an axial electric field component appears, which is a very favorable situation for launching a surface wave (which comprises both an axial component and a radial component) in the section of the dielectric tube 3 beyond the outlet plane Y of the applicator.

Thus, unlike existing techniques, in which an electromagnetic wave is launched into the dielectric tube tangentially to it, the invention proposes to launch an electromagnetic wave in the longitudinal direction of said tube from an electromagnetic wave introduced in the section. dielectric tube.

The efficiency of the system is thus substantially improved since, assuming a perfect impedance matching, all the incident electromagnetic power is introduced and then propagates in the dielectric tube.

To achieve optimum impedance matching, it is desirable to place the impedance matching device - which is in itself a device known to those skilled in the art - in the coaxial assembly, as close as possible to the plasma.

For example, the figure 7 discloses an example where the impedance matching between the plasma impedance Z _p and the characteristic impedance Z _c of the coaxial assembly is obtained by a quarter-wave transformer with impedance Z _i where: $${{\mathrm{<figure-callout\; id="2"\; label="Z\; i"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">Z\; </figure-callout>}}_{\mathrm{i}}}^{\mathrm{2}}={\mathrm{Z}}_{\mathrm{vs}}{\mathrm{Z}}_{\mathrm{p}}$$

In this case, the dielectric tube 3 must be introduced into the coaxial assembly over a length corresponding to a quarter wavelength (λ / 4) in the dielectric.

More generally, those skilled in the art are able to determine the impedance matching means between a given coaxial structure and a given load impedance.

For the implementation of the invention, it is possible to use an electromagnetic wave in a frequency range covering the radiofrequency (RF) and microwave domains.

Within this range, which is very extensive, it is possible to use the ISM frequencies (acronym for "industrial, scientific and medical) such as 13.56 MHz, 27.12 MHz or 40.68 MHz for the RF domain, and 433 MHz, 2.45 GHz or 5.80 GHz for the microwave field.

Naturally, this list is not limiting and the person skilled in the art can choose any other frequency in the RF range (that is to say between 1 and 100 MHz) or in the microwave domain (that is, ie between 100 MHz and 10 GHz) without departing from the scope of the present invention.

Depending on the application, the power applied may be between 1 or a few watts (for example lighting) and a few hundred watts, or more (eg treatment of gaseous effluents).

The skilled person is able to determine the appropriate power depending on the intended application.

Under the effect of the electromagnetic wave propagating in the dielectric tube 3, plasma is generated in the plasma gas which is in contact with the tube 3.

As mentioned above, said plasmagenic gas may be located inside and / or outside the dielectric tube 3.

The plasma gas may be any gas whose components make it possible to generate a plasma under the effect of the electromagnetic wave propagating in the dielectric tube 3.

In applications relating to lighting, the plasma gas may thus be conventionally constituted by one or more rare gases (in particular argon) and mercury.

As non-limiting examples, gases such as nitrogen, oxygen, halogenated gases, or any other gas having physicochemical properties of interest for targeted application can also be envisaged.

According to one embodiment of the invention, the plasmagenic gas is confined inside the dielectric tube 3, which is sealed at its end 33 opposite to the coaxial assembly 2.

The dielectric tube 3, once inserted into the coaxial assembly 2, thus forms a sealed enclosure for the generation of plasma.

The figure 5 illustrates an example of such an embodiment.

On it, the plasma gas 4 is enclosed in the dielectric tube 3 which is sealed at one of its ends around the central core 20 and at its other end 33 by a sealed wall.

As can be seen in this figure, the outer conductor 21 may at least partially surround the dielectric tube 3, beyond the exit plane of the applicator which, in this embodiment, corresponds to the end of the central core 20.

This configuration makes it possible, for example, to form a shield at the level of the exit plane of the applicator and thus to prevent the transmission of the electromagnetic radiation to the outside.

According to another embodiment of the invention, illustrated on the figure 6 , the plasma gas 4 is confined in an enclosure (not shown) and the dielectric tube 3 is itself inserted into said enclosure.

It is thus possible to form plasma outside the dielectric tube 3.

This embodiment is particularly advantageous insofar as the plasma generated outside the dielectric tube, the plasma absorbs the electromagnetic radiation.

A particular example is that of lighting, where the bulb constitutes said enclosure containing the plasma gas, the dielectric tube being arranged inside the bulb.

If the tube 3 is open at its end 33 and thus communicates with the volume of the chamber, plasma can also be formed inside said tube 3.

Optionally, as can be seen on the figure 6 , the central core 20 can occupy at least partly the inside of the dielectric tube 3, beyond the exit plane of the applicator which, in this embodiment, corresponds to the end of the outer conductor 21.

This embodiment is particularly advantageous for cooling the central core 20 by means of an internal circulation of water or any heat transfer fluid in the case of a heat pipe)

Concretely, the sealing of the plasma volume can be achieved by known techniques.

Thus, the sealing of the plasma volume vis-à-vis the applicator can be ensured by the establishment of O-rings between the dielectric tube and the central core and the outer conductor of the coaxial assembly.

Alternatively or additionally, the dielectric tube may be brazed to the central core and the outer conductor of the coaxial assembly.

Optionally, the dielectric tube may be sealed, near its end inserted into the annular volume of the coaxial assembly, by a plug of dielectric material.

On the other hand, when the plasma is to be generated outside the dielectric tube, the dielectric tube 3 can be inserted inside a sealed enclosure, the outer conductor of the coaxial assembly preferably flush with the inner wall of the said enclosure (as shown in figure 8 for example).

The seal between the coaxial assembly and the wall of the enclosure through which it passes is ensured by any appropriate means, such as O-rings, brazing, etc.

In the case of application to lighting, the applicator operates in static mode, that is to say without plasma gas flow.

Alternatively, the applicator can be implemented in dynamic mode, that is to say in an enclosure containing a device for pumping plasma gas from outside to inside the chamber.

This particular embodiment is illustrated in figure 8 , where a pumping device 5 has been schematized in the enclosure.

Optionally (also illustrated in figure 8 ), the core may comprise a conduit 23 for introducing plasma gas into the chamber.

This embodiment of the invention is advantageous when a chemical reaction is carried out in the plasma (for example for the treatment of effluents), since a renewal of the plasma gas and the evacuation of the products of the reaction are then necessary.

Thanks to the pumping device 5 and, if appropriate, to the conduit 23, it is possible to control the working pressure or the gas flow rate in dynamic mode.

In the case of using high electromagnetic power, it may be necessary to cool the applicator.

This cooling can be effected by circulating a suitable fluid (for example, water) inside the central core and / or the outer conductor of the coaxial assembly.

It is also possible to circulate a dielectric cooling fluid in the microwave propagation space 22.

The definition and the realization of the channels allowing this circulation is known in itself and within the reach of the person skilled in the art according to the technical constraints encountered.

When working at very high power, it may be necessary to cool the dielectric tube as well.

This can be achieved by circulating a dielectric fluid in the thickness of said tube, and / or inside the dielectric tube (in the case where the plasma is produced outside the tube)

In general, surface wave plasmas are produced in the absence of a static magnetic field, except at low pressure where an axial magnetic field (in the tube direction) can be applied to improve the radial confinement of the plasma (decrease in plasma losses on the walls of the tube) and / or produce a plasma excitation at the electron cyclotron resonance.

A first simplified embodiment, illustrated at figure 9 can be obtained by inserting at the end of the central core 20 of the coaxial structure a cylindrical magnet 200 of axial magnetization.

Another embodiment, even more advantageous, makes it possible to benefit from the electronic cyclotron resonance (ECR) mode.

où m_e est la masse de l'électron, -e est la charge de l'électron et B₀ l'intensité du champ magnétique correspondant à la résonance cyclotronique électronique (RCE) pour la fréquence micro-onde f₀.At electron cyclotron resonance, electrons are accelerated very efficiently by the microwave electric field if the intensity of the magnetic field (which can be produced by coils or permanent magnets) is such that the frequency of electron gyration in the magnetic field is equal to the frequency f ₀ of the microwave electric field, namely: $${\mathrm{<figure-callout\; id="0"\; label="f"\; filenames="imgf0002.png"\; state="\{\{state\}\}">f</figure-callout>}}_{\mathrm{0}}={\mathrm{<figure-callout\; id="0"\; label="eB"\; filenames="imgf0002.png"\; state="\{\{state\}\}">eB</figure-callout>}}_{\mathrm{0}}/\mathrm{2}{\mathrm{\pi m}}_{\mathrm{e}}$$

where m _e is the mass of the electron, e is the charge of the electron and B _{0 is} the intensity of the magnetic field corresponding to the electron cyclotron resonance (ECR) for the microwave frequency f ₀ .

In the absence of collisions, the trajectory of so-called fast electrons, thus accelerated to electron cyclotron resonance in the magnetic field, then winds in a helical motion around a magnetic field line.

• un aimant permanent cylindrique 200, agencé à l'extrémité de l'âme centrale 20 et dont la direction d'aimantation (schématisée par une flèche) est parallèle à l'axe X ; ledit aimant présente un rayon sensiblement identique à celui de l'âme centrale 20 (concrètement, l'aimant cylindrique peut présenter un rayon légèrement inférieur à celui de l'âme centrale et être logé dans un logement cylindrique ménagé à l'extrémité de l'âme centrale) ;
• un aimant annulaire 201, agencé à l'extrémité du conducteur externe 21 de l'ensemble coaxial et dont la direction d'aimantation (schématisée par une flèche) est parallèle à l'axe X et de même sens que celle de l'aimant cylindrique 200.

To implement this resonance mode, the applicator comprises, as illustrated in FIG. figure 10 :

• a cylindrical permanent magnet 200, arranged at the end of the central core 20 and whose magnetization direction (represented by an arrow) is parallel to the axis X; said magnet has a radius substantially identical to that of the central core 20 (concretely, the cylindrical magnet may have a radius slightly less than that of the central core and be housed in a cylindrical housing provided at the end of the central soul);
• an annular magnet 201, arranged at the end of the outer conductor 21 of the coaxial assembly and whose magnetization direction (represented by an arrow) is parallel to the axis X and in the same direction as that of the cylindrical magnet 200.

Preferably, said annular magnet has an inner radius substantially equal to that of the outer conductor 21, which corresponds to the outer radius of the annular volume 22 of propagation of microwaves, noted R. Concretely, the annular magnet may have an inner radius slightly greater than that of the outer conductor and outer radius less than that of the outer conductor and be housed in an annular housing provided at the end of the outer conductor.

The magnets can be made integral with the coaxial assembly by any appropriate means.

The magnetization of the cylindrical magnet 200 and the annular magnet 201 is chosen so as to form a magnetic field suitable for providing, in a zone distant from the plane Y output of the applicator, an electron cyclotron resonance coupling with the microwave electric field generated by the applicator.

This assumes that the magnetization of said magnets 200 and 201 is sufficient to generate, at a distance from the output plane Y of the applicator, a magnetic field having the intensity B ₀ allowing electronic cyclotron resonance as a function of the microwave frequency according to formula (1) above.

For excitation of the electron cyclotron resonance plasma by microwaves at 2.45 GHz, the resonance condition (B ₀ = 875 gauss) can be obtained by conventional permanent magnets, for example samarium-cobalt.

On the other hand, the cylindrical magnet 200 and the annular magnet 201 make it possible to generate magnetic field lines that pass through the electron cyclotron resonance coupling zone in a direction substantially parallel to the X axis of the applicator.

This effect can be obtained by a judicious choice of the outer radius and the magnetization of the annular magnet 201.

In fact, the larger the outside radius of the annular magnet 201, the more the iso-intensity lines of the magnetic field generated at a distance from the applicator remain parallel to the output plane Y of the applicator over a large radius.

Since the electron cyclotron resonance zone is delimited, in the radial direction, by the zone in which the microwave electric field is the strongest, the use of an annular magnet whose outside radius is much greater than the radius of this zone makes it possible to obtain a zone of ECR substantially parallel to the output plane Y of the applicator.

This zone of strong electric field is considered to extend over a radius of the order of twice the radius of the applicator.

Therefore, if the annular magnet 201 has an outer radius greater than the radius of the strong electric field area, the ECR region is substantially parallel to the exit plane of the applicator over its entire radius 2R range.

On the other hand, because of the presence of the annular magnet 201 having an outer radius greater than 2R, the field lines that start from the pole located at the exit plane of the applicator to reach the opposite pole remain substantially parallel to the axis X of the applicator during their crossing of the zone Z _RCE of radius 2R, including the periphery of this zone.

In other words, the annular magnet has the effect of "straightening" the field lines at the periphery of the ECR area.

According to another embodiment of the invention, in the case where it is desired to produce an annular surface wave plasma along the outer wall of the dielectric tube, it may be advantageous to confine said plasma by means of a tube dielectric confinement having a diameter greater than that of the tube along which the plasma is produced, and arranged concentrically therewith.

The electromagnetic field, which is maximum at the interface between the plasma and the dielectric tube along which said plasma is generated, can thus be absorbed by the annular volume of gas extending around said tube.

This limits the electromagnetic radiation to the outside.

This embodiment is illustrated on the figure 11A .

The dielectric tube for confining the plasma is designated by the reference 6.

The end of the tube 6 opposite to the coaxial assembly is closed, so that the tube 6 constitutes an enclosure capable of enclosing plasma gas.

Thus, according to a particular embodiment, the dielectric confinement tube may constitute the envelope of a light bulb.

The dielectric tube 3 along which the plasma is generated can be open or closed at its opposite end to the coaxial assembly.

It is thus possible to obtain three plasma generation configurations.

In a first case, the tube 3 is open so that the inside of the tube 3 communicates with the outside of said tube, which makes it possible to generate plasma both inside and outside the tube 3, said plasma being confined externally by the tube 6.

In a second case, the tube 3 is closed and evacuated or filled with a dielectric material, for example in liquid form, the plasmagenic gas being contained in the confinement tube 6, outside the tube 3. thus plasma in an annular volume between the tubes 3 and 6.

Finally, in a third case, the tube 3 is closed and contains the plasma gas, the tube 6 not containing plasma gas. Thus plasma is formed in the tube 3 only.

The dielectric confinement tube 6 is advantageously embedded in the outer tubular conductor 21 of the coaxial assembly, to a depth p.

This embedding has the effect of promoting the formation of the two axial and radial components of the electric field HF in the exit plane of said confinement tube, as occurs for the dielectric tube 3 along which the plasma is generated.

The depth p is advantageously substantially equal to (2k + 1) λ / 4, where k is an integer and λ is the wavelength of the electromagnetic wave propagating within the dielectric tube 3 inserted into the coaxial set.

This gives a belly (maximum) electric field in the output plane of the applicator.

où λ₀ est la longueur d'onde de l'onde électromagnétique se propageant dans le vide ou dans l'air, et ε est la permittivité relative du matériau diélectrique du tube de confinement 6 par rapport à la permittivité du vide.Said wavelength λ is given by the formula: $$\mathrm{\lambda }={\mathrm{<figure-callout\; id="0"\; label="\lambda "\; filenames="imgf0002.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}_{\mathrm{0}}/{\mathrm{<figure-callout\; id="1"\; label="\epsilon "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}}^{\mathrm{1}/\mathrm{2}}$$

where λ ₀ is the wavelength of the electromagnetic wave propagating in the vacuum or in the air, and ε is the relative permittivity of the dielectric material of the confinement tube 6 with respect to the permittivity of the vacuum.

Preferably, to favor the compactness of the device, k = 0 is chosen, ie a embedment depth of the confinement tube of the order of λ / 4.

As can be seen on the figure 11A the external electrical conductor may have a shoulder 21a protruding from the outlet plane Y of the applicator.

This shoulder makes it possible to prevent the electromagnetic wave from propagating radially outside the dielectric confinement tube 6 in the exit plane.

The figure 11 B presents for comparison a situation in which the confinement tube 6 is simply in contact with the exit surface of the external electrical conductor 21.

In this case, only the axial component of the electric field HF, normal to the surface of the external electrical conductor 21, is present, which is not very favorable to the launching of a surface wave; on the contrary, this configuration promotes the radial propagation of the wave, on the surface of the outer conductor 21.

Applicators according to the various embodiments of the invention can be advantageously used, unitarily or in combination to form extended sources, in multiple applications.

Among these, there may be mentioned in a nonlimiting manner lighting, the formation of plasma sources extended to perform surface treatments (by combining several applicators in the same enclosure), etching for microelectronics and nanotechnologies, the treatment of gaseous effluents, plasma sterilization, sources of neutral species, photon sources, or ion propulsion.

The invention makes it possible to remedy the disadvantages of the existing devices described above.

In particular, the applicator has a design and manufacture substantially simpler than existing devices, and adapted to a wide range of frequencies (RF and microwave).

Furthermore, the radial size of the applicator is determined by the overall size of the coaxial assembly (typically the external diameter of the outer tubular conductor), which is generally substantially smaller than that of tangential wave launch devices. such as surfatron and surfaguide illustrated in Figures 2A and 2B .

For example, the diameter of a coaxial applicator is of the order of 1 to 2 cm while the dimensions of a surfaguide are of the order of the wavelength of the electromagnetic wave.

On the other hand, the applicator works with conventional impedance matching devices, depending on the frequency of the electromagnetic wave employed, and therefore does not require the implementation of bulky and expensive devices.

The surface wave being launched in a single direction (that is, the direction of the end 33 of the dielectric tube 3 opposite the coaxial assembly 2), there is no loss of energy.

The energy efficiency of the applicator is therefore optimal.

Finally, as mentioned above, the applicator can be readily adapted to cyclotron electron resonance (ECR) coupling to form and maintain the plasma at low pressure.

The structural modifications to be made to the applicator are indeed minimal, since it suffices, as we saw above, to have permanent magnets at the end of the central core and the outer conductor of the assembly. coaxial.

REFERENCES

1. [1] M. Moisan, J. Pelletier, Collisional Plasma Physics, EDP Sciences, Les Ulis, France (2006), pp 405-408
2. [2] M. Moisan, A. Shivarova, AW Trivelpiece, "Experimental investigations of the propagation of surface waves along a plasma column," Plasma Physics, Vol. 24, No. 11, pp. 1331-14000, 1982
3. [3] M. Moisan, Z. Zakrzewski, Surface wave plasma sources, in "Microwave Excited Plasmas", edited by M. Moisan and J. Pelletier, Elsevier, Amsterdam (November 1992) Chapter 5, pp 123-180, Fig. 5.13
4. [4] M. Moisan, J. Margot, Z. Zakrzewski, Surface Wave Plasma Sources, in "High Density Plasma Sources", edited by Oleg A. Popov, Noyes Publication, Park Ridge, New Jersey (1995), Chap 5, pp. 191- 250

Claims (19)

1. A surface wave applicator (1) for plasma production, including:

   - an electrically conductive coaxial assembly (2), consisting of a central core (20) and of an outer tubular conductor (21) surrounding the central core (20) and separated therefrom by an annular volume (22) for propagation of an electromagnetic wave (W), and

   - a dielectric tube (3) inserted, at the end of said coaxial assembly (2), into said annular volume (22) for propagation of the electromagnetic wave,

   said applicator being characterized in that the dielectric tube extends beyond the exit plane (Y) of the applicator for a length at least equal to twice the outer diameter of said tube (3), so that the electromagnetic wave (W) propagating in the coaxial assembly (2) is introduced into the section of said dielectric tube (3) in the longitudinal direction (X) of said tube (3) so as to produce a surface wave plasma along the portion of the dielectric tube the inner wall (30) and/or the outer wall (31) whereof is in contact with a plasma gas (4).
2. The applicator according to Claim 1, characterized in that the ends of the central core (20) and of the outer conductor (21) of the coaxial assembly (2) are coplanar.
3. The applicator according to Claim 1, characterized in that the outer conductor (21) surrounds at least partially the dielectric tube (3) beyond the plane of the end of the central core (20).
4. The applicator according to Claim 1, characterized in that the central core (20) occupies at least partially the interior volume of the dielectric tube (3) beyond the plane of the end of the outer conductor (21).
5. The applicator according to one of Claims 1 to 4, characterized in that the coaxial assembly comprises an impedance-matching device, the length of the dielectric tube (3) inserted into the coaxial assembly (2) is chosen so as to provide impedance matching between the impedance of the plasma (Z_p) and the characteristic impedance (Z_c) of the coaxial assembly (2).
6. The applicator according to one of Claims 1 to 5, characterized in that the coaxial assembly (2) includes a circuit for circulating a cooling fluid provided in the central core (20) and/or in the outer conductor (21) and/or the dielectric tube (3) includes a circuit for circulating a dielectric cooling fluid provided inside or inside the thickness of said tube.
7. The applicator according to one of Claims 1 to 6, characterized in that it further includes a cylindrical permanent magnet with its magnetization direction parallel to the axis of the applicator, positioned at the end of the central core.
8. The applicator according to one of Claims 1 to 6, characterized in that it further includes:

   - a cylindrical permanent magnet with its magnetization direction parallel to the axis of the applicator, positioned at the end of the central core, and

   - at least one annular permanent magnet with its magnetization direction parallel to the axis of the applicator and concurrent with the magnetization of the central cylindrical magnet, positioned around the end of the outer conductor,

   the magnetization of said magnets being chosen so as to form a magnetic field suitable for obtaining, in a region remote from the end of the applicator, electron cyclotron resonance coupling with the microwave electric field generated by said applicator,
   the outer radius and the magnetization of the annular magnet also being selected so that the magnetic field lines generated by said magnets pass through the electron cyclotron resonance coupling region in a direction substantially parallel to the axis of the applicator.
9. The applicator according to one of Claims 1 to 8, characterized in that it includes a confinement tube (6) made of a dielectric material extending concentrically about the dielectric tube (3), said confinement tube (6) being embedded in the outer electrical conductor (21) of the coaxial assembly.
10. A surface wave plasma production device, including an enclosure containing a plasma gas (4) and at least one applicator (1) according to one of Claims 1 to 9, wherein a portion of the inner wall (30) and/or of the outer wall (31) of the dielectric tube (3) extending beyond the exit plane of the applicator is in contact with the plasma gas (4).
11. The device according to Claim 10, characterized in that the dielectric tube (3) is located inside the enclosure.
12. The device according to Claim 11, characterized in that the enclosure includes a confinement tube (6) made of a dielectric material extending concentrically about the dielectric tube (3), said confinement tube being embedded into the outer electrical conductor (21) of the coaxial assembly of the applicator.
13. The device according to Claim 12, characterized in that the depth (p) of embedment of said dielectric confinement tube (6) is equal to (2k + 1) λ/4, where k is an integer, λ is the wavelength of the electromagnetic wave propagating within the dielectric tube (6) inserted into the coaxial assembly, said wavelength (λ) being given by the formula λ = λ₀ / ε^1/2, where λ₀ is the wavelength of the electromagnetic wave propagating in a vacuum or in air and ε is the relative permittivity of the dielectric material of the confinement tube (6) relative to the permittivity of a vacuum.
14. The device according to one of Claims 11 to 13, characterized in that the dielectric tube (3) is open at its end (33) opposite to the coaxial assembly (2), the plasma gas (4) being in contact with the inner wall (30) and the outer wall (31) of the tube (3).
15. The device according to one of Claims 11 to 13, characterized in that the dielectric tube (3) is closed at its end (33) opposite to the coaxial assembly (2), the plasma gas (4) being in contact only with the outer wall (31) of the tube (3).
16. The device according to one of Claims 11 to 13, characterized in that the dielectric tube (3) is closed at its end (33) opposite to the coaxial assembly (2), said tube (3) being placed under vacuum or filled with a dielectric material.
17. The device according to one of Claims 11 to 16, characterized in that the enclosure includes a device for introducing plasma gas into the enclosure and a device (5) for pumping plasma gas (4) from the interior to the exterior of the enclosure.
18. The device according to one of Claims 14 to 17, characterized in that the central core (20) includes a duct (23) for introducing plasma gas into the enclosure.
19. A method for producing a surface wave plasma along a dielectric tube (3), the inner wall (30) and/or the outer wall (31) whereof is in contact with a plasma gas (4), characterized in that it includes:

    - propagation of an electromagnetic wave (W) in an electrically conductive coaxial assembly (2), consisting of a central core (20) and of an outer conductor (21) surrounding the central core (20) and separated therefrom by an annular volume (22) for propagation of the electromagnetic wave and

    - introduction of said electromagnetic wave (W) into the section of said dielectric tube (3) in the longitudinal direction (X) of said tube (3), said dielectric tube (3) being inserted, at the end of said coaxial assembly (2), into the annular volume (22) for propagation of the electromagnetic wave and extending beyond the exit plane of the coaxial assembly over a length at least equal to twice the outer diameter of said tube (3).

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