FR3038488A1 - COOLING A COAXIAL LINE TRUNK AND A PLASMA PRODUCTION DEVICE - Google Patents

Abstract

The invention relates to the cooling of a coaxial line section and a plasma generating device comprising the coaxial line section. The coaxial line section (10) comprising a core (11) and an outer conductor (12) separated by a dielectric medium (13), the core (11) being cooled by circulating a coolant circulating in a channel ( 20) internal to the core (11), the section (10) further comprising at least one tube (21, 22) passing through the dielectric medium (13) and the outer conductor (12), the at least one tube (21) , 22) supplying the channel (20) with heat transfer fluid.

Description

FIELD OF THE INVENTION The invention relates to the cooling of a coaxial line section and a plasma production device comprising the coaxial line section.

The production of plasma can be done from a gas enclosed in a chamber, the gas being excited by microwave energy. The plasma production device comprises one or more applicators for introducing the microwave energy into the enclosure. The applicator or applicators are generally formed by a coaxial line section receiving the microwave energy by a connector. One end of the section opens into the chamber to excite the gas.

During the operation of the plasma generating device, the coaxial line section heats up and it is necessary to cool it. It is known to cool the soul of the section by circulating water.

Microwave plasma production devices often use magnetrons as a microwave source. The most widespread magnetrons operate in a frequency band around 2.45 GHz. These magnetrons are commonly used in domestic microwave ovens. At this frequency a phase shift appears between the electric field generated by the microwave source and the poles of the water molecule. This phase shift generates a heat-generating dielectric loss that is used in microwave ovens.

This dielectric loss represents a disadvantage for water used to cool the core of a coaxial line section. It is therefore avoided to cross the dielectric medium by the cooling water.

In a plasma generator, we therefore seek to maintain the flow of water always inside the soul. The supply of water into the applicator is done by the opposite end to that which opens into the enclosure. To enable this assembly, the connector by which the microwave energy is injected is arranged radially with respect to the axis of the coaxial line section and at the level of the connector, the section is equipped with a short circuit between the core and the outer conductor of the coaxial line, at a distance equal to a quarter of the wavelength (λ / 4) of the microwave source in the dielectric medium considered. This λ / 4 short circuit behaves like an open circuit at the connector. At the short circuit, it is then easy to introduce the water directly into the core of the coaxial line.

This structure is adapted to a frequency of 2.45 GHz. If we move away from this frequency, the open circuit then loses its electrical characteristics, causing losses at the connector. In addition, to obtain an open circuit with good performance, manufacturing tolerances of short circuit at λ / 4 are tight, which tends to increase the cost of implementation of the applicator. The invention aims to overcome all or part of the problems mentioned above by proposing a new coaxial line section cooled by means of a coolant circulating in the core and without short circuit at λ / 4. This line section is advantageously used as an applicator in a plasma production device. For this purpose, the subject of the invention is a coaxial line section comprising a core and an outer conductor separated by a dielectric medium, the core being cooled by circulation of a coolant circulating in a channel internal to the core, the section further comprising at least one tube passing through the dielectric medium and the outer conductor, the at least one tube supplying the heat transfer fluid channel.

It is accepted to circulate water (or other heat transfer fluid) partly in the dielectric medium during a short path with respect to the length of the channel flowing in the core. Although the water is slightly heated during this journey, it still retains sufficient thermal properties to cool the soul.

Crossing the dielectric medium to supply the cooling channel of the core makes it possible to dispense with the short circuit at λ / 4. It is thus possible with the same coaxial line section structure to provide broadband use. More precisely, the invention makes it possible to avoid the stress related to the wavelength imposed by the short circuit. In addition, the short circuit at λ / 4 unnecessarily increases the length of the coaxial line section. For frequencies below 1 GHz, the length of the short circuit at λ / 4 may be prohibitive. The invention thus makes it possible to reduce the total length of the coaxial line section.

The coaxial line section may comprise two tubes passing through the dielectric medium and the outer conductor, the fluid entering the channel through a first of the two tubes and leaving the channel by a second of the two tubes.

A first of the two tubes may be disposed in a second of the two tubes.

The outer conductor is coaxial with the soul around an axis. The tube or tubes advantageously cross the dielectric medium and the outer conductor radially relative to the axis. The core and the outer conductor extend between two ends of the section along the axis. The channel can be folded inside the core The two tubes can be located in the same section of the coaxial line section, the section being perpendicular to the axis.

The section is advantageously disposed in the vicinity of one end of the section and the channel may be arranged on one side of the section.

The at least one tube may comprise a tubular-shaped envelope of dielectric material inside which the fluid circulates, the envelope being in contact with the outer conductor.

The at least one tube may comprise a tubular metal material surrounded by the envelope of dielectric material.

The outer conductor is coaxial with the soul around an axis. The channel may include an axially extending central tube, a cavity disposed at the end and a cavity formed around the central tube.

The outer conductor is coaxial with the soul around an axis. The core and the outer conductor extending between two ends of the section along the axis. The section may comprise a coaxial connector connected to the core and to the outer conductor, the coaxial connector being disposed coaxially with the section at a first of its ends. The subject of the invention is also a device for producing plasma from a gas comprising a chamber containing the gas and at least one coaxial applicator of a microwave energy comprising a coaxial line section according to the invention, microwave energy being provided to the device by the coaxial connector, a second of the two ends of the section opening into the enclosure. The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given by way of example, a description illustrated by the attached drawing in which: FIG. 1 schematically represents a section of a line coaxial embodying the invention; Figures 2a and 2b show two alternative embodiments of tubes for cooling the coaxial line section of Figure 1; FIG. 3 schematically represents a plasma production device in which the invention can be implemented; FIGS. 4 and 5 show a microwave energy applicator implemented in the device of FIG. 3; FIG. 6 represents a variant of a coaxial line section.

For the sake of clarity, the same elements will bear the same references in the different figures.

FIG. 1 schematically represents a coaxial line section 10 comprising a core 11 and an outer conductor 12 separated by a dielectric medium 13. The outer conductor 12 is coaxial with the core 11 about an axis 14. The core 11 and the outer conductor 12 extend between two ends 15 and 16 of the section 10 along the axis 14. The coaxial line section 10 can be implemented in many uses where transmission of an electromagnetic wave is required between the two ends 15 and 16. This type of coaxial line section is used in particular for the transmission of microwaves. The frequency band generally used for microwaves is between 300 MHz and 3 GHz. The invention works well in this frequency band. It is nevertheless possible to implement the invention beyond or below this frequency band.

In the example shown, a coaxial connector 17 makes it possible to introduce the electromagnetic wave at the end 16 of the coaxial line section 10. The axis of the coaxial connector 17 is the axis 14 of the section 10. The other end 15 of the section 10 can also be equipped with a connector or any other means allowing the electromagnetic wave to leave the section 10. The core 11 comprises an internal channel 20 in which circulates a coolant, such as water, for cooling the core 11. Any other heat transfer fluid can be implemented within the scope of the invention. Fluids such as methane, freon, helium ... can be implemented. In the case of the water used as heat transfer fluid, additives can be used as alcohols which allow to lower its solidification temperature.

The channel 20 is supplied with heat transfer fluid by tubes, two tubes 21 and 22 in the example shown. The tube 21 is used to bring the fluid to the channel 20 and the tube 22 is used to bring out the fluid of the channel 20. It is also possible to provide a greater number of tubes for the fluid supply of the channel 20. The number of tubes and their arrangement are defined according to the location of hot zones along the core 11.

According to the invention, at least one of the tubes 21 and 22 passes through the dielectric medium 13 and the outer conductor 12. In the example shown, the two tubes 21 and 22 pass through the dielectric medium 13 and the outer conductor 12. In this arrangement the tubes 21 and 22 do not leave the coaxial line section 10 by one of the ends 15 or 16 which is thus released, for example to accommodate more easily the coaxial connector 17. It is thus possible to implement a standard connector not provided for the circulation of the coolant. Having a connector coaxially to the axis of the section 10 facilitates the introduction of the electromagnetic wave in the section 10 without breaking in the direction of the wave. This makes it possible, for example, to promote a TEM mode wave transmission in section 10. The TEM mode is a transmission mode in which the electric (E) and magnetic (H) fields of the electromagnetic wave are perpendicular to the axis 14 of section 10.

In order to facilitate the crossing of the dielectric medium by the tubes 21 and 22, they traverse it radially with respect to the axis 14. As a function of the configuration of the coaxial line section 10 and the equipment in which the section is implemented, it is possible, for the tubes 21 and 22, to cross the dielectric medium 13 and the outer conductor 12 in a direction inclined relative to a direction radial to the axis 14. Nevertheless, the use of the radial direction reduces the distance traveled by the coolant in the dielectric medium 13. This radial arrangement is also easier to manufacture.

The two tubes 21 and 22 may be located in the same section 25 of the coaxial line section 10. The section 25 is perpendicular to the axis 14. This arrangement facilitates the production of mechanical parts allowing the transition between the tubes 21 and 22 on the one hand and channel 20 on the other. Still depending on the configuration of the coaxial line section 10 and the equipment in which the section 10 is implemented, it is possible for the tubes 21 and 22 to pass through the dielectric medium 13 and the outer conductor 12 each according to a different section.

Advantageously, the section 25 is disposed in the vicinity of the end 16. In addition, the channel 20 is arranged on one side of the section 25. This facilitates the realization of the channel 20 which follows two paths 20a and 20b parallel to the 14 and a folding axis 20c disposed in the vicinity of the end 15. The heat transfer fluid moves in the coaxial line section 10 in the following order, the tube, 21, the forward path 20a, the folding 20c, the return path 20b and finally the tube 22 to exit the coaxial line section 10.

Alternatively, the section 25 may be disposed at a distance from the two ends 15 and 16. The channel 20 may then comprise several circulations of heat transfer fluid in particular on either side of the section 25.

FIGS. 2a and 2b show two alternative embodiments of the tubes 21 and 22. The tubes 21 and 22 may be made of a homogeneous dielectric material in order to avoid any direct electrical contact between the outer conductor 12 and the core 11. For example, the tubes 21 and 22 can be made of polytetrafluoroethylene or polypropylene homopolymer which are low loss dielectric materials. The dielectric material forms an envelope 21a and 22b respectively in which circulates the coolant. The envelope 21a or 22a can be directly in contact with the outer conductor 12 without the risk of a short circuit with the core 11.

It is possible to stiffen the tubes 21 and 22 by arranging a metal tube respectively 21b and 22b, for example copper inside the envelope 21a or 22a of dielectric material. A metal tube whose potential is floating, that is to say not electrically connected to the core 11 or the outer conductor 12, makes it possible to mask the dielectric constant of the coolant and thus makes it possible to limit the dielectric losses due to the passing through the dielectric medium by the coolant.

FIG. 3 schematically represents a device 30 for producing plasma in which the invention can be implemented. The device comprises a sealed enclosure 31 in which a gas is confined. One or more applicators 32 of microwave energy are used to excite the gas. Each applicator 32 is formed from a coaxial line section 10 as previously described. Depending on the pressure range prevailing in the enclosure 31 and the presence or absence of a magnetic field, the plasma can be produced at the electron cyclotron resonance (ECR), when the frequency of the applied microwave electric field is equal at the rate of gyration of electrons in the magnetic field. Plasma can be produced at the electron cyclotron resonance (ECR), but also by any other process of absorption of microwaves, collisional or not. The microwave energy may be, for example, generated by magnetrons or microwave generators with transistors, not shown, connected to the applicators 32 by the connectors 17. The applicators are cooled by means of a heat transfer fluid. We find the tubes 21 and 22 previously described. The ends 15 of each of the coaxial line sections 10 open into the enclosure 31.

FIG. 4 represents in more detail a microwaves energy applicator 32 implemented in the plasma production device 30 of FIG. 3. The core 11, the outer conductor 12 and the dielectric medium 13 are found. end 15 is intended to open into enclosure 31 and end 16 is provided with coaxial connector 17.

The channel 20, for cooling the core 20, is formed of a central tube 35 extending along the axis 14. The central tube 35 forms the path 20a bringing the fluid towards the end 15. The return path 20b is here formed in a cavity 36 formed around the central tube 35. The cavity 36 is coaxial with the central tube 35. Alternatively, it is possible to circulate the fluid in the cavity 36 for the forward path 20a and in the central tube 35 for the return trip 20b. The paths 20a and 20b have been defined in FIG. 1. The channel 20 is folded in a cavity 38 disposed at the end 15 and into which both the central tube 35 and the cavity 36 open. This coaxial arrangement of the paths 20a and 20b can be implemented for any coaxial line section implementing the invention.

A gun 40, belonging to the core 11, makes it possible to make the connection on the one hand between the tube 21 and the central tube 35 and on the other hand between the tube 22 and the cavity 36. The tubes 21 and 22 pass through the air here forming the dielectric medium 13. Fittings 41 and 42, respectively, make it possible to connect the tubes 21 and 22 to a heat exchange circuit, not shown, and making it possible to exchange the heat recovered in the outside environment. 11 by the heat transfer fluid.

FIG. 5 represents an alternative embodiment of microwave energy applicator 45 that can be implemented in the plasma production device 30 of FIG. 3. This variant differs from that of FIG. 4 essentially in that the medium dielectric 13 is formed either of air, marked 13a, or of suitable rigid material identified 13b.

The tubes 21 and 22 pass through the air 13a forming here the dielectric medium 13. Alternatively, it is possible to pass through the rigid material 13b through the tubes 21 and 22. In the rigid material 13b, the tubes 21 and 22 can be directly formed by holes made in the rigid material 13b.

In this variant, the tubes 21 and 22 pass through an intermediate mechanical part 47 fixed to the outer conductor 12 and to the coaxial connector 17. In the variant of FIG. 4, the tubes pass directly through the outer conductor 12. It is also possible to develop a particular coaxial connector whose outer conductor is traversed by the tubes 21 and 22. The piece 47 and the outer conductor of the connector are all electrically connected and all fulfill the function of the outer conductor of the coaxial line section.

It is also possible to cool the outer conductor 12 by another circulation of heat transfer fluid. The outer conductor 12 comprises a cavity fed by channels made in the outer conductor 12 and similar connections to the connectors 41 and 42. The circulation of the coolant in the outer conductor 12 is easier to achieve than in the core 11. In indeed, in the outer conductor 12, it is not necessary to pass through the dielectric medium 13.

The connectors 41 and 42 as well as the connectors used for cooling the outer conductor 12 may be arranged radially with respect to the axis 14.

FIG. 6 is a partial sectional view of a variant of the coaxial line section 50 in which the tube 21 is disposed in the tube 22. Alternatively, it is possible to arrange the tube 22 in the tube 21. The barrel 51 for connecting the tubes 21 and 22 to the channel 20 formed here by the central tube 35 and the cavity 36 is adapted to receive these tubes 21 and 22. The variant of the coaxial line section 50 simplifies the crossing of the dielectric medium 13 by the tubes 21 and 22 .

Claims (11)

1. A coaxial line section (10; 50) comprising a core (11) and an outer conductor (12) separated by a dielectric medium (13), the core (11) being cooled by circulation of a circulating heat transfer fluid in a channel (20) internal to the core (11), the section (10; 50) further comprising at least one tube (21, 22) passing through the dielectric medium (13) and the outer conductor (12), the at least one tube (21, 22) supplying the heat transfer fluid channel (20). 2. The coaxial line section according to claim 1, comprising two tubes (21 and 22) passing through the dielectric medium (13) and the outer conductor (12), the fluid entering the channel (20) through a first (21) of the two tubes and leaving the channel (20) by a second (22) of the two tubes. 3. The coaxial line section according to claim 2 wherein a first (21) of the two tubes is disposed in a second (22) of the two tubes. 4. The coaxial line section according to one of the preceding claims wherein the outer conductor (12) is coaxial with the core (11) about an axis (14) and wherein the or the tubes (21, 22) ) pass through the dielectric medium (13) and the outer conductor (12) radially with respect to the axis (14). 5. The coaxial line section according to claims 2 and 4, wherein the core (11) and the outer conductor (12) extend between two ends (15, 16) of the section (10) along the axis ( 14), the channel (20) being folded inside the core (11) and wherein the two tubes (21, 22) are located in a same section (25) of the coaxial line section (10), the section being perpendicular to the axis (14). 6. The coaxial line section according to claim 5, wherein the section (25) is disposed in the vicinity of one of the ends (16) and wherein the channel (20) is arranged on one side of the section. (25). 7. The coaxial line section according to one of the preceding claims, wherein the at least one tube (21, 22) comprises a casing (21a, 22a) of tubular form of dielectric material inside which circulates the fluid. , the envelope (21a, 22a) being in contact with the outer conductor (12). 8. The coaxial line section according to claim 7, wherein the at least one tube (21, 22) comprises a tubular metal material (21b, 22b) surrounded by the envelope (21a, 22a) of dielectric material. 9. The coaxial line section according to one of the preceding claims, wherein the outer conductor (12) is coaxial with the core (11) about an axis (14) and wherein the channel (20) comprises a central tube (35) extending along the axis (14), a cavity (38) disposed at the end (15) and a cavity (36) formed around the central tube (35). 10. The coaxial line section according to one of the preceding claims, wherein the outer conductor (12) is coaxial with the core (11) about an axis (14), the core (11) and the conductor. outside (12) extending between two ends (15, 16) of the section (10) along the axis (14), and wherein the section (10) comprises a coaxial connector (17) connected to the core (11). ) and to the outer conductor (12), the coaxial connector (17) being disposed coaxially with the section (10) at a first end thereof (16). 11. A device for producing plasma (30) from a gas comprising an enclosure (31) containing the gas and at least one coaxial applicator (32; 45) of a microwave energy comprising a coaxial line section (10) according to claim 10, the microwave energy being supplied to the device by the coaxial connector (17), a second (15) of the two ends of the section (10) opening into the enclosure (31).