RU2468544C1 - Device for exciting and maintaining microwave discharges in plasma chemical reactors - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: device for exciting long-distance discharges in gas flows includes source of electromagnetic microwave oscillations, which is interconnected with metal waveguide of rectangular section, with two opposite located holes in the centre of wide walls. Inside waveguide there arranged is concentrator of electric field in the form of hollow current-conducting flattened cone with diameters of bases D and d (D>d), which has electric contact to one wide wall of waveguide along the boundary of hole of larger diameter and gap z between the edge of base with lower diameter and opposite wall of waveguide. Dielectric reactor tube with gas flow flowing through it passes across waveguide through concentrator and hole in opposite wide wall. Value of gap z lies in the range of λ/50 to λ/5, where λ - length of wave of used source of microwave oscillations in vacuum.

EFFECT: increasing operating stability of plasmatron on surface plasma wave in plasma chemical processes in wide range of pressures of plasma forming gas from several Pascal (Pa) values to atmospheric value.

5 cl, 2 dwg

Description

FIELD OF THE INVENTION

The scope of the invention. If there are several such areas, priority ones are indicated.

The device relates to microwave generators of low-temperature plasma (microwave plasmatrons), intended primarily for use in plasma chemical processes with an increased requirement for the purity of the synthesized product. Such processes include, in particular, synthesis of highly pure and specially doped silicon dioxide from the gas phase with deposition of the synthesized material in the form of highly transparent amorphous layers on the side surface of quartz glass pipes and rods for the purpose of subsequent fabrication of optical waveguides for integral and fiber from the prepared blanks. optics.

State of the art

Information about the known analogues with the selection of the analogue closest to the invention (prototype).

The device’s operation is based on the creation of an extended plasma column in the gas stream, which plays the role of a plasma waveguide, in which the plasma is supported by the transfer of microwave energy by surface plasma waves excited in the plasma column and propagating along it from the point of application of the electric microwave field.

Several devices are known in which discharges of this type are implemented. Their common useful property is the absence of electrodes, as well as any other plasma contact with the conductive parts of the plasmatron.

A device is known for exciting and maintaining an extended discharge on a surface plasma wave in a gas-filled dielectric tube intersecting the gap created between the metal parts of the plasmatron, due to which an increased microwave electric field is generated in the wall region of the tube parallel to the tube axis. This configuration leads to the transfer of microwave energy from the plasma source due to the excitation of surface plasma waves in it (US patent 4810933).

A known type of such a plasmatron in which a gap with an increased electric microwave field is formed due to a reduction in the distance between the wide walls of a metal waveguide of rectangular cross section. In this design, the waveguide is equipped with two smooth transitions to the cross section with a smaller width of the narrow wall (US patent 6224836 B1).

A plasmatron is known in which surface plasma waves are excited on the surface of a cylindrical dielectric rod located inside a dielectric tube with gas under reduced pressure. The rod will perform the function of a dielectric microwave waveguide, through which the plasma transfers energy from a source of microwave oscillations (patent US 5597624 A).

The closest analogue (prototype) to the invention is a plasmatron based on a rectangular waveguide in which a plasma is formed inside a dielectric tube crossing the waveguide through holes in the center of the wide walls. Inside the waveguide, in the region where the tube intersects the wide walls, a device of a special form is made for concentration of the electric microwave field in the wall region of the tube. A special form of the device concentrating the microwave field of the device provides efficient energy transfer from the source of microwave oscillations to surface plasma waves (patent US 7799119 B2).

Disclosure of invention

The invention as a technical solution is expressed in the aggregate of essential features to achieve the result provided by the invention.

The essence of the claimed invention as a technical solution consists in the special design of the microwave plasmatron, with which it is possible to excite an extended plasma column in the gas stream with the supply of microwave energy to the plasma due to the excitation of a surface plasma wave in the plasma column.

The problem to which the invention is directed, indicating the technical result provided by it.

The task to which the invention is directed is to ensure stable operation of the plasmatron on a surface plasma wave in plasma-chemical processes occurring in a wide range of plasma-forming gas pressures from several Pa to atmospheric.

Another task is to ensure unidirectional propagation of the surface wave in the plasma column in order to most efficiently use microwave energy supplied to the plasma in plasma-chemical processes.

The features used to characterize the invention.

The basis of the design of the plasmatron is a segment of a rectangular waveguide in which the H ₁₀ wave is excited, carrying energy from a source of microwave oscillations to the plasma. In the center of the wide walls of the waveguide, opposite each other, there are holes with diameters D and d (D> d) through which a dielectric tube resembles perpendicular to the plane of the wide wall. The tube performs the function of a plasma chemical reactor in which plasma is excited and through which the plasma-forming gas flows together with gaseous chemicals.

The first sign of a technical solution characterizing the claimed invention is a hollow hub in the form of a truncated cone made of conductive material. The concentrator forms a gap z inside the waveguide with increased intensity of the microwave electric field directed along the axis of the reactor tube. The presence of such a gap is necessary for the excitation of a surface plasma wave. In the patented design, the gap z is formed by the inner surface of the section of the wide waveguide wall near the boundary of the hole with a smaller diameter and the end face of the hub connected to the opposite wall of the waveguide along the boundary of the hole of larger diameter and having electrical contact with it. The inner diameter of the cone concentrator in the region of the gap z coincides with the diameter of the hole in the wide wall of the waveguide adjacent to the end of the concentrator. The optimal matching corresponding to the maximum microwave energy transfer coefficient to the plasma corresponds to the range of ratios of the diameters of the base D and the vertex d of the truncated conical concentrator D / d from 1.1 to 5 and the gap value z in the range from λ / 50 to λ / 5, where λ - the wavelength of the used source of microwave oscillations in vacuum.

The second feature that characterizes the invention is the presence in the plasmatron of a plasma terminating device, with the help of which a break in the plasma column is achieved in a given cross section of the reactor tube. This ensures the unidirectional propagation of the surface plasma wave along the plasma column and thereby sets the direction of plasma propagation in the reactor tube. The termination device is a coaxial type resonator open on one side, formed by pieces of metal tubes connected to each other by a back wall of conductive material so that the length of the inner tube segment L with an error of no more than 5% of the working wavelength λ is N * λ / 4 where N is an odd number. The resonator is located on top of the reactor tube, coaxial with it so that the reactor tube passes inside the resonator. The action of the termination device is based on the fact that as a result of destructive interference between the exciting and reflected from the back wall of the wave resonator microwave field strength near the free end of the inner tube of the coaxial resonator is minimal. The low intensity of the microwave electric field, insufficient to maintain the plasma, leads to the breakdown of the plasma column, which makes it impossible for the plasma to propagate in the reactor tube towards the rear wall of the termination resonator.

Depending on the preferred purpose of the plasmatron, the coaxial termination resonator is positioned relative to the waveguide with the reactor tube in two ways.

Option 1, optimal for plasma-chemical processes occurring under reduced pressure, when heating the walls of the tube-reactor with plasma does not lead to their thermal destruction. In this case, the length of the plasma column can reach several meters. Preferred for this option from the point of view of optimal transfer of microwave energy to the plasma is the connection of the outer tube of the termination resonator from the outside to the wide wall of the waveguide from the side of the hole of smaller diameter d. A column of plasma arising in the reactor tube extends outward from the point of application of the microwave field in the gap z to the side of the hole with a larger diameter D, passing through the horn extension formed by the inner surface of the concentrator forming the gap z, as shown schematically in Figure 1.

Option 2, optimal for plasma-chemical processes occurring at pressures close to atmospheric. In this case, the gas temperature in the plasma is high, and the contact of the plasma with the tube wall leads to its thermal destruction. Therefore, the plasma-forming gas is supplied in a swirling mode relative to the axis of the tube-reactor. The swirl centers the plasma on the axis of the reactor tube and thereby prevents its contact with the walls. For this version of the technological plasmatron, an important factor is the possibility of substrate access to the region of the plasma jet with a maximum temperature, which rapidly decreases with distance from the point of application of the microwave field in the gap z. Therefore, it is preferable for this option to attach a terminating resonator from the outside to a wide waveguide wall from the side of the hole with a larger diameter D, as shown schematically in Figure 2.

Brief Description of the Drawings

Figure 1. Microwave plasmatron for exciting plasma on a surface wave under reduced pressure of a plasma-forming gas.

Figure 2. Microwave plasmatron for excitation of a plasma on a surface wave at pressures close to atmospheric.

1 - metal microwave waveguide of rectangular cross section; 2 - a concentrator of an electric microwave field; 3 - dielectric tube reactor; 4 - plasma; 5 - plasma terminating resonator of the coaxial type.

The implementation of the invention

How the invention can be implemented with the implementation of the destination indicated by the applicant.

Example 1. Plasmatron on a surface plasma wave for operation under reduced pressure of a plasma-forming gas. Suppose that you want to excite a plasma column in an oxygen stream inside a tube with an inner diameter of 16 mm and a wall thickness of 2 mm at a pressure of 5 kPa. To do this, in a rectangular copper waveguide with an internal cross section of 45 * 90 mm, we excite the microwave wave H ₀₁ using a magnetron operating at a frequency of 2450 MHz (the wavelength in vacuum corresponding to this frequency is λ = 122 mm). In the plasmatron design, schematically shown in Figure 1, we set the following dimensions of the parts: D = 40 mm, d = 22 mm, L = 30 mm. The gap value z is equal to 12 mm. With a magnetron power of 1-5 kW, a plasma column of 200-700 mm long appears in the tube.

Example 2. Plasmatron on a surface plasma wave for operation at a plasma-forming gas pressure close to atmospheric. Suppose that you want to excite a plasma column in an oxygen stream inside a tube with an inner diameter of 16 mm and a wall thickness of 2 mm at a pressure of 100 kPa. To do this, in a rectangular copper waveguide with an internal cross section of 45 * 90 mm, we excite the microwave wave H ₀₁ using a magnetron operating at a frequency of 2450 MHz (the wavelength in vacuum corresponding to this frequency is λ = 122 mm). In the plasmatron design, schematically shown in Figure 2, we set the following dimensions of the parts: D = 40 mm, d = 22 mm, L = 30 mm. The gap value z is equal to 12 mm. We supply oxygen to the tube with a flow swirl around the axis of the tube. With a magnetron power of 1-5 kW, a plasma column with a length of 120-100 mm appears at the exit from the tube.

Claims (5)

1. A device for exciting extended discharges in gas flows, including a microwave electromagnetic source coupled to a rectangular metal waveguide with two opposite openings in the center of the wide walls and an electric field concentrator located inside the waveguide in the form of a hollow, conductive truncated cone with diameters of the bases D and d (D> d) having electrical contact with one wide wall of the waveguide along the boundary of the hole of larger diameter and the gap z between the end of the base with less diameter and opposite wall of the waveguide, a dielectric tube-reactor with a gas stream flowing through it, passing across the waveguide through a hub and an opening in the opposite wide wall, characterized in that, in order to create a plasma in the tube-reactor in the form of an extended column supported by due to the excitation of surface plasma waves in it, the gap value z lies in the range from λ / 50 to λ / 5, where λ is the wavelength of the used microwave oscillation source in vacuum. 2. The device according to claim 1, characterized in that the ratio of the diameters of the bases of the hollow conical concentrator D / d lies in the range from 1.1 to 5. 3. The device according to claim 1, characterized in that, in order to forcibly break off the plasma column in a given cross section of the reactor tube and thereby create a unidirectional propagation mode of the surface plasma wave in the plasma column from the outside of the waveguide, on top of the dielectric tube reactor a plasma column terminating device is coaxially placed in the form of a coaxial type resonator open on one side, made in the form of two pieces of conductive tubes connected between a conductive base, so that the length of the inner tube segment L with an error of not more than 5% of the wavelength of the used microwave oscillation source in vacuum λ is N · λ / 4, where N is an odd number. 4. The device according to claim 3, characterized in that the plasma terminating resonator of the coaxial type is connected with the open side from the outside to the waveguide from the side of the wide wall forming a gap z with the concentrator so that the outer resonator tube has electrical contact with it. 5. The device according to claim 3, characterized in that the plasma terminating resonator of the coaxial type is connected with the open side from the outside to the waveguide from the side of the wide wall connected to the concentrator so that the outer resonator tube has electrical contact with it.

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