Microwave Plasmatron

Abstract

A device for producing low-temperature plasma of microwave discharge at atmospheric pressure of a plasma-forming gas suitable for conducting chemical reactions of extreme purity, depositing thin films, growing crystals, producing powders and other technological purposes. The device includes a spherical or radial waveguide wherein there is excited a converging symmetrical electromagnetic wave, and a discharge tube disposed on the axis of symmetry of the waveguide.

Description

The present invention relates to devices for generating low-temperature plasma in a microwave-frequency discharge at atmospheric or a nearly-atmospheric pressure, and more specifically to plasmatrons which are used, for example, to carry out chemical reactions of extreme purity, to deposit thin films, and to clean powders and gases.

There exist microwave-discharge plasmatrons comprising a tube which encloses the region of the gas discharge and is placed in a waveguide or a cavity resonator coupled to a feeding waveguide by a coupling transformer intended to provide optimum conditions for energy transfer from the waveguide into the resonator.

A disadvantage of this type of plasmatron is in that microwave energy is transferred to the plasma from one side only, namely, from that of the waveguide. Because of this, the parameters (temperature, ionization level, etc.) of the plasma column vary across its section.

It is known that a gas-discharge plasma in microwave field tends to shift in the direction of the energy source, which fact handicaps confinement of the plasma on the axis of the gas-discharge tube.

Another disadvantage of existing plasmatrons is that the coupling transformer, usually having the form of a radiating slot, probe, or loop, has finite dimensions determined by the frequency of the microwave source. As the power of a plasmatron increases, the strength of the electric field also increases such that the field intensity at the transformer often rises to a value sufficient to cause a spontaneous breakdown and a discharge. This parasitic discharge may lead to the dissipation of the input energy at the coupling transformer, resulting in its destruction and failure of the entire plasmatron.

An object of the present invention is to provide a microwave plasmatron in which the parameters of the gas-discharge plasma are the same across the entire section of the plasma column and in which the probability of breakdowns occurring at the coupling transformer is very remote.

This object is accomplished by the fact that in a microwave plasmatron whose gas-discharge tube is arranged along the axis of symmetry of a cavity resonator electromagnetically coupled to a microwave-feed waveguide, the waveguide is, according to the invention, located at right angles to the longitudinal axis of the gas-discharge tube and outside the cavity resonator entirely surrounding its perimeter and shares with it a wall which carries electromagnetic-coupling elements equidistantly separated from the axis of the gas-discharge tube and separated from one another by a distance providing for oscillations in phase produced therein.

Such an arrangement of the plasmatron materially simplifies confinement of the plasma at the center of the gas-discharge tube, because energy is fed into the plasma uniformly from all sides, and the tendency of the plasma to shift in the direction of the energy source is nonexistent. Another advantage is that the power of the plasmatron can be markedly increased, since each of the electromagnetic-coupling elements has to accommodate only part of the total energy input.

It is preferable to arrange electromagnetic-coupling elements along the entire length of the waveguide, separated (from center to center) of a distance equal to a half-wavelength of the input energy, adjacent elements being located on opposite sides of the long axis of the waveguide.

As an alternative, electromagnetic-coupling elements may be separated by a distance equal to the wavelength of the input microwave energy, in which case they should be all located on one side of the long axis of the waveguide.

In each case, the waveguide may be a hollow torus.

The invention will be best understood from the following description of preferred embodiments when read in connection with the accompanying drawings in which:

FIG. 1 is a cross section through the spherical cavity resonator, gas-discharge tube, and part of the feeding waveguide of a plasmatron according to the invention:

FIG. 2 is section taken along line II-- II of FIG. 1;

FIG. 3 shows the cylindrical cavity resonator, gas-discharge tube and part of the feeding waveguide of a plasmatron according to the invention, and

FIG. 4 shows a toroidal waveguide.

Referring to FIG. 1, there is a tube 1 enclosing the gas-discharge region, which is fabricated from a heat-resistant glass of low RF loss and placed inside a cavity resonator 2. For practical reasons, preference should be given to cavity resonators having axial symmetry, such as spherical, cylindrical, etc. When such a resonator is excited in the dominant (symmetrical) mode of oscillation, the maximum of the electric field intensity is coincident with the axis of symmetry along which the tube 1 is also located.

Holes 3 through which the tube 1 passes inside the cavity resonator 2 are fitted with stubs 4 whose diameter is less than the critical dimension. The function of these stubs 4 is to prevent the emission of microwave energy outside the plasmatron. Instead of stubs, use may be made of chokes or any other elements having the property of microwave filters.

Placed outside the cavity resonator 2 surrounding its perimeter and at right angles to the longitudinally axis of the gas-discharge tube 1 is a ring-shaped, rectangular waveguide 5 which shares one wall 6 (FIG. 2) with the cavity resonator 2.

The wall 6 has slots 7 which serve as electromagnetic coupling elements to transfer microwave energy from the waveguide into the cavity resonator 2.

The slots 7 are equidistantly spaced from the longitudinally axis of the gas-discharge tube 1 and are separated (from center to center) by a distance equal to a half-wavelength of the input microwave energy, adjacent slots being located on opposite sides of the longitudinally axis of the waveguide 5. This arrangement of the coupling slots provides for the occurrence of the oscillations in-phase produced therein. The slot farthest in the direction of propagation of microwave energy in the waveguide is within a quarter-wavelength of the wall 8 of the waveguide. The function of the wall 8 is to split the input microwave energy equally between all the slots.

As an alternative, the coupling elements may be loops or probes, in which case the waveguide with probes or loops is arranged in the way identical to the one described above.

The amount of coupling between the waveguide 5 and the coupling elements is selected such that each will transfer an equal share of the total input energy into the cavity resonator. In the preferred embodiment just described, the amount of coupling is governed by the offset of slots relative to the longitudinal axis of the waveguide.

FIG. 3 shows a plasmatron with a cylindrical resonator 2. The waveguide 5 is arranged to surround the perimeter of the cylindrical resonator 2 and at right angles to the longitudinal axis of the gas-discharge tube 1. The coupling elements, which are likewise slots 7 separated (center to center) by a distance equal to the wavelength of the input microwave energy, are arranged along the entire length thereof, and all on one side of the longitudinal axis of the waveguide. This arrangement of coupling elements provides for the occurrence of the oscillations in-phase produced therein.

It should be borne in mind that in a plasmatron using a cylindrical resonator, the coupling elements may likewise be spaced at a half-wavelength of the input microwave energy apart.

Instead of a waveguide with the wall 8, use may be made of a toroidal waveguide. In this case, one utilized the property of two electromagnetic waves travelling towards each other to form an electrical wall.

The microwave energy fed into the waveguide 5 (FIG. 4) is split into two equal parts which propagate towards each other. In the area located across the diameter from the entry into the waveguide, the two waves produce an electrical wall which acts exactly as the wall 8 in the above-described preferred embodiments of the plasmatron. The layout of such a waveguide relative to the cavity resonator and the gas-discharge tube and the arrangement of coupling elements, notably slots 7, are analogous to what has been already described. In this case, too, the slots nearest to the electrical wall are separated from it by a quarter-wavelength of the microwave energy propagating in the waveguide.

The plasmatron disclosed herein operates as follows.

A plasma-forming gas is passed through the gas-discharge tube. The microwave energy propagating through the waveguide 5 passes through the slots 7 into the resonator 2 where it is concentrated on the axis along which is arranged the gas-discharge tube 4 where the plasma absorbing microwave energy is produced.

If the dimensions of the resonator are such that the operating frequency is equal to its resonant frequency, then at the instant of ignition the electric field intensity will be a maximum at the axis of the resonator and a minimum at the slots 7. This field-intensity distribution facilitates the ignition and reduces electrical stresses that occur in the resonator due to its resonance properties, especially when there is no absorption of radiation energy.

To keep the plasma from touching the walls of the tube 4, the latter is subjected to a turbulent jet of plasma-forming gas. The turbulence produces a low-pressure region along the axis of the tube 1, which prevents the plasma from contacting the walls.

The present invention has been embodied in several prototypes which have passed tests successfully. Using a microwave power input of about 3 kW. in CW operation and a quartz discharge-tube with a diameter of 500 mm., the resultant plasma column was 300 mm. long and 40 mm. in diameter, having a temperature of 3000.degree.--5000.degree. K.

Claims

I claim:

1. A microwave plasmatron comprising: a cavity resonator of symmetrical configuration and having an axis of symmetry; a gas-discharge tube through which a plasma-forming gas is passed and which is supported inside said cavity resonator and along said axis of symmetry; a feeding waveguide for transmitting microwave energy into said cavity resonator, said waveguide being mounted on said cavity resonator at right angles to the longitudinal axis of said gas-discharge tube externally of said cavity resonator and sharing a common wall with the latter; a plurality of slots provided in said common wall of said waveguide and resonator and intended for reirradiating microwave energy from said waveguide into said cavity resonator, said slots being equidistantly spaced from the longitudinal axis of said gas-discharge tube and separated from one another by a distance providing for the occurrence of oscillations in phase with one another.

2. A plasmatron, as claimed in claim 1, in which the slots are arranged along the entire length of the waveguide and separated from center to center by a distance equal to a half-wavelength of the input microwave energy, adjacent slots being located on opposite sides from the longitudinal axis of the waveguide.

3. A plasmatron, as claimed in claim 1, in which the slots are arranged along the entire length of the waveguide and separated from center to center by a distance equal to the wavelength of the input microwave energy, all said slots being located on one side from the long axis of the waveguide.

4. A plasmatron, as claimed in claim 1, in which the waveguide is a hollow torus in which microwave energy propagates in equal amounts both ways from the place of entry.