High-power Plasma Generator Employed As A Source Of Light Flux At Atmospheric Pressure

Abstract

A multimode cavity resonator includes a duct system for directing a gas stream through the resonator. Electromagnetic power is supplied to the resonator for establishing a plasma discharge in the air stream within the resonator. Light flux generated by the plasma at atmospheric pressure is transmitted through an optically transparent wall portion of the cavity to a utilization region outside of the cavity. The gas ducts and the optically transparent wall of the cavity include a cluster of conductive tubes dimensioned to be cut off at the operating frequency of the cavity.

Description

DESCRIPTION OF THE PRIOR ART

Heretofore, multimode cavity resonators have been excited with electromagnetic energy at atmospheric pressure and such multimode cavities have experienced a plasma discharge therein, giving off light. However, such plasma discharges have been unstable resulting in the plasma discharge operating in an intermittent fashion on an unpredictable basis. Moreover, the plasma discharge was a phenomenon to be avoided in use and no utilization was made thereof. A typical example of such a multimode cavity is disclosed in U.S. Pat. No. 3,478,188, issued Nov. 11, 1969 and assigned to the same assignee as the present invention.

SUMMARY OF THE PRESENT INVENTION

The principal object of the present invention is the provision of an improved high power plasma generator useful as a source of light flux at atmospheric pressure.

One feature of the present invention is the provision of a multimode cavity resonator dimensioned to have a volume of many cubic wavelengths at the operating frequency of the cavity for establishing a plasma discharge therein at atmospheric pressure. A ducting system is provided for directing a gas stream through the cavity to support the plasma discharge therein and to stabilize and confine the plasma to the gas stream. The cavity includes an optically transparent wall portion facing the plasma discharge for passage of light flux from the plasma to a utilization region located outside of the cavity.

Another feature of the present invention is the same as the preceding feature wherein the optically transparent wall portion of the cavity includes a cluster of parallel conductive tubes dimensioned to be cut off at the operating frequency of the cavity, such tubes being directed at the plasma discharge with the inner ends of the tubes defining the inside optically transparent wall of the cavity.

Another feature of the present invention is the same as the first feature wherein the ducts for directing the gas stream through the cavity include a pair of ducts having end portions defining gas-permeable wall portions of the cavity with the gas-permeable wall portions each including a cluster of parallel conductive tubes dimensioned to be cut off at the operating frequency of the cavity, and such tubes being aligned in the direction of gas flow in the ducts.

Another feature of the present invention is the same as any one or more of the preceding features wherein the cavity resonator is dimensioned to have transverse inside dimensions in excess of five free-space wavelengths at the operating frequency of the cavity, whereby substantial mode overlap is obtained to facilitate excitation of the cavity by an electromagnetic power generator.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawing wherein:

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic perspective line diagram, partially broken away, depicting a plasma generator employed as a source of light flux at atmospheric pressure and incorporating features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing, there is shown a plasma generator 1 employed as a high-power source of light flux at atmospheric pressure. The plasma generator 1 includes a relatively large multimode cavity resonator 2, as of aluminum. The cavity 2 is preferably of rectangular, including cubic, dimensions dimensioned for l, m, and n wavelengths on a side where l, m, and n are each greater than 5. In a typical example at S-band, the cavity was 4 feet deep, 4 feet wide, and 10 feet long.

Electromagnetic energy at the operating frequency of the cavity 2 is supplied to the cavity 2 via 3 input waveguides 3, 4 and 5, respectively, preferably located at 3 different corners of the cavity 2 and being directed with their longitudinal axes in the direction of power flow therethrough along mutually orthogonal axes in three dimensional space. Three sources of power 6, 7 and 8, such as high-power klystrons or magnetrons, are connected to the waveguides 3, 4 and 5, respectively, for supplying relatively large amounts of power to the cavity 2. In a typical example, each of the power sources 6, 7 and 8 delivers 30 kilowatts at S-band to the cavity 2 such that a total of 90 kilowatts average power is supplied to the cavity 2.

Three input ducts 9, 11 and 12, located near the bottom of the cavity 2 are arranged for directing an air stream into the cavity 2 as supplied via air blower 13, ducting 14, and a manifold 15 to the ducts 9, 11 and 12. Three output gas ducts 16, 17 and 18 are disposed vertically over the respective input ducts 9, 11 and 12 for directing the gas stream out of the cavity 2. In a typical example, 2,000 cubic feet per minute of gas is directed via the ducts through the cavity 2. Each of the ducts 9-12 and 16-18 includes a section adjacent the cavity 2 which is formed by a cluster of conductive tubes 19. Each of the conductive tubes in the cluster has transverse cross-sectional dimensions to be cut off at the operating frequency of the cavity 2, and are made to be greater than 3 diameters long such that energy is not lost from the cavity 2 via the ducts 9-12 and 16-18. The inner ends of the tubes 19 are preferably flush with the inside wall of the cavity to form a gas-permeable wall portion of the cavity.

High-intensity standing-wave electric fields are established within the cavity by electromagnetic power fed to the cavity 2. These high-intensity electric fields produce a breakdown in the air streams passing through the cavity 2 to establish plasma discharge regions in the gas streams. In a cavity 2 dimensioned as aforedescribed, the plasma discharge regions are approximately 1 foot in cross section and 2 feet long extending in the gas streams between the input and output ducts. With vertical gas streams, gas heated by the plasma rises within the gas stream rather than rising out of the gas stream, which latter effect would tend to make the plasma discharge unstable.

Large amounts of visible and ultraviolet light flux are generated in the plasma discharge regions. These discharge regions are operating at atmospheric pressure and the total light flux emitted from these discharge regions is proportional to the volume of the plasma discharge regions. Thus, by employing a relatively large cavity 2 with a relatively large ducting system, a large amount of light flux is obtained from the plasma discharge.

The light flux is extracted from the cavity 2 via an optically transparent wall portion 21 of the cavity 2, formed by a relatively large cluster of electrically conductive tubes 22, such tubes 22, being dimensioned in cross section to be cut off at the operating frequency of the cavity. The tubes 22 are parallel to each other and directed at the plasma discharge regions. In this manner, the transparency of the transparent wall 21 can be relatively high, such as greater than 90 percent, without introducing any substantial loss into the cavity, whereby the Q of the cavity can be maintained at a relatively high value. The inner ends of the tubes 22 define the inside wall of the cavity and form the optically transparent wall portion of the cavity 2.

The source of high-power light flux may be employed to advantage for stimulating chemical reactions in a utilization region disposed externally of the cavity 2. By dimensioning the total volume of the cavity to have a relatively high number of cubic wavelengths, a relatively high mode density is obtained inside the cavity to facilitate excitation of the cavity over a band of frequencies. A degree of mode density is preferably which will have mode overlap at 3 db. points for each of the modes. This requires that the cavity 2 have dimensions that are several wavelengths on a side. Moreover, the Q of the cavity should remain relatively high such that the standing-wave electric fields build up to a sufficiently high value to break down the air and to initiate the plasma discharge. A relatively high volume of gas should flow through the cavity for stabilizing the plasma discharge in the air streams. The exhaust gas may be recirculated, however, it becomes hot due to the plasma discharge. In order to prevent overheating of certain of the gas ducts and the blower, fresh gas may be passed through the cavity resonator rather than recirculating the exhaust air.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. A high-power source of light flux comprising an envelope forming an electromagnetic multimode cavity resonator, means for introducing electromagnetic power into said resonator, gas inlet port means opening directly to the interior of said envelope for passing gas into said resonator, gas outlet port means opening directly to the interior of said envelope for passing gas out of said resonator, means for causing gas flow through said inlet and outlet port means and through the interior of said resonator to form within said resonator a gas flow stream which supports and confines a continuous stable plasma discharge in the resonator, said gas in the resonator being at substantially atmospheric pressure, and said envelope having a substantially optically transparent wall portion facing the plasma discharge to a utilization region located outside of said cavity.

2. The apparatus of claim 1 wherein said optically transparent wall portion of said cavity includes a cluster of parallel conductive tubes dimensioned to be cut off at the operating frequency of said cavity, said tubes being directed at the plasma discharge region in said cavity resonator, said tubes terminating within said cavity such that their inner ends define the inside optically transparent wall portion of said cavity, and the outer ends of said tubes opening to the atmosphere.

3. The apparatus of claim 1 wherein the inside of said cavity resonator envelope is rectangular with its three dimensions each being in excess of five free-space wavelengths at the operating frequency of said cavity.

4. The apparatus of claim 1 in which said means for introducing electromagnetic power comprises a plurality of waveguides opening into said cavity resonator, said waveguides having their axes defined by the direction of power flow into said cavity and being directed orthogonally to each other.

5. The apparatus of claim 1 wherein one of said gas port means is disposed above the elevation of the other such that the direction of air flow of the gas stream within said cavity includes a substantial component in the vertical direction.

6. The apparatus of claim 1 in which the power delivered by said means for introducing electromagnetic power is substantially 90 kilowatts average power, and the gas flow delivered by said gas-flow means is substantially 2,000 cubic feet per minute.

7. The apparatus of claim 1 wherein said gas is air.

8. The method of operating an electromagnetic cavity resonator as a high power source of light flux in which said resonator includes an optically transparent wall portion, comprising the steps of introducing electromagnetic power into said resonator, creating a gas stream flow directly into, through and out of the interior of said resonator and thus creating solely by said electromagnetic power and gas flow a continuous stable plasma discharge in said gas stream, and utilizing the output from said transparent wall as a source of high power light flux.