Very high power microwave-induced plasma

Details Very high power microwave-induced plasma Very high power microwave-induced plasma

1998-08-12

2002-03-26

Wahlberg, Teresa (Department: 3742)

Electric heating

Metal heating

By arc

C219S121480, C356S316000

Reexamination Certificate

active

06362449

ABSTRACT:

TECHNICAL FIELD
This invention relates to apparatus for generating very high power plasmas, and more specifically to such apparatus for generating very high power plasmas induced by microwave electromagnetic radiation with high levels of microwave power coupled into the plasma.
BACKGROUND OF THE INVENTION
Most current thermo-plasma technologies are electrically generated and can be characterized either as direct current (DC) or alternating current (AC) plasma arcs requiring electrodes, or as electrodeless radio frequency (RF) induced plasma torches.
DC and AC arcs become plasma torches when the electric arc is blown out by rapid gas flow. The electrodes in DC and AC generated arcs have a limited lifetime. Thus, they require frequent replacement which increases costs and maintenance and reduces reliability. During material processing, eroded material from the electrodes in DC and AC plasma arc technologies can contaminate materials that require high purity. Some plasma arc systems use metallic electrodes cooled by water. Water cooling, however, increases the lifetime of the electrodes to only a few hundred hours and electrode erosion still contaminates processed material. Furthermore, the water introduces a safety concern because water leaking into the plasma can produce an explosion. Plasma arc systems that use graphite electrodes can operate only in a non-oxidizing environment, otherwise the electrodes burn up. Even if the graphite electrode system is purged of oxygen, oxidizing material can be introduced by the materials being treated, e.g., wet municipal waste or hydrocarbon plastics.
RF induced plasmas are relatively inefficient in coupling RF power into the plasma. High power RF induction torches typically have coupling efficiencies of less than fifty percent. In addition, radiated RF power from the induction coil must be shielded for safety. This shielding prevents the possibility of combining RF torches to increase power.
Known microwave-induced plasma generators, like those that are RF induced, are electrodeless, and avoid material contamination and electrode maintenance problems. Thus, they are cleaner, more reliable, and more cost effective. However, physical principles expressed in the prior art would lead to a conclusion that maximum power was limited by requirements of minimum plasma skin depth, i.e., the length over which plasma absorbs power. Thus, conventional wisdom assumed the maximum power and the maximum dimensions of microwave-induced plasma generators to be limited. U.S. Pat. No. 5,671,045 issued Sep. 23, 1997, provides such an example of a microwave-induced plasma generator with limited power and dimension.
U.S. Pat. No. 5,468,356 issued Nov. 21, 1995, discloses a microwave plasma generator using eight kilowatts of microwave power. The waveguide structure, however, includes a cavity to concentrate microwave power and facilitate plasma startup. Waveguide restrictions that effect microwave resonance, e.g., cavities and antennae, limit maximum useable microwave power unlike a fundamental mode waveguide or a quasi-optical overmoded waveguide without restrictions between the microwave source of power and plasma.
Jinsong Zhang, et al., “Step Sintering of Microwave Heating and Microwave Plasma Heating for Ceramics,”
Institute of Metal Research
, Chinese Academy of Sciences (1998), describes a microwave-induced plasma using no more than ten kilowatts of power input into the microwave generator. Based on a private conversation between the authors of the paper and one of the inventors herein, the authors indicated that the coupling efficiency did not exceed forty percent. Thus, power coupled into the plasma does not exceed four kilowatts. Furthermore, this embodiment does not have unlimited maximum power, because there is a danger of arcing with the internal antenna.
In the global effort to protect the environment, there exists the need to minimize waste production in manufacturing and to improve waste destruction processes. Legislation now discourages landfill for all but the least hazardous materials. Thus, there is a strong shift towards incineration. Incineration, widely used for waste destruction, is a chemical combustion process requiring fuel and large quantities of air. Environmental groups state that many new toxic products are formed in incineration, and these and other unwanted materials are present in the effluent steams of even the most modern incinerators. In addition, incinerators cannot reduce the volume of waste composed of certain kinds of materials, such as metal.
Electrically generated plasmas offer the advantage of higher operating temperatures for more complete and universal waste destruction, significantly reducing the volume of off-gas emissions and off-gas toxic compounds. DC and AC plasma arc technologies have been around for almost a century and are used in many thermal processes including waste destruction and materials manufacturing. But, DC and AC plasma arc technologies have not yet replaced incineration for waste destruction because, among other reasons, their reliability and maintenance costs are unproven in commercial use.
Since RF induced plasma technology does not require electrodes, it is presently used in manufacturing processes where electrode contamination cannot be tolerated, such as the semiconductor and fiber optics industries. However, RF induced plasmas have limited maximum achievable coupling efficiency levels of 40-60% which decrease with power. Thus, their applications are limited to processes with low power requirements. The limited maximum achievable efficiency rules out their use in waste destruction.
There exists a need for reliable and cost effective plasma torches that can be scaled to unlimited power outputs as compared to existing plasma generators. Furthermore, there is also a need for such very high power plasma torches to have a high level of coupling efficiency. In many manufacturing applications, there is also a need to limit contamination by the plasma apparatus.
SUMMARY OF THE INVENTION
In accordance with the above, one aspect of the invention is a high power microwave plasma torch which includes a source of microwave energy which is propagated by a waveguide. The waveguide has no structural restrictions effecting resonance and is configured such that at least five kilowatts of microwave power is coupled into a gas flowing through the waveguide to create a plasma.
In one embodiment, the waveguide is a fundamental mode waveguide. In a preferred embodiment, the maximum internal dimension of the waveguide is less than the wavelength of the microwave energy. The fundamental mode waveguide can be constructed of electrically conducting walls which are smooth. In a preferred embodiment, the fundamental mode waveguide is shorted to facilitate plasma startup. A dielectric tube, transparent to microwaves, can traverse the fundamental mode waveguide to contain the gas flow. In one embodiment, the dielectric tube traverses the fundamental mode waveguide ¼ of the microwave wavelength back from the short.
In an alternative embodiment of the invention, the waveguide is a quasi-optical overmoded waveguide. In a preferred embodiment, the minimum internal dimension of the quasi-optical overmoded waveguide is greater than the wavelength of the microwave energy. The internal walls of a quasi-optical overmoded waveguide can be constructed of either corrugated, electrically conducting material or of a smooth, non-conducting material. The quasi-optical overmoded waveguide can be adapted to propagate in the HE
11
mode. In a preferred embodiment, a focusing mirror at one end of the quasi-optical overmoded waveguide facilitates plasma startup. A dielectric tube, transparent to microwaves, can traverse the quasi-optical overmoded waveguide to contain the gas flow. In a further embodiment, the dielectric tube traverses the overmoded waveguide at the focus of the focusing mirror.
The preferred embodiment of the invention also includes a reflected power protector to protect the microwave generator from returned power. In

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