US3862393A

US3862393A - Low frequency induction plasma system

Info

Publication number: US3862393A
Application number: US327082A
Authority: US
Inventors: Peter H Dundas; John W Poole; Charles E Vogel
Original assignee: Humphreys Corp
Current assignee: Humphreys Corp
Priority date: 1971-08-20
Filing date: 1973-01-26
Publication date: 1975-01-21
Anticipated expiration: 1992-01-21

Abstract

A low frequency induction plasma system includes a plasma chamber and energizing coil dimensioned as a function of frequency. Structure for lowering the reluctance of the magnetic path surrounds the coil and the chamber so as to enhance the flux in the chamber produced by the energized coil. A thermal plasma in a flowing system is established by initiating a glow discharge with an auxiliary high frequency power supply and then creating a thermal plasma assisted by the enhanced flux in the chamber.

Description

United States Patent [1 1 Dundas et a1.

LOW FREQUENCY INDUCTION PLASMA SYSTEM Inventors: Peter H. Dundas, Andover, Mass;

John W. Poole; Charles E. Vogel, both of Bow, N.l-l.

Assignee: Humphreys Corporation, Bow, N.l-l.

Filed: Jan. 26, 1973 Appl. No; 327,082

Related US. Application Data Continuation of Ser. No. 173,500, Aug. 20, 1971,

abandoned.

us. c1. 219/121 1 1111.01 823k 9/00 Field of Search 219/121 R, 121 P, 383,

References Cited UNITED STATES PATENTS 12/1959 Giannini et a1. 219/121 R X 11/1964 Gross 219/121 PX Jan. 21, 1975 3,264,508 8/1966 Lai et al. 219/121 P X 3,296,410 1/1967 Hedger 219/121 P 3,543,084 11/1970 Michaelis... 219/121 R X 3,571,772 3/1971 Bennon 336/84 3,620,008 11/1971 Newbold 219/121 P X 3,639,872 2/1972 Hessen et a1... 336/84 3,646,308 2/1972 Splessens 219/121 P Primary Examiner.l. V. Truhe Assistant Examiner--Ga1e R. Peterson 1 ABSTRACT A low frequency induction plasma system includes a plasma chamber and energizing coil dimensioned as a function of frequency. Structure for lowering the reluctance of the magnetic path surrounds the coil and the chamber so as to enhance the flux in the chamber produced by the energized coil. A thermal plasma in a flowing system is established by initiating a glow discharge with an auxiliary high frequency power supply and then creating a thermal plasma assisted by the enhanced flux in the chamber.

12 Claims, 3 Drawing Figures Patentd Jan. 2-1, 1975 r r 3,862,393

2 Sheets-Sheet 1 LOW FREQUENCY INDUCTION PLASMA SYSTEM This application is a continuation of Ser. No. 173,500, filed 'Aug. 20, 1971, now abandoned, entitled Low Frequency Induction Plasma System.

. The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of section 305 of, the National Aeronautics and Space Act of' 1958, Public Law 85568 (72 Stat. 435; 42 U.S.C. 2457).

SUMMARY OF INVENTION This invention relates to induction plasma systems and to methods of operating such systems.

A thermal plasma is created and maintained in a system of the induction type by providing an intense electromagnetic field which produces an electrodeless discharge (thermal plasma) inan ionized medium. Such a thermal plasma has a temperature in the range of 8,0001 1,000K and is useful for a variety of purposes including the performance of chemical reactions, the working ofmetallic and/or refractory materials, heaters for hypersonic tunnels and simulator devices that utilize the high temperatures produced by such systems. A number of arrangements have been proposed for generating such thermal plasmas, including both closed and flowing systems in which the plasma has been stably maintained both in reduced pressure environments and in atmospheric pressure environments. In general, such devices have the electromagneticfield produced by power sources operating in the megahertz and near megahertz frequency range, and as such require expensive electronic circuitry of relatively low efficiency.

It is an'object of this invention to provide a novel and improved induction plasma system operable at frequencies well below the megahertz range.

Another object of the invention is to provide novel and improved induction plasma systems of the flowing type. A flowing type of system facilitates the performance of useful work, either by the plasma within the plasma chamber or by the effluent from the plasma chamber. In such flowing systems, the stabilization of the plasma, both spacially and electrically, becomes more critical, as that plasma must be stabilized under the dynamic flow conditions against drift which would damage the integrity of the apparatus and also to maintain the necessary electrical characteristics of the system so that the thermal plasma is not extinguished. The extinction of the thermal plasma is a function of the energizing frequency and the characteristic decay time of the plasma.

According to theory, the maintenance of an induction plasma is dependent on a circulating current which should circulate near the wall of the plasma chamber.

' The depth of penetration of such current may be termed skin depth" and is represented by the formula:

8 skin depth 1- conductivity of load ,1 magnetic permeability of load f= frequency of applied energy For coupling efficiency the skin depth should be less than the radius of the plasma and therefore the plasma diameter should be larger at lower frequencies of energization. A minimum voltage gradient around the plasma is required to sustain the plasma, this gradient being 2 volts/cm in a high current argon arc, and higher for air. Thus the requisite voltage increases with larger diameter plasmas. These additional considerations have lead experimenters in induction plasma systems to employ high frequency energizing sources. Through our invention involving particular ignition and magnetic enhancement techniques, we have successfully operated stable flowing induction plasma systems at frequencies far below the frequencies conventionally employed.

In accordance with our invention, there is provided a plasma generation system that includes an elongated plasma defining chamber having exhaust port structure at one end and injector structure at the one end for introducing material plasma forming material at one end of the chamber for flow through the chamber and exit through the exhaust port structure. An electrical coil surrounds the chamber and is connected to a relatively low frequency power supply (that is 20 kilohertz or below) for creating an intense electromagnetic field within the chamber. Magnetic enhancement structure that at least doubles the flux densityin the chamber surrounds the electrical coil. In a particular embodiment this magnetic enhancement structure is formed of a series of discrete, spaced groups of clamped, axially extending electrical steel laminations, with cooling structures on the sides of the groups adjacent the leads of the electrical coil. I

Plasma initiation in this low frequency system is preferably accomplished by initially reducing the pressure in the chamber and applying a high frequency (e.g., 20 megahertz) signal across the chamber to establish a glow discharge that substantially fills the chamber and then coupling the low frequency power supply to the electrical coil to convert the glow discharge to a thermal plasma. In a particular embodiment this glow discharge condition is established by short duration pulsed application of high frequency energy.

The methods and apparatus in accordance with the invention provide a simple, reliable, and relatively easy to operate induction plasma system of the flowing type operable to provide a stable thermal plasma energized at low frequencies of less than 20 kilohertz in a system.

Other objects, features and advantages of the invention will be seen as the following description of a particular embodiment progresses in conjunction with the drawings, in which:

FIG. 1 is a diagrammatic view of an induction plasma system constructed in accordance with the invention;

FIG. 2 is a sectional view showing details of the induction plasma apparatus shown in FIG. 1 taken along the line 2-2 of FIG. 1; and

FIG. 3 is a diagram of an induction plasma system employing the apparatus as shown in FIG. 1.

DESCRIPTION OF PARTICULAR EMBODIMENT The induction plasma generator apparatus shown in FIG. 1 is designed for operation at 9600I-Iz and includes an opaque fused quartz tube I0 having an inner diameter of five and one-half inches and a length of twenty and one-half inches. In a system designed for operation at 960I-Iz a tube 10 that has a diameter of twelve inches and a length of sixty inches is employed. The upstream end 12 of tube 10 is seated on flange 14 of end plate structure 16 and sealed by O-ring l8, and the downstream end 20 of tube 10 is similarly seated within recess 22 of end plate 24 and sealed by O-ring 26.

lnjector structure 30 and flow stabilizer structure 32 are positionedin end plate 16 and clamped plate 34 and bolts 36. A first inlet port 38 supplies gas through channel 40 to annular chamber 42 for distribution through radially directed ports 44. A second inlet port 46 supplies gas through passage 48 to annular chamber 50 for distribution through swirl ports 52. Axial ports '54 are supplied through a third passage (not shown) that is angularly offset from

passages

40 and 48. The upper end of injector structure 30 is cooled by water flowing through passages 56 and chamber 58 which flow stabilizer structure 32 is cooled by water flowing through passages 60 and chamber 62.

A cast acrylic cylinder 64 surrounds tube with its upstream end seated in recess 66 in end plate 16 and its downstream end seated in recess 68 in end plate 24. Pressurized coolant is supplied through passage 70 for flow throughspace 72 and exhaust through passage 74. An induction coil 80 is formed of rectangular (one inch by one-half inch) one sixteenth inch wall copper tubing in a coil having an inner diameter of seven and onequarter inches and a length of seven inches. Surrounding coil 80 is a magnetic enhancement structure 82 in the form of a series of six electrical steel lamination assemblies 84, as indicated in FIG. 2. Each such assembly includes two clamp plates 86 between which are disposed a stack of Armco Tran-cor T electricalsteel laminations 88, each lamination being 0.007 inch thick, seven-eighths inch wide and ten and one-half inches long, with inwardly extending ear portions 89 at either end. Each stack has a depth of two and one-half inches. Each lamination assembly 84 is mounted on support 90. Cooling tubes 92 are secured to the clamp plates 86 on either side of coil terminals 94 by silver brazing and cooling water is flowed through tubes 94 during operation of the apparatus. This magnetic enhancement structure more than trebles the flux density in chamber End plate 24 has an evacuation port 100 which is arranged for connection via valve 102 to a vacuum systern and an exit port 104 across which is disposed a grid 106 of cooling pipes 107 supported on ring 108. Glass plate 110 functions as a valve and as a sight port when the system is operating under reduced pressure conditions isseated on ring 108 and is sealed by O-rings 112.

A diagram of the operating system is shown in FIG. 3. The injector assembly 30 is connected to a suitable source 120 of gas. Top flange 24 is connected to vacuum system 122 as controlled by valve 102. Exit port 104 is controlled by valve 110. A 450 kilowatt 9600 Hertz motor generator power supply 124 and 9200 KVAR capacity is connected to coil 80 and high frequency MHz) auxiliary ignition supply 126 connected through switch 128 across

end plates

16 and 24.

To form a thermal plasma 130 in chamber 10, valve 110 is closed and the pressure in the chamber 10 is reduced to a pressure of one Torr.,. the RF power supply ignition source 126 is energized and a glow discharge is produced whichfills chamber 10. With the main (LF) power supply 124 energized, the glow discharge provides a sufficient load for coupling by the low frequency power and a thermal plasma 130 is established as signified by a dramatic change in brightness. Before establishment of the plasma 130 the power supply 124 was operating at full voltage, 60 percent current and 6 percent lead power factor. After establishment of plasma, voltage dropped to 40-50 percent current increased to percent and the power factor shifted to 10 percent lag. With initiation of the thermal plasma, argon gas flow is increased, increasing the pressure in the chamber 10. Plasma is stably maintained as the pressure is increased to atmospheric pressure in a flowing mode of operation. With an argon flow rate of approximately 700 SCFH at one atmosphere pressure, the minimum sustaining power was KW.

In a 960 Hz system, a 1250 KW motor generator power supply 124' was employed with a capacitor bank of 22,000 KVAR capacity connected to coil 80'. The high frequency ignition supply was a 25 KW 4 MHZ supply. Glow discharge at 0.35 Torr was produced by energizing coil 80 and then applying a pulse (172 millisecond in duration in one sequence) of ignition energy across end plates 16', 24' to create a glow discharge that fills chamber 10' and the low frequency enhanced electromagnetic field couples to the discharge and a plasma 130 is established.

While a particular embodiment of the invention has been shown and described, various modifications thereof will be apparent to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodiment or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

- What is claimed is:

1. An induction plasma system for use with an ignition source comprising:

structure defining an elongated plasma chamber,

an electrical coil surrounding said chamber,

' means for supplying a plasma forming material for flow into said chamber and conversion to plasma condition under the influence of an electromagnetic field produced by said electrical coil, and exhaust port structure at one end of said chamber defining a port for flow of said material from said chamber, wherein the improvement comprises a low frequency power supply connected to said coil for creating an intense electromagnetic field within said chamber, and

magnetic enhancement structure disposed adjacent said coil, said magnetic enhancement structure providing magnetic flux density in said chamber that is at least twice the flux density that exists in said chamber in the absence of said magnetic enhancement structure.

2. The system as claimed in claim 1 wherein said magnetic enhancement structure comprises a series of spaced groups of axially extending laminations of electrical steel that surround said coil.

3. The system as claimed in claim 2 and further including cooling means disposed between the leads of said coil and each immediately adjacent group of laminations.

4. The system as claimed in claim 1 and further including an auxiliary high frequency power supply and means to connect said auxiliary power supply across the ends of said plasma chamber and establish a glow discharge in said plasma chamber.

5. The system as claimed in claim 1 and further including means for sealing said exhaust port structure and further including auxiliary exhaust port structure of substantially smaller dimension than the dimension of saidmain exhaust port structure.

6. The system as claimed in claim 5 wherein said plasma chamber defining structure is a tubular member of uniform cross-sectional configuration throughout its eluding means to'reduce the pressure in said plasma chamber, an auxiliary high frequency power supply and means to connect said auxiliary power supply across the ends of said plasma chamber and establish a glow discharge in a reduced pressure environment is said plasma chamber.

9. A method for initiating operation of a low frequency induction flowing plasma system having a plasma defining chamber having a gas' inlet port at one end and a plasma outlet port at the opposite end, and an' electrical coil surrounding said chamber and adapted to be connected to a suitable low frequency power supply for creating an intense electromagnetic field within said chamber comprising the steps of continuously applying said low frequency energy to said coil, reducing the pressure in said chamber,

establishing a glow discharge in said chamber by applying a discharge pulse of high frequency energy across the ends of said plasma chamber to create a conductive condition in the reduced pressure environment in said chamber, said conductive condition enabling a thermal plasma to be established in said chamber under the influence of the electromagnetic field created by the low frequency energy applied to said electrical coil, and after said thermal plasma is established, increasing the pressure in said chamber to atmospheric pressure while continuously flowing gas through said inlet port into said chamber so that plasma flows through said outlet port.

10. The system as claimed in claim 1 wherein said power supply has an operating frequency of less than 20 kilohertz for creating an electromagnetic field within said plasma chamber inside said electrical coil.

11. An induction plasma system for use with an ignition source comprising:

structure defining an elongated plasma chamber,

an electrical coil surrounding and extending along an axial length of said plasma chamber,

inlet port structure for supplying a plasma forming material for flow into said chamber and conversion to plasma condition under the influence of an electromagnetic field produced by said electrical coil,

, and exhaust port structure at one end of said chamber defining a port for flow of said material in plasma condition from said chamber, wherein the improvement comprises a low frequency power supply connected to said coil for creating an intense electromagnetic field within said chamber, and

magnetic enhancement structure disposed adjacent I and outside of and extending around said coil and said chamber, said magnetic enhancement structure including a main portion extending the length of said coil and portions at either end of said main portion that overlie the-ends of said coil and extend inwardly from said main portion towards said plasma chamber.

12. The system as claimed in claim 11 wherein said magnetic enhancement structure comprises a series of spaced axially extending members of magnetic material, said series of members surrounding said coil and each said member including an inwardly extending end portion that overlies the adjacent end of said coil, said magnetic enhancement structure being isolated from the high voltage that appears across the ends of said coil.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 62,393

DATED 1 January 21, 1975 INVENTOR(S) Peter H. Dundas et a1.

it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, lines 5-6, delete "for creating, an

electromagnetic field within said nlasma. chamber inside said electrical coil".

Signed and sealed this 6th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks

Claims

1. An induction plasma system for use with an ignition source comprising: structure defining an elongated plasma chamber, an electrical coil surrounding said chamber, means for supplying a plasma forming material for flow into said chamber and conversion to plasma condition under the influence of an electromagnetic field produced by said electrical coil, and exhaust port structure at one end of said chamber defining a port for flow of said material from said chamber, wherein the improvement comprises a low frequency power supply connected to said coil for creating an intense electromagnetic field within said chamber, and magnetic enhancement structure disposed adjacent said coil, said magnetic enhancement structure providing magnetic flux density in said chamber that is at least twice the flux density that exists in said chamber in the absence of said magnetic enhancement structure.

5. The system as claimed in claim 1 and further including means for sealing said exhaust port structure and further including auxiliary exhaust port structure of substantially smaller dimension than the dimension of said main exhaust port structure.

6. The system as claimed in claim 5 wherein said plasma chamber defining structure is a tubular member of uniform cross-sectional configuration throughout its axial length and said exhaust port sealing means includes a removable plate member disposed across the downstream end of said plasma chamber defining structure.

7. Ths system as claimed in claim 5 wherein said magnetic enhancement structure comprises a series of spaced groups of axially extending laminations of electrical steel that surround said coil and further including cooling means disposed between the lead of said coil and each immediately adjacent group of laminations.

8. The system as claimed in claim 7 and further including means to reduce the pressure in said plasma chamber, an auxiliary high frequency power supply and means to connect said auxiliary power supply across the ends of said plasma chamber and establish a glow discharge in a reduced pressure environment is said plasma chamber.

9. A method for initiating operation of a low frequency induction flowing plasma system having a plasma defining chamber having a gas inlet port at one end and a plasma outlet port at the opposite end, and an electrical coil surrounding said chamber and adapted to be connected to a suitable low frequency power supply for creating an intense electromagnetic field within said chamber comprising the steps of continuously applying said low frequency energy to said coil, reducing the pressure in said chamber, establishing a glow discharge in said chamber by applying a discharge pulse of high frequency energy across the ends of said plasma chamber to create a conductive condition in the reduced pressure environment in said chamber, said conductive condition enabling a thermal plasma to be established in said chamber under the influence of the electromagnetic field created by the low frequency energy applied to said electrical coil, and after said thermal plasma is established, increasing the pressure in said chamber to atmospheric pressure while continuously flowing gas through said inlet port into said chamber so that plasma flows through said outlet port.

11. An induction plasma system for use with an ignition source comprising: structure defining an elongated plasma chamber, an electrical coil surrounding and extending along an axial length of said plasma chamber, inlet port structure for supplying a plasma forming material for flow into said chamber and conversion to plasma condition under the influence of an electromagnetic field produced by said electrical coil, and exhaust port structure at one end of said chamber defining a port for flow of said material in plasma condition from said chamber, wherein the improvement comprises a low frequency power supply connected to said coil for creating an intense electromagnetic field within said chamber, and magnetic enhancement structure disposed adjacent and outside of and extending around said coil and said chamber, said magnetic enhancement structure including a main portion extending the length of said coil and portions at either end of said main portion that overlie the ends of said coil and extend inwardly from said main portion towards said plasma chamber.