System and method for plasma generation of nitric oxide

Abstract

Plasmatron includes an anode having a cylindrical proximal portion and a cylindrical distal portion, the distal portion having a smaller diameter than the proximal portion; a connecting portion connecting the proximal and distal portions and having walls oriented at 40-60 degrees to a center axis of the anode; a cathode having a generally cylindrical shape in its proximal portion and a tapering at a 30-45 degree angle to the center axis of the anode in its distal portion, with a cylindrical rod on its tip. Gap between the connecting portion of the anode and the distal portion of the cathode is double the gap between the proximal portion of the anode and the proximal portion of the cathode. High voltage power supply provides an operating voltage of 800-2500 volts and a current of 0.3-0.7 A. Length of the rod is approximately 1.5 times its diameter.

Claims

1. A plasma generation system, comprising: an anode having a generally cylindrical proximal portion and a generally cylindrical distal portion, the distal portion having a smaller diameter than the proximal portion; a connecting portion connecting the proximal and distal portions and having walls oriented at 40-60 degrees to a center axis of the anode; a cathode having a generally cylindrical shape in its proximal portion and a tapering at a 30-45 degree angle to the center axis of the anode in its distal portion, with a cylindrical rod on its tip, wherein a gap between the connecting portion of the anode and the distal portion of the cathode is at least twice as large as a gap between the proximal portion of the anode and the proximal portion of the cathode; and a power supply providing an operating voltage of 800-2500 volts to the cathode; wherein the power supply comprises two transistors in a push-pull configuration connected to a powered transformer, with a midpoint primary winding connected to parallel capacitors having electric capacitance value such as to stimulate oscillations of an output voltage within opening and closing transistors cycles.

2. The plasma generation system of claim 1, wherein the power supply is configured to provide the operating voltage at a current of 0.3-0.7 A.

3. The plasma generation system of claim 1, wherein a length of the cylindrical rod is 1.5 times a diameter of the cylindrical rod.

4. The plasma generation system of claim 1, wherein a diameter of the cylindrical rod is from D=?{square root over (V0.003*P)} to D=?{square root over (0.03*P)}, and length of the cylindrical rod is L=1.5*D, where P is power in watts, and D and L are in mm.

5. The plasma generation system of claim 1, wherein the anode and the cathode are coaxial.

6. The plasma generation system of claim 1, wherein the cathode is movable along the center axis.

7. The plasma generation system of claim 1, further comprising a screw for moving the cathode along the center axis.

8. The plasma generation system of claim 1, wherein both the cathode and the anode are made from stainless steel.

9. The plasma generation system of claim 1, wherein the cathode is made of copper and the cylindrical rod is made of hafnium.

10. The plasma generation system of claim 1, wherein the anode is made of stainless steel.

11. The plasma generation system of claim 1, wherein the cathode is made of stainless steel and the cylindrical rod is made of hafnium.

12. The plasma generation system of claim 1, wherein the power supply is configured to provide a moment of transistor opening and closing at a minimum of voltage during oscillation.

Description

BRIEF DESCRIPTION OF THE ATTACHED FIGURES

(1) The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

(2) In the drawings:

(3) FIG. 1 shows jumping place of secondary breakdown from hot wide place to cold narrow place and back.

(4) FIG. 2 shows a cathode of a conventional plasmatron in detail.

(5) FIG. 3 shows a cathode of the proposed plasmatron.

(6) FIG. 4 shows a power curve as an average power of time for conventional and proposed plasmatrons.

(7) FIG. 5 shows a fixed place of secondary breakdown by thermal accumulation in hot end of rod at installed on the end of cathode.

(8) FIG. 6 shows a sectional view of the cathode and anode of the proposed plasmatron.

(9) FIG. 7 shows another sectional view of the cathode and anode of the proposed plasmatron.

(10) FIG. 8 shows an illustration of heat balance calculation for the cathode of the proposed plasmatron.

(11) FIG. 9 shows a power supply of the proposed plasmatron.

(12) FIG. 10 shows a voltage waveform for the power supply of the proposed plasmatron.

(13) FIG. 11 shows ferrite gap core configurations of the power supply.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(14) Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

(15) The results of the proposed design according to the ideas described above are as follows. Plasmatron operation with completely different level of output power stability and region of fluctuations of average power FIG. 4, which shows average power dependence on time of conventional (top) and new proposed (bottom) plasmatrons. As this figure shows, the average power fluctuation region is about two times higher in conventual plasmatron than in modified plasmatron.

(16) The invention solves the problem of instability by modification of the power supply and by modification of plasmatron design. Modified power supply should be made on the base of push-pull schematic consists from high voltage transformer with gap core, midpoint primary winding and high voltage secondary winding loaded to reactive current limitation schematic, based on capacitor or inductor and high voltage rectifier connected with plasmatron. Both ends of primary windings connected with transistors (IGBT, field or bipolar transistors) with anti-parallel diode, as shown in FIG. 9. When the system is in operation, both transistors are opened and closed alternately with necessary dead time between closing first and opening second transistor like in regular push-pull schematic. Both transistors are connected with parallel capacitors with electric capacitance value C enough to stimulate voltage U oscillations inside of opening and closing transistors cycle as shown in FIG. 10. To adjust the frequency of oscillations, a gap thickness of high voltage transformer gap core is used. The gap thickness can control effective inductance of primary windings and adjust oscillations frequency to provide two effects which are necessary for effective and stable operation of plasmatron. One effect is an increase of high voltage maximum by a factor of 2.

(17) Maximum voltage determines the maximum length of the plasma filament, stability of electric discharge ignition and stability of frequency of secondary breakdowns. Another effect of adjustment of oscillation frequency is energy efficiency of the power supply, by adjustment of the moment of transistor opening/closing to the moment of minimum of voltage during oscillation. By the opening of transistor at the moment when parallel capacitor voltage (equal to the voltage on transistor) is minimal we minimize loses of energy storage in capacitor which is equal C*U.sup.2/2. This way, a positive effect of parallel capacitor on plasmatron operation stability is reached, and at the same time energy loses at the moment of short circuit of capacitor by opening of transistor are minimized.

(18) The problem of thermal instability described above can be solved by changing of plasmatron cathode design to make a thermal accumulator, which cannot prevent fast changing of temperature of plasma channel close to cathode tip.

(19) Instead of the cathode shown in FIG. 2, the proposed cathode has a rod 302 on the end of the conical tip, as shown in FIG. 3. Isometric cross-sectional views of the cathode structure are shown in FIG. 6 and FIG. 7.

(20) The rod 302 can work like a thermal accumulator. When the end of this rod will be heated it can be cooled only after some minimal time because of thermal resistant of rode caused by its length and diameter rate. To calculate parameters of the rod 302, consider long rod with a length greater than the diameter. Once of rod is heated up to some temperature, we need to calculate how temperature T(X,t) of rod points will be changed as a function of time t and length x of this tip in a process of thermal transfer through the rod. Here,

(21) ddiameters of rod, and rod cross section is

(22) $S=\frac{?d^2}{4},$

(23) Divide rod length to elements dx.

(24) Mass of element dx is dm=?Sdx.

(25) Energy balance of dx is:
?Q=?Q.sub.+??Q.sub.?where

(26) $\begin{array}{cc}?Q_{+}=\frac{\left(T\left(x-dx\right)-T\left(x\right)\right).Math.S.Math.?.Math.\mathrm{dt}}{dx};& \end{array}$

(27) ?thermal conductivity coefficient of rod metal.

(28) $\begin{array}{cc}?Q_{-}=\frac{\left(T\left(x\right)-T\left(x+dx\right)\right).Math.S.Math.?.Math.\mathrm{dt}}{dx};& \end{array}$

(29) As shown in FIG. 8, P is power of electric discharge dissipated in rod tip. Therefore,

(30) $?Q=?Q_{+}-?Q_{-}=\frac{\left(T\left(x-dx\right)-T\left(x\right)\right).Math.S.Math.?.Math.\mathrm{dt}}{dx}-\frac{\left(T\left(x\right)-T\left(x+dx\right)\right).Math.S.Math.?.Math.\mathrm{dt}}{dx};$$\mathrm{dT}=\frac{?Q}{C.Math.\mathrm{dm}};$$?Q=\mathrm{Cdm}.Math.\mathrm{dT}=\frac{\left(T\left(x-dx\right)-T\left(x\right)\right).Math.S.Math.?.Math.\mathrm{dt}}{dx}-\frac{\left(T\left(x\right)-T\left(x+dx\right)\right).Math.S.Math.?.Math.\mathrm{dt}}{dx};$$\mathrm{And}\mathrm{then}\text{:}\frac{dT}{dt}=\frac{d^{2}T}{d{x}^2}.Math.\frac{?}{C.Math.?};$

(31) Solving of this equation with boundary conditions characteristic for our case give us that for a hafnium rod with diameter 2 mm and length 3 mm, the temperature of the tip is below the melting temperature of hafnium and at the same cooling time is more than 0.5 millisec and this rod can work like thermal accumulator for stabilization of temperature near rod end. The result of operation of the cathode with the rod 302 we can see in the graphs of FIG. 4, showing the average power of low current plasmatron without rod in the top graph, and with the rod 302 in the bottom graph. As can be seen from these graphs, adding the rod 302 decreases power fluctuations by more than 2?.

(32) The following are examples of rod dimensions for various operating conditions:

(33) 1. Power 300 Watts, insert (rod) made of hafnium of diameter 2 mm and length (beyond the conical portion) 3 mm Range of fluctuations reduced by 2.1? compared to a cathode without the insert.

(34) 2. Power 300 Watts, insert (rod) made of stainless steel of diameter 2 mm and length (beyond the conical portion) 3 mm. Range of fluctuations reduced by 2? compared to a cathode without the insert.

(35) 3. Power 500 Watts, insert (rod) made of hafnium of diameter 2.6 mm and length (beyond the conical portion) 4 mm Range of fluctuations reduced by 2.2? compared to a cathode without the insert.

(36) 4. Power 100 Watts, insert (rod) made of hafnium of diameter 1 mm and length (beyond the conical portion) 1.5 mm Range of fluctuations reduced by 2.2? compared to a cathode without the insert.

(37) Generally, with a power range of 100 to 1000 Watt the insert (rod) made from material with heat conductivity 15-30 W/(m*K) should have diameter D=?{square root over (0.01*P)} mm and length L=1.5*D (P is power in watts).

(38) Having thus described a preferred embodiment, it should be apparent to those skilled in the art that certain advantages of the described method and apparatus have been achieved.

(39) It should also be appreciated that various modifications, adaptations and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.