Tunable plasma method and apparatus using radio frequency heating and electron beam irradiation

A method and apparatus for the pyrolytic destruction or synthesis of gases via a highly tunable combination of radio frequency heating and electron beam irradiation is disclosed. The method is appropriate for destroying toxic gases emanating from hazardous wastes and for synthesizing new molecules from the molecules of a gas. The method is also appropriate for creating scavenger gases and hot gases with large enthalpy for use in sterilization procedures, for example. Embodiments are disclosed employing inductive or direct waveguide/cavity coupling of radio frequency power to the gas. In embodiments of the invention, magnetic fields are used to modify the paths of the electrons in the beam to facilitate tuning and improve the energy efficiency of the system. In a two-stage system, solid and/or liquid wastes are first heated in order to vaporize the toxic materials. Then, the gases produced in the first stage are destroyed by the combination of radio frequency heating and electron beam irradiation of the invention.

BACKGROUND OF THE INVENTION 
This invention relates to systems for breaking down and synthesizing 
molecules, in particular to systems employing a tunable combination of 
radio frequency heating and electron beam irradiation. 
In one aspect, the invention relates to systems for breaking down complex 
organic toxins. The use of electron beam irradiation to eliminate 
pollutants in smoke stack emissions is well-known. However, the use of 
electron beams to provide all the energy required is expensive and 
inflexible. Pure thermal plasma approaches wherein a plasma is created by 
heating a waste gas to a temperature sufficient to cause break-down of the 
molecules of the gas have been attempted. As an example, plasma torches 
have been used to destroy toxic wastes. A problem with such devices is 
that the electrodes degrade and must be replaced after repeated use. The 
use of electrodeless radio frequency-generated plasmas for toxic waste 
destruction at atmospheric pressures is limited by plasma stability and 
has not been employed in the field. Furthermore, both electrode and 
electrodeless thermal plasma approaches require higher temperatures (up to 
20,000K) than are needed for optimum processing in many situations. These 
methods are expensive and inefficient since all molecules, even nontoxic 
components of the gas, are destroyed in the process. 
SUMMARY OF THE INVENTION 
In one aspect, the invention is a method for transferring energy to a 
gaseous medium by simultaneously coupling radio frequency energy to the 
gaseous medium and irradiating the gaseous medium with electron beam 
energy to create a plasma and to control the plasma chemistry. In one 
embodiment, the method is adapted for breaking down molecules, 
particularly complex organic toxins emanating from toxic wastes. Example 
toxins which can be destroyed using the invention are carbon 
tetrachloride, PCBs, nerve gas, and dioxin. In another embodiment, the 
method is adapted for the synthesis of molecules. The invention can be 
used, for example, to synthesize titanium oxide, a pigment in paint, as a 
particulate or aerosol. In yet another embodiment, the method is adapted 
for the creation of scavenger gases for destroying toxins or for 
sterilization procedures, for example. In another embodiment, the method 
is adapted to create a hot gas of large enthalpy for external materials 
processing and sterilization. 
Other aspects of the invention are energy transfer apparatuses including a 
structure for containing a gaseous medium with means for transferring 
gases into and out of the structure, apparatus for coupling radio 
frequency energy to the gaseous medium, and apparatus for irradiating the 
gaseous medium with electron beam energy. The combination of radio 
frequency heating and electron beam irradiation creates a plasma and 
controls the chemistry of the plasma in the structure. 
In some embodiments, radio frequency power is inductively coupled to the 
gas in the containing structure. In one example configuration, a coil 
encircling the containing structure is connected to a radio frequency 
power source for inductively heating the gas. In another configuration, 
the containing structure is annular and a radio frequency magnetic field 
is imposed in the core of said annular structure for inductively heating 
the gas. 
In other embodiments, radio frequency power is coupled to the gas via 
waveguides and/or cavities. In one configuration, a radio frequency power 
source is connected by a conducting waveguide to a cavity adapted to 
couple the power to the gaseous medium. In another configuration, a radio 
frequency power source is connected to a conducting waveguide which is 
adapted to couple the power to the gas. 
In preferred embodiments, the combination of radio frequency heating and 
electron beam irradiation is tunable, increasing the versatility of the 
method and apparatus. In one variation, the electron beam irradiation is 
used to ionize the gas and the radio frequency heating is used to drive 
selective chemical reactions at temperatures including those that are too 
low to sustain the plasma by radio frequency heating alone. In another 
variation, the electron beam irradiation is used to ionize the gas and to 
drive selective non-equilibrium chemical reactions and the radio frequency 
energy is used to tune the temperature of the gas to increase the 
efficiency of the chemical reactions. In a third variation, the radio 
frequency heating is used to ionize the gas and the electron beam 
irradiation is used to stabilize the plasma and to drive selective 
non-equilibrium chemical reactions. 
In embodiments of the invention, a magnetic field is employed to affect the 
path of electrons in the electron beam irradiation. The magnetic field can 
be used to prevent electrons in the beam from striking the walls of a 
structure containing the gas. The magnetic field can further be used to 
control deposition of electrons in the beam so that the gas is uniformly 
affected by the electron beam irradiation, thus increasing electron beam 
efficiency. 
A further embodiment of the invention is a two-chamber system for the 
destruction of toxic wastes. The first chamber heats solid or liquid 
wastes, preferably without the use of fossil fuels, to cause vaporization 
of toxic materials and the second chamber destroys the toxic gases using 
the tunable combination of inductive heating and electron beam 
irradiation. The second chamber can be, for example, any of the 
embodiments described above.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention involves the use of an externally-driven radio 
frequency-heated plasma. By externally-driven it is meant that substantial 
ionization is provided by another source in addition to the radio 
frequency heating. In a particular embodiment, radio frequency power is 
delivered to the gas in the form of inductive discharges. Inductive 
discharges by themselves are very sensitive to the nature and properties 
of the gas, and if not controlled properly, they can contract or 
extinguish. It is necessary to have an externally-driven inductive 
discharge in order to make a robust system. 
According to one aspect of the invention, an externally-generated electron 
beam is used to provide the ionized gas medium that is heated by the radio 
frequency energy. The electron beam provides stability in terms of both 
electrical and thermal avalanching. Only small amounts of electron beam 
power are required to control the heating characteristics of the plasma. 
By varying the radio frequency heating and the electron beam powers it is 
possible to vary the nonthermal composition of the resulting plasma. 
Electron beams can be used to increase the density of the plasma, 
radicals, and excited atoms or molecules well beyond what is possible when 
the plasma is in thermal equilibrium. It is possible to make large 
densities of very reactive gases that, in conjunction with the additional 
radio frequency heating, can be used to break up molecules at relatively 
low temperatures. 
Moreover, the use of the electron beam for ionization extends the operating 
temperature and pressure ranges of heated plasmas well beyond that 
possible with radio frequency heating alone, allowing operation at 
relatively low temperatures (as low as room temperature) and at pressures 
above atmospheric. 
According to the invention, the combination of radio frequency heating and 
electron beam irradiation is tunable and the functions of the radio 
frequency heating and the electron beam are variable. In one variation, 
the primary function of the electron beam is to ionize the gas. The radio 
frequency power heats the ionized gas, thereby forming new products via 
high temperature reactions. The radio frequency power does not need to be 
high, as it does not have to produce electrons to sustain the plasma. 
Thus, selective chemical reactions can be driven at temperatures too low 
to sustain the plasma by radio frequency heating alone. In a second 
variation, radio frequency heating is used to tune the gas temperature to 
maximize the efficiency of chemical processes driven by highly reactive 
species generated by electron beam irradiation. In a third variation, the 
radio frequency heating ionizes the gas. The electron beam energy acts to 
stabilize the plasma and to create a large concentration of radicals and 
excited atoms and molecules in the radio frequency-produced plasma. These 
highly reactive species enhance the formation of new chemical products. 
In all these cases, the resulting plasma is not purely thermal due to the 
presence of the electron beam. By adjusting the relative amount of radio 
frequency and electron beam power imparted to the gas, the chemistry of 
the plasma can be controlled. Further tuning can be accomplished by 
adjusting the energy of the electrons in the beam and the electron beam 
spatial distribution. Externally-applied magnetic fields can be used to 
control the trajectories of the electrons in the beam, as described in 
detail below. Tuning of the temperature is yet another way in which the 
present invention can be easily optimized and reoptimized for breaking 
down and synthesizing molecules of interest. 
A further capability of the invention is the creation of scavenger gases. 
In one variation, the radio frequency energy is used to heat the gas, and 
the electron beam is used to create a non-equilibrium distribution of 
highly reactive species which can destroy toxic gases. For example, O and 
O.sup.+ oxidize PCBs, and can also be used for sterlizing medical 
equipment and wastes. 
The tunable system can also be used as a source of hot gas for various 
types of processing of external solids, liquids and gases, and for 
sterilization procedures. 
Advantages of combining radio frequency heating with electron beam 
irradiation include the electrodeless operation, which removes the need 
for maintaining the electrodes and eliminates the electrode waste heat, 
decreasing downtime and increasing the efficiency of the burner. For 
synthesis applications, the absence of electrodes removes a source of 
contamination. Further, the power input can be varied at will by changing 
the radio frequency power or the electron beam power. The use of two power 
sources provides the control needed for reliable and efficient processing 
of a wide range of materials. The electron energy can be varied to 
accommodate different gas species. Large high temperature volumes, at high 
pressures, can be created, with good uniformity. Limits on heating volume 
in pure radio frequency discharges due to skin depth effects can be 
greatly alleviated. Additional control of the temperature/density of the 
plasma is provided, making possible operation with non-thermal plasmas; 
the electron beams can be used to facilitate certain chemical reactions by 
producing selective reactive species. 
In one embodiment of the invention, an inductively coupled radio frequency 
discharge is applied to a gas using the arrangement shown in FIG. 1. As 
indicated, a gas 10 flows through a chamber 12 around which coupling coils 
14 are wrapped. The chamber is preferably made out of a non-conducting 
material such as quartz. The partially conducting gas acts as the 
secondary of a transformer. A time-varying current through the coils 
imparts energy to the gas, resulting in the linear plasma 16. In this 
embodiment, the time-varying current is preferably a sinusoid at 
frequencies between 60 Hz and 100 MHz. The approach can also be extended 
to higher frequencies by use of cavities and waveguides to couple 
electromagnetic energy to the conducting gas, an embodiment described in 
detail below. 
The electron beam can be applied to the gas using, for example, the 
configuration shown in FIG. 2. As shown, an electron beam generator 18 is 
configured to transmit the electron beam 20 through the window 22. 
A system combining the inductive discharge and electron beam configurations 
of FIGS. 1 and 2 is illustrated in FIG. 3. 
The inductive discharge can also be applied using the configuration shown 
in FIG. 4. In this embodiment, a gas 24 flows through an annular chamber 
26. The chamber is preferably made out of a non-conducting material such 
as quartz. The core of the annular chamber is filled with a material of 
high permeability 28, such as soft iron. A time varying magnetic field 30 
induces a toroidal plasma 31 which is held in a stable position along the 
length of the chamber by the external field 32. 
The electron beam can be applied to the gas using one or both of the 
configurations shown in FIG. 5. As shown, electron beam generators 34 and 
36 are configured to transmit electron beams 38 and 40 through the windows 
42 and 44. A linear arrangement of electron beam generators, rather than 
point sources, is recommended. Preferably, the generators 34 and 36 form 
continuous annuli around the chamber. 
A system combining an inductive heating and electron beam configurations of 
FIGS. 4 and 5 is illustrated in FIG. 6. It will be recognized by those 
skilled in the art that other configurations combining inductive heating 
and electron beam irradiation, beyond those illustrated in FIGS. 3 and 6, 
are within the scope of the invention. 
For pure inductive heating in the kilohertz to 100 megahertz range, the 
skin depth becomes an issue. If the frequency is too low, the coupling 
will not be adequate; if the frequency is too high, too much energy will 
be in the skin. Further, tunable radio frequency sources are expensive. 
However, the tunability provided by the use of the electron beam allows 
the use of a single frequency radio frequency source for a variety of 
gases by matching the skin depth of the electron-beam produced plasma to 
the radio frequency source. 
The method of the invention can be extended to higher frequencies (up to 
about 3000 MHz using currently practical sources of radio frequency power) 
by use of cavities and waveguides to couple electromagnetic energy to the 
gas. FIG. 7 illustrates an embodiment of the present invention which 
employs a conducting waveguide and cavity to couple electromagnetic power 
to the electron beam-driven conducting gas. As indicated, the radio 
frequency power is provided by a high power tube 50 and travels as an 
electromagnetic wave through a waveguide 52 into a cavity 54. The cavity 
and the waveguide are designed to optimize coupling of the power to a gas 
56 in a non-conducting chamber 58. In preferred embodiments, the waveguide 
52 and the cavity 54 are made of copper, and the chamber 58 is made of 
quartz. As in the previous embodiments, an electron beam generator or 
generators 60 is configured to transmit an electron beam or beams 62 
through the window or windows 64. 
In a variation of this embodiment, the radio frequency power is coupled to 
the gas by the waveguide alone, as illustrated in FIG. 8. As shown, the 
radio frequency power is provided by a high power tube 70 and travels as 
an electromagnetic wave through a waveguide 72 to a dump 74. The waveguide 
is designed to optimize coupling of the power to the gas 76 in the 
non-conducting chamber 78. In preferred embodiments, the waveguide 72 is 
made of copper, and the chamber 78 is made of quartz. An electron beam 
generator or generators 80 is configured to transmit an electron beam or 
beams 82 through the window or windows 84. 
In all of the embodiments described above, the energy of the electrons in 
the electron beam is determined by the size of the chamber, the pressure, 
and to a lesser degree, the nature of the gas. Energies between 100-500 
keV are appropriate and are preferably selected to provide adequate 
penetration. The mean free path of the electron beam is preferably 
comparable to or smaller than the size of the chamber, in order to prevent 
large shine-through. The mean free path should also not be much smaller 
than the chamber size, because the electron beam deposition will not be 
uniform. The ratio of the electron mean free path in the gas at the 
injection energy and the effective size of the chamber is preferably 
between two and six. 
The electron current that can be extracted through a window is about 1 
mA/cm in a linear beam configuration. Assuming 200 keV electron beam 
energy, then the power per unit beam width is 200 W/cm. The beam injection 
units can be stacked next to each other to increase the total power per 
unit length. 
In order to increase the efficiency of the system, and to protect as much 
as possible the electron beam window, it is desirable to operate at low 
temperature. The temperature of the gas operation is preferably 
1000.degree.-4000.degree. C. The required power levels depend upon the 
flow rate of the gas. Typical power levels are about 100 kW for 10 
tons/day of throughput. 
The window of the electron beam injector is subjected to the damaging 
action of the beam itself and the corrosive environments in the chamber. 
As shown in FIG. 9, one way to minimize the damage is to protect the 
window 90 by a slow flow of gas 92 (such as nitrogen) in the area near the 
window. 
Another way to minimize window damage is to fabricate the window of 
materials that are resistant to corrosion, such as thin films made of 
materials such as diamond or diamond-like film, silicon carbide, boron 
nitride, aluminum oxide, or silicon nitride. these films are easy to 
manufacture and can be supported against the atmospheric load by a grid or 
mesh. The grid or mesh can be made either separate from the film, or as an 
integrated part of the film by growing the film on a substrate and then 
etching away most of the substrate material. The remaining substrate 
material will provide the required support and cooling. The advantages of 
diamond and diamond-like carbon films are their very large thermal 
conductivity, high resistance to corrosion, very large strength, and low 
interception of the electron beam resulting in low window heating. Silicon 
carbide, boron nitride, aluminum oxide, and silicon nitride share these 
properties (with the exception of the high thermal conductivity) but have 
the advantage of good high-temperature properties. 
Applied magnetic fields of different magnitudes can be used to modify the 
electron beam path for optimum operation. This adds yet another tuning 
mechanism. An applied magnetic field can be used to prevent electrons from 
reaching the walls, thus minimizing shine-through. This is especially 
important when high energy electron beams are used which might hit the 
other side of the chamber, creating x-rays. This mechanism also can be 
used to control the deposition of the electrons to ensure that the gas is 
uniformly affected by the electron beam. The magnetic field is preferably 
constant or very slowly time-varying (with period on the order of 
seconds). The magnitude of the magnetic field can be varied to allow 
optimum use of electron beams of varying energy. 
One use of magnetic fields to alter the paths of electrons is illustrated 
in FIG. 10. In this embodiment, the magnetic field direction is roughly 
perpendicular to path of the electrons. As shown, an electron source 100 
injects electrons 102 into the chamber 104. A magnetic field 106, having 
the dependence on height shown in the figure, is applied inside the 
chamber. As the electrons slow down, the magnetic forces change the 
direction of the motion in a spiral-like pattern 108 with an axis 110 
inside the plasma 112. The electrons spin while slowing down until they 
have lost all their energy. Low values of magnetic field are required. As 
the motion of an electron in a magnetic field depends on its energy, 
tuning of the electron beam energy and the magnetic field strength in 
tandem can be used to obtain the desired spatial electron deposition. At 
100 G, the gyro radius of a 500 keV electron is about 10 cm. 
In an alternative embodiment, a magnetic field parallel to the direction of 
electron beam injection is used to prevent electrons from striking the 
side walls and producing large amounts of x-rays. An embodiment where the 
electron beam path and the magnetic field are parallel to the gas flow is 
illustrated in FIG. 11. As shown, gases 120 enter the chamber 122 and flow 
in an upward direction. Electrons 124 are injected in an upward direction 
through the window 126. Induction coils 128 provide the inductive heating. 
A second coil 130 provides the magnetic field 132 having an upward 
direction. This applied magnetic field acts to prevent scattering of the 
electrons, keeping them on an upward path 134 and away from the walls of 
the chamber. Magnetic fields on the order of 1 kilogauss (corresponding to 
Larmor radius or .about.1 cm for .about.100 KeV electrons) are appropriate 
for this embodiment. 
An embodiment where the electron beam path and the magnetic field are 
perpendicular to the gas flow is illustrated in FIG. 12. As shown, gases 
140 flow upward in the chamber 142. Electrons 144 are injected 
perpendicular to the flow through the window 146. An applied magnetic 
field 148 parallel to the path 150 of the electrons acts to prevent 
scattering. 
A two chamber system for the destruction of wastes according to the present 
invention is shown in FIG. 13. The first chamber 160 heats the 
solid/liquid wastes, which may not be well characterized, and the second 
chamber 162 destroys the toxic gases generated by the heating of the 
material. The second chamber uses the combined radio frequency-electron 
beam system described above. The secondary system can be, for example, one 
of the embodiments illustrated in FIGS. 3, 6, 7, or 8, optionally 
employing an imposed magnetic field as in FIGS. 10-12. 
The purpose of the first chamber is to heat the solids and/or liquids to 
temperatures such that the organic contaminants vaporize, leaving a sludge 
composed of metals and inorganic materials. This sludge can be allowed to 
solidify in a separate chamber, immobilizing the heavy metals. The 
temperature of the first chamber preferably should be sufficiently high to 
vaporize all toxic organic materials in the mixed waste (so that all 
toxins can be treated in the secondary chamber) and to melt the solids (so 
that the non-volatile toxic metals become immobilized after the slag 
solidifies). 
Ideally, the heating of the solid/liquid wastes in the first chamber should 
be done without the use of fossil fuels that would increase the gas 
loading of the secondary chamber. Any of a large number of methods can be 
used, such as an electric discharge operating with electrodes (arc, 
torch), inductively heated plasmas at AC frequencies, or radio frequency 
heating. 
The advantages of using plasmas or radio frequency heating for the primary 
combustion chamber include the absence of the burning of fossil fuels 
which minimizes the gas throughput in the secondary chamber and in the gas 
treatment system following the secondary chamber. Further, in comparison 
to incinerators, relatively small size units are possible. With the 
present invention, power input can be varied widely in short periods of 
time, including fast turn-on and shut-down phases. The temperature of the 
solid and liquid waste can be adjusted by controlling the power in the 
first chamber. 
As discussed above, systems for waste destruction employing electron beam 
irradiation or pure thermal plasmas have been developed in the past. The 
details of the physical embodiments and the use of these units are 
relevant to aspects of the present invention. The following articles, 
herein incorporated by reference, describe existing plasma arc and 
electron beam units. 
"High-power selfshielded electron processors and their application to stack 
gas treatment" by J. Hiley et al., in Nuclear Instruments and Methods in 
Physics Research, B24/25, pp. 985-989, 1987. 
"Operating and testing a combined SO.sub.2 and NO.sub.x removal facility" 
by N. W. Frank et al., in Environmental Progress, Vol. 6, pp. 177-182, 
August 1987. 
"Stack testing of the mobile plasma arc unit" by M. Gollands et al., in EPA 
Project Summary, EPA/600/S2-87/013 May 1987. 
"Trial burns-plasma arc technology" by N. P. Kolak et al., in Nuclear and 
Chemical Waste Management, Vol. 7, pp. 37-41, 1987. 
It is recognized that modifications and variations of the invention will 
occur to those skilled in the art, and it is intended that all such 
modifications and variations be within the scope of the claims.