Corona source for producing corona discharge and fluid waste treatment with corona discharge

A corona source suitable for use in vehicle ignition systems uses a conductive coil that receives an RF input at one end and has a corona discharge site at the other end, with a reference electrode capacitively coupled to the coil. The pitch and the length of the coil are selected to produce a corona discharge in response to an RF input signal at a predetermined frequency and voltage, through quarter wavelength resonation. Either the new resonant coil or other corona discharge devices can be used to remediate fluid-borne wastes by initiating and sustaining RF corona discharges within the fluid. The pulses used to initiate the corona discharge preferably have alternating positive and negative components, with high initial voltages on the positive components to initiate the discharge, followed by lower positive voltage levels to sustain the discharge. Unipolar pulses, preferably with progressively decreasing voltage levels, can also be used.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the treatment of liquid and gaseous wastes, and 
more particularly to the treatment of hydrocarbon waste materials in a 
fluid through a corona discharge, and the generation of a corona discharge 
with a radio frequency (RF) operated helical coil quarter-wave resonator. 
2. Description of the Related Art 
Corona scrubbers have been used to activate chemical reactions that break 
down pollutants in a gas discharge. Such systems, which are described for 
example in Nunez et al. "Corona Destruction: an Innovative Control 
Technology for VOCs and Air Toxics", Air & Waste, Vol. 43, February 1993, 
pages 242-247 and Yamamoto et al., "Control of Volatile Organic Compounds 
by an AC Energized Ferroelectric Pellet Reactor and a Pulsed Corona 
Reactor", Industry Applications Society Annual Meeting 1989, Vol. 2, pages 
2175-2179, employ relatively low frequency energizing signals to produce a 
corona discharge. Energizing frequencies that have been used generally 
range from the standard line frequency of 60 hz up to about 200 Hz. 
Unfortunately, this corona discharge technique has not been found to be 
applicable for the treatment of waste products contained in a liquid. 
In the field of liquid waste treatment, chlorination is commonly used for 
potable water and sewage. However, it has serious disadvantages in terms 
of safety, handling complexity, and the generation of undesirable 
chlorinated hydrocarbons as byproducts; other chemical treatments have 
similar drawbacks. Another approach uses ultraviolet (UV) excitation to 
destroy biologically active viral and bacterial agents remnant in sewage. 
However, UV has not been shown to be of utility in the decontamination of 
industrial or sewage waste requiring the removal of destruction of 
carcinogenic or toxic compounds. Thermal treatments, such as distillation, 
have also been investigated but are very expensive. 
Another area of investigation is the treatment of contaminated wastewater 
with electron beams. This type of treatment has been demonstrated using a 
1.5-MeV electron beam scanned across a thin sheet of flowing water, as 
described in W. J. Cooper, et al. "Treatment of Industrial Hazardous 
Wastes With High Energy Electrons", presented to Hazardous Materials 
Control Research Institute's 7th National RCRA/Superfund Conference, May 
2-3, 1990, St. Louis, Mo., pages 1-15. The technique has been shown to be 
effective against chlorinated hydrocarbons and many other organic 
contaminates, which are reduced or oxidized to inert compounds by the 
action of free radicals and free electrons induced as secondaries within 
the water by the beam. High energy electrons deposit energy into the water 
by bremsstrahlung radiation, which creates low energy ionizing x-rays, and 
by ionizing collisions. The exact chemical processes are complex, but they 
are believed to lead to the formation of a variety of reactive species 
within the water, in particular to free thermal electrons and to OH 
radicals, that are highly reactive. 
Electron beam treatment has required a high beam energy to obtain a 
suitable penetration depth. This in turn involves a high cost to shield 
x-rays and to erect the structure. At lower electron beam energies, in the 
100-150 keV range, the power supply and electron gun become of more 
manageable size and the x-ray hazard becomes manageable. However, beam 
losses in the window that protects the electron gun vacuum become serious 
below 150 kV, with the robustness of the window vanishing below about 100 
kV if it is designed to be thin enough to efficiently transmit electrons. 
In addition to the window injury, the low beam energy results in a 
penetration depth that is very short and requires exotic capillary fluid 
flow apparatus (with high viscous flow losses) to ensure that the fluid 
cross section is thin enough to be successfully irradiated. Furthermore, 
there is a serious foil-heating problem that arises with high duty use. 
In addition to the gas scrubbing application mentioned above, corona 
discharge devices have been developed which have other potential 
environmental uses. For example, conventional spark plugs used in internal 
combustion engines typically deliver energies of about 20-30 mJ per pulse. 
If a higher energy ignition system could be developed, it would offer 
several advantages. First, with current systems not all of the burned gas 
is ejected from the cylinder during idle, resulting in a rough idle; 
increased idle stability could be achieved with a higher energy ignition. 
Second, higher fuel economies are available through exhaust gas 
recirculation (EGR) systems. With a spark energy of about 75 mJ, the EGR 
can be increased to its optimum level, and the gas mileage improved on the 
order of 1 MPG. In addition, NOx emissions would be reduced. Third, a 
higher energy spark would allow the fuel mixture at startup to be run 
leaner. Since most hydrocarbon emissions occur at startup, a significant 
drop in hydrocarbon emissions could be expected. Unburned fuel in the 
exhaust manifold, particularly when the catalytic converter is cold, is 
also an environmental problem that can be addressed by an efficient corona 
discharge. 
One corona discharge device that is of interest is described in Bonnazza et 
al., "RF Plasma Ignitions System Concept for Lean Burn Internal Combustion 
Engines", Society of Automotive Engineers, Paper No. 929416, 1992, pages 
4.315-4.319. It uses a co-axial, quarter-wave resonator with a solid inner 
conductor surrounded by an outer conductive cylinder. The outer cylinder 
is grounded, while a high frequency RF signal, in the hundreds of 
megahertz range, is applied to the inner conductor. The apparatus extends 
for one-quarter the length of the excitation wavelength, resulting in a 
co-axial cavity resonator with a maximum voltage at the opposite end of 
the device from where the RF input is applied. This produces a step-up 
transformation of the voltage at the opposite end of the device when 
resonance occurs. 
The article reports the testing of a model, approximately 38 cm long, with 
a 200 MHz input signal that was applied through a rectangular loop feed. 
While successful ignitions were observed, the 38 cm length of the device 
was much longer than what is practical for a vehicle ignition system. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide an improved liquid waste treatment 
technique that avoids the drawbacks of chemical treatment, has a wider 
range of application than UV treatment, is less expensive than thermal 
treatment, and is operable at lower energy levels than electron beam 
treatment systems. This is accomplished with an RF corona discharge 
system. In a related vein, the invention further seeks to provide an 
improved RF corona discharge mechanism that can be used for both liquid 
waste and exhaust gas treatment and in vehicle ignition systems, and is 
substantially smaller than previous devices. 
Liquid waste treatment, or other chemical reactions, are achieved with the 
invention by initiating an RF corona discharge within a liquid, and 
sustaining the discharge at a level and for a duration sufficient to 
induce the desired chemical reaction, such as breaking down hydrocarbon 
waste materials. In the preferred embodiment, RF actuating pulses are 
applied to a corona discharge electrode within the liquid. An efficient 
operation is achieved by providing the pulses with alternating positive 
and negative components to initiate a corona discharge at a high 
potential, and then draw corona energy out from the fluid before arcing 
occurs. Various pulse forms can be used, such as positive pulse components 
that have high initial voltages to initiate the corona discharge followed 
by lower voltages to sustain the discharge, pulses at a single RF 
frequency, and pulses formed from a range of signals at different RF 
frequencies that are superimposed to provide the pulses with 
non-sinusoidal shapes. Unipolar pulses can also be used, in which case 
efficiency is enhanced by spacing the pulses close enough together so that 
their peak voltages can be progressively reduced. A corona discharge site 
is provided that establishes an electric field greater than about 30 to 
100 kV/cm with an applied voltage of less than about 50 kV; this is 
sufficient to initiate the desired corona discharge. 
The invention also provides a new quarter-wavelength RF excited resonant 
corona source that uses a length contracted center conductor, preferably a 
helical coil. This allows for the production of corona discharges with a 
device having an axial length that is significantly shorter than what has 
previously been achievable. The new corona source is applicable to vehicle 
ignition and remediation systems, gaseous and liquid waste treatment and 
other uses. 
These and other features and advantages of the invention will be apparent 
to those skilled in the art from the following detailed description, taken 
together with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
A novel corona source in accordance with the invention is shown in FIG. 1. 
It consists of a coaxial quarter-wavelength resonator that has an outer 
conductive cylindrical shield 2, preferably metal, and a length contracted 
inner conductive helically coiled wire 4 that is preferably wrapped around 
a dielectric core 6. The shield 2 serves as a reference electrode that is 
capacitively coupled to the coil 4. If enough stray capacitance between 
the coil and ground is present, the device will operate even without the 
outer conductor 2. 
An RF signal is applied to the corona source over an input line 8, which 
makes a loop around the lower end of the coil 4 and is then connected to 
the lower end of the grounded shield 2. In this manner the lower ends of 
both the coil and shield are held at ground potential, while the RF input 
feed 8 functions as a resonant coupler. The coil functions simultaneously 
as a resonant circuit and as a single turn transformer winding, thereby 
inducing a stepped up voltage in the multi-turn coil that is peaked at the 
electrode end. 
In one embodiment, the upper end of the coil is provided with an outward 
extending electrode 10 that has a sharp tip which functions as a corona 
discharge site, acting as a focal point for a field enhancement that leads 
to corona formation. The electrode may have a single discharge point as 
shown in FIG. 2, or it can be shaped more generally to produce a desired 
corona distribution or be fit into a sparkplug with either a monopolar 
discharge or a more conventional spark gas discharge, as shown in FIG. 1. 
RF energy is delivered to the corona source via a coaxial feedthrough 12 
and the power feed loop at the grounded end of the cavity. If the cavity 
were extended into an infinite transmission line, the RF feed signal would 
propagate along the line with an axial phase velocity that is a function 
of the relative diameters of the coil 4 and shield 2, and of the helix 
pitch. In general, the phase velocity decreases as the helix pitch angle 
decreases. Calculation of the phase velocity is difficult, and is 
discussed in the literature on traveling wavetubes, such as Mishra et al., 
"Effect of Plasma and Dielectric Loading on the Slow-Wave Properties of a 
Traveling Wave Tube", IEEE Transactions on Electronic Devices, Vol. 37, 
No. 6, June 1990, pages 1561-1565. However, an estimate of the phase 
velocity can be obtained by considering the limiting case in which the 
reference electrode shield 2 is located close to the coil 4. In this 
limit, the RF axial phase velocity v.sub.p is given by: 
EQU v.sub.p =c sin .phi. 
where c is the speed of light in vacuum and .phi. is the helix pitch angle. 
For a given geometric configuration, there is a frequency f.sub.o at which 
RF resonance will occur along the coil 4. The resonant frequency f.sub.o 
is determined in the corona source by the boundary conditions at the 
opposite ends of the coil, the resonator length L and the phase velocity 
v.sub.p along the resonator: 
EQU f.sub.o =(2n+1)v.sub.p /4L, n=0,1,2, . . . 
The lowest resonant frequency occurs when n=0 and the resonator's length is 
equal to 1/4 of the RF wavelength. 
Since the potential difference between the coil 4 and outer shield 2 is 
zero at the grounded end and a maximum at the opposite end, there is a 
voltage gain between these two extremities. The absolute voltage 
difference obtained at the corona discharge end is determined by the feed 
loop's coupling efficiency, the input power and the resonator Q, which in 
turn is dependent upon the materials' electrical properties, the drive 
frequency, and the cavity feed loop and electrode geometries. 
Instead of providing the RF input as a single turn transformer loop 8, the 
RF input line can be connected directly to the coil. This is illustrated 
in FIG. 2, in which the input line 8' is connected to the near lower end 
of the coil, which in turn is connected to the bottom of the grounded 
electrode shield 2. This version, however, does not provide for the 
transformer isolation that is inherent in the design of FIG. 1. 
FIG. 3 illustrates the application of the resonator coil corona source to a 
vehicle ignition application. The vehicle battery 14 is connected to 
energize an RF oscillator 16. The oscillator output is applied over an 
output line 18 to a spark plug implemented as a resonator coil corona 
source 20 as described above. In this case, however, the corona tip 10' is 
turned upward so that a corona 22 is produced above the outer electrode. 
In the preparation for this set up, the RF source 16 would preferably be 
tuned to the corona source's resonant frequency by sweeping the source's 
frequency until the reflected power from the resonator is minimized. Phase 
adjusters could also be added to provide for impedance matching, tuning 
the phase adjusters until the reflector power is again minimized. 
In the automotive application, the engine timing requires proper 
synchronization of the spark. It is possible to use a number of corona 
spark plugs, each powered by a separate RF power supply to assure proper 
timing. Alternately, one may use a single RF power supply and RF switches 
to select each plug in sequence. However, a much more cost effective means 
is to use a common tunable RF power supply in combination with a number of 
corona spark plugs, with each plug tuned to a different resonant 
frequency. The RF supply is repetitively pulsed at the ignition repetition 
rate and swept in frequency. Each pulse of the output frequency matches to 
the correct spark plug at the time required by the engine timing sequence. 
In this manner, a number of spark plugs can be operated with a common 
transmission line. Plugs that are not in the resonant range do not draw 
power. 
FIG. 4 illustrates a series of spark plugs 20a-20d supplied from a 
frequency-sweeping RF power supply 23 in this manner. FIGS. 5 and 6 
illustrate the frequency sweeping, with the frequency for each spark plug 
identified by the plug's reference number 20a-20d. 
In an implementation of the invention, a resonator length of 11.6 cm was 
employed, with an outer reference electrode diameter of 3.8 cm and a 
helical coil diameter of 0.6 cm. The helix pitch was 0.25 cm, with a pitch 
angle of 5.2.degree.. An RF feed loop as illustrated in FIG. 1 was used; 
the loop diameter was 1.57 cm, offset from the bottom of the resonator by 
0.3 cm. The wire size employed in the helical coil was 0.05 cm. Corona was 
first observed at 77 MHz with 5 W of net input power. The corona ignited 
spontaneously, and was approximately 0.5 cm in diameter. 
The corona may be arranged to be localized at one point, or to fill a gap 
between electrodes. The corona is found to be chemically active, as though 
it were a flame. It is capable of fusing plastics, or igniting flammable 
materials. 
Another aspect of the invention involves the use of this type of RF corona 
source, or other RF corona sources, for the treatment of liquid wastes. 
Previously, corona has been observed in a liquid in conjunction with water 
discharge devices used as high voltage switches. In these devices, a 
water-filled gap is charged with a pulse that exceeds the gap's normal 
breakdown voltage, producing a corona discharge that rapidly results in an 
arc across the gap. The idea is to transition from corona to an arc as 
rapidly as possible. 
With the present invention, on the other hand, it has been discovered that 
hydrocarbon waste material in a liquid can be broken down by the 
application of a sustained corona discharge within the liquid. The process 
involved is believed to be similar to that postulated for the use of 
electron beams in liquid waste treatment. Specifically, the principle 
treatment process is believed to stem from emitted electrons hitting water 
molecules to form OH radicals, which are highly reactive. The radicals 
diffuse through the liquid, reacting with and breaking down organic 
molecules. When an OH radical hits a hydrogen atom on a hydrocarbon 
molecule, water is produced. When the radical hits a carbon atom on the 
molecule, carbon monoxide is produced and then converted to carbon dioxide 
upon being hit by another radical. 
A corona is present in a medium when the medium is exposed to a flux of 
non-thermal electrons with sufficient energy to excite high level 
transitions and ionization events, characteristically producing a visible 
diffuse glow. Generally, a corona is time dependent and can pass through 
noisy growth and decay phases, or may transition into an arc. On a 
microscopic scale, a corona pattern is similar to a fractal, with 
filamentary ionization paths propagating outward into a volume. These 
filaments are referred to as streamers and brushes, depending upon their 
shape and polarity. Visually, the geometry of a streamer in an insulating 
fluid is similar to that in a solid. Although the damage pattern in a 
solid is generally permanent, a gas or liquid generally recovers its 
dielectric properties after a pulse. 
For this reason, the corona activated region can be repetitively pulsed in 
a liquid. The activated region can be made to grow statistically from 
pulse to pulse, thereby involving a large fraction of the liquid volume. 
On a repetitive basis, the pattern statistically covers all of the liquid 
a certain distance away from the electrode. This distance depends upon the 
field, the pulse waveform and the liquid characteristics. Activation of 
all of the fluid can be assisted with the use of a turbulent flow, or by 
reversing polarity. 
FIG. 7 shows a simplified system in which quarter-wave resonant corona 
sources of the type described above are used to induce a corona discharge 
in a flowing liquid to treat organic wastes contained in the liquid. The 
liquid flows through a dielectric tube 24 such as glass. An RF power 
supply 26 provides RF signals, with a frequency on the order of about 
fifty to several hundred MHz, to a series of resonators 28 through 
respective tuning stubs 30. The helical coil 32 within each resonator is 
brought out into the center of the liquid flow path through the tube 24, 
where it extends along the tube axis with a series of projecting tines 34 
which provide corona discharge sites at their outer tips. A reference 
electrode can be stationed either outside and surrounding the tube, as 
indicated by dashed line 36, or as an elongated cylinder within the tube 
as indicated by dashed line 38; positioning it outside the tube is 
generally preferred in order to eliminate bipolar arcs to electrode 38 and 
also to avoid disrupting the liquid flow and restricting the effective 
flow passageway. 
A closed loop system for treating liquid waste is illustrated in FIG. 8. A 
pump 40 circulates the liquid around the closed conduit 42, with valves 44 
and 46 providing fluid inlets and outlets, as desired. An optional high 
voltage power supply 48 or other voltage source provides a DC bias voltage 
that is modulated by an RF modulator 50, which is illustrated as being 
connected to provide an output signal to anode electrodes 52 and 54 on 
opposite sides of the conduit. The conduit sections 55 on either side of 
each anode are formed from a dielectric material, as illustrated in FIG. 
9, to separate the anodes from the grounded metal pipe 42. A series of 
cathode electrode tines 56, the distal ends of which are distributed 
across the conduit's cross-section, extend into the conduit 42 from metal 
cathode rings 57 at the ends of the insulating sections 55. The cathode 
electrodes are typically grounded by connecting them to metallic portions 
of the conduit that run to and from the pump 40 and also to the RF 
modulator 50 and negative polarity side of the optional voltage source 48. 
The output RF modulator 50, together with an optional DC bias, is applied 
to the anode electrodes located within the two insulating conduit 
sections. It is not necessary that the anode be physically inside the 
insulating conduit and in contact with the liquid. The anode's function is 
to develop electric fields at the cathode electrodes. The liquid is 
circulated, with turbulent flow if necessary, until the desired level of 
treatment has been achieved. 
An open loop system with another corona source configuration, in which the 
flow is regulated at a speed that achieves the necessary excitation and 
treatment in a single pass through four corona regions, is illustrated in 
FIG. 10. In this case an open-ended conduit 58 has input and outlet valves 
60, 62 which control the flow of liquid through the conduit. An optional 
DC source 64, which is illustrated as being variable so that the corona 
discharge power can be controlled, biases the output from an RF modulator 
66 which provides an RF output to anode electrode frames 68, 70 on 
opposite sides of the conduit. In this embodiment, separate grounded 
reference electrodes are provided within the conduit in the form of 
cylinders 72, 74 with rounded ends. A series of electrode tines 76 extend 
from each frame towards their respective ground electrodes, producing 
corona discharges 78 in the vicinity of the distal ends of the tines and 
the ground electrodes. With a proper positioning of the tines, 
substantially all of the flowing liquid can be treated by passing it 
sequentially through the four regions of corona discharge 78. Appropriate 
seals would be provided where the frames enter the conduit to prevent 
leaks. 
It is important, for reasons of energy conservation, to avoid continuing 
the corona current flow beyond the point at which the corona streamers and 
brushes merge, or one or the other reaches the opposite electrode. This is 
because further current will lead to a concentration of the current into a 
resistively heated arc channel of very small cross section, and will not 
contribute to chemical excitation of the bulk of the remaining liquid. 
Based upon the work done with electron beam treatment, approximately 8 
J/cc is required to treat typical contaminates. 
In accordance with the invention, the duration of high electric fields is 
limited so that the motion of corona streamers or brushes is truncated 
before arcing begins. To accomplish this, pulses must be applied rapidly 
and then removed rapidly. The surface properties of the electrodes play a 
role in the onset of the corona. There is an advantage to providing a well 
defined structure with field enhancement at periodic or random locations 
to help reduce the necessary applied voltage. However, more uniform 
propagation and activation is achieved by using the maximum fields 
obtainable. Typically, fields well in excess of 100 kV/cm will ensure 
corona inception and a correspondingly rapid closure of the gap between 
electrodes. 
Arcing in the gap is inhibited by adjusting the pulse shape, which also 
limits the required power. Limiting the required power is very important, 
in that if the required electrical power exceeds some maximum amount (for 
example, 50% of the engine power for automobile applications), the system 
will be deemed to be impractical by its users. For this purpose, the 
external power is reduced as the corona bridges the gap. The initial 
energy U that is stored in the capacitance Co of the gap is given by: 
EQU U=1/2C.sub.o V.sub.o.sup.2 
where V.sub.o is the initial voltage. A crude rule of thumb for propagation 
near the threshold for instability is that the charge remains a constant 
as the corona front moves. In this case, if the electrodes are first 
charged to the level V.sub.o and then disconnected from the power supply, 
the stored energy is just enough to supply a constant E field as the front 
sweeps across. The energy dissipated per unit length is then also a 
constant that is just equal to the initial energy density stored in the 
gap. At lower E fields the corona may die out, while at higher fields the 
front may accelerate and the charge build up. Alternately, providing power 
to the electrodes during the discharge leads to enhancement of the energy 
in the front, and a higher likelihood of arcing at an earlier time. Once a 
corona stream is initiated, the electric field at the discharge tip is 
enhanced by a field enhancement factor, thereby reducing the required 
average value of the E field. 
One method for determining how much energy will lead to an arc involves 
measuring the minimum voltage for the onset of an arc. At this voltage 
level the formation of corona will be at a minimum, and the delay between 
the application of the voltage and the glow-to arc transition is highly 
unpredictable. It has been discovered empirically that, when the voltage 
is raised to twice the minimum arc voltage, the delay time between the 
application of the voltage and the onset of an arc falls to a minimum, and 
the statistics become much more stable. If it is desired to ensure corona 
inception, the voltage should exceed about twice the minimum threshold. 
However, to avoid putting excess power into an arc, the circuit should be 
timed to first reduce and then remove external power before the minimum 
time delay is reached. 
To rapidly initiate a corona discharge, but still prevent arcing all the 
way between opposed electrodes, the pulses applied to the discharge 
electrode can have an initial high voltage level to initiate the corona 
streamers, rapidly followed by a reduction in voltage to prevent the 
streamers from crossing all the way between the electrodes to produce an 
arc. The net result is to sustain an efficient corona discharge for a 
longer period than would be the case if the initial high voltage were 
maintained. The reduction in voltage is rapidly followed by a negative 
voltage, which pulls energy back out of the electrode gap before an arc is 
reached. This type of pulse is indicated by reference number 80 in FIG. 
11a, with an initial high positive voltage 80a followed by a lower 
positive voltage level 80b, and then a negative voltage 80c. By reversing 
the current flow to the electrodes during each pulse, energy losses 
consist mainly of current that is used for the actual formation of corona. 
The pulses 80 in FIG. 11a are spaced fairly wide apart. If the pulses are 
moved closer together in time, as illustrated in FIG. 11b, a lower initial 
voltage can be employed because each pulse 82 is initiated while there is 
still some ionization from the previous pulse. This lower initial voltage 
results in a reduction in wasted energy. A relatively complex pulse 
forming network would be required to produce the pulses of FIG. 11a or 
11b. Alternately, these pulses could be generated by taking their Fourier 
transform, and superimposing a range of RF signals which correspond to the 
transform. In this case the period "a" of the initial high voltage pulse 
82a corresponds to the period of a single RF frequency at the high 
frequency end of this range, while the period "b" between successive 
pulses corresponds to the fundamental RF frequency. 
Using RF signals rather than a switching circuit to produce the pulses 
makes it possible to obtain closer spaced pulses with a lower voltage. The 
lower voltage in turn provides a higher degree of control over the corona 
discharge, so that more corona power per unit volume can be sustained 
without arcing. Instead of the shaped pulses of FIG. 11a and 11b, an 
energizing signal at a single RF frequency can be used, as illustrated in 
FIG. 11c. The alternating positive and negative pulses of this signal 84 
can be used to alternate the initiation of corona discharges from opposing 
electrodes. The pulse period is shorter than for the spaced pulses of 
FIGS. 11a and 11b, resulting in a reduction in wasted energy. Only a 
simple oscillator is needed to produce this waveform. 
FIG. 11d illustrates a waveform 86 that can be produced by superimposing RF 
harmonic frequencies to yield a continuous series of shaped pulses, 
without any gaps between successive pulses. The individual pulse shapes 
are similar to those of FIGS. 11a and 11b, with initial peak positive 
voltages followed by a lower positive voltage, and then a negative 
portion. This type of waveform will generally require the lowest average 
voltage (although the initial peak may be higher than in the single 
frequency waveform of FIG. 11c), and consequently the lowest amount of 
wasted energy. 
The optimum RF frequency for fluid waste treatment depends upon a number of 
factors. One is the nature of the medium, which effects the electron 
mobility and thus the propagation of corona streamers. Higher electron 
mobilities require higher frequencies to avoid arcing during a given 
period. Another factor is the density of the liquid, since the speed at 
which the ionization front moves varies in a positive fashion with the 
density. Accordingly, liquids with higher densities will require higher 
frequencies. A third factor is the operating voltage. Since the corona 
discharge will be established faster and propagate faster for higher 
voltages, the frequency should be increased as the voltage goes up. In 
general, operating frequencies for gaseous waste treatment will range from 
about 50 MHz to several hundred MHz, and for liquid waste treatment will 
range from several hundred MHz into the microwave band. 
The optimum voltage also depends upon the particular application. The 
voltage level can be established empirically by turning on the power and 
adjusting the voltage until a corona discharge is observed which fills as 
much of the electrode gap voltage as possible, without arcing. The 
required voltage will generally increase as the gap spacing is enlarged. 
The voltage is also a function of the electrode shape. Pointed electrodes 
produce a high electric field, making it easier to form a corona discharge 
at a given voltage level. However, pointed electrodes tend to wear, and 
the corona discharge from a pointed electrode tends to be localized; 
operating a smooth electrode at a higher voltage will produce a more 
uniform corona. In any event, the actuating voltage can be kept to much 
less than 50 kV, considerably less than that required with the electron 
gun treatment technique, with the electrode selected to produce an 
electric field greater than 100 kV/cm to initiate the corona discharge. 
For the helical coil resonator described above, which employs a pointed 
electrode, the excitation voltage will generally be on the order of about 
500-1,000 V in a gas and about 1500-3000 V in many liquids. 
While it is normally desirable to alternate positive and negative pulses, 
unipolar pulses such as all-positive may also be employed by biasing the 
RF field with a DC voltage (48, 64). This is less energy efficient than an 
RF excitation, since the corona streamers must be made to lose their 
energy by the time they approach the opposite electrode. Otherwise, all of 
the energy put into the electrode gap is lost, without pulling any back as 
with pulses of alternating polarity. Since the stored energy varies with 
the square of the applied voltage, a negative pulse is particularly 
important if a high voltage is needed to initiate a corona discharge. 
However, the DC bias approach enables easier initiation of a discharge to 
start the corona, after which it may be reduced. 
Although unipolar pulses generally involve a loss of energy efficiency as 
compared to an alternating RF signal, it is easier to implement a unipolar 
pulse forming circuit than one that alternates positive and negative 
pulses. FIGS. 12a and 12b compare the energy situation for a 
single-frequency RF signal 88 and a series of spaced positive pulses 90. 
In the RF signal 88, the energy that goes into the corona discharge is 
illustrated by shaded areas 92 around the peaks of alternating positive 
and negative pulses. This corona energy is equal for each successive 
period. With the DC biased pulses of FIG. 12b, by contrast, if the pulses 
are applied at a high enough frequency, each pulse will leave residual 
energy in the corona gap that will still be present when the next pulse 
arrives. Accordingly, subsequent pulses can have successively lower peak 
voltages, since less energy is needed for the successive discharges. This 
is illustrated by the successive reduction in pulse heights and corona 
energy 94 in FIG. 12b. The reduction in the average DC pulse voltage 
mitigates the lower efficiency of the unipolar approach. 
Shaped pulses with an initial high voltage level, as illustrated in FIGS. 
11a, 11b and 11d, can be generated either with high power RF oscillators 
or resonant RF circuits as described above, or with conventional pulse 
forming networks, Bleumleins, Marx banks, or more modern pulsers or laser 
activated switches. Generally, the use of a matched pulse forming network 
limits the voltage applied to the corona discharge electrode to one-half 
of the voltage applied to the pulse forming network. However, a Bleumlein 
that uses two pulse forming networks allows the full initial voltage to be 
applied. 
This type of pulse forming device is illustrated in FIG. 13. A pair of 
pulse forming networks in the form of cable sections 96 and 98 surround 
respective inner conductors 100 and 102, and are energized at the opposite 
ends of cables 96 and 98 by a power supply 103 that is applied through a 
resonant charging network composed of inductors 104 and diodes 105. An 
impedance matching resistor 106 is connected between the inner conductors 
of the two cables. Corona discharge electrodes 108 and 109 are connected 
between the opposing sides of cable conductors 100 and 102. Pulses are 
applied to the electrodes at a periodic rate by periodically triggering a 
spark gap 110 between the outside end of cable 96 and ground. The trigger 
circuit consists of a trigger pulse generator 112 that is connected to a 
low transformer winding 114, with a high voltage winding 116 coupled to 
the low voltage winding so as to trigger the spark gap when a trigger 
pulse is applied. The cable lengths and trigger pulse rate are selected to 
resonantly charge the cable transmission lines, which generate high 
voltage corona pulses at resonance. 
A more efficient pulse forming circuit is shown in FIG. 14. It employs an 
RF switch tube 118 between a DC power supply 120 and the corona chamber 
122. The RF switch tube 118 is typically a vacuum tube capable of 
switching on and off at the repetition rate (such as 100 MHz) required at 
the power levels noted above. A resonant inductor L1 and a capacitor C1 
are added as required to compensate for variations in lead inductance and 
load capacitance to tune the circuit to resonate at the desired frequency. 
A resonant feedback loop is used to sense oscillating current in the load, 
phase shift it with a capacitor C2, and drive the grid potential of the 
switch tube 118 at an optimum phase delay. In one implementation of the 
circuit, the feedback is sensed by making L1 the primary winding of a 
resonant transformer T1, with the secondary winding L2 connected in a loop 
with C2 and the primary winding of a pickup transformer T2; the secondary 
winding of T2 is connected back to the grid of switch tube 118. Numerous 
other feedback options are also available, such as a capacitive pickup at 
point A using an additional capacitor C3, or a pickup at point B using an 
optional capacitive C4 or an inductor L3 to generate a voltage of a 
desirable level and phase. 
The present invention thus provides both an improved corona discharge 
source, and a corona-based fluid waste treatment technique that avoids the 
high voltages required by electron beam treatment, the high cost of 
thermal treatment, the limited utility of UV treatment and the safety, 
handling and undesirable byproduct problems of chemical treatment. While 
several illustrative embodiments of the invention have been shown and 
described, numerous variations and alternative embodiments will occur to 
those skilled in the art. Such variations and alternate embodiments are 
contemplated, and can be made without departing from the spirit and scope 
of the invention as defined in the appended claims.