Patent ID: 12256484

DETAILED DESCRIPTION OF THE INVENTION

FIG.1Ais a lengthwise cross-sectional view of an example of an atmospheric pressure (AP) plasma source102. The AP plasma source102includes an axially elongated plasma-generating chamber104or other structure that serves as a ground electrode for generating a plasma and that serves as a conduit for flowing gases into the plasma. The plasma-generating chamber104may be enclosed in an electrically- and thermally-insulating housing (not shown). A “hot” or powered electrode106is located in the plasma-generating chamber104. Electrical connections to the hot electrode106may be made through a dielectric structure108located at the proximal end of or in the plasma-generating chamber104. One or more gas inlets110may be formed through the dielectric structure108in fluid communication with the plasma-generating chamber104. The gas inlets110may be placed in fluid communication with a gas supply source. Accordingly, the gas inlets110provide a flow path for plasma-generating gas fed to plasma-generating region112within the plasma-generating chamber104proximate to the hot electrode106. In operation, the plasma is generated in region112and subsequently flows (with the gas flow) toward a nozzle114positioned at a distal end of the plasma-generating chamber104.

Generally, operating parameters associated with the AP plasma source102are selected so as to produce a stable plasma discharge. Control116having a processor is used for setting and controlling the operating parameters which depend on the particular application ranging, for example, from nanoscale etching of micro-fabricated structures or devices (e.g., microelectromechanical systems (MEMS) devices) to removing large areas of paint from aircraft carriers. Examples of operating parameters are provided below with the understanding that the teachings herein are not limited by such examples. In the case of generating an air plasma, the rate at which the air is fed to the AP plasma source102may range from 1×10−6SCCM to 1×106SCCM. The feed pressure into the AP plasma source102may range from 1 Pa to 1×107Pa. The power level of the electrical field driving the plasma may range from 1×10−6W to 1×106W. The drive frequency of the electrical field may range from direct current (DC) (0 GaHz) to 100 GHz. The separation distance, i.e, the distance from the nozzle exit to the exposed surface of the material to be removed, may range from 1×10−6m to 40 cm. The scan speed, i.e. the speed at which the AP plasma source102may be moved across (over) the surface of the material, may range from 1×10−9s to 103s. Related to the scan speed and power is the time averaged power density. Also related to the scan speed is the dwell time, i.e., the period of time during which a particular area of the material is exposed to the plasma plume, which may range from 1×10−9s to 1×103s.

In one embodiment of the present invention, AP plasma source102has a converging nozzle (i.e., a straight conical cross-sectional flow area without being followed by a diverging section), has been fabricated and evaluated. The AP plasma source repeatably and reliably produces a plasma plume which may include the production of shock waves. The AP plasma source generates an air plasma using air at about room temperature as the feed gas. The air may be fed to an AP plasma source of this type at a pressure ranging from 30-110 psi and at a flow rate ranging from 1-7.5 CFM. In another example, the pressure range is 65-95 psi. In another example, the flow rate range is 1-4 CFM. Pressures higher than 110 psi may also be implemented to produce shock waves. In a more general example, the pressure may be 30 psi or greater and the flow rate may be 1 CFM or greater.

Under these conditions, at plasma ignition, there is a (typically small) arc from the driven or “electrically hot” electrode to the chamber wall, and the gas flow “expands” the spatially confined arc into a diffused volume of plasma or plasma plume118extending out of the outlet114. The electrical impedance before and after plasma ignition and during the expansion of the arc vary greatly as detailed below.

The present invention provides as shown inFIG.1Ba system for providing power to the plasma during these changing load resistance conditions by way of an inverter120(controlling for example the AC frequency of a square wave pulse signal) and a ballast transformer122. In this system, a leakage inductance of the ballast transformer122serves the purposes of both a) limiting the current into a variable load when driven by a fixed voltage AC source and b) providing a resonance with cable capacitance and therefore can provide a high voltage to ignite a plasma.

FIG.2is a schematic circuit model of asymmetric ballast transformer122of the present invention for coupling to a variable load Rv such as a plasma load, including but not limited to an atmospheric pressure plasma pen (discussed above) or a cutting torch (discussed below). As shown inFIG.2, an AC voltage source130is coupled to transformer122by coupling connections134. The AC voltage source can provide an AC waveform which may be sinusoidal, square wave, or other arbitrary pulsed or bi-polar waveform, and provide a waveform whose frequency can be varied. The voltage source130supplies current which flows through the primary windings1. The current through windings1induce current flow through the secondary windings2of transformer102, producing a step up or step down AC voltage which appears across the variable plasma load Rv. A coaxial connection140is used in this circuit to connect the transformer122to the variable plasma load Rv, but other types of electrical interconnects with or without filtering could be used in addition or instead of the coaxial connection140. As shown inFIG.2, a leakage inductance138and a cable capacitance142(from the coaxial cable) appear in this circuit.

In general, ballast transformers have a leakage inductance that appears in a simple analysis as a separate inductor (leakage inductor) in series with the primary and or the secondary. If the leakage inductance is sufficiently large, the present inventors have realized (as noted above) that this leakage inductance will serve both to a) limit the current into a variable load when driven by a fixed voltage AC source and b) provide a resonance (with the cable capacitance) and therefore can provide a high voltage to ignite a plasma.

Existing transformers with a two pole or three pole transformer core require either a larger core with lower magnetic path length to cross sectional area ratio and extra magnetic path extension in the transformer core in order to reduce coupling to an acceptable value where a transient load would not adversely affect a voltage source such as voltage source130. Alternatively, the transformer would need finer wire with more turns and thick bobbin walls for a coaxial design on a two pole transformer core in order to suppress current surges. Both of these approaches are undesirable.

Accordingly, the present inventors have realized that, for a conventional two pole core design to suppress current surges, a set of large bobbins along with a fine wire size would be necessary. Indeed, because of the limited wire sizes that are practical, many turns would be necessary to achieve a sufficient flux density. Yet, this approach comes with excessive wire heating even for a 1-3 kW transformer for example having a ˜50-100 mm (height and width) 2 pole transformer core, with a core area of each pole being ˜320 mm2-600 mm2. Furthermore, the present inventors have realized that, if only a single primary winding were placed on one pole of the core and only a secondary winding were on the other pole, then it is impossible to obtain coupling as high as 0.97.

Accordingly, using conventional measures, one either a) obtains a transformer with limited power rating or b) cannot obtain enough coupling. These deficiencies are especially problematic when the variable load is a plasma, where the on state and the off state present a tremendous change in impedance nearly instantly, which can result in excessive current flow and damage to the power supply and power coupling equipment.

Asymmetric Ballast Transformer

In view of the problems noted above for the ignition and operation of an atmospheric pressure plasma, the present inventors utilized a two pole winding design with a coaxial winding of a second primary winding on the secondary side of a transformer, This solution provides an asymmetric ballast transformer permitting adjustment of the primary windings so that some of the primary windings are on the primary pole and the rest of the primary windings are disposed in a vicinity of and preferably coaxial with the high voltage secondary coil on the second pole.

FIG.3is a schematic of a ballast transformer of the present invention. As shown inFIG.3, a magnetic flux circuit comprises a transformer core300forming a magnetic loop (which could include air gaps not shown) linking a primary side302of the transformer to a secondary side312of the transformer. The primary side302comprises a first primary winding304of wire W1on bobbin306. Wire W1connects to an AC power source (not shown inFIG.3), but similar to voltage source130inFIG.2. The secondary side312comprises a second winding314of wire W2on bobbin316. Wire W2connects to a variable load not shown inFIG.3, but similar to variable load Rv inFIG.2.

InFIG.3, bobbin316is illustrated for a high voltage secondary. Bobbin316has wire W1wound around it. In one embodiment of the invention, bobbin316fits inside primary bobbin326to provide coupling thereto. The position of primary bobbin326on the second pole can vary from design to design to provide an adjustable coupling to the secondary winding324and/or to the transformer core300. Primary bobbin326typically has a lower coupling to the transformer core than either the secondary winding324or the primary winding304on bobbins316and306, respectively.

In one embodiment of the invention, the required number of turns for the transformer's primary are distributed between the two primary bobbins306and326in order to set the coupling for an appropriate leakage inductance, while the total number of windings on the primary bobbins remains the same as if there were only one primary bobbin, thus obtaining appropriate excitation or magnetization inductance, and thereby controlling maximum flux while allowing larger wires on the bobbins than otherwise would be the case when the primary windings were coaxial on only one pole. In one embodiment of the invention, bobbin326is insulated although insulation may not be necessary if bobbin326is of a size to where it can reside at the bottom of the secondary winding where the voltage is lower than at the top side of the secondary windings.

Accordingly, in one embodiment of the invention, the primary winding on transformer core300is split by the presence of second primary winding324in proximity to (e.g., wrapped around or coaxially surrounding) the secondary winding314. This second primary winding324(connected in series with the winding primary winding304) can be a non-coaxial and/or a coaxial winding relative to the secondary winding314so that it is possible to control the coupling coefficient (leakage inductance) and optimize the trade-off between maximum flux density, core heating, and wire losses without the necessity of auxiliary adjustable flux paths. In one embodiment, the relative positions of bobbin306, bobbin316, and/or bobbin326to the transformer core (and/or to each other) can be adjusted or can otherwise be fixed at different relative positions.

In one embodiment of the invention, by keeping the number of turns constant, the exact coupling may be adjusted by moving turns from primary1(winding304) to primary2(winding324) or vice versa. In effect, turns can be moved back and forth between primary bobbins to adjust the coupling and leakage inductance. If more turns are on primary2and less on primary1, then the coupling is increased without affecting the turns ratio or open circuit (no load) output voltage. Reversing the situation, more turns on primary1and less on primary2decreases the coupling. Less coupling makes the leakage inductance increase while more coupling makes it decrease.

The numerical values given below are merely illustrative and not limiting of a asymmetrical ballast transformer of the present invention. Typical values for operation of the ballast transformer of the present invention are 0-350 mTeslas, 0.97 coupling on primary, net loss <50 W between 20-500 kHz, 1 kV-50 kV peak volts pre-ignition, 0.50-5 kV volt peak operating, 0 volts output post-ignition state.

Below are details of a constructed asymmetrical ballast transformer of the present invention.

Ballast Transformer Design

Operational Input: Pulsed 300 V above ground signal at pulse frequency from 20-500 kHzTransformer Design:Primary Rating: 230 VACEpoxy coating or other coating to hold primary wire and secondary wire in place on bobbins and to prevent vibration in use.First Primary Winding: wire size #12 AWG, 1-15 turnsFirst Primary Winding Inductance: 0.5-10 μH at 10 KHz, no coreSecond Primary Winding: wire size #12 AWG, 1-15 turnsSecond Primary Winding Inductance: 0.5-10 μH at 10 KHz, no coreTotal Primary Windings: 2-30 turnsTotal Primary Winding Inductance: 500-2000 μH at 10 KHz, with core, Q=300Secondary Winding: wire size #22 AWG, ˜200 turns, layered windingsSecondary Winding Inductance: 100 to 1000 μH at 10 kHz, no core50 to 5000 mH at 10 kHz, with core, Q=500Measured Leakage Inductance: 5-100 μH at 10 kHz, with core, Q=30

A further embodiment of the present invention is that bobbin326can be disposed offset from the transformer core, that is that the primary wire W1on bobbin326is moved away from the core (where the operating flux and thus heating is the greatest). Flux near wire W1is somewhat higher than elsewhere in the core.

Further, in another embodiment, the bobbin may be perforated for air flow or liquid cooling tubes along the core inside of the winding. In some cases, it is also advantageous to offset bobbin326on the core to allow more wire exposure into the window region330of the transformer core, and thereby move the magnetic flux distribution in the core to prevent localized saturation of the core.FIG.3shows bobbin326in one offset position, but it is also possible to have bobbin326centered around bobbin316and to only offset bobbin306on the primary side of the transformer.

Coil layers of the windings on the bobbins may also be separated for better cooling and less current crowding. Flux is not the same around the core during any mode of operation since the circuit with capacitive output causes significant circulating current so that the circulating power is typically 1.6 times the real output power. Such a relationship is necessary for ballasting. The transformer core may be un-gapped for maximum power output, but in another embodiments a gapped core is utilized to minimize saturation. This design does not necessarily have, but could utilize, a center tapped primary although center tapping would reduce power handling and/or increase size.

In one embodiment of the invention, the primary bobbin306(as noted above) is offset from primary side302of the transformer core. This offset allows magnetic flux to leak out and be intercepted by second primary winding324wound on bobbin326.

In one embodiment of the invention, one of the primary or secondary windings provides tight coupling while the other provides loose coupling while simultaneously providing a) enough leakage inductance to limit flux density to a safe level, b) at least a turns ratio to develop the operating or developed plasma voltage and more, and c) a reasonable leakage inductance for resonance condition for ignition and use that same leakage inductance for ballast when there is a developed plasma. In one embodiment of the invention, the leakage is adjusted by construction of the ballast transformer components so as not to change the turns ratio all the while keeping the transformer compact while avoiding extra ferrite flux path elements.

In one embodiment of the invention, it is desirable to minimize interwinding capacitance. As shown inFIG.3, there is a two ‘pole’ ferrite core but the primary is not wound over the top of the ferrite core and the secondary is not wound over the top of the ferrite core. This is to avoid electrostatic stray capacitive coupling between the primary and secondary. Furthermore, in one embodiment of the invention, one part of the primary coaxial to the secondary is at the ‘grounded’ or low voltage end. This arrangement further reduces the effect of stray capacitive coupling by grounding that end of the secondary winding. At plasma ignition, the plasma impedance transitions very quickly between very high (megOhms or greater) impedance to a near short circuit (less than an Ohm). This means the load voltage on the plasma electrode drops rapidly (for example in a very few nanoseconds) which can generate a large capacitively coupled transient into the primary which can damage the drive devices in control116or inverter120like IGBTs or FETs. In one embodiment of the invention, this effect is minimized by the open top ferrite core design shown inFIG.3, reducing the interwinding capacitance between the primary and secondary winding.

FIG.4is a graph of frequency vs impedance sweep characteristic of the ballast transformer circuit for a “no plasma” case and for a developed plasma case. Two points/frequencies marked are for plasma operation (to the left, 90.27 kHz) and pre-ignition (to the right, 149.6 kHz). These frequencies will vary with a particular coax cable type and length (FIG.2, coaxial cable140and its cable capacitance142). If the cable capacitance changes, the operating points/frequencies change for the same output. Note the scale depicted is magnitude dB relative to 1 volt at the output of the cable (i.e. at the pen/torch/plasma port). 80 dB volt is 10,000 volts. 68 dB volt is 2,500 volts amplitude or peak. InFIG.4, the no plasma curve represents a very high load impedance (here calculated for 100 k Ohms, but it may be 1 Meg Ohm or higher). InFIG.4, the fully developed plasma curve represents a load of 2000 Ohms. The type of plasma pen/torch used for these calculations was assumed to be a high voltage plasma type with a relatively high impedance while running.

In one embodiment of the invention, the frequency of operation can be moved from 149.6 kHz toward a lower frequency (toward the peak resonance frequency) in order to develop higher ignition voltages (than would exist at 149.6 kHz) and thereafter moved to even lower frequencies (once ignited) to couple more plasma power once ignited and developed.

One plasma condition that is not shown inFIG.4is the state immediately post ignition when the plasma is spatially confined into a conductive filament, small in size, and has an impedance of 1 Ohm or less. This condition is depicted inFIG.5, a graph comparing input current for pre-ignition and post-ignition. InFIG.5, the scale on the right is in dB Amperes relative to 1 Ampere peak. Specifically,FIG.5shows the input current as a function of frequency for a no load pre-ignition, and shows the input current as a function of frequency for a 1 Ohm post-ignition state (not a fully developed plasma). At startup, the input current with no load condition (simulation is 100 k Ohms) is at about 32 dB or 39.8 Amperes peak. When the plasma ignites, the input current is relatively small and has a relatively low impedance as compared to the fully developed plasma state and simulated here with a resistance 1 Ohm. Note that the input current at plasma ignition actually drops to about 21 dB or 11.2 Amperes peak. Thereafter, as shown inFIG.6, the plasma develops and the input current increases as the frequency of operation is lowered to 90.27 kHz or lower.

More specifically, as the plasma develops, the impedance increases moving the current from the post-ignition current curve to the plasma-run current curve, and the frequency is adjusted to 90.27 kHz in this example to develop full power. Thus, the ballast transformer is used to permit the system to generate ignition voltages (FIG.4), withstand the sudden load transition from very high to near shorted conditions (FIG.5) and then smoothly move to full power plasma (FIG.6).

FIG.7is a schematic depicting a measured current trace at the moment of ignition taken with a current sample transformer (CST) plus load (current sample transformer and resistive load). The schematic shows that the current was “kicked” very far off nominal to around double normal peaks.

FIG.8is a schematic depicting measured current traces when the ignition and current kick have produced transformer core saturation resulting in current spikes. After ignition, the current and therefore core flux goes high enough that the leakage inductance begins to saturate and disappear leading to extremely high currents since at this time the steady-state plasma condition is not developed and its impedance is just one or two Ohms. Such conditions can destroy the transistor bridge devices.

Moreover, 1) ringing, 2) current reversals, and 3) high ‘saturation’ currents can damage commutator diodes in the active devices if the voltage and current are reversed quickly before forward conduction recovery time, resulting in a high power dissipation condition. A common method to combat this problem is to put capacitors across the active devices and thus diodes. Unfortunately, the current kick and fly-back currents from leakage inductance and capacitors can compromise dead time and produce bridge shoot through.

FIG.9is a schematic depicting simulation of ignition with an “offset current kick.” This ignition simulation shows output voltage trace and output device currents. Such systems commonly often use full pulse width drive with only enough dead time to avoid ‘shoot through’ in a bridge transistor string. However, the current ‘kick’ (called DC offset) will change the effective dead time by extending commutator diode conduction time which risks shoot through in the bridge transistor string. This situation occurs because transistor current was not zero and changed slope or di/dt at the moment of ignition. That is, when plasma ignition occurs at some unknown time in the drive cycle with other than a zero magnitude of the instantaneous bridge/transformer plasma drive current, the transistor bridge is at risk of self-destruction.

The present invention addresses this problem by providing a minimal amount of voltage to ignite a plasma or spark in the electrode gap and no more, thereby avoiding transistor bridge drive damage by using the present invention's frequency chirp ignition.FIG.10is a schematic of a reduced equivalent secondary circuit of a ballast transformer plasma generator circuit. This circuit has the transfer voltage response inFIG.4, with the tallest voltage peak for 105 Ohms (or larger). If this circuit is driven by a voltage source near (but offset from) resonance, then the voltage across the spark gap will increase until a loss occurs in the system, for example resistive and/or inductive losses, which consumes the input energy. As the operating frequency is brought closer to the resonant frequency, at some point, there is enough quasi-steady state voltage to ignite plasma. At that time, the ballast transformer output current and the output voltage will nearly have the relationship as they would have at resonance, as shown inFIG.9in which the instantaneous voltage and current are out of phase with the peak voltage occurring when the instantaneous current is near or substantially zero.

FIG.11is a schematic depiction of the voltage and current in the circuit ofFIG.10at 170 kHz. The resonant frequency in this example is 150 kHz. The markers onFIG.10are placed at peak output voltage, which represents just enough voltage (electric field in the gas) to ignite the plasma. Even though somewhat off-peak resonance, output current is nearly zero and thus there is very low flux density and stored magnetic energy. The stored magnetic energy is substantially all in the cable capacitance. If an arc initiates in the electrode gap and becomes a short (low impedance), current will increase smoothly from zero and not be affected by a reverse slope from a non-zero output, and therefore not drive input current to high values.

If the initial drive frequency is set at well above the resonance (1.5 to 2.5 times) or even higher, depending on bandwidth (BW) of the resonance, and set to an operational frequency well outside any significant resonance response, then the sudden appearance of drive will not have sideband components that excite the resonance frequency of the secondary side of the ballast transformer. This is an important (but non-limiting) aspect of this invention which can prevent premature ignition. After plasma ignition, the drive or operational frequency is gradually reduced at a rate where frequency modulation components will not tend to excite the resonance as it approaches the resonance frequency, else the voltage increase will not be smooth, but will have a ringing envelope. A rate of frequency change of 10× slower than the BW/2 Hz per 0.6/BW seconds is a suitable rate of frequency change.

In about 1 millisecond to 100 ms, the system comes to the near resonance condition inFIG.11. In one embodiment of the invention, a dwell time of 0.1 to 50 milliseconds at the near resonance condition is enough time for the electrode gap to ignite for a few nanoseconds but at a peak voltage that alone is not enough to guarantee ignition. In one embodiment of the invention, 100's of cycles over a few milliseconds at the same voltage will give enough time to ignite. Also, the sweep step size or rate must not be so high that there is a danger that a single step will make a very large increase in voltage else the plasma system will be in the same dangerous condition as originally mentioned. In one embodiment of the invention, a step size that makes 10% voltage change near ignition would be the maximum step size as this would allow current to rise to 10% of maximum before ignition. In one embodiment of the invention, since the operational frequency where ignition is expected to occur is not precise, the operational frequency (where plasma ignition is expected) can be sweep below the predetermined ignition frequencies, but not below resonance.

Chirp Plasma Ignition and Plasma Maintenance Control

FIG.12is a schematic of a system on a chip (SOC) integrated circuit for control plasma ignition and maintenance. The SOC has an advanced reduced-instruction-set machine (ARM) microcontroller which generates a drive signal for a transistor bridge which is connected to the ballast transformer shown on the right. Before ignition, Rpen will be 100 k Ohms or higher. A current sense transformer (CST) is connected between the gate/drive transistors. The CST has an appropriate internal load resistance such that maximum current in Amperes is scaled to within range of the analogue to digital converter (ADC) on the SOC. Other items shown are common but not required. Software in the SOC starts the chirp ignition sequence as described above and initiates analog to digital conversion (ADC) of voltage and therefore current measurements. The initial drive frequency is as described above. This is reduced to perhaps 30% of the resonance which is 50% of the initial drive frequency. Current and/or voltage is monitored for ignition. It is not desirable to sweep below resonance because the circuit is capacitive reactive there and produces strong switching current spikes.

An inverter current waveform output, hence the transformer input current and a H-bridge gate drive voltage trace of a chirp ignition using the present invention is shown inFIG.13. The fast and small value signal superimposed on the current trace is inconsequential, and can be filtered out of all circuits and data elsewhere. The bottom trace is a ‘zoom’ view of the top trace about the point in time for plasma ignition where the increasing sinusoid pattern for the current waveform (prior to plasma ignition) transitions suddenly to a lower amplitude sawtooth waveform (after plasma ignition). The bottom trace shows no current spikes during this transition, and the drive current waveform transitions to more of a sawtooth wave than a sinusoid wave (which existed prior to plasma ignition).

Looking at the top half, the input current and therefore the output voltage gradually increase. At resonance, a resonant circuit has the surge impedance of:

Zo=(LsCcable)
Then Vout=Iout×Zo where Ioutis output current Voutis output voltage. Ioutis Iin/Trwhere Iinis bridge transistor supplied input current which has been measured by the microcontroller via the ADC, and Tris the known output XFMR turns ratio.

In one embodiment of the invention, the microcontroller can calculate Voutapproximately to confirm that the sudden change in current characteristics is the ignition. Note inFIG.13that the spike or transient in the gate voltage drive waveform is the plasma ignition, and that the plasma ignition occurred near zero current with no large current transient occurring. Furthermore, the current sample is clipped at the bottom by a clamp diode so that the zero point current is slightly offset downwards.

The advantages of using this method for controlled high to low frequency chirp drive plasma ignition are:there is no significant current kick during ignition which would jeopardize drive devices from this high current;there is no significant current kick during ignition which would possibly saturate a ballast transformer core which then could jeopardize drive devices through high current from loss of ballasting inductance and current flow under conditions with a very low, near zero load after plasma ignition;a current transient is not propagated backwards to the drive and microcontroller circuits causing possible resets and improper operation; andit is possible to calculate the approximate ignition voltage from input current alone to detect ignition gap wear.
Computer Control

It will be understood that the control116schematically illustrated inFIGS.1A and1Bmay also be representative of one or more types of user devices, such as user input devices (e.g., keypad, touch screen, mouse, and the like), user output devices (e.g., display screen, printer, visual indicators or alerts, audible indicators or alerts, and the like). Control116may have a graphical user interface (GUI) controlled by software for display by an output device, and one or more devices for loading media readable by the controller212(e.g., logic instructions embodied in software, data, and the like). The control116may include an operating system (e.g., Microsoft Windows® software) for controlling and managing various functions thereof.

FIG.14is a flowchart detailing a method of the present invention for powering a plasma load.

In step1401, coupling power from an AC source to a variable plasma load via an asymmetric ballast transformer having a leakage inductance and a coaxial capacitance to ground. The variable plasma load comprising for example the atmospheric pressure plasma source or the low impedance cutting torches discussed above.

In step1403, while in a no-plasma state, generating a near resonance-voltage on the secondary side due to the leakage inductance and the capacitance.

In step1405, ignite a plasma at the near-resonance-voltage, and thereafter lower the operational frequency driving the plasma. In this step, the fully developed plasma load is resistive with the leakage inductance acting as a low pass filter preventing high frequency transients from propagating backwards into the AC source.

Furthermore, control116can suppress current spiking at the time of the plasma ignition by having the operational frequency fopoffset from resonant frequency f1and by having a duty cycle of the voltage pulses being less than 50%, less than 40%, less than 30%, less than 20%, or less than 10% of a total period of the cycle. Control116can be programmed to identify the plasma ignition at the operational frequency fopby a voltage spike on a voltage drive signal to the first (primary) side of the transformer. To suppress current spiking, the voltage spike occurs when an instantaneous value of AC current to the first (primary) side of the transformer is substantially near zero.

Furthermore, control116can be programmed with a predetermined plasma initiation frequency f2which is offset from the resonant frequency f1, and can be configured to at least one of:a) provide repeated cycles of the drive voltage at the initiation frequency f2to provide time for the plasma ignition,b) change the operational frequency fopof the AC power source by a step size that makes no more than a 10% voltage change near the initiation frequency f2, andc) change the operational frequency fopof the AC power source to values of frequency above or below the initiation frequency f2.

It will be understood that one or more of the processes, sub-processes, and process steps described herein may be performed by hardware, firmware, software, or a combination of two or more of the foregoing, on one or more electronic or digitally-controlled devices for example adjusting the variable capacitors and/or the relative bobbin positions and/or the power level of the AC source. The software may reside in a software memory (not shown) in a suitable electronic processing component or system such as, for example, control116schematically depicted inFIGS.1A and1B. The software memory may include an ordered listing of executable instructions for implementing logical functions (that is, “logic” that may be implemented in digital form such as digital circuitry or source code, or in analog form such as an analog source such as an analog electrical, sound, or video signal). The instructions may be executed within a processing module, which includes, for example, one or more microprocessors, general purpose processors, combinations of processors, digital signal processors (DSPs), or application specific integrated circuits (ASICs). Further, the schematic diagrams describe a logical division of functions having physical (hardware and/or software) implementations that are not limited by architecture or the physical layout of the functions. The examples of systems described herein may be implemented in a variety of configurations and operate as hardware/software components in a single hardware/software unit, or in separate hardware/software units.

The executable instructions may be implemented as a computer program product having instructions stored therein which, when executed by a processing module of an electronic system (e.g., the control116depicted inFIGS.1A and1i), direct the electronic system to carry out the instructions. The computer program product may be selectively embodied in any non-transitory computer-readable storage medium for use by or in connection with an instruction execution system, apparatus, or device, such as a electronic computer-based system, processor-containing system, or other system that may selectively fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this disclosure, a computer-readable storage medium is any non-transitory means that may store the program for use by or in connection with the instruction execution system, apparatus, or device. The non-transitory computer-readable storage medium may selectively be, for example, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device. A non-exhaustive list of more specific examples of non-transitory computer readable media include: an electrical connection having one or more wires (electronic); a portable computer diskette (magnetic); a random access memory (electronic); a read-only memory (electronic); an erasable programmable read only memory such as, for example, flash memory (electronic); a compact disc memory such as, for example, CD-ROM, CD-R, CD-RW (optical); and digital versatile disc memory, i.e., DVD (optical). Note that the non-transitory computer-readable storage medium may even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner if necessary, and then stored in a computer memory or machine memory.

It will also be understood that the term “in signal communication” as used herein means that two or more systems, devices, components, modules, or sub-modules are capable of communicating with each other via signals that travel over some type of signal path. The signals may be communication, power, data, or energy signals, which may communicate information, power, or energy from a first system, device, component, module, or sub-module to a second system, device, component, module, or sub-module along a signal path between the first and second system, device, component, module, or sub-module. The signal paths may include physical, electrical, magnetic, electromagnetic, electrochemical, optical, wired, or wireless connections. The signal paths may also include additional systems, devices, components, modules, or sub-modules between the first and second system, device, component, module, or sub-module.

More generally, terms such as “communicate” and “in . . . communication with” (for example, a first component “communicates with” or “is in communication with” a second component) are used herein to indicate a structural, functional, mechanical, electrical, signal, optical, magnetic, electromagnetic, ionic or fluidic relationship between two or more components or elements. As such, the fact that one component is said to communicate with a second component is not intended to exclude the possibility that additional components may be present between, and/or operatively associated or engaged with, the first and second components.

Exemplary Statements of the Invention

The following numbered statements of the invention set forth a number of inventive aspects of the present invention:

Statement 1. A system for plasma ignition and maintenance of an atmospheric pressure plasma, the system comprising:a variable frequency alternating current (AC) power source;a transformer havinga magnetic core (optionally including gaps),a primary winding on a first (primary) side of the magnetic core, anda secondary winding on a second (secondary) side of the magnetic core;a cable connected to the secondary winding to output power from the transformer, and the cable having a capacitance to ground; anda programmed microprocessor for control of power to the atmospheric pressure plasma,whereinthe primary winding is connectable to the power source,the cable is connectable to electrodes of the atmospheric pressure plasma,the secondary winding and the cable are resonant at a resonant frequency f1, andthe programmed processor is configured toa) at pre-ignition, power the AC power source at an operational frequency fophigher than the resonant frequency fr,b) decrease the operational frequency fopof the AC power source until there is plasma ignition, andc) after the plasma ignition, further decrease the operational frequency fopof the AC power source to a frequency lower than the resonant frequency fr.

Statement 2. The system of statement 1, wherein the programmed processor is configured to set the operational frequency fopto at least 1.5×f1to mitigate against current spiking occurring in the AC power supply as a result of an arc forming at the time of the plasma ignition and presenting a short-circuit load for the transformer.

Statement 3. The system of any of the statements above, wherein the programmed processor is configured to adjust the operational frequency for, until fopequals the resonant frequency f1, wherein a rate of frequency change is 10× slower than the BW/2 Hz per 0.6/BW seconds.

Statement 4. The system of any of the statements above, wherein the AC power source comprises a transistor bridge configured to produce voltage pulses at the operational frequency fopfor application to the primary winding of the transformer.

Statement 5. The system of any of the statements above, wherein the programmed processor is configured to suppress current spiking at the time of the plasma ignition by having the operational frequency fopoffset from resonant frequency f1and optionally may operate with a duty cycle of the voltage pulses being nearly 100%, less than 75%, less than 50% less, than 25%, or less than 10% of a total period of the cycle.

Statement 6. The system of statement 1, wherein the programmed microprocessor is configured to identify the plasma ignition by monitoring for a change in primary current wave-shape on the primary side of the transformer.

Statement 7. The system of statement 6, wherein a potentially damaging current spike is minimized because the plasma ignition occurs when an instantaneous value of AC current to the primary side of the transformer is substantially near zero.

Statement 8. The system of statement 1, whereinthe programmed processor is programmed with a predetermined plasma initiation frequency f2which is offset from the resonant frequency f1, andthe programmed processor is configured to at least one of:d) provide repeated cycles of the drive voltage at the initiation frequency f2to provide time for the plasma ignition,e) change the operational frequency fopof the AC power source by a step size that makes no more than a 10% voltage change near the initiation frequency f2, andf) change the operational frequency fopof the AC power source to values of frequency above or below the initiation frequency f2.

Statement 9. The system of any of the statements above, wherein the plasma ignition occurs when stored energy in the secondary side of the transformer is substantially all in the capacitance of the cable.

Statement 10. The system of any of the statements above, wherein the transformer comprises a resonant transformer having a resonance associated with the capacitance of the cable and an inductance of the transformer.

Statement 11. The system of any of the statements above, wherein the secondary winding has more turns than the primary winding such that the transformer comprises a step-up transformer for supplying current to the atmospheric pressure plasma.

Statement 12. The system of any of the statements above, wherein the transformer comprises a ballast transformer in which the primary winding comprises a first primary winding and a second primary winding.

Statement 13. The system of any of the statements above, wherein the first primary winding and the second primary winding provide an inductive impedance that opposes current surges when a load is introduced.

Statement 14. The system of any of the statements above, wherein the second primary winding is displaceable from the secondary winding to alter a coupling coefficient of the transformer.

Statement 15. The system of any of the statements above, wherein the second primary winding coaxially surrounds the secondary winding.

Statement 16. The system of any of the statements above, wherein the second primary winding is offset axially from and surrounds the secondary winding or wherein the first primary winding is offset axially from the magnetic core.

Statement 17. The system of any of the statements above, wherein the programmed processor is configured to produce a drive signal for a transistor bridge connected to the transformer.

Statement 18. The system of any of the statements above, wherein the programmed processor comprises an analogue to digital converter (ADC) in electrical communication with current and voltage sampling points.

Statement 19. The system of any of the statements above, further comprising a current sense transformer, wherein the transistor bridge is coupled to the current sense transformer, and the current sense transformer is connected in series with the primary winding of the transformer.

Statement 20. The system of any of the statements above, wherein the processor, the transistor bridge, the current sense transformer, and the analogue to digital converter (ADC) comprise a chirp plasma ignition and plasma maintenance controller for the system.

Statement 21. A (computerized) method for plasma ignition and maintenance of an atmospheric pressure plasma using any of the systems and programmed processors described in the statements above.

Statement 22. The method of statement 21, comprising setting the operational frequency fopto at least 1.5×f1to mitigate against current spiking occurring in the AC power supply as a result of an arc forming at the time of the plasma ignition and presenting a short-circuit load for the transformer.

Statement 23. The method of statement 21, comprising adjusting the operational frequency for, until fopequals the resonant frequency f1, wherein a rate of frequency adjustment is less than 0.6×(a bandwidth in Hz of the resonance at f1) per second to avoid or minimize current spikes after plasma ignition.

Statement 24. The method of statement 21, comprising controlling a transistor bridge so that the bridge produces voltage pulses at the operational frequency fopfor application to the primary winding of the transformer.

Statement 25. The method of statement 21, comprising suppressing current spiking at the time of the plasma ignition by having the operational frequency fopoffset from resonant frequency f1.

Statement 26. The method of statement 21, comprising detecting the plasma ignition by a current spike on the primary side of the transformer.

Statement 27. The method of statement 21, comprising programming the processor with a predetermined plasma initiation frequency f2which is offset from the resonant frequency f1, and can be configured to at least one of:a) provide repeated cycles of the drive voltage at the initiation frequency f2to provide time for the plasma ignition,b) change the operational frequency fopof the AC power source by a step size that makes no more than a 10% voltage change near the initiation frequency f2, andc) change the operational frequency fopof the AC power source to values of frequency above or below the initiation frequency f2.

Statement 28. A ballast transformer as described above in any of the statements 11-18, where the plasma ignition and maintenance is controlled in part by the systems and programmed processors described in any of the statements above.

Numerous modifications and variations of the invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.