SUPPRESSION OF SELF PULSING DC DRIVEN NONTHERMAL MICROPLASMA DISCHARGE TO OPERATE IN A STEADY DC MODE

Abstract

The current disclosure relates to a suppressor circuit configuration for extending the stable region of operation of a DC driven micro plasma discharge at atmospheric and higher pressures.

Claims

1. An instability suppressor circuit for self-pulsing direct current driven microplasma discharge comprising: a power supply; a ballast resistor; a plasma discharge; an inductor connected in series with the power supply, ballast resistance and plasma discharge; wherein the suppressor circuit adds a positive impedance making plasma from the plasma discharge less sensitive to a change in voltage with respect to a change in current; and wherein the suppressor circuit functions at atmospheric pressure and above.

2. The suppressor circuit of 1, wherein the inductor increases the combined response time of the plasma and the inductor, such that t L/R.sub.discharge>t R.sub.ballastC.sub.p.

3. The suppressor circuit of 1, wherein the plasma discharge characteristics are obtained from the solution of the below equation: $V=L_x.Math.\frac{\mathrm{dI}}{\mathrm{dt}}+R_{\mathrm{discharge}}.Math.I$ $V=V_s-{\mathrm{IR}}_{\mathrm{discharge}}-R_{\mathrm{ballast}}.Math.C_p.Math.\frac{\mathrm{dV}}{\mathrm{dt}}$

4. The suppressor circuit of 1, wherein the inductor shifts a negative differential resistance region into lower current regimes.

5. The suppressor circuit of X, wherein two electrodes having a separation distance of from 100 μm to 400 μm form the plasma discharge.

6. A system for suppressing a self-pulsing regime of a direct current driven microplasma discharge comprising: a power supply; a ballast resistor; a plasma discharge; an inductor connected in series with the ballast resistance and plasma discharge; wherein the inductor suppresses oscillation of the plasma discharge, thereby establishing a steady plasma discharge; wherein the system comprises a positive impedance making plasma from the plasma discharge less sensitive to a change in voltage with respect to a change in current; and wherein the system functions at atmospheric pressure and above.

7. The system of 6, wherein varying an inductance value increases a response time of plasma to a value wherein t Lx/R.sub.discharge>t R.sub.ballastC.sub.p thereby making a driving circuit response time shorter.

8. The system of 6, wherein the system shifts a negative differential resistance region into lower current regimes.

9. The system of 6, wherein the inductor increases the combined response time of the plasma and the inductor, such that t L/R.sub.discharge>t R.sub.ballastC.sub.p.

10. The system of 6, wherein the plasma discharge characteristics are obtained from the solution of the below equation: $V=L_x.Math.\frac{\mathrm{dI}}{\mathrm{dt}}+R_{\mathrm{discharge}}.Math.I$ $V=V_s-{\mathrm{IR}}_{\mathrm{discharge}}-R_{\mathrm{ballast}}.Math.C_p.Math.\frac{\mathrm{dV}}{\mathrm{dt}}$

11. The system of 6, wherein the plasma discharge is formed between two electrodes having a separation distance of from 100 μm to 400 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The construction designed to carry out the invention will hereinafter be described, together with other features thereof. The invention will be more readily understood from a reading of the following specification and by reference to the accompanying drawings forming a part thereof, wherein an example of the invention is shown and wherein:

[0020] FIG. 1 shows an experimental setup wherein R.sub.ballast, L.sub.x, and R.sub.shunt represents ballast resistance, external inductor and shunt resistance respectively.

[0021] FIG. 2 illustrates voltage current characteristics of a DC driven micro plasma discharge operating in helium at atmospheric pressure (d.sub.inter-electrode=200 μm, R.sub.ballast=100 kΩ, pd=15.2 Torr-cm) in presence of external inductor.

[0022] FIG. 3 shows voltage current characteristics of a DC driven micro plasma discharge operating in helium at atmospheric pressure with and without the suppression circuit for different inter-electrode separation distance a) dinter-electrode=400 μm, R.sub.ballast=100 kΩ, pd=30.4 Torr-cm, b) d.sub.inter-electrode=100 μm, R.sub.ballast=100 kΩ, pd=7.6 Torr-cm.

[0023] FIG. 4 illustrates a stability map based on the suppression of oscillatory discharge with the inclusion of external inductance (R.sub.ballast=100 kΩ, Cp=100 pF). Each symbol represents an individual microplasma discharge state.

[0024] FIG. 5 shows: (a) transient discharge voltage and current profile in the NDR region without the presence of any suppressing circuit element; and (b) time dependent voltage versus time dependent current (d.sub.inter-electrode=200 □m, R.sub.ballast=100 kΩ, pd=15.2 Torr-cm).

[0025] FIG. 6 shows a discharge current profile with the presence of inductor showing: (a) self-pulsation (L.sub.x=0.01 H); and (b) damped oscillation (L.sub.x=20H) (R.sub.ballast=100 kΩ, C.sub.p=100 pF).

[0026] It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all its respects, to every aspect of this invention. As such, the preceding objects can be viewed in the alternative with respect to any one aspect of this invention. These and other objects and features of the invention will become more fully apparent when the following detailed description is read in conjunction with the accompanying figures and examples. However, it is to be understood that both the foregoing summary of the invention and the following detailed description are of a preferred embodiment and not restrictive of the invention or other alternate embodiments of the invention. In particular, while the invention is described herein with reference to a number of specific embodiments, it will be appreciated that the description is illustrative of the invention and is not constructed as limiting of the invention. Various modifications and applications may occur to those who are skilled in the art, without departing from the spirit and the scope of the invention, as described by the appended claims. Likewise, other objects, features, benefits and advantages of the present invention will be apparent from this summary and certain embodiments described below, and will be readily apparent to those skilled in the art. Such objects, features, benefits and advantages will be apparent from the above in conjunction with the accompanying examples, data, figures and all reasonable inferences to be drawn therefrom, alone or with consideration of the references incorporated herein.

SUMMARY OF THE INVENTION

[0027] In a first embodiment, the current disclosure provides an instability suppressor circuit for self-pulsing direct current driven microplasma discharge comprising. The circuit comprises a power supply, a ballast resistor, a plasma discharge, an inductor connected in series with the power supply, ballast resistance and plasma discharge. The suppressor circuit adds a positive impedance making plasma from the plasma discharge less sensitive to a change in voltage with respect to a change in current. The suppressor circuit functions at atmospheric pressure and above. Further, the inductor increases the combined response time of the plasma and the inductor, such that t L/R.sub.discharge>t R.sub.ballastC.sub.p. Even further, the plasma discharge characteristics are obtained from the solution of the below equation:

[00001]$V=L_x.Math.\frac{\mathrm{dI}}{\mathrm{dt}}+R_{\mathrm{discharge}}.Math.I$$V=V_s-{\mathrm{IR}}_{\mathrm{discharge}}-R_{\mathrm{ballast}}.Math.C_p.Math.\frac{\mathrm{dV}}{\mathrm{dt}}$

Still further, the inductor shifts a negative differential resistance region into lower current regimes. Further yet, two electrodes having a separation distance of from 100 μm to 400 μm form the plasma discharge.

[0028] In an alternative embodiment, a system for suppressing a self-pulsing regime of a direct current driven microplasma discharge is provided. The system comprises a power supply, a ballast resistor, a plasma discharge, and an inductor connected in series with the ballast resistance and plasma discharge. The inductor suppresses oscillation of the plasma discharge, thereby establishing a steady plasma discharge. The system comprises a positive impedance making plasma from the plasma discharge less sensitive to a change in voltage with respect to a change in current. Also, the system functions at atmospheric pressure and above.

[0029] Further, varying an inductance value increases a response time of plasma to a value wherein t Lx/R.sub.discharge>t R.sub.ballastC.sub.p thereby making a driving circuit response time shorter. Still further, the system shifts a negative differential resistance region into lower current regimes. Yet further, the inductor increases the combined response time of the plasma and the inductor, such that t L/R.sub.discharge>t R.sub.ballastC.sub.p. Furthermore, the plasma discharge characteristics are obtained from the solution of the below equation:

[00002]$V=L_x.Math.\frac{\mathrm{dI}}{\mathrm{dt}}+R_{\mathrm{discharge}}.Math.I$$V=V_s-{\mathrm{IR}}_{\mathrm{discharge}}-R_{\mathrm{ballast}}.Math.C_p.Math.\frac{\mathrm{dV}}{\mathrm{dt}}$

Even further, the plasma discharge is formed between two electrodes having a separation distance of from 100 μm to 400 μm.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0030] With reference to the drawings, the invention will now be described in more detail. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the presently disclosed subject matter belongs. Although any methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of the presently disclosed subject matter, representative methods, devices, and materials are herein described.

[0031] Unless specifically stated, terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. Likewise, a group of items linked with the conjunction “and” should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as “and/or” unless expressly stated otherwise. Similarly, a group of items linked with the conjunction “or” should not be read as requiring mutual exclusivity among that group, but rather should also be read as “and/or” unless expressly stated otherwise.

[0032] Furthermore, although items, elements or components of the disclosure may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated. The presence of broadening words and phrases such as “one or more,” “at least,” “but not limited to” or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent.

[0033] The current disclosure provides suppression of the self-pulsing regime of a DC driven microplasma discharge in a parallel plate, pin to plate, or similar configuration by employing an external suppressor circuit. The external circuit, which is an integral part of the discharge system, has often been considered to characterize and study the self-pulsing oscillatory region. From the external circuit constraint, self-pulsing in the NDR region is obtained when the external circuit response time becomes higher than the ion transit time, i.e. t.sub.R.sub.ballast.sub.C.sub.p>t.sub.plasma. FIG. 1 shows a schematic of the experimental setup of the current disclosure wherein R.sub.ballast, L.sub.x, R.sub.shunt represent ballast resistance, the external inductor and shunt resistance respectively. The external inductor is “open bracket” channel mount type that is vacuum impregnated with polyurethane varnish for long operation life. The inductor has an iron core, coil composition. As FIG. 1 illustrates, the experimental set-up may include a high voltage power supply 10, an oscilloscope 20, a micropositioner 30, a gas supply 40, which may be helium, a throttle valve 50, a microscope 60, a camera 70, a pressure gauge 80, and a voltage probe 90, and current 100 in order to examine R.sub.ballast 110, L.sub.x 120, and R.sub.shunt 130. The set-up may be connected to a monitoring device 140 to view the system in progress.

[0034] The suppression circuit of the current disclosure comprises an inductor connected in series with the ballast resistance and the discharge, which increases the combined response time of the plasma and the inductor, such that t L/R.sub.discharge>t R.sub.ballastC.sub.p. As a test case, helium micro plasma operating at atmospheric pressure was studied. However, higher pressures are considered within the scope of this disclosure. Three inter-electrode separation distances were investigated 100 μm, 200 μm and 400 μm corresponding to pd values of 7.6, 15.2, and 30.4 Torr-cm. The electrode arrangement consisted of a spherical anode and a flat cathode disk having diameters of 12.7 and 10 mm respectively.

[0035] The spherical anode was used to maintain the discharge in the central region (i.e. the smallest gap) to ease the visualization process. It should be noted that despite the sphere-plate type electrode design the radial size of the discharge is sufficiently small such that the electrode configuration can be considered to be a parallel-plate arrangement. The anode electrode was attached to a micro-positioner for varying the inter electrode separation distance. The electrodes were contained inside a stainless pressure chamber with quartz window viewports for discharge visualization. The chamber is sealable and there are gas inlets and outlets for testing in a variety of pressures and discharge gases.

[0036] The experiments were conducted using a Spellman SL20P2000 DC power supply 20 setup connected in series to a 100 kΩ ballast resistor, an inductor (oscillation suppression experiments) and the discharge. For time dependent current measurements a current shunt (10 kΩ) was placed between one electrode of the discharge and the ground. A North Star PVM-4 high impedance 1000:1 voltage probe was placed directly adjacent to the anode to measure the discharge voltage. Both the voltage probe and the current shunt are connected to an oscilloscope (Agilent Technologies InfiniiVision MSO7054B) for DC or time dependent measurements. The parasitic capacitance of the external circuit was measured as 40 pF. Experiments were conducted with high purity helium feed gas (AirGas, 99.997% purity level). For visualizing the discharge, a Nikon D7000 camera was mounted on a microscope focused on the discharge. The microscope-camera setup provided a variable magnification.

[0037] FIG. 2 shows the voltage-current (VI) characteristics with and without the presence of the inductor element. Due to the oscillatory nature of the discharge in the NDR regime, the VI characteristics in the NDR region is obtained from the RMS voltage and current. The VI curves manifest the usual shape that corresponds to the NDR regime, i.e. subnormal mode at lower currents and then attains the “flat” normal glow as the discharge current increases. Though self-pulsing is a NDR phenomenon, the region near the inflection point (i.e. transition from ‘subnormal’ to ‘normal’) attains a steady “non-pulsing” discharge condition.

[0038] The presence of an inductor was found to extend the normal glow region operation to lower currents—shifting the NDR region. The measurement with different inductance value shows that, the ‘normal’ glow region of the discharge can be extended to lower currents with increasing inductance value. The transition from ‘subnormal’ to ‘normal’ glow occurs at 0.8 mA in absence of any external inductor element. The transition point shifted to 0.65 mA and 0.40 mA for a 1 H and 40 H respectively The NDR region is still retained with the different inductors however the slope changes significantly. The slope of the NDR region varies from 440 kΩ, 305 kΩ, and 225 kΩ for an inductance value of 0 H, 1 H, and 40 H, respectively. The decrement of the slope of the NDR region is also an indication of the fact that the inductor element is extenuating the NDR response of the system. The suppressing circuit element adds a positive impedance to the system and the plasma become less sensitive to the change in voltage with respect to the change in current.

[0039] Images of the discharges for different discharge currents in both the steady and pulsing regime for the same pd value are provided as insets in FIG. 2. False coloring scaled as a function of emission intensity is employed to obtain a better insight of the discharge structure. The time averaged image of a pulsing discharge shows that the discharge has a uniform intensity (both the positive column and negative glow being equally bright) without the presence of a Faraday dark space. The discharge images in presence of the inductor clearly shows that the classical and distinctive DC glow structure is attained—anode glow, Faraday dark space and negative glow. The ‘normal’ glow is also retained which is confirmed by the increasing cathode spot and the constant current density of 1.8 mA/cm.sup.2. For identical discharge current of 0.65 mA, the inductor suppresses the oscillation of the discharge and establishes a steady discharge that has the distinctive steady DC glow characteristics. The discharge images further confirms that even at the lowest discharge current the plasma is operating in the ‘normal’ mode. The VI characteristics for two other inter-electrode spacing, 100 μm and 400 μm corresponding to pd values of 7.6 and 30.4 Torr-com show similar instability suppression and extension of the normal glow regime in the presence of external inductor element (FIG. 3). The inclusion of 40 H inductor altered the normal glow inception from 0.46 mA to 0.30 mA and from 0.90 mA to 0.50 mA for electrode spacing of 100 μm and 400 μm respectively.

[0040] Based on the interaction of the different circuit element, especially the representative characteristics response time, a stability map denoting regimes of pulsing and stable operation can be proposed. FIG. 4 shows such a stability regime map where each of the symbols represents individual simulated cases. The 45 degree line in the stability plot represents the condition where t Lx/R.sub.discharge=t R.sub.ballastCp and demarcates stable and unstable operation regimes. For conditions where t Lx/R.sub.discharge<t R.sub.ballastC.sub.p a self-pulsing DC discharge that undergoes relaxation type oscillation is observed. By varying/incrementing the inductance value, the combined time response of plasma with inductor can be increased to a value where t Lx/R.sub.discharge>R.sub.ballastC.sub.p making the driving circuit response time comparably shorter and establishing a stable DC operation can be obtained.

[0041] As explained herein, an instability suppressor circuit for self-pulsing DC driven microplasma discharge has been tested over a range of pressure and electrode separation distance. The external circuit configuration was successful in suppressing self-pulsing of the discharge, extending the normal glow regime to lower currents. The negative differential resistance (NDR) region was observed to shift further left in the voltage-current parametric space (i.e. lower current) and the slope of the NDR region was decreased substantially. Currently there are no existing technologies aimed at suppressing the instability at atmospheric and higher pressures. The current disclosure employs a simple external circuit configuration that is inexpensive to implement. The potential user for this technology is in the field of plasma enhanced chemical vapor deposition (PECVD), plasma surface treatment, plasma lighting, etc.

[0042] The temporal evolution of the voltage and current for a standard self-pulsing discharge (without any suppressing circuit element) is presented in FIG. 5. FIG. 5 shows: graph (a) illustrating transient discharge voltage and current profile in the NDR region without the presence of any suppressing circuit element; and graph (b) that shows time dependent voltage versus time dependent current (d.sub.inter-electrode=200 □m, R.sub.ballast=100 kΩ, pd=15.2 Torr-cm). In the pulsing mode the voltage and current exhibit a phase difference of 15°. The temporal profiles have similarity to those obtained for a moderate pressure self-pulsing MHCD.

[0043] In a single pulse, the discharge voltage exhibits three different stages. During the current spike, the discharge voltage shows a sudden dip which is followed by a gradual increase to a moderate voltage that is maintained for a significant duration. A linear ramping to the highest voltage is observed soon after. The phase space diagram for the voltage-current is presented in FIG. 5, see graph b, which is found to attain a triangular shape. The phase space diagram has three different regions. During the extremely low current stage the voltage sharply increases from 500V to 1250V (stage 1). One can then see a decrease in discharge voltage with an increase in the discharge current (stage 2). Following this stage the current decreases sharply followed with a slight increase in the voltage (stage 3).

[0044] Based on the experimental results, a circuit model is solved to investigate the stability condition in details. Circuit models have been widely used to study the instability in the NDR region for parallel plate or MHCD geometry but not distinctively on stability suppression concepts. The discharge characteristics in the presence of the suppression element (i.e. inductor) can be obtained from the solution of Eq. (1) and (2).

[00003]$V=L_x.Math.\frac{\mathrm{dI}}{\mathrm{dt}}+R_{\mathrm{discharge}}.Math.I$$V=V_s-{\mathrm{IR}}_{\mathrm{discharge}}-R_{\mathrm{ballast}}.Math.C_p.Math.\frac{\mathrm{dV}}{\mathrm{dt}}$

[0045] The solution of the circuit model is based on the choice of the discharge resistance. It is a common norm to model the nonlinear NDR discharge resistance as a function of discharge current. For example, prior work modeled the NDR resistance with a second order polynomial expression which predicted the discharge instability and the temporal profile of the discharge voltage for low pressure systems. However, the polynomial form of expression was unable to predict the current pulse shape which has a distinctive spike followed by a very low current stage.

[0046] More recent studies have proposed a hyperbolic tangent form of discharge resistance profile, which resulted in better agreement between experiments and predictions. However, these discharge resistance were suggested for moderate pressure MHCD geometry.

[0047] For the current model the discharge resistance is expressed as:

[00004]$R_{\mathrm{discharge}}=C_1.Math.\tanh \left(\frac{I-I_{\lim }}{p}\right)+C_2$

Where, the constants, C.sub.1=−1920Ω, C.sub.2=2000Ω, I.sub.lim=0.317 mA, and p=0.45 mA, were obtained from experimental fits. The system of equations for the circuit model is solved with an implicit Runge-Kutta solver in MATLAB with the accuracy level of 10.sup.−3˜10.sup.−6.

[0048] The transient discharge voltage and current profile from the circuit equation is shown in FIG. 6. The numerical results are found to predict the experimental trends. For an inductance value of L.sub.x=0.01 H, a pulsing of voltage and current (i.e. an oscillatory discharge) is observed in FIG. 6, graph (a). This lower value of inductance corresponds to a value, where τ.sub.L.sub.x.sub./R.sub.discharge<τ.sub.R.sub.ballast.sub.C.sub.p, as a result the discharge shows oscillation without any amplitude attenuation. The discharge voltage has a sawtooth like pattern in close resemblance to those of the experiments. Simulation is also conducted in the presence of an inductor of higher magnitude, such that τ.sub.L.sub.x.sub./R.sub.discharge<τ.sub.R.sub.ballast.sub.C.sub.p; the oscillation for both the current and voltage is damped resulting in a steady discharge voltage/current at the end (FIG. 6(b)). This high inductance value 20 H was chosen within the experiment range of 1 H to 40 H. The model predictions are in qualitative agreement with the experimental results. The presence of an inductor acts as a damping element in the coupled plasma-external circuit. Based on the interaction of the different circuit element, especially the representative characteristics response time, a stability map denoting regimes of pulsing and stable operation can be proposed.

[0049] While the present subject matter has been described in detail with respect to specific exemplary embodiments and methods thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art using the teachings disclosed herein.