Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Plasma generating
Reexamination Certificate
2002-07-11
2004-08-10
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Plasma generating
C315S111710, C313S231310
Reexamination Certificate
active
06774569
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to discharge plasma systems and methods, and more specifically to an apparatus and method for generating a glow discharge plasma under atmospheric pressure conditions.
BACKGROUND OF THE INVENTION
Atmospheric Pressure Glow (APG) discharge is used in practise, among others, for non-destructive material surface modification. Glow discharge plasmas are relatively low power density plasmas, typically generated under atmospheric pressure conditions or partial vacuum environments.
In practise, the plasma is generated in a plasma chamber or plasma discharge space between two oppositely arranged parallel plate electrodes. The plasma is generated in a gas or a gas mixture in the discharge space by energizing the electrodes from AC power means.
It has been observed that a stable and uniform plasma can be created in, for example, a pure Helium or a pure Nitrogen gas. However, as soon as impurities or other gasses or chemical compositions at ppm level are present in the gas, the stability of the plasma will decrease significantly. Typical examples of stability destroying components are O
2
, NO, CO
2
, etc.
It has been observed that instabilities in the plasma will develop either in a high current density plasma or will extinguish locally the plasma. As a result, current arcing occurs and the glow discharge plasma can not be maintained. Instead, a combination of filamentary and glow discharge is generated.
Filamentary discharges between parallel plate electrodes in air under atmospheric pressure have been used to generate ozone in large quantities but are of limited use for the surface treatment of materials, since the plasma filaments tend to puncture or treat the service unevenly and are associated with relatively high plasma currents.
U.S. Pat. No. 5,414,324 discloses a plasma chamber having a first or upper electrode through which air is injected and a second or counter electrode on which the substrate to be treated is located. For protecting the substrate to be treated against sparks, i.e. current arcing in the plasma, an electrically conductive grid is provided between the oppositely spaced electrodes, which grid connects by an inductor with a large inductance (choke coil).
From experiments disclosed by U.S. Pat. No. 5,414,324 it appears that for practical use an inductor having an inductance of 100 H and even higher has to be applied.
Those skilled in the art will appreciate that the combination of conductive grid and high inductance is a design which can be used only for over-current protection, being in fact a solution for damping the current of the sparks developed in the plasma and preventing that the sparks will reach the counter electrode where the material to be treated in plasma is located. The solution disclosed in U.S. Pat. No. 5,414,324 can have only a small effect at normal low current operating conditions and can affect only slightly the plasma stability and will not protect against the formation of sparks. A spark is already there when the inductor starts to react.
Further, inductors of the size disclosed above require a relatively large amount of space and, due to the magnetic core required, have a significant weight. Further, the conductive grid limits the effective discharge space for treating a substrate by the plasma.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a novel and improved apparatus and method for producing and sustaining a uniform glow discharge plasma.
This and other objects and features of the invention are achieved according to the present invention by an apparatus for producing and sustaining a glow discharge plasma in a plasma discharge space comprising at least two oppositely spaced electrodes, means for supplying a gas or a gas mixture under atmospheric pressure conditions in the discharge space, AC power supply means for energizing the electrodes, and electric stabilisation means for stabilizing current variations in the plasma, the stabilisation means connect between the electrodes and the power supply means, and are arranged for providing a negative feedback opposed to a positive feedback of the plasma to instabilities in the plasma.
Compared to the prior art solution as presented by U.S. Pat. No. 5,414,324 in the present invention, the plasma discharge space is not obstructed by the introduction of an electrically conductive screen and the like.
Electrically, an APG system can be represented by an equivalent RLC circuit diagram comprised of a capacitance C, mainly due to the capacitance of the spaced electrodes, an inductance L and a plasma resistance R. The inductance component includes APG self-inductance and the equivalent inductance of system coupling to the power supply. When the APG is coupled to a power supply by a transformer, the equivalent inductance of the coupling system is of a significant order of at least a few mH.
At plasma breakdown large current oscillations can be generated as a consequence of the current resonance generated by the RLC circuit. These current oscillations set a ground for the generation of waves in the plasma and the constriction of the plasma column. Due to the relatively low value of the APG self-inductance, the resonance frequency is high and is preferentially excited at breakdown. Accordingly, during the plasma breakdown high amplitude and current oscillations are developed which are a major risk for plasma stability. Also a strong electromagnetic emission is developed due to the self-resonance of the APG inductance and capacitance.
In a further embodiment of the invention, the stabilisation means are arranged for decreasing plasma voltage proportional to the rate of plasma current variation and/or to an electromagnetic field radiated by the plasma.
The present invention is based on the insight that the insertion of a negative feedback stabilisation means is functional at the plasma breakdown when the voltage applied to the plasma is decreased on a short time scale. To prevent the development of undesired high current oscillations, in a preferred embodiment of the invention the inductive component of the impedance of the stabilization means has to decreased substantially after the plasma breakdown. Also in order to prevent plasma instabilities initiated by low current streamers, the stabilization means are arranged in such a way that a negative feedback not only provided to the plasma current but also to the plasma electromagnetic radiation too. In this manner filamentary discharges in plasmas can be effectively avoided.
From experimentally obtained data, it has been shown that the solution according to the present invention results in a very stable plasma for a variety of different supply gases, excitation conditions, electrode designs and electrode spacings.
During breakdown, the plasma is in its most unstable phase in which even small voltage variations can easily affect plasma stability. By introducing a relatively large inductance, such as disclosed by U.S. Pat. No. 5,414,324, the voltage variations will be large and, as consequence, the plasma stability will be adversely affected.
In order to prevent the generation of the plasma oscillations and the reduction of the current amplitude after the critical phase of plasma breakdown, in the preferred embodiment of the present invention, the stabilization means are arranged for a substantial decrease of inductance after the plasma breakdown. Preferably, the stabilisation means are arranged for a substantial decrease of inductance at a plasma current density above approximately 1 mA/cm
2
. A decrease of inductance of at least one order of magnitude appears to be sufficient.
In a practical embodiment of the apparatus according to the present invention, the stabilisation means comprise inductor means. Those skilled in the art will appreciate that inductor means generate voltages opposed to the rate or speed of APG magnetic flux variation and APG current variation, dependent on their inductance.
By series connecting the inductor means and at least one
Aldea Eugen
de Vries Hindrik Willem
Mori Fuyuhiko
van de Sanden Mauritius Cornelius Maria
Davidson Berquist Klima & Jackson LLP
Dinh Trinh Vo
Fuji Photo Film B.V.
Wong Don
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