Electric lamp and discharge devices – With gas or vapor – Having particular electrode structure
Reexamination Certificate
2000-05-27
2002-08-13
Bruce, David V. (Department: 2882)
Electric lamp and discharge devices
With gas or vapor
Having particular electrode structure
C313S306000
Reexamination Certificate
active
06433480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to plasma generation.
2. Description of Related Art
Generally, it is known to generate low pressure, plasma glow discharges. These low pressure, plasma glow discharges are created in a vacuum and are difficult to sustain for any length of time. Typically, the low pressure, plasma glow discharges can be maintained in a stable, uniform state.
Plasma is a term used to describe an electrically neutral, partially ionized gas composed of ions, electrons, and neutral particles. Furthermore, plasma is a phase of matter that is distinct from solids, liquids, and normal gases because plasma is produced by either high temperatures or strong constant or time varying electric fields.
Generally, discharge plasmas are produced when free electrons are energized by electric fields in a background of neutral particles, such as atoms and/or molecules. When these free electrons are energized, the free electrons collide with the neutral particles. These collisions transfer energy from the free electrons to the neutral particles and, as a result, the originally neutral background gas becomes at least partially, and in some cases fully ionized. When this occurs, the ionized gas is able to conduct currents.
A specific class of plasmas, known as current-maintained plasmas, is produced by a glow plasma discharge or an arc plasma discharge. These current-maintained plasmas are only maintained, and are only conductive, while a current is passed through the generated plasma. Thus, if the current is removed, the plasma quickly becomes nonconductive. This is because the relaxation time for most plasmas is between &eegr;s and &mgr;s.
The characteristics that distinguish an arc plasma discharge from a glow plasma discharge are a high gas temperature and a low cathode fan potential. However, it is possible to have a high gas temperature associated with a high cathode fall, and vice versa. Furthermore, as would be expected, there are transition points at which the characteristics of the discharge plasma change from glow plasma discharge characteristics to arc plasma discharge characteristics.
A plasma discharge encounters several intermediate stages during the transition from a glow plasma discharge to arc plasma discharge. Some of these intermediate stages are relatively stable, while other of the stages are not. However, unless the current is limited by an external method, such as, for example, a large series resistance, the final transition from a glow plasma discharge to arc plasma discharge is usually an unstable change.
Furthermore, if the current is not limited by an external method, the final transition from a glow plasma discharge to arc plasma discharge is quite rapid, and equilibrium is typically not achieved in any of the intermediate transition stages. This final transition accelerates as the pressure of the background neutral gas increases towards atmospheric pressure.
There are two basic methods that can be used to generate discharge plasmas. The first method is through external ionization. External ionization produces plasmas by using photons or charged particles. However, this method produces a weakly ionized gas at high (atmospheric) pressure. Additionally, the efficiency of external ionization is rather low, and therefore the cost of such a method relatively high.
The second method that can be used to generate plasmas is internal ionization The method of internal ionization generates electrons and ions in a self-sustained gas discharge. If the current is strongly limited, or the applied voltage is smaller than the breakdown voltage, this method can produce large volumes of weakly ionized gas. However, if the current is not limited and the voltage is high enough, there is a transition to an arc.
For example, U.S. Pat. No. 5,939,829 to Schoebach et al. merely discloses a discharge device for operation in gas at a prescribed pressure including a single cathode having a plurality of microhollows, and a single anode spaced from the cathode.
Additionally, U.S. Pat. No. 6,005,349 to Kunhardt et al. merely discloses a method and apparatus for stabilizing glow plasma discharges by suppressing the transition from glow-to-arc. The Kunhardt et al. apparatus includes a single upper electrode, a single cathode, a collar, and a perforated dielectric plate positioned over the cathode. The holes in the perforated dielectric plate provide a narrow channel to limit the overall current density. Additionally, the collar is positioned between the upper electrode and the cathode to contain the glow discharge.
Thus, methods for generating large volumes of ionized gas at high pressure typically involve ionization mechanisms wherein the energy required for the ionization is drawn from electrical energy. Examples of this kind of ionization mechanism are radio frequency (RF) and microwave discharges, barrier discharges, and pulsed corona discharges where the discharge is sustained by either alternating or pulsed fields, and steady state discharges wherein the discharge is driven by a direct current (dc) power source.
Much of the efforts in generating stable glow discharges at high pressure have focused on preventing the onset of instabilities in the regions near the electrodes, particularly in the cathode region. These regions near the electrodes are regions of higher electric field and consequently higher power density compared to the positive column of the discharge. These regions are, therefore, a cradle of instabilities, which lead to constrictions and arc formation in the discharge. The glow-to-arc transition (GAT), the development of a highly conductive channel, which shorts out the glow discharge, shows the first visible evidence near the cathode. Other instabilities which may develop in the positive column of discharges in electronegative gases, such as the attachment instability, are generally more benign than the GAT.
Segmentation of the cathode, and ballasting the individual discharge resistively has been used to prevent the onset of the GAT instability in atmospheric pressure glow discharges. The current density in the bulk of the glow discharge is known to increase linearly with pressure, whereas the current density in the cathode layer for normal mode operation increases quadratically with pressure.
In order to make the conditions in the bulk and at the cathode compatible, the current cross-section at the cathode must be reduced with increasing pressure. This was achieved by using ballasted pins as individual cathodes with cross-sections that are small compared to the area of the cathode segment. The onset of bulk instabilities can be prevented by flowing air with such a speed through the discharge that the plasma is replaced by cold air on a time scale that is small when compared to the inverse of the growth rate of the bulk instabilities.
None of these previous efforts disclose all of the benefits of the present invention, nor does the prior art teach or suggest all of the elements of the present invention.
SUMMARY OF THE INVENTION
None of the known methods for producing a glow discharge produce a high pressure, uniform, stable plasma glow discharge. Specifically, none of the known methods for producing a glow discharge produce a dc driven, high pressure, uniform, stable plasma glow discharge with relatively high electron densities, such as, for example, 10
13
cm
−3
in air. It should be understood that the term “high pressure” refers to pressures that are approximately atmospheric pressures and could range from between slightly less than one atmosphere to several atmospheres.
Research on high pressure glow discharges is motivated by applications such as instantly activated reflectors and absorbers for electromagnetic radiation, surface treatment, thin film deposition, remediation and detoxification of gaseous pollution and gas lasers.
Therefore, a direct current high pressure glow discharger, according to this invention, allows simultaneous generation of relatively high electron densities at relatively low temperaures with
Schoenbach Karl H.
Stark Robert H.
Bruce David V.
Hobden Pamela R.
Kaufman & Canoles
Old Dominion University
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