JP2010536124A - Cathode assembly and method for pulsed plasma generation - Google Patents

Abstract

A cathode assembly and method for pulsed plasma generation is disclosed. The cathode assembly comprises a cathode holder (2) aligned in the longitudinal direction, preferably of the same diameter and connected to a plurality of cathodes (10, 20, 30) of different lengths. The method is characterized by forming an electric arc between the cathode (10, 20, 30) and the anode (4) in the assembly by passing a predetermined amount of DC current. As the arc is created, the current is reduced to a magnitude that is sufficient or slightly larger to sustain the electric arc, thereby reducing the area of the arc attachment to a single cathode. When the area of attachment is reduced, the current is increased to the operating level of the pulse, but the area of attachment does not increase significantly.
[Selection] Figure 2

Description

  The present invention relates to a cathode assembly of a plasma generator and a method for generating plasma, in particular pulsed plasma.

  The generation of a pulsed plasma with a pulse and a relatively short off period presents a unique set of challenges. Currently known plasma generators have several limitations that make it impractical to use them to generate pulsed plasma.

  In general, a plasma generator comprises a cathode and an anode. A plasma-generating gas, typically a noble gas, flows in a channel extending longitudinally through the anode between the cathode and the anode. As the plasma generating gas traverses the plasma channel, the plasma generating gas is heated by an electric arc created between the cathode and anode and converted to plasma. The portion of the plasma channel may be formed by one or more intermediate electrodes.

  Plasma generation occurs in three stages. The first stage, called spark discharge, occurs when an electrical spark is created between the cathode and anode. The second stage, called glow discharge, occurs when positively charged ions formed as a result of the movement of negatively charged electrons in the electric spark strike the cathode. The third stage, called arc discharge, occurs after the portion of the cathode has been sufficiently heated by ion bombardment, which ion bombardment is sufficient to sustain a current between the cathode and anode to heat the plasma generating gas. Begins to emit a large number of electrons. The electric arc heats the plasma generating gas that forms the plasma. Each time a hot plasma is generated, the plasma generating gas must go through all three stages.

  In prior art devices, at start-up, the current passing between the cathode and anode is simply increased to the desired operating level. However, this rapidly increased current is not sustained between the spark discharge phase and the glow discharge phase. The applied operating level current begins to flow between the cathode and anode only if the arcing phase is reached and the cathode begins to emit thermal ions of electrons at a rate sufficient to support such current. Attempting to pass such current through the cathode before the cathode begins to emit hot ions at a sufficiently high rate to maintain a high operating level of current, stress the cathode and compare After a small number of start-ups, the cathode will eventually be destroyed.

  Generation of pulsed plasma requires frequent start-up of a rapid and continuous plasma generator. For example, in skin treatment, a single session treatment with a pulsed plasma may require thousands of pulses, resulting in thousands of activations. Prior art methods of starting the plasma generator are not suitable for pulsed plasma generation because the cathode may be damaged during the session.

  Currently, two types of devices may be used for generating pulses of ionized gas. The apparatus disclosed in Patent Document 1 is an example of the first type. In this type of device, a corona discharge is generated by passing a plasma generating gas, such as nitrogen, through an alternating magnetic field. An alternating magnetic field produces a fast movement of free electrons in the gas. The rapidly moving electrons knock out other electrons from the gas atoms, forming a so-called electron avalanche, which in turn causes a corona discharge. A pulse corona discharge is generated by applying an electric field in pulses. The advantages of this method of generating a pulsed corona discharge among others are (1) the absence of impurities in the flow, and (2) a short start time that allows the generation of a truly pulsed flow. is there. For purposes of this disclosure, a truly pulsed flow refers to a flow that stops completely during the off period of the pulse.

  The disadvantage of the first type of apparatus and method is that the generated corona discharge has a constant maximum temperature of about 2000 ° C. The corona discharge formed in the device is not heated by the electric arc, so it never becomes a hot plasma. Therefore, an apparatus that generates a pulse corona discharge cannot be used for some applications that require temperatures above 2000 ° C. Thus, the application of the first type of device is limited by the nature of the discharge process, which is capable of causing corona discharge but not high temperature plasma.

  A second type of device generates plasma by heating a flow of plasma-generating gas that passes through the plasma channel by an electric arc created between the cathode and the anode that forms the plasma channel. An example of the second type of device is disclosed in US Pat. According to the disclosure of U.S. Pat. No. 6,057,049, a pulsed DC voltage is applied between the anode and cathode when a plasma generating gas, preferably argon, traverses the plasma channel. A predetermined constant bias voltage may or may not be added to the pulsed DC voltage. During the voltage pulse, the number of free electrons in the plasma generating gas increases, resulting in a decrease in the resistance of the plasma and an exponential increase in the current flowing in the plasma. During the off period, the number of free electrons in the plasma product gas decreases, resulting in an increase in plasma resistance and an exponential decrease in the current flowing through the plasma. The current is relatively low during the off period, but the current never disappears completely. This low current, called standby current, is undesirable because no truly pulsed flow is generated. During the off period, a continuous low power plasma flow is maintained. In essence, the device does not generate a pulsed plasma, but generates a continuous plasma flow with power spikes called pulses, thereby simulating the pulsed plasma. Since the off period is substantially longer than the pulse, the device outputs a significant amount of energy during the off period, and therefore the device is effectively utilized for applications that require a truly pulsed plasma flow. I can't. For example, if the device is used for skin treatment, the device may be separated from the skin surface after each pulse so that the skin is not exposed to low power plasma during the off period . This detracts from the usefulness and safety of the device.

  Reducing the current through the plasma to zero between pulses and restarting the device with each plasma pulse is not practical when using the device disclosed in US Pat. Restarting the device after each pulse does not guarantee that the cathode will discharge enough electrons to support the high current through the cathode, and as a result of passing a high current through the cathode. , Resulting in rapid destruction of the cathode. Attempting to pass a high current through the cathode before it begins to emit electrons at a high enough rate to sustain such a current will stress the cathode and eventually destroy the cathode. cause. Alternatively, both the voltage between the cathode and anode and the current passing through the plasma can be increased slowly. This alternative is not practical because the start of the device per pulse is unacceptably long.

  It is due to the configuration of the device that the device disclosed in U.S. Pat. No. 6,057,056 and other devices of this type known at present do not have the ability to generate a truly pulsed plasma stream. When this type of device starts up, there is some electrode corrosion due to sputtering. This corrosion results in electrode materials such as separated metal particles that flow in the plasma. When a continuous plasma flow is used, activation impurities are a relatively unimportant drawback because activation and associated impurities occur only once per treatment. It is therefore possible to wait a few seconds after activation for the electrode particles to exit the device before the device starts the actual treatment. However, when using a pulsed plasma flow, it is not practical to wait for impurities to escape from the device. This is because particles separate from the electrode every pulse.

  When the plasma stream is pre-created, it takes only a few microseconds to increase or decrease the current in the plasma stream. Furthermore, since there is no activation during the treatment, impurities do not enter the plasma flow and the cathode is not stressed. However, maintaining a current through the plasma continuously even at low currents is an optimal device for some applications that require a truly pulsed plasma flow, as described above. That's not true.

  It is mainly due to the nature of the process that occurs at the cathode and anode that it is difficult to produce a truly pulsed plasma stream by heating the plasma-generating gas with an electric arc. In general, and especially for medical applications, it is important to ensure operation without erosion of the anode and cathode when the current increases rapidly. During rapid current increases, the cathode temperature is low and may not be easily controlled during the repetition of subsequent pulses. During the generation of an electric arc between the cathode and anode, the area of arc attachment to the cathode is strongly dependent on the initial temperature of the cathode. When the cathode is cold, the area of attachment is relatively small. After a few pulses, the temperature of the cathode rises, so that during a rapid current increase, the area of attachment expands to the entire surface area of the cathode and then to the cathode holder. Under these circumstances, the cathode fall begins to fluctuate and cathodic corrosion begins. Furthermore, if the area of attachment of the electric arc reaches the cathode holder, the cathode holder begins to melt and introduces unwanted impurities into the plasma stream. For proper cathode functionality, it is necessary to control the exact location and size of the area of the electrical arc attachment to the cathode surface during the rapid current increase in each pulse of the plasma.

  Electric arcs tend to adhere to imperfect surfaces (sometimes called irregularities) of the cathode. In the prior art, such surface imperfections were created by changing the shape of the cylindrical cathode. A typical surface imperfection used in the prior art is cathode tapering. Cathode tapering creates a tip where the arc tends to adhere. Another way to produce imperfections is by cutting the cylindrical cathode diagonally. This method also creates imperfections where the arc tends to adhere. Although these methods control the position of the electric arc attachment between successive plasma flow sessions, as described above, the arc attachment area expands in stages, as opposed to pulsed plasma operation. Because of this, it is not enough to control the size of the area.

  Regardless of these attempts to control the location and size of the arc attachment region, some prior art devices use multiple cathodes for various purposes. For example, in Patent Document 3, a plurality of cathodes are used in plasma-based light bulbs to generate sparks between the cathodes. In Patent Document 4, a plurality of cathodes are divided into three groups. Since three stages of power are distributed between the cathodes, one group is used during the stage of providing a quasi-continuous mode of operation. In U.S. Patent No. 6,057,836, a pair of cathodes is used to sputter metal traces from the cathode using particles separated between the cathodes by a magnetic field. In Patent Document 6, a plurality of cathodes are provided on the rotating drum so that the cathodes can be replaced without breaking the vacuum seal of the vacuum chamber in which discharge occurs. U.S. Patent No. 6,057,031 discloses a water purification system in which a plurality of cathodes are spaced around the anode. This multi-cathode configuration is used to reduce water flow turbulence through the purifier. In U.S. Patent No. 6,057,051, an electrically isolated cathode multi-cathode assembly is used to extend the life of an ion beam device by alternating cathodes involved in electric arc generation. In Patent Document 9, a plurality of parallel cathodes spaced apart from each other are used in a plasma spray device to enlarge the cross section of the plasma flow in order to prevent clogging of the plasma channel by powder particles. In general, prior art documents disclosing multiple cathodes are not related to problems associated with the generation of a pulsed plasma.

US Pat. No. 6,629,974 US Pat. No. 6,475,215 US Pat. No. 1,661,579 US Pat. No. 2,615,137 US Pat. No. 3,566,185 US Pat. No. 4,785,220 US Pat. No. 4,713,170 US Pat. No. 5,089,707 US Pat. No. 5,225,625

  Thus, there is currently a need for a cathode assembly and a method of operating an apparatus that uses the cathode assembly that overcomes the limitations of the prior art for truly pulsed plasma generation.

  A cathode assembly for pulsed plasma generation includes a cathode holder connected to a plurality of cathodes aligned in the longitudinal direction. Preferably, the cathodes in the assembly are clustered as close together as possible. The cathode is preferably made of tungsten containing lanthanum. The cathodes preferably have the same diameter but different lengths. Optimally, the length difference between the two cathodes closest in length is approximately equal to the diameter of the cathode in the assembly, preferably 0.5 mm. A cathode assembly according to an embodiment of the present invention is used in an apparatus that generates a pulsed plasma based on heating of a plasma-generating gas by an electric arc created between one of the cathodes and the anode. In particular, the cathode assembly includes (a) a cathode holder, (b) a plurality of cathode clusters that are longitudinally aligned and connected to the cathode holder, each cathode in physical contact with at least one other cathode. And comprising.

  In a preferred embodiment, in operation, plasma generated gas is passed between the cathode and anode, preferably through the plasma channel. A large number of free electrons are generated by applying a high-frequency, high-amplitude voltage wave between the anode and the cathode. These electrons form a spark discharge. The spark ionizes the plasma generating gas and enters the glow discharge phase. During the glow discharge, positive ions formed due to ionization of gas atoms collide with the cathode, thereby heating the cathode. When the anode end of the cathode reaches the temperature of thermionic electron emission, the plasma-generating gas enters an arc discharge phase and an arc is created between the cathode and anode. The arc adheres to all cathodes in the assembly.

  After the arc is created between the cathode and anode, the current is reduced to a magnitude sufficient to sustain the arc, or to a slightly larger magnitude. This reduces the arc attachment area. The area of attachment is reduced so that the arc adheres to a single cathode. After this low current is maintained for a period of time, the current is raised to the operating level of the pulse. The area of attachment does not increase significantly and electron emission occurs only from a single cathode. After the operating current is maintained for the desired interval, the device enters an off period in which no current and voltage are applied.

  This method of operation avoids the problem of unstable operation associated with prior art methods. If the multi-cathode assembly is operated in this manner, the cathode will not overheat and the area of attachment will not extend to the cathode holder. This ensures a stable operation of the plasma generator. This method of operation further provides certain advantages when used in a cathode assembly having a single cathode.

  A method for generating a pulse of plasma includes: (a) passing a first current through one or more cathodes and anodes; and (b) a second current having a current magnitude less than the first current magnitude. Passing one or more cathodes and anodes; (c) passing a third current having a current magnitude greater than the first current magnitude through the one or more cathodes and anodes; and (d). Stopping a third current through the one or more cathodes and anodes.

It is a figure which shows the basic apparatus for pulse plasma generation. FIG. 3 illustrates a cathode assembly of a preferred embodiment in three dimensions. It is a figure which shows the apparatus which produces | generates the pulse plasma employ | adopted for skin treatment. It is a figure which shows the pattern of the voltage between the anode and cathode for the production | generation of each pulse. It is a figure which shows the pattern of the electric current applied to the cathode for production | generation of each pulse, the plasma production gas in a plasma channel, and an anode. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. FIG. 3 shows a process that occurs in a plasma channel during pulse generation. In the prior art, it shows the temperature of the cathode and the area of the arc attachment in a single cathode assembly after a certain number of pulses generated by a currently known method. FIG. 4 shows the cathode temperature and arc attachment region in a multi-cathode assembly after a certain number of pulses generated by an embodiment of the present invention. Figure 2 is a schematic view of a microscopic field of a single cathode assembly after 500 pulses generated by a prior art method. FIG. 6 is a schematic view of a microscopic field of view of a multi-cathode assembly after 40000 pulses produced by an embodiment of the method of the present invention.

  In an exemplary embodiment, a cathode assembly having a plurality of cathodes is part of a plasma generator. As long as there are at least two, there is no theoretical limit to the number of cathodes in the assembly. FIG. 1 is a schematic longitudinal sectional view of such a device. The cathode holder 2 holds three cathodes 10, 20 and 30 which are aligned with each other in the longitudinal direction. The anode 4 is arranged away from the cathode. In the preferred embodiment, initially each cathode has a flat surface 12, 22 and 32, respectively, at the end closest to the anode 4 (“anode end”). The flat surfaces form edges 14, 24 and 34, respectively. FIG. 2 shows a three-dimensional view of the cathode assembly.

  In terms of geometry, the cathode should be clustered. Clustering means that all cathodes are in longitudinal contact with at least one other cathode, and all of the cathodes are arranged as a single group, with no cathodes separated from the group. The cathodes are preferably clustered as close as possible to each other. However, it is sufficient that each cathode in the assembly is in physical contact with at least one other cathode in the cluster. Theoretically, the cathodes in the assembly may have different diameters. However, in a preferred embodiment, the cathodes 10, 20, 30 preferably have the same diameter, which is 0.5 mm. In some embodiments, at least one cathode in the assembly has a length that is different from the length of at least one other cathode. In a preferred embodiment, all cathodes in the assembly have different lengths. Preferably, the minimum length difference between the two cathodes is approximately equal to the cathode diameter, which in the preferred embodiment of the assembly is 0.5 mm.

  In some embodiments, an apparatus employing a cathode assembly further comprises a plasma channel 6 extending between the cathode 10, 20, 30 and the anode 4 and passing through the anode 4. In some embodiments, the plasma channel is formed by one or more intermediate electrodes. In some embodiments, the anode ends of the cathodes 10, 20, 30 are located in a plasma chamber connected to the plasma channel. The cathode assembly may be used in other devices, such as, for example, the pulsed plasma generator shown in FIG.

  Devices that may employ a cathode assembly, however, are not limited to plasma generators. In some embodiments, the cathode assembly may be used for a light source or as part of a communication device. In general, the cathode assembly may be used in devices that require creating a short-spaced electric arc between the cathode and the anode.

  For the purpose of describing the method of operation, the embodiment of the apparatus shown in FIG. 3 is used. However, it should be noted that the method of operation described below provides the same advantages if used in conjunction with a multi-cathode assembly in other devices. Furthermore, although operating methods may be used in conjunction with single cathode assemblies, it is more effective to use these operating methods with multi-cathode assemblies. The apparatus shown in FIG. 3 includes a cathode assembly having the cathode holder 2 and the cathodes 10, 20 and 30 shown in FIG. The apparatus further comprises an anode 4 and one or more intermediate electrodes 42a-42e that are electrically insulated from the anode 4 and electrically insulated from each other. The plasma channel 6 is formed by the intermediate electrodes 42 a to 42 e and the anode 4. In some embodiments, the intermediate electrode 42a further forms the plasma chamber 8. During operation of the device, plasma generated gas (typically a noble gas such as argon) is taken into the device through opening 72. The plasma generating gas flows along the cathodes 10, 20, 30 into the plasma chamber 8 and then into the plasma channel 6, after which the plasma generating gas passes through the opening in the anode 4. Exit the device.

  In some embodiments, an extension nozzle is attached to the anode end of the device. The extension nozzle forms an extension channel connected to the plasma channel. A tubular insulator element covers the longitudinal portion of the inner surface of the extension channel. Further, in some embodiments, the extension nozzle has one or more oxygen carrying gas inlets.

  A plasma generating device, such as the device shown in FIG. 3, typically includes (1) a voltage applied between the anode 4 and the cathode 10, 20, 30, and (2) a cathode 10, 20, 30, connected to one or more electronic circuits that control the plasma generating gas in the plasma channel 6 and the current passing through the anode 4. The circuit for controlling the current is a known current source. These circuits are used for the generation of each pulse of plasma. Since all the cathodes in the assembly are electrically connected to each other and connected to the same circuit, the cathodes 10, 20, 30 have the same potential, there is no voltage between the individual cathodes, and the anode 4 And between the cathodes 10, 20 and 30. The process of plasma pulse formation is controlled by (1) applying a voltage between the cathode and anode, and (2) controlling the current passing through the plasma generating gas.

  In summary, the plasma generation process includes three stages: (1) spark discharge, (2) glow discharge, and (3) arc discharge. The electric arc in the arc discharge stage heats the plasma generation gas flowing in the plasma channel 6 to form plasma. The generation of each plasma pulse requires the plasma generating gas to go through all three stages. Before the generation of the pulse, the resistance of the plasma generating gas is nearly infinite. A small number of free electrons are present in the plasma product gas due to the ionization of atoms by cosmic rays.

  In order to create a spark discharge, a high-amplitude, high-frequency voltage wave is applied between the anode 4 and the cathodes 10, 20, 30. This voltage wave increases the number of free electrons in the plasma channel 6 between the cathodes 10, 20, 30 and the anode 4. When a sufficient number of free electrons are formed, a DC voltage is applied between the anode 4 and the cathodes 10, 20, 30, and the DC current is applied to the cathodes 10, 20, 30, the plasma generating gas, and the anode 4. And a spark discharge is formed between the cathode 10, 20, 30 and the anode 4.

  After the spark discharge, the resistance of the plasma generating gas decreases and the glow discharge phase begins. During the glow discharge phase, positively charged ions are attracted to the cathode under the influence of the electric field created by the voltage between the cathode 10, 20, 30 and the anode 4. When ions collide with the cathodes 10, 20, and 30, the temperature at the anode end of the cathode rises. When the temperature rises to the temperature of thermionic electron emission, the arc discharge phase begins. Initially, the arc attaches to all cathodes in the assembly. The current passing through the plasma generating gas is then reduced, so that the area of attachment is reduced to an almost minimal area of the attachment that can sustain the arc discharge. Because the area of the arc attachment is small, the area of attachment is limited to a single cathode in the assembly. Therefore, the current required to sustain the arc discharge and depending on the cathode diameter is relatively low. After the current is lowered and held at that level for a period of time, the current is rapidly increased to the operating level of the pulse. The area of arc attachment increases slightly and only a single cathode continues to emit electrons during the remainder of the pulse. Decreasing the arc attachment area so that only a single cathode emits electrons from the controlled area, and then maintaining that small area is crucial to the operation of a truly pulsed plasma device It is.

  More specifically, the following description of the pulsed plasma generation method refers to FIGS. 4A-B, where FIG. 4A shows the voltage applied between the anode 4 and the cathodes 10, 20, 30, and FIG. The current flowing through the plasma from one or more of the cathodes 10, 20, 30 to the anode 4 through the plasma generating gas in the plasma channel 6 is shown. The voltage, current, and time values described below are preferred values for this method when used in conjunction with a three cathode assembly in the pulsed plasma apparatus shown in FIG. When this method is used for other embodiments of the multi-cathode assembly, or when the multi-cathode assembly is used in another device, other values of voltage, current, and time may be preferred. is there.

FIG. 4A shows a graph of the voltage applied between the anode 4 and the cathodes 10, 20, 30. A bias voltage 202 is generated at time t ₀ before generating the plasma pulse. The bias voltage may be 100 to 1000 volts, but is preferably 400 to 500 volts. Between t ₀ and t ₁ , a bias voltage is applied between the anode 4 and the cathode 10, 20, 30 by an electronic circuit. However, the generation of the bias voltage 202 does not generate a current through the plasma generation gas in the plasma channel 6 because the resistance of the plasma generation gas is nearly infinite. In one embodiment, a capacitor is used to sustain the bias voltage. FIG. 5A shows that there is no current flowing in the plasma channel 6 between t ₀ and t _1, and slightly between the cathode 10, 20, 30 and the anode 4 in the plasma channel 6. It shows that there are only a few free electrons.

At time t ₁ , a high frequency, high amplitude voltage wave 204 is applied between the anode 4 and the cathodes 10, 20, 30. The amplitude of the voltage wave is at least 1 kV, but is preferably about 5 kV. In some embodiments, the high frequency, high amplitude voltage wave 204 is attenuated with an exponentially decreasing amplitude, as shown in FIG. 4A. The frequency of the voltage wave is at least 300 kHz, preferably about 500 kHz. The duration of the high voltage, high frequency wave is at least two wavelengths. For example, the duration of a wave having a frequency of 500 kHz is at least 0.4 microseconds, but longer waves of 15 to 20 microseconds are preferred. Note that the high frequency, high amplitude voltage wave 204 is the only voltage controlled part of the pulsed plasma generation. During the rest of the pulse, the voltage is simply between the anode 4 and the cathode 10, 20, 30 as a result of the current passing through the plasma generating gas between the cathode 10, 20, 30 and the anode 4. Maintained.

  The high-frequency, high-amplitude voltage wave 204 creates a rapid alternating movement of free electrons in the plasma product gas inside the plasma channel 6. The rapidly moving free electrons knock out electrons from the atoms of the plasma generation gas flowing in the plasma channel 6. This process is known as electronic avalanche. As a result of the avalanche, the amount of free electrons reaches a number sufficient to create a spark discharge between the cathodes 10, 20, 30 and the anode 4, as shown in FIG. 5B.

  In an embodiment having a plasma channel 6 formed by one or more intermediate electrodes, such as the embodiment shown in FIG. 3, the spark is first between the cathode and the intermediate electrode 42a closest to the cathode. Produced. Other sparks are created between the free electrons in the plasma generation gas flowing through the plasma channel 6 and the other intermediate electrodes 42 b to 42 e forming the plasma channel 6. Eventually, a spark discharge is created between the cathodes 10, 20, 30 and the anode 4 shown in FIG. 5C.

  Spark discharge ionizes a certain number of atoms in the plasma generated gas, thus increasing the conductivity of the plasma generated gas and reducing the resistance of the plasma generated gas, preferably to 200-1000Ω. Free electrons created as a result of ionization are confined in the relatively small volume 302 shown in FIG. 5C.

After the high frequency, high amplitude voltage wave 204 is terminated at time t ₂ , a voltage 206 falling between 100 and 1000 volts, preferably about 400 to 500 volts is applied between the anode 4 and the cathode 10, 20, 30. To be applied. In some embodiments, the voltage applied at time t ₂ is equal to the bias voltage 202 of the high frequency, high amplitude voltage wave 204. In some embodiments, voltage 206 decreases exponentially with time, as shown in FIG. 4A.

Once t _2, the plasma generation gas has a sufficient free electrons to conduct electricity. However, the cathodes 10, 20, 30 are thermionic electron emission that allows for a sustainable electric arc that maintains the generation of a plasma stream with the properties required for a particular application, eg, skin treatment. Is not heated enough to realize. The discharge voltage 206 starts the glow discharge phase. As the cathode 10, 20, 30 begins to emit electrons thermionicly, the cathode surfaces 12, 22 and 32 reach a temperature inherent to the cathode material called the thermionic electron emission temperature, or the temperature of thermionic electron emission. Must. For example, for a tungsten cathode containing lanthanum, such as the cathode used in the preferred embodiment, the temperature of electron emission is about 2800-3200K. Under the influence of the electric field created by the voltage between the anode 4 and the cathode 10, 20, 30, free electrons present in the plasma channel 6 are attracted toward the anode 4, and ions are attracted to the cathode 10, 20, Attracted toward 30. The glow discharge shown in FIG. 5D is a self-sustained discharge that uses a cold cathode that emits electrons mostly due to ion bombardment due to secondary emission. The salient features of this discharge are, in the preferred embodiment, a positive space charge layer at the cathode with a strong electric field at the surface and a significant potential drop of 100 to 400 volts. This drop is known as cathode fall. If the current is increased, the glow discharge goes to an arc discharge at a certain level, by which time it has reached a sufficient surface temperature to emit electrons to hot ions.

At time t ₃ , when the voltage between the anode 4 and the cathodes 10, 20, 30 decreases to a predetermined value, the cathodes 10, 20, 30, the plasma generating gas in the plasma channel 6, the anode 4, The current passing through increases from 0A to a predetermined first current, preferably in the range of 4-6A. Preferably, this current is maintained for 1 to 10 milliseconds. When the current starts to increase, the predetermined voltage, but to no e ^-0.5 voltage at time t ₂ is ^-1.5 times e, preferably, about e ^-1 times the voltage at time t ₂ (Note that e is the base of the natural logarithm and is approximately equal to 2.718). For example, in one embodiment, voltage applied between at time t ₂ and the anode 4 and the cathode 10,20,30 is about 400 volts. As the voltage drops to about 150 volts, the current through the plasma generating gas is increased to about 5A. In some embodiments, the current increase is a lamp 208 with a 300 to 500 microsecond interval between t ₃ and t ₄ .

At some point after t ₄ , the cathode begins to emit electrons from the cathode surfaces 12, 22 and 32 as shown in FIG. 5E. The electron emission at this point is sufficient to sustain the electric arc required to produce a plasma with the desired characteristics. At this point, the arcing phase begins and an arc is created along the plasma channel 6 between the cathodes 10, 20, 30 and the anode 4. The resistance of the plasma in the flow is about 1 to 3Ω. At this point, theoretically, the current can be increased to the required operating level for a particular application as shown in FIG. 5F. However, increasing the current to the operating level at this point causes the following undesirable effects: As shown in FIGS. 5D-F, all cathodes in the assembly are involved in the glow discharge phase and then the arc discharge phase. The body of the cathode 10, 20, 30 continues to be bombarded by positively charged ions during the glow discharge phase, and the arc adheres to the surface area of the entire cathode during the arc discharge phase. During the off-period between pulses, the temperature of the cathode 10, 20, 30 does not drop to the original non-operating level, so if the cathode is still heated by the previous pulse, the glow discharge phase and arc discharge Stages occur. The area of the plasma attachment is increased because the larger part of the cathode becomes sufficiently heated to emit electrons with each pulse. At some point after about 300 to 500 pulses, the plasma will adhere to the entire surface area of the cathode and will begin to adhere to the cathode holder 2 as well.

  As the arc adheres to the cathode holder 2, the cathode holder is heated to the point where it begins to sputter and emit electrons with the electrode material. This will introduce impurities into the plasma stream and is unacceptable for some applications, particularly medical applications. Furthermore, a cathode holder having a melting point sufficiently lower than that of the cathode starts to melt. When the portions of the cathode holder that are in contact with one or more cathodes begin to melt, these cathodes are damaged. This damage causes imperfections where the electric arc can adhere during subsequent pulses. Attachment of the arc to this imperfection at the base of one or more cathodes can cause an electric arc that terminates outside the plasma channel. As a result, it becomes impossible to control whether a plasma is formed in the plasma channel. Furthermore, the uncontrolled surface of the attachment causes fluctuations in the cathode potential. In general, the expansion of the uncontrollable arc attachment area causes unstable operation of the apparatus.

  It has been found that extending the length of the cathode and thus moving the cathode holder 2 away from the anode end of the cathode 10, 20, 30 where the arc first deposits is not an optimal solution. Several experiments have shown that extending the cathode does not remove the undesirable process described above, but only slightly delays.

According to a preferred method, at time t ₅ , the current is reduced to a second current. In some embodiments, the current reduction is a lamp 209 with an interval of 300 to 500 microseconds. The current is preferably reduced to a level between the minimum current required to sustain the arc discharge and approximately three times this minimum current. For some embodiments, this current is in the range of 0.33 to 1.0 A. Preferably, the second current is maintained for 5 to 20 milliseconds. The current drop results in a reduction in the cross section of the electric arc between the cathodes 10, 20, 30 and the anode 4, as well as a reduced area of the arc attachment. Although it is not necessary to reduce the attachment area to the minimum required to sustain the arc, reducing the current reduces the area of the attachment to a size that does not significantly exceed the minimum area. As shown in FIG. 5G, the arc does not adhere to the entire surface area of the cathode. Indeed, to sustain the electric arc, the emitted electrons are concentrated in a relatively small volume and emitted from a small area, as shown in FIG. 5G. The ionic current heating the cathode remains strong enough to sustain thermionic electron emission from the cathode due to the high current density flux through the small area of the attachment. This ionic current causes very high temperatures in the area of the arc attachment and the surrounding volume. Reducing the current applied to the cathodes 10, 20, 30 and the plasma generating gas and the anode 4 in this way ensures that the arc adheres only to a single cathode and that the arc attachment is relatively Guarantees that it is constrained to a small area.

It has been experimentally found that the cathode diameter has the most significant effect on the minimum sustainable current that is passed through the cathode while still maintaining the electric arc between the cathode and anode. For example, the minimum current for a 1.0 mm diameter and 5 mm long cathode is about 1A. The minimum current for a 0.5 mm diameter and 5 mm long cathode is about 0.5A. The minimum current for a 0.5 mm diameter and 35 mm long cathode is about 0.3A. During the second reduced current period between t ₆ and t ₇ , the plasma adheres to only one cathode, so that, for example, between t ₄ and t ₅ , an electric arc is generated in the assembly. When attached to all of the cathodes, it is possible to sustain the electric arc using a relatively small current compared to the current required to sustain the arc. Referring to the preferred embodiment of the cathode assembly, the diameter of a single cathode in the assembly is about half of the total diameter of all cathodes in the assembly so that the arc adheres to a single cathode. The current required to sustain the arc is approximately half that required when the arc is deposited on all three cathodes.

Once t _7, the current third current, that is, the operation level required for a particular application, preferably 10 to be increased to a current in the range of 80A. In some embodiments, the current increase is a lamp 211 with an interval between 300 and 500 milliseconds between t ₇ and t ₈ . The rate of increase is 1000 to 10,000 A per second. Up to time t ₈ , an operating voltage, preferably in the range of 30 to 90 volts, is the result of the device geometry and the current passing between one of the cathodes 10, 20, 30 and the anode 4, It is maintained between the anode 4 and the cathode 10, 20, 30.

At time t ₈ , the current reaches the operating level and the fully grown plasma flow is maintained at operating current level 214 and operating voltage level 216, which are preferably 10 to 80 A and 30 to 90 volts, respectively. These operational levels are maintained over a desired interval for a particular application. For example, for skin treatment, it is preferred spacing t ₇ -t ₈ from 5 to 100 milliseconds. FIG. 5H shows an electric arc that sustains a fully grown plasma flow between the cathode 10 and the anode 4, one of the cathodes. During the operating period of the pulse, the electric arc has a cross section that is not significantly larger than the cross section of the arc during the period t ₆ -t ₇ when the second current is passed.

At t _9, when the plasma flow has been lasted over a desired interval, the current flowing through the plasma generation gas in the plasma channel 6 is switched off, as a result, the anode 4 and the cathode 10, 20 , 30 is stopped and the device enters the off period shown in FIG. 5I until the next plasma pulse is generated.

The use of the method described above avoids escalating the arc attachment area as described above. The glow discharge that occurs from t ₂ to t ₄ where the plasma can adhere to the entire exposed surface area of the cathode lasts up to 10 milliseconds in the preferred embodiment. The temperature increase obtained during the glow discharge is lost between the rest of the pulse and the off period. As a result, the cathode is cooled by the time a new pulse is to be generated. FIG. 6A schematically illustrates the temperature and area of the single cathode assembly attachment for a series of pulses generated by a prior art method. The upper graph represents current as a function of time. The middle graph represents the cathode temperature as a function of time. The lower graph represents the area of arc attachment to the cathode assembly as a function of time. Although FIG. 6A represents only four pulses for purposes of illustration, the actual process may occur over a period of about 300 to 500 pulses. Thus, for example, the first illustrated pulse may be the first actual pulse, the second illustrated pulse may be the 150th actual pulse, and the third illustrated pulse may be the 300th actual pulse. The fourth illustrated pulse may be the 450th actual pulse. During the first illustrated pulse, the cathode is cold and the arc adheres to a small area of the cathode surface. However, the current passing through the cathode during the first illustrated pulse raises the temperature of the cathode. The cathode temperature drops slightly before the next pulse, but does not drop to the cathode's original non-operating temperature. During the second illustrated pulse, the area of the arc attachment does not increase, but the cathode temperature rises even further. After the second illustrated pulse, the temperature drops slightly, but does not drop to the temperature of the cathode before the second pulse. During the third illustrated pulse, the temperature rises further and exceeds the critical temperature T _0, and the entire cathode body is capable of emitting hot ions when the critical temperature is exceeded. After the cathode temperature exceeds T ₀ , the area of attachment increases rapidly with each subsequent pulse. As shown in FIG. 6A, by the fourth illustrated pulse, the area of arc attachment covers the entire cathode surface.

FIG. 6B schematically illustrates the temperature and attachment regions of a preferred embodiment of a multi-cathode assembly for a series of pulses generated by an embodiment of the present invention. The current pulses are shown in FIG. 4B and correspond to the pulses described above. The illustrated pulse corresponds to an actual pulse as in the case of FIG. 6A. As described above, at each pulse of current, after the arc is started, the arc attaches to all cathodes in the assembly. The current then decreases to reduce the attachment area to only a single cathode, and only thereafter the current is increased to the operating level. Since the arc adheres to a small area for substantially the entire pulse interval, the entire body of the cathode is not significantly heated. During the off period, the cathode cools rapidly because most of the cathode assembly cools significantly during the pulse. As shown in FIG. 6B, after the first illustrated pulse, the cathode temperature drops to the non-operating temperature before the next actual pulse. Thus, when the next actual current pulse begins, the cathode in the assembly has an initial non-operating temperature. During the off period following the pulse, the cathode temperature drops again to the original non-operating level. Since the cathode temperature never exceeds T ₀ , the attachment area does not increase and remains approximately the same for tens of thousands of pulses, as shown in the lower graph of FIG. 6B.

FIG. 7A is a schematic view of a microscope field of a single cathode assembly after 500 pulses generated by a prior art method. Region 350 is the region of electrical arc attachment during the last pulse of a 500 pulse session. Cathode holder 352 melts and region 350 contains the entire cathode. Microscopic examination of the cathode revealed that the attachment region was severely corroded due to cathode temperature instability resulting from operating methods that did not take into account control of the attachment region. FIG. 7B is a schematic representation of a microscopic field of view of a multi-cathode assembly after 40000 pulses generated by an embodiment of the method of the present invention. Region 360 is the region of attachment in the last pulse of a 40,000 pulse session. As can be seen from FIG. 7B, the cathode holder and the longitudinal portion of the cathode closest to the holder are unaffected because the arc never adheres. Similarly, the portion of the cathode covered by the area of attachment will have little effect by the arc, as shown in FIG. 5F, as the arc will only adhere to that area between t ₄ and t _5. After t ₅ , the area of the attachment is reduced to a small area in contact with one of the cathodes so that the rest of the cathode is not affected by the arc.

  For the cathode assembly shown in FIG. 2, it has been experimentally found that the arc attaches to the shortest cathode 10 during the first thousands of pulses. During these pulses, the anode end of the cathode 10 is heated significantly. As a result, some melting occurs at the anode end of the cathode 10. The cathode 10 loses a well-defined surface imperfection at the edge 14. If the surface imperfection is not very well defined, the arc will begin to attach to the second shortest cathode 20 with the anode end still having a well defined edge 24. After thousands of pulses, the end of the cathode 20 loses a well-defined edge 24. Thereafter, the arc begins to attach to the next short cathode, ie, cathode 30. After thousands of pulses, the end of the cathode 30 loses the well-defined edge 34 of the cathode 30 as well. In embodiments of the cathode assembly with four or more cathodes, the arcs are attached to different cathodes in order of increasing length. After the arc has adhered to the longest cathode, all the cathode ends closest to the anode are clearly melted at the cathode end due to some melting because of the heat absorbed by the anode end of the longest cathode. Lose defined edges.

  Once this happens, the arc begins to adhere to the shortest cathode again. The arc attaches to the cathode 10 during thousands of pulses until the anode further loses the definition of the edge 14. At this point, the arc begins to attach to the second shortest cathode, ie, cathode 20, having an anode end that includes a well-defined edge 22 rather than edge 12. In thousands of pulses, the arc attaches to the next short cathode, etc.

  For the cathode assembly shown in FIG. 2, experiments have shown that an arc attaches to cathode 10 during about 10,000 pulses, after which an arc attaches to cathode 20 during the next about 10,000 pulses. After that, it was found to adhere to the cathode 30 during the next approximately 10,000 pulses. The arc then reattaches to the cathode 10 etc. during the next approximately 10,000 pulses. The cathode assembly shown in FIG. 2 has been shown to function in this manner for a session of 60000 pulses, which is sufficient for most pulsed plasma applications.

  Although the above method has been shown to give the best results when used with a multi-cathode assembly, the use of this method is equally beneficial for a single cathode assembly.

  The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described for best explanation of the principles of the invention and practical applications of the invention, thereby enabling those skilled in the art to understand the invention. Various embodiments and modifications suitable for a particular use are contemplated. It is intended that the scope of the invention be defined by the appended claims and their equivalents.

Claims (14)

a. A cathode holder;
b. A plurality of cathodes that are longitudinally aligned and connected to the cathode holder as a cluster, each cathode in physical contact with at least one other cathode;
A cathode assembly comprising:   The cathode assembly of claim 1, wherein the cathodes are electrically connected to each other.   The cathode assembly of claim 1, wherein at least one of the cathodes has a length that is different from a length of at least one other cathode.   The cathode assembly of claim 3, wherein all of the cathodes have different lengths.   The cathode assembly of claim 4, wherein each of the plurality of cathodes has substantially the same diameter.   The cathode assembly of claim 5, wherein the minimum difference in length between the pair of cathodes is equal to the diameter of the cathode.   The cathode assembly according to claim 5, wherein the cathode has a diameter of 0.5 mm. A method for generating a pulse of plasma in an apparatus comprising an anode and a cathode assembly including a cathode holder connected to one or more cathodes, comprising:
a. Passing a first current through the one or more cathodes and the anode;
b. Passing a second current having a magnitude of current less than the magnitude of the first current through the one or more cathodes and the anode;
c. Passing a third current having a magnitude of current greater than the magnitude of the first current through the one or more cathodes and the anode;
d. Turning off the third current through the one or more cathodes and the anode;
A method comprising:   The method of claim 8, wherein the second current is passed through one cathode and the third current is passed through the same cathode.   The method of claim 9, further comprising applying an alternating voltage between the anode and the one or more cathodes prior to passing the first current.   The magnitude of the second current is between 1 and 3 times the minimum current required to sustain an electric arc between the cathode and the anode. Method.   The method of claim 11, wherein the magnitude of the second current is 0.33 to 1.0 A.   The method of claim 12, wherein the magnitude of the first current is 4.0 to 6.0 A.   The method of claim 13, wherein the third current magnitude is between 10 and 80 A.

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