Patent Publication Number: US-2023143330-A1

Title: Plasma generator

Description:
TECHNICAL FIELD 
     The present invention relates to a plasma generator, and particularly to a dielectric barrier discharge type plasma generator capable of generating plasma in substantially atmospheric pressure. 
     BACKGROUND ART 
     Conventionally, in order to suppress emission of exhaust gas discharged from a diesel engine or the like into the atmosphere in a state where particulate matter (PM) such as soot is contained, an exhaust gas treatment apparatus including a plasma generator is provided in a flow path of the exhaust gas (see, for example, Patent Literature 1). Plasma is generated in such a flow path of the exhaust gas, and the PM is decomposed into carbon dioxide and the like by bringing the PM into contact with the plasma. 
     In many plasma generators, plasma is generated in a plasma generation chamber (vacuum container) close to vacuum, but since the pressure in the flow path of the exhaust gas is higher than vacuum and close to atmospheric pressure, an apparatus capable of generating plasma in substantially atmospheric pressure is used as the plasma generator used in the exhaust gas treatment apparatus. One such apparatus is a dielectric barrier discharge type plasma generator for generating plasma using dielectric barrier discharge. 
     In a dielectric barrier discharge type plasma generator, a side of at least one electrode of a pair of electrodes is coated with an insulating material, the side facing the other electrode. When an AC voltage having a frequency in a range of several tens Hz to 100 kHz and an amplitude in a range of 500 V to 10 kV is applied between adjacent electrodes in a state where the pressure between these electrodes is set to approximately atmospheric pressure, discharge occurs between the adjacent electrodes when the absolute value of the potential difference between the adjacent electrodes exceeds a threshold within one cycle of AC. By this discharge, charges attach themselves on the insulating material, and a potential difference between the insulating materials of both electrodes decreases, and the discharge stops. When the absolute value of the potential difference between the adjacent electrodes increases within the one cycle from that state, discharge occurs again, but charges further attach themselves on the insulating material, the potential difference between the insulating materials of both electrodes decreases, and the discharge stops again. As described above, pulsed discharge occurs at a repetition frequency higher than the frequency of the AC voltage while the absolute value of the voltage between the electrodes increases within one cycle of the AC voltage. 
     One of a pair of electrodes constituting such a dielectric barrier discharge type plasma generator is disposed in a gas flow path of an exhaust gas treatment apparatus, and the other is disposed as a conductive wall constituting the gas flow path. As a result, discharge occurs in the gas flow path, which is a space between adjacent electrodes, and gas flowing in the gas flow path is ionized to generate plasma. Then, when the PM comes into contact with the plasma, the PM is decomposed. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2018-071403 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the plasma generator described in Patent Literature 1, the electrodes are connected to an AC power supply through AC wires or to ground through ground wires. A cable in which a flexible metal wire is covered with a flexible covering material is usually used for the AC wires in order to facilitate handling. In such a cable, the covering material deteriorates over time during long-term use. When the electrode in the gas flow path or the electrode that is the wall of the gas flow path receives vibration from the flow of the gas in the gas flow path, the vibration is also transmitted to the cable via the electrode connected thereto. When the cable in which the covering material is deteriorated over time comes into contact with or comes close to a member other than the electrode to which the cable is connected due to the vibration, electric leakage or undesirable discharge (other than discharge for generating plasma) may occur. 
     Here, the exhaust gas treatment apparatus for decomposing the PM in the exhaust gas discharged from the diesel engine or the like has been described as an example, but the same problem also occurs in a dielectric barrier discharge type plasma generator provided in a gas treatment apparatus for performing various treatments on a gas flowing in a gas flow path by ionizing the gas to generate plasma. 
     An object of the present invention is to provide a dielectric barrier discharge type plasma generator that is provided in a gas treatment apparatus and can prevent electric leakage and undesired discharge from occurring. 
     Solution to Problem 
     The present invention made to solve the above problems is a plasma generator provided in a gas treatment apparatus for generating plasma by ionizing gas flowing in a gas flow path, the plasma generator including: 
     a) an AC power supply; 
     b) a power supply electrode and a ground electrode, one of which is disposed in the gas flow path and the other of which is a conductive wall constituting the gas flow path; 
     c) an inflexible connection member configured to electrically connect the AC power supply and the power supply electrode; and 
     d) an insulating material covering a side of one of the power supply electrode and the ground electrode, the side facing the other electrode. 
     In the plasma generator according to the present invention, an inflexible connection member is used to electrically connect the AC power supply and the power supply electrode. The term “inflexible” as used herein means that it is not easily deformed, and more specifically means that it vibrates within an elastic range even when vibration is applied, and the original installation state is maintained. In other words, if it is initially installed so as not to come into contact with another member or the like, a state in which it does not come into contact with another member is maintained even if it receives vibration or the like for a long period of time. Therefore, even if vibration is transmitted from the gas flowing in the gas flow path to the connection member via the power supply electrode (electrode disposed in gas flow path, or electrode that is conductive wall constituting gas flow path), the connection member does not unexpectedly come into contact with or does not come close to a member other than the power supply electrode in the plasma generator, so that it is possible to prevent electric leakage and undesirable discharge from occurring. 
     In the plasma generator according to the present invention, it is not necessary to cover the connection member with the covering material in order to prevent electric leakage and undesired discharge by using the inflexible connection member as described above. On the other hand, the connection member may be covered with a covering material in consideration of safety at the time of inspection or the like. Alternatively, a protective cover may be provided separately from the connection member to cover the connection member. 
     The insulating material may be provided only on one of the power supply electrode and the ground electrode, or may be provided on both of them. 
     In order to ground the ground electrode, an inflexible connection member similar to the connection member may be used. 
     As the AC power supply, similarly to a conventional dielectric barrier discharge type plasma generator, one for generating an AC voltage having a frequency in a range of several tens of Hz (including 50 Hz and 60 Hz that are commercial frequencies in Japan) to 100 kHz and an amplitude in a range of 500 V to 10 kV can be used. 
     The plasma generator according to the present invention may further include a power measurement unit configured to measure AC power output from the AC power supply, and a voltage control unit configured to control an AC voltage of the AC power according to the AC power measured by the power measurement unit. As a result, when the AC power changes due to a change in the density, component, or the like of the gas between the power supply electrode and the ground electrode, the AC power can be controlled to be within a predetermined range. 
     The plasma generator according to the present invention may further include an electric current waveform acquisition unit configured to acquire the waveform of the AC electric current output from the AC power supply, a pulse electric current detection unit configured to detect a pulse electric current due to discharge from the waveform of the AC electric current measured by the electric current waveform acquisition unit, and a second voltage control unit configured to control the AC voltage of the AC power according to a pulse repetition frequency of the pulse electric current detected by the pulse electric current detection unit. As a result, when the pulse repetition frequency changes due to a change in the density, component, or the like of the gas between the power supply electrode and the ground electrode, the pulse repetition frequency can be controlled to be within a predetermined range. 
     In the plasma generator according to the present invention, it may be possible to employ a configuration of including a plurality of sets of the combination of the power supply electrode and the ground electrode, in which a common connection member is connected to each of the power supply electrodes. According to this configuration, plasma can be simultaneously generated between a plurality of sets of the power supply electrode and the ground electrode, so that the processing capability of the gas can be increased. 
     In the case where a plurality of sets of the combination of the power supply electrode and the ground electrode are provided as described above, it may be possible to employ a configuration in which one of the power supply electrode and the ground electrode is a linear tubular electrode, and the plasma generator further includes a connection flow path configured to connect two of a plurality of the tubular electrodes. This makes it possible to lengthen the flow path of the gas while suppressing the size of the tubular electrode in the longitudinal direction, so that the gas treatment can be performed more reliably. 
     In the plasma generator according to the present invention, it may be possible to employ a configuration in which a plurality of the power supply electrodes and a plurality of the ground electrodes are alternately arranged, and a common connection member is connected to each of the power supply electrodes. As a result, plasma is generated between the power supply electrode and the ground electrode adjacent to each other, and plasma can be simultaneously generated between adjacent electrodes of the plurality of sets, so that the processing capability of the gas can be increased. In each power supply electrode, plasma is generated between the ground electrodes on both sides (that is, two ground electrodes). 
     In the case where a plurality of the power supply electrodes and a plurality of the ground electrodes are alternately arranged, it may be possible to employ a configuration in which the power supply electrodes and the ground electrodes are flat plate electrodes, and the plasma generator further includes a connection flow path configured to connect adjacent gas flow paths each formed between one of the power supply electrode and the ground electrode and the other of the power supply electrode and the ground electrode. This makes it possible to lengthen the gas flow path while suppressing the size of the flat plate electrode in the direction parallel to the plate, so that the gas treatment can be performed more reliably. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to prevent electric leakage and undesired discharge from occurring in a plasma generator provided in a gas treatment apparatus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic view illustrating a first embodiment of a plasma generator according to the present invention. 
         FIG.  2    is a schematic view illustrating a modification of the plasma generator of the first embodiment. 
         FIG.  3    is a schematic view illustrating another modification of the plasma generator of the first embodiment. 
         FIG.  4    is a cross-sectional view taken along line A-A illustrating a second embodiment of the plasma generator according to the present invention. 
         FIG.  5    is a cross-sectional view taken along line B-B of the plasma generator of the second embodiment. 
         FIG.  6    is a cross-sectional view taken along line A-A illustrating a modification of the plasma generator of the second embodiment. 
         FIG.  7    is a cross-sectional view taken along line A-A illustrating a third embodiment of the plasma generator according to the present invention. 
         FIG.  8    is a cross-sectional view taken along line B-B of the plasma generator of the third embodiment. 
         FIG.  9    is a cross-sectional view taken along line A-A illustrating a modification of the plasma generator of the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a plasma generator according to the present invention will be described with reference to  FIGS.  1  to  9   . 
     (1) Plasma Generator of First Embodiment 
     (1-1) Configuration of Plasma Generator of First Embodiment 
       FIG.  1    illustrates a schematic configuration of a plasma generator  10  of a first embodiment. The plasma generator  10  of the first embodiment is provided in a gas treatment apparatus, and includes a tube serving as a flow path of a gas which is to be treated (gas to be treated). A tube wall of the tube is made of a conductor and is grounded. This tube wall corresponds to a ground electrode  112  of the plasma generator  10 . A power supply electrode  111  is disposed in a tube of the ground electrode  112 , that is, in a gas flow path. In the present embodiment, the tube of the ground electrode  112  is a cylinder, and the power supply electrode  111  is a cylindrical conductor disposed at the center of the cylinder. One end (left side in  FIG.  1   ) of the power supply electrode  111  extends to one end (left side) of the tube of the ground electrode  112 , and the other end (right side) extends to the outside of the other end (right side) of the tube of the ground electrode  112 . 
     On the side face of the cylinder of the power supply electrode  111 , a power supply side insulating material  121  made of an insulator (dielectric) is provided so as to cover the entire side face. On the inner face of the tube of the ground electrode  112 , a ground side insulating material  122  made of an insulator (dielectric) is provided so as to cover the entire inner face. In the present embodiment, the power supply side insulating material  121  and the ground side insulating material  122  are provided, but only one of them may be provided. 
     One end (lower side in  FIG.  1   ) of a connection member  13  that is a conductor and is a rod made of an inflexible material is connected to a portion of the power supply electrode  111  extending to the outside of the tube of the ground electrode  112 . The plasma generator  10  includes an AC power supply  14 , and the other end (upper side) of the connection member  13  is connected to one electrode  141  of the AC power supply  14 . The connection member  13  is not covered with a covering material, and is not in contact with members other than the power supply electrode  111  and the electrode  141  of the AC power supply  14 . 
     An other electrode  142  of the AC power supply  14  is formed so as to cover the periphery of the tube of the ground electrode  112 , and is grounded together with the ground electrode  112 . As the AC power supply  14 , one having a frequency in a range of several tens Hz to 100 kHz and an output voltage of 500 V to 10 kV is used. A Japanese commercial power supply (having a frequency of 50 Hz or 60 Hz and a voltage of 100 V or 200 V) may be used for the AC power supply  14 . 
     For example, copper or stainless steel can be used as a material of each of the power supply electrode  111 , the ground electrode  112 , and the connection member  13 . 
     On the outer side of the connection member  13 , a protective cover  16  made of a plate material of an insulator (dielectric) is provided so as to be separated from the connection member  13  and cover the connection member  13 . When there is no possibility that a person touches the connection member  13  while the connection member  13  is energized at the time of inspection or the like, the protective cover  16  may be omitted. In addition, instead of providing the protective cover  16 , the connection member  13  may be covered with a covering material. 
     The other end of the tube of the ground electrode  112  is provided with a feedthrough  17  for airtightly closing an opening at the other end of the tube while allowing the power supply electrode  111  to pass therethrough. An opening is provided in the tube wall of the tube of the ground electrode  112  in front of the other end, and this opening serves as a gas discharge port  182 . The opening at the one end of the tube of the ground electrode  112  serves as a gas introduction port  181 . 
     (1-2) Operation of Plasma Generator of First Embodiment 
     The operation of the plasma generator  10  of the first embodiment will be described. A gas to be treated (for example, exhaust gas discharged from a diesel engine) is introduced from the gas introduction port  181  into a pipe of the ground electrode  112  serving as a gas flow path. At the same time, an AC voltage is applied between the power supply electrode  111  and the ground electrode  112  by the AC power supply  14 . As a result, similarly to a conventional dielectric barrier discharge type plasma generator, pulsed discharge occurs at a repetition frequency higher than the frequency of the AC voltage while the absolute value of the voltage between the electrodes increases within one cycle of the AC voltage. By the pulsed discharge, the gas to be treated flowing in the tube of the ground electrode  112  is ionized to generate plasma, and the content of the decomposition target such as PM in contact with the plasma is decomposed. The gas to be treated that has been treated with plasma in this manner is discharged from the gas discharge port  182 . 
     When the gas to be treated thus treated flows in the tube of the ground electrode  112 , the power supply electrode  111  in the gas flow path receives vibration from the flow of the gas to be treated. This vibration is transmitted from the power supply electrode  111  to the connection member  13 . 
     In a plasma generator provided in a conventional gas treatment apparatus, since a power supply electrode and an AC power supply are connected by a cable in which a flexible metal wire is coated with a flexible coating material, there is a possibility that a cable in which the coating material is deteriorated over time comes into contact with or comes close to a member other than an electrode in the plasma generator due to vibration received from the power supply electrode, and electric leakage or undesirable discharge occurs. On the other hand, in the plasma generator  10  of the present embodiment, since the power supply electrode  111  and the AC power supply  14  are electrically connected by the inflexible connection member  13 , the connection member  13  does not come into contact with or does not come close to a member other than the electrode in the plasma generator  10  even when receiving vibration from the power supply electrode  111 , and it is possible to prevent electric leakage and undesirable discharge from occurring. 
     (1-3) Modification of Plasma Generator of First Embodiment 
       FIG.  2    illustrates a schematic configuration of a plasma generator  10 A of a modification of the first embodiment. The plasma generator  10 A is obtained by additionally installing a power measurement unit  191  and a voltage control unit  192  in the plasma generator  10  of the first embodiment. 
     The power measurement unit  191  has an electric current input terminal  1911  and a voltage input terminal  1912 . The connection member  13  and the one electrode  141  of the AC power supply  14  are connected to the electric current input terminal  1911 . Two cables electrically connected to the connection member  13  and the ground electrode  142  are connected to the voltage input terminal  1912 . The electric current flowing through these two cables is sufficiently smaller than the electric current flowing through the connection member  13 . The power measurement unit  191  obtains power on the basis of an electric signal indicating the magnitude of the electric current input from the electric current input terminal  1911  and the amplitude of the voltage input from the voltage input terminal  1912 , and outputs an electric signal corresponding to the obtained power from an output terminal  1913 . The output terminal  1913  is connected to the voltage control unit  192 . The voltage control unit  192  controls a voltage output from the AC power supply  14  as described later according to an output signal from the power measurement unit  191 . 
     The plasma generator  10 A of the modification generates plasma in the tube of the ground electrode  112  by the same operation as the plasma generator  10  of the first embodiment. While the plasma is generated, the power measurement unit  191  measures the power output from the AC power supply  14  as needed, and transmits an output signal indicating the measurement result to the voltage control unit  192 . Based on the signal input from the power measurement unit  191 , the voltage control unit  192  transmits a signal of an instruction to lower the voltage to the AC power supply  14  when the value of the power output from the AC power supply  14  exceeds a predetermined range, and transmits a signal of an instruction to increase the AC voltage to the AC power supply  14  when the value of the power is below the predetermined range. As a result, even if the AC power output from the AC power supply  14  changes due to a change in the density, component, or the like of the gas between the power supply electrode  111  and the ground electrode  112 , the AC power can be controlled to be within a predetermined range. 
       FIG.  3    illustrates a schematic configuration of a plasma generator  10 B as another modification of the first embodiment. The plasma generator  10 B is obtained by additionally installing an electric current waveform acquisition unit  193 , a pulse electric current detection unit  194 , and a second voltage control unit  195  in the plasma generator  10  of the first embodiment. 
     The electric current waveform acquisition unit  193  is provided with an electric current input terminal  1931  and an output terminal  1932 , acquires a waveform of an AC electric current input from the electric current input terminal  1931 , converts the waveform into an electric signal indicating a magnitude of the electric current, and outputs the electric signal from the output terminal  1932 . The connection member  13  and the one electrode  141  of the AC power supply  14  are connected to the electric current input terminal  1931 . The pulse electric current detection unit  194  is connected to the output terminal  1932 . The pulse electric current detection unit  194  detects a pulse of an electric current on the basis of an electric signal input from the electric current waveform acquisition unit  193 . The second voltage control unit  195  controls the voltage output from the AC power supply  14  as described later on the basis of the repetition frequency of the pulse of the detected electric current. 
     The plasma generator  10 B of this modification generates plasma in the tube of the ground electrode  112  by the same operation as the plasma generator  10  of the first embodiment. While the plasma is generated, the electric current waveform acquisition unit  193  acquires a waveform of the AC electric current as needed, and the pulse electric current detection unit  194  detects a pulse of the electric current. When the repetition frequency of the pulse of the electric current detected by the pulse electric current detection unit  194  changes outside the predetermined range, the second voltage control unit  195  increases or decreases the voltage output from the AC power supply  14  so that the pulse repetition frequency is within the predetermined range. As a result, even if the pulse repetition frequency changes due to a change in the density, component, or the like of the gas between the power supply electrode  111  and the ground electrode  112 , the pulse repetition frequency can be controlled to be within a predetermined range. 
     Note that the power measurement unit  191  and the voltage control unit  192  included in the plasma generator  10 A, and the electric current waveform acquisition unit  193 , the pulse electric current detection unit  194 , and the second voltage control unit  195  included in the plasma generator  10 B may be provided together. In this case, the power measurement unit  191  and the electric current waveform acquisition unit  193  can be used as one unit by using a unit having a function of acquiring the waveform of the AC electric current input from the electric current input terminal  1911  as the power measurement unit  191 . In addition, the voltage control unit  192  and the second voltage control unit  195  may also be used as one unit. 
     (2) Plasma Generator of Second Embodiment 
     (2-1) Configuration of Plasma Generator of Second Embodiment 
     A plasma generator of a second embodiment will be described with reference to FIGS.  4  to  6 . The plasma generator of the second embodiment includes a plurality of power supply electrodes and a plurality of ground electrodes. 
       FIGS.  4  and  5    are diagrams illustrating a schematic configuration of a plasma generator  20  of a second embodiment.  FIG.  4    illustrates a configuration in the cross section taken along line A-A illustrated in  FIG.  5   , and  FIG.  5    illustrates a configuration in the cross section taken along line B-B illustrated in  FIG.  4   . 
     In the plasma generator  20 , a plurality of holes are provided in a conductor (for example, stainless steel) block  201 , and one set of a combination of a power supply electrode  211  and a ground electrode  212  is inserted into each hole one by one. Each power supply electrode  211  and each ground electrode  212  has the same configuration as the power supply electrode  111  and the ground electrode  112  of the first embodiment. That is, the ground electrode  212  has a tubular shape, and the power supply electrode  211  is inserted into the tube of the ground electrode  212 . The ground electrode  212  is in contact with the block  201 , and the ground electrode  212  is also grounded by grounding the block  201 . A power supply side insulating material  221  is provided on a side face of the power supply electrode  211 , and a ground side insulating material  222  is provided on an inner face of the tube of the ground electrode  212 . 
     One end of each power supply electrode  211  extends to the outside of the tube of each ground electrode  212 , and is electrically connected to a common connection member  23 . The connection member  23  is connected to one electrode  241  of the AC power supply  24 . An other electrode  242  of the AC power supply  24  is grounded. Although not provided in the example illustrated in  FIG.  4   , the connection member  23  may be covered with a non-contact protective cover, or the connection member  23  may be covered with a covering material. 
     The block  201  is further provided with a gas introduction path  251  communicating with a gas introduction port  281  that is an opening at one end (left side in  FIG.  4   ) of the ground electrode  212 , and a gas discharge path  252  communicating with a gas discharge port  282  that is an opening at the other end (right side in  FIG.  4   ). The gas introduction path  251  communicates with all of the gas introduction ports  281  of the plurality of ground electrodes  212 , and the gas discharge path  252  communicates with all of the gas discharge ports  282  of the plurality of ground electrodes  212 . 
     Although  FIGS.  4  and  5    illustrate the example in which twelve sets of the power supply electrode  211  and the ground electrode  212  are provided, the number of combinations of the power supply electrode  211  and the ground electrode  212  is not limited thereto. One of the power supply side insulating material  221  and the ground side insulating material  222  may be omitted. Furthermore, in the present embodiment, the ground electrode  212  is provided separately from the block  201 , but only the power supply electrode  211  (covered with the power supply side insulating material  221  as necessary) may be inserted into the hole provided in the block  201 , and the block  201  itself may be used as the ground electrode. In this case, the ground side insulating material can be formed by covering the inner face of the hole provided in the block  201  with the insulating material. 
     (2-2) Operation of Plasma Generator of Second Embodiment 
     The operation of the plasma generator  20  of the second embodiment will be described. When the gas to be treated is introduced into the gas introduction path  251 , the gas to be treated is divided into the pipes of the plurality of ground electrode  212 , flows in the pipes, and is discharged from the common gas discharge path  252 . Meanwhile, an AC voltage is applied between each power supply electrode  211  and each ground electrode  212  by the AC power supply  24 . As a result, as in the case of the first embodiment, pulsed discharge occurs between each power supply electrode  211  and each ground electrode  212 , and the gas to be treated is ionized to generate plasma. The content of the decomposition target in contact with the plasma is decomposed. 
     According to the plasma generator  20  of the second embodiment, since plasma can be simultaneously generated between a plurality of sets of the power supply electrode  211  and the ground electrode  212 , the processing capability of the gas to be treated can be increased. 
     (2-3) Modification of Plasma Generator of Second Embodiment 
       FIG.  6    is a cross-sectional view taken along line A-A of a plasma generator  20 A of a modification of the second embodiment. A cross section taken along line B-B of the plasma generator  20 A is similar to that illustrated in  FIG.  5   . In the plasma generator  20 A, sets of the power supply electrode  211  and the ground electrode  212  adjacent to each other are inserted into the holes of the block  201  in directions opposite to each other. Specifically, the gas introduction port  281 , which is an opening of the ground electrode  212 , which is a linear tube, is arranged on the left side of  FIG.  6    in one set, and is arranged on the right side of  FIG.  6    in the other set. Each power supply electrode  211  extends to the outside of the tube of the ground electrode  212  on the right side of  FIG.  6    (regardless of whether it is on the gas introduction port  281  side or the gas discharge port  282  side), and is electrically connected to the common connection member  23 . 
     Since the sets of each power supply electrode  211  and each ground electrode  212  are arranged as described above, the gas introduction port  281  of one set and the gas discharge port  282  of the other set are adjacent to each other between the adjacent sets. In the block  201 , a connection flow path  253  for connecting the gas introduction port  281  of one set and the gas discharge port  282  of the other set adjacent to each other is provided. 
     As a result, the four tubes of the ground electrodes  212  illustrated in  FIG.  6    are connected by the connection flow path  253 , and one gas flow path is formed. Three gas flow paths each including a set of four tubes of the ground electrodes  212  are formed in the depth direction of  FIG.  6    (the lateral direction of  FIG.  5   ). A hole may be provided in the block  201  so as to further connect these three gas flow paths, and one gas flow path may be formed by the entire plasma generator  20 A. 
     By connecting the pipes of the plurality of ground electrodes  212  to form the gas flow path as described above, the gas to be treated can be brought into contact with the plasma for a longer time while the size of the ground electrode  212  in the longitudinal direction is suppressed, so that the content of the decomposition target in the gas to be treated can be more reliably decomposed. 
     (3) Plasma Generator of Third Embodiment 
     (3-1) Configuration of Plasma Generator of Third Embodiment 
     A plasma generator of a third embodiment will be described with reference to  FIGS.  7  to  9   . The plasma generator of the third embodiment includes a plurality of power supply electrodes  311  and a plurality of ground electrodes  312  each having a flat plate shape. 
       FIGS.  7  and  8    are diagrams illustrating a schematic configuration of a plasma generator  30  of a third embodiment.  FIG.  7    illustrates a configuration in the cross section taken along line A-A illustrated in  FIG.  8   , and  FIG.  8    illustrates a configuration in the cross section taken along line B-B illustrated in  FIG.  7   . 
     In the plasma generator  30 , three holes having a flat plate shape are provided in a conductor block  301  side by side in the longitudinal direction from the right side to the left side in  FIG.  8   . One power supply electrode  311  having a flat plate shape is inserted into each of the three holes in parallel to the flat plate having the shape of the hole. The upper and lower faces of the block  301  and the conductor of the block  301  left between the holes serve as the ground electrodes  312  having a flat plate shape. Therefore, in this embodiment, the power supply electrodes  311  and the ground electrodes  312  having a flat plate shape are alternately arranged in parallel. A power supply side insulating material  321  is provided on both faces of the power supply electrode  311 , and a ground side insulating material  322  is provided on a face of the ground electrode  312  facing the power supply electrode  311 . Openings of these holes are airtightly closed by a lid  331  made of a conductor. The lid  331  is electrically insulated from the block  301  by an insulating material  37 . Each power supply electrode  311  is in contact with the lid  331 . A rod-shaped connection member  33  is further in contact with the lid  331 . The connection member  33  is connected to one electrode  341  of an AC power supply  34 . An other electrode  342  of the AC power supply  34  is grounded. Note that the connection member  33  may be covered with a non-contact protective cover, or the connection member  33  may be covered with a covering material. 
     A flow path through which the gas to be treated flows is formed between each power supply electrode  311  and each ground electrode  312 . In  FIG.  7   , the left end of each power supply electrode  311  and each ground electrode  312  is a gas introduction port  381 , and the right end is a gas discharge port  382 . A gas introduction path  351  communicating with each gas introduction port  381  is provided on the left side of each power supply electrode  311  and each ground electrode  312 , and a gas discharge path  352  communicating with each gas discharge port  382  is provided on the right side. 
     In  FIGS.  7  and  8   , three sets of the power supply electrode  311  and the ground electrode  312  are provided, but the number of sets is not limited to three. One of the power supply side insulating material  321  and the ground side insulating material  322  may be omitted. Further, in the present embodiment, a part of the block  301  is used as the ground electrode  312 , but the ground electrode  312  may be provided separately from the block  301 . 
     (3-2) Operation of Plasma Generator of Third Embodiment 
     The operation of the plasma generator  30  of the third embodiment will be described. When the gas to be treated is introduced into the gas introduction path  351 , the gas to be treated separately flows in the gas flow paths between the plurality of power supply electrodes  311  and the plurality of ground electrodes  312 , and is discharged from the common gas discharge path  352 . Meanwhile, an AC voltage is applied between each power supply electrode  311  and each ground electrode  312  by the AC power supply  34 . As a result, as in the case of the first and second embodiments, pulsed discharge occurs between each power supply electrode  311  and each ground electrode  312 , and the gas to be treated is ionized to generate plasma. The content of the decomposition target in contact with the plasma is decomposed. 
     According to the plasma generator  30  of the third embodiment, since plasma can be simultaneously generated between a plurality of sets of the power supply electrode  311  and the ground electrode  312 , the processing capability of the gas to be treated can be increased. 
     (3-3) Modification of Plasma Generator of Third Embodiment 
       FIG.  9    is a cross-sectional view taken along line A-A of a plasma generator  30 A of a modification of the third embodiment. A B-B cross section of the plasma generator  30 A is similar to that illustrated in  FIG.  8   . In the plasma generator  30 A, a gas flow path formed on both upper and lower sides of the first power supply electrode  311  from the top among the three power supply electrodes  311  and a gas flow path formed on both upper and lower sides of the second power supply electrode  311  from the top are connected by providing a connection flow path  353  on the right side of the power supply electrodes  311 . Similarly, the gas flow path formed on both upper and lower sides of the second power supply electrode  311  from the top and a gas flow path formed on both upper and lower sides of the third power supply electrode  311  from the top are connected by providing a connection flow path  353  on the left side of the power supply electrodes  311 . As a result, a zigzag gas flow path is formed from the first power supply electrode  311  from the top toward the third power supply electrode  311  from the top. In the example of  FIG.  9   , the case where the number of power supply electrodes  311  is three has been described, but a zigzag gas flow path can be similarly formed in the case where the number of power supply electrodes is two or four or more. 
     By generating pulsed discharge between each power supply electrode  311  and each ground electrode  312  while causing the gas to be treated to flow through such a zigzag gas flow path, the gas to be treated can be brought into contact with the plasma for a longer time while the size in the direction parallel to the power supply electrode  311  is suppressed, so that the content of the decomposition target in the gas to be treated can be more reliably decomposed. 
     Although the embodiments and the modifications of the present invention have been described above, it is also possible to combine, for example, a plurality of embodiments and/or modifications or to add and/or change further components within the scope of the gist of the present invention, other than the examples described above. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  10 A,  10 B,  20 ,  20 A,  30 ,  30 A . . . Plasma Generator 
           111 ,  211 ,  311  . . . Power Supply Electrode 
           112 ,  212 ,  312  . . . Ground Electrode 
           121 ,  221 ,  321  . . . Power Supply Side Insulating Material 
           122 ,  222 ,  322  . . . Ground Side Insulating Material 
           13 ,  23 ,  33  . . . Connection Member 
           14 ,  24 ,  34  . . . AC Power Supply 
           141 ,  241 ,  341  . . . Electrode of AC Power Supply 
           142 ,  242 ,  342  . . . Ground Electrode of AC Power Supply 
           16  . . . Protective Cover 
           17  . . . Feedthrough 
           181 ,  281 ,  381  . . . Gas Introduction Port 
           182 ,  282 ,  382  . . . Gas Discharge Port 
           191  . . . Power Measurement Unit 
           1911  . . . Electric Current Input Terminal 
           1912  . . . Voltage Input Terminal 
           1913  . . . Output Terminal 
           192  . . . Voltage Control Unit 
           193  . . . Electric Current Waveform Acquisition Unit 
           1931  . . . Electric Current Input Terminal 
           1932  . . . Output Terminal 
           194  . . . Pulse Electric Current Detection Unit 
           195  . . . Second Voltage Control Unit 
           201 ,  301  . . . Block 
           251 ,  351  . . . Gas Introduction Path 
           252 ,  352  . . . Gas Discharge Path 
           253 ,  353  . . . Connection Flow Path 
           33  . . . Connection Member 
           331  . . . Lid 
           37  . . . Insulating Material