Patent Publication Number: US-2018038337-A1

Title: Plasma jet plug

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase Application under 35 U.S.C. § 371 of International Patent Application No. PCT/JP2016/000563, filed Feb. 3, 2016, and claims the benefit of Japanese patent application Nos. 2015-036010, filed Feb. 26, 2015, 2015-075551, filed Apr. 2, 2015, and 2015-105326, filed May 25, 2015, all of which are incorporated herein by reference in their entireties. The International Application was published in Japanese on Sep. 1, 2016 as International Publication No. WO/2016/136149 under PCT Article 21(2). 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a plasma jet plug that ignites an air-fuel mixture by jetting plasma. 
     BACKGROUND OF THE INVENTION 
     A plasma jet plug is a spark plug having a space called “cavity” for generating plasma (Japanese Patent Application Laid-Open (kokai) No. 2008-045449). An orifice electrode (also called “ground electrode”) having an opening is provided at an exit of the cavity, and a center electrode is provided inside the cavity, with a gap interposed between the orifice electrode and the center electrode. The portion, other than the orifice electrode and the center electrode, of the wall surface in the cavity is constituted by an insulator. An air-fuel mixture is ignited by supplying a large current to the cavity so as to fill the cavity with a large amount of plasma, and ejecting the plasma. At the time of supplying a large current to the cavity, first, dielectric breakdown is caused by applying a high voltage between the orifice electrode and the center electrode so as to form discharge paths in the cavity, and thereafter, a large current is superposed with a low voltage. 
     PROBLEM TO BE SOLVED BY THE INVENTION 
     As the discharge paths in the cavity, an aerial path that is a path in a space away from the wall surface of the cavity, and a surface path that extends along the wall surface of the cavity (in particular, the surface of the insulator) may be formed. Usually, the surface path can be more easily formed than the aerial path. Once a surface path has been formed, a phenomenon called “channeling” occurs in which the insulator surface in contact with the surface path is melted to form a groove by the current generated during dielectric breakdown. When channeling occurs, the shape of the cavity is significantly changed, resulting in deterioration in the plasma ejection performance. Furthermore, discharge is concentrated at the groove formed as a result of channeling, giving rise to a problem that a deeper groove may be formed. Therefore, there is a need for a technique that is able to reduce the likelihood of occurrence of surface discharge so as to allow aerial discharge to occur in a stable manner, thus suppressing the occurrence of channeling. 
     The present inventor found that when the length of the exposed portion of the center electrode in the cavity is large, the area in which the center electrode is in contact with plasma is increased, which poses a problem that erosion of the center electrode caused by the heat of plasma becomes excessively significant. The present inventor also found that when the inner surface of the orifice electrode is exposed into the cavity, the inner surface of the orifice electrode undergoes excessive erosion by the heat of plasma. 
     SUMMARY OF THE INVENTION 
     Means For Solving the Problem 
     The present invention has been made to solve the above-described problems, and can be embodied in the following modes. 
     (1) According to a first mode of the present invention, a plasma jet plug is provided. The plasma jet plug includes a tubular insulator having an axial hole extending along an axial direction; a center electrode disposed inside the axial hole; a metal shell disposed on an outer circumference of the insulator; and an orifice electrode electrically connected to the metal shell and disposed on a front side of the insulator. In the plasma jet plug according to the first mode, a plasma generating cavity is formed by a surface of the center electrode, an inner surface of the insulator, and an inner surface of the orifice electrode and a shortest path length D 1  of a surface path is greater than or equal to 5 times an aerial gap G, the surface path extending, inside the cavity, from a surface of the center electrode via an inner surface of the insulator to an inner surface of the orifice electrode, the aerial gap G being a shortest distance between the center electrode and the orifice electrode. 
     With the plasma jet plug, the shortest path length D 1  of the surface path is sufficiently larger than the aerial gap G. Accordingly, it is possible to reduce the likelihood of occurrence of surface discharge so as to allow aerial discharge to occur in a stable manner, thus suppressing the occurrence of channeling. 
     (2) In the above-described plasma jet plug, the inner surface of the insulator may include at least one groove portion that forms a recessed path on the surface path, and the groove portion may have a groove width of 0.1 mm or more. 
     With this configuration, it is possible to keep the capacity of the cavity small by providing the groove portion in the inner surface of the insulator, thus increasing the shortest path length D 1  of the surface path while facilitating ejection of plasma. It is also possible to adjust the effective length of the shortest path length D 1  of the surface path to be a length along the groove portion by setting the groove width of the groove portion to 0.1 mm or more. Accordingly, it is possible to allow aerial discharge to occur in a more stable manner. 
     (3) In the above-described plasma jet plug, the groove portion may have a depth that is less than or equal to 3 times the groove width. 
     With this configuration, by setting the depth of the groove portion to be less than or equal to 3 times the groove width, it is possible to keep the capacity of the cavity small while increasing the shortest path length D 1  of the surface path, thus facilitating ejection of plasma. 
     (4) In the above-described plasma jet plug, a side surface of the center electrode that faces the cavity may have a surface area of 20 mm 2  or less. 
     With this configuration, by setting the surface area of the side surface of the center electrode that faces the cavity to be 20 mm 2  or less, it is possible to suppress a phenomenon in which plasma is cooled by the center electrode, thus facilitating ejection of plasma. 
     (5) In the above-described plasma jet plug, a portion of the insulator that faces the cavity may be formed of a plurality of members. 
     With this configuration, when a portion of the insulator that faces the cavity is formed of a plurality of members, the inner surface shape of the insulator that faces the cavity can be easily formed so as to increase the path length D 1  of the surface path. 
     (6) In the above-described plasma jet plug, the plurality of members of the insulator may include a first member provided on an outer circumferential side of the center electrode, and a second member provided on an outer circumferential side of the first member, and the first member may be formed from a first insulating material having a higher coefficient of thermal conductivity than the second member, and the second member may be formed from a second insulating material having a higher dielectric strength than the first member. 
     With this configuration, the coefficient of thermal conductivity of the first member is higher than the coefficient of thermal conductivity of the second member, so that it is possible to increase the heat conduction from the center electrode by the first member, thus enhancing the durability of the center electrode. Furthermore, the dielectric strength of the second member is higher than that of the first member, so that it is possible to enhance the voltage endurance of the insulator as a whole. 
     (7) In the above-described plasma jet plug, a side surface of the center electrode in the cavity may be covered with an insulating material, and a distance L from a front end of the insulating material provided on the side surface of the center electrode to a front end of the center electrode may be 0.4 mm or less. 
     With this configuration, the length L of the front end portion of the center electrode that is exposed from the insulating material is as short as 0.4 mm or less, so that it is possible to suppress the erosion of the center electrode caused by the heat of plasma. 
     (8) In the above-described plasma jet plug, a distance H between the side surface of the center electrode and an inner wall surface of the cavity, as measured along a direction perpendicular to the axial direction, may be is larger than the aerial gap G. 
     With this configuration, surface discharge is less likely to occur along a path from the side surface of the center electrode to the inner wall surface of the cavity along a direction perpendicular to the axial direction, so that it is possible to allow aerial discharge to occur in a stable manner. 
     (9) In the above-described plasma jet plug, the inner surface of the orifice electrode around a through hole of the orifice electrode may be covered with an insulating material so as to leave an exposed surface adjacent to the through hole, and a distance J between an outermost circumferential position of the exposed surface and the side surface of the center electrode, as measured along a direction perpendicular to the axial direction, may be smaller than the distance H. 
     With this configuration, the inner surface of the orifice electrode is covered with the insulating material so as to leave an exposed surface adjacent to the through hole, so that it is possible to suppress the erosion of the inner surface of the orifice electrode caused by plasma. 
     (10) In the above-described plasma jet plug, a distance K between the outermost circumferential position of the exposed surface and the front end of the center electrode may be larger than the aerial gap G. 
     With this configuration, surface discharge is less likely to occur along a path from the front end of the center electrode to the insulating material covering the inner surface around the through hole of the orifice electrode, so that it is possible to allow aerial discharge to occur in a stable manner. 
     (11) According to a second mode of the present invention, a plasma jet plug is provided. The plasma jet plug includes a tubular insulator having an axial hole extending along an axial direction; a center electrode disposed inside the axial hole; a metal shell disposed on an outer circumference of the insulator; and an orifice electrode electrically connected to the metal shell and disposed on a front side of the insulator, a plasma generating cavity being formed by a surface of the center electrode, an inner surface of the insulator, and an inner surface of the orifice electrode. In the plasma jet plug according to the second mode, a relationship between the aerial gap G that is the shortest distance between the center electrode and the orifice electrode and the shortest distance Dr between a front end edge of the center electrode and the inner surface of the insulator satisfies 1.5×G≦Dr. The feature portions of the plasma jet plug according to the second mode can be used in combination with the plasma jet plug according to the first mode described above, or may be used regardless of the presence of the feature portions of the plasma jet plug according to the first mode. 
     With the plasma jet plug according to the second mode, the shortest distance Dr between the front end edge of the center electrode and the inner surface of the insulator is sufficiently larger than the aerial gap G. Accordingly, surface discharge is less likely to occur, and aerial discharge is allowed to occur in a stable manner, thus making it possible to suppress the occurrence of channeling. 
     (12) In the above-described plasma jet plug, the inner surface of the insulator that faces the cavity may include a reduced diameter portion provided such that the inner surface of the insulator is reduced in diameter toward a rear side of the insulator, and the cavity may include a first cavity portion located on a front side relative to a rear end of the reduced diameter portion of the insulator, and a second cavity portion located on a rear side relative to the rear end of the reduced diameter portion. 
     With this configuration, the shortest distance Dr between the front end edge of the center electrode and the inner surface of the insulator can be increased by the second cavity portion having a small capacity, so that it is possible to keep the overall capacity of the cavity small while suppressing the occurrence of surface discharge, thus facilitating ejection of plasma. 
     (13) In the above-described plasma jet plug, a radial spatial distance Dp may be 0.1 mm or more, the radial spatial distance Dp being a distance between the surface of the center electrode and the inner surface of the insulator in the second cavity portion, as measured in a radial direction perpendicular to the axial direction. 
     With this configuration, it is possible to suppress the occurrence of surface discharge in the second cavity portion so as to allow aerial discharge to occur in a stable manner, thus suppressing the occurrence of channeling. 
     (14) In the above-described plasma jet plug, a depth Dq of the second cavity portion, as measured along the axial direction, may satisfy 0&lt;Dq≦3×Dp. 
     With this configuration, by setting the depth Dq of the second cavity portion within this range, it is possible to increase the tendency that aerial discharge is more likely to occur than surface discharge, and also to prevent the capacity of the second cavity portion from being excessively increased, thus facilitating ejection of plasma. 
     (15) In the above-described plasma jet plug, a relationship between the radial spatial distance Dp of the second cavity portion and the shortest distance Dr between the front end edge of the center electrode and the inner surface of the insulator may satisfy Dp/Dr≦0.5. 
     With this configuration, by setting Dp/Dr within this range, it is possible to further facilitate ejection of plasma. 
     The present invention can be embodied in various forms. For example, the invention may be embodied in forms such as a plasma jet plug, an ignition device using a plasma jet plug, an internal combustion engine having the plasma jet plug mounted therein, an internal combustion engine having mounted therein an ignition device using the plasma jet plug, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawing(s), wherein like designations denote like elements in the various views, and wherein: 
         FIG. 1  is a partial cross-sectional view of a plasma jet plug according to an embodiment. 
         FIG. 2  is an enlarged cross-sectional view of a front end portion of the plasma jet plug. 
         FIG. 3  is a block diagram of an ignition device. 
         FIGS. 4A and 4B  are enlarged views, in cross section, of front end portions of plasma jet plugs according to the first embodiment and a modified embodiment thereof. 
         FIG. 5  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to a second embodiment. 
         FIG. 6  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to a third embodiment. 
         FIG. 7  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to a fourth embodiment. 
         FIG. 8  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to a fifth embodiment. 
         FIG. 9  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to a sixth embodiment. 
         FIGS. 10A AND 10B  are explanatory diagrams showing test results for D 1 /G. 
         FIGS. 11A AND 11B  are explanatory diagrams showing test results for the groove width. 
         FIGS. 12A and 12B  are explanatory diagrams showing test results for a relationship between the groove depth and the groove width. 
         FIGS. 13A and 13B  are explanatory diagrams showing test results for the surface area of a side surface of a center electrode that faces a cavity. 
         FIG. 14  is a enlarged view, in cross section, of a front end portion of a plasma jet plug according to a seventh embodiment. 
         FIG. 15  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to an eighth embodiment. 
         FIG. 16  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to a ninth embodiment. 
         FIG. 17  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to a tenth embodiment. 
         FIGS. 18A and 18B  are explanatory diagrams showing test results for the exposed length of a center electrode. 
         FIGS. 19A and 19B  are explanatory diagrams showing test results for covering of an inner surface of an orifice electrode by an insulator. 
         FIG. 20  is an enlarged cross-sectional view of a front end portion of a plasma jet plug according to an eleventh embodiment. 
         FIG. 21  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to an eleventh embodiment. 
         FIG. 22  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to a twelfth embodiment. 
         FIGS. 23A-23D  are explanatory diagrams showing test results for Dr/G. 
         FIGS. 24A-24C  are explanatory diagrams showing test results for a radial spatial distance Dp of a second cavity portion. 
         FIGS. 25A and 25B  are explanatory diagrams showing test results (No. 1) for Dq/Dp. 
         FIGS. 26A and 26B  are explanatory diagrams showing test results (No. 2) for Dq/Dp. 
         FIGS. 27A and 27B  are explanatory diagrams showing test results for Dp/Dr. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. Overall Configuration: 
       FIG. 1  is a partial cross-sectional view of a plasma jet plug  100  according to an embodiment of the present invention.  FIG. 2  is an enlarged cross-sectional view of a front end portion of the plasma jet plug  100 . In  FIGS. 1 and 2 , the lower side along a direction of an axial line O of the plasma jet plug  100  is referred to as a front side of the plasma jet plug  100 , and the upper side is referred to as a rear side. In addition, a direction intersecting the axial line O and extending perpendicular to the axial line O is referred to as “radial direction”. 
     In  FIG. 1 , the right side of the axial line O shows an external view of the plasma jet plug  100 , and the left side of the axial line O shows a cross-sectional view. The plasma jet plug  100  includes an insulator  10 , a metal shell  50  that holds the insulator  10 , a center electrode  20  held inside the insulator  10 , an orifice electrode  30  disposed at a front end portion  57  of the metal shell  50 , and a metal terminal  40  disposed at a rear end portion of the insulator  10 . 
     The insulator  10  is a tubular insulating member formed by baking a ceramic material such as alumina, and has an axial hole  12  extending in the direction of the axial line O. A flange portion  19  having the largest outer diameter is formed at substantially the center in the direction of the axial line O, and a rear trunk portion  18  is formed on the rear side thereof. A front trunk portion  17  having a smaller outer diameter than the rear trunk portion  18  is formed on the front side relative to the flange portion  19 , and a long nose portion  13  having an even smaller outer diameter than the front trunk portion  17  is formed on the front side relative to the front trunk portion  17 . A portion between the long nose portion  13  and the front trunk portion  17  is formed in a stepped shape. A portion of the axial hole  12  that corresponds to an inner circumference of the long nose portion  13  is formed as an electrode housing portion  15 . The electrode housing portion  15  is made smaller in diameter than the inner circumferential portion of each of the front trunk portion  17 , the flange portion  19 , and the rear trunk portion  18 . The center electrode  20  is held inside the electrode housing portion  15 . An enlarged inner diameter portion  16  having a larger inner diameter than the long nose portion  13  is formed on the front side of the long nose portion  13  of the insulator  10 . 
     The center electrode  20  is a bar-shaped conductive member extending along the axial line O, and is disposed inside the axial hole  12  of the insulator  10 . In the present embodiment, the center electrode  20  is an integrally molded article formed from a high melting point material such as tungsten. However, various other configurations may be used as the configuration of the center electrode  20 . For example, it is possible to use a configuration having a double structure composed of a base material and a core material embedded in the base material. 
     As shown in  FIG. 2 , the center electrode  20  includes a head portion  21  on the rearmost side, and a nose portion  22  having a smaller outer diameter than the head portion  21  and located on the front side relative to the head portion  21 . The nose portion  22  of the center electrode  20  is housed in the electrode housing portion  15 , and the head portion  21  of the center electrode  20  is housed in a portion on the rear side from a reduced inner diameter portion  10   z  of the axial hole  12 . The front side surface of the head portion  21  and the rear side surface of the reduced inner diameter portion  10   z  are closely attached to each other, and are sealed around the entire circumference in the circumferential direction thereof. 
     As shown in  FIG. 1 , the center electrode  20  is electrically connected to the metal terminal  40  on the rear side via a conductive seal member  4  that is made of a mixture of a metal and glass and provided inside the axial hole  12 . The seal member  4  causes the center electrode  20  and the metal terminal  40  to be fixed inside the axial hole  12  and to be electrically connected to each other. A high-voltage cable (not shown) is connected to the metal terminal  40  via a plug cap (not shown). 
     The metal shell  50  is a cylindrical metal member for fixing the plasma jet plug  100  to an engine head of an internal combustion engine, and holds the insulator  10  so as to surround the insulator  10 . The metal shell  50  includes a tool engagement portion  51  to which a plug wrench is fitted, and a thread portion  52  that is screwed to the engine head. A crimp portion  53  is provided on the rear side relative to the tool engagement portion  51  of the metal shell  50 . Circular ring members  6  and  7  are interposed between a portion of the metal shell  50  that extends from the tool engagement portion  51  to the crimp portion  53  and the rear trunk portion  18  of the insulator  10 , and the space between the two ring members  6  and  7  is filled with powder of talc  9 . Then, by crimping the crimp portion  53 , the insulator  10  is pressed inside the metal shell  50  toward the front side via the ring members  6  and  7  and the talc  9 . Thus, the stepped portion between the long nose portion  13  and the front trunk portion  17  of the insulator  10  is supported via an annular packing  80  by a locking portion  56  formed in a stepped shape on the inner circumferential surface of the metal shell  50 , and the metal shell  50  and the insulator  10  are integrated with each other. The packing  80  maintains the airtightness between the metal shell  50  and the insulator  10 , preventing the outflow of the combustion gas. In addition, a flange portion  54  is formed between the tool engagement portion  51  and the thread portion  52 , and a gasket  5  is inserted in the vicinity of the rear side of the thread portion  52 , or in other words, at a seating portion  55  of the flange portion  54 . 
     The orifice electrode  30  is provided at the front end portion  57  of the metal shell  50 . As shown in  FIG. 2 , a recess  57 A is formed on the inner circumferential side of the front end portion  57  of the metal shell  50 , and the orifice electrode  30  is fitted into the recess  57 A. The orifice electrode  30  is a circular plate-shaped member having a through hole  31  at the center thereof. The through hole  31  functions as an ejection hole for ejecting plasma. The circumferential edge of the orifice electrode  30  is joined by laser welding or the like to the metal shell  50  around the entire circumference thereof. The metal shell  50  and the orifice electrode  30  are electrically connected. Since the metal shell  50  is screwed to the engine head and is grounded, the orifice electrode  30  is also grounded. In addition, the orifice electrode  30  covers an opening, in the front direction, of the metal shell  50 . 
     As shown in  FIG. 2 , the cavity CV for generating plasma is formed between the inner surface of the front end portion of the insulator  10 , the surface of the front end portion of the center electrode  20 , and the inner surface of the orifice electrode  30 . Plasma is generated by applying a voltage between the center electrode  20  and the orifice electrode  30 . 
       FIG. 3  is a block diagram showing a configuration of an ignition device  120  that ignites the plasma jet plug  100 . The ignition device  120  includes a spark discharge circuit portion  140 , a plasma discharge circuit portion  160 , and two control circuit portions  130  and  150  that control the spark discharge circuit portion  140  and the plasma discharge circuit portion  160 . The control circuit portions  130  and  150  are connected to an ECU of an automobile. 
     The spark discharge circuit portion  140  is a power circuit for performing the so-called triggered discharge in which dielectric breakdown is caused by applying a high voltage to a gap between the center electrode  20  and the orifice electrode  30  of the plasma jet plug  100 , thereby starting spark discharge. The plasma discharge circuit portion  160  is a power circuit for supplying a large current to the gap in which dielectric breakdown is caused by the triggered discharge. The plasma discharge circuit portion  160  includes a condenser  162  in which electric energy is stored, and a high voltage generation circuit  161  for charging the condenser  162 . One end of the condenser  162  is grounded, and the other end thereof is connected to the center electrode  20 . When discharge occurs in the gap between the center electrode  20  and the orifice electrode  30 , the gas inside the cavity CV is excited by the large current supplied from the ignition device  120 , thus forming plasma. When the pressure inside the cavity CV is increased as a result of the expansion of the plasma that has been formed in the cavity CV, the plasma in the cavity CV is ejected from the through hole  31  of the orifice electrode  30 . The ejected plasma ignites an air-fuel mixture in a combustion chamber of the internal combustion engine. 
     B. Various Embodiments of Front End Portion of Plasma Jet Plug: 
       FIG. 4A  is an enlarged view, in cross section, of a front end portion of a plasma jet plug according to the first embodiment, and  FIG. 4B  is an enlarged view, in cross section, of a front end portion of a modified embodiment thereof. Note that  FIGS. 4A AND 4B  are shown upside down relative to  FIGS. 1 and 2 . That is, the upper side of  FIGS. 4A and 4B  correspond to the front side of the plasma jet plug, and the lower side of  FIGS. 4A and 4B  correspond to the rear side of the plasma jet plug. 
     In the plasma jet plug  100  according to the first embodiment shown in  FIG. 4A , a front end portion of the center electrode  20  is formed as a columnar nose portion  22 . At the long nose portion  13  in the vicinity of the front end of the insulator  10 , the enlarged inner diameter portion  16  having a larger inner diameter than the long nose portion  13  is formed. Note that the long nose portion  13  is also referred to as “small inner diameter portion  13 ”. A reduced diameter portion  14  is formed between the long nose portion  13  and the enlarged inner diameter portion  16 . In this example, the reduced diameter portion  14  is formed as a surface perpendicular to the axial line O, but the reduced diameter portion  14  may be tapered. A circular groove portion Gr 1  that is recessed toward the rear side relative to the surface of the reduced diameter portion  14  is formed at an outer edge of the reduced diameter portion  14  of the insulator  10 . The groove portion Gr 1  forms a recessed path on the surface path. The size of the groove portion Gr 1  is defined by a width Wa 1  and a depth Wd 1  of the groove portion Gr 1 . The effect achieved by forming the groove portion Gr 1  will be described later. 
     The cavity CV is a space surrounded by a surface  20   s  of the center electrode  20 , an inner surface  10   in  of the insulator  10 , and an inner surface  30   in  of the orifice electrode  30 . However, the cavity CV does not include a portion constituted by the through hole  31  of the orifice electrode  30 , and means a space inside the inner surface  30   in  of the orifice electrode  30 , assuming that the through hole  31  is not provided. Between the outer circumferential surface of the nose portion  22  of the center electrode  20  and the inner surface of the insulator  10 , a minute clearance (less than 0.06 mm) is formed for assembly of the two components. A space with a clearance of less than 0.06 mm is a minute space in which no plasma will be generated, and therefore does not function as a part of the cavity CV. As used herein, “cavity” means a space in which plasma can be generated, and also means a space having a clearance of 0.06 mm or more. To be more specific, the “cavity” in the first embodiment shown in  FIG. 4A  means a space that can be formed between the inner surface  10   in  of the front end portion of the insulator  10 , the surface of the front end portion of the center electrode  20 , and the inner surface  30   in  of the orifice electrode  30  and has a clearance of 0.06 mm or more, and the “cavity” does not include a space having a clearance of less than 0.06 mm. 
     In  FIG. 4A , the following dimensions are further given. 
     (1) D 1 : the shortest length (hereinafter referred to as “shortest surface path length”) of a surface path from the surface  20   s  of the center electrode  20  via the inner surface of the insulator  10  to the inner surface  30   in  of the orifice electrode  30 . In  FIG. 4A , the shortest surface path length D 1  includes the length of the recessed path along the groove portion Gr 1 . 
     (2) E: the inner diameter of the through hole  31  of the orifice electrode  30 . 
     (3) G: a distance G, in the axial direction, between the inner surface  30   in  of the orifice electrode  30  and a front end surface  20   t  of the center electrode  20 . The distance G is also referred as “aerial gap G”. A typical range of values of the aerial gap G is, for example, 0.3 mm to 1.5 mm. 
     Note that the inner diameter E of the through hole  31  of the orifice electrode  30  is preferably smaller than the outer diameter of the nose portion  22  located at the front end of the center electrode  20 . This is to facilitate aerial discharge in the aerial gap G. 
       FIG. 4B  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   r  according to a modified embodiment. The plasma jet plug  100   r  corresponds to the plasma jet plug  100  of the first embodiment from which the groove portion Gr 1  of the insulator  10  has been omitted, and the rest of the configuration is the same as that of the first embodiment. The shortest surface path length D 1   r  in this modified embodiment is shorter than the shortest surface path length D 1  in the first embodiment by the length (=Wa 1 +2×Wd 1 ) of the groove portion Gr 1 . 
     In the first embodiment shown in  FIG. 4A , the groove portion Gr 1  is provided in a portion of the inner surface  10   in  of the insulator  10 , so that the shortest surface path length D 1  can be longer than that in the modified embodiment. As a result, it is possible to reduce the likelihood of occurrence of surface discharge so as to allow aerial discharge to occur in a stable manner. In this respect, it is particularly preferable that the shortest surface path length D 1  is greater than or equal to 5 times the aerial gap G. However, when the shortest surface path length D 1   r  is set to be greater than or equal to 5 times the aerial gap G in the modified embodiment shown in  FIG. 4B , the modified embodiment can also make it possible to reduce the likelihood of occurrence of surface discharge so as to allow aerial discharge to occur in a stable manner, and therefore can be used as an embodiment of the present invention. However, it is preferable to provide a groove portion Gr 1  that forms a recessed path on the surface path as in the first embodiment shown in  FIG. 4A , since the shortest surface path length D 1  can be increased without excessively increasing the capacity of the cavity CV. 
     The groove width Wa 1  of the groove portion Gr 1  may be 0.06 mm or more, but is preferably 0.1 mm or more. The reason is that, when the groove width Wa 1  is excessively small, the groove portion Gr 1  may not have a function of extending the surface path (i.e., discharge occurs so as to jump over the groove portion Gr 1 ). It is preferable to provide a groove portion Gr 1  having a groove width Wa 1  of 0.1 mm or more, since the shortest surface path length D 1  can be increased while keeping the capacity of the cavity small. Although the maximum value of the groove width Wa 1  is not particularly limited, the groove width Wa 1  is, for example, preferably 0.5 mm or less, more preferably 0.3 mm or less. 
     The depth Wd 1  of the groove portion Gr 1  is preferably less than or equal to 3 times the groove width Wa 1 . This makes it possible to keep the capacity of the cavity CV small, while increasing the shortest surface path length D 1 , thereby facilitating ejection of plasma. 
       FIG. 5  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   a  according to a second embodiment. The plasma jet plug  100   a  corresponds to the plasma jet plug  100  ( FIG. 4A ) of the first embodiment to which a circular second groove portion Gr 2  has been additionally provided in the inner surface  10   in  of the insulator  10 , and the rest of the configuration is the same as that of the first embodiment. That is, in the plasma jet plug  100   a  according to the second embodiment, two groove portions Gr 1  and Gr 2  are provided in the inner surface  10   in  of the insulator  10 . Although the groove depth Wd 1  of the second groove portion Gr 2  is the same as the groove depth of the first groove portion Gr 1  in the example shown in  FIG. 5 , their depths may be changed. The groove width Wa 2  of the second groove portion Gr 2  may be either the same as or different from the groove width Wa 1  of the first groove portion Gr 1 . Furthermore, three or more groove portions may be provided. Although the groove portions Gr 1  and Gr 2  are provided at the reduced diameter portion  14  of the insulator  10  in the example shown in  FIG. 5 , the groove portions may be formed in a cylindrical inner surface of the insulator  10  that extends along the axial line O. 
       FIG. 6  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   b  according to a third embodiment. The plasma jet plug  100   b  has a configuration in which the portion of the cavity CV of the plasma jet plug  100  ( FIG. 4A ) according to the first embodiment has been extended in the direction of the axial line O, and the rest of the configuration is the same as that of the first embodiment. That is, in the plasma jet plug  100   b  of the -third embodiment, a side surface  20   f  of the center electrode  20  that faces the cavity CV is longer than that in the first embodiment. The surface area S 20f  of the side surface  20   f  of the center electrode  20  can be expressed as follows: 
         S   20f =2π R·L    (1),
 
     where R represents the radius of the exposed portion of the center electrode  20 , and L represents the length, in the axial direction, of the exposed portion of the center electrode  20 . A typical range of values of the radius R is, for example, 0.25 mm to 1 mm. A typical range of values of the length L is, for example, 0 mm to 5 mm. 
     When the surface area S 20f  of the side surface  20   f  of the center electrode  20  is excessively increased, plasma is cooled by the center electrode  20 , which may result in deterioration in the plasma ejection performance. In view of this, the surface area S 20f  of the side surface  20   f  of the center electrode  20  is preferably 20 mm 2  or less. This can suppress a phenomenon in which plasma is cooled by the center electrode  20 , making it possible to facilitate ejection of plasma. 
       FIG. 7  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   c  according to a fourth embodiment. In the plasma jet plug  100   c,  a portion of the insulator  10  that faces the cavity CV is formed of a plurality of members  13   c  and  16   c,  and the rest of the configuration is the same as that of the first embodiment. More specifically, the portion constituted by the long nose portion  13  of the insulator  10  and the enlarged inner diameter portion  16  that is continuous with the front side thereof is divided into two members, namely, a first member  13   c  provided on the outer circumferential side of the center electrode  20  and a second member  16   c  provided on the outer circumferential side thereof. The first member  13   c  corresponds to the long nose portion  13  shown in  FIG. 4A , and is formed so as to have a smaller outer diameter than the long nose portion  13 . The second member  16   c  is a substantially circular member, and is fixed by being fitted to the outer circumferential side of the first member  13   c.    
     In  FIG. 7 , a groove portion Gr 1  that forms a recessed path on the surface path is formed at a position at which the first member  13   c  is in contact with the second member  16   c . The groove portion Gr 1  is formed at a boundary portion between the two members  13   c  and  16   c.  Forming a portion of the insulator  10  that faces the cavity CV by using the plurality of members  13   c  and  16   c  offers an advantage that the groove portion Gr 1  can be more easily formed. However, the cavity CV may be formed in the same shape as that shown in  FIG. 4B  by omitting the groove portion Gr 1 . 
     An additional advantage can be achieved by forming a portion of the insulator  10  that faces the cavity CV by using the plurality of members  13   c  and  16   c  and also changing the materials of these members. For example, the first member  13   c  on the inner circumferential side may be formed from a first insulating material (e.g., aluminum nitride (AlN)) having a higher coefficient of thermal conductivity than the second member  16   c  on the outer circumferential side, and the second member  16   c  on the outer circumferential side may be formed from a second insulating material (e.g., alumina (Al 2 O 3 )) having a higher dielectric strength than the first member  13   c  on the inner circumferential side. By using such a configuration, it is possible to increase the heat conduction from the center electrode  20  by the first member  13   c,  making it possible to enhance the durability of the center electrode  20 . Since the dielectric strength of the second member  16   c  is higher than that of the first member  13   c , it is possible to enhance the voltage endurance of the insulator  10  as a whole. 
       FIG. 8  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   d  according to a fifth embodiment. In the plasma jet plug  100   d,  a portion of the insulator  10  that faces the cavity CV is formed of a plurality of members  13   d  and  16   d,  similarly to the fourth embodiment ( FIG. 7 ). In  FIG. 8 , a first groove portion Gr 1  is provided in the first member  13   d  of the insulator  10 , and a second groove portion Gr 2  is provided at a boundary position between the first member  13   d  and the second member  16   d.  In other words, a portion of the wall surface of the second groove portion Gr 2  is constituted by a surface of the first member  13   d,  and the other portions are constituted by a surface of the second member  16   d.  As a result, the shortest surface path length D 1  can be sufficiently increased with the plurality of groove portions Gr 1  and Gr 2 . 
       FIG. 9  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   e  according to a sixth embodiment. In the plasma jet plug  100   e,  a portion of the insulator  10  that faces the cavity CV is formed of a plurality of members  13   e  and  16   e,  similarly to the fourth embodiment ( FIG. 7 ) and the fifth embodiment ( FIG. 8 ).  FIG. 9  is different from  FIG. 7  in that a front end opening portion  16   p  having a small opening is provided at a front end of the second member - 16   e  so as to cover the inner surface  30   in  of the orifice electrode  30 . Note that the front end opening portion  16   p  of the second member  16   e  may cover the inner surface  30   in  of the orifice electrode  30  either entirely or partially. By providing the front end opening portion  16   p  with the second member  16   d  so as to cover the inner surface  30   in  of the orifice electrode  30  in this manner, it is possible to further increase the shortest surface path length D 1 . 
     As can be understood from the above-described embodiments shown in  FIGS. 4A to 9 , the shortest surface path length D 1  can be sufficiently increased by providing at least one groove portion in the inner surface of the insulator  10  in a portion located on the shortest surface path from the surface  20   s  of the center electrode  20  via the inner surface of the insulator  10  to the inner surface  30   in  of the orifice electrode  30 . As a result, it is possible to reduce the likelihood of occurrence of surface discharge so as to allow aerial discharge to occur in a stable manner. As can also be understood from the examples shown in  FIGS. 7 to 9 , forming a portion of the insulator  10  that faces the cavity CV by using a plurality of members offers an advantage that the inner surface shape of a portion of the insulator that faces the cavity can be easily formed so as to increase the shortest surface path length D 1 . 
     C. Test Results: 
     In the following, test results for preferable dimensions of the plasma jet plugs shown in  FIGS. 4A to 9  will be described sequentially. 
       FIGS. 10A and 10B  show explanatory diagrams showing test results for a ratio D 1 /G between the shortest surface path length D 1  and the aerial gap G.  FIG. 10A  is a schematic plan view of a testing apparatus. In this test, an insulator  210  having a groove portion  212  was placed in a pressure chamber, and a first electrode  220  and a second electrode  230  were placed on the insulator  210  so as to oppose each other with the groove portion  212  interposed therebetween. The insulator  210  was formed from alumina. A gap Dg between the two electrodes  220  and  230  was set to a fixed value of 0.5 mm. A groove width Da of the groove portion  212  was set to a fixed value of 0.2 mm, and the groove portion path length DL was varied by changing the groove depth Dd of the groove portion  212 . The “groove portion path length DL” is the shortest path length that follows the inner surface of the groove portion  212 , and is given by DL=Da+2Dd. 
     The two electrodes  220  and  230  simulate the center electrode  20  and the orifice electrode  30 . As a discharge path between the two electrodes  220  and  230 , the following two discharge paths may be produced. 
     (1) First discharge path RT 1 : a discharge path (indicated by the solid arrow in  FIG. 10A ) that jumps over the groove portion  212  in the vicinity of an upper surface  210   s  of the insulator  210 . 
     (2) Second discharge path: a surface path (not shown) that follows the upper surface  210   s  of the insulator  210  and the groove portion path length DL. 
     Since these two discharge paths are the same in the configuration of the path portion along the upper surface  210   s  of the insulator  210 , the only difference is that the first discharge path RT 1  passes along an aerial path following the groove width Da, and the second discharge path passes along a recessed surface path following the groove portion path length DL. Therefore, by applying this structure to the structure shown in  FIG. 4 , it can be understood that the groove width Da serves as a dimension that simulates the aerial gap G shown in  FIG. 4 , and the groove portion path length DL serves as a dimension that simulates the shortest surface path length D 1 . 
     In the discharge path confirmation test shown in  FIGS. 10A and 10B , discharge was performed 100 times for each case, with the interior of the pressure chamber being pressurized to 0.4 MPa, 1.2 MPa, and 2.0 MPa (all in atmosphere). Then, the discharge path was imaged by using a high-speed camera, and the percentage of times that discharge had occurred on the above-described second discharge path, out of 100 times of discharge, was determined, and the determined percentage was used as “surface discharge rate”. Here, “surface discharge” means discharge along the above-described second discharge path, and “aerial discharge” means discharge along the first discharge path RT 1 . 
       FIG. 10B  shows a relationship between the value of the ratio DL/Da and the surface discharge rate. According to the test results, the surface discharge rate decreased with an increase in the value of the ratio DL/Da. When DL/Da was 5 or more, no surface discharge occurred, and all the discharges were aerial discharges. The results can be understood as follows. That is, as the groove portion path length DL shown in  FIG. 10A  increases, the above-described surface discharge via the second discharge path is less likely to occur, and aerial discharge via the first discharge path RT 1  is more likely to occur. Therefore, by setting DL/Da to 5 or more, it is possible to allow aerial discharge to occur in a stable manner. Meanwhile, as described previously, the groove portion path length DL simulates the shortest surface path length D 1  shown in  FIG. 4 , and the groove width Da simulates the aerial gap G. Accordingly, the horizontal axis shown in  FIG. 10B  can be considered to simulate the ratio D 1 /G between the shortest surface path length D 1  and the aerial gap G. In view of the test results, in the plasma jet plug, it is preferable that the value of the ratio D 1 /G between the shortest surface path length D 1  and the aerial gap G is set to 5 or more. In other words, it is preferable that the shortest surface path length D 1  is greater than or equal to 5 times the aerial gap G. This makes it possible to reduce the likelihood of occurrence of surface discharge in the cavity CV so as to allow aerial discharge to occur in a stable manner. 
       FIGS. 11A and 11B  show explanatory diagrams showing test results for the groove width Wa 1  of the groove portion Gr 1 . Although the testing apparatus shown in  FIG. 11A  is the same as that shown in  FIG. 10A , the dimensions are set differently from those in the test shown in  FIGS. 10A and 10B . That is, in the test shown in  FIGS. 11A and 11B , the groove width Da was changed to several values, and the groove depth Dd was also changed such that the groove depth Dd was equal to the groove width Da. In addition, the aerial gap Dg was set to a value obtained by adding 0.3 mm to each of the values of the groove width Da. In this test, the groove width Da simulates the groove width Wa 1  of the groove portion Gr 1  in  FIGS. 4A and 4B . In the discharge path confirmation test, discharge was performed  100  times for each case, with the interior of the pressure chamber being pressurized to 0.8 MPa (in atmosphere), the percentage of times that discharge had occurred on the first discharge path RT 1 , out of 100 times of discharge, was determined, and the determined percentage was used as “aerial discharge rate”. 
       FIG. 11B  shows a relationship between the value of the groove width Da and the aerial discharge rate. According to the test results, the aerial discharge rate decreased with an increase in the value of the groove width Da. When the groove width Da became 0.1 mm or more, no aerial discharge occurred, and all the discharges were surface discharges. The results can be understood as follows. That is, when the groove width Da is small, aerial discharge is likely to occur along the first discharge path RT 1 , not via the recessed surface path (second discharge path) along the groove portion  212 . On the other hand, as the groove width Da increases, surface discharge along the recessed surface path along the groove portion  212  is more likely to occur. In other words, when the groove width Da of the groove portion  212  is less than 0.1 mm, the recessed path along the groove portion  212  is less likely to serve the function of a discharge path. On the other hand, when the groove width Da becomes 0.1 mm or more, the recessed path along the groove portion  212  sufficiently serves the function of a discharge path. In view of the test results, in the plasma jet plug shown in  FIG. 4 , it is preferable that the groove width Wa 1  of the groove portion Gr 1  is set to 0.1 mm or more. The same applies to the groove widths of the other groove portions Gr 2  ( FIG. 5  and  FIG. 8 ). When the groove width Wa 1  is set to 0.1 mm or more, the surface path can be sufficiently increased with the groove portion Gr 1 . Accordingly, it is possible to sufficiently suppress the occurrence of surface discharge in the cavity CV so as to allow aerial discharge to occur in a stable manner. 
       FIGS. 12A and 12B  show explanatory diagrams showing test results for the groove depth Wd 1  and the groove width Wa 1  of the groove portion Gr 1 . In this test, a plurality of types of samples including groove portions Gr 1  having different groove depths Wd 1  and groove widths Wa 1  were produced. For each of these samples, L+G (L is the length of the exposed portion of the center electrode  20 , G is the aerial gap) was set to 3.5 mm, the outer diameter 2R of the center electrode  20  was set to 1.5 mm, and the inner diameter of the enlarged inner diameter portion  16  of the insulator  10  was set to 3.5 mm. The groove width Wa 1  was set to three values, namely, 0.2 mm, 0.3 mm, and 0.5 mm, and the groove depth Wd 1  was set such that the value of Wd 1 /Wa 1  was in the range of 0.5 to 5.0. Then, each sample of the plasma jet plug was discharged, with the interior of the pressure chamber being pressurized to 0.6 MPa (in atmosphere), and plasma that had been ejected from the through hole  31  of the orifice electrode  30  was imaged from the side, to obtain a schlieren image. Then, the schlieren image was binarized so as to be classified into pixels representing a high density portion and pixels representing a low density portion, and the number of pixels representing a high density portion was calculated as the size of the ejected plasma. Note that the schlieren imaging was performed 10 times for each sample, and an average of the number of pixels of the plasma calculated by the 10 times of imaging was determined as the ejection area. 
       FIG. 12B  shows a relationship between the value of the ratio Wd 1 /Wa 1  between the groove depth Wd 1  and the groove width Wa 1  and the ejection area of plasma. According to the test results, when the value of Wd 1 /Wa 1  became 3 or more, the ejection area of plasma was reduced with an increase of the value of Wd 1 /Wa 1 , regardless of the value of the groove width Wa 1 . The reason is presumably that when the groove depth Wd 1  is excessively increased, the capacity of the cavity CV is excessively increased, making plasma less likely to be ejected. In view of the test results, it is preferable that the depth Wd 1  of the groove portion Gr 1  is less than or equal to 3 times the groove width Wa 1 . The same applies to the groove widths of the other groove portions Gr 2 . This makes it possible to keep the capacity of the cavity CV small, while increasing the shortest surface path length D 1 , thereby facilitating ejection of plasma. 
       FIGS. 13A and 13B  show results of a plasma ejection test for the surface area of the side surface of the center electrode that faces the cavity. In this test, as shown in  FIG. 6 , a plurality of types of samples having different surface areas S 20f  of the side surface  20   f  of the center electrode  20  that faces the cavity CV were produced by changing the length L of the center electrode  20  exposed in the cavity CV. For these samples, the aerial gap G was set to 0.5 mm or 1.0 mm, the outer diameter 2R of the center electrode  20  was set to a fixed value of 1 mm, the groove width Wa 1  was set to a fixed value of 0.2 mm, and the groove depth Wd 1  was set to a fixed value of 0.4 mm. Then, schlieren imaging was performed under the same conditions as those shown in  FIGS. 12A and 12B , and an average of the number of pixels of plasma calculated by 10 times of imaging was determined as the ejection area. 
       FIG. 13B  shows a relationship between the surface area S 20f  of the side surface  20   f  of the center electrode  20  that faces the cavity CV and the ejection area of plasma. As can be understood from the test results, the ejection area of plasma tends to decrease with an increase of the value of the surface area S 20f  of the side surface  20   f  of the center electrode  20 . In view of the test results, it is preferable that the value of the surface area S 20f  of the side surface  20   f  of the center electrode  20  is small. However, even when the value of the surface area S 20f  becomes less than 20 mm 2 , the ejection area of plasma will not increase very much. Accordingly, it is sufficient that the value of the surface area S 20f  is 20 mm 2  or less. Note that it is possible to adopt a shape in which the length L of the center electrode  20  that faces the cavity CV has a negative value (shape in which a portion of the nose portion  22  located at the front end of the center electrode  20  is recessed to the rear side relative to the reduced diameter portion  14  of the recessed insulator  10 ). However, such a shape may, on the contrary, make surface discharge more likely to occur. In view of this, it is preferable that the length L of the center electrode  20  that faces the cavity CV is 0 mm or more, or in other words, the surface area S 20f  of the side surface  20   f  of the center electrode  20  that faces the cavity CV is 0 mm 2  or more. 
     D. Other Embodiments: 
       FIG. 14  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   f  according to a seventh embodiment. The plasma jet plug  100   f  is the same as the fourth embodiment ( FIG. 7 ) in that a portion of the insulator  10  that faces the cavity CV is formed of a plurality of members  13   f  and  16   f,  and is different from the fourth embodiment in the following two respects. The first difference is that a reduced diameter portion  14   f  of the insulator  10  extends so as to cover the side surface of the front end portion (nose portion  22 ) of the center electrode  20  in a state in which a part of the front end portion of the center electrode  20  is exposed. In this case, it is preferable that the distance L from a front end  14   t  of the reduced diameter portion  14   f  (insulating material) provided on the side surface of the center electrode  20  to the front end of the center electrode  20  is set to 0.4 mm or less. By doing so, the distance L (referred to as “exposed length L of the center electrode  20 ”) becomes sufficiently short, so that it is possible to suppress the erosion of the center electrode caused by the heat of plasma. The second difference is that the distance H between the side surface of the center electrode  20  and the inner wall surface of the cavity CV, as measured along a direction perpendicular to the direction of the axial line O, is smaller than that in the fourth embodiment ( FIG. 7 ). However, in this case as well, it is preferable that the distance H is larger than the aerial gap G. This can reduce the likelihood of occurrence of surface discharge in a direction perpendicular to the direction of the axial line O along a path from the side surface of the center electrode  20  to the inner wall surface of the cavity CV, so that it is possible to allow aerial discharge to occur in a stable manner. Here, it is preferable that the condition that G&lt;H is satisfied by the other various embodiments. 
       FIG. 15  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   g  according to an eighth embodiment. The plasma jet plug  100   g  is different from the seventh embodiment ( FIG. 14 ) in that an insulating member  14   g  different from the insulator  10 , in place of the reduced diameter portion  14   f  of the insulator  10 , covers the side surface of the front end portion (nose portion  22 ) of the center electrode  20 , and the rest of the configuration is the same as that of the seventh embodiment. The insulating member  14   g  can be formed from any insulating material such as alumina. The insulating member  14   g  can be formed so as to cover around the center electrode  20  by any method such as plating. 
       FIG. 16  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   h  according to a ninth embodiment. The plasma jet plug  100   h  is different from the seventh embodiment ( FIG. 14 ) in the following two respects. The first difference is that a reduced diameter portion  14   h  of the insulator  10  covers, at a front end portion  14   e  thereof, the front end portion of the center electrode  20 , but a gap GP is formed on the lower side (rear side) of the front end portion  14   e.  However, the gap GP may be omitted. The second difference is that the distance H between the side surface of the center electrode  20  and the inner wall surface of the cavity CV, as measured along a direction perpendicular to the direction of the axial line O, is larger than that in the seventh embodiment ( FIG. 14 ). However, the second difference is of low importance, and therefore may not be provided. 
     As can be understood from the above-described seventh to ninth embodiments, as the insulating material for covering the side surface of the center electrode  20  in the cavity CV, it is possible to use a part of the insulator  10 , or to use an insulating material (e.g., the insulating member  14   g  shown in  FIG. 15 ) different from the insulator  10 . According to these embodiments, the exposed length L of the center electrode  20  is sufficiently short, and it is therefore possible to suppress the erosion of the center electrode caused by the heat of plasma. 
       FIG. 17  is an enlarged view, in cross section, of a front end portion of a plasma jet plug  100   j  according to a tenth embodiment. The plasma jet plug  100   j  is different from the seventh embodiment ( FIG. 14 ) in that, similarly to the sixth embodiment shown in  FIG. 9 , a front end opening portion  16   p  having a small opening is provided at the front end of a second member  16   j  of the insulator  10  so as to cover the inner surface of the orifice electrode  30 . However, the opening of the front end opening portion  16   p  is larger than the through hole  31  of the orifice electrode  30 , and an exposed surface  32  that is not covered with the front end opening portion  16   p  is left on the inner surface of the orifice electrode  30 . The exposed surface  32  is located at a position adjacent to the through hole  31  of the orifice electrode  30 . It is preferable that an outermost circumferential position  32   e  of the exposed surface  32  is located outward in a radial direction relative to an edge portion of the front end of the center electrode  20 . Here, the “radial direction” means a direction perpendicular to the direction of the axial line O. In this case, it is preferable that a distance J between the outermost circumferential position  32   e  of the exposed surface  32  and the side surface of the center electrode  20 , as measured along the radial direction, is smaller than the distance H between the side surface of the center electrode  20  and the inner wall surface of the cavity CV. This allows the inner surface of the orifice electrode  30  to be covered with the insulating material so as to leave the exposed surface  32  adjacent to the through hole  31 , so that it is possible to suppress the erosion of the inner surface of the orifice electrode  30  caused by plasma. 
     The tenth embodiment also has an additional feature that a linear distance K between the outermost circumferential position  32   e  of the exposed surface  32  and the front end of the center electrode  20  is larger than the aerial gap G. If the condition G&lt;K is satisfied, surface discharge is less likely to occur along a path from the front end of the center electrode  20  to the insulating material (front end opening portion  16   p ) covering around the inner surface of the through hole  31  of the orifice electrode  30 , and it is thus possible to allow aerial discharge to occur in a stable manner. Although the front end opening portion  16   p  of the second member  16   j  that constitutes a part of the insulator  10  is used as the insulating material for covering the inner surface around the through hole  31  of the orifice electrode  30  in the tenth embodiment, it is possible to use an insulating material different from the insulator  10  instead. 
     Although the insulator  10  is formed of a plurality of members (e.g., the two members  13   f  and  16   f  in  FIG. 14 ) in the seventh to tenth embodiments, it is possible to form the insulator  10  by a single member instead. 
       FIGS. 18A and 18B  show explanatory diagrams showing test results for the exposed length L of the center electrode  20 .  FIG. 18A  shows the shape of samples, and this shape corresponds to the shape of the seventh embodiment as shown in  FIG. 14 . In this test, the following parameters were used.
         The shortest surface path length D 1 : 3.5 mm   The inner diameter E of the through hole  31  of the orifice electrode  30 : 0.5 mm   The aerial gap G: 0.5 mm   The outer diameter 2R of the center electrode  20 : 1.5 mm   The inner diameter Dcv (inner diameter of the enlarged inner diameter portion  16   f ) of the cavity CV: 3.5 mm   The distance H between the side surface of the center electrode  20  and the inner wall surface of the cavity CV: 1.0 mm   The exposed length L of the center electrode  20  (with shielding by the insulating member  14   f ): 0 to 0.6 mm   The exposed length L of the center electrode  20  (without shielding by the insulating member  14   f ): 2.0 mm       

       FIG. 18B  is a graph showing test results for a relationship between the exposed length L of the center electrode  20  and the erosion volume of the front end of the center electrode  20 . The vertical axis represents a ratio obtained by dividing the erosion volume of the front end of the center electrode  20  with shielding on the side surface of the center electrode  20  by the erosion volume thereof without shielding on the side surface of the center electrode  20 . Here, “with shielding on the side surface of the center electrode  20 ” means that the side surface of the front end portion of the center electrode  20  is covered with the reduced diameter portion  14   f  of the insulator  10  (L=0 to 0.6 mm). On the other hand, “without shielding on the side surface of the center electrode  20 ” means that the side surface of the front end portion of the center electrode  20  is not covered with the reduced diameter portion  14   f  of the insulator  10  (L=2.0 mm). The “erosion volume” is a value obtained by determining a volume that has been lost from the front end portion of the center electrode  20  after performing a spark discharge durability test at 30 Hz for 30 hours. 
     As can be understood from the results shown in  FIG. 18B , when the side surface of the center electrode  20  is shielded by the insulating material, the erosion volume at the front end of the center electrode  20  is reduced as compared with when no shielding is provided. In particular, when the exposed length L of the center electrode  20  is set to 0.4 mm or less, a significant effect of suppressing the erosion of the center electrode caused by plasma is achieved. 
       FIGS. 19A and 19B  show explanatory diagrams showing test results for covering of an inner surface of the orifice electrode  30  by an insulator.  FIG. 19A  shows the shape of samples, and this shape corresponds to the shape of the tenth embodiment as shown in  FIG. 17 . In this test, the following parameters were used.
         The shortest surface path length D 1 : 4.0 mm   The inner diameter E of the through hole  31  of the orifice electrode  30 : 0.5 mm   The aerial gap G: 0.5 mm   The outer diameter 2R of the center electrode  20 : 1.5 mm   The inner diameter Dcv (inner diameter of the enlarged inner diameter portion  16   j ) of the cavity CV: 3.5 mm   The distance H between the side surface of the center electrode  20  and the inner wall surface of the cavity CV: 1.0 mm   The outer diameter D 32  of the exposed surface  32  on the inner surface of the orifice electrode  30 : 1.4 to 1.7 mm       

     Here, the outer diameter D 32  of the exposed surface  32  is the same as the inner diameter of the front end opening portion  16   p  that covers the inner surface around the through hole  31  of the orifice electrode  30 . The distance J between the outermost circumferential position  32   e  of the exposed surface  32  and the side surface of the center electrode  20 , as measured along the radial direction, is equal to J=(D 32 −2R)/2. 
       FIG. 19B  is a graph showing test results for a relationship between the outer diameter D 32  of the exposed surface  32  on the inner surface of the orifice electrode  30  and the erosion volume of the inner surface of the orifice electrode  30 . The vertical axis represents a ratio obtained by dividing the erosion volume of the inner surface of the orifice electrode  30  with shielding on the inner surface of the orifice electrode  30  by the erosion volume thereof without shielding on the inner surface of the orifice electrode  30 . Here, “with shielding on the inner surface of the orifice electrode  30 ” means that the inner surface of the orifice electrode  30  is covered with the front end opening portion  16   p  of the insulator  10 . On the other hand, “without shielding on the inner surface of the orifice electrode  30 ” means that the inner surface of the orifice electrode  30  is not covered with the front end opening portion  16   p  of the insulator  10 . The “erosion volume” is a value obtained by determining a volume that has been lost from the inner surface of the orifice electrode  30  after performing a spark discharge durability test at 30 Hz for 30 hours. 
     As shown at the lower end of  FIG. 19B , when D 32 =1.4 mm or 1.5 mm, the distance K between the outermost circumferential position  32   e  of the exposed surface  32  and the front end of the center electrode  20  is equal to the aerial gap G. In these cases, slight channeling occurred in the front end opening portion  16   p  of the insulator  10 . On the other hand, when D 32 =1.6 mm or 1.7 mm, the distance K between the outermost circumferential position  32   e  of the exposed surface  32  and the front end of the center electrode  20  is larger than the aerial gap G. In these cases, no channeling occurred in the front end opening portion  16   p  of the insulator  10 . The reason is presumably that if G&lt;K is satisfied, surface discharge is less likely to occur along a path from the front end of the center electrode  20  to the insulating material (front end opening portion  16   p ) covering the inner surface around the through hole  31  of the orifice electrode  30 . 
     As can be understood from the results shown in  FIG. 19B , it is preferable that the inner surface of the orifice electrode  30  is shielded with the insulating material, since the erosion volume on the inner surface of the orifice electrode  30  is reduced as compared with when no shielding is provided. It can also be understood that, in order to reduce the likelihood of occurrence of surface discharge, it is preferable that inner surface around the through hole  31  of the orifice electrode  30  is covered with an insulating material so as to satisfy G&lt;K. 
     E. Still Other Embodiments: 
       FIG. 20  is an enlarged cross-sectional view of a front end portion of a plasma jet plug  100   k  according to an eleventh embodiment. In the plasma jet plug  100   k,  a center electrode  20   k  includes a head portion  21  located on the rearmost side, a nose portion  22  located on the front side relative to the head portion  21  and having a smaller outer diameter than the head portion  21 , and a front end small diameter portion  27  located on the frontmost side and having the smallest outer diameter. The rest of the configuration of the plasma jet plug  100   k  is substantially the same as that shown in  FIG. 2 , and therefore, the description thereof has been omitted here. 
       FIG. 21  is an enlarged view, in cross section, of a front end portion of the plasma jet plug  100   k  according to the eleventh embodiment. Note that  FIG. 21  is shown upside down relative to  FIGS. 1 and 20 . That is, the upper side of  FIG. 21  corresponds to the front side of the plasma jet plug  100   k,  and the lower side of  FIG. 21  corresponds to the rear side of the plasma jet plug  100   k.    
     As described previously, the nose portion  22  and the front end small diameter portion  27  are formed in the vicinity of the front end of the center electrode  20   k.  Each of the nose portion  22  and the front end small diameter portion  27  has a columnar shape. A reduced diameter portion  28  is provided between the nose portion  22  and the front end small diameter portion  27 . Although the reduced diameter portion  28  is tapered in the example shown in  FIG. 21 , the reduced diameter portion  28  may be formed so as to constitute a surface perpendicular to the axial line O instead of being tapered. 
     At a long nose portion  13  in the vicinity of the front end of the insulator  10 , an enlarged inner diameter portion  16  having a larger inner diameter than the long nose portion  13  is formed. Note that the long nose portion  13  is also referred to as “small inner diameter portion  13 ”. A reduced diameter portion  14  is formed between the long nose portion  13  and the enlarged inner diameter portion  16 . Although the reduced diameter portion  14  is tapered in this example, the reduced diameter portion  14  may be formed so as to constitute a surface perpendicular to the axial line O instead of being tapered. The reduced diameter portion  14  of the insulator  10  is provided on the front side relative to the reduced diameter portion  28  of the center electrode  20   k.  The outer circumference of the front end small diameter portion  27  of the center electrode  20   k  and the inner surface of the long nose portion  13  of the insulator  10  are spaced apart by a distance Dp. The circular groove portion having a width equal to the distance Dp corresponds to a second cavity portion CV 2  described below. 
     The cavity CV is a space surrounded by a surface  20   s  of the center electrode  20   k,  an inner surface  10   in  of the insulator  10 , and an inner surface  30   in  of the orifice electrode  30 . However, the cavity CV does not include a portion constituted by the through hole  31  of the orifice electrode  30 , and means a space inside the inner surface  30   in  of the orifice electrode  30 , assuming that the through hole  31  is not provided. Between the outer circumferential surface of the nose portion  22  of the center electrode  20   k  and the inner surface of the insulator  10 , a minute clearance (less than 0.06 mm) is formed for assembly of the two components. A space with a clearance of less than 0.06 mm is a minute space in which no plasma will be generated, and therefore does not function as a part of the cavity CV. As used herein, “cavity” means a space in which plasma can be generated, and also means a space having a clearance of 0.06 mm or more. To be more specific, the “cavity” in the eleventh embodiment shown in  FIG. 21  means a space that can be formed between the inner surface  10   in  of the front end portion of the insulator  10 , the surface of the front end portion of the center electrode  20   k,  and the inner surface  30   in  of the orifice electrode  30  and has a clearance of 0.06 mm or more, and the “cavity” does not include a space having a clearance of less than 0.06 mm. The cavity CV can be classified into the following two cavities. 
     (a) First cavity portion CV 1 : a cavity portion present on the front side relative to a rear end  14   e  of the reduced diameter portion  14  of the insulator  10 . 
     (b) Second cavity portion CV 2 : a cavity portion present on the rear side relative to the rear end  14   e  of the reduced diameter portion  14  of the insulator  10 . 
     In  FIG. 21 , the following dimensions are further given. 
     (1) Dp: the distance (referred to as “radial spatial distance Dp”) between the outer circumference of the front end small diameter portion  27  of the center electrode  20   k  and the long nose portion  13  of the insulator  10 . The radial spatial distance Dp corresponds to the width of the second cavity portion CV 2 . 
     (2) Dq: the distance between a rear end  28   e  of the reduced diameter portion  28  of the center electrode  20   k  and the rear end  14   e  of the reduced diameter portion  14  of the insulator  10 . The distance Dq corresponds to the depth, in the axial direction, of the second cavity portion CV 2 . 
     (3) Dr: the shortest distance between the front end edge  20   c  of the center electrode  20   k  and the inner surface  10   in  of the insulator  10 . Note that the “shortest distance” means a minimum value obtained when the distance from the front end edge  20   c  of the center electrode  20   k  to the inner surface  10   in  of the insulator  10  was measured in a given direction. 
     (4) Ds: a difference between the inner radius of the enlarged inner diameter portion  16  of the insulator  10  and the inner radius of the long nose portion  13 . The difference Ds corresponds to a difference between the inner radius of the enlarged inner diameter portion  16  of the insulator  10  and the outer radius of the nose portion  22  of the center electrode  20   k.    
     (5) D 27 : the outer diameter of the front end small diameter portion  27  of the center electrode  20   k.    
     (6) D 22 : the outer diameter of the nose portion  22  of the center electrode  20   k.    
     (7) E: the inner diameter of the through hole  31  of the orifice electrode  30 . 
     (8) G: the distance, in the axial direction, between the inner surface  30   in  of the orifice electrode  30  and the front end surface  20   t  of the center electrode  20   k.  The distance G is also referred as “aerial gap G”. 
     (9) Z: the distance between the inner surface  30   in  of the orifice electrode  30  and the rear end  14   e  of the reduced diameter portion  14  of the insulator  10 . The distance Z corresponds to the depth, in the axial direction, of the first cavity portion CV 1 . 
     It is preferable that the inner diameter E of the through hole  31  of the orifice electrode  30  is smaller than the outer diameter D 27  of the front end small diameter portion  27  of the center electrode  20   k.  This is to facilitate aerial discharge in the aerial gap G. 
       FIG. 22  is an enlarged view, in cross section, of a front end portion a plasma jet plug  100   m  according to a twelfth embodiment. A center electrode  20   m  of the plasma jet plug  100   m  does not include the front end small diameter portion  27  included in the center electrode  20   k  of the plasma jet plug  100   k  shown in  FIG. 21 , and has a shape in which the nose portion  22  is directly extended to the front end. Accordingly, the second cavity portion CV 2  present in the plasma jet plug  100   k  shown in  FIG. 21  is not present in the plasma jet plug  100   m  shown in  FIG. 22 . 
     Also in the plasma jet plug  100   m  shown in  FIG. 22 , which does not have the second cavity portion CV 2 , by ensuring a sufficiently large shortest distance Dr between the front end edge  20   c  of the center electrode  20   m  and the inner surface  10   in  of the insulator  10 , it is possible to reduce the likelihood of occurrence of surface discharge so as to allow aerial discharge to occur in a stable manner. However, by providing the second cavity portion CV 2  as in  FIG. 21 , the shortest distance Dr between the front end edge  20   c  of the center electrode  20   m  and the inner surface  10   in  of the insulator  10  can be increased, so that it is possible to suppress the occurrence of surface discharge, and to keep the overall capacity of the cavity CV small, making it possible to facilitate ejection of plasma. 
     In the following, results of several tests performed by using the dimensions for the plasma jet plugs shown in  FIGS. 21 and 22  as parameters will be described sequentially. 
       FIGS. 23A-23D  show results of a discharge path confirmation test for the relationship between the shortest distance Dr between the front end edge  20   c  of the center electrode  20   n  and the inner surface  10   in  of the insulator  10 , and the aerial gap G. Here,  FIG. 23A  shows a vertical cross-sectional view of a plasma jet plug  100   n  for the discharge path confirmation test, and  FIG. 23B  shows a plan view thereof. The plasma jet plug  100   n  has a configuration in which the orifice electrode  30  of the plasma jet plug  100   m  shown in  FIG. 22 , which has no second cavity portion CV 2 , has been replaced by a bar-shaped electrode  30  bar. The reason is that it is difficult to image the inside of the cavity CV from the through hole  31  ( FIG. 22 ) of the orifice electrode  30 . In the discharge path confirmation test, the plasma jet plug  100   n  was mounted in a pressure chamber, and discharge was performed 100 times, with the interior of the pressure chamber being pressurized to 1.0 MPa (in atmosphere). At this time, the discharge path in the cavity CV was imaged by using a high-speed camera, and the percentage of times that surface discharge had occurred, out of 100 times of discharge, was determined. 
       FIG. 23C  shows various dimensions of samples S 101  to S 104  for which the value of the ratio Dr/G between the shortest distance Dr between the front end edge  20   c  of the center electrode  20   n  and the inner surface  10   in  of the insulator  10  and the aerial gap G was used as parameters. Since the samples S 101  to S 104  do not have a second cavity portion CV 2 , the dimensions Dp and Dq related to the second cavity portion CV 2  have a value of zero, and Dr=Ds. In this test, the four samples S 101  to S 104 , for which the aerial gap G was fixed at 0.5 mm and the value of the shortest distance Dr was changed in the range of 0.25 mm to 1.00 mm, were used. 
       FIG. 23D  shows the surface discharge rate obtained by the discharge path confirmation test. According to these test results, the surface discharge rate decreased with an increase in the value of Dr/G. When Dr/G became 1.5 or more, no surface discharge occurred, and all the discharges were aerial discharges. In view of the results, it is preferable that the value of the ratio Dr/G of the shortest distance Dr to the aerial gap G is as large as possible. It is particularly preferable that the following relationship is satisfied. 
       1.5× G≦Dr    (1)
 
     If the expression (1) is satisfied, the shortest distance Dr between the front end edge  20   c  of the center electrode  20   n  and the inner surface  10   in  of the insulator  10  is sufficiently larger than the aerial gap G, so that surface discharge is less likely to occur, making it possible to allow aerial discharge to occur in a stable manner. As a result, it is possible to suppress the occurrence of channeling. 
     The relationship represented by the above expression (1) is presumably applicable not only to the plasma jet plug  100   m  as shown in  FIG. 22 , which does not have the second cavity portion CV 2 , but also to the plasma jet plug  100   k  as shown in  FIG. 21 , which has the second cavity portion CV 2 . The reason is that if the above expression (1) is satisfied, the shortest distance Dr is also sufficiently larger than the aerial gap G when the second cavity portion CV 2  is present, so that it can be expected that surface discharge is less likely to occur, allowing aerial discharge to occur in a stable manner. 
     In the sense of making aerial discharge more likely to occur than surface discharge, the value Dr is preferably set so as to satisfy the above expression (1). However, on the other hand, the value of Dr preferably falls within such a range that the capacity of the cavity CV will not be excessively increased. The reason is that when the capacity of the cavity CV is excessively increased, the plasma ejection performance may be deteriorated. In this sense, the value of Dr is, for example, preferably 2 mm or less, more preferably 1.5 mm or less, most preferably 1 mm or less. 
       FIGS. 24A-24C  show explanatory diagrams showing discharge test results for the radial spatial distance Dp of the second cavity portion CV 2 .  FIG. 24A  is a schematic plan view of a testing apparatus, and  FIG. 24B  is a cross-sectional view thereof taken along the line B-B. In this test, a first electrode  310  was placed in a pressure chamber  300 , an insulator  320  having a rectangular parallelepiped shape is fitted into a recess in the upper surface of the first electrode  310 , and a columnar second electrode  330  was placed on the insulator  320 . A wall portion  312  extending vertically upward was formed at one end of the first electrode  310 , and a spatial distance Dp was set between the wall portion  312  and the insulator  320 . Of a surface path extending from a side surface of the second electrode  330  toward the wall portion  312  of the first electrode  310 , the surface distance on the insulator  320  was set to 0.5 mm. Additionally, the distance Dq between the upper surface of the first electrode  210  and the upper surface of the insulator  320  was varied by changing the thickness of the insulator  320  to several values. The spatial distance Dp was adjusted such that the distance Dq was equal to the spatial distance Dp. The wall portion  312  of the first electrode  310  simulates the center electrode  20   k  shown in  FIG. 21 , the groove portion GV between the wall portion  312  of the first electrode  310  and the insulator  320  simulates the second cavity portion CV 2  shown in  FIG. 21 . That is, the spatial distance Dp shown in  FIG. 24B  simulates the radial spatial distance Dp of the second cavity portion CV 2  ( FIG. 21 ), and the distance Dq shown in  FIG. 24B  simulates the depth Dq of the second cavity portion CV 2 . 
     In the discharge test, discharge was performed 100 times for each case, with the interior of the pressure chamber being pressurized to 0.2 mPa, 0.6 MPa, and 1.0 MPa (all in atmosphere). Then, the discharge path was imaged by using a high-speed camera, and the percentage of times that aerial discharge had occurred, out of 100 times of discharge, was determined. Here, “aerial discharge” means a discharge not passing through a surface path along the surface of the insulator  320 , and “surface discharge” means a discharge passing through a surface path along the surface of the insulator  320 . 
       FIG. 24C  shows a relationship between the spatial distance Dp and the aerial discharge rate. According to the test results, the aerial discharge rate decreased with an increase in the value of the spatial distance Dp. When the spatial distance Dp became 0.1 mm or more, no aerial discharge occurred, and all the discharges were surface discharges. The results can be understood as follows. That is, as the spatial distance Dp shown in  FIG. 24B  increases, aerial discharge that laterally arrives at the second electrode  330  through the air from the surface of the wall portion  312  of the first electrode  310  is less likely to occur. By applying this to the plasma jet plug  100   k  as shown in  FIG. 21 , it can be presumed that as the radial spatial distance Dp of the second cavity portion CV 2  increases, discharge along the surface path of the inner surface of the insulator  10  after jumping over the groove portion of the second cavity portion CV 2  through the air from the side surface of the center electrode  20   k  is less likely to occur. Accordingly, in order to suppress surface discharge in the plasma jet plug  100   k  as shown in  FIG. 21 , which has the second cavity portion CV 2 , the radial spatial distance Dp of the second cavity portion CV 2  is preferably large, particularly 0.1 mm or more. This makes it possible to suppress the occurrence of surface discharge so as to allow aerial discharge to occur in a stable manner. 
     In the sense of making aerial discharge more likely to occur than surface discharge, the radial spatial distance Dp of the second cavity portion CV 2  is preferably 0.1 mm or more. However, on the other hand, the value of Dp preferably falls within such a range that the capacity of the second cavity portion CV 2  will not be excessively increased. In this sense, the value of Dp is, for example, preferably 1 mm or less, more preferably 0.7 mm or less, more preferably 0.5 mm or less. 
       FIGS. 25A and 25B  show test results for the ratio Dq/Dp between the depth Dq and the radial spatial distance Dp of the second cavity portion CV 2 . In this test, a plasma jet plug  100   k  was mounted in a pressure chamber, and discharge was performed 100 times, with the interior of the pressure chamber being pressurized to 1.0 MPa (in atmosphere), and the discharge voltage was measured. Here, the “discharge voltage” means a voltage at which dielectric breakdown has occurred as a result of application of a high voltage. 
       FIG. 25A  shows the dimensions of samples S 201  to S 216 . The sample S 201  is a plug having the shape shown in  FIG. 22 , which does not have the second cavity portion CV 2 . The samples S 202  to S 206  are samples for which the radial spatial distance Dp of the second cavity portion CV 2  was set to a fixed value of 0.1 mm and the depth Dq of the second cavity portion CV 2  was varied. The samples S 207  to S 211  are samples for which the radial spatial distance Dp of the second cavity portion CV 2  was set to a fixed value of 0.3 mm and the depth Dq of the second cavity portion CV 2  was varied. The samples S 212  to S 216  are samples for which the radial spatial distance Dp of the second cavity portion CV 2  was set to a fixed value of 0.5 mm and the depth Dq of the second cavity portion CV 2  was varied. Note that the difference in the radial spatial distance Dp between the three sample groups S 202  to S 206 , S 207  to S 211 , and S 212  to S 216  was adjusted by changing the outer diameter D 27  of the front end small diameter portion  27  of the center electrode  20   k.  In this test, for all the samples S 201  to S 216 , the inner diameter E of the through hole  31  of the orifice electrode  30  was set to 2.5 mm, which was an excessively larger value than a normal value (about 1.0 mm). The reason for this is to make it certain that surface discharge always occurs, without the occurrence of aerial discharge from the center electrode  20   k  to the orifice electrode  30 . 
       FIG. 25B  shows a relationship between the value of Dq/Dp and the discharge voltage. As can be understood from this result, the discharge voltage tends to increase with an increase in the value of Dq/Dp. As described above, the samples used in this test have a shape in which surface discharge always occurs without the occurrence of aerial discharge from the center electrode  20   k  to the orifice electrode  30 . Accordingly, the higher the discharge voltage in  FIG. 25B , the higher the likelihood that aerial discharge from the center electrode  20   k  to the orifice electrode  30  occurs in the actual plasma jet plug  100   k.  Therefore, it is preferable that the discharge voltage in this test is high, since aerial discharge is likely to occur and surface discharge is less likely to occur. Specifically, it is preferable that the value of Dq/Dp of the ratio between the depth Dq and the radial spatial distance Dp of the second cavity portion CV 2  exceeds 0 (i.e., that the second cavity portion CV 2  is present). Even when the value of Dq/Dp becomes 3 or more, the discharge voltage will not increase further. Accordingly, it is sufficient that the value of Dq/Dp is 3 or less. 
       FIGS. 26A and 26B  show results of a plasma ejection test for the ratio Dq/Dp between the depth Dq and the radial spatial distance Dp of the second cavity portion CV 2 . In this test, the plasma jet plug  100   k  was discharged, with the interior of the pressure chamber being pressurized to 0.6 MPa (in atmosphere), and the plasma that had been ejected from the through hole  31  of the orifice electrode  30  was imaged from the side, to obtain a schlieren image. Then, the schlieren image was binarized so as to be classified into pixels representing a high density portion and pixels representing a low density portion, and the number of the pixels representing a high density portion was calculated as the size of the ejected plasma. Note that the schlieren imaging was performed 10 times for each sample, and an average of the number of pixels of the plasma calculated by the 10 times of imaging was determined as the ejection area. 
       FIG. 26A  shows the dimensions of samples S 302  to S 316 . The dimensions of the samples S 302  to S 316  are the same as those of the samples S 202  to S 216  shown in  FIG. 25A  except that the inner diameter E of the through hole  31  of the orifice electrode  30  was set to 1.0 mm (normal value). The reason that the inner diameter E of the through hole  31  of the orifice electrode  30  was set to 1.0 mm for the samples S 302  to S 316  shown in  FIG. 26A  is to cause aerial discharge in the aerial gap G between the center electrode  20   k  and the orifice electrode  30 . 
       FIG. 26B  shows a relationship between the value of Dq/Dp and the ejection area of plasma. As can be understood from the test results, the ejection area of plasma tends to decrease with an increase in the value of Dq/Dp. Therefore, according to the result shown in  FIG. 26B , it is preferable that the value of the ratio Dq/Dp between the depth Dq and the radial spatial distance Dp of the second cavity portion CV 2  is small. However, even when the value of Dq/Dp becomes smaller than 3, the ejection area of plasma will not increase very much. Accordingly, it is sufficient that the value of Dq/Dp is 3 or less. 
     In view of the results shown in  FIGS. 25 and 26  described above, it is preferable that the radial spatial distance Dp and the depth Dq of the second cavity portion CV 2  satisfy the following relationship. 
       0&lt; Dq≦ 3× Dp    (2)
 
     By setting the depth Dq of the second cavity portion CV 2  in the range represented by the expression (2), it is possible to increase the tendency that aerial discharge is more likely to occur than surface discharge ( FIG. 25B ). Further, it is possible to prevent the capacity of the second cavity portion from being excessively increased, thus facilitating ejection of plasma ( FIG. 26B ). 
       FIGS. 27A and 27B  show results of a plasma ejection test for the ratio Dp/Dr between the radial spatial distance Dp of the second cavity portion CV 2  and the shortest distance Dr between the front end edge  20   c  of the center electrode  20   k  and the inner surface  10   in  of the insulator  10 . The plasma ejection test was performed under the same conditions as those used in  FIGS. 26A and 26B , except for the shape of samples. 
       FIG. 27A  shows the dimensions of samples S 401  to S 405 . For the samples S 401  to S 405 , the radial spatial distance Dp of the second cavity portion CV 2  was varied by changing the outer diameter D 22  of the nose portion  22  of the center electrode  20   k.  However, the difference Ds between the inner radius of the enlarged inner diameter portion  16  and the inner radius of the long nose portion  13  of the insulator  10  was adjusted such that the shortest distance Dr between the front end edge  20   c  of the center electrode  20   k  and the inner surface  10   in  of the insulator  10  had a fixed value (1.0 mm). 
       FIG. 27B  shows a relationship between the value of Dp/Dr and the ejection area of plasma. As can be understood from the test results, the ejection area of plasma tends to decrease with an increase of the value of Dp/Dr. Therefore, according to the results shown in  FIG. 27B , it is preferable that the value of Dp/Dr is small. However, even when the value of Dq/Dp becomes smaller than 0.5, the ejection area f plasma will not increase further. Accordingly, it is sufficient that the value of Dq/Dp is 0.5 or less. Note that in the structure shown in  FIG. 21 , Dp represents the distance from the side surface (outer circumferential surface) of the center electrode  20   k  to the wall surface of the insulator  10  that constitutes the outer circumference of the second cavity portion CV 2 . Dr represents the shortest distance from the front end edge  20   c  of the center electrode  20   k  to the inner surface  10   in  of the insulator  10  that constitutes the outer circumference of the first cavity portion CV 1 . The results shown in  FIG. 27B  can be understood to mean that when the value of the ratio Dp/Dr between these distances exceeds 0.5, plasma becomes likely to spread deeply into the second cavity portion CV 2 , so that ejection force for ejecting plasma from the through hole  31  of the orifice electrode  30  to the outside is reduced. 
     In view of the test results as shown in  FIG. 27B , it is preferable that the relationship between the radial spatial distance Dp of the second cavity portion CV 2  and the shortest distance Dr between the front end edge  20   c  of the center electrode  20   k  and the inner surface  10   in  of the insulator  10  satisfies the following relationship. 
       ( Dp/Dr )≦0.5   (3)
 
     By setting Dp/Dr so as to satisfy the expression (3), it is possible to further facilitate ejection of plasma. 
     MODIFIED EMBODIMENTS 
     The present invention is not limited to the above embodiments and modes and may be embodied in various other forms without departing from the scope of the invention. 
     Modified Embodiment 1 
     The structural features described with reference to  FIGS. 4 to 19  and the structural features described with reference to  FIGS. 21 to 27  may be used together or separately. 
     Modified Embodiment 2 
     As the configuration of the plasma jet plug, it is possible to use various configurations other than those shown in  FIGS. 4 to 9 ,  FIGS. 14 to 17 , and  FIGS. 21 to 22 . For example, the center electrode  20  in the vicinity of the front end may not have a simple columnar shape, and may be provided with irregularities on the surface thereof. 
     The front end of the center electrode  20  may not have the shape of a sharp edge, and may be chamfered (e.g., R-chambered or C-chambered). This makes electric field concentration less likely to occur, so that it is possible to further suppress the erosion of the center electrode  20  caused by the heat of plasma. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           4 : seal member 
           5 : gasket 
           6 : ring member 
           9 : talc 
           10 : insulator 
           10   z : reduced inner diameter portion of insulator 
           10   in : inner surface of insulator 
           12 : axial hole of insulator 
           13 : long nose portion (small inner diameter portion) of insulator 
           13   c,    13   d,    13   e : first member 
           14 : reduced diameter portion of insulator 
           15 : electrode housing portion of insulator 
           16 : enlarged inner diameter portion of insulator 
           16   c  to  16   j : second member of insulator 
           16   p : front end opening portion of insulator 
           17 : front trunk portion of insulator 
           18 : rear trunk portion of insulator 
           19 : flange portion of insulator 
           20 ,  20   k  to  20   n : center electrode 
           20   f : side surface of center electrode 
           20   s : surface of center electrode 
           20   t : front end surface of center electrode 
           21 : head portion of center electrode 
           22 : nose portion of center electrode 
           30 : orifice electrode 
           30   in : inner surface of orifice electrode 
           31 : through hole of orifice electrode 
           32 : exposed surface of orifice electrode 
           32   e : outermost circumferential position of exposed surface of orifice electrode 
           40 : metal terminal 
           50 : metal shell 
           51 : tool engagement portion of metal shell 
           52 : thread portion of metal shell 
           53 : crimp portion of metal shell 
           54 : flange portion of metal shell 
           55 : seating portion of metal shell 
           56 : locking portion of metal shell 
           57 : front end portion of metal shell 
           57 A: recess of front end portion of metal shell 
           80 : packing 
           100 ,  100   a  to  100   n,    100   r : plasma jet plug 
           120 : ignition device 
           130 : control circuit portion 
           140 : spark discharge circuit portion 
           160 : plasma discharge circuit portion 
           161 : high voltage generation circuit 
           162 : condenser 
           210 : insulator 
           210   s : upper surface of insulator 
           212 : groove portion of insulator 
           220 : first electrode 
           230 : second electrode