Patent Publication Number: US-7218047-B2

Title: Indirectly heated electrode for gas discharge tube

Description:
TECHNICAL FIELD 
   The present invention concerns an indirectly heated electrode for gas discharge tube. 
   BACKGROUND ART 
   A known example of the abovementioned indirectly heated electrode for gas discharge tube is that which is disclosed in Japanese Examined Patent Publication No. 62-56628 (U.S. Pat. No. 4,441,048). The indirectly heated electrode for gas discharge tube (indirectly heated cathode for gas discharge tube) that is disclosed in Japanese Examined Patent Publication No. 62-56628 has an arrangement wherein a double coil is wound a plurality of turns around and fixed closely to the outer wall of a cylinder of good thermal conductivity, a uniform cathode surface is formed by applying a paste-form cathode material in the space inside the primary coil and between the secondary coil of the double coil, and providing a heater inside the cylinder. 
   DISCLOSURE OF THE INVENTION 
   An object of the present invention is to provide an indirectly heated electrode for gas discharge tube that can realize long electrode service life and stable discharge. 
   As a result of research, the present inventors made the following new findings. In a case where the potential distribution of the electrode (cathode) surface is non-uniform, since the heat generation amount is non-uniform accordingly, the density of thermion generation is also non-uniform and localized discharge (skewing of the discharge position) occurs. Localized discharge causes the cathode material (metal oxide that is the material likely to emit electrons) to undergo removal (sputtering) and stabilization (mineralization) by oxidation with the reduced metal, that is, causes degradation of the thermionic emission ability and movement of the discharge position to another position with better thermionic emission characteristics. By thus repeating localized degradation of thermionic emission, the electrode surface becomes degraded. The abovementioned movement of the discharge position also causes the discharge itself to become unstable. 
   Based on the above research results, the present invention provides an indirectly heated electrode for gas discharge tube comprising: a coil member, wound in coil form; a heater, disposed at the inner side of the coil member and having an electrical insulating layer formed on a surface thereof; a metal oxide, serving as a material likely to emit electrons and held by the coil member; and an electrical conductor, having a predetermined length and disposed at the inner side of the coil member and so as to be in contact with the coil member. 
   In the indirectly heated electrode for gas discharge tube of the present invention, since an equipotential surface is effectively formed at the rear surface (surface at the opposite side of the discharge surface) of the coil member by the electrical conductor and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened. The occurrence of localized discharge can thus be restrained and long service life of the electrode can be realized. Since the movement of the discharge position is also restrained, stable discharge can be obtained over along period of time. Also due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art, thus enabling the provision of an indirectly heated electrode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and enabling realization of pulse operation and large current operation. 
   Also, the electrical conductor is preferably put in contact with the metal oxide and in contact with a plurality of coil parts of the coil member. In this case, the potential of the discharge surface that is made up of a plurality of discharge points or discharge lines by the electrical conductor is made substantially uniform. The sputtering of the metal oxide and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can thus be restrained, that is, the degradation of the thermionic emission ability can be restrained and the movement of the discharge position can be restrained as well. As a result, long service life and stable discharge of an electrode can be realized by a simple arrangement in which an electrical conductor is disposed so as to be in contact with a metal oxide. 
   Also, the electrical conductor is preferably a high-melting-point metal that has been formed to a mesh, a wire, or a plate. By the electrical conductor being such a high-melting-point metal that has been formed to a mesh, a wire, or a plate, an electrical conductor of an arrangement that can restrain the degradation of the thermionic emission ability and movement of the discharge position can be realized at low cost and in simpler manner. Also, since the electrical conductor will be a rigid body, it can be processed readily and can be put in close contact with the metal oxide. With the present application, “plate” shall refer inclusively to such shapes as a ribbon shape, foil shape, etc. 
   Also, the coil member is preferably a multiple coil, arranged by winding a coil having a mandrel in coil form. With such an arrangement, when a multiple coil is used, the metal oxide that is the material likely to emit electrons is held in a manner where it is sandwiched between the pitches (spacings), which are the gaps between the wire material that forms the coil. Since the distance between pitches is small and gap-like, the falling off the metal oxide due to vibration can be restrained. Also, since a plurality of pitches of gap-like structure exist, a large amount of metal oxide can be held, providing the effect of replenishing the metal oxide loss that accompanies the degradation with time during discharge. Furthermore, since a mandrel is provided, deformation of the multiple coil during processing can be restrained. 
   Also, the metal oxide is preferably an oxide of a single metal among barium (Ba), strontium (Sr), and calcium (Ca) or a mixture of oxides of these metals or contains an oxide of a rare earth metal. By the metal oxide being an oxide of a single metal among barium, strontium, and calcium or a mixture of oxides of these metals or containing an oxide of a rare earth metal, the work function of the electron emitting part can be made small effectively and the emission of thermions can thus be facilitated. 
   The present invention provides an indirectly heated electrode for gas discharge tube comprising: a coil member, wound in coil form; a heater, disposed at the inner side of the coil member and having an electrical insulating layer formed on a surface thereof; a high-melting-point metal, formed to a mesh, a wire, or a plate and disposed along the length direction of the coil member at the inner side of the coil member; and a metal oxide, serving as a material likely to emit electrons and held by the coil member so as to be in contact with the high-melting-point metal; and wherein the high-melting-point metal forms a plurality of contacts with the coil member and the coil member is grounded. 
   In the indirectly heated electrode for gas discharge tube of the present invention, since an equipotential surface is effectively formed at the rear surface (surface at the opposite side of the discharge surface) of the coil member by the high-melting-point metal that has been formed to a mesh, a wire, or a plate and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened. The sputtering of the metal oxide and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can thus be restrained, that is, the degradation of the thermionic emission ability can be restrained and long service life of the electrode can be realized. Since the movement of the discharge position is also restrained, stable discharge over a long period of time can be realized. Also, since the high-melting-point metal is a rigid body, it is easy to process and can be put in close contact with the metal oxide. Also, due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art, thus enabling the provision of an indirectly heated electrode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and enabling realization of pulse operation and large current operation. 
   The present invention provides an indirectly heated electrode for gas discharge tube comprising: a coil member, wound in coil form; a heater, disposed at the inner side of the coil member and having an electrical insulating layer formed on a surface thereof; a high-melting-point metal, formed to a mesh, a wire, or a plate and disposed along the length direction of the coil member at the inner side of the coil member; and a metal oxide, serving as a material likely to emit electrons and held by the coil member so as to be in contact with the high-melting-point metal; and wherein the high-melting-point metal forms a plurality of contacts with the coil member and the high-melting-point metal is grounded. 
   In the indirectly heated electrode for gas discharge tube of the present invention, since an equipotential surface is effectively formed at the rear surface (surface at the opposite side of the discharge surface) of the coil member by the high-melting-point metal that has been formed to a mesh, a wire, or a plate and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened. The sputtering of the metal oxide and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can thus be restrained, that is, the degradation of the thermionic emission ability can be restrained and long service life of the electrode can be realized. Since the movement of the discharge position is also restrained, stable discharge over a long period of time can be realized. Also, since the high-melting-point metal is a rigid body, it is easy to process and can be put in close contact with the metal oxide. Also, due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art, thus enabling the provision of an indirectly heated electrode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and enabling realization of pulse operation and large current operation. 
   The present invention provides an indirectly heated electrode for gas discharge tube comprising: a coil member, having a mandrel and wound in coil form; a heater, disposed at the inner side of the coil member and having an electrical insulating layer formed on a surface thereof; a high-melting-point metal, formed to a mesh, a wire, or a plate and disposed along the length direction of the coil member between the coil member and the heater; and a metal oxide, serving as a material likely to emit electrons and disposed so as to be in contact with the coil member; and wherein the high-melting-point metal forms a plurality of electrical contacts with the coil member and the coil member is grounded. 
   In the indirectly heated electrode for gas discharge tube of the present invention, since the coil member is grounded, thermions, secondary electrons, etc. are supplied via this coil member. Also, at the rear surface (surface at the opposite side of the discharge surface) of the coil member, since an equipotential surface is effectively formed on the cathode surface by the high-melting-point metal and the inner side part of the coil member and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened. The sputtering of the metal oxide and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can thus be restrained, that is, the degradation of the thermionic emission ability can be restrained and long service life of the electrode can be realized. Since the movement of the discharge position is also restrained, stable discharge over a long period of time can be realized. Since a mandrel is provided, deformation of the coil member during processing can be restrained. Also, due to the increase of the discharge area, even if the current density is slightly increased and the load is some what increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art, thus enabling the provision of an indirectly heated electrode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and enabling realization of pulse operation and large current operation. 
   The present invention provides an indirectly heated electrode for gas discharge tube comprising: a coil member, having a mandrel and wound in coil form; a heater, disposed at the inner side of the coil member and having an electrical insulating layer formed on a surface thereof; a high-melting-point metal, formed to a mesh, a wire or a plate and disposed along the length direction of the coil member at the outer side of the coil member between the coil member and the heater; and a metal oxide, serving as a material likely to emit electrons and disposed so as to be in contact with the coil member; and wherein the high-melting-point metal is in electrical contact with the coil member at a plurality of locations and the high-melting-point metal is set to a ground potential. 
   In the indirectly heated electrode for gas discharge tube of the present invention, since the high-melting-point metal is grounded, thermions, secondary electrons, etc. are supplied via this high-melting-point metal and the coil member. Also, since an equipotential surface is effectively formed at the cathode surface by the high-melting-point metal and the inner side part of the coil member and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened. The sputtering of the metal oxide and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can thus be restrained, that is, the degradation of the thermionic emission ability can be restrained and long service life of the electrode can be realized. Since the movement of the discharge position is also restrained, stable discharge over a long period of time can be realized. Since a mandrel is provided, deformation of the coil member during processing can be restrained. Also, due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art, thus enabling the provision of an indirectly heated electrode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and enabling realization of pulse operation and large current operation. 
   It is favorable for the coil member to be a multiple coil arranged by winding a coil in coil form. With such an arrangement, the metal oxide that is the material likely to emit electrons is held in a manner where it is sandwiched between the pitches (spacings), which are the gaps between the wire material that forms the coil. Since the distance between pitches is small and gap-like, the falling off the metal oxide due to vibration can be restrained. Also, since a plurality of pitches of gap-like structure exist, a large amount of metal oxide can be held, providing the effect of replenishing the metal oxide loss that accompanies the degradation with time during discharge. 
   The present invention provides an indirectly heated electrode for gas discharge tube comprising: a coil member, wound in single coil form; a heater, disposed at the inner side of the coil member and having an electrical insulating layer formed on a surface thereof; a high-melting-point metal, formed to a mesh, a wire, or a plate, and disposed along the length direction of the coil member between the coil member and the heater; and a metal oxide, serving as a material likely to emit electrons and disposed so as to be in contact with the coil member; and wherein the high-melting-point metal forms a plurality of electrical contacts with the coil member and the coil member is grounded. 
   In the indirectly heated electrode for gas discharge tube of the present invention, since the coil member is grounded, thermions, secondary electrons, etc. are supplied via this coil member. Also, at the rear surface (surface at the opposite side of the discharge surface) of the coil member, since an equipotential surface is effectively formed on the cathode surface by the high-melting-point metal and the inner side part of the coil member and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened. The sputtering of the metal oxide and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can thus be restrained, that is, the degradation of the thermionic emission ability can be restrained and long service life of the electrode can be realized. Since the movement of the discharge position is also restrained, stable discharge over along period of time can be realized. Also, due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art, thus enabling the provision of an indirectly heated electrode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and enabling realization of pulse operation and large current operation. 
   The present invention provides an indirectly heated electrode for gas discharge tube comprising: a coil member, wound in single coil form; a heater, disposed at the inner side of the coil member and having an electrical insulating layer formed on a surface thereof; a high-melting-point metal, formed to a mesh, a wire, or a plate, and disposed along the length direction of the coil member between the coil member and the heater; and a metal oxide, serving as a material likely to emit electrons and disposed so as to be in contact with the coil member; and wherein the high-melting-point metal forms a plurality of electrical contacts with the coil member and the coil member is grounded. 
   In the indirectly heated electrode for gas discharge tube of the present invention, since the high-melting-point metal is grounded, thermions, secondary electrons, etc. are supplied via this high-melting-point metal and the coil member. Also, since an equipotential surface is effectively formed at the cathode surface by the high-melting-point metal and the inner side part of the coil member and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened. The sputtering of the metal oxide and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can thus be restrained, that is, the degradation of the thermionic emission ability can be restrained and long service life of the electrode can be realized. Since the movement of the discharge position is also restrained, stable discharge over along period of time can be realized. Also, due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art, thus enabling the provision of an indirectly heated electrode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and enabling realization of pulse operation and large current operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic front view, showing an indirectly heated cathode for gas discharge tube of a first embodiment. 
       FIG. 2  is a schematic side view, showing an indirectly heated cathode for gas discharge tube of the first embodiment. 
       FIG. 3A  is a schematic top view, showing an indirectly heated cathode for gas discharge tube of the first embodiment. 
       FIG. 3B  is a schematic top view, showing an indirectly heated cathode for gas discharge tube of the first embodiment. 
       FIG. 4  is a schematic sectional view, showing an indirectly heated cathode for gas discharge tube of the first embodiment. 
       FIG. 5  is a schematic sectional view, showing an indirectly heated cathode for gas discharge tube of the second embodiment. 
       FIG. 6  is a schematic sectional view, showing an indirectly heated cathode for gas discharge tube of the third embodiment. 
       FIG. 7  is a schematic sectional view, showing an indirectly heated cathode for gas discharge tube of the forth embodiment. 
       FIG. 8  is an overall perspective view, showing a first embodiment&#39;s gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 9  is an exploded perspective view of the light emitting part of the first embodiment&#39;s gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 10  is a transverse sectional view of the light emitting part of the first embodiment&#39;s gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 11  is an arrangement diagram, showing a lamp with one outer electrode that uses an indirectly heated cathode for gas discharge tube of the first embodiment. 
       FIG. 12  is a circuit diagram, showing a first embodiment&#39;s lighting circuit for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 13  is a circuit diagram, showing a first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 14A  is a timing chart, showing the operation voltage characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 14B  is a timing chart, showing the operation voltage characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 14C  is a timing chart, showing the operation voltage characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 14D  is a timing chart, showing the operation voltage characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 14E  is a timing chart, showing the operation voltage characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 14F  is a timing chart, showing the operation voltage characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 15A  is a timing chart, showing the operation current characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 15B  is a timing chart, showing the operation current characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 15C  is a timing chart, showing the operation current characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 15D  is a timing chart, showing the operation current characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
       FIG. 15E  is a timing chart, showing the operation current characteristics of the first embodiment&#39;s lighting device for gas discharge tube using an indirectly heated cathode for gas discharge tube. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   Preferred embodiments of this invention&#39;s indirectly heated electrode for gas discharge tube shall now be described in detail with reference to the drawings. In the following description, the same symbol shall be used for the same elements or elements with the same functions and redundant description shall be omitted. 
   (First Embodiment) 
     FIG. 1  is a schematic front view of an indirectly heated cathode for gas discharge tube of a first embodiment,  FIG. 2  is likewise a schematic side view of an indirectly heated cathode for gas discharge tube of the first embodiment,  FIG. 3  is likewise a schematic top view of indirectly heated cathodes for gas discharge tube of the first embodiment, and  FIG. 4  is likewise a schematic sectional view of an indirectly heated cathode for gas discharge tube of the first embodiment. With  FIGS. 1 ,  2 , and  3 , illustrations of an electrical insulating layer  4  and a metal oxide  10  are omitted for the sake of description. This embodiment is an example of application of an indirectly heated electrode for gas discharge tube to a cathode (indirectly heated cathode for gas discharge tube). 
   As shown in  FIGS. 1 through 4 , an indirectly heated cathode for gas discharge tube C 1  has a heater  1 , a double coil  2  as a coil member, a plate member  3  as an electrical conductor, and metal oxide  10  as a material likely to emit electrons (cathode material). Heater  1  comprises a filament coil, with which a tungsten element wire of 0.03 to 0.1 mm diameter, that is for example, a tungsten element wire of 0.07 mm diameter is wound in double, and an electrical insulating material (for example, alumina, zirconia, magnesia, silica, etc.) is coated by electrode position, etc. and formed as electrical insulating layer  4  on the surface of this tungsten filament coil. Also, an arrangement, which uses a cylindrical pipe of an electrical insulating material (for example, alumina, zirconia, magnesia, silica, etc.) and with which heater  1  is inserted inside this cylindrical pipe insulate heater  1 , maybe employed in place of electrical insulating layer  4 . 
   Double coil  2  is a multiple coil arranged from a coil that is wound in coil form, and a tungsten element wire of 0.091 mm diameter is formed into a primary coil with a diameter of 0.25 mm and a pitch of 0.146 mm and this primary coil is formed into a double coil with a diameter of 1.7 mm and a pitch of 0.6 mm. Heater  1  is inserted into and disposed at the inner side of double coil  2 . As a coil member, a triple coil, etc. may be used in place of double coil  2 . 
   Plate member  3  is a conductive rigid body (metal conductor) formed to plate, high-melting-point metal (with a melting point of at least 1000° C.) selected from among groups IIIa to VIIa, VIII, and Ib of the periodic table or, more specifically, from among tungsten, tantalum, molybdenum, rhenium, niobium, osmium, iridium, iron, nickel, cobalt, titanium, zirconium, manganese, chromium, vanadium, rhodium, rare earth metals, etc. or an alloy of these metals. With the present embodiment, a tungsten plate member of 1.5 mm width and 25.4 μm thickness is used. 
   Plate member  3  is disposed so as to be substantially orthogonal to the discharge direction along the length direction of double coil  2  at the inner side of double coil  2  (between heater  1  and double coil  2 ). Plate member  3  is in a state where it is electrically connected to double coil  2 . Also, plate member  3  contacts a plurality of coil parts at the inner side of double coil  2  and thus forms a plurality of contacts with double coil  2 . Plate member  3  is grounded (set to GND) by being connected to the ground terminal of heater  1 . By plate member  3  being grounded, double coil  2  is grounded as well. Also, in place of using plate member  3 , a wire member that has been formed to a wire (for example, a tungsten element wire of approximately 0.1 mm diameter) may be used. Also, plate member  3  and double coil  2  may be welded together at the respective contact points. 
   Metal oxide  10  is held by double coil  2  and heater  1  and is put in contact with plate member  3 . The surface of metal oxide  10  and the surface of double coil  2  are exposed to the outer side of indirectly heated cathode for gas discharge tube C 1 , and the surface part of double coil  2  is put in contact with the surface part of metal oxide  10 . 
   As metal oxide  10 , a single oxide of a metal selected from among barium (Ba), strontium (Sr), and calcium (Ca), or a mixture of such oxides, or an oxide, with which the principle component is a single oxide of a metal selected from among barium, strontium, and calcium or a mixture of such oxides and a sub-component is an oxide of a metal selected among rare earth metals including lanthanum (metals of group IIIa of the periodic table), is used. Each of barium, strontium, and calcium is low in work function, can emit thermions readily, and enable the thermion supply amount to be increased. Also, in a case where a rare earth metal (metal of group IIIa of the periodic table) is added as a sub-component, the thermion supply amount can be increased further and the sputter resistance can be improved as well. 
   As the cathode material, metal oxide  10  is coated in the form of a metal carbonate (f or example, barium carbonate, strontium carbonate, calcium carbonate, etc.) and obtained by vacuum thermal decomposition of the coated metal carbonate. If vacuum thermal decomposition is to be performed by passage of electricity through the heater  1 , AC thermal decomposition is preferred over DC thermal decomposition. In the final stage, the metal oxide  10  that is thus obtained becomes the material likely to emit electrons. 
   The metal carbonate that is to be the cathode material is coated from the surface side of double coil  2  with heater  1  being positioned at the inner side of double coil  2  and plate member  3  being positioned at the inner side of double coil  2  that is to be the discharge surface side as shown in  FIGS. 1 through 3B . The metal carbonate need not be coated so as to cover the entire periphery of indirectly heated cathode for gas discharge tube C 1  (double coil  2 ) but may be coated onto just the part at the side at which plate member  3  is provided and which is to be the discharge surface side. 
   As shown in  FIGS. 3B and 4 , heater  1  is put in contact with metal oxide  10  and double coil  2  via electrical insulating layer  4 . The heat of heater  1  can thus be transferred definitely and efficiently to metal oxide  10  and double coil  2  in the preheating process. Also, in comparison to an arrangement having a cylinder of good thermal conductivity, such as that of the indirectly heated cathode for gas discharge tube disclosed in Japanese Examined Patent Publication No. Sho 62-56628, the loss of the heat amount necessary for hot cathode operation can be restrained. This enable designs, which require neither the supplying of heat to the electrode from the exterior nor forced heating and with which the electrode will operate with just the heat amount provided by self-heating. When electrons are emitted from an electrode in a gas discharge tube, the ionized gas in the discharge space collides and causes electrical neutralization, and here, “self-heating” refers to the heat that is generated by the impact of collision of the gas molecules with the electrode. 
   Though besides the abovementioned metal oxides, the use of a metal boride, such lanthanum boride, a metal carbide, a metal nitride, etc. as the thermion supply source may be considered, metal borides, metal carbides, metal nitrides are poor in performance as a thermion supply source that can serve as a hot cathode for gas discharge tube and there is no meaning in adding such compounds as a principle component or a sub-component. However, such compounds may be used at peripheral parts of the cathode for effects besides the effect as a thermion supply source, such as for improving the insulation effect in order to restrain the amount of heat dissipation to parts besides the discharge part. 
   Here, consider the discharge at three predetermined discharge parts (designated as point  1 A, point  1 B, and point  1 C, starting from the point closer to the ground (GND) that is the electron supply source) on the surface of double coil  2 . The respective discharge parts  1 A,  1 B, and  1 C have resistances R 1 A, R 1 B, and R 1 C corresponding to winding resistances from plate member  3  to double coil  2 . Though the discharge current amount will differ according to the work function of each location, supposing that:
 
I1A&gt;I1B&gt;I1C  (1)
 
   when the main discharge occurs at discharge part  1 A, having a winding resistance R 1 A, the generation of heat (W) due to Joule heat, expressed by the following Equation (2), increases:
 
 W=I 1 A   2   ×R 1 A   (2)
 
   and the lowering of the work function due to temperature rise occurs. Thus much of the discharge concentrates at this discharge part  1 A, causing the degree of concentration of discharge to increase, and the discharge distribution becomes a continuous distribution of peaks with gradual unevenness. The greater the value of the winding resistance R 1 A, the greater the slope of the discharge distribution, and oppositely, as the value of the winding resistance R 1 A becomes small, the discharge distribution converges to a broad, gradual, single-peak continuous distribution. 
   Thus with indirectly heated cathode for gas discharge tube C 1  of the present embodiment, since plate member  3  is disposed in contact with metal oxide  10  and double coil  2 , plate member  3 , along with the inner side part of double coil  2 , effectively forms an equipotential surface at the rear surface (surface at the opposite side of the discharge surface) of double coil  2 . That is, plate member  3  and the inner side part of double coil  2  are arranged with a plurality of electrical wiring (conduction paths) and do not restrict the flow of electrical current to a single direction. The electrical resistances across the ends of the surface of plate member  3  are thus considerably low, the surface of plate member  3  is put in a substantially equipotential state, and the potential of the discharge surface that comprises a plurality of discharge points or discharge lines will be substantially uniform. In other words, a plurality of electrical circuits, which enable discharge current to flow in directions parallel to the discharge surface, that is, a plurality of paths (equipotential circuits) for the discharge electrons (emission) are formed by plate member  3 . 
   Thus with indirectly heated cathode for gas discharge tube C 1 , since an equipotential surface is formed effectively at the rear surface (surface at the opposite side of the discharge surface) of double coil  2  by plate member  3  and double coil  2 , and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened, and the sputtering of metal oxide  10  and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can be restrained, in other words, the lowering of the thermionic emission ability can be restrained. As a result, the occurrence of localized discharge can be restrained and long service life of the cathode can be realized. Since the movement of the discharge position is also restrained, stable discharge over a long period of time can be realized. Also, since the discharge area is increased, the operation voltage and the generated heat amount of indirectly heated cathode for gas discharge tube C 1  can be reduced. 
   Also, with indirectly heated cathode for gas discharge tube C 1 , due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art. This enables the provision of an indirectly heated cathode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and the realization of pulse operation and large current operation. 
   Also since plate member  3  is used as the electrical conductor, an electrical conductor of an arrangement, which can restrain the degradation of the thermionic emission ability and the movement of the discharge position, can be realized at low cost and in a simpler manner. Also, since plate member  3  (electrical conductor) is a rigid body, it is easy to process and can be put in close contact with metal oxide  10 . Furthermore, the locations at which plate member  3  contacts metal oxide  10  can be made numerous readily. 
   With indirectly heated cathode for gas discharge tube C 1  of the present embodiment, since heater  1  is used as a core at the outer side of which double coil  2 , which holds metal oxide  10 , is positioned in a surrounding manner, and plate member  3  is positioned at the inner side of double coil  2  so as to be in contact with metal oxide  10 , the vibration restraining effect of double coil  2  is put to work and the falling off of metal oxide  10  is thereby prevented. Also, since a large amount of metal oxide  10  will be held between the pitches of double coil  2 , the effect of replenishing the metal oxide loss that accompanies the degradation with time during discharge is provided. 
   (Second Embodiment) 
     FIG. 5  is a schematic sectional view of an indirectly heated cathode for gas discharge tube of a second embodiment. The second embodiment differs from the first embodiment in that the double coil has a mandrel and the electrical conductor is a mesh member. 
   As shown in  FIG. 5 , indirectly heated cathode for gas discharge tube C 2  has a heater  1 , a double coil  41  as a coil member, a mesh member  21  as an electrical conductor, and a metal oxide  10  as a material likely to emit electrons. 
   Double coil  41 , like double coil  2  of the first embodiment, is a multiple coil arranged from a coil wound in coil form and has a mandrel  42 . Heater  1  is disposed at the inner side of double coil  41 . Here, the mandrel is a core wire that serves the role of a mold that determines the winding diameter in the process of preparing the filament coil. As the material of the mandrel, for example, molybdenum is used. 
   Mesh member  21  is a conductive rigid body (metal conductor) formed of a single, high-melting-point metal (with a melting point of at least 1000° C.) selected from among groups IIIa to VIIa, VIII, and Ib of the periodic table or, more specifically, from among tungsten, tantalum, molybdenum, rhenium, niobium, osmium, iridium, iron, nickel, cobalt, titanium, zirconium, manganese, chromium, vanadium, rhodium, rare earth metals, etc. or an alloy of these metals. With the present embodiment, a mesh member made by weaving tungsten element wires of 0.03 mm diameter into mesh form is used. The mesh size of mesh member  21  is set to 80 mesh. Mesh member  21  has a prescribed length. 
   Mesh member  2  is disposed across the length direction of double coil  41  at the inner side of double coil  41  (between heater  1  and double coil  41 ) so as to be substantially orthogonal to the discharge direction. Mesh member  21  is put in a state where it is in electrical contact with double coil  41 . Mesh member  21  also contacts a plurality of coil parts at the inner side of double coil  41  and forms a plurality contacts with double coil  41 . Mesh member  21  is connected to the ground terminal of heater  1  and is thereby grounded (set to GND). By mesh member  21  being grounded, double coil  41  is also grounded. 
   Metal oxide  10  is held by double coil  41  and heater  1 . The surface part of double coil  41  and metal oxide  10  are exposed to the outer side of indirectly heated cathode for gas discharge tube C 2  so that the surface of metal oxide  10  and the surface part of double coil  41  make up a discharge surface and the surface part of metal oxide  10  is put in contact with the surface part of double coil  41 . Metal oxide  10  is disposed in the same manner as in the first embodiment. 
   As shown in  FIG. 5 , heater  1  is in contact with metal oxide  10  and double coil  41  via electrical insulating layer  4 . The heat of heater  1  can thus be transferred definitely and efficiently to metal oxide  10  and double coil  41  in the preheating process. Also, as with the first embodiment, the loss of the heat amount necessary for hot cathode operation can be restrained, and this enables designs, which require neither the supplying of heat to the electrode from the exterior nor forced heating and with which the electrode will operate with just the heat amount provided by self-heating. 
   Thus with indirectly heated cathode for gas discharge tube C 2  of the present embodiment, since mesh member  21  is put in contact with metal oxide  10  and with double coil  41 , mesh member  21  effectively forms an equipotential surface at the rear surface (surface at the opposite side of the discharge surface) of double coil  41 . That is, mesh member  21  is arranged with a plurality of electrical wiring (conduction paths) and does not restrict the flow of electrical current to a single direction. The electrical resistances across the ends of the surface of mesh member  21  are thus considerably low, the surface of mesh member  21  is put in a substantially equipotential state, and the potential of the discharge surface that comprises a plurality of discharge points or discharge lines will be substantially uniform. In other words, a plurality of electrical circuits, which enable discharge current to flow in directions parallel to the discharge surface, that is, a plurality of paths (equipotential circuits) for the discharge electrons (emission) are formed by mesh member  21 . 
   Thus with indirectly heated cathode for gas discharge tube C 2 , since an equipotential surface is formed effectively at the rear surface (surface at the opposite side of the discharge surface) of double coil  41  by mesh member  21  and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened, and the sputtering of metal oxide  10  and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can be restrained, in other words, the lowering of the thermionic emission ability can be restrained. As a result, the occurrence of localized discharge can be restrained and long service life of the cathode can be realized. Since the movement of the discharge position can also be restrained, stable discharge over a long period of time can be realized. Also since the discharge area is increased, the operation voltage and the generated heat amount of indirectly heated cathode for gas discharge tube C 2  can be reduced. 
   Also, with indirectly heated cathode for gas discharge tube C 2 , due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art. This enables the provision of an indirectly heated cathode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and the realization of pulse operation and large current operation. 
   Also since mesh member  21  is used as the electrical conductor, an electrical conductor of an arrangement that can restrain the degradation of thermionic emission ability and the movement of the discharge position can be realized at low cost and in a simpler manner. Also, since mesh member  21  (electrical conductor) is a rigid body, it is easy to process and can be put in close contact with metal oxide  10 . Furthermore, the locations at which mesh member  21  contacts metal oxide  10  can be made numerous readily. 
   With indirectly heated cathode for gas discharge tube C 2  of the present embodiment, since heater  1  is used as a core at the outer side of which double coil  41 , which holds metal oxide  10 , is positioned in a surrounding manner, and mesh member  21  is positioned at the inner side of double coil  41  so as to be in contact with metal oxide  10 , the vibration restraining effect of double coil  41  is put to work and the falling off of metal oxide  10  is thereby prevented. Also, since a large amount of metal oxide  10  will be held between the pitches of double coil  41 , the effect of replenishing the metal oxide loss that accompanies the degradation with time during discharge is provided. 
   Also, since double coil  41  has a mandrel, the additional effect that the deformation of double coil  41  during processing can be restrained is provided. 
   (Third Embodiment) 
     FIG. 6  is a schematic sectional view of an indirectly heated cathode for gas discharge tube of a third embodiment. The third embodiment differs from the first and second embodiments in that the coil member is a single coil and the electrical conductor is a wire member. 
   As shown in  FIG. 6 , indirectly heated cathode for gas discharge tube C 3  has a heater  1 , a single coil  45  as a coil member, a wire member  23  as an electrical conductor, and a metal oxide  10  as a material likely to emit electrons. 
   Single coil  45  is a coil member arranged as a coil wound in the form of a single coil and is formed by winding a tungsten element wire of 0.15 mm diameter at a diameter of 1.7 mm and a pitch of 0.18 mm. Heater  1  is disposed at the inner side of single coil  45 . 
   Wire member  23 , which is formed to a wire and has a predetermined length, is, like mesh member  21 , a conductive rigid body (metal conductor) formed of a single, high-melting-point metal (with a melting point of at least 1000° C.) selected from among groups IIIa to VIIa, VIII, and Ib of the periodic table or, more specifically, from among tungsten, tantalum, molybdenum, rhenium, niobium, osmium, iridium, iron, nickel, cobalt, titanium, zirconium, manganese, chromium, vanadium, rhodium, rare earth metals, etc. or an alloy of these metals. With the present embodiment, a wire member made of tungsten is used. The diameter of wire member  23  is set to approximately 0.1 mm. 
   Wire member  23  is disposed at the inner side of single coil  45  (between heater  1  and single coil  45 ) and along the length direction of single coil  45  so as to be substantially orthogonal to the discharge direction. Wire member  23  is put in a state where it is electrically connected with single coil  45 . Also, wire member  23  contacts a plurality of coil parts at the inner side of single coil  45  and thus forms a plurality of contacts with single coil  45 . Wire member  23  is grounded (set to GND) by being connected, along with the ground terminal of heater  1 , to a lead rod. By wire member  23  being grounded, single coil  45  is grounded as well. 
   Metal oxide  10  is held by single coil  45  and heater  1 . The surface part of single coil  45  and metal oxide  10  are exposed to the outer side of indirectly heated cathode for gas discharge tube C 3  so that the surface of metal oxide  10  and the surface part of single coil  45  make up a discharge surface and the surface part of metal oxide  10  is put in contact with the surface part of single coil  45 . Metal oxide  10  is disposed in the same manner as in the first embodiment. 
   As shown in  FIG. 6 , heater  1  is in contact with metal oxide  10  and single coil  45  via electrical insulating layer  4 . The heat of heater  1  can thus be transferred definitely and efficiently to metal oxide  10  and single coil  45  in the preheating process. Also, as with the first embodiment, the loss of the heat amount necessary for hot cathode operation can be restrained, and this enables designs, which require neither the supplying of heat to the electrode from the exterior nor forced heating and with which the electrode will operate with just the heat amount provided by self-heating. 
   Thus with indirectly heated cathode for gas discharge tube C 3  of the present embodiment, since wire member  23  put in contact with metal oxide  10  and with single coil  45 , wire member  23 , along with the inner side part of single coil  45 , effectively forms an equipotential surface at the rear surface (surface at the opposite side of the discharge surface) of single coil  45 . That is, wire member  23  and the inner side part of single coil  45  are arranged with a plurality of electrical wiring (conduction paths) and do not restrict the flow of electrical current to a single direction. The electrical resistances across the ends of the surface of wire member  23  are thus considerably low, the surface of wire member  23  is put in a substantially equipotential state, and the potential of the discharge surface that comprises a plurality of discharge points or discharge lines will be substantially uniform. In other words, a plurality of electrical circuits, which enable discharge current to flow in directions parallel to the discharge surface, that is, a plurality of paths (equipotential circuits) for the discharge electrons (emission) are formed by wire member  23 . 
   Thus with indirectly heated cathode for gas discharge tube C 3 , since an equipotential surface is formed effectively at the rear surface (surface at the opposite side of the discharge surface) of single coil  45  by wire member  23  and the inner side part of single coil  45  and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened, and the sputtering of metal oxide  10  and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can be restrained, in other words, the lowering of the thermionic emission ability can be restrained. As a result, the occurrence of localized discharge can be restrained and long service life of the cathode can be realized. Since the movement of the discharge position can also be restrained, stable discharge over a long period of time can be realized. Also since the discharge area is increased, the operation voltage and the generated heat amount of indirectly heated cathode for gas discharge tube C 3  can be reduced. 
   Also, with indirectly heated cathode for gas discharge tube C 3 , due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art. This enables the provision of an indirectly heated cathode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and the realization of pulse operation and large current operation. 
   Also since wire member  23  is used as the electrical conductor, an electrical conductor of an arrangement that can restrain the degradation of thermionic emission ability and the movement of the discharge position can be realized at low cost and in a simpler manner. Also, since wire member  23  (electrical conductor) is a rigid body, it is easy to process and can be put in close contact with metal oxide  10 . Furthermore, the locations at which wire member  23  contacts metal oxide  10  can be made numerous readily. 
   Also with indirectly heated cathode for gas discharge tube C 3  of the present embodiment, since heater  1  is used as a core at the outer side of which single coil  45 , which holds metal oxide  10 , is positioned in a surrounding manner, and wire member  23  is positioned at the inner side of single coil  45  so as to be in contact with metal oxide  10 , the vibration restraining effect of single coil  45  is put to work and the falling off of metal oxide  10  is thereby prevented. 
   (Fourth Embodiment) 
     FIG. 7  is a schematic sectional view of an indirectly heated cathode for gas discharge tube of a fourth embodiment. The fourth embodiment differs from the first to third embodiments in having a base metal. 
   As shown in  FIG. 7 , indirectly heated cathode for gas discharge tube C 4  has a heater  1 , a double coil  41 , a metal oxide  10  as a material likely to emit electrons, and a base metal  31 . 
   Base metal  31  is formed to a tubular form and is conductive. Base metal  31  is formed for example of molybdenum, etc. Heater  1  is inserted into and positioned at the inner side of this base metal  31 . Double coil  41  is wound a plurality of times around and fixed to the outer surface of base metal  31 . Base metal  31  functions as a barrier between metal oxide  10 , which is the material likely to emit electrons, and electrical insulating layer  4 , which is formed on heater  1 . As base metal  31 , a medium- to high-melting-point metal with a melting point that is higher than the cathode temperature during operation may be used. Also, though a tubular member of cylindrical shape is generally used as base metal  31 , a tubular member with an arcuate shape with a notch (an open shape) may be used instead. 
   Base metal  31  is disposed at the inner side of double coil  41  (between heater  1  and double coil  41 ) and along the length direction of double coil  41  so as to be substantially orthogonal to the discharge direction. Base metal  31  is put in a state where it is electrically connected to double coil  41 . Also, base metal  31  contacts a plurality of coil parts at the inner side of double coil  41  and thus forms a plurality of contacts with double coil  41 . Base metal  31  is grounded (set to GND) by being connected, along with the ground terminal of heater  1 , to a lead rod. By base metal  31  being grounded, double coil  41  is grounded as well. 
   Metal oxide  10  is held by double coil  41 . The surface part of double coil  41  and metal oxide  10  are exposed to the outer side of indirectly heated cathode for gas discharge tube C 4  so that the surface of metal oxide  10  and the surface part of double coil  41  make up a discharge surface and the surface part of metal oxide  10  is put in contact with the surface part of double coil  41 . 
   Thus with indirectly heated cathode for gas discharge tube C 4  of the present embodiment, since base metal  31  is disposed in contact with metal oxide  10  and with double coil  41 , base metal  31 , along with the inner side part of double coil  41 , effectively forms an equipotential surface at the rear surface (surface at the opposite side of the discharge surface) of double coil  41 . That is, base metal  31  and double coil  41  are arranged with a plurality of electrical wiring (conduction paths) and do not restrict the flow of electrical current to a single direction. The electrical resistances across the ends of the surface of base metal  31  are thus considerably low, the surface of base metal  31  is put in a substantially equipotential state, and the potential of the discharge surface that comprises a plurality of discharge points or discharge lines will be substantially uniform. In other words, a plurality of electrical circuits, which enable discharge current to flow in directions parallel to the discharge surface, that is, a plurality of paths (equipotential circuits) for the discharge electrons (emission) are formed by base metal  31 . 
   Thus with indirectly heated cathode for gas discharge tube C 4 , since an equipotential surface is formed effectively at the rear surface (surface at the opposite side of the discharge surface) of double coil  41  by base metal  31  and double coil  41 , and thermionic emission thus occurs over a wide region of the equipotential surface that is formed, the discharge area is increased, the electron emission amount per unit area (electron emission density) is increased, and the load placed on the discharge position is lightened, and the sputtering of metal oxide  10  and stabilization (mineralization) due to oxidation with the reduced metal, which are degradation factors, can be restrained, in other words, the lowering of the thermionic emission ability can be restrained. As a result, the occurrence of localized discharge can be restrained and long service life of the cathode can be realized. Since the movement of the discharge position can also be restrained, stable discharge over a long period of time can be realized. Also since the discharge area is increased, the operation voltage and the generated heat amount of indirectly heated cathode for gas discharge tube C 4  can be reduced. 
   Also, with indirectly heated cathode for gas discharge tube C 4 , due to the increase of the discharge area, even if the current density is slightly increased and the load is somewhat increased, that is, even if the discharge current is increased, the damage can be made less than that of the prior art. This enables the provision of an indirectly heated cathode for gas discharge tube of large discharge current with substantially the same shape as that of the prior art and the realization of pulse operation and large current operation. 
   Also, since double coil  41  has a mandrel, the additional effect that the deformation of double coil  41  during processing can be restrained is provided. 
   Next, a gas discharge tube, which uses indirectly heated cathode for gas discharge tube C 1  of the above-described arrangement, shall be described based on  FIGS. 8 to 10 .  FIG. 8  is an overall perspective view of a gas discharge tube that uses indirectly heated cathode for gas discharge tube C 1 ,  FIG. 9  is an exploded perspective view of the light emitting part of the gas discharge tube, and  FIG. 10  is a transverse sectional view of the light emitting part. With this embodiment, indirectly heated cathode for gas discharge tube C 1  is applied to a side-on type deuterium gas discharge tube. As the indirectly heated cathode for gas discharge tube, any of the indirectly heated cathodes for gas discharge tube C 2  to C 4  maybe used in place of indirectly heated cathode for gas discharge tube C 1 . 
   A deuterium gas discharge tube DT 2  has a glass outer container  61 . As shown in  FIG. 8 , a light emitting part assembly  62  is housed inside outer container  61  and the bottom part of outer container  61  is sealed in an airtight manner by a glass stem  63 . Four lead pins  64   a  to  64   d  extend from the lower part of light emitting part assembly  62  and are exposed to the exterior upon passing through stem  63 . Light emitting part assembly  62  has a shielding box structure, formed by adhering together a discharge shielding plate (discharge shielding part)  71  and a supporting plate  72 , both made of alumina, and a metal front cover  73 , which is mounted to the front face of discharge shielding plate  71 . 
   As shown in  FIG. 9 , a through hole is formed in the vertical direction at the rear part of supporting plate  72 , having a protruding cross-sectional shape, and lead pin  64   a  is inserted through this through hole and held by stem  63 . An indented groove, which extends vertically downwards, is formed on the front face of supporting plate  72 , and lead pin  64   b , which extends from stem  63 , is set inside this groove, and by these parts, supporting plate  72  is fixed to stem  63 . A flat, rectangular anode  74  is fixed facing forward on lead pin  64   b  and is held by being in contact with two protrusions formed on the front face of supporting plate  72 . 
   Also as shown in  FIG. 9 , discharge shielding plate  71  is arranged as a structure with a protruding cross-sectional shape that is thinner and wider in comparison to supporting plate  72 , and a through hole  71   a  is formed at a central position corresponding to anode  74 . A through hole is formed in the vertical direction to a side of the protruding part of discharge shielding plate  71 , and an electrode rod  81 , which has been bent to an L-shape, is inserted through this through hole. In the condition where discharge shielding plate  71  and supporting plate  72  are adhered together, the lower end of electrode rod  81  and the tip of lead pin  64   c , which has been bent into an L-shape, are welded together. An upper electrode rod  82  of an indirectly heated cathode for gas discharge tube C 1  is welded to the tip part of electrode rod  81  that extends to the side, and in the condition where discharge shielding plate  71  and supporting plate  72  are adhered together, a lower electrode rod  83  is welded to the tip of lead pin  64   d , which has been bent into an L-shape. 
   As shown in  FIG. 9 , a metal focusing electrode  76  is arranged by preparing an L-shaped metal plate, having a focusing aperture  76   a  formed coaxial to through hole  71   a  of discharge shielding plate  71  at a middle part, and bending this metal plate towards the rear at the upper part and towards the front at a side part in the direction of indirectly heated cathode for gas discharge tube C 1 , and at a side part, an aperture  76   b , which has a rectangular shape that is long in the vertical direction and faces indirectly heated cathode for gas discharge tube C 1 , is formed. Each of discharge shielding plate  71 , supporting plate  72 , and focusing electrode  76  has four through holes formed at corresponding positions. Thus by inserting two metal pins  84  and  85  in the condition where discharge shielding plate  71 , supporting plate  72 , and focusing electrode  76  are adhered together, these components can be fixed to stem  63 . 
   As shown in  FIGS. 8 and 9 , metal front cover  73  has a U-shaped cross section that is formed by bending in four stages and has an aperture window  73   a  for light projection formed at a central part. Two protrusions  73   b  are formed at each end part and these correspond to four through apertures  71   b  that are formed at the end parts of the front face of discharge shielding plate  71 . Here, by inserting these protrusions  73   b  into through apertures  71   b , front cover  73  is fixed to discharge shielding plate  71 , and in this condition, the front end part of focusing electrode  76  contacts the inner face of front cover  73 , and the space in which indirectly heated cathode for gas discharge tube C 1  is disposed is separated from the light emitting space. 
   As shown in  FIGS. 9 and 10 , focusing electrode  76  has, at its central part, a focusing aperture  76   a  that is coaxial to through hole  71   a  of discharge shielding plate  71 , and here, an aperture restricting plate  78  for restricting the aperture diameter is fixed by welding. Aperture restricting plate  78  is bent in the direction of anode  74  at the periphery of focusing aperture  76   a  and thus the distance between anode  74  and the aperture of aperture restricting plate  78  is less than the thickness of discharge shielding plate  78 . 
   The respective electrodes inside light emitting part  62 , which is assembled in the above-described manner, are positioned as shown in  FIG. 10 . Anode  74  is fixed by being sandwiched by discharge shielding plate  71  and supporting plate  72 , and aperture restricting plate  78 , which is welded to focusing electrode  76  is fixed to discharge shielding plate  71  at a position at which it faces anode  74  via through hole  71   a  of discharge shielding plate  71 . Indirectly heated cathode for gas discharge tube C 1  is positioned within a space surrounded by discharge shielding plate  71 , front cover  73 , and the surface of focusing electrode  76  provided with rectangular aperture  76   b  and at a position at which it faces aperture restricting plate  78  via rectangular aperture  76   b.    
   The operation of deuterium gas discharge tube DT 1  shall now be described with reference to  FIG. 10 . After indirectly heated cathode for gas discharge tube C 1  has been heated adequately, a trigger voltage is applied across anode  74  and indirectly heated cathode for gas discharge tube C 1  and discharge is thereby started. The flow path of thermions at this time is restricted to just the single path  91  (illustrated as the part sandwiched by broken lines) by the focusing by aperture restricting plate  78  of focusing electrode  76  and the shielding effect by discharge shielding plate  71  and supporting plate  72 . That is, the thermions (not shown) emitted from indirectly heated cathode for gas discharge tube C 1  pass through aperture restricting plate  78  from rectangular aperture  76   b  of focusing electrode  76 , pass through the through hole  71   a  of discharge shielding plate  71  and reaches anode  74 . An arc ball  92  due to arc discharge is generated at a space in front of aperture restricting plate  78  and at the side opposite anode  74 . The light taken out from arc ball  92  is emitted substantially in the direction of arrow  93  through aperture window  73   a  of front cover  73 . 
   Thus with deuterium gas discharge tube DT 1  of the present embodiment, a deuterium gas discharge tube of long service life and stable operation can be realized by the use of any of indirectly heated cathodes for gas discharge tube C 1 . 
   Any of indirectly heated cathodes for gas discharge tube C 1  to C 4  may be used as an electrode (indirectly heated cathode for gas discharge tube) in a gas discharge tube besides the above-described deuterium gas discharge tube DT 1 , for example, in a head-on type deuterium gas discharge tube, with which light is taken out from a top part of the tube, a rare gas fluorescent lamp, a mercury fluorescent lamp, etc. Specifically, gas discharge tubes using this invention&#39;s indirectly heated electrode for gas discharge tube include a rare gas fluorescent lamp, which has discharge electrodes, forming a pair and including this invention&#39;s indirectly heated electrode for gas discharge tube, has a sealed container, on the inner surface of which is formed a fluorescent film, and with which a rare gas is sealed inside the sealed container. Gas discharge tubes using this invention&#39;s indirectly heated electrode for gas discharge tube include a mercury lamp, which has discharge electrodes, forming a pair and including this invention&#39;s indirectly heated electrode for gas discharge tube, has a sealed container, and with which a rare gas and mercury are sealed inside the sealed container. Gas discharge tubes using this invention&#39;s indirectly heated electrode for gas discharge tube include a fluorescent lamp, which has discharge electrodes, forming a pair and including this invention&#39;s indirectly heated electrode for gas discharge tube, has a sealed container, on the inner surface of which is formed a fluorescent film, and with which a rare gas and mercury are sealed inside the sealed container. 
   Also, making use of the characteristic of the dispersion of discharge, this invention&#39;s indirectly heated electrode for gas discharge tube may be employed in a lamp with one outer electrode, which has an electrode  42  at the exterior of a container  41 , has any of indirectly heated cathodes for gas discharge tube C 1  to C 4  disposed inside container  41 , has a rare gas sealed inside container  41 , and is driven using a high-frequency power supply  43  as shown in  FIG. 11 . This invention&#39;s indirectly heated electrode for gas discharge tube can thus be used in the above-described low-pressure gas lamp, etc. 
   As a lighting circuit for the above-described gas discharge tube DT 2 , which maybe a rare gas fluorescent lamp, mercury lamp, fluorescent lamp, etc., a known, starter (preheating starting) type lighting circuit, having a glow tube  53 , ballast  54 , and AC power supply  55  as shown in  FIG. 12 , may be used. In place of a starter type, a rapid start type lighting circuit may also be used as the lighting circuit. As the driving method, a type specialized to high-frequency lighting (Hf) may also be used. 
   With a gas discharge tube using this invention&#39;s indirectly heated electrode for gas discharge tube, in the case of AC operation, each of the pair of electrodes (indirectly heated cathodes for gas discharge tube C 1  to C 4 ) alternatingly serves, as the main functions, the role of a cathode that emits electrons and an anode into which electrons flow. When functioning as an anode, a large amount of heat is generated at an electrode due to the voltage drop that occurs when the electrons flow in. By using the heat amount, which is generated when an electrode functions as the anode, as the heat amount necessary for thermionic emission when the electrode functions as the cathode, stable, sustained discharge can be realized without the supply of heat from heater  1  or with a lower supply of heat in comparison to DC operation during sustained discharge of the gas discharge tube. 
   A lighting device suitable for deuterium gas discharge tube DT 1  that uses indirectly heated cathode for gas discharge tube C 1  shall now be described based on  FIG. 13 .  FIG. 13  is a circuit diagram, showing a lighting device for deuterium gas discharge tube DT 1  that uses indirectly heated cathode for gas discharge tube C 1 . 
   A lighting device  101  comprises a constant current power supply  103 , connected as a power supply between indirectly heated cathode for gas discharge tube C 1  and anode  74  of deuterium gas discharge tube DT 1 , an auxiliary lighting circuit unit  111 , connected between anode  74  and focusing electrode  76  in order to generate a trigger discharge across indirectly heated cathode for gas discharge tube C 1  and focusing electrode  76 , a make-and-break switching circuit unit  121 , connected between indirectly heated cathode for gas discharge tube C 1  and anode  74  and supplying electricity to a heater  1  for a predetermined period and then cutting off the supply of electricity to heater  1  after the elapse of the predetermined period, and a fixed resistor  131  for current detection, serially connected and installed between anode  74  and constant current power supply  103 . 
   Constant current power supply  103  supplies a DC open voltage of approximately 160V and a steady-state current of approximately 300 mA. A negative resistance  105  and a diode  107  for discharge stabilization are connected serially to this constant current power supply  103 . Negative resistance  105  is set to approximately 50 to 150 Ω. 
   Auxiliary lighting circuit unit  111  includes a fixed resistor  113 , which is serially connected and installed between anode  74  and focusing electrode  76 , and a capacitor  115 , which is connected in parallel to this fixed resistor  113 . Make-and-break switching circuit unit  121  includes a glow tube  123 . A switch, which is opened after operation (lighting) of deuterium gas discharge tube DT 1  may be provided between auxiliary lighting circuit unit  111  and focusing electrode  76 . Also, in place of a glow starter system using glow tube  123 , an electronic starting system using a semiconductor element with a timer function or a mechanical (contact) switch, which may or may not have a timer function, may be used. 
   The operation of lighting device  101  shall now be described based on  FIGS. 14A to 14F  and  15 A to  15 E. 
   Though not illustrated in  FIG. 13 , when a main power switch of lighting device  101  for deuterium gas discharge tube DT 1  is switched ON (start), power is supplied from constant current power supply  103  to glow tube  123 , glow discharge occurs at glow tube  123 , and by mutual contact of the electrodes of glow tube  123 , power is supplied to heater  1  of indirectly heated cathode for gas discharge tube C 1 , and indirectly heated cathode for gas discharge tube C 1  is thereby preheated (period A1 in  FIGS. 14A to 14F  and  15 A to  15 E). At this point, a voltage of approximately 130V is applied across indirectly heated cathode for gas discharge tube C 1  and anode  74  from constant current power supply  103  and an electric field directed from anode  74  to indirectly heated cathode for gas discharge tube C 1  is generated. 
   When these preparations for trigger discharge have been made, the glow discharge at glow tube  123  stops and by the separation of the electrodes of glow tube  123 , a potential of approximately 130V is generated at focusing electrode  76  from constant current power supply  103  and via the parallel-connected capacitor  115  and fixed resistor  113 , and a trigger discharge is generated across indirectly heated cathode for gas discharge tube C 1  and focusing electrode  76  (period A2 in  FIGS. 14A to 14F  and  15 A to  15 E). 
   By thus causing a trigger discharge to occur, an arc discharge is made to occur across indirectly heated cathode for gas discharge tube C 1  and anode  74 , and based on the current of approximately 300 mA that is supplied across indirectly heated cathode for gas discharge tube C 1  and anode  74  from constant current power supply  103 , arc discharge is sustained in a stable manner until the main power switch is turned OFF (period A3 in  FIGS. 14A to 14F  and  15 A to  15 E). During operation (lighting) of deuterium gas discharge tube DT 1 , the voltage applied to deuterium gas discharge tube DT 1  from constant current power supply  103  is lowered, by fixed resistor  131 , from the approximately 160V in the starting process to approximately 120V. 
   Since deuterium gas discharge tube DT 1  using indirectly heated cathode for gas discharge tube C 1  can be driven in accordance to the relationships expressed by the following equations (3) and (4);
 
I f0 =Ip  (3)
 
V f1 =0  (4)
 
   In the above, I f0 : initial supply current to the heater in the starting state 
   Ip: discharge current 
   V f1 : voltage applied to the heater during operation 
   With lighting device  101 , a lighting device for lighting deuterium gas discharge tube DT 1  using indirectly heated cathode for gas discharge tube C 1  can be realized. Also, since a single constant current power supply  103  can be used for the preheating of indirectly heated cathode for gas discharge tube C 1 , for the starting of the trigger discharge (discharge by initial gas ionization), and for the main discharge, a power supply for preheating (heater) of indirectly heated cathode for gas discharge tube C 1  is made unnecessary in particular, thus enabling significant reduction of the number of parts and simplification of arrangement. 
   Also, with lighting device  101 , since make-and-break switching circuit unit  121  includes a glow tube  123 , make-and-break switching circuit unit  121  can be realized simply and at low cost. Furthermore, since auxiliary lighting circuit unit  111  includes fixed resistor  113  and capacitor  115 , auxiliary lighting circuit unit  111  can be realized simply and at low cost. 
   Also, with lighting device  101 , since a fixed resistor  131  for current detection is provided, the voltage during operation of deuterium gas discharge tube DT 1  can be lowered and the consumption power of deuterium gas discharge tube DT 1  can thus be lowered. 
   Though in the present embodiment, a high-melting-point metal is used as the electrical conductor, a porous metal of low thickness, carbon fibers, etc. may be used in place of a high-melting-point metal. Also for improvement of the sputter resistance and improvement of the discharge performance of metal oxide  10 , a nitride or carbide of tantalum, titanium, niobium, etc. maybe attached to the surface of metal oxide  10  or to double coil  2  or  41 , single coil  45 , plate member  3 , mesh member  21 , or wire member  23 . 
   Also, though the surface part of double coil  2  or  41  or single coil  45  is exposed in the present embodiment, this does not have to be exposed necessarily, and as long as the surface part of double coil  2  or  41  or single coil  45  is in contact with metal oxide  10 , the surface part of double coil  2  or  41  or single coil  45  may be covered with metal oxide  10 . By exposing the surface part of double coil  2  or  41  or single coil  45 , the discharge property can be improved. 
   INDUSTRIAL APPLICABILITY 
   This invention&#39;s indirectly heated electrode for gas discharge tube can be used as an indirectly heated electrode (indirectly heated cathode) of a rare gas lamp, rare gas fluorescent lamp, mercury lamp, mercury fluorescent lamp, deuterium lamp, etc