Gas discharge tube

Abstract: A gas discharge tube comprises an envelope (11) for sealing a gas, lead pins (13-16) and a light-emitting assembly (20). The light-emitting assembly includes: a focusing electrode support member (21) of a conductive material; a hot cathode (31) for emitting thermoelectrons; an anode (24) for receiving thermoelectrons; a focusing electrode (29) for converging paths of thermoelectrons; a spacer (50) of an insulator being arranged between the focusing electrode support member and the anode; and an anode support member (22) of an insulator for pushing the anode onto the focusing electrode support member through the spacer. A gas discharge tube having a long service life and capable of improving the operational stability during long-time continuous light emission can be provided.

Description: Field of the invention The present invention relates to a gas discharge tube used as an ultraviolet light source for a spectrophotometer, liquid chromatography, or the like. Related background art A gas discharge tube is a discharge light source using positive column light emission by arc discharge of a gas sealed in a tube. As a typical gas discharge tube, a deuterium discharge tube in which ultraviolet light is emitted by discharge of sealed deuterium is known. This deuterium discharge tube is mainly used as an ultraviolet continuous spectrum source for a spectrophotometer or the like. Note that such a deuterium discharge tube is described in, e.g., Japanese Patent Application Laid-Open Gazette No. 4-255662. Other gas discharge tubes are to be found in DE-A-3908553 and EP-A-0473378. In such a gas discharge tube, since very small variations of, e.g., 0.01% or 0.001% in output is undesirable during long-time continuous light emission, strict characteristics are requested in many cases. Further, it is necessary to maintain the electrical insulating property between the focusing electrode and anode of the gas discharge tube during long-time continuous light emission. SUMMARY OF THE INVENTION It is an object of the present invention to provide a gas discharge tube which has a longer service life than the conventional gas discharge tube and is capable of improving the operational stability during long-time continuous light emission. In accordance with the present invention there is provided a gas discharge tube comprising: a cathode for emitting thermoelectrons; an anode for receiving the thermoelectrons emitted from said cathode; a focusing electrode between the anode and the cathode having a focusing opening for converging the paths of the thermoelectrons; and a focusing electrode support member formed of an electrically conductive material, said focusing electrode support member having a front surface and a rear surface which is opposite to said front surface and which is in communication with the front surface by a through hole aligned with said focusing opening; as known from DE-A-3908553 for example, which is characterised by a spacer formed of an electrically insulating material, said spacer being arranged between said focusing electrode support member and said anode, and being in contact with both the rear surface of said focusing electrode support member and a front surface of said anode; and an anode support member formed of an electrically insulating material, said anode support member being located on the opposite side of the anode with respect to said focusing electrode support member and having a surface which is in contact with a rear surface of said anode for determining the position of said anode with respect to the rear surface of said focusing electrode support member by means of said spacer, whereby the spacing between said focusing electrode and said anode is defined by said focusing electrode support member and said spacer. As will be described hereinafter, the present invention enables a gas discharge tube to be provided which has a longer service life than the conventional gas discharge tube and has improved operational stability during long-time continuous light emission. The present invention enables a gas discharge tube to be provided which enables a short circuit between the focusing electrode and the anode of the discharge tube to be fully prevented. In one embodiment of a gas discharge tube in accordance with the present invention, said spacing between the focusing electrode and the anode is defined by a surface of the spacer, the rear surface of the focusing electrode support member and an inner wall surface of the focusing electrode support member, the inner wall surface defining the through hole in the focusing electrode support member. The focusing electrode support member and the focusing electrode may be made of metal. Preferably, the anode comprises an anode fixing plate formed of an electrically conductive material, the anode fixing plate having a front surface being in contact with the spacer, and a rear surface being in contact with the anode support member; and an anode plate for receiving the thermoelectrons, the anode plate being made of a high melting point metal, and being fixed on the front surface of the anode fixing plate. The anode fixing plate may be partially covered by the anode plate, the spacer is in contact with the uncovered portion of the fixing plate, and a space is defined between the surface of the spacer, the surface of the fixing plate and a surface of the anode plate. In another embodiment of a gas discharge tube according to the present invention the spacer has a shape which enables it to form a space defined by a surface of the spacer and the rear surface of the focusing electrode support member. The spacer preferably has a shape selected from the group of a columnar shape, a prismatic shape, a spherical shape and a block-like shape. In yet another embodiment of a gas discharge tube according to the present invention the anode support member may have a depression at its surface which faces, the focusing electrode support member, for accommodating the anode and the spacer therein, the depression extending from the surface of the anode support member in a vertical direction with respect to the rear surface of the focusing electrode support member. A gas discharge tube in accordance with the present invention may comprise a discharge straightening plate for discharge shielding, the discharge straightening plate being positioned and mounted on the front surface of the focusing electrode support member, and having a slit for passing through thermoelectrons emitted from the cathode. Preferably, a gas discharge tube may further comprise a front cover for discharge shielding, the front cover being positioned and mounted on the front surface of the focusing electrode support member so as to accommodate the cathode and the discharge straightening plate in a space defined by the front cover and the focusing electrode support member, the front cover having a window located at a position which faces the focusing opening of the focusing electrode, for outputting light caused by discharge. The anode support member and the spacer according to the present invention are preferably made of a ceramic, and the focusing electrode support member and the focusing electrode are preferably made of a metal. In such a gas discharge tube, when arc discharge occurs among the cathode, the focusing electrode and the anode, the anode generates heat upon reception of thermoelectrons, and the focusing electrode also generates heat upon bombardment of cations. However, in the gas discharge tube of the present invention, the anode is pressed against and fixed to the anode support member by the focusing electrode support member through the spacer, and the focusing electrode is supported on the front surface of the focusing electrode support member. For this reason, a predetermined interval between the focusing electrode and the anode can be strictly defined by the focusing electrode support member and the spacer. In accordance with another aspect of the present invention the gas discharge tube as described above may be used as an ultraviolet light source. Therefore, even when the focusing electrode and the anode are heated during use of the gas discharge tube as mentioned above, the predetermined distance between the focusing electrode and the anode can be maintained constant thereby to prevent deformation of the path of the thermoelectrons between the focusing electrode and the anode. Thus the arc discharge state can be held stable, and the stability of light emission of the discharge tube will not be impaired. Additionally, it is achieved by the present invention to prevent shortening of the service life of the discharge tube, such shortening would be caused by an increase in loss of the anode, and the like. Further, in the gas discharge tube of the present invention, although the focusing electrode support member is conductive, the anode support member and the spacer have electrical insulating properties. For this reason, the conductive anode can be electrically insulated from the conductive focusing electrode as well as the conductive focusing electrode support member by the spacer and the anode support member which have electrical insulating properties. Furthermore, since the focusing electrode support member according to the present invention is made of a conductive material such as a metal, the focusing electrode can be fixed to the focusing electrode support member by welding. In a case where the discharge straightening plate and the front cover are made of a conductive material, these members can also be fixed to the focusing electrode support member by welding. Therefore, the gas discharge tube of the present invention is easy and cheap to produce. Into such a space as is defined by the surfaces of the spacer and the focusing electrode support member and such a space as is defined by the surfaces of the spacer and the anode, an electrode material which would be sputtered from the anode and/or the focusing electrode by thermoelectrons during light emission of the gas discharge tube is hardly deposited. Particularly, such an electrode material is hardly deposited into a space defined by a surface of the spacer, the anode fixing plate and the anode plate. Therefore, in the gas discharge tube of the present invention, a short circuit between the focusing electrode and the anode can be fully prevented. Further, since the above relatively complex structure for preventing the deposition of the material sputtered is formed by the spacer and the focusing electrode support member both of which have relatively simple structures, the present invention achieves not only prevention of the short circuit but also more efficient production. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing an embodiment (first embodiment) of a side-on type gas discharge tube according to the present invention. Fig. 2 is an exploded perspective view of a light-emitting assembly of the gas discharge tube shown in Fig. 1. Fig. 3 is a perspective view showing a focusing electrode support member and an anode in the light-emitting assembly shown in Fig. 2. Fig. 4 is a perspective view showing an anode support member and the anode in the light-emitting assembly shown in Fig. 2. Fig. 5 is a horizontal sectional view of the light-emitting assembly of the gas discharge tube shown in Fig. 1 (taken along a line A - A in Fig. 1). Fig. 6 is a partial sectional view for explaining a structure of the anode accommodation recess portion in the light-emitting assembly shown in Fig. 5. Fig. 7 is a partial sectional view of another embodiment of the light-emitting assembly according to the present invention. Fig. 8 is a perspective view showing an embodiment (second embodiment) of a head-on type gas discharge tube according to the present invention. Fig. 9 is a vertical sectional view of the gas discharge tube shown in Fig. 8 (taken along a line B - B in Fig. 8). Fig. 10 is a vertical sectional view of a light-emitting assembly of the gas discharge tube shown in Fig. 8 (taken along a line C - C in Fig. 9). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The arrangement and function of a gas discharge tube according to the first embodiment of the present invention will be described below in detail with reference to Figs. 1 to 6. The gas discharge tube of this embodiment is a side-on type deuterium discharge tube which emits light (ultraviolet light) from the side portion of the tube. Note that, in this embodiment, the front and rear sides are defined on the basis of the light emission direction. In a deuterium discharge tube 10 shown in Fig. 1, a light-emitting assembly (light-emitting section) 20 is accommodated in a cylindrical glass envelope 11 while being supported by lead pins 13 to 16. Deuterium gas is sealed in the glass envelope 11 at about several Torr. The envelop 11 with its head portion sealed has a cylindrical shape, and the bottom portion of the envelope 11 is hermetically sealed by a glass stem 12. The envelop 11 is made of ultraviolet light-transmitting glass, quartz glass or the like which has a high permeability to ultraviolet light. The four lead pins 13 to 16 which are in parallel aligned in line extend through the glass stem 12 from the exterior of the envelope 11, and are covered by insulating members 130, 140, 150, and 160, respectively. Terminals 131, 141, 151, and 161 will be connected to terminals of an external power supply. The light-emitting assembly 20 is positioned at distal ends of the lead pins 13-16 and is spaced from an inner side wall of the envelope 11. The light-emitting assembly 20 comprises a front cover 23 which is made of a metal such as Ni or SUS, or a ceramic; an anode support member 22 which is made of a ceramic; and a focusing electrode support member 21 arranged between the anode support member 22 and the front cover 23 and made of a metal (Ni or SUS). Specifically, the light-emitting assembly 20, as shown in Fig. 2, comprises: a hot cathode 31 for emitting thermoelectrons; an anode 24 for receiving the thermoelectrons emitted from the hot cathode 31; a focusing electrode 29 having a focusing opening 29a for converging and passing through the thermoelectrons; a focusing electrode support member (discharge shielding member) 21 for supporting the focusing electrode 29; two columnar spacers 50 made of a ceramic and located between the focusing electrode support member 21 and the anode 24; and an anode support member 22 located on an opposite side to the focusing electrode support member 21 through the anode 24 in order to push the anode 24 onto the focusing electrode support member 21 through the spacers 50. The light-emitting assembly 20 further comprises: a discharge straightening plate 32 mounted on the front surface 21a of the focusing electrode support member 21 and having a slit 32a for passing through the thermoelectrons emitted from the hot cathode 31; and a front cover 23 mounted on the front surface 21a of the focusing electrode support member 21 and having a window 30 for outputting ultraviolet light (UV) caused by arc discharge. The structure of the light-emitting assembly 20 will be described below in detail. As shown in Figs. 2, 3 and 5, an anode (anode portion) 24 is fixed at the distal end of the lead pin 14. This anode 24 is constituted by a rectangular anode fixing plate 24a made of a metal such as Ni and fixed at the distal end of the lead pin 14, and a plate-like anode (anode plate) 24b fixed on a front surface 24aB of the anode fixing plate 24a. A part of the front surface 24aB of the anode fixing plate 24a is not covered with the anode plate 24a. The anode plate 24b is made of a high melting point metal such as molybdenum or tungsten. In this specification, the high melting point metal means one of a group of materials having a melting point higher than that (1414°C) of sillicon. Further, the anode support member 22 consisting of a prism having an almost convex section has, at its front portion, an anode accommodation recess portion (depression) 25 for accommodating the anode fixing plate 24a, and a lead pin accommodation recess portion (depression) 26 for accommodating the distal end portion of the lead pin 14 located behind the anode 24. Therefore, when the lead pin 14 is accommodated in the lead pin accommodation recess portion 26 while fixing the anode 24 to the lead pin 14, the anode support member 22 can be held in the envelope 11 by the lead pin 14. A rear surface 24aA of the anode fixing plate 24a is in contact with and supported by a bottom surface 25a (constituting a part of a front surface K of the anode support member 22) of the anode accommodation recess portion 25. The anode support member 22 is integrally formed of a ceramic having electrical insulating properties and a high thermal conductivity. A so-called conductive ceramic such as beryllium oxide or aluminum nitride is preferably used. Therefore, the anode support member 22 acts as a heat sink for the anode 24 to-be heated to a high temperature, thereby efficiently dissipating the heat accumulated in the light-emitting assembly 20. As shown in Figs. 2, 4, and 5, the plate-like focusing electrode support member 21 arranged in front of the anode 24 has a rectangular opening portion (through hole) 27 at a position facing the anode plate 24b. A focusing electrode fixing plate 28 made of a metal is arranged to contact the focusing electrode support member 21. A focusing electrode 29 made of a metal is fixed to a front surface 28a of the focusing electrode fixing plate 28. The focusing electrode fixing plate 28 is fixed to a front surface 21a of the focusing electrode support member 21. A focusing opening 29a of the focusing electrode 29 is arranged in the opening portion (through hole) 27 of the focusing electrode support member 21 and opposes the anode plate 24b. In this case, the focusing electrode 29 can be fixed to the focusing electrode support member 21 through the focusing electrode fixing plate 28 by welding. Further, two columnar spacers 50 made of a ceramic are arranged between the focusing electrode support member 21 and the anode 24, and are each in contact with both the rear surface 21b of the focusing electrode support member 21 and a front surface 24aB of the anode fixing plate 24a. Therefore, as shown in Fig. 5, the interval between the focusing electrode 29 and the anode 24 is defined by the focusing electrode support member 21 and the spacers 50. Specifically, the above interval is defined by a surface 50a of the spacer 50, the rear surface 21b of the focusing electrode support member 21 and an inner wall surface 21c of the focusing electrode support member 21, the inner wall surface 21c defining the through hole 27. As shown in Fig. 6, since the spacers 50 have a columnar shape and the anode plate 24b is nor in contact with the spacers 50, spaces (depressions) D1 each defined by the surface 50a of the spacer 50, the front surface 24aB of the anode fixing plate 24a and the surface of the anode plate 24b are formed. Further, spaces (depressions) D2 each defined by the surface 50a of the spacer 50 and the rear surface 21b of the focusing electrode support member 21 are also formed. The anode accommodation recess portion 25 of the anode support member 22, as shown in Fig. 6, has a depth W25 matching a sum of a diameter W50 of the spacer 50 and a thickness W24a of the anode fixing plate 24a, thereby an edge of the front surface K of the anode support member 22 is in directly contact with the rear surface 21b of the focusing electrode support member 21. As shown in Figs. 2 and 5, the front cover 23 has an almost U-shaped section and is fixed to the front surface 21a of the focusing electrode support member 21. A window 30 for passing through light caused by discharge is formed around the center portion of the front cover 23 so that the window 30 opposes to the focusing opening 29a and the anode plate 24b. A spiral hot cathode (filament) 31 for generating thermoelectrons is arranged in a space S formed between the front cover 23 and the focusing electrode support member 21. The hot cathode 31 is arranged outside the optical path of the ultraviolet light to-be emitted, i.e., on one side in the front cover 23. The hot cathode 31 has electrode rods 31a and 31b at its two ends. A discharge straightening plate 32 made of a metal (Ni or SUS) or a ceramic is arranged outside the optical path of the ultraviolet light to-be emitted and between the hot cathode 31 and the focusing electrode 29. One end of the discharge straightening plate 32 is fixed to the front surface 21a of the focusing electrode support member 21 while the other end is in contact with an inner wall 23a of the front cover 23. The discharge straightening plate 32 also has a slit 32a for causing the hot cathode 31 to communicate with the focusing electrode 29. Thermoelectrons generated from the hot cathode 31 are passed through the slit 32a thereby to be straightened. Assembling of the light-emitting assembly 20 will be described below. In a case where the front cover 23 and the discharge straightening plate 32 are made of a metal, as shown in Fig. 2, a pair of left and right flange portions 23b are integrally formed at the two ends of the metal front cover 23. The flange portions 23b are brought into contact with the front surface 21a of the focusing electrode support member 21 and thereafter fixed to the focusing electrode support member 21 by welding. Further, a projecting piece 32b is integrally formed at one end of the metal discharge straightening plate 32. The projecting piece 32b is brought into contact with the front surface 21a of the focusing electrode support member 21 inside the flange portions 23b and fixed to the focusing electrode support member 21 by welding. On the other hand, in a case where the front cover 23 and the discharge straightening plate 32 are made of a ceramic, the front cover 23 and the discharge straightening plate 32 are fixed to the focusing electrode support member 21 by using rivets or the like. Although not illustrated, a bendable pawl piece is formed on the focusing electrode support member 21, and pawl through holes for receiving the pawl piece are formed in the front cover 23 and the discharge straightening plate 32. When the pawl piece of the focusing electrode support member 21 is inserted into the pawl through holes of the front cover 23 and the discharge straightening plate 32 and bent, the front cover 23 and the discharge straightening plate 32 can be fixed to the focusing electrode support member 21. As shown in Figs. 2 to 4, rivet through holes 41, 42 and 43 are formed in the focusing electrode fixing plate 28, the focusing electrode support member 21 and the anode support member 22, respectively. The rivet through holes 41 to 43 extend in a direction of assembling to be coaxially arranged. Therefore, when the rivet through holes 41 to 43 are positioned, rivets 44 are then inserted into the rivet through holes 41 to 43, and the end portions of the rivets 44 are caulked, the focusing electrode fixing plate 28, the focusing electrode support member 21 and the anode support member 22 are integrally assembled, as shown in Fig. 5. A vertical through hole 46 for receiving the lead pin 13 is formed in the anode support member 22 at its rear portion. The distal end of the lead pin 13 inserted in this vertical through hole 46 is welded to a tongue piece 47 integrally formed at the head portion of the focusing electrode fixing plate 28 (shown in Fig. 1). A vertical through hole 48a for receiving an L-shaped electrode rod 48 is formed in the anode support member 22 at its side portion. To fix the hot cathode 31 in the space S, the electrode rod 48 is inserted into the vertical through hole 48a, and thereafter, the lower end of the electrode rod 48 is welded to the distal end of the lead pin 15. Additionally, the electrode rod 31b of the hot cathode 31 is welded to the distal end of the electrode rod 48 while the electrode rod 31a of the hot cathode 31 is welded to the distal end of the lead pin 16. As shown in Figs. 4, 5 and 6, two columnar spacers 50 made of a ceramic are arranged between the focusing electrode support member 21 and the anode fixing plate 24a of the anode 24. The spacers 50 are each in contact with the rear surface 21b of the focusing electrode support member 21 and the front surface 24aB of the anode fixing plate 24a on both sides in the anode accommodation recess portion 25. When the focusing electrode support member 21 and the anode support member 22 are assembled through the rivets 44, the rear surface 24aA of the anode fixing plate 24a of the anode 24 is pressed against and fixed to, by the pressing force given thereto through the spacers 50, the bottom surface 25a of the anode accommodation recess portion 25 constituting a part of the front surface K of the anode support member 22. Therefore, a predetermined interval between the focusing electrode 29 and the anode 24 can always be maintained constant by the spacers 50 and the focusing electrode support member 21. Although columnar spacers are used in the embodiment mentioned above, spacers having a spherical, prismatic, or block-like shape may be used as spacers 50. Fig. 7 shows another embodiment using therein octagonal prismatic spacers 50. Next, the operation of the above-described side-on type deuterium discharge tube 10 will be described below. A power of about 10 W is supplied from the external power supply (not shown) to the hot cathode 31 about 20 seconds before discharge to preheat the hot cathode 31. Thereafter, a DC open-circuit voltage of about 150 V is applied between the hot cathode 31 and the anode 24, thereby preparing for arc discharge. Upon completion of the above preparation, a trigger voltage of about 350 to 500 V is applied between the hot cathode 31 and the anode 24. Under this condition, thermoelectrons emitted from the hot cathode 31 pass through the elongated slit 32a of the discharge straightening plate 32 toward the anode plate 24b while being converged by the focusing opening 29a of the focusing electrode 29. Arc discharge occurs in front of the focusing opening 29a. Ultraviolet light emitted from an arc ball (high-density discharge area) generated by this arc discharge passes through the window 30 and is then projected outward through the circumferential surface of the glass envelope 11. The anode 24 and the focusing electrode 29 are heated to a high temperature exceeding several hundreds °C. This heat is dissipated through the above-described members made of a ceramic or the like as needed. Since the anode 24 is firmly held by the anode support member 22 and the focusing electrode support member 21 through the spacers 50, and the focusing electrode 29 is firmly held by the focusing electrode support member 21, deformation of the above members hardly occurs even in a high temperature state during long-time continuous light emission. Therefore, a satisfactory positional precision between the anode 24 and the focusing electrode 29 can be maintained constant. Further, an electrode material sputtered from the anode plate 24b by thermoelectrons during light emission of the gas discharge tube is hardly deposited into the depressions D1 as well as the depressions D2. Therefore, a short circuit between the focusing electrode 29 and the anode 24 can be fully prevented. A gas discharge tube according to the second embodiment of the present invention will be described below. The gas discharge tube of this embodiment is a head-on type deuterium discharge tube which emits light from the head portion of the tube. Note that, in this embodiment, the front and rear sides are defined on the basis of the light emission direction. In a deuterium discharge tube 60 shown in Fig. 8, a light-emitting assembly 70 is accommodated in a cylindrical glass envelope 51. Deuterium gas (not shown) is sealed in the envelope 61 at about several Torr. The envelope 61 has a disk-like light-emitting surface 62 at the head portion and a disk-like stem 63 at the bottom portion. A tip tube 64 for exhausting/sealing a gas is provided to the stem 63. Upon completion of exhausting and then sealing of a gas in the envelope 61 through the tip tube 64, the tip tube 64 can be closed to hermetically seal the envelope 61. The envelope 61 is formed of an ultraviolet light-transmitting glass or quartz glass having a high transmissivity to ultraviolet light. Six lead pins 65a to 65f are fixed to the stem 63. The lead pins 65a to 65f extend through the stem 63 and are connected to an external power supply (not shown) while being covered with insulating members 120a to 120f, respectively (in Fig. 8, the insulating members 120b to 120d are hidden by the light-emitting assembly 70). The light-emitting assembly 70 has a front cover 66 arranged in the front and made of a metal (Ni or SUS) or a ceramic; an anode support member 67 arranged at the rear of the front cover 56 and made of a ceramic; and a focusing electrode support member 68 fixed between the anode support member 67 and the front cover 66 and made of a metal (Ni or SUS). The structure of the light-emitting assembly 70 will be described below in detail. As shown in Figs. 9 and 10, an anode 71 is fixed at the distal end of the lead pin 65c extending through the anode support member 67. This anode 71 is constituted by a rectangular anode fixing plate 71a fixed at the distal end of the lead pin 65c, and a plate-like anode (anode plate) 71b fixed on a front surface 71aB of the anode fixing plate 71a. A circumferential portion of the front surface 71aB of the anode fixing plate 71a is not covered with the anode plate 71b. The anode fixing plate 71a is made of a metal such as Ni, and the anode plate 71b is made of a high melting point metal such as molybdenum or tungsten. The cylindrical anode support member 67 having an almost concave section has, at its front portion, an anode accommodation recess portion 72 for accommodating the anode 71. Therefore, a rear surface 71aA of the anode fixing plate 71a can be brought into contact with and supported by a bottom surface 72a (constituting a part of a front surface G of the anode support member 67) of the anode accommodation recess portion 72. To arrange the anode support member 67 in the envelope 61, the anode support member 67 is fixed at the distal end of the lead pin 65f. The plate-like focusing electrode support member 68 arranged in front of the anode 71 has a rectangular opening portion (through hole) 73 at a position opposing the anode plate 71b. A metal focusing electrode 74 is fixed to a front surface 68a of the focusing electrode support member 68. A focusing opening 74a of the focusing electrode 74 is arranged in the opening portion 73 of the focusing electrode support member 68 and opposes the anode plate 71b. In addition, the focusing electrode 74 is welded to the distal end of the lead pin 65a extending through the anode support member 57 and the focusing electrode support member 68. To arrange the focusing electrode support member 68 in the envelope 61, the focusing electrode support member 68 is fixed at the distal end of the lead pin 65e. The front cover 66 has a cup-like section and is fixed to the front surface 68a of the focusing electrode support member 68 by rivets or pawl pieces (not shown). A window 75 for passing through light caused by discharge is formed around the center portion of the front cover 66 so that the window 75 opposes to the focusing opening 74a and the anode plate 71b. A hot cathode 76 for generating thermoelectrons is arranged in a space P formed between the front cover 66 and the focusing electrode support member 68. The hot cathode 76 is arranged outside the optical path of the ultraviolet light to-be emitted, i.e., on one side in the front cover 66. The hot cathode 76 has electrode rods 76a and 76b at its two ends. The electrode rods 76a and 76b are respectively welded to the distal ends of the lead pins 65b and 65d extending through the anode support member 67 and the focusing electrode support member 68. A discharge straightening plate 77 made of a metal (Ni or SUS) or a ceramic is arranged outside the optical path of the ultraviolet light to-be emitted, and between the hot cathode 76 and the focusing electrode 74. The discharge straightening plate 77 stands on the front surface 68a of the focusing electrode support member 68 while contacting the focusing electrode 74. Four spherical spacers 80 made of a ceramic are arranged between the focusing electrode support member 68 and the anode fixing plate 71a of the anode 71. The spacers 80 are each in contact with both a rear surface 68b of the focusing electrode support member 68 and the front surface 71aB of the anode fixing plate 71a, around the focusing opening 74a in the anode accommodation recess portion 72. When the focusing electrode support member 68 and the anode support member 67 are assembled and fixed, the rear surface 71aA of the anode fixing plate 71a is pressed against and fixed to, by the pressing force given thereto through the spacers 80, the bottom surface 72a of the anode accommodation recess portion 72 constituting a part of the front surface G of the anode support member 67. Therefore, a predetermined interval between the focusing electrode 74 and the anode 71 can always be maintained constant by the spacers 80 and the focusing electrode support member 68. Note that the spacers 80 may also have a columnar, prismatic, or block-like shape. Next, the operation of the above-described head-on type deuterium discharge tube 60 will be described below. A power of about 10 W is supplied from the external power supply (not shown) to the hot cathode 76 about 20 seconds before discharge to preheat the hot cathode 76. Thereafter, a DC open-circuit voltage of about 150 V is applied between the hot cathode 76 and the anode 71, thereby preparing for arc discharge. Upon completion of the above preparation, a trigger voltage of about 350 to 500 V is applied between the hot cathode 76 and the anode 71. Under this condition, thermoelectrons emitted from the hot cathode 76 are straightened by the discharge straightening plate 77 toward the anode plate 71b while being converged through the focusing opening 74a of the focusing electrode 74. Arc discharge occurs in front of the focusing opening 74a. Ultraviolet light emitted from an arc ball generated by this arc discharge passes through the window 75 and is then projected outward through the light-emitting surface 62 of the glass envelope 61. The anode 71 and the focusing electrode 74 are heated to a high temperature exceeding several hundreds °C. This heat is dissipated through the above-described members formed of a ceramic or the like as needed. Since the anode 71 is firmly held by the anode support member 67 and the focusing electrode support member 68 through the spacers 80, and the focusing electrode 74 is firmly held by the focusing electrode support member 68, deformation of the above members hardly occurs even in a high temperature state during long-time continuous light emission. Therefore, a satisfactory positional precision between the anode 71 and the focusing electrode 74 can be maintained constant. Further, an electrode material sputtered from the anode plate 71b by thermoelectrons during light emission of the gas discharge tube is hardly deposited into the depressions D1 each defined by the surface of the spacer 80, the front surface 71aB of the anode fixing plate 71a and the surface of the anode plate 71b, as well as the depressions D2 each defined by the surface of the spacer 80 and the rear surface 68b of the focusing electrode support member 68. Therefore, a short circuit between the focusing electrode 74 and the anode 71 can be fully prevented. The gas discharge tube of the present invention is not limited to the above embodiments, and various changes and modifications can also be made. As has been described above in detail, in the gas discharge tube according to the present invention, the spacer is brought into contact with both the rear surface of the focusing electrode support member and the front surface of the anode to press the anode against the front surface of the anode support member, thereby holding constant the interval between the focusing electrode and the anode through the spacer and the focusing electrode support member. With this arrangement, the anode is firmly held by the spacer and the anode support member. Deformation of the above members hardly occurs even in a high temperature state during long-time continuous light emission, and satisfactory positional precision between the anode and the focusing electrode can be maintained constant thereby to prevent deformation of the path of the thermoelectrons between the focusing electrode and the anode. Thus the arc discharge state can be held stable, and the stability of light emission of the discharge tube will not be impaired. Therefore, the operational stability of the gas discharge tube can be improved, and the service life thereof can also be prolonged. In addition, in the gas discharge tube of the present invention, since the anode support member and the spacer are made of an insulator such as a ceramic, the electrical insulating effect between the focusing electrode and the anode, as well as the heat dissipation effect from the anode and the focusing electrode, can be promoted. Further, in the gas discharge tube of the present invention, a portion such as a depression on which the sputtered electrode material hardly deposits can be easily formed in the space formed between the focusing electrode support member and the anode support member, by means of appropriate selection of the shape and the position of the spacer to-be used. For this reason, a short circuit between the focusing electrode and the anode can be easily and fully prevented by only selection of the shape and the position of the spacer to-be used. Therefore, a gas discharge tube having a long service life and capable of improving the operational stability during long-time continuous light emission can be easy provided. From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. The basic Japanese Application No. 7-29564 (29564/1995) filed on February 17, 1995 is hereby incorporated by reference.

Claims: A gas discharge tube comprising: a cathode (31; 76) for emitting thermoelectrons; an anode (24; 71) for receiving the thermoelectrons emitted from the cathode; a focusing electrode (29; 74) between the anode (24) and the cathode (31) having a focusing opening (29a) for converging the paths of the thermoelectrons; and a focusing electrode support member (21; 68) formed of an electrically conductive material, the focusing electrode support member having a front surface (21a) and a rear surface (21b) which is opposite to the front surface and which is in communication with the front surface (21a) by a through hole (27) aligned with the focusing opening (29a); characterised by a spacer (50; 80) formed of an electrically insulating material, the spacer being arranged between the focusing electrode support member (21) and the anode (24), and being in contact with both the rear surface of the focusing electrode support member and a front surface of the anode; and an anode support member (22; 67) formed of an electrically insulating material, the anode support member being located on the opposite side of the anode (24) with respect to the focusing electrode support member (21; 68) and having a surface which is in contact with a rear surface (21b; 68b) of the anode for determining the position of the anode with respect to the rear surface of the focusing electrode support member by means of the spacer (50), whereby the spacing between the focusing electrode and the anode is defined by the focusing electrode support member and the spacer. A gas discharge tube according to claim 1, wherein the spacing between the focusing electrode (29; 74) and the anode (24) is defined by a surface of the spacer, the rear surface (21b) of the focusing electrode support member (21) and an inner wall surface of the focusing electrode support member, the inner wall surface defining the through hole in the focusing electrode support member. A gas discharge tube according to claim 1 or 2, wherein the anode support member (22; 67) and the spacer (50; 80) are made of a ceramic. A gas discharge tube according to any preceding claim, wherein the focusing electrode support member (21; 68) and the focusing electrode (29; 74) are made of a metal. A gas discharge tube according to any preceding claim, wherein the anode (24; 71) comprises: an anode fixing plate (24a; 71a) formed of an electrically conductive material, the anode fixing plate having a front surface (24aB; 71aB) being in contact with the spacer (50), and a rear surface (24aA; 71aA) being in contact with the anode support member (22; 67); and an anode plate (24b) for receiving the thermoelectrons, the anode plate being made of a high melting point metal, and being fixed on the front surface of the anode fixing plate. A gas discharge tube according to claim 5, wherein the anode fixing plate (24a) is partially covered by the anode plate (24b), the spacer (50) is in contact with the uncovered portion of the fixing plate, and a space (D1) is defined between the surface (50a) of the spacer, the surface (24aB) of the fixing plate and a surface of the anode plate. A gas discharge tube according to claim 6, wherein the spacer (50) has a shape which enables it to form a space (D2) defined by a surface of the spacer and the rear surface of the focusing electrode support member (21). A gas discharge tube according to any preceding claim, wherein the spacer (50) has a shape selected from the group of a columnar shape, a prismatic shape, a spherical shape and a block-like shape. A gas discharge tube according to any preceding claim, wherein the anode support member (22) has a depression (25; 72) at its surface which faces to the focusing electrode support member (21), for accommodating the anode (24) and the spacer (50) therein, the depression extending from the surface of the anode support member in a vertical direction with respect to the rear surface of the focusing electrode support member. A gas discharge tube according to any preceding claim, further comprising a discharge straightening plate (32; 77) for discharge shielding, the discharge straightening plate being positioned and mounted on the front surface (21a) of the focusing electrode support member (21), and having a slit (32A) for passing through thermoelectrons emitted from the cathode (31). A gas discharge tube according to any preceding claim, further comprising a front cover (23) for discharge shielding, the front cover being positioned and mounted on the front surface (21a) of the focusing electrode support member (21) so as to accommodate the cathode (31) and the discharge straightening plate (32; 77) in a space defined by the front cover and the focusing electrode support member, the front cover having a window (30; 75) located at a position which faces the focusing opening of the focusing electrode, for outputting light caused by discharge. Use of a gas discharge tube according to any one of claims 1 to 11 as an ultraviolet light source.

Inventors: IKEDO TOMOYUKI, ITO YOSHINOBU, MATUI RYOTARO, IKEDO, TOMOYUKI, ITO, YOSHINOBU, MATUI, RYOTARO
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1996
Main classification: H01J61
IPC codes: H01J61, G01J3, G01N21