Patent Publication Number: US-2002011771-A1

Title: Gas discharge tube

Description:
RELATED APPLICATIONS  
     [0001] This is a Continuation-In-Part application of International Patent application serial No. PCT/JP99/06909 filed on Dec. 9, 1999, now pending. 
    
    
     
       BACK GROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to a gas discharge tube and, more particularly, to a gas discharge tube used as an ultraviolet light source or the like for spectrophotometer, liquid chromatography, and so on.  
       [0004] 2. Related Background Art  
       [0005] The conventional technology in this field includes the technique disclosed in Japanese Patent Application Laid-Open No. H04-147557. In a deuterium discharge tube described in this application, a focusing opening as a small hole of slit shape is formed in a focusing electrode plate interposed between anode and cathode. This focusing opening is formed in such a strip shape as to match with a slit shape of an analyzer, thereby increasing utilization efficiency of light emitted from the discharge tube.  
       SUMMARY OF THE INVENTION  
       [0006] In the conventional gas discharge tube, however, since the focusing opening of the small hole had the size of 0.4 mm horizontal and 2.5 mm vertical, there arose the problem that it was hard to obtain a beam with high luminance and in uniform slit shape though it was easy to induce arc discharge.  
       [0007] The present invention has been accomplished in order to solve the above problem and a specific object of the invention is to provide a gas discharge tube that is able to supply an emission with high luminance and in uniform slit shape.  
       [0008] In order to achieve the above object, a gas discharge tube according to the present invention comprises a hot cathode for generating thermoelectrons, an anode for receiving the thermoelectrons, and a focusing electrode provided between the hot cathode and the anode, for converging the thermoelectrons, wherein the focusing electrode has a focusing opening of a slit shape, wherein B/A is in a range of 0.1 to 0.5, where A is an opening length in a longitudinal direction of the focusing opening and B an opening length thereof in a direction perpendicular to the longitudinal direction, and wherein an opening area of the focusing opening is in a range of 0.15 to 0.5 mm 2 .  
       [0009] In the case of the commonly known focusing openings, the limit is normally a circular hole having the diameter of 0.5 mm because of increase in discharge starting voltage or occurrence of abnormal discharge. This is because decrease of the diameter of the focusing opening to below 0.5 mm will result in increasing the barrier between the hot cathode and the anode and thus necessitating high energy for a start of discharge. With increase in this energy (for example, with increase in discharge voltage), there will occur an event of failure in lighting of the gas discharge tube because of the abnormal discharge. In order to assure a stable discharge start, the inventor noted the area of the focusing opening of the slit shape. It was then verified by experiments that increase in the area of the focusing opening surely made it easier to induce arc discharge between the hot cathode and the anode but the luminance of emission became lower. For ensuring high luminance while enabling lighting of the gas discharge tube, the opening area was thus narrowed down into the range of 0.15 to 0.5 mm 2 . In addition, while taking the aforementioned opening area into consideration, the inventor also noted the relation between the opening length A in the longitudinal direction of the focusing opening and the opening length B in the direction perpendicular to the longitudinal direction, in order to obtain the uniform emission with high luminance. Then the shape of the focusing opening was specified using the equation of relation of B/A and the value thereof was narrowed into the range of 0.1 to 0.5. The inventor succeeded in specifying the uniform slit light with a good lighting property and with high luminance in the gas discharge tube, by limiting the focusing opening by the various parameters as described above, which will provide an aid for use of the emission.  
       [0010] The focusing opening is preferably formed so that B/A is in a range of 0.1 to 0.25 and the opening area is in a range of 0.15 to 0.25 mm 2 . This permits the discharge tube to supply light with uniform luminance distribution and with extremely high luminance, thereby enhancing the intensity of the spot emission so as to meet market needs.  
       [0011] The present invention will become more fully understood from the detailed description and the accompanying drawings which follow. These are to be considered in all respects as illustrative and not restrictive to the present invention.  
       [0012] The scope of further application of the present invention will become apparent from the detailed description of the invention which follows. However, the detailed description and specific examples are presented only for the purpose of illustration while demonstrating preferred embodiments of the present invention, and it is clear that various modifications and improvements within the spirit and scope of the invention are obvious to those skilled in the art from the detailed description. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013]FIG. 1 is a perspective view to show an embodiment of the gas discharge tube according to the present invention.  
     [0014]FIG. 2 is an exploded perspective view of a light-emitting section in the gas discharge tube of FIG.  
     [0015]FIG. 3 is a perspective view to show a state before assembly of a support plate and an anode plate in the light-emitting section of FIG. 2.  
     [0016]FIG. 4 is a perspective view to show a state before assembly of a discharge shielding member and the anode plate in the light-emitting section of FIG. 2.  
     [0017]FIG. 5 is a plan view to show the positional relation among the discharge shielding member, the anode plate, and the support member in the light-emitting section of FIG. 2.  
     [0018]FIG. 6 is a cross-sectional view along a line VI-VI of FIG. 5.  
     [0019]FIG. 7 is a cross-sectional view along a line VII-VII of FIG. 5.  
     [0020]FIG. 8 is a perspective view to show an example of an opening limiter applied to the gas discharge tube of the present invention.  
     [0021]FIG. 9 is a cross-sectional view along a line IX-IX of FIG. 8.  
     [0022]FIG. 10 is a schematic diagram to show a focusing opening.  
     [0023]FIG. 11 is a graph to show the relationship between opening area and aspect ratio of aperture in the focusing opening.  
     [0024]FIG. 12 a schematic diagram to show another example of the focusing opening. 
    
    
     DESCTIPRION OF THE PREFERRED EMBODIMENTS  
     [0025] The preferred embodiments of the gas discharge tube according to the present invention will be described hereinafter in detail with reference to the accompanying drawings.  
     [0026]FIG. 1 shows a side-on type deuterium lamp as an example of the gas discharge tube. In this gas discharge tube  10 , a light-emitting section  20  is housed inside an envelope  11  of glass and deuterium gas (not illustrated) is confined under the pressure of about several Torr. The envelope  11  is formed in a cylindrical shape with its head portion being sealed and the bottom portion of the envelope  11  is hermetically sealed by a glass stem  12 . The envelope  11  is made of ultraviolet-transmitting glass or silica glass having a high UV transmittance.  
     [0027] Four lead pins  13  to  16  juxtaposed on a straight line extend from the bottom portion of the light-emitting section  20  and penetrate the stem  12 . These lead pins  13  to  16  are covered by insulating members  130 ,  140 ,  150 ,  160 , respectively, and connected to a predetermined lighting circuit. The light-emitting section  20  is constructed in a shielding box structure in which a ceramic support member  22  and a metal front window electrode  23  are bonded to each other with a discharge shielding member (spacer)  21  in between.  
     [0028] The structure of the light-emitting section  20  will be described below in detail with reference to FIGS.  2  to  7 .  
     [0029] As shown in FIG. 2 and FIG. 3, the support member  22  of a prism of a  -shaped cross section is provided with a vertical through hole  220 , concave grooves  221  to  223 , a depression  224 , four projections  225 , and four horizontal through holes  226 . The vertical through hole  220  vertically extends through a projected portion  22 A in the rear part of the support member  22  of the  -shaped section. The concave groove  221 , depression  224 , and concave grooves  222 ,  223  are depressed from the surface of a front flat portion  22 B and extend in succession toward the bottom portion of the envelope  11 . This allows the lead pin  14  and insulating member  141  to be appropriately accommodated. The four projections  225  project from the surface of the flat portion  22 B two each in the vicinity of opening edges of the concave grooves  221 ,  222  so as to be opposed to the respective corners of an anode plate  24 . The four horizontal through holes  226  horizontally extend to penetrate the support at two positions each in the upper end portion and in the lower end portion.  
     [0030] This support member  22  is held by the stem  12  through the lead pin  13  penetrating the vertical through hole  220  and through the lead pin  14  fitted in the concave grooves  221  to  223 . The anode plate  24  formed in a rectangular flat plate shape is welded and fixed to the distal end of the lead pin  14  and supported from the back by the four projections  225 . A heat-radiating space is ensured behind the anode  24  by the depression  224  having an aperture substantially equivalent to the surface area of the anode  24 .  
     [0031] As shown in FIG. 2 and FIG. 4, the discharge shielding member  21  formed in a flat plate shape is of a  -shaped cross section thinner and wider than the support member  22  and is provided with a through hole  210 , a recess  211 , a vertical through hole  212 , four horizontal through holes  213 , two horizontal through holes  214 , and four horizontal through holes  215 . The through hole  210  penetrates almost the center of the discharge shielding member  21  so as to be opposed to the anode  24 . The recess  211  is depressed from the surface of a flat portion  21 A in the back of the discharge shielding member  21  in order to accommodate the anode  24 , and includes a first opening edge of the through hole  210  located on the back side. The vertical through hole  212  penetrates a projected portion  21 B on the front side. The four horizontal through holes  213  horizontally extend to be opposed to the four horizontal through holes  226  of the support member  22 . The two horizontal through holes  214  of the discharge shielding member  21  are formed at positions to accept lock pawls  271  of a cathode slit electrode  27  described hereinafter, and the four horizontal through holes  215  at positions to accept lock pawls  231  of the front window electrode  23  described hereinafter.  
     [0032] A bent portion of a substantially L-shaped electrode rod  216  is fitted in the vertical through hole  212  and the lower end thereof is exposed from the discharge shielding member  21 . The lower end of the electrode rod  216  is welded and fixed to the distal end of the lead pin  15 . Thus the discharge shielding member  21  is held by the stem  12  through the electrode rod  216 . Electrode rods  250 ,  251  are welded to the two ends of a hot cathode (filament)  25 , respectively. Then the distal end of the electrode rod  250  is welded to the electrode rod  216 , and the distal end of the electrode rod  251  to the distal end of the lead pin  16 . In this structure, the hot cathode  25  is held by the stem  12 .  
     [0033] As shown in FIGS.  5  to  7 , the rectangular anode  24  indicated by the dashed line is received in the recess  211  of the discharge shielding member  21  and the corner portions of the anode  24  are sandwiched by cooperation of the bottom surface of the recess  211  of the discharge shielding member  21  and the four projections  225  of the support member  22 . Most of the four sides of the anode  24  match the through hole  210  of the substantially rectangular shape slightly rounded, and the other portions of the first opening edge are joined to the four corner portions of the anode  24 . The four projections  225  with the circular surface are joined to the four corner portions of the anode  24  so as to press the anode  24 . In particular, as illustrated in FIG. 7, the rectangular recess  211  has a depth equal to the sum of the height of the four projections  225  and the thickness of the anode  24 , so that the peripheral region in the front surface of the support member  22  can abut on the back surface of the discharge shielding member  21 .  
     [0034] As shown in FIG. 2 and FIG. 6, a focusing electrode  26  is formed by bending a metal plate into a substantially L-shape, and is provided with an opening  260  and four horizontal through holes  263 . This opening  260  is arranged coaxial with the through hole  210  of the discharge shielding member  21 . An opening limiter  261  for limiting the opening diameter is welded to the peripheral area of the opening  260 . The opening limiter  261  is provided with an arc ball receiving recess  262  projecting toward the anode  24  so as to pass the opening  260  and a focusing opening  40  of a slit shape is formed in the center of the recess  262 . The four horizontal through holes  263  are formed through the thickness of the focusing electrode  26  so as to be opposed to the four horizontal through holes  213  of the discharge shielding member  21 .  
     [0035] This focusing electrode  26  is set in contact on the projected part  21 B of the discharge shielding member  21  and a distal end  26 A bent backward is welded to the distal end of the lead pin  13  projecting from the support member  22 . In this way the focusing electrode  26  is fixed to the discharge shielding member  21  and to the support member  22 . It is noted here that the distance between the opening limiter  261  and the anode  24  is smaller than the thickness of the discharge shielding member  21 . Here the horizontal through holes  226 ,  213 ,  263  of the discharge shielding member  21 , the support member  22 , and the focusing electrode  26  are aligned in line in each set. Therefore, these members can be fixed together to the stem  12  by inserting four metal rivets  28  into the through holes in a bonded state of the discharge shielding member  21 , the support member  22 , and the focusing electrode  26 .  
     [0036] As illustrated in FIGS. 2, 6, and  7 , the metal cathode slit electrode  27  is bent corresponding to the shape of the stepped region of the discharge shielding member  21  and has an opening  270  and two lock pawls  271 . The opening  270  formed in a vertically long rectangular shape is made in the front part of the cathode slit electrode  27 . The two lock pawls  271  formed at the upper and lower ends of the cathode slit electrode  27  are bent backward.  
     [0037] This cathode slit electrode  27  faces the hot cathode  25  and is placed on the front surface on one side of the discharge shielding member  21 . The cathode slit electrode  27  is fixed to the discharge shielding member  21  by inserting the two lock pawls  271  into the two horizontal through holes  214  of the discharge shielding member  21 . The opening  270  is located between the hot cathode  25  and the opening limiter  261 .  
     [0038] The front window electrode  23  of metal is formed in a substantially U-shaped cross section bent at four positions and is also provided with an opening window  230  and four lock pawls  231 . The opening window  230  formed in a rectangular shape is arranged coaxial with the arc ball receiving recess  262  of the focusing electrode  26 . The four lock pawls  231  formed in the upper and lower parts on the both side ends of the front window electrode  23  project backward. The opening window  230  is located at the position to project ultraviolet light from the space in front of the arc ball receiving recess  262 .  
     [0039] This front window electrode  23  is fixed to the discharge shielding member  21  by inserting the four lock pawls  231  into the four horizontal through holes  215  of the discharge shielding member  21 . Then the front end of the cathode slit electrode  27  is brought into contact with the internal surface of the front window electrode  23 , whereby the space for placement of the hot cathode  25  can be separated from the emission space for occurrence of arc discharge.  
     [0040] With the focusing electrode  26 , cathode slit electrode  27 , and front window electrode  23  constructed in this structure, the focusing electrode  26  is electrically insulated through the discharge shielding member  21  from the cathode slit electrode  27  and the front window electrode  23 . On the other hand, the cathode slit electrode  27  and the front window electrode  23  are in contact with each other and set at a common potential.  
     [0041] The operation of the gas discharge tube  10  described above will be briefly described below.  
     [0042] First, an unrepresented trigger switch is set in an off state and a luminance control switch is set in an on state with respect to a discharge starting circuit. This results in applying the voltage of about 2.5 V from a cathode-heating voltage supply to the hot cathode  25  for about 20 seconds before discharge, to preheat the hot cathode  25 . After the hot cathode  25  is heated well up to the temperature of about 1100° C., the voltage of about 150 V is applied from a field-generating voltage supply to between the hot cathode  25  and the anode  24 , thereby generating an electric field directed from the anode  24  to the hot cathode  25 .  
     [0043] After completion of preparation for trigger discharge in this manner, the trigger switch is turned on to bring the focusing electrode  26  into the potential of about 150 V, thereby inducing trigger discharge between the hot cathode  25  and the focusing electrode  26 .  
     [0044] Since the focusing electrode  26  is electrically insulated from the cathode slit electrode  27  and the front window electrode  23 , the focusing electrode  26  can be set at a positive potential higher than the cathode slit electrode  27  and the front window electrode  23  set at the potential of approximately 0 V. For this reason, as illustrated in FIG. 6, a trigger discharge area  30  is generated so as to extend from the hot cathode  25 , and thus the trigger discharge area  30  extends from the space surrounded by the front window electrode  23  and the cathode slit electrode  27 , i.e., from the inside of the cathode box up to the focusing electrode  26 . The trigger discharge is induced between the hot cathode  25  and the opening limiter  261  in this way, so that an oblate arc ball Y is generated in the arc ball receiving recess  262 . UV light from this arc ball (i.e., positive column light) Y travels through the opening window  230  of the front window electrode  23  to emerge in the form of slit light.  
     [0045] As illustrated in FIG. 8 and FIG. 9, the opening limiter  261  disposed on the focusing electrode  26  has a rectangular flat substrate  42  of molybdenum, which is a refractory metal. The arc ball receiving recess  262  of a cup shape, which is made by press work of the substrate  42 , is provided in the center of the opening limiter  261 . The aperture diameter D of the arc ball receiving recess  262  is approximately 4 mm and a flat portion  41  is formed in the bottom portion of the arc ball receiving recess  262 . A focusing opening  40  of a slit shape is formed in the center of the flat portion  41 . In the opening limiter  261  used in the present embodiment, the size of the substrate  42  is 8×8 mm and the thickness thereof is in the range of approximately 0.3 to 0.7 mm. However, the material of the substrate can also be another refractory metal such as tungsten or the like.  
     [0046] For the commonly known focusing openings  40 , the limit was normally a circular hole having the diameter of 0.5 mm because of the increase of discharge start voltage or the occurrence of abnormal discharge. This is because decrease in the diameter of the focusing opening  40  to below 0.5 mm will increase the barrier between the hot cathode  25  and the anode  24  and raise the need for high energy upon a start of discharge. With increase in this energy (for example, with increase in the discharge voltage), there will occur an event of failure in lighting of the gas discharge tube  10  due to abnormal discharge.  
     [0047] In order to ensure a stable discharge start, the inventor thus noted the area S of the focusing opening  40 . The inventor verified by experiments that increase in the area S of the focusing opening  40  surely made it easier to induce arc discharge between the hot cathode  25  and the anode  24  but the luminance of emission decreased so as to become dim as a whole. Thus the opening area S was narrowed into the range of 0.15 to 0.5 mm 2  in order to ensure high luminance while enabling lighting of the gas discharge tube  10  by the rectangular focusing opening  40 . It was verified by experiments that steady lighting of the gas discharge tube  10  was difficult when the opening area S was less than 0.15 mm 2  and that the light became too spread and it was difficult to utilize the light as a spotlike beam when the opening area S exceeded 0.5 mm 2 .  
     [0048] Further, in order to obtain a uniform slit emission with high luminance and with a clear outline, while taking the aforementioned opening area S into consideration, the inventor noted the relation between the opening length A in the longitudinal direction of the focusing opening  40  and the opening length B thereof in the direction perpendicular to the longitudinal direction, as shown in FIG. 10. Then the inventor attempted to specify the shape of the focusing opening  40  by use of an equation defining the relation of B/A (aspect ratio). As a result, the inventor confirmed by experiments that with the emission made by the rectangular focusing opening  40 , the light could be utilized as a spotlike beam when the value of B/A was set in the range of 0.1 to 0.5, in order to ensure uniform light with high luminance and with a clear outline.  
     [0049] In particular, it is preferable that the value of B/A be in the range of 0.1 to 0.25 and that the opening area S be in the range of 0.15 to 0.25 mm 2 . In this case, the light can be obtained with uniform luminance distribution and with extremely high luminance, thereby succeeding in providing the strong spot of slit emission so as to meet the market needs. These relations are presented in FIG. 11.  
     [0050] An example satisfying the relations is the slit focusing opening  40  having the opening length B of 0.15 mm and the opening length A of 1 mm. With this example, the light output was actually measured with a spectrophotometer and it was proved that the gas discharge tube of the example provided the output approximately three times higher than the conventional tube with the circular hole of the diameter of 0.5 mm. This is extremely slender and strong light output, which was unable to be obtained before. In order to make slender slit light, where the value of B is not more than 0.5 mm, other examples of the focusing opening  40  are, for example, a shape in which A is 1.0 mm and B 0.2 mm, a shape in which A is 1.25 mm and B 0.2 mm, and so on.  
     [0051] The slit light emitted from such a gas discharge tube  10  is used as a light source in spectrophotometers, and analyzers for liquid chromatography, capillary electrophoresis, and so on. In recent years, spotlike light is used as a light source for analysis in order to prevent influence from optics, stray light, etc., but there is the desire for stronger and smaller slitlike spot light with decrease in the size of cells of analyzers. Then the gas discharge tube  10  described in the aforementioned embodiment fully satisfies this desire.  
     [0052] The present invention is not limited to the aforementioned embodiment, but the invention also permits application of the graph illustrated in FIG. 11, for example, to a configuration as illustrated in FIG. 12 wherein the opening limiter  261  is constructed with the focusing opening  60  of an elongated elliptic shape.  
     [0053] The embodiment was described as a side-on type deuterium lamp, but the present invention can also be applied to head-on type deuterium lamps, for example, like the one as described in FIG. 9 and FIG. 10 of U.S. Pat. No. 5,587,625.  
     [0054] The gas discharge tube of the present invention is able to provide the emission with a good lighting property, with high luminance, and in uniform slit shape. Accordingly, the invention is suitably applicable to the light sources of the spectrophotometers, and the analyzers for liquid chromatography, capillary electrophoresis, and so on.  
     [0055] It is apparent from the above description of the invention that the present invention embraces various modifications. Such modifications should be regarded as not departing from the spirit and scope of the invention, and it is understood that all improvements obvious to those skilled in the art should be included in the scope of claims below.