Directly heated dispenser cathode with improved helix element and rare gas ion laser generating system using the same

Abstract: A directly heated dispenser cathode (13) is installed in a rare gas ion laser generating system, and faces with an anode (12e). The directly heated dispenser cathode comprises a plurality of helical elements (13a) merged into one another, and a flange (13f) is formed in the outermost helical element facing the anode so that ion bombardment causes less damage to the rear helical elements behind the flange, thereby prolonging the service time period.

Description: FIELD OF THE INVENTION This invention relates to a directly heated dispenser cathode and, more particularly, to the structure of a helical shaped directly heated dispenser cathode used in a rare gas ion laser generating system. DESCRIPTION OF THE RELATED ART The directly heated dispenser cathode allows a large amount of current to flow therethrough rather than an oxide coated cathode, and provides prolonged service time period. Other attractive features of the directly heated dispenser cathode are resistivities against ion-impact and arc discharge. For this reason, the directly heated dispenser cathode is used in high-power laser tubes. Fig. 1 shows a typical example of the directly heated dispenser cathode used in the prior art high-power laser tube (see e.g. US-A-5 050 184). The prior art directly heated dispenser cathode is shaped into a helix, and the helix consists of a plurality of helical elements 1 merged into one another. The outermost helical elements are welded or brazed to lead wires 2a and 2b, and the welded spots are represented by dots 3a and 3b in Fig. 1. The cross section 4 of each helical element 1 is generally rectangular as shown in Fig. 2, and is uniform over the helix. The helix is formed of sintered tungsten, and the sintered tungsten is impregnated with electron emitting substance such as mixture of barium oxide (BaO), calcium oxide (CaO) and aluminum oxide (Al₂O₃). When current flows through the helix, the electron emitting substance is heated, and electrons are radiated therefrom. The prior art directly heated dispenser cathode is fabricated through the following process sequence. The process sequence starts with preparation of tungsten powder, and the tungsten powder is pressed into a thick straight rod. The rod-shaped tungsten powder is sintered under appropriate conditions, and a porous sintered body is produced through the sintering process. The sintered body is, thereafter, infiltrated with inert plastic filler, and the inert plastic filler enhances the machinability of the sintered body. Then, the rod-shaped sintered body thus infiltrated with the inert plastic filler is machined into the helix, and the inert plastic filler is removed in vacuum or air. The lead wires 2a and 2b are welded at the spots 3a and 3b to the outermost helical elements, respectively, and the sintered body is covered with powder of the electron emitting substance. The powder of the electron emitting substance is heated over a certain temperature in vacuum or hydrogen atmosphere, and the powder of the electron emitting substance is melted so as to impregnate into the sintered body. The prior art directly heated dispenser cathode thus fabricated is installed in an argon gas ion laser generating system as shown in Fig. 3. The argon gas ion laser generating system comprises a cathode tube 5, and a laser capillary 6 projecting from the cathode tube 5. A cathode chamber 5a is defined in the cathode tube 5, and the prior art directly heated dispenser cathode is accommodated in the cathode chamber 5a. A partition member 6a is inserted into the laser capillary 6, and separates an anode chamber 6b from the cathode chamber 5a. A through-hole 6c and gas return paths 6d are formed in the partition member 6a, and an anode 6e is provided in the anode chamber 6b. The argon gas ion laser generating system thus constructed behaves as follows. First, the cathode and anode chambers 5a and 6b are evacuated, and vacuum is developed in those chambers. When current is supplied to the prior art directly heated dispenser cathode, the helical elements 1 are heated to a certain temperature, and argon gas is introduced into the chambers 5a and 6b. Arc discharge takes place, and argon gas laser is produced in the prior art argon gas ion laser generating system. However, a problem is encountered in the prior art directly heated dispenser cathode in relatively short service time period. This is because of the fact that argon ion damages the surface of the porous sintered body, and the electrons are hardly emitted due to melted sintered substance covering the porous surfaces of the helical elements. The service time period of the directly heated dispenser cathode is inversely proportional to the discharge current. In fact, while the current is about 6 amperes, the service time period is of the order of ten thousand hours. However, if the current is increased to 10 amperes, the prior art directly heated dispenser cathode serves over only five hundred hours. SUMMARY OF THE INVENTION It is therefore an important object of the present invention to provide a directly heated dispenser cathode which serves over prolonged time period. The present inventor discovers that only one or two helical elements on the anode side are rapidly damaged, and the damage proceeds to the rear helical elements. In accordance with one aspect of the present invention, there is provided a directly heated dispenser cathode comprising: a) a helix member formed of porous sintered substance impregnated with electron emitting substance, and having a plurality of helical elements merged into one another and broken down into a first group faced to an anode and a second group on the opposite side to the anode, a flange projecting from the outermost helical element of the first group in such a manner as to face to the anode; b) a first lead wire coupled with the outermost helical element of the first group; and c) a second lead wire coupled with the outermost helical element of the second group. In accordance with another aspect of the present invention, there is provided a rare gas ion laser generating system comprising: a) a first tube member having an anode chamber; b) an anode provided in the anode chamber; c) a second tube member having a cathode chamber connected with the anode chamber; and d) a directly heated dispenser cathode provided in the cathode chamber, and comprising d-1) a plurality of helical elements formed of porous sintered substance impregnated with electron emitting substance, and having the plurality of helical elements merged into one another and broken down into a first group faced to the anode and a second group on the opposite side to the anode, a flange projecting from the outermost helical element in such a manner as to face to the anode, d-2) a first lead wire coupled with the outermost helical element of the first group, and d-3) a second lead wire coupled with the outermost helical element of the second group. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the directly heated dispenser cathode according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: Fig. 1 is a side view showing the structure of the prior art directly heated dispenser cathode; Fig. 2 is a cross sectional view showing the helical component of the prior art directly heated dispenser cathode; Fig. 3 is a cross sectional view showing the structure of the prior art rare gas ion laser generating system; Fig. 4 is a cross sectional view showing the structure of a rare gas ion laser generating system according to the present invention; Fig. 5 is a side view showing the structure of a directly heated dispenser cathode according to the present invention; Fig. 6 is a cross sectional view showing the structure of a helix element of the directly heated dispenser cathode; and Fig. 7 is a side view showing the structure of another directly heated dispenser cathode according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment Referring to Fig. 4 of the drawings, a rare gas ion laser generating system embodying the present invention comprises a cathode tube 11, and a laser capillary 12 projecting from the cathode tube 11. A cathode chamber 11a is defined in the cathode tube 11, and a directly heated dispenser cathode 13 is accommodated in the cathode chamber 11a. A partition member 12a is inserted into the laser capillary 12, and separates an anode chamber 12b from the cathode chamber 11a. A through-hole 12c and gas return paths 12d are formed in the partition member 12a, and an anode 12e is provided in the anode chamber 12b. Turning to Fig. 5, the directly heated dispenser cathode 13 comprises a helix member 13a or helical elements 131 to 137 merged into one another, and lead wires 13b and 13c of molybdenum respectively arc-welded to the helical elements 131 and 137 at welded spots 13d and 13e. However, the lead wires may be brazed to the helix member 13a. The helix member 13a is formed of porous sintered tungsten, and electron emitting substance such as mixture of BaO, CaO and Al₂O₃ is impregnated into the porous sintered tungsten. A flange 13f projects from both sides of a part of the helix member 13a as shown in Fig. 6, and extends over the entire helical element 131 and a half of the next helical element 132. However, the remaining helical elements do not have any flange, and have a rectangular cross section. In this instance, the helical elements 131 and 132 belong to a first group, and the other helical elements 133 to 137 form in combination a second group. The directly heated dispenser cathode 13 is fabricated through the following process sequence. The process sequence starts with preparation of tungsten powder, and the tungsten powder is pressed into a thick straight rod. The rod-shaped tungsten powder is sintered under appropriate conditions, and a porous sintered body is produced through the sintering process. The porous sintered body is, thereafter, infiltrated with inert plastic filler, and the inert plastic filler enhances the machinability of the sintered body. Then, the rod-shaped porous sintered body thus infiltrated with the inert plastic filler is machined into a generally helix configuration. Namely, the helical elements 131 and 132 are shaped into a rectangular cross section with the flange 13f, and the other helical elements 133 to 37 are shaped into the rectangular cross section. After the shaping, the inert plastic filler is removed in vacuum or air. The lead wires 13b and 13c are welded at the spots 3a and 3b to the outermost helical elements 131 and 137, respectively, and the porous sintered body is covered with powder of the electron emitting substance. The powder of the electron emitting substance is heated over a certain temperature in vacuum or hydrogen atmosphere, and the powder of the electron emitting substance is melted so as to impregnate into the sintered body. The rare gas ion laser generating system thus constructed behaves as follows. First, the cathode and anode chambers 11a and 12b are evacuated, and vacuum is developed in those chambers. When current is supplied to the directly heated dispenser cathode 13, the helix 13a is heated to a certain temperature, and rare gas such as, for example, argon is introduced into the chambers 11a and 12b. Arc discharge takes place, and argon gas laser is produced in the rare gas ion laser generating system. Since the flange 13f projecting from the helical elements 131 and 132 is effective against the ion bombardment, the helical elements 133 to 137 are less liable to be damaged. In fact, when the flange 13f is provided to 1.5 turns of the helical elements of the directly heated dispenser cathode with the outer diameter of 12.7 millimeters, the service time period is twice prolonged at discharge current of 10 amperes with respect to the prior art shown in Fig. 3, and the flange 13f is surely effective against the ion bombardment. Second Embodiment Turning to Fig. 7 of the drawings, another directly heated dispenser cathode 23 embodying the present invention is illustrated. The directly heated dispenser cathode is similar to the first embodiment except for a flange 23f projecting from the first and second helical elements 131 and 132 on the anode side. For this reason, components of the directly heated dispenser cathode 23 are labeled with the same references as those designating the corresponding elements of the first embodiment without detailed description. The flange 23f is formed of tantalum, and is shaped into a ring. The ring-shaped flange 23f is laser welded to the entire periphery of the helical elements 131 and 132, and the other helical elements 133 to 137 are protected against ion bombardment by virtue of the protective flange 23f projecting from the first and second turns of the helix member 13a. As will be understood from the foregoing description, the flange effectively protects the helical elements from the ion bombardment, and the directly heated dispenser cathode according to the present invention is improved in service time period. Although particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made. For example, the directly heated dispenser cathodes according to the present invention are available for krypton ion laser generating systems. The flange may be brazed to the helical elements, and some helical elements may be directly machined so as to form flanges.

Claims: A directly heated dispenser cathode (13; 23) comprising: a) a helix member (13a) formed of porous sintered substance impregnated with electron emitting substance, and having a plurality of helical elements (131 to 137) merged into one another and broken down into a first group (131/132) faced to an anode (12e) and a second group (133 to 137) on the opposite side to said anode; b) a first lead wire (13b) coupled with the outermost helical element (131) of said first group; and c) a second lead wire (13c) coupled with the outermost helical element (137) of said second group, characterized in that a flange (13f; 23f) projects from said outermost helical element (131) of said first group in such a manner as to face to said anode. A directly heated dispenser cathode as set forth in claim 1, in which said flange extends to the helical element (132) of said first group next to the outermost helical element. A directly heated dispenser cathode as set forth in claim 2, in which said flange (23f) is formed from a ring-shaped tantalum member, and is welded to said outermost helical element (131) and to said helical element (132) next to said outermost helical element. A directly heated dispenser cathode as set forth in claim 1, in which said directly heated dispenser cathode forms a rare gas ion laser generating system together with a) a first tube member (12) having an anode chamber (12b), b) an anode (12e) provided in said anode chamber, and c) a second tube member (11) having a cathode chamber (11a) connected with said anode chamber, and accommodating said directly heated dispenser cathode (13). A rare gas ion laser generating system comprising: a) a first tube member (12) having an anode chamber (12a); b) an anode (12e) provided in the anode chamber; c) a second tube member (11) having a cathode chamber connected with the anode chamber; and d) a directly heated dispenser cathode (13) provided in the cathode chamber, and comprising d-1) a plurality of helical elements (131 - 137) formed of porous sintered substance impregnated with electron emitting substance, and having the plurality of helical elements merged into one another and broken down into a first group (131 - 132) faced to the anode and a second group (133 - 137) on the opposite side to the anode, d-2) a first lead wire (13b) coupled with the outermost helical element of the first group, and d-3) a second lead wire (13c) coupled with the outermost helical element of the second group, characterized by a flange (13f) projecting from the outermost helical element in such a manner as to face to the anode.

Inventors: SEURA RYOICHI C O NEC CORPORAT, SEURA, RYOICHI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H01S3
IPC codes: H01S3, H01J1