Gas laser apparatus

Abstract: In a gas laser, the surface of either or both of discharge electrodes (2, 3) are formed from titanium oxide which provides fine sputtering proof. The discharge electrodes (2, 3) are formed from titanium as a base material having an oxidized surface. Alternatively, the discharge electrodes (2, 3) are formed from molybdenum as a base material with titanium coated and the surfaces are oxidized. In the gas laser of the direct current discharge type, the cathode (3) is formed as described above. Both of anode (2) and cathode (3) are formed as described above in the gas laser of the alternate current discharge type.

Description: The present invention relates to a gas laser apparatus according to the preamble of claim 1. Such a gas laser apparatus is disclosed in EP-A-0 235 788. As the gas laser of the above-mentioned type, a CO₂ laser has been used for material processing. The CO₂ laser generally comprises a discharge section filled with a gas mixture as a laser medium, a plurality of anode bars provided in the discharge section so as to be arranged in the shape of a reed screen, and a plurality of generally L-shaped cathode bars disposed in opposite relation to the anode bars. A high voltage DC power supply is connected between the discharge electrodes through ballast resistances. DC voltage is applied across the discharge electrodes, causing a glow discharge to take place between the electrodes and thereby exciting the gas mixture. The gas mixture is recirculated through a heat exchanger from the cathode side to the anode side for the purpose of preventing the temperature thereof from being increased. Molybdenum has been conventionally employed as a material of both of the discharge electrodes because of its high melting point. In the laser apparatus introduced to a production line of a works for the purpose of the material processing not the studying, it is desirable that the maintenance cycle of the laser apparatus be as long as possible since the maintenance cycle affects productivity of the production line. While, in the conventional CO₂ laser, the discharge electrodes are deteriorated and the electric discharge in the discharge section of the laser is altered from the glow to arc discharge in case that the maintenance of the electrodes is not executed. Under these circumstances, the maintenance of the discharge electrodes is reiteratively executed in a relatively short cycle. For example, the conventional transverse flow 5-kW gas laser is continuously used for about 200 hours at the maximum. The inventors of the present invention, made experiments to investigate the mechanism of deteriorating the discharge electrodes. From the experiments, the inventors have found that a physical sputtering action is applied to the cathode formed from molybdenum owing to collision of positive ions during the glow discharge. Further, since molybdenum has a sublimation temperature as low as below 700°C, molybdenum is also likely to suffer from the chemical sputtering action. Consequently, the discharge electrodes are deteriorated owing to both physical and chemical sputtering actions. Inspection of the molybdenum cathode after a long time of use shows that net-shaped projections are formed along grain boundaries on the surface of the molybdenum cathode, which makes it clear that the sputtering acts on the cathode ununiformly. Further, a thin oxidized film is formed on the surface of the cathode. It is considered that the thin oxidized film results from an extremely small amount of oxygen contained in the gas mixture in which He, N₂ and CO₂ are in the ratios of 50:45:5. It is further considered that the oxygen results from leakage through sealing members of an airtight vessel of the gas laser apparatus or decomposition of CO₂ contained in the gas mixture. On the other hand, inspection of th molybdenum anode after a long time of use shows that a piled or stacked substance about 1 or 2 µm thick is found on the side thereof confronting the cathode and that black projections, the outer diameter of each of which is about 100 µm long are found on the surface thereof. Since the piled substance is amorphous MoO₃ and partial to the side of the anode confronting the cathode, it is consider that the above-described sputtering in the cathode causes cathode material particles to scatter and move to the anode with a flow of the gas mixture, resulting in the substance piled up on the anode. Analysis shows that projections formed on the piled substance is composed of carbon. Consequently, it is considered that micro-arcs are generated from the projections. From the above-described results of analysis, mechanism leading from the deterioration of the discharge electrodes to the generation of micro-arcs is considered as follows: the molybdenum cathode suffers from the sputtering action owing to the glow discharge in the discharge section of the gas laser, resulting in the substance ununiformly piled up on the anode. Since the piled substance is a molybdenum oxide, it has an insulating property. Concentration of the electric field takes place on some portions of the anode surface. While, the net-shaped projections are formed on the surface of the cathode owing to the ununiform sputtering action, which projections also cause concentration of the electric field. The micro-arcs are generated as the result of the electric field concentration and the electric field tends to be concentrated on the micro-arc generating points as the number of the micro-arc generating points is increased, whereby ununiformity of the discharge input progresses. The discharge input locally reaches the arc limit or above and at last, the glow discharge is altered to the arc discharge throughout the discharge section. The above-described condition is applied not only to the gas lasers of the DC discharge type but to those of the AC discharge type, wherein both of the discharge electrodes alternately suffer from the sputtering to thereby be worn and deteriorated. So far as described, the conventional gas laser has a problem that the oxide produced owing to the sputtering at one discharge electrode side is piled up on the surface of the other discharge electrode and that deterioration of the electrode lowers the arc limit, thereby requiring the maintenance in a short period. Therefore, an object of the present invention is to provide a gas laser apparatus wherein the discharge electrodes can be prevented from being deteriorated such that the maintenance cycle thereof can be improved. US-A-4 122 411 discloses a variety of materials for use in a sputter-resistant cathode, including Ti and Mo as a base material, preferably coated on the working surface with an oxide, e.g. of Ti. Gas laser apparatus of the present invention are specified in claims 1 to 4. According to an embodiment the surfaces of both of discharge electrodes are formed from titanium oxide instead of the conventional electrodes formed from molybdenum. Since titanium oxide is an exceedingly stable material, it has a small sputtering rate and is not likely to suffer from the physical sputtering. Furthermore, since the vapor pressure is relatively low when the atmospheric temperature is high, titanium oxide is not likely to suffer from the chemical sputtering. The vapor pressure takes the value of 10⁻⁶torr when the temperature of TiO₂ is approximately 1530°C, while the temperature of MoO₃ is increased only to 490 °C under the same vapor pressure condition. Consequently, the piled substance resulting from the sputtering at one discharge electrode may be prevented from occurring on the other discharge electrode, thereby preventing the electric field concentration. Furthermore, the state of the surface of titanium is stable, which further prevents the electric field concentration and therefore, occurrence of micro-arcs. Thus, since the discharge electrodes may be prevented from being deteriorated, a stable discharge may be ensured for a long period. The discharge electrodes having the surfaces of titanium oxide may be obtained in the following methods. In a first method, the discharge electrodes are formed from titanium as a base material. Prior to incorporation of the discharge electrodes into the gas laser apparatus, the electrodes are baked in the atmosphere so that the surfaces thereof are oxidized. In a second method, the discharge electrodes are formed from titanium as a base material and incorporated in the gas laser apparatus. When the electric discharge is initiated between the discharge electrodes in the usual manner, heat owing to the discharge is applied to the base material of titanium and an extremely small amount of oxygen contained in the gas mixture as the laser medium causes the base material to be oxidized, thereby obtaining the surface of titanium oxide. In a third method, the discharge electrodes are formed from molybdenum as a base material. Titanium is then affixed to the surfaces of the electrodes by way of sputtering, ion plating or laser cladding. Then, the electrodes are previously baked or incorporated in the gas laser apparatus so that the electric discharge is initiated, whereby the surfaces of the electrodes are oxidized. In the case of the gas laser apparatus of the DC discharge type, either one of a pair of discharge electrodes may have the surface of titanium oxide. However, in the case of the gas laser apparatus of the AC discharge type, both of the discharge electrodes need to have the titanium oxide surfaces, respectively. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a partially cross sectional view of a cathode employed in the gas laser apparatus of a first embodiment to which the invention as claimed, can be applied; FIG. 2 is a perspective view of the cathode; FIG. 3 is a cross sectional view of the discharge section of the gas laser apparatus; FIG. 4 is a graph representing the result of elemental analysis in the direction of the depth of the cathode before the discharge is initiated; FIG. 5 is a graph representing the result of elemental analysis in the direction of the depth of the cathode at the time when 200 hours elapses from the initiation of discharge; FIG. 6 is a graph representing the relationship between practical discharge input and discharge period; FIG. 7 is a perspective view of a discharge electrode employed, in accordance with the invention as claimed, in the gas laser apparatus of a second embodiment; FIG. 8 is a partially cross sectional view of a cathode employed in the pulsed laser apparatus of a third embodiment; and FIG. 9 is a cross sectional view of the discharge section of the pulsed laser apparatus. A first embodiment wherein the gas laser apparatus is a CO₂ laser for material processing will now be described with reference to FIGS. 1 to 4 of the accompanying drawings. Referring first to FIG. 3, a gas mixture as a laser medium is caused to flow from the left-hand side to the right-hand side of an inner cavity of a discharge section 1, as viewed in FIG. 3. The gas mixture is cooled by a heat exchanger (not shown) and recirculated through the cavity of the discharge section 1. He, N₂ and CO₂ composing the gas mixture are in the ratios of 50:45:5. It is to be considered that an extremely small amount of oxygen or the like is contained in the gas mixture. The pressure of the gas mixture is about 4x10³ Pascal (30 torr) and the flow rate thereof in the discharge section is about 70 m/sec. in the center of the cavity. Each anode bar 2 of discharge electrodes comprises a slender bar electrode extending vertically. A plurality of such anode bars 2 are aligned across the cavity of the discharge section 1. Each cathode bar 3 of the discharge electrodes comprises a generally L-shaped slender bar electrode. A plurality of such cathode bars 3 are disposed so that one ends thereof are directed to the anode bars 2. A DC power supply 5 is connected between the electrodes 2 and 3 through ballast resistances 4 each having the resistance of 20 KΩ so that high DC voltage is applied across the electrodes 2 and 3. When the DC voltage is applied across the electrodes 2 and 3 with the gas mixture being flown through the cavity of the discharge section 1, a glow discharge is initiated. An area in which a glow exists is shown by crossed oblique lines in FIG. 3. A lightening portion of each cathode bar 3 which is covered with the glow is subject to an influence of the gas pressure or the like. In the embodiment, the lightening portion of each cathode bar 3 is in the ranges between 10 mm and 20 mm from the distal end of each cathode bar 3. Upon occurrence of the glow discharge, CO₂ in the gas mixture as the laser medium is excited and laser oscillation takes place between conventional resonators each provided with a reflector (not shown). A beam axle is designated by reference numeral 6 in FIG. 3. Titanium is employed as the base material 7 of each cathode bar 3 and a thin titanium oxide film 8 is formed on the surface of the lightening portion, as is schematically shown in FIG. 1. Each cathode bar 3 provided with the titanium oxide film 8 is manufactured as follows: the whole cathode bars 3 are first formed from titanium and incorporated in the discharge section 1 of the gas laser apparatus. The electric discharge is then initiated between the discharge electrodes 2 and 3 in the same manner as in the normal usage of the apparatus. The cathode bars 3 are heated by the glow and the surface of each cathode bar 3 is oxidized owing to an extremely small amount of oxygen contained in the gas mixture, thereby forming a thin titanium oxide film on the surface of each cathode bar 3. For reference, FIGS. 4 and 5 show the results of an elemental analysis of the cathode in the direction of its depth before occurrence of the electric discharge and after the discharge for 200 hours, in accordance with an analysis method based on the Auger effect, respectively. The transverse axis represents the depth from the surface of the cathode and the vertical axis the elemental ratio based on the spectral intensity, in each graph. The graphs show that a thin titanium oxide film of about 50 nm (500 Å) thick is formed on the surface of the cathode formed from titanium as the base material after the electric discharge for 200 hours. It is considered that the thickness of the titanium oxide film thus formed is nearly proportional to the discharging period. According to the above-described construction, since the lightening portion surface of each cathode bar 3 is covered with the thin titanium oxide film 8 the sputtering rate of which is considerably low, the physical sputtering is almost prevented even when the electric discharge causes the cathode bars 3 to suffer from the sputtering action. Further, since titanium oxide is a stable material and has a low vapor pressure in the atmosphere of a high temperature, the chemical sputtering is also prevented. Accordingly, an amount of the substance piled on the anode bar 2 is considerably reduced as compared with the case of the conventional cathode formed from molybdenum, whereby concentration of the electric field on tne surface of the anode and hence, occurrence of micro-arcs is prevented. Further, since the titanium oxide film 8 is stable, the electric field concentration on the surface of each cathode bar 3 due to ununiform sputtering as seen in the case of the conventional molybdenum cathode is prevented, thereby further restraining the occurrence of micro-arcs. As the result of the above-described construction, the electrodes 2 and 3 may be prevented from being deteriorated and therefore, a stable discharge may be maintained for a long period. Further, the maintenance cycle of the gas laser apparatus may be improved and the productivity of a production line where the gas laser apparatus is employed may be improved. FIG. 6 shows the relationship between the practical discharge input and discharging period in the case of a transverse flow 5-kW CO₂ laser. As shown in FIG. 6, the practical discharge input is gradually reduced in the conventional molybdenum cathode and the electric discharge is maintained only for about 100 hours. While, in the titanium cathode of the present invention, the practical discharge input may be maintained at a large value even after the discharge for 200 hours. The transverse axis in FIG. 6 denotes logarithmic scale. The diameter of the titanium cathode is reduced by about 9 µm after the electric discharge for 200 hours. Based on the result, the operational period of the titanium cathode employed in the embodiment is about 10,000 hours. Conventionally, the substance piled on the cathode is required to be removed at an interval of bout 200 hours at the longest. While, in the apparatus of the present invention, the interval may be lengthened to about 1,500 hours 7.5 times as long as in the conventional apparatus. As understood from FIG. 6, the titanium cathode 3 shows a tendency that the practical discharge input is low at an initial stage of the electric discharge and increased as the discharge progresses. However, when the baking is executed for the cathode 3 for a longer period than in the conventional apparatus in the test operation of the apparatus at the time of forwarding or shipment from a works, the practical discharge input may be maintained at a sufficiently large value at the first. Although the invention is preferably applied to a biaxial cross flow CO₂ laser employing the direct-current discharge in the foregoing embodiment, the invention may be applied to other laser apparatus of the type that the discharge electrodes are placed in the atmosphere of the gas mixture as the laser medium without regard to the kind of the gas mixture, for example, axial flow gas lasers, triaxial cross flow gas lasers or the like. The discharge cathode is formed into a general Σ-shape as a cathode 9 shown in FIG. 7 as a second embodiment, which is in accordance with the invention as claimed. As is shown in FIG. 8 illustrating a third embodiment, each cathode bar 3 may be formed from molybdenum as a base material 10. A titanium coating 11 may be applied to each cathode bar 3 and then oxidized so that titanium oxide film is formed on the surface of each cathode bar 3. The method of applying the titanium coating 11 to each cathode bar 3 includes the sputtering, ion plating and laser cladding as well known in the art. Of these methods, the laser cladding is considered to be the best method since it provides for the maximum coating thickness of about 0.4 mm. Further, in order to oxidize the titanium coating 11, the same method as employed in the foregoing embodiment may be applied. Alternatively, the cathode may be heated before the assembling of the laser apparatus so that the titanium coating is oxidized. Although the invention is preferably applied to the direct current discharge type gas laser in the foregoing embodiment, the invention may be applied to an alternate current discharge type gas laser. Since both of the electrodes alternately suffer from the sputtering action in the gas laser of the alternate current discharge type, both of the cathode and anode need to have the titanium oxide surface. Since it is considered that the construction that both of the cathode and anode have the titanium oxide surface may be easily understood from the description of the first embodiment with reference to FIGS. 1 to 4, a detailed description and the drawings for the construction are eliminated. Furthermore, in order to achieve the object of the invention, the discharge electrodes may be formed from titanium as a base material or from molybdenum as a base material with titanium coated thereon instead of previously forming titanium oxide on the surface of the discharge electrodes as described above. When use of the gas laser apparatus is initiated, the electric discharge induced between the discharge electrodes causes the titanium surface thereof to be oxidized and accordingly, the surface of each electrode is covered with titanium oxide at last. Since this construction is also considered to be easily understood from the first embodiment, a detailed description and drawings are eliminated. FIGS. 8 and 9 show a fourth embodiment related to a pulsed laser employing pulse output. Cathode bars 13 and anode bars 14 both formed from titanium as a base material are oppositely disposed at the upper and lower sides of the discharge section 12. A pulsed power supply is connected across the electrodes. The titanium oxide film is formed on the surface of the lightening portion of each electrode as the result of oxidization of titanium as the base material. The gas mixture as the laser medium may flow in either of the right-hand and left-hand directions, as viewed in FIG. 9. The same effect may be achieved in the fourth embodiment as in the first embodiment.

Claims: A gas laser apparatus wherein a pair of discharge electrodes (2, 3) are disposed so as to be opposed to each other and placed in an atmosphere of a gas mixture as a laser medium and a DC voltage is applied across the discharge electrodes (2, 3) so that an electric discharge is initiated, thereby exciting the gas mixture, wherein said discharge electrodes (2, 3) comprise a number of anode bars (2) arranged into the form of a reed screen and a number of cathode bars (3) arranged at an upstream side of a flow of the gas mixture relative to the anode bars (2), characterized in that said cathode bars are bent into a general Σ-shape whose free end faces away from the anode, and stacked in respective planes parallel to one another with a predetermined space between each cathode bar (3) and the adjacent cathode bars (3), said anode bars (2) extending in a direction in which the cathode bars (3) are stacked, the cathode (3) of said discharge electrodes (2, 3) is formed from titanium as a base material (7) and that the surface of a lightening portion of said cathode (3) is oxidized into titanium oxide, the lightening portion corresponding to a portion covered with a glow during the electric discharge. A gas laser apparatus wherein a pair of discharge electrodes (2, 3) are disposed so as to be opposed to each other and placed in an atmosphere of a gas mixture as a laser medium and a DC voltage is applied across the discharge electrodes (2, 3) so that an electric discharge is initiated, thereby exciting the gas mixture, wherein said discharge electrodes (2, 3) comprise a number of anode bars (2) arranged into the form of a reed screen and a number of cathode bars (3) arranged at an upstream side of a flow of the gas mixture relative to the anode bars (2), characterized in that said cathode bars (3) are bent into a general Σ-shape whose free end faces away from the anode, and stacked in respective planes parallel to one another with a predetermined space between each cathode bar (3) and the adjacent cathode bars (3), said anode bars (2) extending in a direction in which the cathode bars (3) are stacked, the cathode (3) of said discharge electrodes (2, 3) is formed from molybdenum as a base material (10) and a titanium coating (11) is applied to the cathode (3), and that the surface of a lightening portion of said cathode (3) is applied to the cathode (3) is oxidized into titanium oxide, the lightening portion corresponding to a portion covered with a glow during the electric discharge. A gas laser apparatus wherein a pair of discharge electrodes (2, 3) are disposed so as to be opposed to each other and placed in an atmosphere of a gas mixture as a laser medium and a DC voltage is applied across the discharge electrodes (2, 3) so that an electric discharge is initiated, thereby exciting the gas mixture, wherein said discharge electrodes (2, 3) comprise a number of anode bars (2) arranged into the form of a reed screen and a number of cathode bars (3) arranged at an upstream side of a flow of the gas mixture relative to the anode bars (3) characterized in that said cathode bars (3) are bent into a general Σ-shape whose free end faces away from the anode, and stacked in respective planes parallel to one another with a predetermined space between each cathode bar (3) and the adjacent cathode bars (3), said anode bars (2) extending in a direction in which the cathode bars (3) are stacked, the cathode (3) of said discharge electrodes (2, 3) is formed from titanium, the surface of which is oxidized during the electric discharge. A gas laser apparatus wherein a pair of discharge electrodes (2, 3) are disposed so as to be opposed to each other and placed in an atmosphere of a gas mixture as a laser medium and a DC voltage is applied across the discharge electrodes (2, 3) so that an electric discharge is initiated, thereby exciting the gas mixture, wherein said discharge electrodes (2, 3) comprise a number of anode bars (2) arranged generally into the shape of a reed screen and a number of cathode bars (3) arranged at an upstream side of a flow of the gas mixture relative to the anode bars (3) characterized in that said cathode bars (3) are bent into a general Σ -shape whose free end faces away from the anode, and stacked in respective planes parallel to one another with a predetermined space between each cathode bar (3) and the adjacent cathode bars (3), said anode bars (2) extending in a direction in which the cathode bars (3) are stacked, the cathode (3) of said discharge electrodes (2, 3) is formed from molybdenum as a base material (7) and a titanium coating (11) is applied to the cathode (3), said titanium coating (11) being oxidized during the electric discharge. A gas laser apparatus according to claim 1, characterized in that both of the discharge electrodes (2, 3) are formed from titanium as a base material and that the surfaces of lightening portions of both said discharge electrodes (2, 3) are oxidized into titanium oxide, each lightening portion corresponding to a portion covered with a glow during an electric discharge. A gas laser apparatus according to claim 2, characterized in that each of the discharge electrodes (2, 3) is formed from molybdenum as a base material and a titanium coating is applied to each discharge electrode (2, 3) is oxidized into titanium oxide, each lightening portion corresponding to a portion covered with a glow during an electric discharge. A gas laser apparatus according to claim 3, characterized in that each of the discharge electrodes (2, 3) is formed from titanium and that the surface of each discharge electrode (2, 3) is oxidized during the electric discharge. A gas laser apparatus according to claim 4, characterized in that each of the discharge electrodes (2, 3) is formed from molybdenum as a base material (7), and a titanium coating (11) is applied to each of said discharge electrodes (2, 3), said titanium coating being oxidized during the electric discharge.

Inventors: NISHIDA KOICHI HAMAKAWASAKI WO, TERAI KIYOHISA HAMAKAWASAKI WO, NISHIDA, KOICHI HAMAKAWASAKI WORKS, K.K.TOSHIBA, TERAI, KIYOHISA HAMAKAWASAKI WORKS, K.K.TOSHIBA
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01S3
IPC codes: H01S3