DISCHARGE TUBE FOR LASER

Abstract: A runoff of an electrode material, a deterioration of an electrode, and the like caused by a corona discharge are prevented, and an injected power is increased by improving the heat-radiation and insulation characteristic of the electrode. A discharge tube (1) for a laser is a pipe of a circular cross section, which is made of a dielectric (e.g. a quartz glass) and on its outer peripheral surface two electrode sections (2, 3) are provided in spiral forms at an identical pitch respectively. The electrode sections (2, 3) comprise electrodes (21, 31) and dielectric layers (22, 32), and the respective electrodes (21 and 31) are formed of silver coating on the outer surface of the tube wall (1A). The dielectric layers (22 and 32) are so formed as to cover the electrodes (21 and 31). This dielectric layers (22 and 32) are made of ceramic, glass or resin by flame spraying, coating, or baking. Alternatively, the dielectric layers are prepared separately and bonded to the electrodes. Their thickness is about 0.1 mm - 5 mm.

Description: The present invention relates to a laser discharge tube used in a laser oscillator, and more specifically, to a laser discharge tube by which the dielectric breakdown and the like of the electrodes disposed on the outside periphery of a tube wall are prevented. A discharge tube used in a laser oscillator induces a discharge by a high voltage applied thereto and outputs a laser beam produced by a laser gas excited by the discharge to the outside. Incidentally, the high voltage is applied to electrodes disposed on the outside periphery of the tube wall of the discharge tube and the discharge takes place between the electrodes confronting each other across the discharge tube. Therefore, a discharge normally takes place within a region determined by the width of the electrodes. Nevertheless, since a very high high-frequency voltage (e.g., 4000 V) is applied to the electrodes, there is a possibility that a dielectric breakdown is caused in the region other than the region between the electrodes and corona discharge takes place. The corona discharge is liable to take place at the portion having a high temperature of the discharge tube such as, for example, at the downstream of a laser gas flowing in the discharge tube and the temperature of the portion where the corona discharge takes place is increased and thus an electrode material is eroded and separated by being melted so that the deterioration of the electrode material is rapidly accelerated. By the way, silver having a good electric conductivity is generally used as the electrode material. The electrodes are formed by depositing the silver on the outside wall of the discharge tube by metalizing or the like. Further, when a high voltage is applied to the electrodes, the edge portion of each electrode has a distribution of a strong field and thus corona discharge takes places. When the electrodes are lifted by thermal strain or the like and an air gap is created, corona discharge also takes place at the lifted portion. The corona discharge travels once along the surface of the outside wall of the discharge tube and then gets into the discharge tube at a position apart from the electrodes by several millimeters. At the time, the silver is flown out by reacting with water and the flown silver grows to a tree-branch-shape along the outside wall of the discharge tube from the electrodes and covers the outside wall of the discharge tube in the vicinity of the electrodes with a width of several centimeters. When such a phenomenon in which the silver serving as the electrode material migrates occurs, a dielectric strength is further lowered and corona discharge is more liable to take place, and as a result the flow-out of the electrode material and deterioration of the electrodes, and the like become more significant. Further, since the vicinity of the electrodes is covered with the melted silver, the width of the electrodes is increased to change the electric characteristics thereof, the life of the discharge tube is also shortened. In addition, the external appearance of the discharge tube becomes bad. Although it suffices to lower a voltage applied to the electrodes for the prevention of the aforesaid corona discharge, this leads to the reduction of a power to be supplied and a laser beam machining requiring a high power cannot be executed. Further, since corona discharge is liable to take place as the temperature of the electrodes increases, there is a method of lowering the temperature of the electrodes by the provision of heat-dissipating plates with the electrodes. However, the occurrence of corona discharge also cannot be prevented at the downstream of a laser gas where the temperature of the discharge tube is increased. Further, although the temperature may be lowered by Lowering an applied voltage, a power to be supplied is also lowered in this case as described above, and a laser beam machining requiring a high power cannot be executed. JP-A-01066983 and EP-A-0382510 both disclose laser discharge tubes in which the electrodes are covered with a ceramic dielectric. Neither document addresses the problems caused by thermal strain. Taking the above into consideration, an object of the present invention is to provide a laser discharge tube capable of preventing the flow out and separation of an electrode material and deterioration of electrodes caused by corona discharge. According to the present invention, there is provided a laser discharge tube apparatus for inducing a discharge by a high-frequency voltage applied thereto and exciting a laser gas, the apparatus comprising a discharge tube having electrodes disposed on the outside periphery of a tube wall; and ceramic dielectric layers deposited on and covering completely the electrodes; characterised in that the ceramic layers are composed of a ceramic material having a low or negative coefficient of thermal expansion; and a glass with a melting point lower than that of the electrodes and having a flowing property is used as a binder; wherein a resulting combined coefficient of thermal expansion of the ceramic material and the binder is substantially equal to the coefficient of thermal expansion of the laser discharge tube. The dielectric layers can prevent the occurrence of corona discharge conventionally induced at the ends of the electrodes, and thus the flow out and separation of the electrode material and deterioration of the electrodes can be prevented. Preferably the ceramic dielectric layers are composed of a material having a melting point temperature lower than that of the electrodes and higher than a heat resistant temperature required by a laser discharge. Further, the heat-dissipating property and insulating property of the electrodes can be improved by using a material having a high coefficient of thermal conductivity and high insulating property as the dielectric layers. This allows the power to be supplied to be increased. In the accompanying drawings: Figure 1 is a diagram showing the arrangement of a laser discharge tube according to an embodiment of the present invention; and Figure 2 is a diagram schematically showing the arrangement of a laser oscillator using the laser discharge tube according to an embodiment of the present invention. Figure 2 is a diagram schematically showing the arrangement of a laser oscillator using the laser discharge tube according to an embodiment of the present invention. A laser discharge tube 1 is a tube with a circular cross section composed of a dielectric material (e.g., quartz). Two electrode units 2, 3 are helically disposed on the outside periphery of the laser discharge tube 1 at the same pitch. The electrode unit 2 is composed of an electrode 21 and a dielectric layer 22 and the electrode unit 3 is composed of an electrode 31 and a dielectric layer 32, in the same way. The details thereof will be described later. A laser gas 10 flows in the laser discharge tube 1 in the direction of a tube axis indicated by an arrow, and when a high-frequency voltage is applied between the electrodes 21 and 31 from a high-frequency power supply (not shown), a discharge takes place in the laser discharge tube 1 between the electrodes confronting each other across the discharge tube 1 and the laser gas 10 is exited. A totally reflecting mirror 4 and output coupling mirror 5 are disposed at the opposite ends of the laser discharge tube 1 to constitute a Fabry-Perot reflector and a beam discharged from the molecules of the exited laser gas is oscillated and a part thereof is output from the output coupling mirror 5 as a laser beam 11. The laser beam 11 is irradiated to a workpiece for the laser machining thereof. Note, the laser discharge tube 1 may be composed of other dielectric materials (e.g., alumina, aluminium titanate) resistant to dielectric breakdown in addition to quartz. Further, although the electrodes are helically disposed to the laser discharge tube 1, plate-shaped electrodes may be disposed thereto. Figure 1 shows the arrangement of the laser discharge tube according to an embodiment of the present invention, which is a cross sectional view taken along the line X - X of Figure 2. As described above, the electrode units 2 and 3 are disposed on the outside periphery 1A of the tube wall of the laser discharge tube 1. The respective electrodes 21 and 31 of the electrode units 2 and 3 are formed by depositing silver as a conductor on the outside periphery 1A of the tube surface. The dielectric layers 22 and 32 are formed to cover the electrodes 21 and 31. The dielectric layers 22 and 32 are formed on the electrodes 21 and 31 disposed on the outside periphery 1A of the tube wall by depositing a dielectric material of a ceramic material, by spray coating, coating, baking or the like or by bonding independently formed dielectric materials to the electrodes 21 and 31, and have the thickness of about 0.1 mm - 5 mm. Here, a case will be described in which the dielectric layers 22 and 32 are composed of a ceramic material having a low coefficient of thermal expansion or negative coefficient of thermal expansion and a low melting point glass is used as a binder. A lead oxide material composed of lead oxide added with B₂O₃ (PbO·B₂ O₃ ) is used as a typical low melting point glass, and aluminium titanate (Al₂TiO₅), boron nitride (BN) and silicon nitride (Si₃N₄) are used as the ceramic material. The lead oxide (PbO·B₂ O₃ ) has the melting point of about 400°C - 450°C which is lower than that of the silver forming the electrodes 21 and 31 and thus it is suitably coated and baked to the electrodes 21 and 31. Further, the lead oxide also has the function to flow into an air gap and return to its original state at the initial stage of an air gap discharge caused by the lifting of the electrodes 21 and 31 due to thermal strain before the electrodes 21 and 31 are melted and broken. A mixing ratio of the lead oxide to the entire material is 50 - 90 wt%. Aluminium titanate and boron nitride have a negative coefficient of thermal expansion and serve to lower the coefficient of thermal expansion of the entire material by being mixed with the lead oxide. A mixing ratio of them to the entire material is adjusted so that a resulting coefficient of thermal expansion is substantially equal to the coefficient of thermal expansion of the laser discharge tube 1 and, for example, the aluminium titanate has the mixing ratio of 18 wt% and the boron nitride has the mixing ratio of 5 wt%. Further, since the aluminium titanate and boron nitride have a coefficient of thermal conductivity higher than that of the low melting point glass, a coefficient of thermal conductivity as a whole is increased. In particular, since the boron nitride has a large coefficient of thermal conductivity, a heat-dissipating property can be greatly improved by mixing the boron nitride. Note, the addition of a slight amount (e.g., 2 wt%) of the silicon nitride enables the ceramic material to be easily coated to the laser discharge tube 1. Silicon carbide may be used in place of the silicon nitride to obtain the same effect. The above lead oxide, aluminium titanate, boron nitride and silicon nitride are mixed together with a solvent and coated to the electrodes 21 and 31 of the laser discharge tube 1 and further baked in a thermostat. The baking conditions thereof are, for example, 450°C × 30 minutes. The electrodes 21 and 31 are completely covered with the thus made ceramic-deposited-layers (dielectric layers) 22 and 32. As a result, the silver is prevented from being melted by coming into contact with water and reacting with it. Further, air gaps conventionally interposed between the electrodes 21 and 31 and the outside periphery 1A of the tube wall are removed and there is no chance that corona discharge conventionally excited by the air gaps takes place. Further, since the lifted air gaps caused by the thermal strain between the laser discharge tube 1 and the electrodes 21 and 31 are returned to their original state, there is also no chance for corona discharge to take place there. Therefore, the flow out and separation of the silver constituting the electrodes 21 and 31, deterioration of the electrodes 21 and 31, and the like can be prevented and the life of the laser discharge tube 1 is prolonged more than ten times. Further, the characteristics of the laser discharge tube 1 are not deteriorated (changed) by the silver flowing out to the vicinity of the electrodes and covering them. Furthermore, the external appearance of the discharge tube is not made bad. Further, since the ceramic-deposited-layers 22 and 32 have a coefficient of thermal expansion adjusted substantially equal to that of the laser discharge tube 1, the thermal strain produced between the ceramic-deposited-layers 22, 32 and the laser discharge tube 1 is minimized and thus the ceramic-deposited-layers 22 and 32 are not separated by the thermal strain. Therefore, the life of the laser discharge tube 1 is also greatly improved in this point. Further, the heat-dissipating property and insulating property of the electrodes 21 and 31 can be improved by using a material having a high coefficient of thermal conductivity and high insulating property to the ceramic-deposited-layers 22 and 32, so that a conventionally restricted power to be supplied (electric power to be sup plied) can be increased. When the same electric power is supplied, a laser oscillation output obtained from the laser discharge tube of the present invention is increased from a conventional output of, for example, 1570 W to 1770 W by about 15% due to the improvement of the heat-dissipating property. The insulating property between the electrodes is increased by five times. The ceramic-deposited-layers 22 and 32 can endure a supplied power generating a temperature up to 400°C. Further, they can endure a rapid cooling/heating test from -200°C to 400°C. Furthermore, even if ceramic-deposited-layers 22 and 32 are partially heated to 1500°C, they can keep a function to return to their original state. Although the aluminium titanate and boron nitride are used as the ceramic material having a negative coefficient of thermal expansion in the above description, other ceramic materials such as, for example, lead titanate may be used. Further, although the aluminium titanate and boron nitride are mixedly used, any one of them may be used. Further, the same effect can be obtained by using a ceramic material having a low coefficient of thermal expansion, in addition to the ceramic material having a negative coefficient of thermal expansion. Further, a ceramic material mainly composed of, for example, titanium composite oxide (TiO, Al₂TiO₅, MgTiO₃, etc.) or a composite ceramic material composed of the titanium composite oxide, and alumina, magnesia, boron nitride and/or the like may be used as a ceramic type paint. In this case, silicon or tyranno-polymer (trade mark, Si-C) may be used as a binder in addition to the above lead oxide. When the composite ceramic material composed of the titanium composite oxide, and alumina, magnesia, boron nitride and/or the like is used, a ceramic material exhibiting a low coefficient of thermal expansion and high coefficient of thermal conductivity can be obtained by regulating the combination of the titanium composite oxide with the alumina, magnesia, boron nitride and/or the like. Further, although silver is used to the electrodes in the above description, they may be composed of other conductive material (e.g., gold). As described above, according to the present invention, since the dielectric material is deposited on the electrodes of the laser discharge tube, the electrodes are completely covered with the dielectric layers thereof. As a result, air gaps conventionally interposed between the electrodes and the outside periphery of the tube wall are removed and there is no chance for corona discharge excited by the air gaps to take place together with the effect of the function of the air gaps to return to their original state. Therefore, the flow out of the electrode material, deterioration of the electrodes, and the like can be prevented. Further, the characteristics of the electrodes are not deteriorated (changed) by that the vicinity of thereof is covered with the flown out silver, and thus the life of the electrodes can be prolonged. Further, since the heat-dissipating property and insulating property of the electrodes can be improved by using a material having a high coefficient of heat conductivity and high insulating property as the dielectric layers, a conventionally restricted power to be supplied can increased.

Claims: A laser discharge tube apparatus for inducing a discharge by a high-frequency voltage applied thereto and exciting a laser gas, the apparatus comprising a discharge tube (1) having electrodes (21,31) disposed on the outside periphery of a tube wall; and ceramic dielectric layers (22,32) deposited on and covering completely the electrodes; characterised in that the ceramic layers (22,32) are composed of a ceramic material having a low or negative coefficient of thermal expansion; and a glass with a melting point lower than that of the electrodes and having a flowing property is used as a binder; wherein a resulting combined coefficient of thermal expansion of the ceramic material and the binder is substantially equal to the coefficient of thermal expansion of the laser discharge tube (1). An apparatus according to claim 1, wherein the ceramic material is a titanium composite oxide. An apparatus according to claim 2, wherein the titanium composite material is aluminium titanate, or lead titanium. An apparatus according to claim 1, wherein the ceramic material is a boron nitride. An apparatus according to claim 1, wherein the ceramic material is titanium composite oxide and boron nitride. An apparatus according to claim 1, wherein the ceramic layers are mixed with silicon nitride or silicon carbide. An apparatus according to any one of the preceding claims, wherein the binder has a mixing ratio of 50 - 90 wt%. An apparatus according to claim 1, wherein the ceramic layers are composed of titanium composite oxide or a composite ceramic material of titanium composite oxide, and magnesia, and/or alumina. An apparatus according to claim 8, wherein tyranno-polymer (Si-C) is used as an additional binder to the titanium composite oxide or the composite ceramic material of titanium composite oxide, and magnesia and/or alumina. An apparatus according to any one of the preceding claims, wherein the ceramic dielectric layers (22,32) are deposited on the electrodes by spray coating, coating or vapour deposition. An apparatus according to claim 1, wherein the ceramic dielectric layers (22,32) are composed of a material having a low coefficient of thermal expansion and a coefficient of thermal conductivity higher than that of said glass. An apparatus according to any one of claims 1 to 9, wherein the ceramic dielectric layers (22,32) are independently formed and deposited by bonding. An apparatus according to claim 1, wherein the ceramic dielectric layers (22,32) are composed of a material having a melting point temperature lower than that of the electrodes and higher than a heat resistant temperature required by a laser discharge.

Inventors: FUJIOKA YOSHIKI FANUC DAI- VIR, KARUBE NORIO - - MINAMITSUKUSH, MANABE MITSUO, FUJIOKA, YOSHIKI FANUC DAI-3 VIRA-KARAMATSU, KARUBE, NORIO 1-9-28, MINAMITSUKUSHINO, MANABE, MITSUO
Applicants: FANUC LTD, FANUC LTD.
Application year: 1992
Main classification: H01S3
IPC codes: H01S3