Abstract:
An excimer laser apparatus in which deterioration of the laser gas in the laser container can be suppressed, and damage to the magnetic bearings caused by the laser gas can be suppressed, and which can be reduced in size and operated efficiently, and has low power consumption. Each electromagnet (stator) ( 7 - 1 ) of a magnetic bearing ( 7 ) for supporting a rotary shaft ( 4 ) of a circulation fan  3  includes excitation coils. Each of the coils is arranged as one unit by embedding a coil body in a ceramic or glass type hardened material which is non-magnetic and has corrosion resistance against a laser gas. The excitation coils are attached to magnetic poles. Alternatively, coil wires ( 108   b ) of a radial magnetic bearing ( 108 ) can be isolated from a corrosive atmosphere by means of a separation wall ( 124 ) while projecting portions of cores ( 108   b ) extend through the separation wall ( 124 ) and are exposed toward a magnetic bearing rotor ( 108   f ).

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to an excimer laser apparatus in which a rotary bearing of a laser gas circulating fan is supported by magnetic bearings. 
     BACKGROUND OF THE INVENTION 
     FIG. 12 is a first example of a basic arrangement of an excimer laser apparatus to which the present invention can be applied. FIG. 13 shows an arrangement of a motor housing of the apparatus of FIG.  12 . In the excimer laser apparatus, a laser gas is sealably contained in a laser container  1 . The laser gas contains a halogen type gas, such as a fluorine gas. A pair of main discharge electrodes  2 ,  2  are disposed in the laser container  1  so as to obtain an electric discharge for performing laser beam oscillation. Further, a circulation fan  3  is disposed in the laser container  1 , so as to generate a flow of laser gas having a high velocity between the main discharge electrodes  2 ,  2 . 
     The circulation fan  3  has a rotary shaft  4  extending therethrough, which is projected beyond opposite end portions of the fan  3 . The rotary shaft  4  is rotatably supported by radial magnetic bearings  7 ,  7 ,  7  without making contact therewith, that is, in a floating condition. A motor  9  is provided so as to operate the circulation fan  3 . 
     A magnetic bearing generally includes, as basic elements, a rotor, stators made of electromagnets for effecting floating support of the rotor and position sensors for detecting the position of the rotor. The radial magnetic bearing  7  shown in FIGS. 12 and 13 comprises a rotor  7 - 3  (FIG. 13) provided on the rotary shaft  4 , stators, i.e., electromagnets  7 - 1 ,  7 - 1  arranged in a spaced relationship around the rotor, and displacement detection sensors  7 - 2 ,  7 - 2  provided around a sensor target  7 - 4  on the rotary shaft so as to detect displacement of the rotary shaft  4 . Displacement signals from the displacement detection sensors  7 - 2 ,  7 - 2  are input to a control circuit (not shown) for phase compensation and gain adjustment. Output from this control circuit is supplied to the electromagnets  7 - 1 ,  7 - 1 , which generate a magnetic attraction force or a magnetic repellent force in accordance with this output, to thereby support the rotary shaft  4  so that it is floated at a predetermined position between the electromagnets  7 - 1 ,  7 - 1 . 
     As mentioned above, the laser gas is a corrosive gas containing, for example, a fluorine gas. Therefore, the electromagnets  7 - 1 ,  7 - 1  providing the radial magnetic bearing are subject to a corrosive environment. As shown in FIG. 14 a  and FIG. 14 b , the electromagnet (stator)  7 - 1  comprises a stator core (iron core)  7 - 1   a  and excitation coils  7 - 1   b  attached to the stator core  7 - 1   a . A separation wall  14  comprising a non-magnetic body is provided on an inner circumferential surface of the electromagnet  7 - 1 , which surface surrounds the magnetic bearing rotor  7 - 3  provided on the rotary shaft  4 . This prevents the electromagnet  7 - 1  (especially the excitation coils  7 - 1   b  which are liable to corrosion) from making contact with the laser gas. In FIGS. 12,  13 ,  14   a  and  14   b , reference numeral  6  denotes a motor housing;  8  a window through which a laser beam is emitted;  10  a protective bearing;  11  a gas inlet chamber;  12  a dust removing filter;  13  a gas inlet tube; and  15  a magnetic bearing frame. 
     When the separation wall  14  is provided on the inner circumferential surface of the electromagnet  7 - 1 , a problem arises, such that a magnetic gap between the electromagnet  7 - 1  and the magnetic bearing rotor  7 - 3  becomes large, thus reducing a magnetic attraction force or a magnetic repellent force obtained for effecting floating support of the rotary shaft  4 . Therefore, in order to obtain a desired magnetic force of the magnetic bearing for controlling floating support of the rotary shaft, it is required to increase the size of the electromagnet  7 - 1 . This is also problematic because the magnetic bearing inevitably becomes large. 
     In view of the above, the present invention provides an excimer laser apparatus in which an excitation coil of an electromagnet of a magnetic bearing has corrosion resistance against a laser gas, thus reducing the size of the magnetic bearing while increasing the life of the magnetic bearing, and preventing contamination of the laser gas. 
     FIG. 15 shows a second example of a basic arrangement of an excimer laser apparatus to which the present invention can be applied. As shown in FIG. 15, in this excimer laser apparatus, a laser gas containing a halogen type gas, such as a fluorine gas, is sealably contained in a laser container  201 . In the laser container  201 , there are provided pre-ionization electrodes (not shown) for pre-ionizing the laser gas and a pair of main discharge electrodes  202 ,  202  for obtaining an electric discharge for performing laser beam oscillation. Further, a circulation fan  203  is provided in the laser container  201 , so as to generate a flow of the laser gas having a high velocity between the main discharge electrodes  202 ,  202 . 
     The circulation fan  203  has a rotary shaft  204  extending therethrough, which is projected beyond opposite end portions of the circulation fan  203 . Radial magnetic bearings  206 ,  207  and an axial magnetic bearing  208  are provided at opposite ends of the laser container  201 . The rotary shaft  204  is rotatably supported by these magnetic bearings  206 ,  207 ,  208  without making contact therewith, that is, in a floating condition. A motor  209  for operating the circulation fan  203  is provided on a shaft end side of the radial magnetic bearing  207 . 
     Displacement sensor targets  206   c ,  207   c ,  208   d  and magnetic bearing rotors  206   d ,  207   d ,  208   e  of the magnetic bearings are secured to the rotary shaft  204 . Further, a rotor  209   b  of the motor  209  is secured to the rotary shaft  204 . Displacement sensors  206   a ,  207   a ,  208   a , electromagnets (i.e., magnetic bearing stators)  206   b ,  207   b ,  208   b ,  208   c  and a stator  209   a  of the motor  209  are provided so as to face the displacement sensor targets  206   c ,  207   c ,  208   d , the magnetic bearing rotors  206   d ,  207   d ,  208   e  and the rotor  209   b  of the motor  209 , respectively. 
     Separation walls  210 ,  211  in the forms of thin-walled cylinders are provided on inner circumferential surfaces of the displacement sensors  206   a ,  207   a  and the electromagnets  206   b ,  207   b  of the radial magnetic bearings  206 ,  207  and the stator  209   a  of the motor  209 . The separation walls  210 ,  211  are made of a material having corrosion resistance against a halogen type gas contained in a laser gas, for example, austenite type stainless steel such as SUS316L. Thus, the displacement sensors  206   a ,  207   a , the electromagnets  206   b ,  207   b  and the stator  209   a  of the motor  209 , which comprise cores (iron cores) and coil wires having poor corrosion resistance against the laser gas, do not make contact with the laser gas. 
     In the axial magnetic bearing  208 , a separation wall  212  is provided so as to prevent the displacement sensor  208   a  from making contact with the laser gas, as in the case of the radial magnetic bearings  206 ,  207 . With respect to the magnetic bearing stators of the axial magnetic bearing  208 , that is, the electromagnets  208   b ,  208   c , the stator cores are made of a ferromagnetic material having corrosion resistance against a halogen type gas contained in a laser gas, such as a permalloy. Therefore, only the excitation coils are protected by separation walls  213 . 
     The displacement sensor targets  206   c ,  207   c ,  208   d  and the magnetic bearing rotors  206   d ,  207   d ,  208   e  of the magnetic bearings, which are secured to the rotary shaft  104 , are disposed within a sealed space communicated with the laser container  101 . Therefore, the displacement sensor targets  206   c ,  207   c ,  208   d  and the magnetic bearing rotors  206   d ,  207   d ,  208   e  are made of a ferromagnetic material having corrosion resistance against a halogen type gas, such as a permalloy. The rotor  209   b  of the motor  209  is made of a composite of silicon steel plates and an aluminum alloy, or a permanent magnet. Therefore, a separation wall  214  in the form of a thin-walled cylinder is provided on a surface of the rotor  209   b , so as to prevent the rotor  209   b  from making contact with the laser gas. 
     However, in the above-mentioned arrangement in which the separation walls  210 ,  211  in the forms of thin-walled cylinders are provided on the inner circumferential surfaces of the electromagnets  206   b ,  207   b  of the radial magnetic bearings  206 ,  207 , the magnetic gap between the electromagnets  206   b ,  207   b  and the magnetic bearing rotors  206   d ,  207   d  of the radial magnetic bearings  206 ,  207  is increased by a distance corresponding to the wall thickness of the separation walls  210 ,  211 . This leads to low efficiency and an increase in size of the magnetic bearings. 
     Generally, a magnetic force of the magnetic bearing decreases in proportion to the square of a gap between the magnet and a target. Therefore, in order to maintain a desired magnetic force while the gap is increased two times, it is required to use a magnetic bearing in which the surface area of the electromagnet or the number of windings of the coil of the electromagnet is increased four times, or the magnitude of a control current applied to the coil is increased two times. 
     In view of the above, the present invention has been made. It is an object of the present invention to provide a long-life excimer laser apparatus in which deterioration of the laser gas in the laser container can be suppressed, and damage to the magnetic bearings caused by the laser gas can be suppressed. 
     It is another object of the present invention to provide an excimer laser apparatus in which the magnetic bearings can be reduced in size and operated efficiently, and which has a low power consumption. 
     DISCLOSURE OF THE INVENTION 
     The present invention provides an excimer laser apparatus comprising: a laser container in which a laser gas is sealably contained; a circulation fan which generates a flow of laser gas between main discharge electrodes; and magnetic bearings which support a rotary shaft of the circulation fan, wherein: each magnetic bearing comprises a magnetic bearing rotor provided on the rotary shaft and magnetic bearing stators provided around the magnetic bearing rotor; and each magnetic bearing stator comprises a stator core at least part of which is exposed toward the magnetic bearing rotor, coils being attached to the stator core, and an isolating member for isolating each coil from the laser gas. By this arrangement, the coils of the magnetic bearing are isolated from a corrosive atmosphere of the laser gas, so that corrosion of the coils can be prevented and the life of the magnetic bearing can be increased. Further, contamination of the laser gas which is caused by corrosion of the coils can be prevented. Therefore, product quality and reliability of the excimer laser can be improved. Because only the coil is protected by the isolating member and at least part of the stator core is exposed toward the magnetic bearing rotor, the magnetic gap between the stator core and the magnetic bearing rotor can be reduced. Therefore, the magnetic bearing can be reduced in size and operated highly efficiently. As a result, it is possible to provide an excimer laser which requires a small installation area and has a low power consumption. 
     In one embodiment of the present invention, the isolating member is made of a corrosion-resistant material having corrosion resistance against the laser gas and each coil is embedded in the isolating member made of the corrosion-resistant material. 
     In another embodiment of the present invention, the corrosion-resistant material is a ceramic or glass type hardened material. 
     In a further embodiment of the present invention, the isolating member comprises a coil case for sealingly enclosing each coil. 
     In a further embodiment of the present invention, the isolating member comprises a sheath covering each electrically conductive wire of which the coil is made. 
     In a further embodiment of the present invention, the stator core comprises a magnetic body which has corrosion resistance against the laser gas or a magnetic body which has been subjected to an anticorrosion treatment against the laser gas. By this arrangement, contamination of the laser gas due to corrosion of the stator core exposed to a corrosive atmosphere can be prevented. 
     In a further embodiment of the present invention, each magnetic bearing is a radial magnetic bearing; the stator core has projecting portions facing the magnetic bearing rotor; the coils are attached to the projecting portions; and at least part of each projecting portion extends through a separation wall and is exposed toward the magnetic bearing rotor, the separation wall comprising the isolating member. By this arrangement, the projecting portions extending through the separation wall comprising the isolating member can be disposed in proximity to the magnetic bearing rotor, thus reducing the magnetic gap between the stator core and the magnetic bearing rotor. 
     In a further embodiment of the present invention, the magnetic bearing stators comprise a plurality of rodlike projecting portions and a base portion to which the projecting portions are connected, the base portion having a ring-shaped cross-section. By this arrangement, electromagnets can be easily manufactured by first attaching the coils to the rodlike projecting portions and then connecting the projecting portions to the ring-shaped base portion. 
     In a further embodiment of the present invention, each magnetic bearing is a radial magnetic bearing; and the stator core has projecting portions facing the magnetic bearing rotor, the coils being attached to the projecting portions, and an end face member attached to a surface of each projecting portion facing the magnetic bearing rotor, the end face member comprising a magnetic body having corrosion resistance against the laser gas, the end face member being exposed toward the magnetic bearing rotor. By this arrangement, the end face member providing part of the stator core can be disposed in proximity to the magnetic bearing rotor, thus reducing the magnetic gap between the stator core and the magnetic bearing rotor. 
     In a further embodiment of the present invention, a plurality of U-shaped cores, each having two projecting portions formed therein, are provided as the stator cores; the coils are attached to each U-shaped core so that the two projecting portions form an N-pole and an S-pole; the plurality of U-shaped cores are provided around the magnetic bearing rotor so that each projecting portion faces the magnetic bearing rotor and that two adjacent projecting portions of two adjacent U-shaped cores have the same polarity; the end face member is provided so as to extend between and onto the projecting portions having the same polarity; and the isolating member is provided so as to extend between the projecting portions having different polarities, the isolating member comprising a non-magnetic body. By this arrangement, the coils of the magnetic bearing can be isolated from a corrosive atmosphere of the laser gas, by means of the end face members and the isolating member. On the other hand, the end face member providing part of the stator core can be disposed in proximity to the magnetic bearing rotor, thus reducing the magnetic gap between the stator core and the magnetic bearing rotor. Because the isolating member extending between the N-polar projecting portion and the S-polar projecting portion comprises a non-magnetic body, no magnetic short circuit is caused. A magnetic flux passes through the magnetic bearing rotor and effectively exerts a magnetic force. 
     In a further embodiment of the present invention, the stator cores comprise a ring-shaped base portion provided around the magnetic bearing rotor and projecting portions extending radially inward from an inner circumferential surface of the base portion, the projecting portions being arranged at substantially equal intervals in a circumferential direction of the base portion; the coils are attached to the projecting portions so that an order of arrangement of the projecting portions is such that the N-pole, the S-pole, the S-pole and the N-pole are repeated as a unit; the end face member is provided so as to extend between and onto the projecting portions having the same polarity; and the isolating member is provided so as to extend between the projecting portions having different polarities, the isolating member comprising a non-magnetic body. By this arrangement, as compared to the end face members being attached only to the surfaces of the projecting portions facing the magnetic bearing rotor, the number of the end face members used in the magnetic bearing can be reduced by half. Therefore, the number of connecting portions between the end face members and the isolating member can be reduced, and the separation wall comprising the end face members and the isolating member can be easily manufactured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing a basic arrangement of an excimer laser apparatus according to a first embodiment of the present invention. 
     FIGS. 2 a  and  2   b  are cross-sectional views showing an example of an arrangement of a magnetic bearing used in the excimer laser apparatus according to the first embodiment of the present invention. 
     FIGS. 3 a  and  3   b  are cross-sectional views showing another example of an arrangement of a magnetic bearing used in the excimer laser apparatus according to the first embodiment of the present invention. 
     FIGS. 4 a  and  4   b  are cross-sectional views showing a further example of an arrangement of a magnetic bearing used in the excimer laser apparatus according to the first embodiment of the present invention. 
     FIG. 5 is a cross-sectional view showing a basic arrangement of an excimer laser apparatus according to a second embodiment of the present invention. 
     FIG. 6 is a cross-sectional view showing a detail of an arrangement of a bearing housing and its vicinities in the excimer laser apparatus according to the second embodiment of the present invention. 
     FIG. 7 is a cross-sectional view showing a detail of an arrangement of a motor housing and its vicinities in the excimer laser apparatus according to the second embodiment of the present invention. 
     FIG. 8 is a diagram showing an example of an arrangement of a magnetic bearing, as viewed in cross-section taken along the line A—A in FIG.  6 . 
     FIG. 9 is a diagram showing another example of an arrangement of a magnetic bearing, as viewed in cross-section taken along the line A—A in FIG.  6 . 
     FIG. 10 is a diagram showing a further example of an arrangement of a magnetic bearing, as viewed in cross-section taken along the line A—A in FIG.  6 . 
     FIG. 11 is a diagram showing a further example of an arrangement of a magnetic bearing, as viewed in cross-section taken along the line A—A in FIG.  6 . 
     FIG. 12 is a cross-sectional view showing an arrangement of an excimer laser apparatus to which the present invention can be applied. 
     FIG. 13 is a diagram showing an arrangement of the inside of a motor housing of the excimer laser apparatus shown in FIG.  12 . 
     FIGS. 14 a  and  14   b  are diagrams showing an arrangement of a magnetic bearing used in the excimer laser apparatus to which the present invention can be applied. 
     FIG. 15 is a cross-sectional view showing an arrangement of a conventional excimer laser apparatus to which the present invention can be applied. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinbelow, embodiments of the present invention are described, with reference to the drawings. 
     FIG. 1 is a cross-sectional view showing a basic arrangement of an excimer laser apparatus according to a first embodiment of the present invention. In FIG. 1, the portions which are the same or correspond to the portions shown in FIGS. 12,  13 ,  14   a  and  14   b  are designated by the same reference numerals and characters as used in FIGS. 12,  13 ,  14   a  and  14   b . As shown in FIGS. 2 a ,  2   b ,  3   a ,  3   b ,  4   a  and  4   b , in the excimer laser apparatus in this embodiment of the present invention, no separation wall facing the magnetic bearing rotor  7 - 3  is provided on a surface of the electromagnet  7 - 1  of the radial magnetic bearing  7 . 
     FIGS. 2 a  and  2   b  are cross-sectional views (one of which is taken along the line A—A in FIG. 1) showing an example of an arrangement of part of a radial magnetic bearing used in the excimer laser apparatus in the first embodiment. As shown in these drawings, the stator of the radial magnetic bearing  7 , that is, the electromagnet  7 - 1 , comprises the stator core  7 - 1   a  formed from a magnetic body and excitation coils  7 - 1   b  attached to the stator core  7 - 1   a . Reference numeral  15  denotes the magnetic bearing frame. The excitation coil  7 - 1   b  is formed, as one unit, from a coil body  7 - 1   c  formed by coiling an electrically conductive wire and a ceramic or glass type hardened material  7 - 1   f  formed by molding around the coil body  7 - 1   c . The hardened material  7 - 1   f  is non-magnetic and has corrosion resistance against a laser gas. 
     Thus, the excitation coil  7 - 1   b  is arranged as one unit by forming, by molding, the ceramic or glass type hardened material  7 - 1   f  which is non-magnetic and has corrosion resistance against a laser gas around the coil body  7 - 1   c . Therefore, the excitation coil  7 - 1   b  itself has corrosion resistance against a laser gas. Therefore, it is unnecessary to provide a separation wall on the surface of the stator core  7 - 1   a  facing the magnetic bearing rotor  7 - 3 , so as to prevent corrosion of the excitation coil  7 - 1   b  due to the laser gas. That is, the magnetic bearing has a simple structure. 
     In the excimer laser apparatus arranged as mentioned above, when the motor  9  is operated to thereby rotate the circulation fan  3 , a laser gas sealably contained in the laser container  1 , which contains a halogen type gas such as a fluorine gas, is caused to flow at a high velocity between the pair of main discharge electrodes  2 ,  2  and circulate through the laser container  1 . That is, the laser gas is guided from a gas outlet opening  1   a  into the gas inlet chamber  1 , in which dust is removed from the laser gas by means of the dust removing filters  12 ,  12 . The laser gas after removal of the dust is further guided through the gas inlet tubes  13 ,  13  into the bearing housing  5  and the motor housing  6 , and returns to the laser container  1 . 
     A laser excitation discharge is caused by applying a high voltage between the main discharge electrodes  2 ,  2 , thus performing laser beam oscillation. The laser beam thus generated is emitted to the outside of the laser container  1  through the windows  8 ,  8  provided in side walls of the laser container  1 . Because the laser gas in the laser container  1  is circulated by means of the circulation fan  3 , the laser gas between the main discharge electrodes  2 ,  2  is replaced with new laser gas for each electric discharge. This enables the laser beam oscillation to be stably repeated. 
     FIGS. 3 a  and  3   b  are cross-sectional views showing another example of an arrangement of part of a radial magnetic bearing used in the excimer laser apparatus in the first embodiment. As shown in the drawings, the excitation coil  7 - 1   b  comprises the coil body  7 - 1   c  formed by coiling an electrically conductive wire and a coil case  7 - 1   d  which sealingly encloses the coil body  7 - 1   c . The coil case  7 - 1   d  is made of a material having corrosion resistance against a laser gas. The stator of the radial magnetic bearing  7 , that is, the electromagnet  7 - 1 , is obtained by attaching the excitation coils  7 - 1   b  to the stator core  7 - 1   a.    
     Thus, the excitation coil  7 - 1   b  is arranged by sealingly enclosing the coil body  7 - 1   c  in the coil case  7 - 1   d  having corrosion resistance against a laser gas. Therefore, the excitation coil  7 - 1   b  itself has corrosion resistance against a laser gas. Therefore, differing from conventional techniques, it is unnecessary to provide a separation wall on the surface of the stator core  7 - 1   a  facing the magnetic bearing rotor  7 - 3 , so as to prevent corrosion of the excitation coil  7 - 1   b  caused by the laser gas. 
     FIGS. 4 a  and  4   b  are cross-sectional views showing a further example of an arrangement of part of a radial magnetic bearing used in the excimer laser apparatus in the first embodiment. As shown in the drawings, the excitation coil  7 - 1   b  is formed by coiling a sheath wire  7 - 1   e . The sheath wire  7 - 1   e  is obtained by covering an electrically conductive wire with an insolating material and further covering the insolating material on the wire with a sheath made of a material having corrosion resistance against a laser gas. The stator of the radial magnetic bearing  7  is arranged by attaching the excitation coils  7 - 1   b  to the stator core  7 - 1   a.    
     Thus, the excitation coil  7 - 1   b  is arranged by coiling the sheath wire  7 - 1   e  having a sheath made of a material having corrosion resistance against a laser gas. Therefore, the excitation coil  7 - 1   b  itself has corrosion resistance against a laser gas. Therefore, differing from conventional techniques, it is unnecessary to provide a separation wall on the surface of the stator core  7 - 1   a  facing the magnetic bearing rotor  7 - 3 , so as to prevent corrosion of the excitation coil  7 - 1   b  due to the laser gas. 
     As a material for the stator core  7 - 1   a  of the electromagnet  7 - 1  of the radial magnetic bearing  7 , use is made of a magnetic body having corrosion resistance against a laser gas, for example, an austenite type magnetic body or a magnetic body a surface of which has been subjected to an anticorrosion treatment, such as nickel plating. 
     In FIG. 1, an axial magnetic bearing of the excimer laser apparatus is omitted. With respect to a stator (an electromagnet) of the axial magnetic bearing, the same effects as described above can be obtained by using an excitation coil in which a ceramic or glass type hardened material which is non-magnetic and has corrosion resistance against a laser gas is formed around the coil body by molding, or an excitation coil in which the coil body is sealingly enclosed in a coil case having corrosion resistance against a laser gas, or an excitation coil obtained by coiling a sheath wire comprising an electrically conductive wire sealingly covered with a corrosion-resistant sheath. 
     FIGS. 5 to  12  show an arrangement of an excimer laser apparatus according to a second embodiment of the present invention. FIG. 5 is a cross-sectional view of the entire excimer laser apparatus. FIG. 6 is a cross-sectional view showing a detail of an arrangement of a bearing housing and its vicinities in the excimer laser apparatus. FIG. 7 is a cross-sectional view showing a detail of an arrangement of a motor housing and its vicinities in the excimer laser apparatus. FIGS. 8 to  12  show examples of arrangements of magnetic bearings, as viewed in cross-section taken along the line A—A in FIG.  6 . 
     In the excimer laser apparatus in this embodiment, a laser gas containing a halogen type gas, such as a fluorine gas, is sealably contained in a laser container  101 . In the laser container  101 , there are provided pre-ionization electrodes (not shown) for pre-ionizing the laser gas and a pair of main discharge electrodes  2 ,  2  for obtaining an electric discharge for performing laser beam oscillation. A circulation fan  103  is provided in the laser container  101 , so as to generate a flow of the laser gas having a high velocity between the main discharge electrodes  102 ,  102 . 
     A laser excitation discharge is caused by applying a high voltage between the discharge electrodes  102 ,  102 , thus performing laser beam oscillation. The laser beam thus generated is emitted to the outside of the laser container  101  through windows  105 ,  105  provided in side walls of the laser container  101 . The laser excitation discharge causes deterioration of the laser gas between the discharge electrodes  102 ,  102 . Consequently, characteristics of electric discharge are deteriorated, so that it becomes difficult to perform repeated oscillation. Therefore, by means of the circulation fan  103 , the laser gas in the laser container  101  is circulated so that the laser gas between the discharge electrodes  102 ,  102  is replaced with new laser gas for each electric discharge, to thereby perform stably repeated oscillation. 
     The distance between the main discharge electrodes  102 ,  102  is 16 mm. The length of the main discharge electrodes  102 ,  102  is about 600 mm. The frequency of oscillation is several thousand times per second. The length of the circulation fan  103  is slightly greater than that of the main discharge electrodes  102 ,  102  so that the velocity of laser gas flow becomes uniform over the entire length of the electrodes. The circulation fan  103  is rotated at a speed of several thousand rpm, to thereby obtain a laser gas flow having the necessary and sufficient velocity for performing repeated oscillation between the main discharge electrodes  102 ,  102 . 
     The circulation fan  103  has a rotary shaft  104  extending therethrough, which is projected beyond opposite end portions of the circulation fan  103 . Radial magnetic bearings  108 ,  109  and an axial magnetic bearing  110  are accommodated in a bearing housing  106  and a motor housing  107  provided at opposite end portions of the laser container  101 . The rotary shaft  104  is rotatably supported by the radial magnetic bearings  108 ,  109  and the axial magnetic bearing  110  without making contact therewith, that is, in a floating condition. A motor  111  applies a torque to the rotary shaft  104  of the circulation fan  103 . 
     Thread groove labyrinths  114 ,  115  are provided on a side of the laser container  101  in the bearing housing  106  and the motor housing  107 , so as to prevent entry of dust into the housings. Thus, it is possible to avoid a situation such that dust generated in the laser container  101  will enter (penetrate) the bearing housing  106  and the motor housing  107  and contaminate rolling surfaces of protective bearings  112 ,  113 . 
     A gas outlet opening  116  is provided in the laser container  101 . The laser gas from the gas outlet opening  116  enters gas inlet chambers  117 ,  117 , in which dust is removed from the laser gas by means of dust removing filters  118 ,  118 . The laser gas then passes through gas inlet tubes  119 ,  119  and is introduced into the bearing housing  106  and the motor housing  107  in the vicinity of the ends of the rotary shaft. That is, the laser gas is circulated as indicated by arrows in FIG. 5, to thereby surely prevent entry (penetration) of dust into the bearing housing  106  and the motor housing  107 . 
     As shown in FIG. 6, the bearing housing  106  comprises a bearing housing body  106   a , a right-hand electromagnet housing  106   b , a left-hand electromagnet housing  106   c  and a bearing cover  106   d . The bearing housing body  106   a  is connected to the side wall of the laser container  101 . The right-hand electromagnet housing  106   b  is connected to the bearing housing body  106   a , and the left-hand electromagnet housing  106   c  is connected to the right-hand electromagnet housing  106   b , and the bearing cover  106   d  is connected to the left-hand electromagnet housing  106   c . Seal recesses  129 ,  131 ,  133 ,  135  are provided at connecting surfaces of the bearing housing body  106   a , the right-hand electromagnet housing  106   b , the left-hand electromagnet housing  106   c  and the bearing cover  106   d . Seal members  130 ,  132 ,  134 ,  136  are attached to these recesses so as to prevent leakage of the laser gas. It is advantageous to use, as the seal members  130 ,  132 ,  134  and  136 , seal members made of a metal which does not produce moisture or other gases which contaminate the laser gas (such as stainless steel or aluminum). 
     Displacement sensors  108   a  and cores (iron cores)  108   b  of electromagnets (stators) of the radial magnetic bearing  108  are accommodated in the bearing housing body  106   a , in which they are disposed in relative positions determined by spacers  121 ,  122  and a side plate  123 . A projecting portion of the core  108   b  extends through a separation wall  124  formed from a plate type non-magnetic body. A distal end surface of the projecting portion is exposed. 
     The separation wall  124  is made of a material having corrosion resistance against the laser gas, such as austenite type stainless steel. Opposite ends of the separation wall  124  are secured by, for example, welding. By providing the separation wall  124 , it is possible to prevent the laser gas from making contact with the displacement sensors  108   a  and electromagnet coils  108   d  which have poor corrosion resistance against the laser gas. 
     The core  108   b  of the electromagnet (stator) makes contact with the laser gas and is therefore required to have corrosion resistance. Therefore, a permalloy (an Fe—Ni alloy comprising 30 to 80% of Ni) having good corrosion resistance against the laser gas was used for the core  108   b . Although a permalloy is used in this embodiment, a magnetic material other than permalloys, a surface of which (a surface making contact with the laser gas) has been subjected to an anticorrosion treatment such as nickel plating, may be used. 
     A right-hand electromagnet  10   b  and a left-hand electromagnet  110   c  of the axial magnetic bearing  110  are disposed so as to face each other and secured to the right-hand electromagnet housing  106   b  and the left-hand electromagnet housing  106   c , respectively, by welding. Coil wires  110   f ,  110   g  are inserted into coil recesses formed in the cores (iron cores) of the right-hand electromagnet  110   b  and the left-hand electromagnet  110   c , and separation walls  127 ,  127  in the forms of thin circular plates are secured by welding, so as to prevent the coil wires  110   f ,  110   g  from making contact with the laser gas. 
     An axial displacement sensor  110   a  is accommodated in the bearing cover  106   d . A separation wall  128  in the form of a thin circular plate is secured, by welding, to the surface of the sensor which otherwise would make contact with the laser gas. Thus, the axial displacement sensor  110   a  is disposed outside a sealed space communicated with the laser container  101 . 
     The right-hand electromagnet  110   b  and the left-hand electromagnet  110   c  make contact with the laser gas. Therefore, as a material for the cores (iron cores) of these electromagnets, use is made of a permalloy (an Fe—Ni alloy comprising 30 to 80% of Ni) having good corrosion resistance against a fluorine gas contained in the laser gas. As a material for the separation walls  127 ,  127 ,  128 , use is made of austenite type stainless steel having corrosion resistance against the laser gas. 
     On the other hand, a displacement sensor target  108   e  and a magnetic bearing rotor  108   f  of the radial magnetic bearing  108  are secured to the rotary shaft  104  of the circulation fan  103 , in a state such that they are disposed in relative positions determined by rotor spacers  125 ,  126 . Further, a displacement sensor target  110   d  and a magnetic bearing rotor  110   e  of the axial magnetic bearing  110  are secured to the rotary shaft  104  and disposed within the sealed space communicated with the laser container  101 . 
     As a magnetic material for the displacement sensor target  108   e  and the magnetic bearing rotor  108   f  of the radial magnetic bearing  108 , and the displacement sensor target  110   d  and the magnetic bearing rotor  110   e  of the axial magnetic bearing  110 , use is made of a permalloy (an Fe—Ni alloy comprising 30 to 80% of Ni) having good corrosion resistance against a fluorine gas contained in the laser gas. 
     In the displacement sensor target  108   e  and the magnetic bearing rotor  108   f , an eddy current is generated due to a change in magnetic field caused by rotation. In order to suppress an eddy current loss, each of the displacement sensor target  108   e  and the magnetic bearing rotor  108   f  is generally formed from a stack of thin plates. In this arrangement, however, the laser gas flow stagnates in a space between the stacked plates, resulting in contamination of the laser gas. Further, when a PB permalloy is used for the displacement sensor target  108   e  and the magnetic bearing rotor  108   f , there is a possibility that uniform, strongly-adhered Ni plating will not be able to be deposited on the surface of each plate. In order to avoid these problems, each of the displacement sensor target  108   e  and the magnetic bearing rotor  108   f  should be formed as an integral body made of a permalloy. With respect to the displacement sensor target  110   d  and the magnetic bearing rotor  110   e  of the axial magnetic bearing  110 , there is no change in magnetic field due to rotation. Therefore, each of the displacement sensor target  110   d  and the magnetic bearing rotor  110   e  was formed as an integral body of a permalloy. 
     As the protective bearing  112 , a rolling bearing was used. The protective bearing  112  comprises a rolling body  112   a  made of alumina ceramics, and an outer ring  112   c  and an inner ring  112   b , each made of stainless steel such as SUS440C. The protective bearing  112  is disposed within the sealed space communicated with the laser container  1 . Therefore, the rolling body  112   a , the outer ring  112   c  and the inner ring  112   b  are made of a material having corrosion resistance against the laser gas. Therefore, the protective bearing  112  in this embodiment is not deteriorated due to contact with the laser gas. 
     The protective bearing  112  is advantageous because the rolling body  112   a  is made of alumina ceramics and therefore the allowable rotation speed of the protective bearing  112  and the allowable load on the protective bearing  112  becomes large. The protective bearing  112  is formed from the above-mentioned materials. However, the rolling body  112   a  may be made of zirconia ceramics. The outer ring  112   c  and the inner ring  112   b  may be made of alumina ceramics or zirconia ceramics. 
     As shown in FIG. 7, the motor housing  107  comprises a bearing housing body  107   a , a motor housing body  107   b  and a bearing cover  107   c . The bearing housing body  107   a  is connected to the side wall of the laser container  101 . The motor housing body  107   b  is connected to the bearing housing body  107   a  and the bearing cover  107   c  is connected to the bearing housing body  107   b . Seal recesses  152 ,  154 ,  156  are provided at connecting surfaces of the bearing housing body  107   a , the motor housing body  107   b  and the bearing cover  107   c . Seal members  153 ,  155 ,  157  are attached to these recesses so as to prevent leakage of the laser gas. It is advantageous to use, as the seal members  153 ,  155 ,  157 , seal members made of a metal which does not produce moisture or other gases which contaminate the laser gas (such as stainless steel or aluminum). 
     Displacement sensors  109   a  and cores (iron cores)  109   b  of electromagnets (stators) of the radial magnetic bearing  109  are accommodated in the bearing housing body  107   a , in which they are disposed in relative positions determined by a spacer  141  and a side plate  142 . A projecting portion of the core  109   b  of the electromagnet (stator) of the radial magnetic bearing  109  is connected to a separation wall  145  comprising a plate type non-magnetic body. A detail of an arrangement of the radial magnetic bearing  109  is the same as that of the radial magnetic bearing  108  and therefore explanation thereof is omitted. 
     A motor stator  111   a  of the motor  101  and a side plate  143  are accommodated in the motor housing body  107   b . The separation wall  145  in the form of a thin-walled cylinder is provided on an inner circumferential surface of the motor stator  111   a , and opposite ends of the separation wall  145  are secured by welding, to thereby prevent the motor stator  111   a  from making contact with the laser gas. For the above-mentioned reasons, the separation wall  145  is made of austenite stainless steel. 
     On the other hand, a displacement sensor target  109   e  and a magnetic bearing rotor  109   f  of the radial magnetic bearing  109  and a motor rotor  111   b  of the motor  111  are secured to the rotary shaft  104  of the circulation fan  103 , in a state such that they are disposed in relative positions determined by rotor spacers  146 ,  147 , and are disposed within the sealed space communicated with the laser container  101 . As a magnetic material for the displacement sensor target  109   e  and the magnetic bearing rotor  109   f , as in the case of the displacement sensor target  108   e  and the magnetic bearing rotor  108   f  of the radial magnetic bearing  108 , use is made of a permalloy (an Fe—Ni alloy comprising 30 to 80% of Ni). 
     The motor rotor  111   b  of the motor  111  is made of a composite of a stack of silicon steel plates and aluminum. Therefore, in Ni plating which is a suitable anticorrosion treatment, a uniform, strongly-adhered Ni layer cannot be formed on the surface of the motor rotor. Therefore, a separation wall  148  is attached to an outer circumferential surface of the motor rotor  111   b  and secured to side plates  149 ,  150  by welding. The side plates  149 ,  150  are secured to the rotary shaft  104  of the circulation fan  103  by welding, to thereby form a sealed space for preventing the motor rotor from making contact with the laser gas. For the above-mentioned reasons, the separation wall  148  is made of austenite type stainless steel. 
     As in the case of the protective bearing  113  provided in the bearing housing  106 , the protective bearing  112  is a rolling bearing comprising the rolling body  112   a  made of alumina ceramics and the outer ring  112   c  and the inner ring  112   b , each made of stainless steel such as SUS 440C. 
     As shown in FIG. 8, the cores  108   b  of the electromagnets (stators) of the radial magnetic bearing  108  comprise U-shaped cores (iron cores), each having two projecting portions. A plurality of such cores  108   b  are arranged at substantially equal intervals in a circumferential direction around the rotor. Each projecting portion faces an outer circumferential surface of the magnetic bearing rotor  108   f . The coil wires  108   d  are attached to the two projecting portions of each core  108   b  so that one projecting portion forms an N-pole and the other projecting portion forms an S-pole. Thus, as indicated by arrows in FIG. 8, magnetic paths are formed in the cores  108   b  and the magnetic bearing rotor  108   f.    
     The projecting portions of the cores  108   b  are connected to a plate type non-magnetic body  124   a  and form the separation wall  124  which surrounds the magnetic bearing rotor  108   f . Thus, the magnetic bearing rotor  108   f  and the rotary shaft  104  are disposed within the sealed space communicated with the laser container  101 , while the coil wires  108   d  are disposed outside the sealed space. 
     Because the N-pole projecting portion and the S-pole projecting portion of the core  108   b  are connected through the non-magnetic body  124   a , there is no possibility of a magnetic short circuit being caused. A magnetic flux generated by energizing the coil wires  108   d  reliably passes through the magnetic bearing rotor  108   f , and effectively exerts a magnetic force on the magnetic bearing rotor  108   f.    
     Further, because the core  108   b  becomes part of the separation wall  124  forming the sealed space communicated with the laser container  101 , a magnetic gap in the magnetic path of the electromagnet of the radial magnetic bearing  108  can be made small. Therefore, a compact and highly efficient radial magnetic bearing can be easily obtained. 
     As shown in FIG. 9, the cores  108   b  of the electromagnets (stators) of the radial magnetic bearing  108  may comprise rodlike projecting cores  108   b - 1  and a base portion (connecting core)  108   b - 2  having a ring-shaped cross-section. Eight rodlike projecting cores  108   b - 1  are arranged at equal intervals in a substantially circumferential direction around the magnetic bearing rotor  108   f  so that one end of each projecting core  108   b - 1  faces the outer circumferential surface of the magnetic bearing rotor  108   f . The projecting cores are connected to the non-magnetic body  124   a  to thereby form the separation wall  124  surrounding the magnetic bearing rotor  108   f . Two adjacent projecting cores are taken as a pair, and the coil wires  108   d  are attached to the pair of projecting cores so that one projecting core forms an N-pole and the other projecting core forms an S-pole. The base portion  108   b - 2  magnetically connects the pair of adjacent projecting cores  108   b - 1 . 
     By this arrangement of the electromagnets of the radial magnetic bearing  108 , the rodlike projecting cores  108   b - 1  and the non-magnetic body  124   a  can be easily connected. That is, when the core  108   b  and the nonmagnetic body are connected by welding or soldering, there is a possibility of the coil wires  108   d  being damaged due to heat generated during a welding or soldering operation. This can be avoided in the above-mentioned arrangement, because the coil wires  108   d  are attached after the welding or soldering operation. This leads to easy manufacture and an increase in yield. Although the base portion  108   b - 2  has an integral body having a ring-shaped cross-section in this embodiment, it may be divided for each electromagnet. 
     As shown in FIG. 10, the electromagnets (stators) of the radial magnetic bearing  108  may comprise the U-shaped cores  108   b , each having two projecting portions, wherein the projecting portions face the outer circumferential surface of the magnetic bearing rotor  108   f  and are arranged at substantially equal intervals in a circumferential direction around the rotor. The coil wires  108   d  are attached to the electromagnet so that one of the two projecting portions forms an N-pole and the other projecting portion forms an S-pole. An end face member (a sheet-like iron core member)  108   c  is provided on the surface of each projecting portion facing the magnetic bearing rotor  108   f , and magnetic paths are formed in the cores  108   b , the end face members  108   c  and the magnetic bearing rotor  108   f  as indicated by arrows in FIG.  10 . 
     The end face members  108   c  are connected to the plate type non-magnetic body (isolating member)  124   a  and form the separation wall  124  surrounding the magnetic bearing rotor  108   f . Thus, the magnetic bearing rotor  108   f  and the rotary shaft  104  are disposed within the sealed space communicated with the laser container  101 , while the U-shaped cores  108   b  and the coil wires  108   d  are disposed outside the sealed space. 
     By this arrangement of the electromagnets of the radial magnetic bearing  108 , the U-shaped cores  108   b  do not make contact with the laser gas. Therefore, any magnetic material can be used for the cores  108   b . Therefore, the core  108   b  can be formed from a stack of silicon steel plates each having a thickness of 0.3 to 0.4 mm, resulting in high rigidity and high density of the magnetic bearing. Further, due to a stacked configuration of the core  108   b , an electric resistance in the core  108   b  can be increased, so that an eddy current loss in the magnetic bearing can be extremely suppressed. 
     As shown in FIG. 11, the cores  108   b  of the electromagnets of the radial magnetic bearing  108  may comprise a ring-shaped member having eight projecting portions extending inward so as to face the outer circumferential surface of the magnetic bearing rotor  108   f . The projecting portions are arranged at equal intervals in a circumferential direction around the rotor. The coil wires  108   d  are attached so that two adjacent projecting portions form an N-pole and an S-pole. That is, four electromagnets are arranged at substantially equal intervals in a circumferential direction around the magnetic bearing rotor  108   f.    
     In order to prevent formation of a magnetic short circuit between the electromagnets at the ring portion of the core, the coil wires  108   d  are attached in a manner such that two adjacent projecting portions of two adjacent electromagnets have the same polarity. The end face member  108   c  is provided in a manner such that it extends between and onto the surfaces of the two adjacent projecting portions having the same polarity. The end face members  108   c  are connected to the non-magnetic body (isolating member)  124   a , thus forming the separation wall  124  having a sealable structure which surrounds the magnetic bearing rotor  108   f.    
     By this arrangement of the electromagnets of the radial magnetic bearing  108 , the electromagnets of the radial magnetic bearing  108  can be easily arranged at substantially equal intervals in a circumferential direction around the rotor. Therefore, magnetic forces generated by the electromagnets are readily exerted uniformly along the entire circumference of the rotor. Further, the number of the end face members  108   c  can be reduced by half as compared to the arrangement of FIG.  10 . Therefore, the number of connecting portions between the non-magnetic body  124   a  and the end face members  108   c  is also reduced, resulting in easy manufacture of the separation wall  124  having a sealable structure. 
     The structure of the radial magnetic bearing  109  is the same as that shown in FIGS. 8 to  11 , and therefore explanation thereof is omitted.