Patent Publication Number: US-2017372864-A1

Title: X-ray tube device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation Application of PCT Application No. PCT/JP2016/052526, filed Jan. 28, 2016 and based upon and claiming the benefit of priority from Japanese Patent Application No. 2015-037843, filed Feb. 27, 2015, the entire contents of all of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an X-ray tube device. 
     BACKGROUND 
     In the fields of medical diagnosis devices and non-destructive testing, testing such as X-ray transmission image photographing and X-ray CT (Computed Tomography) which uses an X-ray tube device is widely conducted. 
     In recent years, in the field of CT, a technique of dual energy imaging has been gathering attention. The dual energy imaging is an imaging technique utilizing a variation in attenuation of a substance in accordance with the average energy of X rays. Depending on two different tube voltages (for example, 140 kV and 80 kV), tissues, for example, a bone, a contrast medium, fat, and a soft tissue exhibit differences in contrast which are dependent on tissue compositions, and thus, the tissues can be imaged so as to be appropriately separated from one another. One of necessary conditions for the dual energy images is application of a sufficient dose to a low energy side in such a manner that images taken with different levels of X-ray energy have equivalent image quality. With low energy, that is, with a low tube voltage, an electron emission surface of a filament has a low field intensity. Thus, in this case, a filament temperature at which the same tube current is obtained needs to be set higher than in the case of a high tube voltage. As a result, a problem occurs in which an operating temperature of the filament increases to shorten the life of the filament. As a method for improving a tube current when a single filament provides an insufficient tube current, a method has been disclosed in which two filaments are prepared and simultaneously operated to generate two electron beams so as to form one small focus on an anode target (for example, Patent Literatures 1 and 2). Utilization of this method as means for solving the above-described problem can easily be envisaged. 
     Literatures related to the above-described technique are listed below and the entire contents thereof are incorporated herein by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing an example of an X-ray tube device of a first embodiment. 
         FIG. 2A  is a cross-sectional view schematically showing an X-ray tube of the first embodiment. 
         FIG. 2B  is a cross-sectional view taken along an IIA-IIA line in  FIG. 2A . 
         FIG. 2C  is an enlarged view of a cathode of the first embodiment. 
         FIG. 2D  is a cross-sectional view taken along an IIB 1 -IIB 1  line in  FIG. 2B . 
         FIG. 3  is a cross-sectional view showing a principle of a quadrupole magnetic-field generator of the first embodiment. 
         FIG. 4A  is a cross-sectional view schematically showing an X-ray tube of a modification example 1 of the first embodiment. 
         FIG. 4B  is a diagram of a cathode of the modification example 1 of the first embodiment. 
         FIG. 4C  is a cross-sectional view taken along an IVA-IVA line in  FIG. 4A . 
         FIG. 5  is a diagram schematically showing an X-ray tube of a second embodiment. 
         FIG. 6A  is a diagram showing a principle of a dipole magnetic field of the second embodiment. 
         FIG. 6B  is a diagram showing a principle of a quadrupole magnetic-field generator of the second embodiment. 
         FIG. 7A  is a cross-sectional view schematically showing an X-ray tube of a modification example 2 of the second embodiment. 
         FIG. 7B  is a cross-sectional view taken along a VIIA 2 -VIIA 2  line in  FIG. 7A . 
         FIG. 7C  is a cross-sectional view taken along a VIIA 1 -VIIA 1  line in  FIG. 7A . 
         FIG. 8A  is a diagram showing a principle of a quadrupole magnetic field of the modification example 2 of the second embodiment. 
         FIG. 8B  is a diagram showing a principle of a dipole magnetic field of the modification example 2 of the second embodiment. 
         FIG. 8C  is a diagram showing a principle of a quadrupole magnetic-field generator of the modification example 2 of the second embodiment. 
         FIG. 9  is a cross-sectional view schematically showing an example of an X-ray tube device of a third embodiment. 
         FIG. 10A  is a cross-sectional view schematically showing an X-ray tube of the third embodiment. 
         FIG. 10B  is a cross-sectional view taken along an XA-XA line in  FIG. 10A . 
         FIG. 10C  is a cross-sectional view taken along an XB 1 -XB 1  line in  FIG. 10B . 
         FIG. 10D  is a cross-sectional view taken along an XB 2 -XB 2  line in  FIG. 10B . 
         FIG. 10E  is a cross-sectional view taken along an XD-XD line in  FIG. 10D . 
         FIG. 11A  is a diagram showing a principle of a quadrupole magnetic field of a third embodiment. 
         FIG. 11B  is a diagram showing a principle of a dipole of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an X-ray tube device comprises: an anode target comprising a target surface bombarded by electrons to generate X rays and a cathode comprising a plurality of electron generation sources configured to emit the electrons; a vacuum envelope configured to house the cathode and the anode target and internally sealed in a vacuum airtight manner; and a quadrupole magnetic-field generator configured to form a magnetic field by being supplied with a current from a power source, the quadrupole magnetic-field generator being installed on an outer side of the vacuum envelope and constituted of a quadrupole surrounding a periphery of electron orbits of the electrons emitted simultaneously from each of the plurality of electron generation sources. 
     An X-ray tube device according to embodiments will be described below in detail with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a cross-sectional view showing an example of an X-ray tube device  10  of a first embodiment. 
     As shown in  FIG. 1 , the X-ray tube device  10  of the first embodiment roughly includes a stator coil  8 , a housing  20 , an X-ray tube  30 , a high-voltage insulating member  39 , a quadrupole magnetic-field generator  60 , receptacles  301 ,  302 , and X-ray shielding portions  510 ,  520 ,  530 ,  540 . For example, the X-ray tube device  10  is a rotating anode-side X-ray tube device. The X-ray tube  30  is, for example, a rotating anode type X-ray tube. For example, the X-ray tube  30  is a neutral grounding type rotating anode type X-ray tube. Each of the X-ray shielding portions  510 ,  520 ,  530 , and  540  is formed of lead. 
     In the X-ray tube device  10 , insulating oil  9  that is a cooling liquid is stored in a space formed between an inner side of the housing  20  and an outer side of the X-ray tube  30 . For example, the X-ray tube device  10  is configured to circulate the insulating oil  9  using a circulative cooling system (cooler) (not shown in the drawings) connected to the housing  20  via a hose (not shown in the drawings). In this case, the housing  20  includes an introduction port and a discharge port for the insulating oil  9 . The circulative cooling system includes, for example, a cooler which radiates heat from the insulating oil  9  and which circulates the insulating oil  9  and conduits (hoses or the like) coupling the cooler to the introduction port and discharge port of the housing  20  in a liquid and air tight manner. The cooler has a circulating pump and a heat exchanger. The circulating pump discharges the insulating oil drawn from the housing  20  side to the heat exchanger, forming a flow of the insulating oil  9  in the housing  20 . The heat exchanger is coupled to between the housing  20  and the circulating pump to emit heat of the insulating oil to the outside. 
     A configuration of the X-ray tube device  10  will be described below with reference to the drawings. 
     The housing  20  is provided with a housing main body  20   e  formed like a tube and covers (side plates)  20   f,    20   g,    20   h.  The housing main body  20   e  and covers  20   f,    20   g,    20   h  are formed of casting using aluminum. If a resin material is used, metal may also be partly used for areas such as threaded parts which need strength, areas which are difficult to form by injection molding of resin, a shielding layer (not shown in the drawings) which prevents leakage of electromagnetic noise to the outside of the housing  20 , and the like. Here, a central axis passing through the center of circle of the cylinder of the housing main body  20   e  is referred to as a tube axis TA. 
     An annular step portion is formed in an opening portion of the housing main body  20   e  as an inner circumferential surface having a smaller thickness than the housing main body  20   e.  An annular groove portion is formed along the inner circumference of the step portion. The groove portion is formed, by machining, at a position located outward of the step of the step portion at a predetermined length therefrom along the tube axis TA. Here, the predetermined length is, for example, substantially equivalent to the thickness of the cover  20   f.  A C-type retaining ring  20   i  is fitted into the groove portion of the housing main body  20   e . That is, the opening portion of the housing main body  20   e  is occluded by the cover  20   f,  the C-type retaining ring  20   i,  and the like in a liquid-tight manner. 
     The cover  20   f  is shaped like a disc. The cover  20   f  is provided with a rubber member j 2   a  along an outer circumferential portion and fitted on the step portion formed in the opening portion of the housing main body  20   e.    
     The rubber member  2   a  is shaped, for example, like an O ring. As described above, the rubber member  2   a  is provided between the housing main body  20   e  and the cover  20   f  to provide a liquid tight seal between the housing main body  20   e  and the cover  20   f.  In a direction along the tube axis TA of the X-ray tube device, a peripheral portion of the cover  20   f  contacts the step portion of the housing main body  20   e.    
     The C-type retaining ring  20   i  is a fixing member. In order to stop the cover  20   f  from moving in a direction along the tube axis TA, the C-type retaining ring  20   i  is fitted into the groove portion of the housing main body  20   e  as described above to fix the cover  20   f.    
     The cover  20   g  and the cover  20   h  are fitted into an opening portion of the housing main body  20   e  opposite to the opening portion thereof where the cover  20   f  is installed. That is, the cover  20   g  and the cover  20   h  are installed at an end of the housing main body  20   e  opposite to the end thereof where the cover  20   f  is installed, so as to lie parallel and opposite to each other. The cover  20   g  is fitted at a predetermined inner position of the housing main body  20   e  and provided in a liquid-tight manner. At the end of the housing main body  20   e  where the cover  20   h  is installed, an annular groove portion is formed in an outer inner circumferential portion adjacent to the installation position of the cover  20   h.  A rubber member  2   b  is installed between the cover  20   g  and the cover  20   h  so as to maintain the liquid tightness in a stretchable manner. The cover  20   h  is provided outward of the cover  20   g  in the housing main body  20   e.  A C-type retaining ring  20   j  is fitted into the groove portion. That is, the opening portion of the housing main body  20   e  is occluded by the cover  20   g,  the cover  20   h,  the C-type retaining ring  20   j,  the rubber member  2   b,  and the like in a liquid tight manner. 
     The cover  20   g  is shaped like a circle having substantially the same diameter as that of the outer circumference of the housing main body  20   e.  The cover  20   g  is provided with an opening portion  20   k  through which the insulating oil  9  is injected and discharged. 
     The cover  20   h  is shaped like a circle having substantially the same diameter as that of the inner circumference of the housing main body  20   e.  The cover  20   h  is provided with a vent hole  20   m  through which air as atmosphere enters and exits. 
     The C-type retaining ring  20   j  is a fixing member which maintains a state where the cover  20   h  is compressed against a peripheral portion (seal portion) of the rubber member  2   b.    
     The rubber member  2   b  is a rubber bellows (rubber film). The rubber member  2   b  is shaped like a circle. Furthermore, the peripheral portion (seal portion) of the rubber member  2   b  is shaped like an O ring. The rubber member  2   b  is provided between the housing main body  20   e  and the cover  20   g  and the cover  20   h  to seal spaces between the housing main body  20   e  and the cover  20   g  and the cover  20   h  in a liquid-tight manner. The rubber member  2   b  is installed along an inner circumference of the end of the housing main body  20   e . That is, the rubber member  2   b  is provided to isolate a partial space in the housing. In the present embodiment, the rubber member  2   b  is installed in the space surrounded by the cover  20   g  and the cover  20   h  to separate the space into two parts in a liquid-tight manner. Here, the cover  20   g -side space is referred to as a first space, and the cover  20   h -side space is referred to as a second space. The first space is joined, via the opening  20   k,  to a space on the inner side the housing main body  20   e  which is filled with the insulating oil  9 . Thus, the first space is filled with the insulating oil  9 . The second space is joined to an external space via the vent hole  20   m.  Thus, the second space is an air atmosphere. 
     The housing main body  20   e  is provided with an opening  20   o  which partly penetrates the housing main body  20   e.  An X-ray radiation window  20   w  and the X-ray shielding portion  540  are installed in the opening  20   o . The opening portion  20   o  is occluded by the X-ray radiation window  20   w  and the X-ray shielding portion  540  in a liquid-tight manner. As described below in detail, the X-ray shielding portions  520  and  540  are installed to shield against X ray radiation to the outside of the housing  20  through the opening  20   o.    
     The X-ray radiation window  20   w  is formed of a member which allows X rays to pass through. For example, the X-ray radiation window  20   w  is formed of a metal which allows X rays to pass through. 
     The X-ray shielding portions  510 ,  520 ,  530 , and  540  may be formed of an X-ray transmission material containing at least lead and may be formed of a lead alloy or the like. 
     The X-ray shielding portion  510  is provided on an inner surface of the cover  20   g.  The X-ray shielding portion  510  shields against X rays radiated from the X-ray tube  30 . The X-ray shielding portion  510  is provided with a first shielding portion  511  and a second shielding portion  512 . The first shielding portion  511  is joined to an inner surface of the cover  20   g.  The first shielding portion  511  is installed so as to cover the entire inner surface of the cover  20   g . Furthermore, the second shielding portion  512  is installed in such a manner that a first end portion thereof is stacked on an inner surface of the first shielding portion  511 , and a second end portion thereof is arranged at a distance from the opening  20   k  toward the inner side of the housing main body  20   e  in a direction along the tube axis TA. That is, the second shielding portion  512  is installed in such a manner that the insulating oil  9  can flow in and out via the opening portion  20   k.    
     The X-ray shielding portion  520  is shaped generally like a cylinder. The X-ray shielding portion  520  is installed on a portion of the inner circumferential portion of the housing main body  20   e . A first end of the X-ray shielding portion  520  is in proximity to the first shielding portion  511 . This allows shielding against X rays which may exit through a gap between the X-ray shielding portion  510  and the X-ray shielding portion  520 . The X-ray shielding portion  520  is shaped like a tube and extends along the tube axis from the first shielding portion  511  to the vicinity of the stator coil  8 . In the present embodiment, the X-ray shielding portion  520  extends from the first shielding portion  511  to the front of the stator coil  8 . The X-ray shielding portion  520  is fixed to the housing  20  as needed. 
     The X-ray shielding portion  530  is shaped like a tube and fitted along an outer circumference of a receptacle  302  located inside the housing  20  and described below. The X-ray shielding portion  530  is provided in such a manner that a first end portion of the cylinder contacts a wall surface of the housing main body  20   e.  In this case, the X-ray shielding portion  520  is provided with a hole through which the first end portion of the X-ray shielding portion  530  is passed. The X-ray shielding portion  530  is fixed to an outer circumference of the receptacle  302  described below, as needed. 
     The X-ray shielding portion  540  is shaped like a frame and provided at a side edge of the opening portion  20   o  of the housing  20 . The X-ray shielding portion  540  is installed along an inner wall of the opening portion  20   o.  An end of the X-ray shielding portion  540  on the inner side of the housing main body  20   e  contacts the X-ray shielding portion  520 . The X-ray shielding portion  540  is fixed to the side edge of the opening portion  20   o  as needed. 
     The receptacle  301  for the anode and the receptacle  302  for the cathode are each connected to the housing main body  20   e.  Each of the receptacles  301 ,  302  is shaped like a bottomed tube provided with an opening portion. Each of the receptacles  301 ,  302  has a bottom portion installed inside the housing  20  and the opening portion is open toward the outer side. For example, the receptacles  301 ,  302  are installed at a predetermined distance from each other in the housing main body  20   e  in such a manner that the opening portions of the receptacles  301 ,  302  face the same direction. 
     A Plug (not shown in the drawings) which is inserted into the receptacle  301  and the receptacle  301  are of a non-surface-pressure type and are removably formed. With the plug coupled to the receptacle  301 , a high voltage (for example, +70 to +80 kV) is supplied to a terminal  201  through the plug. 
     The receptacle  301  is installed on the cover  20   f  side of the housing  20  and inward of the cover  20   f . The receptacle  301  has a housing  321  as an electric insulating member and the terminal  20  as a high-voltage supply terminal. 
     The housing  321  is formed of an insulating material, for example, resin. The housing  321  is shaped like a bottomed cylinder and has a plug slot which is open to the outer side. The housing  321  is provided with the terminal  201  at a bottom portion thereof. The housing  321  is provided with an annular protruding portion on an outer surface of an opening-side end of the housing  321 . The protruding portion of the housing  321  is formed to be fitted into a step portion  20   ea  which is a step formed at an end portion of a protruding portion of the housing main body  20   e . The terminal  201  is attached to the bottom portion of the housing  321  in a liquid-tight manner and penetrates the above-described bottom portion. A terminal  401  is connected to a high-voltage supply terminal  44  described below, via an insulating coated wire. 
     Furthermore, a rubber member  2   f  is provided between the protruding portion of the housing  321  and the housing main body  20   e.  The rubber member  2   f  is installed between the protruding portion of the housing  321  and a step portion of the step portion  20   ea  to provide a liquid tight seal between the protruding portion of the housing  321  and the housing main body  20   e.  In the present embodiment, the rubber member  2   f  is formed of an O ring. The rubber member  2   f  prevents leakage of the insulating oil  9  to the outside of the housing  20 . The rubber member  2   f  is formed of, for example, sulfur vulcanizable rubber. 
     The housing  321  is fixed by a ring nut  311 . The ring nut  311  is provided with a threaded groove in an outer circumferential portion thereof. For example, the outer circumferential portion of the ring nut  311  is machined into an external thread, and an inner circumferential portion of the step portion  20   ea  is machined into an internal thread. Therefore, screwing the ring nut  311  allows the protruding portion of the housing  321  to be pressed against the step portion  20   ea  via the rubber member  2   f.  As a result, the housing  321  is fixed to the housing main body  20   e.    
     The receptacle  302  is installed on the cover  20   g  side of the housing  20  and inward of the cover  20   g . The receptacle  302  is formed substantially equivalently to the receptacle  301 . The receptacle  302  has a housing  322  as an electric insulating member and a terminal  202  as a high-voltage supply terminal. 
     The housing  322  is formed of an insulating material, for example, resin. The housing  322  is shaped like a bottomed cylinder and has a plug slot which is open to the outer side. The housing  322  is provided with the terminal  201  at a bottom portion thereof. The housing  322  is provided with an annular protruding portion on an outer surface of an opening-side end of the housing  322 . The protruding portion of the housing  322  is formed to be fitted into a step portion  20   eb  which is a step formed at an end portion of a protruding portion of the housing main body  20   e . The terminal  202  is attached to the bottom portion of the housing  321  in a liquid-tight manner and penetrates the above-described bottom portion. The terminal  202  is connected to a high-voltage supply terminal  54  described below, via an insulating coated wire. 
     Furthermore, a rubber member  2   g  is provided between the protruding portion of the housing  322  and the housing main body  20   e.  The rubber member  2   g  is installed between the protruding portion of the housing  322  and a step portion of the step portion  20   eb  to provide a liquid tight seal between the protruding portion of the housing  321  and the housing main body  20   e.  In the present embodiment, the rubber member  2   g  is formed of an O ring. The rubber member  2   g  prevents leakage of the insulating oil  9  to the outside of the housing  20 . The rubber member  2   g  is formed of, for example, sulfur vulcanizable rubber. 
     The housing  322  is fixed by a ring nut  312 . The ring nut  312  is provided with a threaded groove in an outer circumferential portion thereof. For example, the outer circumferential portion of the ring nut  312  is machined into an external thread, and an inner circumferential portion of the step portion  20   eb  is machined into an internal thread. Therefore, screwing the ring nut  312  allows the protruding portion of the housing  322  to be pressed against the step portion  20   eb  via the rubber member  2   g.  As a result, the housing  322  is fixed to the housing main body  20   e.    
       FIG. 2A  is a cross-sectional view schematically showing the X-ray tube  30  of the first embodiment.  FIG. 2B  is a cross-sectional view taken along an IIA-IIA line in  FIG. 2A .  FIG. 2C  is an enlarged view of the cathode of the first embodiment.  FIG. 2D  is a cross-sectional view taken along an IIB-IIB line in  FIG. 2B . In  FIG. 2D , a straight line which is orthogonal to the tube axis TA is designated as a straight line L 1 , and a straight line which is orthogonal to the tube axis TA and the straight line L 1  is designated as a straight line L 2 . 
     The X-ray tube  30  is provided with a fixed shaft, a rotating body  12 , a bearing  13 , a rotor  14 , a vacuum envelope  31 , a vacuum container  32 , an anode target  35 , a cathode  36 , the high-voltage supply terminal  44 , and the high-voltage supply terminal  54 . 
     In  FIG. 2D , a straight line which is orthogonal to a straight line passing through the center of the cathode  36  and which is parallel to the straight line L 2  is designated as a straight line L 3 . 
     The fixed shaft  11  is shaped like a cylinder. The fixed shaft  11  rotatably supports the rotating body  12  via the bearing  13 . The fixed shaft is provided, at a first end thereof, with a protruding portion attached to the vacuum envelope  31  in a liquid-tight manner. The protruding portion of the fixed shaft  11  is fixed to the high-voltage insulating member  39 . In this case, a tip portion of the protruding portion of the fixed shaft  11  penetrates the high-voltage insulating member  39 . The high-voltage supply terminal  44  is electrically connected to the tip portion of the protruding portion of the fixed shaft  11 . 
     The rotating body  12  is shaped like a bottomed tube. The fixed shaft  11  is inserted into the rotating body.  12  so that the rotating body  12  is installed coaxially with the fixed shaft  11 . The rotating body  12  is connected to the anode target  35  described below at a bottom portion-side tip portion thereof and is provided so as to be rotatable along with the anode target  35 . 
     The bearing  13  is installed between an inner circumferential portion of the rotating body and an outer circumferential portion of the fixed shaft  11 . 
     The rotor  14  is provided so as to lie on an inner side of the stator coil  8  shaped like a cylinder. 
     The high-voltage supply terminal  44  applies a relatively positive voltage to the anode target  35  via the fixed shaft  11 , the bearing  13 , and the rotating body  12 . The high-voltage supply terminal  44  is connected to the receptacle  301  and supplied with a current when a high-voltage supply source such as a plug not shown in the drawings is connected to the receptacle  301 . The high-voltage supply terminal  44  is a metal terminal. 
     The anode target  35  is shaped like a disc. The anode target  35  is connected to the bottom portion-side tip portion of the rotating body  12  coaxially with the rotating body  12 . For example, the rotating body  12  and the anode target  35  are installed in such a manner that center axes thereof extend along the tube axis TA. That is, the axes of the rotating body  12  and the anode target  35  are parallel to the tube axis TA. In this case, the rotating body  12  and the anode target  35  are provided so as to be rotatable around the tube axis TA. 
     The anode target  35  has an umbrella-shaped target layer  35   a  provided in a portion of an outer surface of the anode target. The target layer  35   a  emits X rays by being bombarded by electrons emitted from the cathode  36 . An outer surface of the anode target  35  and a surface of the anode target  35  opposite to the target layer  35   a  are blackened. The anode target  35  is formed of a member which is a nonmagnetic substance and has a high electric conductivity (electric conduction property). For example, the anode target  35  is formed of copper, tungsten, molybdenum, niobium, tantalum, nonmagnetic stainless steel, or the like. The anode target  35  may be configured in such a manner that at least a surface portion thereof is formed of a metal member which is a nonmagnetic substance and which has a high electric conductivity. Alternatively, the anode target  35  may be configured in such a manner that the surface portion thereof is coated with a coating member formed of a metal member which is a nonmagnetic substance and which has a high electric conductivity. 
     When arranged in an AC magnetic field, the nonmagnetic substance allows lines of magnetic force resulting from the action of an opposite AC magnetic field based on an eddy current to be more intensively distorted in a case where the electric conductivity is high than in a case where the electric conductivity is low. Since the lines of magnetic force are thus distorted, the lines of magnetic force flow along a surface of the anode target  35  even if the quadrupole magnetic-field generator  60  described below is in proximity to the anode target  35  and the quadrupole magnetic-field generator  60  generates an AC magnetic field. Thus, the magnetic field (AC magnetic field) in the vicinity of the surface of the anode target  35  is intensified. 
     The cathode  36  is provided at a position opposed to the target layer  35   a.  The cathode  36  is installed at a predetermined distance from the surface of the anode target  35 . The cathode  36  emits electrons to the anode target  35 . For example, the cathode  36  is shaped like a cylinder and emits electrons to the surface of the anode target  35  through a filament provided at the center of the circle of the cylinder. In this case, a straight line passing through the center of the cathode  36  is parallel to the tube axis TA. The directions of electrons emitted from the cathode  36  and orbits of the electrons may hereinafter be described as electron orbits. A relatively negative voltage is applied Lo the cathode  36 . The cathode  36  is attached to a cathode support portion (cathode support body, cathode support member)  37  described below and connected to the high-voltage supply terminal  54  passing through the inside of the cathode support portion  37 . The cathode  36  may be referred to as an electron generation source. The center of the cathode  36  may hereinafter include a straight line passing through the center. 
     The cathode  36  is provided with a plurality of filaments (hereinafter referred to as filaments)  361   a ,  361   b,  a plurality of converging grooves (hereinafter referred to as converging grooves (converging groove portions))  362   a,    362   b,  and a plurality of converging surfaces (hereinafter referred to as converging surfaces)  363   a,    363   b.    
     When a negative high voltage is applied to each of the filaments  361   a  and  361   b,  the filament emits electrons (beams). For example, each of the filaments  361   a  and  361   b  is a filament for a small focus. Furthermore, each of the filaments  361   a,    361   b  is provided with a converging electrode around a periphery thereof to converge emitted electron beams. For example, as shown in  FIG. 2A , each of the filaments  361   a,    361   b  is shaped to be elongate in a direction perpendicular to the central axis of the cathode  36 , for example, shaped like a rectangle. Each of the filaments  361   a,    361   b  may be formed to have a circular shape, a square shape, or any other shape. Furthermore, each of the filaments  361   a,    361   b  may be a coil filament or a planar filament. 
     Each of the converging grooves  362   a,    362   b  is formed by hollowing out an anode target  35 -side portion of the cathode  36  into a rectangular groove. The converging grooves  362   a,    362   b  are obtained by forming the converging surfaces  363   a,    363   b  described below into recessed shapes. The converging grooves  362   a,    362   b  house the filaments  361   a,    361   b,  respectively. In this case, in the focusing grooves  362   a,    362   b,  each of the filaments  361   a,    361   b  is provided in the center of the corresponding groove, and a focusing electrode is installed along an inner circumference of the groove. 
     Each of the converging surfaces  363   a,    363   b  is an anode target  35 -side end face of the cathode  36  formed to allow the foci of a plurality of electron beams to overlap on the anode target  35 . For example, the converging surfaces  363   a,    363   b  are formed to incline symmetrically with respect to the central axis of the cathode  36 . In this case, the filaments  361   a,    361   b  and the converging grooves (converging groove portions)  362   a,    362   b  are provided symmetrically with respect to the central axis of the cathode  36 . The shapes and angles of the converging surfaces  363   a,    363   b  are changed as needed in accordance with a distance between the filaments  361   a,    361   b  and the anode target  35 , the size of the filaments  361   a,    361   b,  and the like. The converging surfaces  363   a,    363   b  are advantageous in terms of tube current characteristics, and are thus preferably set at as shallow an angle as possible with respect to a plane parallel to a surface (tip surface) of the cathode  36  opposed to the anode target  35 . 
     Here, the shallow angle of the converging surfaces  363   a,    363   b  indicates that, in  FIG. 2B  and  FIG. 2C , each of the converging surfaces  363   a,    363   b  is formed at an angle close to parallelism to the tip surface. Furthermore, the deep angle of the converging surfaces  363   a,    363   b  indicates that, in  FIG. 2B  and  FIG. 2C , each of the converging surfaces  363   a,    363   b  is formed at an angle close to parallelism to the central axis of the cathode  36 . 
     In  FIG. 2C , an emission angle which is an inclination angle from the central axis of the cathode  36  to the converging surface  363   a  is referred to as al, and an emission angle which is an inclination angle from the central axis of the cathode  36  to the converging surface  363   b  is referred to as α 2 . Each of the emission angles α 1  and α 2  is set to form the focus of a plurality of electron beams at a desired position with the action of a magnetic field from the quadrupole magnetic-field generator  60  taken into account. That is, the converging surfaces  363   a,    363   b  of the cathode  36  are formed at predetermined emission angles α 1  and α 2  so as to form a focus at the desired position. For example, the emission angles α 1  and α 2  are formed in such a manner that 45°&lt;α 1 &lt;90° and 45°&lt;α 2 &lt;90°. Suitably, the emission angles α 1  and α 2  are formed in such a manner that 50°&lt;α 1 &lt;70° and 50°&lt;α 2 &lt;70°. Such setting of the emission angles α 1  and α 2  is known to allow a plurality of electron beams to overlap without being enlarged. 
     Electron (emitted thermal electron) beams emitted from the filaments travel from the converging electrodes to the anode in circles. Thus, if the distance between the converging grooves  362   a,    362   b  and the anode target  35  is far, the angle of the inclined surface of each of the converging surfaces  363   a,    363   b  is shallow with respect to the plane parallel to the central axis (or a deep angle with respect to the central axis). If the distance between the converging grooves  362   a,    362   b  and the anode target  35  is near, the angle is deep with respect to the plane parallel to the central axis (or a shallow angle with respect to the central axis). On the other hand, the distance between the converging electrodes and the anode target  35  is set to a minimum distance needed to avoid high-voltage breakdown. In terms of avoidance of high-voltage breakdown, this distance is advantageously far. However, if the distance is far, the rate at which electron beams from the filaments arrive at the anode target  35  decreases, resulting in disadvantageous tube current characteristics (a prescribed tube current is not obtained without an extra increase in filament current, leading to a shortened life of the filaments). 
     The cathode support portion  37  has a first end portion provided with the cathode  36  and a second end portion connected to an inner wall of the vacuum envelope  31  (vacuum container  32 ). Furthermore, the cathode  36  is internally provided with the high-voltage supply terminal  54 . As shown in  FIG. 2A , the cathode support portion  37  extends from an inner wall surface of the vacuum envelope  31  (vacuum container  32 ) to a surface of the cathode  36  toward the anode target  35 . For example, the cathode support portion  37  is shaped like a cylinder and provided coaxially with the cathode  36 . In this case, the cathode support portion  37  has a first end face connected to a surface of the vacuum envelope  31  (vacuum container  32 ) and a second end face connected to the surface of the cathode  36 . 
     The cathode  36  is provided with a nonmagnetic-substance cover which covers the entire outer circumference. The nonmagnetic-substance cover is provided like a cylinder so as to enclose a periphery of the cathode  36 . The nonmagnetic-substance cover is formed of a nonmagnetic metal material such as one of copper, tungsten, molybdenum, niobium, tantalum, and nonmagnetic stainless steel, or a metal material the principal ingredient of which is one of these materials. Suitably, the nonmagnetic-substance cover is formed of a member with a high electric conductivity. When arranged in an AC magnetic field, the nonmagnetic-substance cover allows lines of magnetic force resulting from the action of the opposite AC magnetic field based on the eddy current to be more intensively distorted in the case where the electric conductivity is high than in the case where the electric conductivity is low. Since the lines of magnetic force are thus distorted, the lines of magnetic force flow along the periphery of the cathode  36  even if the quadrupole magnetic-field generator  60  described below is in proximity to the cathode  36  and the quadrupole magnetic-field generator  60  generates an AC magnetic field. Thus, the magnetic field (AC magnetic field) in the vicinity of the surface of the cathode  36  is intensified. At least a surface portion of the cathode  36  may be formed of a metal member which has a high electric conductivity and which is a nonmagnetic substance. 
     The high-voltage supply terminal  54  has a first end portion connected to the cathode  36  through the inside of the cathode support portion  37  and a second end portion connected to the receptacle  301 . The high-voltage supply terminal  54  supplies a current to the cathode  36  when a high-voltage supply source such as a plug is connected to the receptacle  302 . The high-voltage supply terminal  54  is a metal terminal. The high-voltage supply terminal  54  applies a relatively negative voltage to the cathode  36 , while supplying a filament current to the filaments (electron radiation source) of the cathode  36 , not shown in the drawings. 
     The vacuum envelope  31  is sealed in a vacuum atmosphere (vacuum airtight atmosphere) to internally house the fixed shaft  11 , the rotating body  12 , the bearing  13 , the rotor  14 , the vacuum container  32 , the anode target  35 , the cathode  36 , and the high-voltage supply terminal  54 . 
     The vacuum container  32  is provided with an X-ray transmission window  38  in a vacuum airtight manner. The X-ray transmission window  38  is provided in a wall portion of the vacuum envelope  31  (vacuum container  32 ) opposed to a target surface of the anode target  35  located between the cathode  36  and the anode target  35 . The X-ray transmission window  38  is formed of metal, for example, beryllium or titanium, stainless steel, and aluminum and provided in a portion of the vacuum container  32  which is opposed to the X-ray radiation window  20   w.  For example, the vacuum container  32  is hermetically occluded by the X-ray transmission window  38  formed of beryllium as a member which allows X rays to pass through. 
     In the vacuum envelope  31 , the high-voltage insulating member  39  is arranged from the high-voltage supply terminal  44  side to the periphery of the anode target  35 . The high-voltage insulating member  39  is formed of an electric insulating resin. 
     The vacuum envelope  31  (vacuum container  32 ) is provided with a housing portion  31   a  in which the cathode  36  is installed. The housing portion  31   a  is provided with a small diameter portion  31   b  in a portion thereof between the anode target  35  and the cathode  36  in such a manner that the small diameter portion  31   b  has a reduced diameter. For example, the housing portion  31   a  is shaped like a cylinder. The housing portion  31   a  is a portion of the vacuum envelope  31  and extends from the vicinity of the X-ray transmission window  38  toward the outer side of the X-ray tube  30  along the direction of a straight line parallel to the tube axis TA. Furthermore, the housing portion  31   a  is provided so as to be opposed to the surface of the anode target  35 . For example, as shown in  FIG. 2A , the housing portion  31   a  is provided so as to be opposed to the surface of a radial end of the anode target  35  and to extend from the vicinity of the X-ray transmission window  38  along the direction of a straight line parallel to the tube axis TA. 
     The small diameter portion  31   b  is provided to enhance the action of a magnetic field on a plurality of electron beams emitted from the cathode  36  when the quadrupole magnetic-field generator  60  is installed. The small diameter portion  31   b  is formed to have a smaller diameter than the peripheral housing portion  31   a.  As shown in  FIG. 2A  and  FIG. 2B , the small diameter portion  31   b  is formed between the anode target  35  and the cathode  36  so as to have a smaller diameter than the peripheral housing portion  31   a.  The small diameter portion  31   b  is provided so as to form the focus of a plurality of electron beams at the desired position. 
     Furthermore, the vacuum envelope  31  captures recoil electrons reflected from the anode target  35 . Thus, the vacuum envelope  31  is likely to have the temperature thereof raised by the bombardment of recoil electrons and is normally formed of a member such as copper which has a high heat conductivity. The vacuum envelope  31  is desirably constituted of a member which does not generate a diamagnetic field if the vacuum envelope  31  is affected by an AC magnetic field. For example, the vacuum envelope  31  is formed of a metal member which is a nonmagnetic substance. Suitably, the vacuum envelope  31  is formed of a high-voltage resist member which is a nonmagnetic substance so as to inhibit an overcurrent from being generated by an alternating current. The high-voltage resist member which is a nonmagnetic substance is, for example, nonmagnetic stainless steel, inconel, inconel X, titanium, conductive ceramics, or non-conductive ceramics the surface of which is coated with a metal thin film. 
     The high-voltage insulating member  39  is shaped like a ring having a first end shaped like a cone and a second end which is occluded. The high-voltage insulating member  39  is fixed directly or indirectly to the housing  20  via the stator coil  8  and the like. The high-voltage insulating member  39  electrically insulates the fixed shaft  11  from the housing  20  and the stator coil  8 . Thus, the high-voltage insulating member  39  is installed between the stator coil  8  and the fixed shaft  11 . That is, the high-voltage insulating member  39  is installed so as to internally house a side of the X-ray tube  30  (vacuum container  32 ) from which the fixed shaft  11  of the X-ray tube  30  protrudes. 
     Referring back to  FIG. 1 , the stator coil  8  is fixed to the housing at a plurality of positions. The stator coil  8  is installed so as to surround an outer circumferential portion of the rotor  14  and the high-voltage insulating member  39 . The stator coil  8  rotates the rotor  14 , the rotating body  12 , and the anode target  35 . A predetermined current is supplied to the stator coil  8  to generate a magnetic field provided to the rotor  14 , allowing the anode target  35  and the like to rotate at a predetermined speed. That is, when a current is supplied to the stator coil  8 , which is a rotational driving device, the rotor  14  rotates and the anode target  35  rotates in conjunction with the rotation of the rotor  14 . 
     A space inside the housing  20  which is surrounded by the rubber bellows  2   b,  the housing main body  20   e , the cover  20   f,  the receptacle  301 , and the receptacle  302  is filled with the insulating oil  9 . The insulating oil  9  absorbs at least a portion of heat generated by the X-ray tube  30 . 
     Referring back to  FIG. 2A  to  FIG. 2D , the quadrupole magnetic-field generator  60  will be described. 
     As shown in  FIG. 2B  and  FIG. 2D , the quadrupole magnetic-field generator  60  is provided with a coil  64  ( 64   a,    64   b,    64   c,  and  64   d ), a yoke  66 , and a magnetic pole  68  ( 68   a,    68   b,    68   c,  and  68   d ). 
     The quadrupole magnetic-field generator  60  generates a magnetic field by being supplied with a current from a power source. The quadrupole magnetic-field generator  60  can vary the intensity (magnetic flux density) of a magnetic field generated, the orientation of the magnetic field, and the like based on the intensity or direction of a supplied current, or the like. The quadrupole magnetic-field generator  60  is formed using four poles (or quadrupole) arranged close to one another in such a manner that the adjacent magnetic poles have different polarities. If two adjacent magnetic poles are considered to be one dipole and the remaining two magnetic poles are considered to be another dipole, magnetic fields generated by the two dipoles act in opposite directions. Therefore, the quadrupole magnetic-field generator  60  acts on the shape of each of a plurality of electron beams such as the width and height thereof based on a magnetic field generated. The “width” and “height” of an electron beam are lengths in directions which are both perpendicular to a straight line following an emission direction of each of a plurality of electron beams and which are orthogonal to each other, regardless of a spatial arrangement of the X-ray tube  30 . In the present embodiment, the quadrupole magnetic-field generator  60  has four magnetic poles  8  arranged in a square form. As described below in detail, in the quadrupole magnetic-field generator  60 , the magnetic poles  68   a,    68   b,    68   c,  and  68   d  are provided on the inner side of the yoke  66  so as to be opposed to one another. 
     The quadrupole magnetic-field generator  60  is installed in such a manner that the small diameter portion  31   b  is surrounded by an inner circumferential portion of the yoke  66  described below. The quadrupole magnetic-field generator  60  is eccentrically installed in such a manner that the center of the quadrupole magnetic-field generator  60  does not overlap the central axis of the cathode  36 . That is, the quadrupole magnetic-field generator  60  is installed in such a manner that a central position of the quadrupole magnetic-field generator  60  is displaced from (is eccentric to) the central axis of the cathode  36 . In this case, the center of the quadrupole magnetic-field generator  60  is substantially the same as the center of the yoke  66  formed by a hollow circle or polygon and described below. For example, as shown in  FIG. 2D , the quadrupole magnetic-field generator  60  is installed at a position resulting from movement from a central position of the cathode  36  toward a central position of the anode target  35  in a radial direction (or along the straight line L 1 ). The quadrupole magnetic-field generator  60  may be installed perpendicularly eccentrically to the central axis of the cathode  36  unlike in the description above. Furthermore, the quadrupole magnetic-field generator  60  is installed in association with the emission angles of the above-described converging surfaces  363   a,    363   b  in order to form the focus of a plurality of electron beams at a desired position. In order to form the focus of a plurality of electron beams at the desired position, the quadrupole magnetic-field generator  60  varies the intensity (magnetic flux density) of a magnetic field generated, the orientation of the magnetic field, and the like based on the intensity or direction of a supplied current, or the like in association with the above-described angle. 
     The coil  64  is supplied with a current from the power source (not shown in the drawings) for the quadrupole magnetic-field generator  60  to generate a magnetic field. For example, the coil  64  is an electromagnetic coil. In the present embodiment, the coil  64  is supplied with a direct current from the power source (not shown in the drawings). The coil  64  is provided with a plurality of coils  64   a,    64   b,    64   c , and  64   d.  Each of the coils  64   a  to  64   d  is wound around a portion of a corresponding one of the magnetic poles  68   a,    68   b,    68   c,  and  68   d  described below. 
     The yoke  66  is shaped like a hollow polygon or a hollow cylinder. The yoke  66  is formed of, for example, a high electric resistor which is a soft magnetic substance and which is unlikely to generate an eddy current in spite of an AC magnetic field. The yoke  66  is formed of, for example, a laminate obtained by laminating thin plates of an Fe—Si alloy (silicon steel), an Fe—Al alloy, electromagnetic stainless steel, an Fe—Ni high-magnetic-permeability stainless steel such as permalloy, an Ni—Cr alloy, an Fe—Ni—Cr alloy, an Fe—Ni—Co alloy, an Fe—Cr alloy, or the like in such a manner that electric insulating films are sandwiched between the thin plates, or an aggregate obtained by covering wire materials of any of the above-described materials and bundling and binding the resultant wire materials. Alternatively, the yoke  66  may be formed of a compact obtained by forming any of the above-described materials into fine power of approximately 1 μm, covering a surface of the powder with an electric insulating film, and then performing compression molding on the resultant powder. Moreover, the yoke  66  may be formed of soft ferrite or the like. 
     The magnetic poles  68  are provided with the plurality of magnetic poles  68   a,    68   b,    68   c,  and  68   d . The magnetic poles  68   a,    68   b,    68   c,  and  68   d  are each provided on an inner circumferential wall of the yoke  66 . The magnetic poles  68   a  to  68   d  are arranged to surround electron orbits of a plurality of electron beams around the small diameter portion  31   b.  For example, in the quadrupole magnetic-field generator  60 , the magnetic poles  68   a  to  68   d  are evenly arranged around the central axis of the cathode  36  at positions perpendicular to the central axis. As shown in  FIG. 2D , that is, the magnetic poles  68   a  to  68   d  are installed so as to be arranged at positions of vertices of a square. Suitably, in order to increase magnetic flux density, the magnetic poles  68   a  to  68   d  are installed close to emission directions (electron orbits) of electrons emitted from the filaments  361   a  and  361   b.    
     The magnetic poles  68   a  to  68   d  are formed to have substantially the same shape. The magnetic poles  68   a  to  68   d  include two dipoles each forming a pair. For example, the magnetic pole  68   a  and the magnetic pole  68   b  are a dipole (magnetic pole pair  68   a,    68   b ), and the magnetic pole  68   c  and the magnetic pole  68   d  are a dipole (magnetic pole pair  68   c,    68   d ). In this case, if a direct current is supplied to the magnetic poles  68  via the coils  64  ( 64   a,    64   b,    64   c,  and  64   d ), the magnetic pole pair  68   a,    68   b  and the magnetic pole pair  68   c ,  68   d  form opposed DC magnetic fields. The magnetic poles  68   a  to  68   d  are each installed to face a surface (end face) where a magnetic field is generated with respect to the electron orbits of electron beams in order to deform the shapes of electron beams emitted from the cathode  36 . 
     The principle of the quadrupole magnetic-field generator  60  of the present embodiment will be described below with reference to the drawings. 
       FIG. 3  is a diagram showing the principle of the quadrupole magnetic-field generator of the present embodiment. In  FIG. 3 , an X direction and a Y direction are directions perpendicular to the direction in which electron beams are emitted, and are orthogonal to each other. Furthermore, the X direction is a direction extending from the magnetic pole  38   b  (magnetic pole  68   a ) side toward the magnetic pole  68   d  (magnetic pole  68   c ) side, and the Y direction is a direction extending from the magnetic pole  38   d  (magnetic pole  68   b ) side toward the magnetic pole  68   c  (magnetic pole  68   a ) side. 
     In  FIG. 3 , an electron beam BM 1  emitted from the filament  361   a  and an electron beam BM 2  emitted from the filament  361   b  are assumed to travel from a side closer to the reader toward a side farther from the reader in the drawing. The electron beam BM 1  and the electron beam BM 2  are each assumed to be emitted in a circle. Furthermore, in  FIG. 3 , the magnetic pole  68   a  generates an N-pole magnetic field, the magnetic pole  68   b  generates an S-pole magnetic field, the magnetic pole  68   c  generates an S-pole magnetic field, and the magnetic pole  68   d  generates an N-pole magnetic field. In such a case, magnetic fields traveling from the magnetic pole  68   a  to the magnetic poles  68   c  and  68   d  and magnetic fields traveling from the magnetic pole  68   d  to the magnetic poles  68   c  and  68   b  are formed. When assumed to pass through the center of a space surrounded by the magnetic poles  68   a  to  68   d,  the electron beam BM 1  and the electron beam BM 2  are moved (polarized) toward each other in the X direction by a Lorentz force of the generated magnetic fields and moved (polarized) in a given direction. In the present embodiment, the quadrupole magnetic-field generator  60  is installed in such a manner that a central position thereof is eccentric to the central axis of the cathode  36  in the radial direction (or the Y direction) of the anode target  35 . Thus, when assumed to pass through the center of the space surrounded by the magnetic poles  68   a  to  68   d,  the electron beam BM 1  and the electron beam BM 2  are significantly subjected to the action of a Lorentz force in opposed directions along the X direction and a Lorentz force applied in one direction along the Y direction. 
     For example, as shown in  FIG. 3 , the electron beam BM 1  and the electron beam BM 2  pass through electron orbits which are symmetric with respect to the central position of the quadrupole magnetic-field generator  60  in the X direction. In this case, the electron beam BM 1  and the electron beam BM 2  are significantly subjected to the action of Lorentz forces applied toward the center of the quadrupole magnetic-field generator  60  in the X direction and Lorentz forces applied in a direction opposite to the direction toward the center of the quadrupole magnetic-field generator  60  along the Y direction. That is, the quadrupole magnetic-field generator  60  varies a position with respect to electron beams emitted from the cathode  36  to vary the intensity of the action of magnetic fields acting on each of the electron beam BM 1  and the electron beam BM 2 . The electron beam BM 1  is significantly subjected to the action of magnetic fields from the magnetic poles  68   a  and  68   b  located in proximity to the electron beam BM 1  in the X direction, and the electron beam BM 2  is significantly subjected to the action of magnetic fields from the magnetic poles  68   c  and  68   d  located in proximity to the electron beam BM 2  in the X direction. As a result, as shown in  FIG. 3 , the electron beam BM 1  and the electron beam BM 2  are polarized in a direction in which the electron beams BM 1  and BM 2  approach each other, with the lengths of the electron beams BM 1  and BM 2  not substantially deformed in the Y direction, and the electron beam BM 1  and the electron beam BM 2  are also polarized in a direction opposite to a direction toward the center of the quadrupole magnetic-field generator  60  in the Y direction. At this time, the electron beam BM 1  and the electron beam BM 2  form a focus at a position resulting from movement on the electron orbit in the radial direction of the anode target  35  with respect to a focus on the anode target  35  formed when no magnetic field acts (a position displaced on the electron orbit in the radial direction of the anode target  35  with respect to the focus on the anode target  35  formed when no magnetic field acts). The intensity of a current supplied to the quadrupole magnetic-field generator  60  is adjusted to allow the quadrupole magnetic-field generator  60  to synthesize the electron beam BM 1  and the electron beam BM 2  and to freely vary a width dimension of a focus resulting from the synthesis (the length of the focus of the beams in a direction perpendicular to a length direction of the focus), with the length dimension of the focus (the length of the focus of the beams extending in the radial direction of the anode target  35 ) maintained. 
     In the present embodiment, if the X-ray tube device  1  is driven, the filaments  361   a  and  361   b  emit the electron beam BM 1  and the electron beam BM 2  toward the focus on the anode target  35  bombarded by electrons. Here, the filaments  361   a  and  362   b  emit electrons (beams) substantially perpendicularly to emission angles α 1  and α 2  of the converging surfaces  363   a  and  363   b.  The plurality of emitted electron beams BM 1  and BM 2  travel to the anode target  35  in parallel. In the quadrupole magnetic-field generator  60 , each of the coils  64  (coils  64   a  to  64   d ) is supplied with a direct current from the power source not shown in the drawings. When a direct current is supplied, the quadrupole magnetic-field generator  60  generates magnetic fields among the magnetic poles  68   a  to  68   d , which are a quadrupole. The plurality of electron beams BM 1  and BM 2  emitted from the cathode  36  passes through magnetic fields generated between the cathode  36  and the anode target  35  and are bombarded on the anode target  35 . Since the quadrupole magnetic-field generator  60  is installed in such a manner that the central position thereof is eccentric in the radial direction of the anode target  35 , the electron beams BM 1  and BM 2  are subjected by the action of magnetic fields from the quadrupole magnetic-field generator  60  to Lorentz forces focused on the center in the X direction and Lorentz forces in a direction opposite to a central direction of the quadrupole magnetic-field generator  60  along the Y direction as shown in  FIG. 3 . At this time, the plurality of electron beams BM 1  and BM 2  is focused by magnetic fields generated by the quadrupole magnetic-field generator  60  to form one synthetic focus in such a manner that the synthetic focus forms a desired width dimension. 
     In the present embodiment, the quadrupole magnetic-field generator  60  is installed in such a manner that the central position thereof is eccentric in the radial direction of the anode target  35 . Thus, the quadrupole magnetic-field generator  60  makes the beam width of each of the plurality of electron beams thinner than in a case where the action of magnetic fields from the quadrupole magnetic-field generator  60  is not provided, and applies such Lorentz forces as focus the plurality of electron beams BM 1  and BM 2  into one electron beam. Furthermore, the quadrupole magnetic-field generator  60  can polarize the plurality of electron beams BM 1  and BM 2  in a predetermined direction. For example, as shown in  FIG. 3 , the quadrupole magnetic-field generator  60  deform the plurality of electron beams emitted in circles by the Lorentz forces of magnetic fields into elliptic shapes and polarize the electron beams BM 1  and BM 2  in a direction along the X direction in which the electron beams BM 1  and BM 2  approach each other. Moreover, the quadrupole magnetic-field generator  60  can polarize each of the plurality of electron beams BM 1  and BM 2  in the direction opposite to the direction of the center of the anode target  35  along the Y direction (the radial direction of the anode target  35 ). In this case, the intensity of the magnetic fields may be adjusted so as to correct focus misalignment resulting from an assembly error in each tube or focus misalignment resulting from a variation in tube voltage. Furthermore, the above-described focus misalignment may be regulated by the angles of the emission angles α 1  and α 2  of the converging surfaces  363   a  and  363   b  of the cathode  36 , the installation position of the quadrupole magnetic-field generator  60 , or the like. 
     According to the present embodiment, the X-ray tube device  1  is provided with the X-ray tube provided with the cathode  36  having the plurality of filaments and the quadrupole magnetic-field generator  60  configured to focus a plurality of electron beams to form a synthetic focus at a desired position in a desired shape. The quadrupole magnetic-field generator  60  is installed so as to form a synthetic focus at the desired position in the desired shape. Furthermore, the quadrupole magnetic-field generator  60  forms magnetic fields among the magnetic poles  68   a  to  68   d  as a result of the supply of a direct current from the power source not shown in the drawings to the coils  64 . At this time, in the quadrupole magnetic-field generator  60 , the current is regulated so as to form a focus at the desired position in the desired shape. Therefore, the X-ray tube device  1  of the present embodiment enables electron beams to overlap accurately on the anode target. As a result, the X-ray tube device of the present embodiment can obtain an X-ray focus having a higher X-ray radiation intensity than an X-ray tube device having the same size as that in the conventional art and forming a focus using conventional small-focus filaments. 
     Furthermore, the X-ray tube device  1  of the present embodiment can superimpose the electron beams BM 1  and BM 2  emitted from each of the plurality of filaments  361   a  and  361   b  and deform the beam shape of each of the electron beams BM 1  and BM 2 . Therefore, the X-ray tube device  1  can obtain a synthetic focus having an optimal size and an optimal X-ray radiation intensity according to the purpose of photographing and photographing conditions. 
     A modification example of the present embodiment will be described below with reference to the drawings. 
     The X-ray tube device  1  of the modification example has a configuration substantially equivalent to the configuration of the X-ray tube device  1  of the first embodiment. Thus, the same components of the X-ray tube device  1  of the modification example as the corresponding components of the X-ray tube device of the first embodiment are denoted by the same reference numerals, and detailed description of these components is omitted. 
     Modification Example 1 
     The X-ray tube device  1  of a modification example 1 of the first embodiment is provided with an additional filament in addition to the configuration of the first embodiment. 
       FIG. 4A  is a cross-sectional view schematically showing an X-ray tube of the modification example 1 of the first embodiment.  FIG. 4B  is a diagram of a cathode of the modification example 1 of the first embodiment.  FIG. 4C  is a cross-sectional view taken along an IVA-IVA line in  FIG. 4A . 
     The cathode of the modification example is provided with a filament  361   c,  a converging groove  362   c,  and a converging surface  363   c.  Here, the filament  361   c  is provided between the above-described filament  361   a  and the filament  361   b  so as to be opposed to the anode target  35 . In the present embodiment, the cathode  36  simultaneously emits all electron beams, but filaments that emit electron beams can adjustably be selected from a plurality of installed filaments. 
     When a negative high voltage is applied to the filament  361   c,  the filament  361   c  emits electrons (beams). For example, the filament  361   c  is a filament for a large focus. Furthermore, each of the filaments  361   a  and  361   b  is provided with a converging electrode configured to converge electron beams emitted to surroundings. For example, like the filaments  361   a  and  361   b,  the filament  361   c  is shaped to be thin in a direction perpendicular to the central axis of the cathode  36 , for example, shaped like a rectangle. 
     The converging groove (converging groove portion)  362   c  is formed by hollowing out a portion of the anode target  35  side of the cathode  36  into a rectangular groove. The converging groove  362   c  has the converging surface  363   c  described below and shaped like a recessed portion. The converging groove  362   c  houses the filament  361   c.  For example, the focusing groove portion  362   c  is provided with the filament  361   c  in the center of the groove and a converging electrode along an inner circumferential portion of the groove. 
     The converging surface  363   c  is an end face provided between the converging surface  363   a  and the converging surface  363   b  so as to lie parallel and opposite to the anode target  35 . In this case, the converging surface  363   a  and the converging surface  363   b  are each formed to incline at a predetermined angle from an end of the converging groove  362   c  to a side portion of the cathode  36 . For example, the converging surface  363   c  has a central axis formed to coincide with the central axis of the cathode  36 . In this case, the converging surfaces  363   a  and  363   b  are formed to incline symmetrically with respect to the central axis of the cathode  36 . The filaments  361   a  and  361   b  are provided symmetrically with respect to the central axis of the cathode  36 , and the converging grooves (converging groove portions)  362   a  and  362   b  are provided symmetrically with respect to the central axis of the cathode  36 . The shapes and angles of the converging surfaces  363   a,    363   b,  and  363   c  are polarized as needed according to distances between each of the filaments  361   a,    361   b,  and  361   c  and the anode target  35  and the sizes of the filaments  361   a,    361   b,  and  361   c.  The converging surfaces  363   a  and  363   b  are preferably set at as shallow an angle as possible with respect to a flat surface parallel to a surface (tip surface) opposed to the anode target  35  of the cathode  36 , for example, the converging surface  363   c.    
     Here, the converging surfaces  363   a  and  363   b  with shallow angles indicate that, in  FIG. 4A  and  FIG. 4B , the converging surfaces  363   a  and  363   b  are formed at an angle close to parallelism to the converging surface  363   c.  Furthermore, the converging surfaces  363   a  and  363   b  with shallow angles indicate that, in  FIG. 4A  and  FIG. 4B , the angle is close to parallelism to the central axis of the cathode  36  or the orbits of electron beams from the filament  361   c.    
     In  FIG. 4B , an emission angle which is an inclination angle from the central axis of the cathode  36  or the orbits of electron beams from the filament  361   c,  to the converging surface  363   c,  is denoted as α 3 , and an emission angle which is an inclination angle from the central axis of the cathode  36  or the orbits of electron beams from the filament  361   c,  to the converging surface  363   b,  is denoted as α 4 . The emission angles α 3  and α 4  are set so as to allow the focus of a plurality of electron beams to be formed at a desired position with the action of magnetic fields from the quadrupole magnetic-field generator  60  described below taken into account. That is, the converging surfaces  363   a  and  363   b  of the cathode  36  are formed at predetermined emission angles α 3  and α 4  so as to generate a focus at a desired position. For example, the emission angles α 3  and α 4  are formed in such a manner that 45°&lt;α 3 &lt;90° and 45°&lt;α 4 &lt;90°. The emission angles α 3  and α 4  are formed in such a manner that 50°&lt;α 3 &lt;70° and 50°&lt;α 4 &lt;70°. Such setting of the emission angles α 3  and α 4  is known to allow a plurality of electron beams to overlap without being enlarged. 
     If a filament for a large focus and two filaments for a small focus are provided, it is important that the filament for a large focus and the corresponding converging electrode be provided in a central portion of the cathode main body of the cathode and at a deep position in a depth direction of the most recessed portion. That is, experiments confirm that, if not provided between the above-described filament for a large focus and each of the above-described filaments for a small focus, electron (thermal electron) beams radiated from the two filaments for a small focus fail to overlap reliably at the focal position on the anode target under the effect of electric fields from the converging electrode covering the periphery of the filament for a large focus and the remaining converging electrode (which covers the filaments for a small focus). 
     The quadrupole magnetic-field generator  60  is installed in such a manner that the small diameter portion  31   b  is surrounded by an inner circumferential portion of the yoke  66  described below. In the present embodiment, the quadrupole magnetic-field generator  60  is installed substantially coaxially with the central axis of the cathode  36 . 
     According to the modification example  1  of the present embodiment, the X-ray tube device  1  is provided with the three filaments so that the filament which emits electron beams can be optionally selected. Therefore, in the X-ray tube device  1  of the modification example  1 , electron beams emitted from at least two filaments are regulated by the quadrupole magnetic-field generator  60  to allow formation of a focus which has a size larger than the size in the case of the cathode  36  of the first embodiment and which provides a high loading capability. Furthermore, the X-ray tube device  1  is provided with the three filaments but may be provided with at least two filaments. 
     The quadrupole magnetic-field generator  60  of the modification example  1  is installed coaxially with the central axis of the cathode, but may be eccentrically installed in such a manner that the center of the quadrupole magnetic-field generator  60  does not overlap the central axis of the cathode. 
     Now, an X-ray tube device according to another embodiment will be described. The same components of the another embodiment as the corresponding components of the above-described first embodiment are denoted by the same reference numerals, and detailed description of these components is omitted. 
     Second Embodiment 
     The X-ray tube device  1  of a second embodiment is further provided with a coil configured to polarize electron beams in addition to the configuration of the first embodiment. 
       FIG. 5  is a diagram schematically showing the X-ray tube device of the second embodiment. 
     As shown in  FIG. 5 , the quadrupole magnetic-field generator  60  of the second embodiment is further provided with polarizing coil portions  69   a,    69   b.    
     The quadrupole magnetic-field generator  60  generates superimposed dipole DC magnetic fields in such a manner that magnetic fields generated from two pairs of magnetic poles act in the same direction. The quadrupole magnetic-field generator  60  is provided with a pair of magnetic poles  68   a  and  68   c  and a pair of magnetic poles  68   b  and  68   d.  The magnetic pole pair  68   a,    68   c  and the magnetic pole pair  68   b,    68   d  each act as a dipole to form magnetic fields. In the quadrupole magnetic-field generator  60 , each of the polarizing coil portions  69   a,    69   b  described below are supplied with a current to form a magnetic field by superimposing a DC magnetic field on a DC magnetic field generated between the magnetic pole pair  68   a,    68   c  and the magnetic pole pair  68   b,    68   d.    
     In the quadrupole magnetic-field generator  60 , a DC current supplied to each of the polarizing coil portions  69   a,    69   b  described below by the power source (not shown in the drawings) is controlled by a polarizing power source control portion (not shown in the drawings). By installing the quadrupole magnetic-field generator  60  in such a manner that the center thereof is perpendicularly eccentric to the central axis of the cathode  36 , electron beams in a desired direction can be deformed and polarized. For example, as shown in  FIG. 5 , the quadrupole magnetic-field generator  60  can deform electron beams emitted from the cathode  36  so as to reduce the width of each electron beam and can correct, by polarization, movement in the radial direction associated with the deformation of the width. That is, the quadrupole magnetic-field generator  60  can adjust the position of the focus on the surface of the anode target  35  bombarded by electron beams and reduce a thermal load on the focus. 
     The polarizing coil portions  69   a,    69   b  (a first polarizing coil portion, a second polarizing coil portion) are electromagnetic coils to which a current is supplied by the power source (not shown in the drawings) and which generate magnetic fields. In the present embodiment, the polarizing coil portions  69   a ,  69   b  are supplied with a direct current from the power source (not shown in the drawings) to generate DC magnetic fields. Currents supplied to the polarizing coil portions  69   a,    69   b  allow the polarizing coil portions  69   a,    69   b  to polarize the orbits of electron beams in a predetermined direction. Each of the polarizing coil portions  69   a,    69   b  are wound between any two of the magnetic poles  68   a  to  68   d  connected to the yoke  66 . As shown in  FIG. 4 , the polarizing coil portion  69   a  is wound around the main body portion of the yoke  66  between the magnetic poles  68   a  and  68   c . The polarizing coil portion  69   b  is wound around the main body portion of the yoke  66  between the magnetic poles  68   b  and  68   d.  In this case, the magnetic pole pair  68   a,    68   c  generates a DC magnetic field between the magnetic poles  68   a  and  68   c,  and the magnetic pole pair  68   b,    68   d  generates a DC magnetic field between the magnetic poles  68   b  and  68   d.    
     The principle of the quadrupole magnetic-field generator  60  of the present embodiment will be described below with reference to the drawings. 
       FIG. 6A  is a diagram showing the principle of dipole magnetic fields of the second embodiment, and  FIG. 6B  is a diagram showing the principle of the quadrupole magnetic-field generator  60  of the second embodiment. In  FIG. 6A  and  FIG. 6B , the X direction and the Y direction are directions perpendicular to the direction in which electron beams are emitted, and are orthogonal to each other. Furthermore, the X direction is a direction extending from the magnetic pole  68   b  (magnetic pole  68   a ) side toward the magnetic poles  68   d  (magnetic pole  68   c ) side, and the Y direction is a direction extending from the magnetic pole  68   d  (magnetic pole  68   b ) side toward the magnetic poles  68   c  (magnetic pole  68   a ) side. 
     In  FIG. 6A  and  FIG. 6B , the electron beam BM 1  emitted from the filament  361   a  and the electron beam BM are assumed to travel from the side closer to the reader toward the side farther from the reader in the drawing. Furthermore, in  FIG. 6A  and  FIG. 6B , the magnetic poles  68   a  and  68   c  are a dipole forming a pair (magnetic pole pair), and the magnetic poles  68   b  and  68   d  are a dipole forming a pair (magnetic pole pair). The magnetic pole pair  68   a,    68   c  generates a DC magnetic field traveling in a direction following the X direction, and the magnetic pole pair  68   b,    68   d  generates a DC magnetic field following the X direction. Here, if not subjected to the action of the polarizing coil portions  69   a,    69   b,  the quadrupole magnetic-field generator  60  generates magnetic fields as shown in  FIG. 3  for the first embodiment. 
     As shown in  FIG. 6A , the polarizing coil portion  69   a  generates an N-pole magnetic field at the magnetic pole  68   a  and generates an S-pole magnetic field at the magnetic pole  68   c.  Similarly, the polarizing coil portion  69   b  generates an N-pole magnetic field at the magnetic pole  68   b  and generates an S-pole magnetic field at the magnetic pole  68   d.  Therefore, the magnetic field traveling from the magnetic pole  68   a  toward the magnetic pole  68   c  and the magnetic field traveling from the magnetic pole  68   b  toward the magnetic pole  68   d  are formed by the polarizing coil portion  69   a  and the polarizing coil portion  69   b , respectively. 
     The quadrupole magnetic-field generator  60  is subjected to the action of magnetic fields from the polarizing coil portions  69   a,    69   b  as shown in  FIG. 6A  to superimpose a magnetic field generated by the polarizing coil portion  69   a  on a magnetic field traveling from the magnetic pole  68   a  toward the magnetic pole  68   c,  while superimposing a magnetic field generated by the polarizing coil portion  69   b  on a magnetic field traveling from the magnetic pole  68   d  toward the magnetic pole  68   b.  Therefore, as shown in  FIG. 6B , the quadrupole magnetic-field generator  60  generates superimposed magnetic fields traveling from the magnetic pole  68   a  toward the magnetic pole  68   c  in addition to magnetic fields from the quadrupole. Here, the magnetic fields between the magnetic poles  68   b  and the magnetic pole  68   d  cancel each other. 
     In the present embodiment, when the X-ray tube device  1  is driven, the filament  361   a  and filament  361   c  of the cathode  36  emit electrons toward electrons on the anode target  35 . In the quadrupole magnetic-field generator  60 , the polarizing coil portions  69   a,    69   b  are supplied with a direct current from the power source not shown in the drawings. For example, when a direct current from the power source is supplied to the quadrupole magnetic-field generator  60 , the quadrupole magnetic-field generator  60  forms a magnetic field by superimposing magnetic fields generated by the polarizing coil portions  69   a,    69   b  on magnetic fields from the quadrupole between the magnetic pole pair  68   a ,  68   c,  which is a dipole, and the magnetic pole pair  68   b ,  68   d,  which is a dipole. Therefore, for example, as shown in  FIG. 6B , when arranged perpendicularly eccentrically to the central axis of the cathode  36 , the quadrupole magnetic-field generator  60  can correct, by polarization, movement (misalignment, eccentricity) of electron beams in the length direction (Y direction) thereof resulting from deformation of the electron beams in the width direction (X direction) by magnetic fields from the quadrupole. 
     According to the present embodiment, the X-ray tube device  1  is provided with the quadrupole magnetic-field generator  60  provided with the polarizing coil portions  69   a,    69   b.  When the polarizing coil portions  69   a  and  69   b  are supplied with a direct current from the power source, the quadrupole magnetic-field generator  60  can generate superimposed magnetic fields. The quadrupole magnetic-field generator  60  of the first embodiment is installed in misalignment with (eccentrically to) the orbits of a plurality of electron beams to achieve polarization in one direction. However, the quadrupole magnetic-field generator  60  of the present embodiment can correct, by polarization, movement (misalignment, eccentricity) of electron beams in the length direction thereof (Y direction) resulting from deformation of the electron beams in the width (X direction). Therefore, the X-ray tube device  1  of the present embodiment can magnetically change the shape of a plurality of electron beams into an optimal shape according to an intended use and focus the plurality of electron beams. 
     In the present embodiment, in the quadrupole magnetic-field generator  60 , the polarizing coil portions  69   a,    69   b  are supplied with a direct current from the power source but may be supplied with an alternating current. 
     In such a case, the quadrupole magnetic-field generator  60  generates dipole AC magnetic fields in such a manner that magnetic fields generated from two pairs of magnetic poles act in the same direction. For example, the quadrupole magnetic-field generator  60  is provided with a pair of the magnetic pole  68   a  and the magnetic pole  68   c  and a pair of the magnetic pole  68   b  and the magnetic pole  68   c.  The magnetic pole pair  68   a ,  68   c  and the magnetic pole pair  68   b,    68   d  each act as a dipole to form magnetic fields. The magnetic pole pair  68   a,    68   c  and the magnetic pole pair  68   b,    68   d  each form an AC magnetic field between the magnetic poles. 
     The quadrupole magnetic-field generator  60  can intermittently or continuously polarize the orbits of electrons based on an AC magnetic field generated between the poles of the dipole as a result of supply of an alternating current. In the quadrupole magnetic-field generator  60 , an alternating current from the power source (not shown in the drawings) supplied to each of the polarizing coil portions  69   a,    69   b  described below is controlled by the polarizing power source control unit (not shown in the drawings) so as to intermittently or continuously move the focus bombarded by a plurality of electron beams emitted from the plurality of filaments from the cathode  36 . The quadrupole magnetic-field generator  60  can polarize electron beams emitted from the cathode  36  in a direction along the radial direction of the anode target  35 . That is, the quadrupole magnetic-field generator  60  can move the position of the focus on the surface of the anode target  35  resulting from focusing of a plurality of electron beams. 
     A modification example of the present embodiment will be described below with reference to the drawings. The X-ray tube device  1  of the modification example has a configuration substantially equivalent to the configuration of the X-ray tube device  1  of the above-described embodiment. Thus, the same components of the X-ray tube device  1  of the modification example as the corresponding components of the X-ray tube device of the above-described embodiment are denoted by the same reference numerals, and detailed description of these components is omitted. 
     Modification Example 2 
     The X-ray tube device  1  of a modification example 2 of the second embodiment is provided with a quadrupole magnetic-field generator  601  provided with polarizing coil portions  69   c   1  and  69   d   1  and a quadrupole magnetic-field generator  602  provided with the above-described polarizing coil portions  69   a   2  and  69   b   2 . 
       FIG. 7A  is a cross-sectional view schematically showing the X-ray tube  30  of the modification example 2 of the second embodiment.  FIG. 7B  is a cross-sectional view taken along a VIIA 2 -VIIA 2  line in  FIG. 7A , and  FIG. 7C  is a cross-sectional view taken along a VIIA 1 -VIIA 1  line in  FIG. 7A . 
     As shown in  FIG. 7A , the X-ray tube  30  of the modification example 2 of the present embodiment is provided with the two quadrupole magnetic-field generators  601  and  602 . 
     As shown in  FIG. 7A  and  FIG. 7C , the quadrupole magnetic-field generator  601  is provided with the polarizing coil portion  69   c   1  and the polarizing coil portion  69   d   1 . 
     Each of the polarizing coil portions  69   c   1 ,  69   d   1  is supplied with a current from the power source (not shown in the drawings) to generate a magnetic field. In the present embodiment, each of the polarizing coil portions  69   c   1 ,  69   d   1  is supplied with a direct current from the power source (not shown in the drawings) to generate a DC magnetic field. The polarizing coil portions  69   c   1 ,  69   d   1  can polarize the orbits of electron beams in a predetermined direction by varying a current ratio of supplied currents. Each of the polarizing coil portions  69   c   1 ,  69   d   1  is wound between any two of the magnetic poles  68   a  to  68   d  connected to the yoke  66 . As shown in  FIG. 6B , the polarizing coil portion  69   c   1  is wound around the main body portion of the yoke  66  between the magnetic poles  68   a   1  and  68   b   1 . The polarizing coil portion  69   d   1  is wound around the main body portion of the yoke  66  between the magnetic poles  68   c   1  and  68   d   1 . In this case, for example, the magnetic pole pair  68   a,    68   b  generates a DC magnetic field between the magnetic poles, and the magnetic pole pair  68   c,    68   d  generates a DC magnetic field between the magnetic poles. 
     The quadrupole magnetic-field generators  601  and  602  are each provided on the small diameter portion  31   b.  That is, the quadrupole magnetic-field generators  601  and  602  are arranged on the small diameter portion  31   b.  The quadrupole magnetic-field generator  601  is installed on the small diameter portion  31   b  on the anode target  35  side, and the quadrupole magnetic-field generator  602  is installed on the small diameter portion  31   b  on the cathode side with respect to the quadrupole magnetic-field generator  601 . 
     Furthermore, the quadrupole magnetic-field generators  601  and  602  are each installed perpendicularly eccentrically to the electron orbits of the electron beams emitted from the cathode  36 . For example, as shown in  FIG. 7C , the quadrupole magnetic-field generator  601  is installed eccentrically in a direction along the straight line L 3 , and as shown in  FIG. 7B , the quadrupole magnetic-field generator  602  is installed eccentrically in a direction along the straight line L 1  (in the radial direction of the anode target  35 ) as is the case with the second embodiment. 
     The quadrupole magnetic-field generator  601  is provided with coils  64  ( 64   a   1 ,  64   b   1 ,  64   c   1 , and  64   d   1 ), a yoke  66   ya , and magnetic poles  68  ( 68   a   1 ,  68   b   1 ,  68   c   1 , and  68   d   1 ). 
     The quadrupole magnetic-field generator  602  has a configuration substantially equivalent to the configuration of the quadrupole magnetic-field generator  60  of the second embodiment. The quadrupole magnetic-field generator  602  is provided with coils  64  ( 64   a   2 ,  64   b   2 ,  64   c   2 , and  64   d   2 ), a yoke  66   yb , and magnetic poles  68  ( 68   a   2 ,  68   b   2 ,  68   c   2 , and  68   d   2 ). 
     The coils  64  ( 64   a   2 ,  64   b   2 ,  64   c   2 , and  64   d   2 ) are substantially equivalent to the coils  64  ( 64   a,    64   b ,  64   c,  and  64   d ) of the second embodiment. 
     The yokes  66   ya  and  66   yb  are substantially equivalent to the yoke  66  of the second embodiment. 
     The magnetic poles  68  ( 68   a   2 ,  68   b   2 ,  68   c   2 , and  68   d   2 ) are substantially equivalent to the magnetic poles  68  ( 68   a,    68   b,    68   c,  and  68   d ) of the second embodiment. 
     In the present embodiment, as shown in  FIG. 7B , the quadrupole magnetic-field generator  602  applies, to a plurality of electron beams, the action of magnetic fields substantially equivalent to the action of magnetic fields in the quadrupole magnetic-field generator  60  of the second embodiment. 
     As shown in  FIG. 7C , the quadrupole magnetic-field generator  601  deforms and polarizes an electron beam BM 4  focused and deformed by magnetic fields from the quadrupole magnetic-field generator  602 . 
     The principle of the quadrupole magnetic-field generator  601  of the modification example 2 of the present embodiment will be described below with reference to the drawings. 
       FIG. 8A  is a cross-sectional view showing the principle of quadrupole magnetic fields of the modification example 2 of the second embodiment,  FIG. 8B  is a cross-sectional view showing the principle of dipole magnetic fields of the modification example 2 of the second embodiment, and  FIG. 8C  is a cross-sectional view showing the principle of a quadrupole magnetic-field generator of the modification example 2 of the second embodiment. In  FIG. 8A  to  FIG. 8C , the X direction and the Y direction are directions perpendicular to the central axis of the cathode  36 , and are orthogonal to each other. Furthermore, the X direction is a direction extending from the magnetic pole  68   b   1  (magnetic pole  68   a   1 ) side toward the magnetic pole  68   d   1  (magnetic pole  68   c   1 ) side, and the Y direction is a direction extending from the magnetic pole  68   a   1  (magnetic pole  68   c   1 ) side toward the magnetic pole  68   b   1  (magnetic pole  68   d   1 ) side. 
     In  FIG. 8A  to  FIG. 8C , the electron beam BM 4 , into which the electron beam BM 1  and the electron beam BM 2  are aggregated by the quadrupole magnetic-field generator  60 , is assumed to travel from the side closer to the reader toward the side farther from the reader in the drawing. Furthermore, in  FIG. 8A  to  FIG. 8C , the magnetic pole  68   a   1  and the magnetic pole  68   b   1  are a dipole forming a pair (magnetic pole pair), and the magnetic pole  68   c   1  and the magnetic pole  68   d   1  are a dipole forming a pair (magnetic pole pair). The magnetic pole pair  68   a   1 ,  68   b   1  generates a DC magnetic field traveling in a direction following the Y direction, and the magnetic pole pair  68   c   1 ,  68   d   1  generates a DC magnetic field traveling in a direction following the Y direction. 
     As shown in  FIG. 8A , in the modification example 2, if not subjected to the action of the polarizing coil portions  69   c   1 ,  69   d   1 , the quadrupole magnetic-field generator  60  generates quadrupole magnetic fields. 
     As shown in  FIG. 8B , the polarizing coil portion  69   c   1  generates an N-pole magnetic field at the magnetic pole  68   a   1  and generates an S-pole magnetic field at the magnetic pole  68   b   1 . Similarly, the polarizing coil portion  69   d   1  generates an N-pole magnetic field at the magnetic pole  68   c   1  and generates an S-pole magnetic field at the magnetic pole  68   d   1 . Therefore, a magnetic field traveling from the magnetic pole  68   a   1  toward the magnetic pole  68   b   1  and a magnetic field traveling from the magnetic pole  68   c   1  toward the magnetic pole  68   d   1  are formed by the polarizing coil portion  69   c   1  and the polarizing coil portion  69   d   1 , respectively. 
     The quadrupole magnetic-field generator  601  is subjected to the action of magnetic fields from the polarizing coil portions  69   c   1 ,  69   d   1  as shown in  FIG. 8B  to superimpose a magnetic field generated by the polarizing coil portion  69   c   1  on a magnetic field traveling from the magnetic pole  68   a   1  toward the magnetic pole  68   b   1 , while superimposing a magnetic field generated by the polarizing coil portion  69   d   1  on a magnetic field traveling from the magnetic pole  68   c   1  toward the magnetic pole  68   d   1 . Therefore, as shown in  FIG. 8C , the quadrupole magnetic-field generator  60  generates superimposed magnetic fields traveling from the magnetic pole  68   a   1  toward the magnetic pole  68   b   1  in addition to magnetic fields from the quadrupole as shown in  FIG. 8A . Here, the magnetic fields between the magnetic poles  68   c   1  and the magnetic pole  68   d   1  cancel each other. 
     In the present embodiment, when the X-ray tube device  1  is driven, the filament  361   a  and the filament  361   b,  included in the cathode  36 , emit the electron beams BM 1  and BM 2 , respectively, toward the focus of electrons on the anode target  35 . The electron beams BM 1  and BM 2  are assumed to travel along a straight line passing through the center of the cathode  36 . In the quadrupole magnetic-field generator  602 , each of the polarizing coil portions  69   a   2 ,  69   b   2  is supplied with a direct current from the power source not shown in the drawings. For example, when a direct current from the power source is supplied to the quadrupole magnetic-field generator  602 , the quadrupole magnetic-field generator  602  forms a magnetic field by superimposing magnetic fields generated by the polarizing coil portions  69   a,    69   b  on magnetic fields from the quadrupole between the magnetic pole pair  68   a ,  68   c,  which is a dipole, and the magnetic pole pair  68   b ,  68   d,  which is a dipole. Upon traversing magnetic fields generated by the quadrupole magnetic-field generator  602 , a plurality of electron beams BM is focused into the electron beam BM 4 . 
     In the quadrupole magnetic-field generator  601 , each of the polarizing coil portions  69   c   1 ,  69   d   1  is supplied with a direct current from the power source not shown in the drawings. For example, when a direct current from the power source is supplied to the quadrupole magnetic-field generator  602 , the quadrupole magnetic-field generator  602  forms a magnetic field by superimposing magnetic fields generated by the polarizing coil portions  69   c   1 ,  69   d   1  on magnetic fields from the quadrupole of the magnetic poles  68   a   1  to  68   d   1 . Therefore, as shown in  FIG. 8C , when the electron beam BM 4  traverses magnetic fields, the quadrupole magnetic-field generator  601  can reduce the length dimension of the electron beam BM 4  (the length of the electron beam BM 4  in the Y direction) focused by having the width dimension thereof (the length of the electron beam BM 4  in the X direction) reduced by the quadrupole magnetic-field generator  602 . In this case, for example, for each of the quadrupole magnetic-field generators  601  and  602 , an installation position, a voltage intensity, a current direction, and the like are regulated to form electron beams of a desired size or a desired shape of the focus of the electron beams. 
     According to the present embodiment, the X-ray tube device  1  is provided with the quadrupole magnetic-field generator  601  provided with the polarizing coil portions  69   a   1 ,  69   b   1  and the quadrupole magnetic-field generator  602  provided with the polarizing coil portions  69   c   2 ,  69   d   2 . In each of the quadrupole magnetic-field generators  601  and  602 , the polarizing coil portions  69   a   1 ,  69   b   1 ,  69   c   2 , and  69   d   2  are supplied with a direct current to enable superimposed magnetic fields to be generated. For each of the quadrupole magnetic-field generators  601  and  602  of the modification example 2, the installation position, the voltage intensity, the current direction, and the like are regulated to form electron beams of a desired size or a desired shape of the focus of the electron beams. Therefore, the X-ray tube device  1  of the modification example 2 can magnetically change the shape of a plurality of electron beams into an optimal shape according to an intended use. 
     In the modification example 2, each of the quadrupole magnetic-field generators  601  and  602  is provided with two polarizing coil portions but may be provided with a further polarizing coil portion. Furthermore, the quadrupole magnetic-field generators  601  and  602  may be installed at opposite positions. 
     In the modification example 2 of the present embodiment, in the quadrupole magnetic-field generators  601  and  602 , the polarizing coil portions  69   a   1 ,  69   b   1 ,  69   c   2 , and  69   d   2  may each be supplied with a direct current from the power source but may be supplied with an alternating current. 
     In such a case, the quadrupole magnetic-field generator  601  generates dipole AC magnetic fields in such a manner that magnetic fields generated from two pairs of magnetic poles act in the same direction. For example, the quadrupole magnetic-field generator  601  is provided with a pair of the magnetic pole  68   a   1  and the magnetic pole  68   b   1  and a pair of the magnetic pole  68   c   1  and the magnetic pole  68   d   1 . The magnetic pole pair  68   a   1 ,  68   b   1  and the magnetic pole pair  68   c   1 ,  68   d   1  each serve as a dipole to form a magnetic field. The magnetic pole pair  68   a   1 ,  68   b   1  and the magnetic pole pair  68   c   1 ,  68   d   1  each form an AC magnetic field between the magnetic poles. 
     Similarly, the quadrupole magnetic-field generator  602  generates dipole magnetic fields in such a manner that magnetic fields generated from the two pairs of magnetic poles act in the same direction. For example, the quadrupole magnetic-field generator  602  is provided with a pair of the magnetic pole  68   a   2  and the magnetic pole  68   c   2  and a pair of the magnetic pole  68   b   2  and the magnetic pole  68   d   2 . The magnetic pole pair  68   a   2 ,  68   c   2  and the magnetic pole pair  68   b   2 ,  68   d   2  each serve as a dipole to form a magnetic field. The magnetic pole pair  68   a   2 ,  68   c   2  and the magnetic pole pair  68   b   2 ,  68   d   2  each form an AC magnetic field between the magnetic poles. 
     The quadrupole magnetic-field generators  601  and  602  can each intermittently or continuously polarize the orbits of electrons based on an AC magnetic field generated between the poles of the dipole as a result of supply of an alternating current. In the quadrupole magnetic-field generators  601  and  602 , an alternating current from the power source (not shown in the drawings) supplied to each of the polarizing coil portions  69   a   2 ,  69   b   2 ,  69   c   1 , and  69   d   1  described below is controlled by the polarizing power source control unit (not shown in the drawings) so as to intermittently or continuously move the focus bombarded by electron beams emitted from the cathode  36 . The quadrupole magnetic-field generators  601  and  602  can perform polarization in a desired direction by controlling a current or the like. That is, when each of the quadrupole magnetic-field generators  601  and  602  is supplied with an alternating current, the X-ray tube device  1  can move the position of the focus on the surface of the anode target  35  bombarded by electron beams. 
     Now, an X-ray tube device according to a third embodiment will be described. The same components of the third embodiment as the corresponding components of the above-described embodiment are denoted by the same reference numerals, and detailed description of these components is omitted. 
     Third Embodiment 
     An X-ray tube device  10  of a third embodiment is different from the X-ray tube devices of the above-described embodiments in that, due to the lack of the housing portion  31   a,  the anode target  35  and the cathode  36  are installed closer to each other. Thus, the X-ray tube device  10  of the third embodiment is different from the X-ray tube devices of the above-described embodiments in the configurations of the vacuum envelope  31  (vacuum container  32 ) and the quadrupole magnetic-field generator, and the like. 
       FIG. 9  is a cross-sectional view showing an example of the X-ray tube device of the third embodiment. 
       FIG. 10A  is a cross-sectional view schematically showing the X-ray tube  30  of the third embodiment,  FIG. 10B  is a cross-sectional view taken along an XIA-XIA line in  FIG. 10A ,  FIG. 10C  is a cross-sectional view taken along an XB 1 -XB 1  line in  FIG. 10B ,  FIG. 10D  is a cross-sectional view taken along an XB 2 -XB 2  line in  FIG. 10B , and  FIG. 10E  is a cross-sectional view taken along an XD-XD line in  FIG. 10D . 
     In  FIG. 10B  and  FIG. 10E , a straight line which is orthogonal to the tube axis TA is designated as the straight line L 1 , and a straight line which is orthogonal to the tube axis TA and the straight line L 1  is designated as the straight line L 2 . In  FIG. 10B  and  FIG. 10E , a straight line which is orthogonal to the center of the cathode  36  or a straight line along the emission direction of electron beams and which is parallel to the straight line L 2  is designated as the straight line L 3 . 
     The X-ray tube  30  is provided with a KOV member  55  in addition to the configurations of the above-described embodiments. 
     The anode target  35  is formed of a member which is a nonmagnetic substance and has a high electric conductivity (electric conduction property). For example, the anode target  35  is formed of copper, tungsten, molybdenum, niobium, tantalum, nonmagnetic stainless steel, or the like. The anode target  35  may be configured in such a manner that at least a surface portion thereof is formed of a metal member which is a nonmagnetic substance and which has a high electric conductivity. Alternatively, the anode target  35  may be configured in such a manner that the surface portion thereof is coated with a coating member formed of a metal member which is a nonmagnetic substance and which has a high electric conductivity. 
     The cathode  36  is attached to the cathode support portion (cathode support body, cathode support member)  37  described below and connected to the high-voltage supply terminal  54  passing through the inside of the cathode support portion  37 . The cathode  36  may be referred to as an electron generation source. In the cathode  36 , an emission position for electron beams coincides with the center of the cathode. The center of the cathode  36  may hereinafter include a straight line passing through the center. 
     The cathode support portion  37  has a first end portion provided with the cathode  36  and a second end portion provided with the KOV member  55 . Furthermore, the cathode  36  is internally provided with the high-voltage supply terminal  54 . As shown in  FIG. 11A , the cathode support portion  37  is installed so as to extend from the KOV member  55  provided around the tube axis TA to the vicinity of the outer circumference of the anode target  35 . Furthermore, the cathode support portion  37  is installed substantially parallel to and at a predetermined distance from the anode target  35 . In this case, the cathode support portion  37  is provided with the cathode  36  at an outer circumferential-side end portion of the anode target  35 . 
     The KOV member  55  is formed of a low-expansion alloy. The KOV member  55  has a first end portion joined to the cathode support portion  37  by brazing and a second end portion joined to a high-voltage insulating member  50  by brazing. The KOV member  55  covers the high-voltage supply terminal  54  in the vacuum envelope  31  described below. 
     The high-voltage supply terminal  54  and the KOV member  55  are joined to the high-voltage insulating member  50 . The high-voltage supply terminal  54  penetrates the vacuum container  32  described below and is inserted into the vacuum envelope  31 . In this case, the high-voltage supply terminal  54  is inserted into the vacuum envelope  31  with an insertion portion of the high-voltage supply terminal  54  sealed in a vacuum airtight manner. 
     The high-voltage supply terminal  54  is connected to the cathode  36  through the inside of the cathode support portion  37 . The high-voltage supply terminal  54  applies a relatively negative voltage to the cathode  36 , while supplying a filament current to the filaments (electron radiation source) of the cathode  36 , not shown in the drawings. The high-voltage supply terminal  54  is connected to the receptacle  302  and supplied with a current when a high-voltage supply source such as a plug not shown in the drawings is connected to the receptacle  302 . The high-voltage supply terminal  54  is a metal terminal. 
     The vacuum envelope  31  is sealed in a vacuum atmosphere (vacuum airtight manner) and internally houses the fixed shaft  11 , the rotating body  12 , the bearing  13 , the rotor  14 , the vacuum container  32 , the anode target  35 , the cathode  36 , the high-voltage supply terminal  54 , and the KOV member  55 . 
     The vacuum container  32  is provided with the X-ray transmission window  38  in a vacuum airtight manner. The X-ray transmission window  38  is provided in the wall portion of the vacuum envelope  31  (vacuum container  32 ) opposed to an area between the cathode  36  and the anode target  35 . The X-ray transmission window  38  is formed of metal, for example, beryllium or titanium, stainless steel, and aluminum and provided in a portion of the vacuum container  32  which is opposed to the X-ray radiation window  20   w.  For example, the vacuum container  32  is hermetically occluded by the X-ray transmission window  38  formed of beryllium as a member which allows X rays to pass through. In the vacuum envelope  31 , the high-voltage insulating member  39  is arranged from the high-voltage supply terminal  44  side to the periphery of the anode target  35 . The high-voltage insulating member  39  is formed of an electric insulating resin. 
     The vacuum envelope  31  (vacuum container  32 ) is provided with a recessed portion in which a tip portion of the quadrupole magnetic-field generator  60  described below is housed. As shown in  FIG. 10B , in the present embodiment, the vacuum envelope  31  (vacuum container  32 ) is provided with a plurality of recessed portions  32   a,    32   b,    32   c,  and  32   d.  Each of the recessed portions  32   a,    32   b,    32   c,  and  32   d  is formed in a portion of the vacuum envelope  31  (vacuum container  32 ). That is, each of the recessed portions  32   a,    32   b,    32   c,  and  32   d  is a portion of the vacuum envelope  31  (vacuum container  32 ) surrounding the recess. For example, the recessed portions  32   a  to  32   d  are formed by externally recessing the vacuum envelope  31  (vacuum container  32 ) in such a manner that the recessed portions  32   a  to  32   d  surround the cathode  36  in a direction perpendicular to the emission direction of electron beams. That is, the recessed portions  32   a  to  32   d  are formed to protrude parallel to the emission direction of electron beams from the cathode  36  if the vacuum envelope  31  (vacuum container  32 ) is internally observed. 
     The recessed portions  32   a  to  32   d  are arranged at an even distance from a predetermined central position (recessed portion center). The recessed portions  32   a  to  32   d  are arranged, for example, around the cathode  36  at equal angular intervals in such a manner that the center of the recessed portions (recessed portion center) coincides with a position displaced perpendicularly from (located perpendicularly eccentrically to) electron orbits. In this case, the recessed portion  32   b  is formed at 90° with respect to the recessed portion  32   a  in a rotating direction (counterclockwise) around the recessed portion center. Similarly, the recessed portion  32   d  is formed at 90° with respect to the recessed portion  32   b  in the rotating direction around the center of the cathode  36 , and the recessed portion  32   c  is formed at 90° with respect to the recessed portion  32   d  in the rotating direction around the center of the cathode  36 . 
     For example, as shown in  FIG. 10B , the recessed portion  32   a  is installed at a position located at 45° from the straight line L 1  in the rotating direction around the recessed portion center, the recessed portion  32   b  is installed at a position resulting from rotation through 90° from the recessed portion  32   a  in the rotating direction around the center of the cathode  36 , the recessed portion  32   d  is installed at a position resulting from rotation through 90° from the recessed portion  32   b  in the rotating direction around the center of the cathode  36 , and the recessed portion  32   c  is installed at a position resulting from rotation through 90° from the recessed portion  32   d  in the rotating direction around the center of the cathode  36 . That is, the recessed portions  32   a  to  32   d  are installed so as to be arranged at the positions of vertices of a square. 
     Furthermore, each of the recessed portions  32   a  to  32   d  is formed so as to avoid lying excessively proximate to the surface of the anode target  35  and the surface of the cathode  36  in order to prevent discharge and the like. For example, the recessed portion  32   a  is formed by being recessed to a position farther from the surface of the anode target  35 , in a direction along the tube axis TA, than the surface of the cathode  36  opposed to the surface of the anode target  35 . Alternatively, the recessed portion  32   a  is formed by being recessed to the same position as that of the surface of the cathode  36  or a position slightly closer to the surface of the anode target  35 , in a direction along the tube axis TA, than to the surface of the cathode  36 . Corner portions of the recessed portions  32   a  to  32   d  which protrude to the anode target  35  side are each formed so as to be curved or inclined to lie away from the target surface of the anode target  35  and the surface of the cathode  36  in order to prevent discharge and the like. For example, as shown in  FIG. 11C , the corner portions of the recessed portions  32   a  to  32   d  are each formed like curved surfaces. The corner portions of the recessed portions  32   a  to  32   d  may each be inclined at an angle along the inclination angle of each of the magnetic poles  68  ( 68   a,    68   b,    68   c , and  68   d ). The corners of the recessed portions  32   a  to  32   d  protruding to the anode target  35  side may not be formed to have an inclination and a diameter. 
     Moreover, the number of the recessed portions may not be four provided that the recessed portions are installed so as to peripherally surround the axis (electron orbits) of the cathode  36  along the emission direction of electron beams. For example, the recessed portions  32   a  to  32   d  may be integrally formed. Furthermore, the recessed portions  32   a  and  32   b  may be integrally formed, while the recessed portions  32   c  and  32   d  may be integrally formed. 
     Furthermore, the vacuum envelope  31  captures recoil electrons reflected from the anode target  35 . Thus, the vacuum envelope  31  is likely to have the temperature thereof raised by the bombardment of recoil electrons and is normally formed of a member such as copper which has a high heat conductivity. The vacuum envelope  31  is desirably constituted of a member which does not generate a diamagnetic field if the vacuum envelope  31  is affected by an AC magnetic field. For example, the vacuum envelope  31  is formed of a metal member which is a nonmagnetic substance. Suitably, the vacuum envelope  31  is formed of a high-electric-resistance member which is a nonmagnetic substance in order to avoid overcurrent resulting from an alternating current. The high-electric-resistance member which is a nonmagnetic substance is, for example, nonmagnetic stainless steel, inconel, inconel X, titanium, conductive ceramics, or non-conductive ceramics the surface of which is coated with a metal thin film. More suitably, the recessed portions  32   a  to  32   d  of the vacuum envelope  31  are formed of a high-electric-resistance member which is a nonmagnetic substance, and the whole vacuum envelope  31  except for the recessed portions  32   a  to  32   d  is formed of a nonmagnetic member such as copper which has a high heat conductivity. 
     With reference to  FIG. 10B  to  FIG. 10E , the quadrupole magnetic-field generator  60  will be described below in detail. 
     As shown in  FIG. 10B  and  FIG. 10E , the quadrupole magnetic-field generator  60  is provided with the coils  64  ( 64   a,    64   b,    64   c,  and  64   d ), the yoke  66  ( 66   a,    66   b ,  66   c,  and  66   d ), the magnetic poles ( 68   a,    68   b,    68   c,  and  68   d ), and the polarizing coil portions  69   a,    69   b.    
     In the present embodiment, the quadrupole magnetic-field generator  60  is installed in such a manner that the central position thereof is perpendicularly eccentric to the central axis of the cathode  36 . For example, as shown in  FIG. 10E , in the quadrupole magnetic-field generator  60 , four magnetic poles  68  are arranged in a square form. As described below in detail, in the quadrupole magnetic-field generator  60 , the magnetic poles  68   a,    68   b,    68   c,  and  68   d  are provided at tips of protruding portions  66   a,    66   b ,  66   c,  and  66   d  protruding from the main body portion of the yoke  66 . 
     As schematically shown in  FIG. 100  and  FIG. 10D , the magnetic pole pair  68   a,    68   c  and the magnetic pole pair  68   b,    68   d  each form a magnetic field between the magnetic poles. In the quadrupole magnetic-field generator  60 , a DC current supplied to each of the polarizing coil portions  69   a,    69   b  described below by the power source (not shown in the drawings) is controlled by the polarizing power source control portion (not shown in the drawings). By installing the quadrupole magnetic-field generator  60  in such a manner that the center thereof is perpendicularly eccentric to the central axis of the cathode  36 , electron beams in a desired direction are deformed and polarized. For example, as shown in  FIG. 10E , the quadrupole magnetic-field generator  60  can deform the electron beams BM 1  and BM 2  emitted from the filaments  361   a  and  361   b , respectively, so as to reduce the width of each of the electron beams, and can correct, by polarization, movement of the focus on the anode target  35  in the radial direction associated with the deformation of the width. That is, the quadrupole magnetic-field generator  60  can adjust the position of the focus on the surface of the anode target  35  where the electron beams BM 1  and BM 2  bombard at the same position so as to overlap each other, and can reduce a thermal load on the focus. 
     Each of the coils  64  is supplied with a current from the power source (not shown in the drawings) for the quadrupole magnetic-field generator  60  to generate a magnetic field. In the present embodiment, each coil  64  is supplied with a direct current from the power source (not shown in the drawings). The coils  64  are provided with a plurality of coils  64   a,    64   b,    64   c,  and  64   d.  Each of the coils  64   a  to  64   d  is wound around a portion of a corresponding one of the protruding portions  66   a,    66   b,    66   c,  and  66   d  of the yoke  66  described below. 
     The yoke  66  is provided with the protruding portions  66   a,    66   b,    66   c,  and  66   d  protruding from the main body portion. The protruding portions  66   a  to  66   d  are provided to protrude in a direction parallel to the emission direction (electron orbits) of the electron beams or the central axis of the cathode  36 . The protruding portions  66   a  to  66   d  protrude in the same direction and are parallel to one another. Furthermore, the protruding portions  66   a  to  66   d  are formed to have the same length and shape. Furthermore, in the yoke  66 , the main body portion is shaped like a polygon or a hollow cylinder. In the present embodiment, the yoke  66  is installed in such a manner that each of the four protruding portions  66   a  to  66   d  is housed in a corresponding one of the recessed portions  32   a  to  32   d.  In this case, the yoke  66  is arranged in such a manner that the four protruding portions  66   a  to  66   d  surround the cathode  36 . Furthermore, the coil  64  is wound around a portion of each of the four protruding portions. 
     More specifically, the coil  64   a  is wound around a portion of the protruding portion  66   a  of the yoke  66 , and a portion of the protruding portion  66   a  around which the coil  64   a  is not wound is housed in the recessed portion  32   a.  Similarly, the coils  64   b,    64   c , and  64   d  are each wound around a portion of a corresponding one of the protruding portions  66   b,    66   c , and  66   d,  and portions of the protruding portions  66   b ,  66   c,  and  66   d  around which the coils  64   b,    64   c,  and  64   d , respectively, are not wound are housed in the recessed portions  32   b,    32   c,  and  32   d,  respectively. 
     The magnetic poles  68  are provided with the plurality of magnetic poles  68   a,    68   b,    68   c,  and  68   d . The magnetic poles  68   a,    68   b,    68   c,  and  68   d  are provided at the tip portions of the protruding portions  66   a ,  66   b,    66   c,  and  66   d,  respectively, of the yoke  66 . The magnetic poles  68   a  to  68   d  are arranged to surround the periphery of the cathode  36 . That is, in the quadrupole magnetic-field generator  60 , the magnetic poles  68   a,    68   b,    68   c,  and  68   d  are arranged at positions perpendicular to the central axis of the cathode  36  and evenly around a predetermined position as a center (magnetic pole center). In this case, the central (magnetic pole center) position of arrangement of the magnetic poles  68   a  to  68   d  is an intersection point between straight lines passing through the centers of the magnetic poles  68   a  to  68   d.    
     For example, as is the case with the above-described recessed portions  32   a  to  32   d,  as shown in  FIG. 10B , the magnetic pole  68   a  is installed at a position located at 45° from the straight line L 1  in the rotating direction (counterclockwise) around a magnetic pole center C 1 , the magnetic pole  68   b  is installed at a position resulting from rotation through 90° from the magnetic pole  68   a  in the rotating direction around the magnetic pole center C 1 , the magnetic pole  68   d  is installed at a position resulting from rotation through 90° from the magnetic pole  68   b  in the rotating direction around the magnetic pole center C 1 , and the magnetic pole  68   c  is installed at a position resulting from rotation through 90° from the magnetic pole  68   d  in the rotating direction around the magnetic pole center C 1 . That is, the magnetic poles  68   a  to  68   d  are installed so as to be arranged at the positions of vertices of a square. 
     Suitably, in order to increase the magnetic flux density, the magnetic poles  68   a  to  68   d  are installed moderately close to the emission direction (electron orbits) of electrons emitted from the filaments included in the cathode  36 . That is, the magnetic pole  68   a  is arranged in the vicinity of a cathode  36 -side curved wall surface of the recessed portions  32   a . Similarly, each of the magnetic poles  68   b  to  68   d  is arranged in the vicinity of a cathode  36 -side curved wall surface of a corresponding one of the recessed portions  32   b  to  32   d.  The recessed portions  32   a  to  32   d  are arranged so as to avoid lying excessively proximate to the cathode  36  in order to prevent discharge and the like. 
     The magnetic poles  68   a  to  68   d  are formed to have substantially the same shape. The magnetic poles  68   a  to  68   d  include two dipoles each forming a pair. For example, the magnetic pole  68   a  and the magnetic pole  68   b  are a dipole (magnetic pole pair  68   a,    68   b ), and the magnetic pole  68   c  and the magnetic pole  68   d  are a dipole (magnetic pole pair  68   c,    68   d ). In this case, when a direct current is supplied to each magnetic pole  68  via the corresponding coil  64 , the magnetic pole pair  68   a,    68   d  and the magnetic pole pair  68   c,    68   d  form opposite DC magnetic fields. Each of the magnetic poles  68   a  to  68   d  is installed in such a manner that the surface (end face) thereof faces the magnetic pole center, in order to regulate the shape and direction of each of the electron beams BM 1  and BM 2  emitted from the filaments  361   a  and  361   c,  respectively, with the magnetic flux density made as high as possible with the magnetic poles  68   a  to  68   d  avoiding lying excessively close to the anode target  35 . In this case, the magnetic poles  68   a  to  68   d  are formed in such a manner that the surfaces thereof are opposed to one another. 
     For example, as shown in  FIG. 10B , each of the magnetic poles  68   a  to  68   d  is defined by an inclined surface inclined at the same angle to a straight line which passes through the magnetic pole center C 1  and which is parallel to the tube axis TA. An inclination angle from the straight line which passes through the magnetic pole center C 1  and which is parallel to the tube axis TA to the surface of the magnetic pole  68   a  is denoted by γ1, and an inclination angle from the straight line which passes through the magnetic pole center C 1  and which is parallel to the tube axis TA to the surface of the magnetic pole  68   d  is denoted by γ4. An inclination angle from the straight line which passes through the magnetic pole center C 1  and which is parallel to the tube axis TA to the surface of the magnetic pole  68   b  is denoted by γ2, and an inclination angle from the straight line which passes through the magnetic pole center C 1  and which is parallel to the tube axis TA to the surface of the magnetic pole  68   c  is denoted by γ3. Therefore, for example, if the magnetic poles  68   a  to  68   d  are installed so as to have the same inclination, γ1=γ2=γ3=γ4. In this case, the inclination angles γ (γ1, γ2, γ3, and γ4) of the magnetic poles  68   a  to  68   d  are set within the range of 0°&lt;γ&lt;90°. In this case, each of the magnetic poles  68   a  to  68   d  is formed in such a manner that the inclination angle γ thereof is set within the range of 0°&lt;γ&lt;90°. For example, if the magnetic poles  68   a  to  68   d  have the same inclination angle (γ1=γ2=γ3=γ4), the inclinations γ1, γ2, γ3, and γ4 of the magnetic poles  68   a  to  68   d  are formed within the range of 30°≦γ≦60°. Moreover, the inclinations γ1, γ2, γ3, and γ4 of the magnetic poles  68   a  to  68   d  may be formed at 45° to the straight line which passes through the magnetic pole center C 1  and which is parallel to the tube axis TA. 
     The polarizing coil portions  69   a,    69   b  (the first polarizing coil portion, the second polarizing coil portion) are electromagnetic coils to which a current is supplied by the power source (not shown in the drawings) and which generate magnetic fields. In the present embodiment, each of the polarizing coil portions  69   a,    69   b  is supplied with a DC power supply from the power source (not shown in the drawings) to generate an AC magnetic field. Each of the polarizing coil portions  69   a,    69   b  is wound between any two of the protruding portions  66   a  to  66   d  of the main body portion of the yoke  66 . As shown in  FIG. 10C  and  FIG. 10D , the polarizing coil portion  69   a  is wound around the main body portion of the yoke  66  between the protruding portions  66   a  and  66   c.  The polarizing coil portion  69   b  is wound around the main body portion of the yoke  66  between the protruding portions  66   b  and  66   d.  In this case, the magnetic pole pair  68   a,    68   c  generates a DC magnetic field between the magnetic poles  68   a  and  68   c , and the magnetic pole pair  68   b,    68   d  generates a DC magnetic field between the magnetic poles  68   b  and  68   d.    
     The polarizing coil portions  69   a,    69   d  generate dipole magnetic fields formed along a direction which is perpendicular to the radial direction of the anode target  35  and which extends along the width direction of the filaments included in the cathode  36 . The polarizing coil portions  69   a,    69   b  can polarize and move the orbits of electron beams in a predetermined direction. 
     The principle of the quadrupole magnetic-field generator  60  of the present embodiment will be described below with reference to the drawings. 
       FIG. 11A  is a diagram showing the principle of quadrupole magnetic fields of the third embodiment, and  FIG. 11B  is a diagram showing the principle of a dipole of the second embodiment. In  FIG. 11A  and  FIG. 11B , the X direction and the Y direction are directions perpendicular to the central axis of the cathode  36 , and are orthogonal to each other. Furthermore, the X direction is a direction extending from the magnetic pole  68   b  (magnetic pole  68   a ) side toward the magnetic pole  68   d  (magnetic pole  68   c ) side, and the Y direction is a direction extending from the magnetic pole  68   a  (magnetic pole  68   c ) side toward the magnetic pole  68   b  (magnetic pole  68   d ) side. 
     In  FIG. 11A  and  FIG. 11B , unlike in  FIG. 3 ,  FIG. 6 ,  FIG. 8 , the electron beam BM 1  and the electron beam BM 2  are assumed to travel from the side closer to the reader toward the side farther from the reader in the drawing. Furthermore, in  FIG. 11A  and  FIG. 11B , the magnetic pole  68   a  and the magnetic pole  68   c  are a dipole forming a pair (magnetic pole pair), and the magnetic pole  68   b  and the magnetic pole  68   d  are a dipole forming a pair (magnetic pole pair). The magnetic poles  68   a,    68   c  generate a DC magnetic field traveling in a direction following the X direction, and the magnetic poles  68   b,    68   d  generate a DC magnetic field following the X direction. 
     As shown in  FIG. 11A , if not subjected to the action of the polarizing coil portions  69   a,    69   b,  the quadrupole magnetic-field generator  60  is assumed to generate an N-pole magnetic field at the magnetic pole  68   a,  generate an S-pole magnetic field at the magnetic pole  68   b,  generate an S-pole magnetic field at the magnetic pole  68   c,  and generate an N-pole magnetic field at the magnetic pole  68   d.    
     As shown in  FIG. 11B , the polarizing coil portion  69   a  generates an N-pole magnetic field at the magnetic pole  68   a  and generates an S-pole magnetic field at the magnetic pole  68   c.  Similarly, the polarizing coil portion  69   b  generates an N-pole magnetic field at the magnetic pole  68   b  and generates an S-pole magnetic field at the magnetic pole  68   d.  Therefore, a magnetic field traveling from the magnetic pole  68   a  toward the magnetic pole  68   c  and a magnetic field traveling from the magnetic pole  68   b  toward the magnetic pole  68   d  are formed by the polarizing coil portion  69   a  and the polarizing coil portion  69   b,  respectively. 
     The quadrupole magnetic-field generator  60  is subjected to the action of magnetic fields from the polarizing coil portions  69   a,    69   b  as shown in  FIG. 11B  to superimpose a magnetic field generated by the polarizing coil portion  69   a  on a magnetic field traveling from the magnetic pole  68   a  toward the magnetic pole  68   c,  while superimposing a magnetic field generated by the polarizing coil portion  69   b  on a magnetic field traveling from the magnetic pole  68   d  toward the magnetic pole  68   b.  Therefore, the quadrupole magnetic-field generator  60  generates superimposed magnetic fields traveling from the magnetic pole  68   a  toward the magnetic pole  68   c  in addition to magnetic fields from the quadrupole. Here, the magnetic fields between the magnetic poles  68   b  and the magnetic pole  68   d  cancel each other. 
     In the present embodiment, when the X-ray tube device  1  is driven, the filament  361   a  and the filament  361   b,  included in the cathode  36 , emit the electron beams BM 1  and BM 2 , respectively, toward the focus of electrons on the anode target  35 . Here, the direction in which electrons are emitted is a direction perpendicular to each of the converging surfaces  363   a  and  363   b.  Furthermore, the inclinations γ1 to γ4 of the magnetic poles  68   a  to  68   d  of the quadrupole magnetic-field generator  60  shown in  FIG. 10B  are the same. In the quadrupole magnetic-field generator  60 , each of the coils  64  is supplied with a direct current from the power source not shown in the drawings. When a direct current from the power source is supplied to the quadrupole magnetic-field generator  60 , the quadrupole magnetic-field generator  60  generates magnetic fields among the magnetic poles  68   a  to  68   d , which are a quadrupole. Upon traversing magnetic fields generated between the anode target  35  and the cathode  36  and cathode support portion  37 , the electron beams BM 1  and the electron beam BM 2  emitted from the filaments  361   a  and  361   b  of the cathode  36  are focused and polarized in a predetermined direction. As a result, the electron beam BM 1  and the electron beam BM 2  bombard at the focus on the anode target  35 . In the present embodiment, for example, as shown in  FIG. 10E , the quadrupole magnetic-field generator  60  acts to deform the electron beams emitted in circles into ellipses which are elongate in the Y direction and to focus each of the electron beams BM 1  and BM 2  on the central side of the cathode  36  along the straight line L 3 . In this case, the quadrupole magnetic-field generator  60  can accurately bombard a plurality of electron beams (electron beams BM 1  and BM 2 ) at the focus on the anode target  35  surface in such a manner that the electron beams have a small apparent focus. 
     According to the present embodiment, the X-ray tube device  1  is provided with the X-ray tube  30  provided with the recessed portions  32   a  to  32   d  and the quadrupole magnetic-field generator  60  provided with the polarizing coil portions  69   a  and  6   b.  When the polarizing coil portions  69   a  and  69   b  are supplied with a direct current from the power source, the quadrupole magnetic-field generator  60  can generate superimposed magnetic fields. The quadrupole magnetic-field generator  60  of the first embodiment is installed perpendicularly eccentrically to the orbits of electron beams to achieve polarization in one direction. However, the quadrupole magnetic-field generator  60  of the present embodiment can perform correction by polarizing movement (misalignment, eccentricity) of electron beams in the length direction thereof (Y direction) resulting from deformation of the electron beams in the width direction(X direction). Therefore, the X-ray tube device  1  of the present embodiment can magnetically change the electron beam shape into the optimal shape according to the intended use. 
     Furthermore, in the X-ray tube device  1  of the present embodiment, the anode target  35  and the cathode  36  are installed more proximate to each other than in the above-described embodiments. Therefore, the X-ray tube device  1  of the present embodiment can reduce possible enlargement of the X-ray focus, a possible blur, possible distortion, a possible decrease in the amount of electrons emitted from the cathode  36 , and the like. 
     The X-ray tube device  1  of the present embodiment may further be provided with the polarizing coil portions  69   c,    69   d.  The polarizing coil portions  69   c ,  69   d  (a third polarizing coil portion, a fourth polarizing coil portion) are supplied with a current from the power source (not shown in the drawings) to generate a magnetic field. In the present embodiment, each of the polarizing coil portions  69   c,    69   d  is supplied with a direct current from the power source (not shown in the drawings) to generate a DC magnetic field. Each of the polarizing coil portions  69   c,    69   d  is wound between any two of the protruding portions  66   a  to  66   d  of the main body portion of the yoke  66 . For example, the polarizing coil portion  69   c  is wound around the main body portion of the yoke  66  between the protruding portions  66   a  and  66   b.  The polarizing coil portion  69   d  is wound around the main body portion of the yoke  66  between the protruding portions  66   c  and  66   d.  In this case, the magnetic pole pair  68   a,    68   b  generates a DC magnetic field between the magnetic poles  68   a  and  68   b,  and the magnetic pole pair  68   c,    68   d  generates a DC magnetic field between the magnetic poles  68   c  and  68   d.    
     The polarizing coil portions  69   c,    69   d  generate a dipole magnetic field formed along a direction along the length direction perpendicular to the width direction of the filaments included in the cathode  36 , which is the radial direction of the anode target  35 . The polarizing coil portions  69   c,    69   d  can polarize and move the orbits of electron beams in a predetermined direction. 
     In the present embodiment, the quadrupole magnetic-field generator  60  may be provided with the polarizing coil portions  69   a,    69   b,    69   c,  and  69   d.  In this case, each of the polarizing coil portions  69   a  to  69   d  may be supplied with an alternating current. In such a case, the quadrupole magnetic-field generator  60  generates a dipole AC magnetic fields in such a manner that magnetic fields generated from two pairs of magnetic poles act in the same direction. 
     If each of the polarizing coil portions  69   a  and  69   b  is supplied with an alternating current, for example, the quadrupole magnetic-field generator  60  is provided with the magnetic pole  68   a  and the magnetic pole  68   c  forming a pair and the magnetic pole  68   b  and the magnetic pole  68   d  forming a pair. The magnetic pole pair  68   a,    68   c  and the magnetic pole pair  68   b,    68   d  each serve as a dipole to form a magnetic field. The magnetic pole pair  68   a,    68   c  and the magnetic pole pair  68   b,    68   d  each form an AC magnetic field between the magnetic poles. 
     If each of the polarizing coil portions  69   c  and  69   d  is supplied with an alternating current, for example, the quadrupole magnetic-field generator  60  is provided with the magnetic pole  68   a  and the magnetic pole  68   b  forming a pair and the magnetic pole  68   c  and the magnetic pole  68   d  forming a pair. The magnetic pole pair  68   a,    68   b  and the magnetic pole pair  68   c,    68   d  each serve as a dipole to form a magnetic field. The magnetic pole pair  68   a,    68   b  and the magnetic pole pair  68   c,    68   d  each form an AC magnetic field between the magnetic poles. 
     The quadrupole magnetic-field generator  60  can intermittently or continuously polarize the orbits of electrons based on an AC magnetic field generated between the magnetic poles of the dipole as a result of supply of an alternating current. An alternating current from the power source (not shown in the drawings) supplied to each of the polarizing coil portions  69   a  to  69   d  described below is controlled by the polarizing power source control portion (not shown in the drawings) so as to intermittently or continuously move the focus bombarded by electron beams emitted from the cathode  36 . The quadrupole magnetic-field generator  60  can polarize electron beams emitted from the cathode  36  in a direction along the radial direction of the anode target  35 . That is, the quadrupole magnetic-field generator  60  can move the position of the focus on the surface of the anode target  35  bombarded by the electron beams. 
     Moreover, the X-ray tube device  1  of the present embodiment is provided with the first quadrupole magnetic-field generator provided with the polarizing coil portions  69   a  and  69   b  and the second quadrupole magnetic-field generator provided with the polarizing coil portions  69   c  and  69   d.  In this case, the quadrupole magnetic-field generator  60  can polarize electron beams emitted from the cathode  36  in any direction of the anode target  35 . 
     According to the above-described embodiments, the X-ray tube device  1  is provided with an X-ray tube provided with a plurality of recessed portions and a quadrupole magnetic-field generator which forms electron beams emitted by the X-ray tube. In the quadrupole magnetic-field generator, a direct current from the power source is supplied to coils to generate magnetic fields between a plurality of magnetic poles. 
     In the quadrupole magnetic-field generator, electron beams emitted from the cathode can be deformed by magnetic fields generated by the plurality of magnetic poles. As a result, the X-ray tube device  1  of the present embodiment can reduce possible enlargement of the X-ray focus, a possible blur, possible distortion, a possible decrease in the amount of electrons emitted from the cathode  36 , and the like. 
     In the above-described embodiment, the X-ray tube device  1  is a rotating anode type X-ray tube, but may be a fixed anode type X-ray tube. 
     In the above-described embodiments, the X-ray tube device  1  is a neutral grounding type X-ray tube device, but may be an anode grounding type or cathode grounding type X-ray tube device. 
     Moreover, in the above-described embodiments, the anode  36  is provided with a nonmagnetic cover surrounding the outer circumferential portion of the anode  36 , but may have an integral structure and may all be formed of a nonmagnetic substance or a metal of a nonmagnetic substance with a high electric conductivity. 
     Furthermore, in the above-described embodiments, the surface of the cathode  36  opposed to the anode target  35  is provided with an inclined portion, and the inclined portion is provided with a plurality of electron generation sources. However, the surface of the cathode  36  opposed to the anode target  35  may have no inclined portion and may be a flat portion provided with a plurality of electron generation sources. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; 
     furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.