Rotating-anode X-ray tube

Abstract

 Disclosed is a rotating-anode X-ray tube which comprises a housing (10), a cathode (22) and a rotating anode (26) arranged in one end side of the housing, a rotating cylinder (40) set in the other end side of the housing (10) and adapted to be rotated by an electric motor (52), the rotating cylinder (40) supporting the rotating anode (26), a magnetic bearing (60) disposed in the other end side of the housing (10) and supporting the rotating cylinder (40) uncontacted in the radial and axial directions thereof, and a shielding wall (34) for dividing the inside space of the housing (10) into a first chamber containing the cathode (22) and the rotating anode (26), and a second chamber containing the magnetic bearing (60), whereby the first chamber is thermally and electromagnetically shielded from the second chamber.

Description

[0001] The present invention relates to a rotating-anode X-ray tube, and more specifically to a rotating-anode X_-ray tube in which a rotating shaft rotated together with its rotating anode is supported by a magnetic bearing.

[0002] In a rotating-anode X-ray tube of this type, thermions emitted from a cathode are caused to strike against the target surface of a rotating anode so that the energy of the thermions is discharged as X-rays. Thus, in such a rotating-anode X-ray tube, the substantial target surface area of the rotating anode struck by thermions can be made wider than that of a stationary anode of a stationary-anode tube, so that heat load applied to the rotating anode can be reduced. To maximize this advantage of the rotating-anode X-ray tube, therefore, the rotating anode should preferably be rotated as fast as possible.

[0003] However, in the rotating-anode X-ray tube in which a rotating shaft rotated together with the rotating anode is supported by a mechanical bearing of a contact type, the inside of the X-ray tube is kept at a vacuum, so that the mechanical bearing cannot be effectively supplied with lubricating oil. If the anode is rotated at a high speed, therefore, the amount of heat applied to the mechanical bearing will increase. Moreover, although the amount of heat on the rotating anode may be smaller than that applied to the stationary anode of the stationary-anode tube, the target surface of the rotating anode is heated to more than a thousand degrees centigrade during use. Thus, the mechanical bearing of the stationary-anode tube will be heated further due to the external factor of heat being conducted from the target surface of the rotating anode.

[0004] In order to avoid overheating which is intrinsic to mechanical bearings, a rotating-anode X-ray tube is proposed in which a magnetic bearing is used in place of the mechanical bearing, whereby the rotating shaft of the rotating anode is supported uncontacted. As is generally known, the magnetic bearing can support the rotating shaft uncontacted in its axial and radial directions, so that only a very small amount of heat is generated from the magnetic bearing during use. Therefore, the amount of heat applied to the magnetic bearing can be greatly reduced. As the magnetic bearing has many advantages in a vacuum, the rotating shaft can be rotated faster by a magnetic bearing than in the case where the rotating shaft is supported by the mechanical bearing. .

[0005] Like the rotating-anode X-ray tube using the mechanical bearing, however, the rotating-anode _X-ray tube using the magnetic bearing has inevitable drawbacks. For stable uncontacted supporting of the rotating shaft, the magnetic bearing is provided with position detectors for detecting the axial and radial displacement of the rotating shaft. Magnetic sensors are generally used as position detectors. Basically, the magnetic sensors electromagnetically detect the displacement of the rotating shaft, ;o that their outputs may greatly be influenced by X-rays or other electromagnetic waves. Also, samarium-cobalt or other rare-earth magnets used in the magnetic bearing will deteriorate if exposed to X-rays or other electromagnetic waves. Thus, when supporting the rotating shaft of the rotating-anode X-ray tube by means of a magnetic bearing using magnetic sensors and rare-earth magnets, X-rays emitted from the rotating anode will be scattered and applied to the magnetic sensors and rare-earth magnets. Accordingly, the outputs of the magnetic sensors will be adversely affected, and the rare-earth magnets will deteriorate in time. Thus, it is difficult to securely support the rotating shaft with stability, that is, to rotate the rotating anode stably.

[0006] The object of the present invention is to provide a rotating-anode X-ray tube capable of checking the bad influences of heat and electromagnetic waves from the rotating anode on the driving members for rotating the rotating anode, thereby ensuring stable rotation of the rotating anode.

[0007] The above object may be achieved by a rotating-anode X-ray tube which comprises a housing provided on one end having an X-ray radiating portion for radiating X-rays, a cathode disposed on one end of the housing, a rotating anode rotatably disposed close to the cathode in the housing and adapted to emit X-rays when struck by thermions radiated from the cathod, the X-ray from the rotating anode being radiated from the housing through the X-ray radiating portion thereof, driving means disposed in the housing on the other end side thereof to rotate the rotating anode, and shielding means for dividing the inside of the housing'into a first chamber containing the rotating anode and a second chamber containing the driving means so that the second chamber is thermally and/or electromagnetically shielded from the first chamber.

[0008] According to the present invention, heat radiated from the rotating anode may be intercepted by the shielding means, preventing the driving means from being heated by the heat from the rotating anode, that is, the amount of heat from the driving means can greatly be reduced.

[0009] If the driving means is provided with a magnetic bearing to support the rotating shaft of the rotating anode, the shielding means can also intercept X-rays from the rotating anode which are to be applied to the magnetic sensors and rare-earth magnets of the magnetic bearing. Thus, the magnetic bearing can stably support the rotating shaft, i.e., the rotating anode.

[0010] This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 a sectional view of a rotating-anode X-ray tube according to a first embodiment of the present invention;

Fig. 2 is a sectional view taken along line II-II of Fig. 1₁.

Fig. 3 is a partial sectional view of a rotating-anode X-ray tube according to a second embodiment of the invention; and

Fig. 4 is a partial sectional view of a rotating-anode X-ray tube according to a third embodiment of the invention.

[0011] Referring now to Figs. 1 and 2, there is shown a rotating-anode X-ray tube according to a first embodiment of the present invention. The rotating-anode _X-ray tube is provided with a hollow cylindrical vacuum housing 10. The vacuum housing 10 comprises first and second metallic shell: 12 and 14 each opening at one end. The first and second shells 12 and 14 are airtightly coupler by means of a plurality of connecting screws ::) so that flange portions 16 and 18 formed on the respective open ends of the first and second shells 12 and 14 are joined together. An exhaust tube (not shown in Fig. 1) is connected to the vacuum housing 10. The exhaust tube is connected to, e.g., a vacuum pump, and is sealed after the vacuum housing 10 is evacuated to a predetermined degree of vacuum. In this first embodiment, the vacuum housing 10 is made of metal. Alternatively, however, it may be formed of glass.

[0012] Inside the first shell 12 of the vacuum housing 10, a cathode 22 having a tungsten coil filament and focusing electrodes (not shown) is disposed close to the peripheral wall of the first shell 12. The cathode 22 is electrically connected to and supported by a stem 24. The stem 24 first extends inward from the cathode 22 in the radial direction of the first shell 12, and is then bent to extend along the axis of the first shell 12. The upper end of the stem 24 penetrates the closed end wall of the first shell 12 in an airtight manner to extend to the outside.

[0013] Inside the first shell 12, moreover, a rotating anode 26 in the form of a flat, circular truncated cone is disposed coaxially with the first shell 12 facing the cathode 22. The rotating anode 26 is formed of tungsten, and the tapered peripheral surface of the rotating anode 26 defines what is called the target surface 28 which is struck by thermions emitted from the cathode 22. The angle between the target surface 28 of the rotating anode 26 and the axis of the first shell 12 is set so that X-rays produced when the thermions from the cathode strike against the target surface 28 are radiated through a glass X-ray window 30 which is attached to the peripheral wall of the first shell 12.

[0014] The rotating anode 26 is connected to a supporting shaft 32 made of, e.g., molybdenum. The supporting shaft 32, which is coaxial with the first shell 12 or the vacuum housing 10, extends into the second shell 14 through an opening in a shielding wall 34 which divides the inside space of the vacuum housing 10 in two.

[0015] The shielding wall 34 is fitted in an annular groove 36 which is defined by a pair of step portions formed along the joined inner peripheries of the respective open ends of the first and second housing shells 12 and 14. Thus, when the first and second shells 12 and 14 are coupled together in the aforesaid manner, the shielding wall 34 is fixed between the first and second shells 12 and 14. The shielding wall 34 is formed of a highly conductive material with a high thermal reflection factor. For example, the shielding wall 34 may be formed by a coating plate made of tungsten, molybdenum or another conductive material with a high thermal-reflection material. The shielding wall 34 is grounded through the vacuum housing 10. Thus, the respective insides of the first and second shells 12 and 14 are thermally and electromagnetically shielded from each other.

[0016] The shaft 32 of the rotating anode 26 extending into the second shell 14 is coupled to a rotating cylinder 40 of a rotating mechanism 38.

[0017] The second shell 14 will now be briefly described. A hollow intermediate cylinder portion 14b coaxial with an outer peripheral wall 14a of the second shell 14 protrudes from the closed end wall of the second shell 14 toward the first shell 12. Also, a hollow inner cylinder portion 14c coaxial with the intermediate cylinder portion 14b protrudes inward from the end wall of the intermediate cylinder portion 14b near the first shell 12. Thus, the second shell 14 has a triple-wall structure, as shown in Fig. 1.

[0018] The rotating cylinder 40 is in the form of a hollow cylinder opened at one end, and is contained in an annular space defined between the outer peripheral wall 14a and the intermediate cylinder portion 14b of the second shell 14, allowing a radial clearance in the space. The shaft 32 of the rotating anode 26 is supported on the end wall of the rotating cylinder 40 beside the shielding wall 34 by means of an electric insulating member 42, axially extending in the inner cylinder portion 14c. Here it is to be noted that mechanical bearings 44 of a contact type are arranged at an axial interval on the inner peripheral surface of the inner cylinder portion 14c. The bearings 44, which are not in contact with the supporting shaft 32 in the normal operating state, serve to support the supporting shaft 32 in case of an emergency.

[0019] A contact pin 90 protrudes downward from the lower end (Fig. 1) of the supporting shaft 32. A contact plate 92 which cooperates with the contact pin 90 is disposed inside the inner cylinder portion 14c, axially spaced from the contact pin 90. The contact plate 92 is electrically connected to a conductive rod 94 which penetrates the bottom wall of the inner cylinder portion 14c in an airtight manner. The conductive rod 94 and the stem 24 of the cathode 22 are electrically connected to a power source for applying a predetermined voltage between the rotating anode 26 and the cathode 22.

[0020] The second shell 14 is covered by an outer housing portion 50 of the rotating mechanism 38 with an annular gap between them. A stator 54 of an induction motor 52 is attached to the inside of the peripheral wall of the outer housing portion 50. The armature coil of the stator 54 is electrically connected to a power source (not shown) to drive a motor. A rotor 56 of the motor 52 is fixed to the outer peripheral surface of the rotating cylinder 40 so as to face the stator 54.

[0021] The rotating cylinder 40, which is contained in the annular space between the outer peripheral wall 14a and the intermediate cylinder portion 14b of the second shell 14 with the radial clearance, as stated before, is supported in the radial direction by a magnetic bearing 60 so that it is neither in contact with the outer peripheral wall 14a of the second shell 14 nor with the outer peripheral surface of the intermediate cylinder portion 14b. The rotating cylinder 40 is supported uncontacted also in the axial direction.

[0022] The magnetic bearing 60 is provided with a yoke 62 which is fitted in a space defined between the inner peripheral surface of the intermediate cylinder portion 14b and the outer peripheral surface of the inner cylinder portion 14c. The yoke 62 is made of, e.g., a magnetic material, and, generally, is in the form of a hollow cylinder which is formed by joining rings with an inside diameter equal to the outside diameter of the inner cylinder portion 14c. Four magnetic poles 64a, 64b, 64c and 64d, and 66a, 66b, 66c and 66d protrude radially outward from each of the upper and lower end portions (Fig. 1) of the yoke 62, respectively. Fig. 2 shows the magnetic poles 64a to 64d at the upper end of the yoke 62. Since the magnetic poles at the upper and lower ends of the yoke 62 have the same construction, only the upper magnetic poles 64a to 64d will be described in detail. The magnetic poles 64a to 64d are arranged at rectangular intervals along the circumference. The outside diameter measured between the peripheral surfaces of the two opposite magnetic poles 64a and 64c or between those of the other two magnetic poles 64b and 64d is equal to the inside diameter of the intermediate cylinder 14b. Conductive coils 80 are individually wound around the magnetic poles 64a to 64d. A radially projecting ring-shaped magnetic pole 68 is formed on the central portion of the yoke 62 between the magnetic poles 64 and 66 at the upper and lower ends. The outside diameter of the magnetic pole 68 is equal to the inside diameter of the intermediate cylinder poriton 14b. A pair of ring-shaped conductive coils 70a and 70b is wound around the outer peripheral surface of the yoke 62 so as to hold the magnetic pole 68 between the two coils 70a and 70b along the axial direction of the yoke 62. The conductive coils 80, 70a and 70b are electrically connected to a power source (not shown).

[0023] Ring-shaped permanent magnets 72 and 74 are fixed on the yoke 62 located between the magnetic poles 64 and 68, and between the magnetic poles 68 and 66, respectively. The permanent magnets 72 and 74 are magnetized in the radial direction.

[0024] Annular grooves are formed in the inner peripheral surface of the intermediate cylinder portion 14b, corresponding to the regions between the magnetic poles 64 and the conductive coil 70a, and between the conductive coil 70b and the magnetic poles 66, individually. Laminated magnetic rings 76 and 78 with high permeability are fixedly fitted in the annular grooves, individually.

[0025] A plurality of displacement sensors 82 for detecting the radial displacement of the rotating cylinder 40 is fixed to the inside of the peripheral wall of the outer housing portion 50, facing the magnetic poles 64a to 64d and 66a to 66d. For example, magnetic sensors are used for the displacement sensors 82 which convert the radial displacement of the rotating cylinder 40 into a quantity of electricity. Also, a plurality of magnetic sensors 84 similar to the sensors 82 and adapted to detect the axial displacement of the rotating cylinder 40 is fixed to the lower portion (_Fig. 1) of the inner peripheral surface of^*the outer housing portion 50. The output ends of the magnetic sensors 82 and 84 are electrically connected to a stabilization control circuit (not shown) which controls the values of currents supplied to the conductive coils 80, 70a and 70b. The stabilization control circuit naturally includes the power source for the conductive coils 80, 70a and 70b.

[0026] According to the magnetic bearing 60 described above, magnetic fluxes delivered from the one permanent magnet 72 form a magnetic circuit Ml which corresponds to a loop connecting the permanent magnet 72, the magnetic ring 76, the magnetic poles 64, and the yoke 62; and a magnetic circuit M2 which corresponds to a loop connecting the permanent magnet 72, the magnetic ring 76, the magnetic pole 68, and the yoke 62. Likewise, magnetic fluxes delivered from the other permanent magnet 74 form a magnetic circuit M3 which corresponds to a loop connecting the permanent magnet 74, the magnetic ring 78, the magnetic pole 68, and the yoke 62; and a magnetic circuit M4 which corresponds to a loop connecting the permanent magnet 74, the magnetic ring 78, the magnetic poles 66, and the yoke 62. In Fig. 1, magnetic circuits Ml to M4 are shown by broken lines, respectively. Please note that they are shown only on the right side in the figure for convenience's sake. Thus, the rotating cylinder 40 is supported uncontacted in both radial and axial directions by the magnetic forces of the magnetic circuits Ml to M4 of the magnetic bearing 60 controlled by adjusting the current supplied to the conductive coils 80, 70a and 70b. If the stator 54 of the motor 52 is energized in this state, the rotor 56 of the motor 52 or the rotating cylinder 40 is rotated uncontacted in the radial and axial directions.

[0027] If the radial position of the rotating cylinder 40 is shifted, that is, if the axis of the rotating cylinder 40 is deviated from that of the vacuum housing 10 by any external force or other factor while the rotating cylinder 40 is being rotated in the uncontacted state, the magnetic sensors 82 can detect the radial displacement of the rotating cylinder 40. Thus, the radial deviation of the rotating cylinder 40 can be corrected to align the axis of the rotating cylinder 40 with that of the vacuum housing 10 by controlling the amount of current flowing through the conductive coils 80 on the magnetic poles 64 and 66 to properly vary the magnetic forces of the magnetic circuits Ml and M4 of the magnetic bearing 60 by means of the stabilization control circuit in accordance with output signals from the magnetic sensors 82. In consequence, the rotating cylinder 40 can stably be supported uncontacted in the radial direction.

[0028] While the rotating cylinder 40 is being rotated while it is stably supported in the radial direction, it can be moved downward (Fig. 1) by the force of attraction by controlling the current supplied to the conductive coils 70a and 70b of the magnetic bearing 60 which in turn varies the magnetic forces from the magnetic circuits M2 and M3, that is, by increasing the magnetic force of the magnetic circuit M3. As a result, the supporting shaft 32 of the rotating anode 26 supported by the rotating cylinder 40 is also moved downward, so that the contact pin 90 of the supporting shaft 32 abuts against the contact plate 9'2 to be electrically connected therewith. Accordingly, a predetermined electric potential difference is caused between the cathode 22 and the rotating anode 26, so that thermions emitted from the filament of the cathode 22 are accelerated to strike against the target surface 28 of the rotating anode 26. As a consequence, X-rays produced by the collision of the thermions are radiated from the target surface 28 of the rotating anode 26 toward the X-ray window 30 of the first shell 12, and are discharged to the outside through the X-ray window 30.

[0029] The magnetic sensors 84 can detect the axial displacement of the rotating cylinder 40 from the position where the contact pin 90 and the contact plate 92 are electrically connected. The axial displacement or deviation can be corrected by suitably controlling the current supplied to the conductive coils 70a and 70b in accordance with the output signals from the magnetic sensors 84. Thus, the electrical connection between the contact pin 90 and the contact plate 92 is prevented from being unexpectedly cut off. Also, the contact pin 90 is prevented from being unduly pressed against the contact plate 92 with excessive force. Here it is to be noted that the connection or disconnection between the contact pin 90 and the contact plate 92 is controlled by controlling the current supply to the conductive coils 70a and 70b.

[0030] In the X-ray tube according to the first embodiment described above, the support of the rotating cylinder 40 by the magnetic bearing 60 will never be adversely affected by heat or _X-rays radiated from the rotating anode 26. Part of the X-rays emitted from the target surface 28 of the rotating anode 26 are normally scattered within the first shell 12 without being radiated through the X-ray window 30. However, since the first and second shell 12 and 14 are divided by the conductive sheilding wall 34, the scattered _X-rays moving from the first shell 12 into the second shell 14 can effectively be absorbed by the shielding wall 34. Accordingly, the magnetic sensors 82 and 84 of the magnetic bearing 60 will never be exposed to X-rays, and so their outputs are protected against the adverse effects of X-rays. Thus, the rotating cylinder 40 can stably be supported in the radial and axial directions by the magnetic bearing 60.

[0031] Since the shielding wall 34 is coated with a material with a high thermal reflection factor, the heat radiated from the rotating anode 26 is intercepted by the shielding wall 34. In other words, the shielding wall 34 can restrain the magnetic bearing 60 in the second shell 14 from being heated by the heat radiated from the rotating anode 26, so that the increase of the temperature of the magnetic bearing 60 can be minimized to reduce the heat load on the magnetic bearing 60.

[0032] The present invention is not limited to the first embodiment described above. Referring now to Figs. 3 and 4, there are shown second and third embodiments of the invention. In the second embodiment shown in Fig. 3, one of the mechanical bearings 44 is attached to the shielding wall 34. By doing this, the axial dimension or length of the supporting shaft 32 can be shortened without changing the axial distance between the two mechanical bearings 44. In other words, the supporting shaft 32 can more securely be supported in an emergency, and besides, the inner cylinder portion 14c can be axially shortened due to the reduction in size of the supporting shaft 32. Thus, the inner cylinder portion 14c places no restrictions on the inside diameter of the yoke 62. If the area of the yoke 62 which allows magnetic fluxes of the magnetic circuits is regarded as fixed, the inside and outside diameters of the yoke 62, and hence the diameter of the whole X-ray tube, can be made smaller than those of the yoke shown in Fig. 1.

[0033] In the third embodiment shown in Fig. 4, a coolant passage 100 is defined in the shielding wall 34. The coolant passage 100 is an annular passage which is axially divided into two parts, i.e., upper and lower annular passage portions 104 and 106, by a partition wall 102. An annular communication hole 108 connecting the upper and lower passage portions 104 and 106 is formed along the inner peripheral edge of the partition wall 102. Inlet ports 110 and outlet ports 112 for a coolant are formed in diametrically opposed portions of the outer peripheral edge of the shielding wall 34. As seen from Fig. 4, the first and second shell 12 and 14 are coupled not directly, but with the shielding wall 34 airtightly sandwiched between them. For example, water or oil may be used for the coolant. Thus, according to the third embodiment, the shielding wall 34 can effectively be cooled by circulating the coolant through the passage 100, so that the heat transferred from the first shell 12 to the inside of the second shell 14 can be more effectively reduced. In the third embodiment, moreover, if the shielding wall 34 is formed from a material with high heat absorptivity and heat conductivity, the temperature of the area around the rotating anode 26 can be lowered, so that the heat load on the rotating anode 26 can be reduced.

[0034] In the embodiments described above, the shielding wall 34 has both thermal and electromagnetic screening functions. If a mechanical bearing is used in place of the magnetic bearing 60, however, the shielding wall 34 need have only the thermal shielding function.

Claims

1. A rotating-anode X-ray tube comprising a housing (10) provided at one end side thereof with an X-ray radiating portion (30) for radiating X-rays, a cathode (22) disposed in the housing on one end side thereof, a rotating anode (26) rotatably disposed close to the cathode in the housing and adapted to emit X-rays when struck by thermions radiated from the cathode, the X-rays from the rotating anode being radiated from the housing through the X-ray radiating portion thereof, and driving means disposed in the housing on the other end side thereof, whereby the rotating anode is rotated,
characterized by further comprising shielding means (34) for dividing the inside space of the housing into a first chamber containing the cathode and the rotating anode and a second chamber containing the driving means so that the first chamber is thermally and/or electromagnetically shielded from the second chamber.

2. The rotating-anode X-ray tube according to claim 1, characterized in that the driving means includes a rotating shaft (40) supporting the rotating anode, an electric motor (52) unit for rotating the rotating shaft, and a magnetic bearing (60) for supporting the rotating shaft uncontacted in the radial and axial directions thereof.

3. The rotating-anode X-ray tube according to claim 2, characterized in that the shielding means includes a shielding wall -{34) formed of a material with high thermal absorptivity and thermal conductivity and adapted to divide the inside space of the housing into a first chamber containing the cathode and the rotating anode and a second chamber containing the magnetic bearing for the rotating shaft, whereby heat radiated from the rotating anode is prevented from being transmitted to the interior of the second chamber.

4. The rotating-anode X-ray tube according to claim 2, characterized in that the shielding means includes a shielding wall (34) having a heat reflecting surface on the first chamber side and adapted to divide the inside space of the housing into the first chamber containing the cathode and the rotating anode and the second chamber containing the magnetic bearing for the rotating shaft, whereby heat radiated from the rotating anode is prevented from being transmitted to the interior of the second chamber.

5. The rotating-anode X-ray tube according to claim 4, characterized in that the shielding wall (34) is formed of a material with a high thermal reflection factor.

6. The rotating-anode X-ray tube according to claim 4, characterized in that the surface of the shielding wall on the first chamber side is covered with a material with a high thermal reflection factor.

7. The rotating-anode X-ray tube according to claim 3, characterized in that a coolant passage (100) for circulating a refrigerant therein is defined in the shielding wall.

8. The rotating-anode X-ray tube according to claim 2, characterized in that the shielding means includes a conductive shielding wall (34) for dividing the inside space of the housing into the first chamber containing the rotating anode and the second chamber containing the magnetic bearing for the rotating shaft, whereby X-rays emitted from the rotating anode are prevented from entering the second chamber.

9. The rotating-anode X-ray tube according to claim 8, characterized in that the shielding wall (34) is formed by coating at least that surface of a plate made of a highly conductive material which faces the first chamber with a material having a high thermal reflection factor.

10. The rotating-anode X-ray tube according to claim 2, characterized in that the housing is provided with a pair of contact-type bearings (44) for mechanically supporting the rotating shaft in case of an emergency.

11. The rotating-anode X-ray tube according to claim 2, characterized in that the housing and the shielding means are each provided with a contact-type bearing (44) for mechanically supporting the rotating cylinder in an emergency.

Drawing