HIGH-INTENSITY X-RAY SOURCE

Abstract: A high-intensity X-ray source generates significant heat at the anode. To help dissipate this heat, the anode is often ro­ tated in the vacuum. Heat must still be radiated from the anode to the exterior walls. An improved X-ray source incorporates the anode in the walls of the vacuum chamber and rotates the entire chamber. The heat is then easily conducted to the exteri­ or where it may be dissipated by connection or forced air cool­ ing.

Description: This invention pertains to apparatus for generating high-intensity X-rays, particularly to apparatus for X-ray generation with forced liquid or gas cooling of the anode while maintaining the high vacuum within the interior of the apparatus without the use of vacuum-tight rotating joints. High intensity X-ray sources are in increasing demand for applications such as for X-ray lithography for producing integrated circuits, computerized tomography for X-ray imaging, and for X-ray diffraction for analyzing materials. High intensity X-ray sources can be constructed by impinging a high intensity beam of electrons on an anode, but cooling the anode becomes a significant technical problem. Some improvement in distributing the heat from the beam can be achieved by steering the electron beam to different parts of the anode. US-A 2,229,152 to Walsweer and US-A 4,336,476 to Holland disclose an anode sealed entirely in the vacuum which rotates in response to the field from coils exterior to the vacuum. The heat from the anode must be conducted through bearings or radiated through the vacuum to an external cap. US-A 4,128,781 to Flisikowski et al discloses an X-ray tube having a cathode rotatable relative to an anode. Electrons from a rotating cathode are incident on a stationary anode ring. The X-rays are emitted from different positions in space the cathode is rotated. For many applications, it is important that the X-rays be emitted from a position fixed in space. FR-A-2329067 discloses an X-ray source comprising a vacuum chamber, means for rotating the vacuum chamber about an axis, means for generating electrons mounted on internal bearings within said vacuum chamber and an anode being an end wall of said vacuum chamber for receiving electrons generated by said means for generating electrons and connecting means including an rf transformer for coupling AC energy (having a frequency of about 25 kHZ) from a source external to the said vacuum chamber through a wall of the vacuum chamber to said electron generating means, said transformer means comprising a primary coil positioned outside of said vacuum chamber and a secondary coil mounted within said vacuum chamber. The coils are located on a radius of the rotatable vacuum chamber. The secondary coil has a core which is magnetically attracted and so is of magnetic material. DE-A-3213644 discloses an X-ray generator including a rotatable anode in a chamber within which a cathode generates X-rays which are projected through an window in the envelope. The cathode is energized by means of coils coaxial with the axis of the chamber. In this device the frequency of energization is limited to audio frequencies. The present invention improves on the prior art arrangements by by providing an X-ray source according to Claim 1. Examples of the invention with now be described with reference to the drawings in which:- FIG. 1 is a schematic view of an X-ray source having an anode at one end of a cylindrical rotating chamber and a fixed cathode on the axis of rotation. FIG. 2 is a schematic view of an X-ray source having an anode in the cylindrical wall of a rotating cylindrical chamber with an internal cathode that is fixed in space. FIG. 3A is a perspective view of an X-ray source having segments on the periphery of the rotating structure. FIG. 3B is a sectional view from the side of the embodiment in FIG. 3A. FIG. 4A is an end view of an X-ray source having a segmented rotating anode with the segments on the end of the rotating structure. FIG. 4B is a sectional view from the side of the embodiment of FIG. 4A. FIG. 5 is a schematic sectional view of an X-ray source with an anode in the internal wall of a rotating vacuum chamber and a liquid cooling system on the external wall of said rotating vacuum chamber. Referring now to the drawings wherein reference numerals are used to designate parts throughout the various figures thereof, there is shown in FIG. 1 a rotating anode X-ray source. The anode 10 is one end wall of an evacuated chamber 12. A dispenser cathode 18 and indirect heater 20 are mounted inside the bearing cathode structure 16. A rotating transformer consisting of primary coil 22 outside the evacuated chamber 12 and secondary coil 23 inside the evacuated chamber couples radio frequency power to the indirect heater 20. The cylindrical wall 24 is made of ceramic material to insulate the ends and to facilitate passage of the X-ray hem 26. A high voltage source 28 is connected across the end walls. A magnetic field normal to the paper bends and focuses the electron beam 30 off axis striking the inside of the anode 10. A stream of cooling gas 32 is used to cool the anode 10. In operation the evacuated chamber 12 including anode 10 is caused to rotate, supported by bearings 14 and 17 which are fixed in the laboratory. The magnetic field is maintained in a fixed position in the laboratory so that the region in which the X-rays are generated does not move as the anode rotates. If desired, the cooling gas stream 32 may be used to spin evacuated chamber 12. Alternatively, an electric motor (not shown) may be mechanically coupled to evacuated chamber 12 to cause it to rotate. Circular fins can be placed on the outside of the vacuum chamber to aid in dissipating heat. Radial fins of semicircular, parabolic, hyperbolic or other curved shape could be used in conjunction with an airstream directed at the device to both cool and drive the rotation of the vacuum chamber. Another embodiment shown in FIG. 2 uses a cylindrical chamber 40 in which a cylindrical anode 42 and window 44 for X-rays form the cylindrical wall. External bearings 46 and 48 permit the entire chamber to rotate. An indirect heater 50 and focusing structure 52 are mounted on internal bearings 54. A pair of magnets, one magnet 56 mounted inside the chamber on the electron source and another magnet 58 fixed outside the chamber 40, is used to prevent the internal structure from rotating as the chamber 40 is rotated. External magnet 58 and bearing 48 are maintained fixed in the laboratory by structural member 49. Internal bearings 54 permit the internal cathode structure 53 to remain fixed relative to the laboratory as the cylindrical chamber 40 rotates. A high voltage supply 60 is connected through bearing 46 or via slip rings (not shown) from the electron source to the anode 42. Although anode 42 rotates, the position of the electron beam 43 remains fixed with respect to the laboratory so that the region in which the X-rays are generated also remains fixed in the laboratory. The external surface of anode 42 may be cooled by gas stream 45 or by a liquid system that will be explained more fully in FIG. 5. Chamber 40 may be rotated by a gas stream or motor as desired. Another embodiment shown in FIG. 3 again uses a cylindrical structure 70 mounted on bearings 72 and 74. The anode 76 is arranged as a series of short segments electrically insulated from each other mounted on insulating cylinder 78. These segments are individually wired to an external commutator 80 to which the anode high voltage is applied through a set of brushes 82. The brushes may cover several commutator strips simultaneously so that the anode voltage remains applied to the anode segments in a fixed spatial location with respect to the laboratory. In this way the electrons which are generated by cathode 84 on the spin axis are focused to the same region (in the fixed coordinate system) as the anode rotates. The individual anode segments are insulated from each other. The metal anode material may be spatially overlapped so that the focused electron beam always strikes anode material and not the insulating material. The X-rays 88 are extracted through a suitable window 90 adjacent to the anode or may be extracted from the back of the material. Power supply 92 supplies a positive voltage to the anode segments 76 as they rotate into position. Focusing and directing the electron beam 94 from cathode structure 84 is achieved by the positive potential supplied by power supply 92. Additional focusing control can be achieved by placing a suitable voltage on focusing electrode 96 and applying suitable voltages upon other anode segments by one or more additional commutator brushes 102. The focusing electrode 96 and commutator brushes 102 receive proper focusing voltages from power supply 104. Cylindrical structure 70 may be rotated by attached pulley 106 coupled by a belt to a motor 108 (not shown in FIG. 3B). An alternative commutator arrangement is shown in FIGS. 4A and 4B. Here the anode 80a and commutator 82a are located on the end of the rotating cylindrical structure. The segmented anode systems described so far had separate anode segments on the inside of an insulating cylinder or disk connected by an electrical feed-through to a commutator segment on the outside of the cylinder or disk. Using brazing techniques, one can construct a cylinder or disk structure that contains anode segments alternating with ceramic insulating segments so that the exterior of the anode segments is used as the commutator. Another embodiment shown in FIG. 5 uses a fluid such as water to provide cooling of the anode. The interior configuration of FIG. 5 is similar to that of FIG. 2. In FIG. 5 the rear of anode 42 is in immediate contact with a fluid 120 which may be water. The fluid flows into a hollow section 120 of the rotating shaft that supports the vacuum chamber 122. The shaft is supported by bearings 46. The fluid enters the hollow section 120 through the chamber 126 of fluid seal 128. The cooling fluid flows within bearing 46 and provides cooling to it if needed, and then flows through structure 130 which channels the water past anode 42, providing cooling to the back side of the anode. The water then flows out through a hollow center section 132 of the rotating shaft and out through chamber 134 of fluid seal 128. This cooling arrangement is extremely effective since any gas bubbles that are formed at the back of the anode surface 42 are immediately swept out by the high centrifugal force on the liquid produced by the rapidly rotating structure. This invention is not limited to the preferred embodiments heretofore described, to which variations and improvements may be made, without leaving the scope of the claims.

Claims: An X-ray source comprising a vacuum chamber rotatable about an axis, means for generating electrons mounted within said vacuum chamber, an anode forming a part of said vacuum chamber for receiving electrons generated by said generating means and connecting means including a transformer for coupling energy from a source external to said vacuum chamber through a wall of said vacuum chamber to said means for generating electrons, said transformer means comprising a primary coil positioned outside of said vacuum chamber and a secondary coil mounted within the vacuum chamber both coils being coaxial with the axis of rotation of the chamber, the energy being coupled inductively through the wall of the chamber, wherein the coils are adapted to couple RF energy through the wall of the chamber. An X-ray source as claimed in Claim 1 wherein said vacuum chamber is formed by a housing, the entirety of which is rotatable about an axis, a portion of said housing being said anode, said generating means being arranged to focus said electrons onto a region off said axis, the source further comprising means for holding said generating means fixed when said housing is rotated about said axis so that said region remains fixed and said anode rotates through said region. A source as claimed in any one of Claims 1 to 2 wherein said anode comprises a plurality of electrically conducting portions off said axis and equidistant from said axis and separated from one another by electrically insulating material. A source as claimed in claim 2 and Claim 3 further including means for applying an electrical potential to selected ones of said electrically conducting portions not in said region so that said electrons generated by said generating means are directed toward said electrically conducting portions passing through said region. An X-ray source as claimed in Claim 3 or Claim 4 wherein said portions overlap so that said electrons do not impinge on said insulating material. An X-ray source as claimed in any one of Claims 1 to 5 wherein said secondary coil consists of a single turn. An X-ray source as claimed in any one of Claims 1 to 6 wherein said vacuum chamber comprises a first end, a second end and a wall joining said first end to said second end so that said vacuum chamber has a generally cylindrical shape. An X-ray source as claimed in Claim 7 wherein said wall includes said anode. An X-ray source as claimed in Claim 8, wherein said anode comprises a conical ring and wherein a window for X-rays generated by said electrons striking said anode is proximate said conical ring. An X-ray source as claimed in Claim 2 and Claim 7, wherein said first end portion comprises said plurality of electrically conducting portions. An X-ray source as claimed in any one of Claims 1 to 10, wherein said generating means is stationarily mounted on a structure supported by bearings, the X-ray source comprising a first magnetic means fixedly mounted on said structure and a second magnetic means fixedly mounted outside said chamber opposite said first magnetic means. An X-ray source as claimed in Claim 3 or any Claim dependent thereon, further including an electrode stationarily mounted in said vacuum chamber for directing electrons generated by said generating means to impinge on those electrically conducting portions passing through said region. An X-ray source as claimed in any one of Claims 1 to 12 further including means for cooling said anode. An X-ray source as claimed in Claim 13, wherein said means for cooling said anode includes means for conveying a fluid to an external side of said anode. An X-ray source as claimed in Claim 14, wherein said means for conveying comprises means for receiving said fluid from an external source, means for returning said fluid to an external sink and channel means for conveying said fluid from said means for receiving to an external side of said anode and from said external side of said anode to said means for returning.

Inventors: ANDERSON, WESTON ARTHUR
Applicants: VARIAN ASSOCIATES, INC.
Application year: 1985
Main classification: H01J35
IPC codes: H01J35