X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2002-08-19
2004-06-15
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S130000
Reexamination Certificate
active
06751292
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention generally relates to x-ray generating devices. More particularly, the present invention relates to an x-ray tube rotor assembly having superior cooling characteristics.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device.
The x-ray tube generally comprises an outer housing in which is disposed a substantially cylindrical vacuum enclosure. The vacuum enclosure has disposed therein a cathode and an anode. The cathode includes a filament that, when heated via an electrical current, emits a stream of electrons. The anode typically comprises a graphite substrate upon which is disposed a heavy metallic target surface that is oriented to receive the electrons emitted by the cathode. Though some x-ray tube anodes are stationary, many are rotatably supported within the vacuum enclosure by a rotor assembly.
The rotor assembly typically comprises a rotor shaft, a rotor hub and sleeve, a bearing assembly and a magnetic sleeve. One end of the rotor shaft supports the rotary anode, while the other end is attached to the rotor hub and sleeve. The rotor hub interconnects the rotor shaft and the rotor sleeve with the bearing assembly, thereby enabling the shaft and sleeve to rotate. The rotor sleeve is rotationally and concentrically disposed about a substantial portion of the bearing assembly. A stator is used to induce rotation of the rotor sleeve, which in turn causes the rotor shaft and anode to rotate. The magnetic sleeve typically attaches to and covers either the outer surface of the bearing housing or the inner surface of the rotor sleeve to assist the stator in inducing rotation of the rotor sleeve.
In order for the x-ray tube to produce x-rays, an electric current is supplied to the cathode filament of the x-ray tube, causing it to emit a stream of electrons by thermionic emission. A high voltage potential placed between the cathode and the anode causes the electrons in the electron stream to gain kinetic energy and accelerate toward the target surface located on the anode. Upon striking the target surface, many of the electrons convert their kinetic energy into electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten carbide or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. Finally, the x-ray beam passes through windows defined in the vacuum enclosure and outer housing, where it is directed to an x-ray subject, such as a medical patient.
A recurrent problem encountered with the operation of x-ray tubes deals with the removal of heat from tube components. In general, only a small percentage of the electrons that impact the anode target surface during x-ray production do, in fact, produce x-rays. The majority of the kinetic energy is instead released as heat that is absorbed into the anode target surface and surrounding areas. This heat must be continuously and reliably removed from the anode and surrounding components in order to prevent damage to critical tube components. To the extent that the heat is efficiently removed, less thermal and mechanical stress is imposed upon the x-ray tube, and its operation and performance will be enhanced. If the heat is allowed to reach detrimental levels, however, it can damage the anode and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Many approaches have been implemented to help alleviate the problems created by heating within the x-ray tube. For instance, as noted the anode in many x-ray tubes is rotatable. During operation of the x-ray tube, the rotary anode is rotated at high speeds, which causes successive portions of the target surface to continuously rotate into and out of the path of the electron beam produced by the cathode filament. In this way, the electron beam is in contact with any given point on the target surface for only short periods of time. This allows the remaining portion of the surface to cool during the time that it takes to rotate back into the path of the electron beam, thereby reducing the amount of heat that is absorbed by the anode at any given location.
While the rotating nature of the anode reduces the amount of heat present at the target surface, a large amount of heat is still absorbed by the anode substrate and other components within the vacuum enclosure. Of particular concern is the heat that is conducted from the anode to the rotor assembly, and specifically to the bearing assembly. Excessively high temperatures produced in the anode and conducted through the rotor shaft to the bearing sets can melt the thin metal lubricant that surrounds the bearings. This can cause the lubricant to disperse and expose the bearings to excessive friction. The lubricant may also form clumps in the presence of excessive heat, which in turn causes the bearing assembly to create excessive noise and mechanical vibration during tube operation. Such conditions can reduce the x-ray tube's operating efficiency and even image quality. Repeated exposure to high temperatures can gradually degrade the integrity of the bearing surfaces and reduce their useful life or even cause premature bearing failure. Therefore, it is important to reliably and continuously dissipate heat from the x-ray tube, and particularly from the bearing assembly.
In an effort to remove large quantities of heat within the x-ray tube, rotor sleeves have been designed to absorb heat from the rotor shaft and then to radiate that heat to the surrounding vacuum enclosure. While assisting in limiting the amount of heat transmitted by the rotor shaft to the bearing assembly, this approach alone may not be sufficient to prevent large quantities of heat from reaching the bearing sets.
Another technique used for removing heat from an x-ray tube is to place the vacuum enclosure within an outer housing, as mentioned above. The outer housing serves as a container for a coolant, such as a dielectric oil, which surrounds and envelops the vacuum enclosure, and which may be continuously circulated by a pump about the outer surface thereof. As heat is emitted from the x-ray tube components (the anode, support shaft, etc.), it is radiated to the outer surface of the vacuum enclosure, and then at least partially absorbed by the dielectric oil. The heated oil is then passed to some form of heat exchange device, such as a radiative surface, to be cooled. The oil is then re-circulated by the pump back through the outer housing and the process repeated.
While assisting greatly in the dissipation of heat from the x-ray tube, the coolant is a only of partial assistance when attempting to directly remove heat from the bearing housing. This is due to the fact that in typical x-ray tubes, the coolant is only able to directly circulate past a small portion of the bearing housing, namely the bearing shank, which is disposed at the bottom of the bearing assembly. The rest of the bearing housing is typically prevented from direct contact with the coolant by the surrounding vacuum enclosure. Because of this typical design, effective cooling of the bearing assembly, and specifically the bearing sets, is difficult to achieve.
In light of the above discussion, a
Andrews Gregory C.
Barrett Vaughn Leroy
Church Craig E.
Varian Medical Systems Inc.
Workman Nydegger
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