X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2003-03-03
2004-11-16
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S143000
Reexamination Certificate
active
06819741
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention generally relates to high voltage devices. More particularly, the present invention relates to an apparatus and method for adjusting voltage potentials on the surface of insulating structures used in high voltage devices.
2. The Relevant 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 a vacuum enclosure, a cathode, and an anode. The cathode generally comprises a metallic cathode head housing a filament that, when heated via an electrical current, emits electrons. The cathode is disposed within the vacuum enclosure, as is the anode that is oriented to receive the electrons emitted by the cathode. The anode, which typically comprises a graphite substrate upon which is disposed a heavy metallic target surface, can be stationary within the vacuum enclosure, or can be rotatably supported by a rotor shaft and a rotor assembly. The rotary anode is typically spun using a stator. Often, the vacuum enclosure is disposed within an outer housing for cooling and insulating purposes.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. A high electric potential, or voltage, placed between the cathode and anode causes the electron stream to gain kinetic energy and accelerate toward the target surface located on the anode. The point at which the electrons strike the target surface is referred to as the focal spot. Upon approaching and striking the focal spot, many of the electrons convert their kinetic energy and either emit, or cause the target surface material to emit, 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. The target surface of the anode is angled to minimize the size of the resultant x-ray beam, while maintaining a sufficiently sized focal spot. The x-ray beam is collimated before exiting the x-ray tube through windows defined in the vacuum enclosure and outer housing. The x-ray beam is then directed to the x-ray subject to be analyzed, such as a medical patient or a material sample.
Several types of x-ray tubes are commonly known in the art. Double-ended x-ray tubes electrically bias both the cathode and the anode with a high negative and high positive voltage, respectively. The voltage applied to the cathode and anode may reach +/−75 kilovolts (“kV”) or higher during tube operation, depending on the type of x-ray tube. In contrast, single-ended x-ray tubes electrically bias only the cathode, while maintaining the anode at the housing or ground potential. In such tubes, the cathode may be biased with a voltage of −150 kV or more during tube operation. In either case, a sufficient differential voltage is established between the anode and the cathode to enable electrons produced by the cathode filament to accelerate toward the target surface of the anode.
Because of the high voltage differential present between them, an electric field is created between the anode and the cathode during tube operation. The high voltages present at the anode and/or cathode also necessitate the use of insulating structures supportably connecting the anode and/or cathode to the vacuum enclosure or outer housing to electrically isolate them from the rest of the tube. These insulating structures are typically composed of an insulative material, such as glass or ceramic, and may comprise a variety of shapes. Regardless of their shape however, the insulating structures must accommodate the reduction in voltage from the high voltage present at the anode and/or cathode to the much lower housing or ground potential typically present at the surface of the vacuum enclosure.
The interaction of the electric field with the insulating structures for the anode and/or cathode creates a voltage potential distribution along the insulating length of the insulating structure. The insulating length is defined as the length of insulating structure existing between the high voltage source and the low voltage device structure. In an x-ray tube, the insulating length of the insulating structure extends from the anode and/or cathode to the vacuum enclosure, with high voltage present in the insulating structure near the anode or cathode, and low voltage in the insulating structure near the enclosure. In this way, the high voltage of the electric field is gradually dissipated along the length of the insulating structure, thereby electrically isolating the anode and/or cathode and protecting other tube components.
It has been discovered that during tube operation, the voltage potential distribution in the insulating structures created by the electric field existing between the anode and the cathode tends to concentrate near the high voltage source, in this case the anode and/or cathode. Among other things, this field concentration causes the overall voltage drop between the high voltage source and the vacuum enclosure to occur over a shorter distance of the insulating structure than the entire length thereof In other words, a portion of length of the insulating structure is not utilized to accommodate the necessary voltage drop between the anode and/or cathode and the enclosure. Several problems are created by this field concentration in the insulating structure. First, a waste of insulating structure occurs because a portion of the structure nearest the vacuum enclosure is not utilized. Worse, however, is an added per unit electric field stress that is imposed on the portion of the insulating structure nearest the anode and/or cathode, where the field concentration occurs. This electric field stress is highly undesirable because it may weaken over time the structural integrity of the x-ray tube. Eventually, the insulating structure may fail, causing substantial damage to the x-ray tube and requiring much time and expense to correct.
Various solutions have been attempted to resolve the effects caused by the electric field concentration near the anode and/or cathode. One attempted solution has involved increasing the size of the insulating structure near the anode and/or cathode in order to spread out the electric field concentration, and thus the electric field stress. Such a solution may be undesirable or impossible, however, given the tight space constraints present in many high voltage devices, especially x-ray tubes.
A need therefore exists to provide a manner by which electric field stress present in insulating structures of high voltage devices, such as x-ray tubes, may be mitigated. More generally, a need exists to enable the shaping of high voltage gradients along the length of an insulating structure in a high voltage device as may be desired by the operators of such devices.
BRIEF SUMMARY OF THE INVENTION
In accordance with the invention as embodied and broadly described herein, the foregoing needs are met by a method and apparatus for modifying the voltage potential distribution in insulating structures, or insulators, employed in high voltage devices. Preferred embodiments of the present invention are directed to altering the boun
Church Craig E.
Varian Medical Systems Inc.
Workman Nydegger
Yun Jurie
LandOfFree
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