X-ray or gamma ray systems or devices – Electronic circuit – X-ray source power supply
Reexamination Certificate
2001-11-09
2003-04-29
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
X-ray source power supply
C439S470000, C439S611000
Reexamination Certificate
active
06556654
ABSTRACT:
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 a system for securing a high voltage cable within an x-ray tube.
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 a vacuum enclosure, a cathode, and an anode. The cathode generally comprises a metallic cathode head and a cathode cup disposed thereon. A rectangular slot formed in the cathode cup typically houses a filament that, when heated via an electrical current, emits a stream of electrons. The cathode is disposed within the vacuum enclosure, as is the anode, which 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 that is circumferentially disposed about the rotor assembly, and is disposed outside of the vacuum enclosure. The vacuum enclosure may be composed of metal (such as copper), glass, ceramic material, or a combination thereof, and is typically disposed within an outer housing.
In operation, 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. The beam of x-rays produced by the electrons then passes through windows defined in the vacuum enclosure and outer housing. Finally, the x-ray beam is 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 the operation of a double-ended 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.
The high voltage applied to the anode and/or cathode is typically supplied via a high-voltage cable. The high-voltage cable typically comprises a plurality of conductive wires protectively covered by an outer covering. In a single-ended tube, one end of the high-voltage cable is attached at one end to an electric power supply, while the other end is typically inserted into a plug connector sufficient to provide the high voltages needed for x-ray tube operation. The plug connector comprises an outer covering and has electrical contacts disposed at one end for electrically connecting the conductive wires of the high voltage cable to the cathode.
Because of the high voltage present in the x-ray tube during operation, the use of insulating structures supportably connecting the anode and/or cathode to the vacuum enclosure or outer housing is necessary to electrically isolate them from the rest of the tube. These insulating structures are typically composed of an electrically 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.
In typical x-ray tubes, a cathode insulating structure comprises a hollow conical shape and is composed of an insulating material such as glass or ceramic. The cathode insulating structure attaches at one end to the housing or vacuum enclosure of the x-ray tube and at the other end to the cathode, which it supports in a position proximate the target surface of the anode as described above. In order to supply the high voltage potential to the cathode, the plug connector of the high-voltage cable is typically disposed within the inner volume defined by the conical cathode insulating structure, where it electrically connects to the cathode. The inner surface of the conical cathode insulating structure defines a frustoconical shape. The outer surface of the plug connector of the high-voltage cable that is disposed within the cathode insulating structure also comprises a frustoconical shape near the end that electrically connects with the cathode. This shape is necessary so as to allow the outer surface of the plug connector to complementarily fit against the inner surface of the cathode insulating structure.
A physically close fit between the outer surface of the plug connector and the inner surface of the cathode insulating structure is necessary in order to avoid electric arcing between the surfaces. If a space develops between the plug connector outer surface and the cathode insulating structure inner surface during tube operation, dangerous electrical arcing may occur, which can damage the highly sensitive components contained within the x-ray tube.
In order to avoid problems associated with electrical arcing between the cathode insulating structure and the high-voltage cable, various assemblies have been used in the past to ensure a tight fit between these two components. For instance, cable clamps have been utilized to secure the high-voltage cable within the inner volume of the cathode insulator. Unfortunately, such clamps have suffered from various setbacks. For instance, it is extremely difficult to ascertain the amount of force that such clamps apply to the high-voltage cable disposed within the cathode insulating structure. If too much force is applied, undue stress is inflicted upon the cathode insulating structure and the high-voltage cable, which may reduce the operating lifetime of one or both of the components. Too little force, on the other hand, opens up the possibility for electrical arcing to occur between the insulating structure and the high-voltage cable which, as explained above, is highly undesirable. Specifically, electrical arcing places an undue amount of electrical stress on the cathode insulating structure and the high-voltage cable, which may affect the performance of the x-ray tube and reduce the operating life of the various components therein.
The inability of known high voltage cable clamp systems to determine the amount of applied force between the high voltage cable and cathode insulating structure is exacerbated by other factors. One
Hansen Wayne R.
Richardson John
Dunn Drew A.
Varian Medical Systems Inc.
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