X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2001-04-09
2003-02-11
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S200000, C378S141000
Reexamination Certificate
active
06519317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, embodiments of the present invention relate to an x-ray tube cooling system that increases the rate of heat transfer from the x-ray tube so as to significantly improve tube performance and at the same time control stress and strain in the x-ray tube structures and thereby extend the operating life of the device.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when electrons are accelerated, and then impinged upon a material of a particular composition.
Typically, this process is carried out within a vacuum enclosure. Disposed within the evacuated enclosure is an electron generator, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes electrons to be emitted. A high voltage potential is then placed between the anode and the cathode, which causes the emitted electrons accelerate towards a target surface positioned on the anode. Typically, the electrons are “focused” into an electron beam towards a desired “focal spot” located at the target surface.
During operation of an x-ray tube, the electrons in the beam strike the target surface (or focal track) at a high velocity. The target surface on the target anode is composed of a material having a high atomic number, and a portion of the kinetic energy of the striking electron stream is thus converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as a patient's body. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be produced in the target anode. As a result, the target anode typically experiences extremely high operating temperatures. At least some of the heat generated in the target anode is absorbed by other structures and components of the x-ray device as well.
A percentage of the electrons that strike the target surface rebound from the surface and then impact other “non-target” surfaces within the x-ray tube evacuated enclosure. These are often referred to as “secondary” electrons. These secondary electrons retain a significant amount of kinetic energy after rebounding, and when they impact these other non-target surfaces, a significant amount of heat is generated. This heat can ultimately damage the x-ray tube, and shorten its operational life. In particular, the heat produced by secondary electrons, in conjunction with the high temperatures present at the target anode, often reaches levels high enough to damage portions of the x-ray tube structure. For example, the joints and connection points between x-ray tube structures can be weakened when repeatedly subjected to such thermal stresses. Such conditions can shorten the operating life of the tube, affect its operating efficiency, and/or render it inoperable.
The consequences of high operating temperatures and inadequate heat removal in x-ray tubes are not limited solely to destructive structural effects however. For example, even in relatively low-powered x-ray tubes, the window area can become sufficiently hot to boil coolant that is adjacent to the window. The bubbles produced by such boiling may obscure the window of the x-ray tube and thereby compromise the quality of the images produced by the x-ray device. Further, boiling of the coolant can result in the chemical breakdown of the coolant, thereby rendering it ineffective, and necessitating its removal and replacement. Also, the window structure itself can be damaged from the excessive heat; for instance, the weld between the window structure and the evacuated housing can fail.
While the aforementioned problems are cause for concern in all x-ray tubes, these problems become particularly acute in the new generation of high-power x-ray tubes which have relatively higher operating temperatures than the typical devices. In general, high-powered x-ray devices have operating powers that exceed 40 kilowatts (kw).
Attempts have been made to reduce temperatures in x-ray tubes, and thereby minimize thermal stress and strain, through the use of various types of cooling systems. However, previously available x-ray tube cooling systems and cooling media have not been entirely satisfactory in providing effective and efficient cooling. Moreover, the inadequacies of known x-ray tube cooling systems and cooling media are further exacerbated by the increased heat levels that are characteristic of high-powered x-ray tubes.
For example, conventional x-ray tube systems often utilize some type of liquid cooling arrangement. In many of such systems, a volume of a coolant is contained inside the x-ray tube housing so as to facilitate natural convective cooling of x-ray tube components disposed therein, and particularly components that are in relatively close proximity to the target anode. Heat absorbed by the coolant from the x-ray tube components is then conducted out through the walls of the x-ray tube housing and dissipated on the surface of the x-ray tube housing. However, while these types of systems and processes are adequate to cool some relatively low powered x-ray tubes, they may not be adequate to effectively counteract the extremely high heat levels typically produced in high-power x-ray tubes.
As suggested above, the ability of conventional cooling systems to absorb heat from the x-ray device is primarily a function of the type of coolant employed, and the surface area of the x-ray tube housing. Most conventional systems have focused on the use of various coolants to effect the required heat transfer.
Coolants typically employed in conventional cooling systems include dielectric, or electrically non-conductive, fluids such as dielectric oils or the like. One important function of these coolants is to absorb heat from electrical and electronic components, such as the stator, disposed inside the x-ray tube housing. In order to effect heat removal from these components, the coolant is typically placed in direct contact with them. If the coolant were electrically conductive, rather than dielectric, the coolant would quickly short out or otherwise damage the electrical components, thereby rendering the x-ray tube inoperable. Thus, the dielectric feature of the coolants typically employed in conventional x-ray tube cooling systems is critical to the safe and effective operation of the x-ray tube.
While dielectric type coolants thus possess some properties that render them particularly desirable for use in x-ray tube cooling systems, the capacity of such coolants to remove heat from the x-ray tube is inherently limited. As is well known, the capacity of a cooling medium to store thermal energy, or heat, is often expressed in terms of the specific heat of that medium. The specific heat of a given cooling medium is at least partially a function of the chemical properties of that cooling medium. The higher the specific heat of a medium, the greater the ability of that medium to absorb heat.
Thus, the relatively low specific heat (c), typically in the range of about 0.4 to about 0.5 BTU/lb. ° F., of the cooling media employed in conventional x-ray tube cooling systems have a significant limiting effect on the ability of those media to effect the heat tra
Andrews Gregory C.
Campbell Allen C.
Miller Robert S.
Richardson John E.
Dunn Drew A.
Kiknadze Irakli
Varian Medical Systems Inc.
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