X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2000-11-28
2001-10-09
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S141000, C378S203000
Reexamination Certificate
active
06301332
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thermal energy management system, and more particularly, to a thermal energy storage and transfer assembly for gathering radiant thermal energy and kinetic energy of electrons, such as within an electron beam generating device.
Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. In an x-ray tube, for example, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube, and is removed from the vacuum vessel by a cooling fluid circulating over the exterior surface of the vacuum vessel. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses which are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary. The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. Further, to accelerate the electrons, a typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports.
As mentioned above, many of the incident electrons are not converted to x-rays, and are deflected away from the target in random directions. For example, up to about 50 percent of the incident primary electrons are back scattered from a tungsten anode target. These back scattered electrons travel on a curvilinear path through the electric field between the cathode and anode until they impact another structure. These electrons interact with the electric field and space charge, causing their initial trajectories to be altered in a complicated, but predictable, manner. The electrons back scatter and bounce off of the internal components of the x-ray tube, transferring kinetic energy, until all of their energy is depleted. In addition to depositing thermal energy into tube components, the impact of back scattered electrons also produces additional off-focal x-rays. This production of off-focal x-ray radiation degrades the image quality if it is allowed to exit the vacuum vessel x-ray transmissive window.
The path of the off-focal radiation and the back scattered electrons may be influenced by the electrical potential configuration of the x-ray tube. In a bi-polar configuration, the cathode is maintained at a negative potential and the anode at a positive potential relative to ground, thereby comprising the total voltage drop across the cathode to anode gap. In this configuration, a large fraction of the initially back scattered electrons from the anode are drawn back to the anode by the electrostatic potential. On the other hand, in a uni-polar design the anode and vacuum vessel are grounded and the cathode is maintained at a high negative potential. In the uni-polar configuration, the back scattered electrons are not drawn back to the anode or attracted to the frame. Therefore, in a uni-polar configuration, a larger fraction of the back scattered electron energy can be beneficially collected and not allowed to return to the anode, thus greatly enhancing the thermal performance of the anode and decreasing the amount of off-focal radiation exiting through the transmissive window.
Since the production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700° C., while the temperature in the other parts of the anode may range up to about 1800° C. Additionally, the components of the x-ray tube must be able to withstand the high temperature exhaust processing of the x-ray tube, at temperatures that may approach approximately 450° C. for a relatively long duration.
To cool the x-ray tube, the thermal energy generated during tube operation must be transferred from the anode through the vacuum vessel and be removed by a cooling fluid. The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bi-polar configuration. The performance of the cooling fluid may be degraded, however, by excessively high temperatures that cause the fluid to boil at the interface between the fluid and the vacuum vessel and/or the transmissive window. The boiling fluid may produce bubbles within the fluid that may allow high voltage arcing across the fluid, thus degrading the insulating ability of the fluid. Further, the bubbles may lead to image artifacts, resulting in low quality images. Thus, the current method of relying on the cooling fluid to transfer heat out of the x-ray tube may not be sufficient.
Similarly, excessive temperatures can decrease the life of the transmissive window, as well as other x-ray tube components. Due to its close proximity to the focal spot, the x-ray transmissive window is subject to very high heat loads resulting from thermal radiation and back scattered electrons. These high thermal loads on the transmissive window necessitate careful design to insure that the window remains intact over the life of the x-ray tube, especially in regard to vacuum integrity. The transmissive window is an important hermetic seal for the x-ray tube. The high heat loads cause very large and cyclic stresses in the transmissive window and can lead to premature failure of the window and its hermetic seals. Further, as mentioned above, direct contact with the cooling fluid can cause the fluid to boil as it flows over the window. Also, direct contact with a window that is too hot can cause degraded hydro
Kendall Charles B.
Lounsberry Brian D.
Rogers Carey S.
Snyder Douglas J.
Bindseil James J.
Calkins Charles W.
General Electric Company
Kilpatrick & Stockton LLP
Porta David P.
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