Electrical resistors – With inductance-reducing
Reexamination Certificate
2000-09-06
2002-09-17
Easthom, Karl D. (Department: 2832)
Electrical resistors
With inductance-reducing
C338S292000, C338S293000, C338S287000, C338S297000, C338S300000, C338S261000
Reexamination Certificate
active
06452477
ABSTRACT:
BACKGROUND
The present invention relates to high voltage low inductance resistors and is particularly related to a resistor used to regulate transient current flow caused by electrical discharge within high voltage electrical equipment. The present invention finds particular application in conjunction with high voltage vacuum tubes, particularly x-ray tubes, and will be described with respect thereto.
Conventional diagnostic use of x-radiation includes radiography, in which a still shadow image of the patient is produced on x-ray film, fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and computed tomography (CT) in which complete patient images are digitally constructed from x-rays produced by a high powered x-ray tube rotated about a patient's body.
Typically, an x-ray tube includes an evacuated envelope made of metal, glass, ceramic materials or combinations thereof which is supported within an x-ray tube housing. The x-ray tube housing provides electrical connections to the envelope and is filled with a fluid such as oil to aid in cooling components housed within the envelope. The envelope and the x-ray tube housing each include an x-ray transmissive window aligned with one another such that x-rays produced within the envelope may be directed to a patient or subject under examination. In order to produce x-rays, the envelope houses a cathode assembly and an anode assembly.
The cathode assembly includes a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode assembly and the anode assembly. This potential causes the electrons to flow from the cathode assembly to the anode assembly through the evacuated region in the interior of the evacuated envelope. A cathode focusing cup housing the cathode filament focuses the electrons onto a small area or focal spot on a target of the anode assembly.
The electron beam impinges the target with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through the x-ray transmissive windows of the envelope and x-ray tube housing to a beam limiting device, or collimator, attached to the x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination thereby allowing images to be constructed.
In order to distribute the thermal loading created during the production of x-rays a rotating anode assembly configuration has been adopted for many applications. In this configuration, the anode assembly is rotated about an axis such that the electron beam focused on a focal spot of the target impinges on a continuously rotating circular path about a peripheral edge of the target. Each portion along the circular path becomes heated to a very high temperature during the generation of x-rays and is cooled as it is rotated before returning to be struck again by the electron beam.
Typically, the anode assembly is mounted to a rotor which is rotated by an induction motor. The anode assembly and rotor are part of a rotating assembly which is supported by a bearing assembly.
During operation, the x-ray tube presents a high impedance of several hundred thousand ohms to the voltage applied between the anode assembly and cathode. This results in a relatively small current flow through the vacuum space between the anode assembly and cathode assembly. Under normal operating conditions, the power source is capable of regulating the current flow between the anode and cathode. Despite the regulation by the power source and the electrical isolation of the anode and cathode, when two elements with such a large difference in potential are placed proximate to each other, there is a tendency to arc. An arc is an undesired surge of electrical current between two elements which are at a different electrical potential.
In an x-ray tube, arcing can occur through residual gas molecules present within the evacuated envelope of the x-ray tube. As an x-ray tube ages, the tendency to arc often increases due to such factors as degradation of the vacuum within the tube resulting in increased gas pressure. The increased gas pressure within the evacuated envelope is due to the existence of additional undesired gas molecules. For example, gas molecules may migrate through the envelope, evolve from the materials within the envelope or are released as a result of damage to the components due to arcing. Consequently, the mean free path between gas molecules is reduced such that a chain reaction is more likely to occur when the gas molecules in the vacuum envelope are ionized by the high electric fields generated during normal tube operation. This chain reaction is called avalanche and is a form of arcing.
Arcing typically occurs in an area of the x-ray tube having the highest electric field strength. As such, arcing in an x-ray tube will commonly occur in the general region where the cathode is supplying the anode with electrons for the production of x-ray emissions. In addition, the structural imperfections of the electrodes contribute to the location where arcing occurs. This is because there are intense electric field gradients caused by contamination, sharp corners or rough edges on the surfaces of the electrodes. In particular, fields are higher where there are surface imperfections on the anode disk.
One consequence of arcing is the radiation and conductance of intense electrical noise on the high voltage electronic components. These noise emissions can cause failure of semiconductor devices in the system circuitry.
Another effect of arcing is the sputtering of metal from the cathode produced during arcing often lands on the internal surface of the glass envelope in proximity to the cathode. The existence of the metal deposits on the glass envelope can deleteriously effect x-ray tube performance for several reasons. First, as arcing occurs from time to time, sputtered metal deposits will continue to grow. As the sputtered metal deposits on the glass envelope gets too thick, an electrical charge may accumulate sufficient to damage the glass envelope thereby rendering the tube nonfunctional. Secondly, sputtered metal deposits on the glass envelope will often attract arcing between the deposits and the cathode. The surges of electrical current produced during arcing can damage the glass envelope, again rendering the tube nonfunctional.
When the x-ray tube arcs, a current on the order of hundreds of amperes can flow between the cathode and the anode. Once an x-ray tube starts to arc, an avalanche type effect may occur sputtering metal and the metal atoms as well as ionizing the contaminants in the vacuum. These events further contribute to yet more frequent arcing. In addition, arcing in an x-ray tube used in a Computed Tomography (CT) imaging system contaminates the x-ray signal collected at the detectors and affects proper image reconstruction. This may result in an un-usable set of data requiring another CT scan of the patient.
As mentioned above, arcing can shorten the useable service life of the x-ray tube. Given the considerable cost of an x-ray tube and the associated service costs for replacement, it is desirable to extend the service life of the x-ray tube.
One known method to extend service life and reduce arcing involves providing getter material inside the glass envelope to help maintain the evacuated state. The getter material binds gases on its surface and absorbs such gases to maintain the vacuum state in the x-ray tube. The process of removing residual gases from an evacuated area by binding and absorbing is known as pumping. By using getter material to maintain a vacuum state, arcing is reduced since there is a reduction in the number of gas molecules through which large current surges may flow. Unfortunately, as the x-ray tube ages the effective
Anderson John R.
Roca Tony W.
Clair Eugene E.
Easthom Karl D.
Marconi Medical Systems, Inc.
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