Spark gap device and method of measurement of X-ray tube vacuum pressure

In the present invention, a pressure measurement device for determining the vacuum level within the evacuated housing of a vacuum electrode device is provided that includes an electrically conductive enclosure secured to an interior surface of the housing, an electrically conductive electrode extending through an aperture in the housing, the electrode having a tip at one end positioned within the interior of the housing inside the enclosure to define a gap between the tip and the enclosure and a conductive lead at a second end disposed outside of the housing, and a voltage source connected to the conductive lead to supply a voltage potential to the tip of the electrode. A voltage difference produced between the electrode and the enclosure ionizes gas within the enclosure causing a measurable current to flow between the electrode and the enclosure which can be used to determine the vacuum level in the housing.

BACKGROUND OF INVENTION

The subject matter disclosed herein relates to vacuum electrode devices in general, including but not limited to x-ray tubes, electron beam source devices, power electronic devices, such as a klystron, ignitron, and others, and more specifically to devices and procedures for determining the pressure present within the vacuum electrode device.

Vacuum electron devices are used in a variety of systems in order to generate electrons for different purposes. In one example, as shown inFIG. 1a vacuum electron device is used in an X-ray system10in the detection of internal structures of components of an object or item16being imaged.

The X-ray system10includes an x-ray source12configured to project a beam of X-rays14through an object16. Object16may include a human subject, pieces of baggage, or other objects desired to be scanned. X-ray source12may be a conventional X-ray tube producing X-rays having a spectrum of energies that range, typically, from 30 keV to 200 keV. The X-rays14pass through object16and, after being attenuated by the object, impinge upon a detector18. Each detector in detector18produces an analog electrical signal that represents the intensity of an impinging X-ray beam, and hence the attenuated beam, as it passes through the object16.

A processor20receives the signals from the detector18and generates an image corresponding to the object16being scanned. A computer22communicates with processor20to enable an operator, using operator console24, to control the scanning parameters and to view the generated image. That is, operator console24includes some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus that allows an operator to control the x-ray system10and view the reconstructed image or other data from computer22on a display unit26. Additionally, console24allows an operator to store the generated image in a storage device28which may include hard drives, flash memory, compact discs, etc. The operator may also use console24to provide commands and instructions to computer22for controlling a source controller30that provides power and timing signals to x-ray source12.

In the X-ray source12, the cathode and anode are disposed within a frame/housing for the X-ray source/tube that is evacuated around the cathode/emitter and the anode in order to remove any gases that would otherwise interfere with the flow of electrons between the cathode and the anode. The housing is desired to enclose a perfect vacuum. However, as a result of imperfections in the materials and processes involved in manufacturing of the housing and the X-ray source12, an amount of a gas, such as N2, H2, Ar, can be present within the housing. In addition, over time, other imperfections or irregularities in the construction of the housing or internal component outgassing can increase the gases in the housing, further compromising the operation of the vacuum electrode device. The reason for this is that when gas molecules are present in the housing, the electrons produced at the cathode can strike the gas molecules, ionizing the gas molecules and preventing the electron from reaching the anode to produce X-rays. Further, the ionized gas molecules can be drawn towards and strike the emitter/cathode, causing damage to the cathode which results in premature failure of the emitter/cathode and X-ray source12. As a result, the presence of significant amounts of gas molecules within the housing presents serious negative effects on the longevity and the performance of the X-ray source12. Thus, it is highly desirable to be able to determine the presence and amount of any gas within the housing in order to maximize the operation of the X-ray source12.

The presence of a gas within the housing can be determined by measuring the gas pressure within the housing. With prior art vacuum electron devices/X-ray sources12, in order to test the vacuum electron device for the amount of gas present in the housing, these prior art devices utilize one of two methods: the devices include a stand-alone pressure gauge100built into the housing for the X-ray source12; or the pressure is determined using the emitter/cathode of the vacuum device directly. In either method, the pressure of the gas within the housing is determined by heating a cathode, either in the pressure gauge100or the cathode, and creating ionized gas particles that are drawn to a corresponding anode (not shown). The current produced by the ionized gas between the cathode and the anode can then be utilized to determine the gas pressure within the housing.

Issues with these prior art methods and devices include the increased complexity and cost associated with the stand-alone pressure gauge100to be attached to the housing and the fact that the vacuum device cathode could potentially be damaged by using it for pressure measurement if the pressure within the housing is high. In the ease of vacuum electrode devices that have been returned for analysis due to poor performance or for testing of the vacuum electron device during manufacture, damaging the cathode is undesirable as it prevents the cathode and other components of the vacuum electrode device from being able to be reused in other devices.

Hence it is desirable to provide a vacuum electrode device such as an X-ray source/tube with a pressure measurement device, system or feature that does not greatly increase the complexity of the device, and that does not need the cathode of the vacuum device in order to determine the pressure within the housing.

BRIEF DESCRIPTION OF THE INVENTION

There is a need or desire for a pressure measurement system or device and associated method to measure the pressure within a vacuum electrode device such as an X-ray tube that does not require a separate pressure gauge included in the device structure or the use of the cathode within the tube for the measurement. The above-mentioned drawbacks and needs are addressed by the embodiments described herein in the following description. In an exemplary embodiment of the invention, a pressure measurement device is provided for a vacuum electrode device in the form of a spark gap device disposed on the housing for the vacuum electrode device. The spark gap device includes an electrode/electron source or pin having one end disposed outside of the housing and a second end positioned within the housing. The electron source extends through an electrically isolating feedthrough into the housing, and is connected on the outside of the housing to a voltage source used to negatively bias the electrode/electron source.

Within the housing, an electrode enclosure is disposed around the electron source. The enclosure is biased oppositely to the electrode source, by either applying voltage to both the housing and the electrode or more conveniently just negatively biasing the electrode and leaving the housing grounded, to create a voltage difference between the electrode and the enclosure. When the voltage difference between the enclosure and the electrode or portion thereof exceeds the breakdown voltage of the gas particles present within the gap between the electrode and the enclosure, a spark forms which ionizes the gas particles and produces electrons. The electrons move towards the enclosure and the positively charged gas ions/ionized particles move towards the electrode. This movement of the electrons and the ionized particles will enable an electric current to flow across the gap between the enclosure and electrode. This current passes through the electrode/electron source outside of the housing and can be measured in conjunction with the voltage applied to the electron source to gauge the approximate vacuum level/gas pressure within the housing, such as to detect tube leaks during manufacture. Based on the measured vacuum level or gas pressure, a subsequent test can be performed on the vacuum electrode device using a prior art method if the pressure is low enough to avoid damaging the cathode within the device.

One exemplary embodiment of the invention is a pressure measurement device for determining the vacuum level within a housing of a vacuum electrode device, the pressure measurement device comprising an electrically conductive enclosure adapted to be positioned on an interior surface of the housing, an electrically conductive electrode adapted to extend through the housing, the electrode having a tip at one end adapted to be positioned within an interior of the housing inside the enclosure to define a gap between the tip and the enclosure and a conductive lead at a second end adapted to be disposed outside of the housing and a voltage source connected to the conductive lead to supply a voltage potential to the tip of the electrode.

Another exemplary embodiment of the invention is a method for determining the vacuum level within a housing of a vacuum electrode device, the method comprising the steps of providing a vacuum electrode device utilized to produce electrons including a housing defining an interior containing a vacuum therein and a pressure measurement device for determining the vacuum level within the housing, the pressure measurement device having an electrically conductive enclosure secured to an interior surface of the housing, an electrically conductive electrode extending through an aperture in the housing, the electrode having a tip at one end positioned within the interior of the housing inside the enclosure to define a gap between the tip and the enclosure and a conductive lead at a second end disposed outside of the housing and a voltage source connected to the conductive lead to supply a voltage potential to the tip of the electrode, biasing the electrode with a voltage from the voltage source to create a voltage difference between the tip and the enclosure, ionizing gas particles within the enclosure by causing the voltage difference to exceed the breakdown voltage of the gas particles and creating a current flow between the enclosure and the electrode as a result of a flow of ionized gas particles and electrons between the electrode and the enclosure.

Another exemplary embodiment of the invention is a vacuum electrode device utilized to produce electrons, the device comprising: a housing defining an interior containing a vacuum therein, a cathode disposed in the interior of the housing and operably connected to a first voltage source, the cathode configured to emit electrons upon application of voltage to the cathode, an anode disposed within the housing and spaced from the cathode, the anode maintained at an electric potential different than the cathode to attract electrons emitted from the cathode and a pressure measurement device for determining the vacuum level within the housing, the pressure measurement device comprising an electrically conductive enclosure secured to an interior surface of the housing, an electrically conductive electrode extending through an aperture in the housing, the electrode having a tip at one end positioned within the interior of the housing inside the enclosure to define a gap between the tip and the enclosure and a conductive lead at a second end disposed outside of the housing and a second voltage source connected to the conductive lead to supply a voltage potential to the tip of the electrode.

DETAILED DESCRIPTION OF THE DRAWINGS

In the illustrated exemplary embodiment ofFIG. 2, while the device can be any vacuum electrode device, such as electron beam source devices, power electronic devices, such as a klystron, or ignitron, among others, the vacuum electron device1001is an X-ray tube12incorporating embodiments of the invention. X-ray tube12, such as can be utilized in the prior art imaging system10, includes a frame50that encloses a vacuum region54, and an anode56and a cathode assembly60are positioned therein. Anode56includes a target57having a target track86, and a target hub59attached thereto. Terms “anode” and “target” are to be distinguished from one another, where target typically includes a location, such as a focal spot, wherein electrons impact a refractory metal with high energy in order to generate X-rays, and the term anode typically refers to an aspect of an electrical circuit which may cause acceleration of electrons theretoward. Target56is attached to a shaft61supported by a front bearing63and a rear bearing65. Shaft61is attached to a rotor62. Cathode assembly60includes a cathode cup85and a flat emitter or filament55formed of any suitable emissive material and coupled to a current supply lead71and a current return75that each pass through a center post51. In operation, electrical current is carried to flat emitter55via the current supply lead71and from flat emitter55via the current return75which are electrically connected to source controller30and controlled by computer22of system10inFIG. 1.

Feedthroughs77pass through an insulator79and are electrically connected to electrical leads71and75. X-ray tube12includes a window58typically made of a low atomic number metal, such as beryllium, to allow passage of x-rays therethrough with minimum attenuation. Cathode assembly60includes a support arm81that supports cathode cup73, an emission source such as a coiled filament or a flat emitter55, as well as other components thereof. Support arm81also provides a passage for leads71and75.

In operation, target56is spun via a stator (not shown) external to rotor62. An electric current is applied to flat emitter55via feedthroughs77to heat emitter55and emit electrons67therefrom. A high-voltage electric potential is applied between anode56and cathode60, and the difference therebetween accelerates the emitted electrons67from cathode60to anode56. Electrons67impinge target57at target track86and X-rays69emit therefrom at a focal spot89and pass through window58.

Looking at the exemplary embodiments of the invention inFIGS. 2-4, the pressure measurement device1000is illustrated on the X-ray tube12. The device1000is mounted directly to the housing50and includes an enclosure1002mounted to the housing50, such as on the interior surface1003of the housing50, that extends into the vacuum region or interior54of the housing50. The enclosure1002is formed of an electrically conductive material, such as a metal, as this material not being an electron source can be made of vacuum grade steel, so that a voltage potential can be applied to the enclosure1002during operation of the device1000. The enclosure1002can have any suitable shape, with the illustrated exemplary embodiment being cylindrical in shape with an open inner end1005and a diameter of between 10 mm-6 mm, though other diameters for the enclosure1002are also contemplated as being within the scope of the invention.

Centered relative to the enclosure1002is au electrically isolating feedthrough1004. The feedthrough1004is formed of any suitable material and shape, with the illustrated exemplary embodiment being cylindrical in shape, and has a diameter less than that of the enclosure1002. The feedthrough1004extends through an aperture1006in the housing50to enable an electrode1008to be positioned within the feedthrough1004and extend into the interior54of the housing50.

The electrode1008is formed of an electrically conductive material such that a voltage potential can be applied to the electrode1008during operation of the device1000. In exemplary embodiments of the invention, the material used for the construction of the electrode1008is a metal, such as a refractory metal including, but not limited to tungsten, molybdenum, nickel and alloys thereof, among others. The electrode1008includes a tip1010at one end. The tip1010is located within the housing50and is disposed approximately at the center of the enclosure1002below the open inner end1005of the enclosure1002with a uniform gap or space1012formed around the electrode1008between the electrode1008and the enclosure1002. The radius1014of the tip1010, which can be different or the same as the radius of the electrode1008, can be from up to 2 mm-3 mm, but can also be up to 0.5 mm to enhance the electron field produced by the tip1010, though other radius sizes for the tip1010are also considered to be within the scope of the invention.

Opposite the tip1010, the other end of the electrode1008includes a high voltage conductive lead1016that is positioned on the exterior of the housing50. The lead1016is operably connected to a voltage source1018capable of applying a voltage to the electrode1008in order to produce electrons at the tip1010of the electrode1008. The lead1016is also operably connected to an ammeter1020that can measure any current passing through the lead1016from the tip1010of the electrode1008.

In operation, after the device1000has been mounted to the housing of the source/tube12, such as during the initial manufacture of the source/tube12, a negative voltage potential V− is applied to the electrode1008and the tip1010from the voltage source1018. This voltage contrasts with the voltage potential V+ at the enclosure1002to create a voltage difference between the tip1010and the enclosure1002. As mentioned previously, this potential at the enclosure also be grounded, in which case, the gap1012only sees a voltage differential of V instead of 2V.

When the voltage difference between the enclosure1002and the tip1010exceeds the breakdown voltage of the gas particles1024present within the gap1012, a spark forms which ionizes the gas particles1024and produces electrons1022. The electrons1022move away from the tip1010towards the enclosure1002which has a positive voltage bias V+ relative to the negative voltage bias V− of the electrode1008/tip1010. The positively charged gas ions/ionized particles1024move towards the negatively biased tip1010. This movement of the electrons1022and the ionized particles1024will enable an electric current or spark/arc to flow across the gap1012between the enclosure1002and tip1010. This current can be measured by the ammeter1020and used to determine the vacuum level/gas pressure within the housing50for the source/tube12.

The dimensions of the tip1010and the enclosure1002that define the size of the gap1012are dependent upon the magnitude of the voltage applied to the electrode1008. If the voltage applied is smaller, e.g., under 1 kV, a smaller gap1012is required to enable the current or spark to reach between the tip1010and the enclosure1002with the levels of gas pressure to be measured. However, as the tolerances of the smaller components for provide the small gap1012render the construction of the device1000potentially more variable, in an exemplary embodiment of the invention a larger voltage, e.g., 1 kV to 5 kV, is applied to the electrode1008and tip1010, allowing for a larger gap1012to be present between the tip1010and the enclosure1002. This larger device1000increase the ease and consistency of the construction of the device1000which, in turn, allows for more consistency in the measurement of the gas pressure using the device1000.

Furthermore, the geometry of the electrode1008, and in particular the tip1010, along with the material used to form the electrode1008and the size of the gap1012determines the pressure at which the device1000allows the spark to form and/or fire across the gap1012. Thus, the size of the gap1012, as well as the corresponding voltage bias applied to the device1000, as well as the geometry of and material forming the electrode1008/tip1010can be varied in the construction of the device1000in order to provide a device1000that fires at a desired pressure level. Thus, the device1000can be used alone, without an ammeter1020, to provide a simple binary check for a good/no good indication of the gas pressure within the housing50, or can be utilized with the ammeter1020to provide this check along with a measurement of the gas pressure within the housing50.

With this device1000, it is possible to assess the vacuum level/quality or gas pressure within the housing50and decide if it is safe for the main cathode55to be energized for better assessment of vacuum level. If the device1000reports a high pressure within the housing50, the main cathode50would not be energized to avoid damage to the cathode/emitter55. In particular, the device1000can be effectively utilized during a rework step in a manufacturing process for the sources12including the housings50where a source/tube12is pulled off-line for quality testing. Also, the device1000can be used for evaluation of a source/tube12being returned from the field or from active use for evaluation, as the device1000provides the ability to test the vacuum level of the source/tube12without use of the cathode55. As a result, it is possible to avoid damage to the cathode55and hence increase the chances of harvesting more components of the source/tube12, including the cathode55, for reuse. In the illustrated exemplary configurations for the device1000, the pressure range that can be determined using the device1000is from 1×10−3to 1×10−5Torr, optionally with emission enhancements to the electrode1008/tip1010.

In another exemplary embodiment of the invention illustrated inFIG. 5, the device1000′ includes an enclosure1002′ formed as a cylindrical mesh enclosure1002′ having a closed inner end1005′. The feedthrough1004′ also extends outwardly from the aperture1006′ in the housing50on both sides of the aperture1006′, such that the feedthrough1004′ extends into the interior54of the housing50within the enclosure1002′. Additionally, the tip1010′ of the electrode1008′ is formed with a sharp point1011′ to enhance the electron emission from the tip1010′. Also, the housing50is shown as being formed of a metal casing, as opposed to the glass housings found in other types of sources12, such as X-ray tubes. In this exemplary embodiment, the device1000′ is configured as a limited use device with a non-uniform electric field configuration created by intentional field enhancement as a result of the electrode geometry at the tip1010′. In an exemplary embodiment of this device1000′, the device1000′ is formed with an inwardly tapered, sharp electrode tip1010′ with a field enhancement factor β>100 and made of one or more refractory materials such as molybdenum, stainless steel, nickel or brass, among others. This device1000′ uses the field enhancement on the electrode tip1010′ to cause gas ionization when sufficient voltage is applied on the electrode1008′. The operating voltage range here is high ˜1 kV to 5 kV and the pressure detection range is low, i.e., on the order of 1×10−3Torr. If this gap1012′ is tested with a simple DC supply, the first breakdown voltage at which the spark is created is a primary measure for the pressure within the housing50as subsequent voltage breakdowns will shift due to damage/conditioning done to the electrode tip1010′. Alternatively, a pulsed power supply (not shown) can be used to avoid damaging the electrode thereby providing prolonged usage life for this device1000′.

In another exemplary embodiment of the invention shown inFIG. 6, the device1000′ is modified to include an enclosure1002″ formed as a solid shield electrode extending over the tip1010′ and electrode1008′. This device1000′ works by applying a voltage differential across the electrode1008′ and the internal shield electrode/enclosure1002″. This can be done by:a. keeping both of the electrode1008′ and shield electrode/enclosure1002″ isolated from the housing50and applying +Ve and −Ve voltages to the electrode1008′ and shield electrode/enclosure1002″, respectively, from two (2) voltage supplies1018with two (2) associated feedthroughs1004′;b. leaving the shield electrode/enclosure1002″ and housing50at the same +Ve voltage and applying a −Ve voltage to the external electrode1008′; orc. leaving the housing50and shield electrode/enclosure1002″ at ground potential and applying a −Ve voltage to the external electrode1008′.

The shield electrode/enclosure1002″ is secured to the interior surface of the housing50, such as by a suitable isolating mounting structure and a feedthrough (not shown) when it is desired to provide a voltage to the shield electrode/enclosure1002″ different from the electrode1008′. Alternatively, the shield electrode/enclosure1002″ can be welded directly to the interior surface of the housing50when the when the shield electrode/enclosure1002″ is left at ground potential. The shield electrode/enclosure1002″ functions identically to the enclosure1002and the mesh electrode1002′, but additionally protects the electrode1008′ and tip1010′ from damage as a result of deposition or contamination from any other ion or electron sources present in the housing50.

In still other exemplary embodiments of the invention shown inFIGS. 3, 4, 7 and 8, the device1000is formed with an electrode1008having a tip1010with a rounded edge1028that provides a lower electrode field enhancement for a multiple use/reuse configuration for the device1000. This embodiment of the device1000involves a fairly dull or rounded electrode tip1010″ rounded as inFIG. 7or a bulbous tip1010as shown inFIGS. 3, 4 and 8, with an enhancement factor β<100 which by itself cannot cause vacuum breakdown in 1 kV-5 kV range. This is overcome by adding an electron enhancement material to the surface of the tip1010or by using a doped material to form the tip1010and the electrode1008. In exemplary embodiments of the invention, electrodes1008can be coated with α or β emission source materials, such as americium or californium, among others or can be doped for longer life. The use of a coated surface electrode1008and/or tip1010can encounter the problem of erosion of the coating leading to limited life of the electrode1008/tip1010, but an exemplary embodiment of a 2% thoriated tungsten electrode1008/tip1010would provide reliable operation for the device1000.