Patent Application
20150022080
This disclosure relates to the field of radiation generators, and, more particularly, to cathode assemblies for use in radiation generators.
Well logging instruments that utilize radiation generators, such as sealed-tube neutron generators, have proven incredibly useful in oil formation evaluation. Such a neutron generator may include an ion source and a target. Some ion sources operate by emitting electrons from a cathode, and accelerating those electrons to suitable energies in the presence of an ionizable gas. Once the ions are created by interactions between the electrons and the ionizable gas, they are accelerated to a target that emits neutrons when struck by the ions. Therefore, the rate of neutron production in such a radiation generator is related to the rate of ion production, which in turn is related to the rate of electron production.
Consequently, it is desirable for the production of electrons to remain substantially constant in such a radiation generator, and the creation of new cathode assemblies that help to provide a substantially constant electron output is desirable.
This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
In accordance with the present disclosure, a cathode assembly for use in a radiation generator may include a support having formed therein a hole and a recess at least partially surrounding the hole. The cathode assembly may also include an ohmically heated cathode, and a mount coupled to the support. The mount may include a larger outer frame positioned within the recess, a smaller inner frame carrying the ohmically heated cathode and spaced apart from the larger outer frame, and a plurality of members coupling the smaller inner frame to the larger outer frame.
Another aspect is directed to a radiation generator that may include a housing to contain an ionizable gas, an extractor electrode carried within the housing, and a cathode assembly carried within the housing. The cathode assembly may include an ohmically heated cathode and support having formed therein a hole and a recess at least partially surrounding the hole. The cathode assembly may also include a mount coupled to the support. The mount may include a larger outer frame positioned within the recess, a smaller inner frame carrying the ohmically heated cathode and spaced apart from the larger outer frame, and a plurality of members coupling the smaller inner frame to the larger outer frame. The cathode assembly may also include a cathode grid downstream of the ohmically heated cathode, with the cathode grid and the ohmically heated cathode having a voltage difference such that the ohmically heated cathode emits electrons in a downstream direction toward the extractor electrode. The cathode grid and the extractor electrode may have a voltage difference such that the electrons are decelerated toward the extractor electrode, at least some of the electrons as they travel interacting with the ionizable gas to form ions. There may be a target carried within the housing downstream of the extractor electrode, and the extractor electrode and the target may have a voltage difference such that the ions are accelerated downstream toward the target, the target to emit radiation when struck by at least some of the ions.
A further aspect is directed to a radiation generator that may include a housing, with an extractor electrode carried within the housing, and a cathode assembly carried within the housing. The cathode assembly may include an ohmically heated cathode, and support having formed therein a hole and a recess at least partially surrounding the hole. A mount may be coupled to the support and may include a larger outer frame positioned within the recess, a smaller inner frame carrying the ohmically heated cathode and spaced apart from the larger outer frame, and a plurality of members coupling the smaller inner frame to the larger outer frame. In addition, there may be a cathode grid downstream of the ohmically heated cathode, and the cathode grid and the ohmically heated cathode may have a voltage difference such that the ohmically heated cathode emits electrons in a downstream direction toward the extractor electrode. Also, the cathode grid and the extractor electrode may have a voltage difference such that the electrons are accelerated toward the extractor electrode. There may be a target carried within the housing downstream of the extractor electrode. The extractor electrode and the target may have a voltage difference such that the electrons are accelerated downstream toward the target, the target to emit radiation when struck by at least some of the electrons.
A method aspect is directed to a method of making a cathode assembly for use in a radiation generator. The method may include forming a hole and a recess at least partially surrounding the hole in a support, and coupling a mount to the support. The mount may be coupled to the support by positioning a larger outer frame within the recess, positioning an ohmically cathode in a smaller inner frame spaced apart from the larger outer frame, and coupling the smaller inner frame to the larger outer frame using a plurality of members.
One or more embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill in the art having the benefit of this disclosure.
When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.
For clarity in descriptions, when the term “downstream” is used, a direction toward the target of a radiation generator tube is meant, and when the term “upstream” is used, a direction away from the target of a radiation generator tube is meant. In addition, when any voltage or potential is referred to, it is to be understood that the voltage or potential is with respect to a reference voltage, which may or may not be ground. The reference voltage may be the voltage of the active cathode as described below, for example. Thus, when a “positive” voltage or potential is referred to, that means positive with respect to a reference voltage, and when a “negative” voltage of potential is referred to, that means negative with respect to a reference voltage.
This disclosure relates to cathode assemblies for use in radiation generators. So that the use of such cathode assemblies, as well as the usefulness thereof, is readily apparent, two types of commonly used radiation generators will now be described.
Referring now to
The cathode assembly 556 contains an active cathode that emits electrons in a downstream direction toward an extractor electrode 558. A voltage difference between the cathode assembly 556 and the cathode grid 574 accelerates the electrons as they travel downstream toward the extractor electrode. As the electrons travel downstream, at least some interact with the ionizable gas to form ions, such as deuterium or tritium ions or molecular ions such as D2+, DT+ or T2+. The positive ions are accelerated towards the extractor electrode opening by the positive voltage applied to the extractor electrode 558 with respect to the cathode grid 574.
The cathode in the cathode assembly 556 is heated ohmically by applying a voltage between the cathode support electrode 570 and the cathode power electrode 572. While the cathode power electrode is shown as an additional metallic ring in the ceramic envelope, the voltage could be supplied by a feed-through through the ceramic wall or through the bottom of the neutron generator tube.
The extractor electrode 558, as well as any other optional electrode such as a cylindrical electrode (not shown) between the extractor electrode and the cathode grid 574, shape the electric field such that the ions are attracted or repelled downstream through the extractor electrode. The ions are further accelerated as they travel downstream by the voltage differences between the extractor electrode 558 and a suppressor electrode 560 as well as a target 562. When the ions strike ions embedded in the target 662, fusion reactions such as deuterium-deuterium (d-D), deuterium-tritium (d-T), and tritium-tritium (t-T) reactions, may occur, depending upon what types of ions are accelerated, and depending upon what types of ions are embedded within the target. A product of these fusion reactions is the creation of neutrons, with a d-D fusion reaction creating a 2.45 MeV neutron, a d-T fusion reaction creating a 14.1 MeV neutron, and a t-T fusion creating a pair of neutrons of an undefined energy (but less than 11.3 MeV combined between the pair).
With reference to
As will be understood by those of skill in the art, the radiation generators 550, 650 can be incorporated into downhole tools , and can be activated when in a hole drilled into a subsurface formation. By detecting incoming radiation resulting from interactions between the neutrons and/or x-rays and the subsurface formation, properties of the subsurface formation, such as porosity, density, and lithology, can be determined.
In either radiation generator 550, 650, consistency in the radiation output (whether neutrons or x-rays), whether the radiation is output in pulses or continuously, is desirable for consistency of results in downhole logging. A factor contributing to consistent radiation output is consistent electron output from the cathode assembly 556, 656.
Moving along, the cathode assembly 100 of the present disclosure helps to provide consistent electron output, and will now be described in detail with initial reference to
A mount 107 is coupled to the support 102. The mount includes a larger outer frame 108 positioned within the recess 104, a smaller inner frame 110 spaced apart from the larger outer frame, and a plurality of members 112 coupling the smaller inner frame to the larger outer frame. The larger outer frame 108 and smaller inner frame 110 may also be hollow cylinders, as illustrated.
The smaller inner frame 110 carries an ohmically heated cathode 114. The ohmically heated cathode 114 emits electrons from the active surface 115 via thermionic emission when properly powered. It should be understood that there may be any number of such members 112 and that they may be of any shape. For example, as shown in
The ohmic heating of the cathode is achieved by applying an electrical potential across the resistive heater inside the cathode and passing a current through it. The potential may be applied between the outer surface of the cathode 114 and an electrical contact 116 that may be at the bottom of the cathode 114. A cathode wire 117 is connected to an additional electrode (not shown) to which the voltage may be applied.
The members 212 may instead extend in a straight line from the smaller inner frame 210 to the larger outer frame 208, and may extend in a direction orthogonal to the ohmically heated cathode 214, both of which are shown in
In another configuration, the members 312 may extend in a direction oblique to the ohmically heated cathode 314, as shown in
As explained above, consistent electron production in a radiation generator is desirable. One way in which to help keep electron production from the ohmically heated cathode 114, 214, 314 constant is to keep the temperature thereof constant. However, the ohmic heating of the cathode 114, 214, 314 is costly in terms of power consumption, and therefore ways at reducing heat loss from the cathode are desirable. The design of the cathode assembly 100, 200, 300 helps to reduce thermal conduction losses of heat away from the ohmically heated cathode 114, 214, 314. In particular, the members 112, 212, 312 help to space the ohmically heated cathode 114, 214, 314 away from the larger outer frame 108, 208, 308, which in return reduces the heat conduction from the ohmically heated cathode into the mount 107, 207, 307 and support 102, 202, 302.
In addition, this design wherein the larger outer frame 108, 208, 308 is fitted into the support 102, 202, 302 (i.e. such that the larger outer frame and support are coplanar) helps to keep the ohmically heated cathode 114, 214, 314 centered in the hole 103, 203, 303, which helps to provide for consistent output between different radiation generators. In addition, the members 112, 212, 312 help to keep the face of the ohmically heated cathode 114, 214, 314 substantially parallel to an optional cathode grid (shown as 420 in
The design of the cathode assembly 100, 200, 300 helps facilitate easy and consistent construction as well. For example, the ohmically heated cathode 114, 214, 314 may be first placed into the smaller inner frame 110, 210, 210, and then brazed thereto so that it remains in place securely. The mount 107, 207, 307 may then be placed into the support and welded into place.
In some applications, the cathode assembly 400 may include an additional support 418 coupled to the support 402 by an insulator 416, as shown in
While the disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be envisioned that do not depart from the scope of the disclosure as disclosed herein.
