CN110867359B

CN110867359B - Microfocus X-ray source

Info

Publication number: CN110867359B
Application number: CN201810994642.6A
Authority: CN
Inventors: 姚智伟; 孙泳海
Original assignee: Individual
Current assignee: Individual
Priority date: 2018-08-28
Filing date: 2018-08-28
Publication date: 2022-02-01
Anticipated expiration: 2038-08-28
Also published as: CN110867359A

Abstract

A microfocus X-ray source comprises a cylindrical glass vacuum cavity wall with an upper layer and a lower layer separated by a preset distance; the field emission cathode is arranged at the wall of the lower layer glass vacuum cavity; the upper part of the field emission cathode is provided with a micro heat pipe anode, and the upper part of the micro heat pipe anode penetrates through the wall of the glass vacuum cavity and extends out of one part upwards; the focusing coils are respectively arranged on the periphery of the part of the anode of the micro heat pipe, which is positioned inside the glass vacuum cavity wall. The field emission electron beam kinetic energy distribution can be smaller in a narrow range of focal points. The heat pipe heat dissipation anode has higher heat conductivity and can bear higher power, and the heat pipe heat dissipation anode is internally provided with cooling fluid, and the two-phase characteristic of the cooling fluid further achieves the purposes that the cooling fluid is positioned in the glass vacuum cavity wall to absorb heat and is positioned outside the vacuum cavity wall to dissipate heat, thereby achieving the heat pipe heat dissipation anode temperature equalizing state.

Description

Microfocus X-ray source

Technical Field

The invention relates to the technical fields of microfocus X-ray imaging, X-ray nondestructive testing, X-ray diffraction, X-ray fluorescence spectroscopy and the like, in particular to a microfocus X-ray source adopting a heat pipe for heat dissipation, magnetic field focusing and a field emission cold cathode.

Background

Field electron emission (field emission) has been studied theoretically for a long time as a pure quantum phenomenon. Many field emission electron sources have been developed, most typically metal pointed cone cathodes and carbon nanotube cathodes. The X-ray cathode based on the field emission technology can be used for developing a high-speed pulse X-ray generating device because the X-ray cathode has a low operating temperature and does not need to be preheated. This is not possible with X-ray sources using conventional tungsten filament hot cathodes.

Microfocus X-ray sources have been developed specifically for X-ray non-destructive testing. The minute focus prevents blurring of the X-ray image and provides a sharp enlarged image. Since a large number of high-speed electrons are focused on the focal point of the target material in the micrometer scale, the temperature of the focal point and the surrounding material can be raised to more than two thousand degrees in a short time. Under the combined action of high temperature and high speed electrons, the molecules at the focal position are bombed out of the target material, so that the surface of the target material becomes rough, and the generation efficiency of X-rays is reduced. Therefore, anode heat dissipation has always been an important part of the design of X-ray sources. In the design of a common X-ray anode, copper with high thermal conductivity is used as an anode material, and tungsten with a high melting point is inlaid at a focal position to be used as a target material, so that high temperature resistance and high thermal conductivity are realized. In addition, in the high-power X-ray source, heat is distributed on an annular area by continuously rotating the anode, so that the heat dissipation area is greatly increased, and the temperature of the anode is obviously reduced. However, in the micro-focus X-ray source, in order to increase the magnification of the image, the distance between the anode and the irradiated object is very short, so the transmissive anode target is widely used, and many micro-focus X-ray sources use the reflective anode target. The structure of a prior art microfocus X-ray source using transmissive and reflective anodes is shown in fig. 1A and 1B, in which a hot cathode 101 is heated and creates an electron cloud 105 around the cathode. Electrons in the electron cloud 105 fly toward the transmissive anode 103 or the reflective anode 113 under the influence of the electric field. The electron beam 106 is initially divergent, and under the focusing action of the magnetic lens 102, the electron beam 106 converges to a tiny focal point 107, and bombards on the transmissive anode 103 or the reflective anode 113 to generate X-rays, which pass through the X-ray window 104 and irradiate on the target object. Since the hot cathode 101 operates at an extremely high temperature, the hot cathode 101 and the transmissive anode 103 or the reflective anode 113 cannot be made close together, and the structure of the magnetic lens 102 is complicated.

In view of the limitations of the existing X-ray source using hot cathode, we invented a micro-focus X-ray source using field emission cold cathode and heat pipe heat dissipation technology to improve the heat dissipation efficiency of the anode.

Disclosure of Invention

Therefore, the technical problem to be solved by the present invention is to overcome the defect of low heat dissipation efficiency of the anode of the X-ray source in the prior art.

To this end, a microfocus X-ray source is provided, comprising:

the glass vacuum tube is cylindrical as a whole and is provided with an upper layer of glass vacuum cavity wall (204) and a lower layer of glass vacuum cavity wall which are separated by a preset distance;

the field emission cathode (201) is arranged at the position of the lower layer glass vacuum cavity wall (204);

a micro heat pipe anode (203) is arranged at the upper part of the field emission cathode (201), and the upper part of the micro heat pipe anode (203) penetrates through the glass vacuum cavity wall (204) and extends out of one part upwards;

the periphery of the part of the micro heat pipe anode (203) positioned inside the glass vacuum cavity wall (204) is respectively provided with a focusing coil (202).

Further, in the above-mentioned case,

the upper part of the micro heat pipe anode (203) is cylindrical, the lower part of the micro heat pipe anode (203) is in an inverted cone shape, a focus (205) is formed at the tip of the inverted cone,

further, in the above-mentioned case,

the interior of the micro heat pipe anode is arranged in a hollow mode, the inner wall of the upper portion of the micro heat pipe anode is fixedly provided with a capillary core, and cooling fluid is arranged inside the micro heat pipe anode.

Further, in the above-mentioned case,

the upper part of the anode of the micro heat pipe penetrates through the wall of the glass vacuum cavity and the part extending upwards is fixedly connected with a radiator.

Further, in the above-mentioned case,

the cooling fluid is any one or more of water, oil, mercury and freon.

Further, in the above-mentioned case,

the field emission cathode (201) adopts any one or more arrays of vertically grown carbon nanotubes, metal nanowires, semiconductor nanowires, graphene and randomly arranged carbon nanotubes.

Further, in the above-mentioned case,

the field emission cathode adopts a single nano structure.

Further, in the above-mentioned case,

the anode of the micro heat pipe is a low-temperature copper heat pipe.

Further, in the above-mentioned case,

and a layer of tungsten is arranged between cones of the inverted cone-shaped part of the micro heat pipe anode (203).

Further, in the above-mentioned case,

the distance between the field emission cathode (201) and the focal point (205) of the micro heat pipe anode (203) is 2 to 10 mm.

The technical scheme of the invention has the following advantages:

1. the field emission electron beam kinetic energy is distributed in a narrow range, so that focusing is easier and the focal point can be smaller.

2. The heat pipe heat dissipation anode has higher heat conductivity and can bear higher power, and the heat pipe heat dissipation anode is internally provided with cooling fluid, so that the purposes that the cooling fluid is positioned in the glass vacuum cavity wall to absorb heat and is positioned outside the vacuum cavity wall to dissipate heat are achieved according to the two-phase characteristics of the cooling fluid, and the heat pipe heat dissipation anode is in a uniform temperature state.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.

FIG. 1A is a block diagram of a microfocus X-ray source employing a transmissive anode;

FIG. 1B is a block diagram of a microfocus X-ray source employing a reflective anode;

FIG. 2 is a block diagram of a microfocus X-ray source in accordance with the invention;

fig. 3 is a schematic diagram of the magnetic field focusing of the microfocus X-ray source of the present invention.

101. A hot cathode; 102. a magnetic lens; 103. a transmissive anode; 104. an X-ray window; 105. an electronic cloud; 106. an electron beam; 107. a focal point; 108. an X-ray beam; 113. a reflective anode; 201. a field emission cathode; 202. a focusing coil; 203. a micro heat pipe anode; 204. a glass vacuum cavity wall; 205. a focal point; 206. a capillary wick; 207. a cooling fluid; 208. a heat sink; 209. an electron beam; 210. an X-ray beam; 301. an electron trajectory helix; 302. magnetic lines of force.

Detailed Description

The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it should be understood that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance. In addition, the technical features involved in the different embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

A micro-focus X-ray source, as shown in FIG. 2, comprises a glass vacuum tube having upper and lower glass vacuum cavity walls 204 separated by a predetermined distance. And the field emission cathode 201 is arranged at the lower layer glass vacuum cavity wall 204, wherein the field emission cathode 201 adopts a vertically grown carbon nanotube array. In the field emission cathode 201, about eight thousand vertically grown carbon nanotubes are distributed in a circle with a diameter of 1 mm, the distance between the carbon nanotubes is about 10 micrometers, the length is about 5 micrometers, and the diameter is 10 to 30 nanometers, and the carbon nanotubes are grown on a conductive substrate, in this embodiment, a moderately doped monocrystalline silicon wafer is used.

The upper part of the field emission cathode 201 is provided with a micro heat pipe anode 203, the upper part of the micro heat pipe anode 203 is cylindrical, the lower part of the micro heat pipe anode 203 is in an inverted cone shape, and a focus 205 is formed at the tip of the inverted cone shape. The upper part of the micro heat pipe anode 203 penetrates through the glass vacuum cavity wall 204 and extends out of a part upwards, the interior of the micro heat pipe anode 203 is arranged in a hollow mode, a capillary core 206 is fixedly arranged on the inner wall of the upper part of the micro heat pipe anode 203, a cooling fluid 207 is arranged inside the micro heat pipe anode 203, a layer of tungsten with the thickness of 10 microns is electroplated at the conical space of the inverted conical part of the micro heat pipe anode 203, and the conical space of the micro heat pipe anode 203 is made into an inverted cone with the cone angle of 90 degrees. The cooling fluid 207 is any one or more of water, oil, mercury, and freon. In this embodiment, the cooling fluid 207 of the micro heat pipe is preferably water, and the working temperature thereof can reach 200 ℃. The focusing coils 202 are respectively arranged on the periphery of the part of the micro heat pipe anode 203 positioned inside the glass vacuum cavity wall 204.

In one embodiment, the upper portion of the micro heat pipe anode 203 penetrates the glass vacuum cavity wall 204 and is fixedly connected with a heat sink 208.

The cooling principle of the micro heat pipe anode 203 in the invention is as follows: the liquid phase cooling fluid 207 evaporates into a vapor phase at the heat absorption end near the focal point 205 and flows at a high speed to the heat release end provided with the radiator 208 under the action of pressure difference, and the vapor phase cooling fluid 207 condenses into a liquid phase at the heat release end and then extends the capillary core 206 to flow back to the heat absorption end under the action of capillary. When the micro heat pipe anode 203 is in an ideal operating state, the cooling fluid 207 is in a state of coexistence of liquid and vapor phases, the two phases have no temperature difference, that is, the whole cavity is in a uniform temperature state, and at this time, although heat energy enters and exits the cavity system, the heat absorption end and the heat release end are isothermal, and a heat superconducting phenomenon of isothermal heat transfer is formed.

In one embodiment, the field emission cathode 201 employs a vertically grown carbon nanotube array, and the distance between the field emission cathode 201 and the tip of the micro heat pipe anode 203 is 4 mm. For circuit simplicity, the anode 203 of the micro heat pipe is grounded, and the field emission cathode 201 is connected with a high voltage pulse of minus 80 kv, and the pulse width is 10 milliseconds. When the magnetic induction of the focusing magnetic field at the tip of the micro heat pipe anode 203 is 1 tesla, the size of the focal point 205 can be as small as 10 microns or less. When the cathode high voltage is turned on, field emission cathode 201 emits 1 milliamp of current at 80 watts and generates approximately 0.8 joules. When the duty ratio of the high-voltage pulse is 0.2, which is equivalent to a heat source with power of 16 watts and continuous work, the radiator 208 adopts an aluminum radiating fin with the diameter of 100 millimeters and a radiating fan, and then the stable work of the microfocus X-ray source can be realized.

In one embodiment, a single nanostructure, such as a single carbon nanotube, a single tungsten filament tip, or a single silicon nanowire, is used as the field emission cathode 201. The micro heat pipe anode 203 adopts a low-temperature copper heat pipe, and a layer of tungsten with the thickness of 500 nanometers is electroplated at the tip. The cooling fluid 207 of the micro heat pipe is water, and the working temperature of the micro heat pipe can reach 200 ℃. The tip of the micro heat pipe anode 203 is made into a right circular cone with a cone angle of 90 degrees, and the diameter of the tip is 100 nanometers. The distance between the field emission cathode 201 and the tip of the micro heat pipe anode 203 is 3 mm. For circuit simplicity, the anode 203 of the micro heat pipe is grounded, and the field emission cathode 201 is connected with a high voltage pulse of minus 100 kv, and the pulse width is 5 milliseconds. When the magnetic induction of the focusing magnetic field at the tip of the micro heat pipe anode 203 is 1 tesla, the size of the focal point 205 can be as small as 50 nanometers or less. When the cathode high voltage is turned on, field emission cathode 201 emits 10 microamps of current, with a power of 1 watt, and generates approximately 5 millijoules of heat. To avoid melting the tungsten of the focal spot 205 by high temperature, the duty cycle of the high voltage pulse is not higher than 0.05. The radiator 208 can be an aluminum fin with a diameter of 20 mm, so that a micro-focus X-ray source with a focus size of nanometer can be realized.

The principle of the focusing part of the magnetic field of the micro-focus X-ray source provided by the invention is shown in figure 3. The focusing magnetic field employs an open magnetic circuit design that focuses the electron beam 209 using magnetic flux inhomogeneities. The magnetic field has a high magnetic flux density at a position close to the magnetic core and a low magnetic flux density at a position far from the magnetic core. In the present invention, the field emission cathode 201 is placed at a position where the magnetic flux density is low, and the micro heat pipe anode 203 is placed at a position where the magnetic flux density is high. After leaving the field emission cathode 201, the electrons fly toward the micro heat pipe anode 203 under the action of the electric field. Due to the presence of the magnetic field, the electrons follow a helix 301 of electron trajectories with magnetic field lines 302 as axes. The kinetic energy of electrons is converted from the potential energy provided by the electric field, and the magnetic field only changes the movement direction of the electrons and does not cause energy change. With the increase of the magnetic flux density, the electrons gradually converge to the focal point 205 while flying to the anode 203 of the micro heat pipe, so as to achieve the purpose of focusing.

The micro-focus X-ray source of the invention adopts the field emission cathode 201 as an electron source, so the working temperature is low, the structure is compact, and the focus 205 can be made smaller than that of a common hot cathode X-ray tube. And the used open magnetic circuit focusing mode is simpler than the common quadrupole magnetic lens structure, compact structure, low cost and no need of adjustment. The micro heat pipe anode 203 used in the invention can bear larger power and emit stronger X-ray beams 210 than the common reflective and transmissive anodes, so the micro-focus X-ray source can greatly improve the imaging speed of X-ray images.

It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications therefrom are within the scope of the invention.

Claims

1. A microfocus X-ray source, comprising:

a micro heat pipe anode (203) is arranged at the upper part of the field emission cathode (201), the micro heat pipe anode (203) is a heat pipe anode, and the upper part of the micro heat pipe anode (203) penetrates through the upper glass vacuum cavity wall (204) and extends out of a part of the upper glass vacuum cavity wall;

the periphery of the part of the micro heat pipe anode (203) positioned inside the glass vacuum cavity wall (204) is respectively provided with a focusing coil (202);

the upper part of the micro heat pipe anode (203) is cylindrical, the lower part of the micro heat pipe anode (203) is in an inverted cone shape, and a focus (205) is formed at the tip of the inverted cone shape;

the interior of the micro heat pipe anode (203) is hollow, a capillary core (206) is fixedly arranged on the inner wall of the upper part of the micro heat pipe anode (203), and cooling fluid (207) is arranged in the micro heat pipe anode (203);

the conical tip of the inverted conical part of the micro heat pipe anode (203) is provided with a layer of tungsten with the thickness of 10 microns, and the conical tip of the micro heat pipe anode (203) is an inverted cone with the cone angle of 90 degrees.

2. The microfocus X-ray source according to claim 1,

the upper part of the micro heat pipe anode (203) penetrates through the glass vacuum cavity wall (204) and the part extending upwards is fixedly connected with a radiator (208).

3. The microfocus X-ray source according to claim 1,

the cooling fluid (207) is one or more of water, oil, mercury and freon.

4. The microfocus X-ray source according to claim 1,

5. The microfocus X-ray source according to claim 1,

the field emission cathode (201) adopts a single nano structure.

6. The microfocus X-ray source according to claim 1,

the micro heat pipe anode (203) is a low-temperature copper heat pipe.

7. The microfocus X-ray source according to claim 1,