CN108493087B

CN108493087B - Field emission self-focusing pulse X-ray generating device integrated with high-voltage power supply

Info

Publication number: CN108493087B
Application number: CN201810535911.2A
Authority: CN
Inventors: 孙泳海; 姚智伟
Original assignee: Individual
Current assignee: Individual
Priority date: 2018-05-30
Filing date: 2018-05-30
Publication date: 2020-01-10
Anticipated expiration: 2038-05-30
Also published as: CN108493087A

Abstract

A field emission self-focusing pulse X-ray generating device integrated with a high-voltage power supply comprises an X-ray generating part and a high-voltage generator, wherein the connection relationship between the X-ray generating part and the high-voltage generator is as follows: the anode of the high-voltage generator is connected with the anode of the X-ray generating part, and the cathode of the high-voltage generator is connected with the cathode of the X-ray generating part; the X-ray generating part adopts a diode structure based on a carbon nano tube field emission device and comprises a cathode of the X-ray generating part and an anode of the X-ray generating part; the carbon nano tube field emission device is used as a cathode of the X-ray generating part; the high-voltage generator comprises a control circuit, a transformer and a high-voltage coupling circuit; the high-voltage generator is powered by an external low-voltage direct-current power supply; the transformer adopts a step-up transformer with high turn ratio; the primary coil of the transformer is connected with the control circuit, and the secondary coil of the transformer is respectively connected with the cathode of the X-ray generating part and the anode of the X-ray generating part through the coupling circuit; the coupling circuit is composed of a rectifier diode and a coupling capacitor.

Description

Field emission self-focusing pulse X-ray generating device integrated with high-voltage power supply

[ technical field ] A method for producing a semiconductor device

The invention discloses a field emission self-focusing pulse X-ray generating device integrated with a high-voltage power supply, which is applied to the technical fields of vacuum electronic equipment, X-rays, an electron gun, computed tomography, nondestructive testing and the like.

[ background of the invention ]

X-rays are generated by high velocity electron bombardment of a high atomic weight metal target. A typical X-ray tube includes a metallic anode target, a cathode electron source, and a high voltage power supply. Conventional X-ray tubes use a thermally emissive tungsten filament cathode to generate an electron beam. Because the temperature rising and temperature lowering processes of the tungsten filament are relatively slow, the traditional X-ray tube cannot be switched on and off at a high speed. The structure of an X-ray tube using a hot cathode is shown in fig. 1A. An X-ray anode 113 and a hot cathode 114 of a tungsten-wire-based X-ray generating section in the hot cathode X-ray tube are disposed in the vacuum chamber 112. A high voltage power supply and a cathode filament power supply 111 supply power to an X-ray anode 113 and a hot cathode 114 of a tungsten filament-based X-ray generating part, respectively.

With the development of carbon nanotube field emission technology, several X-ray tubes using field emission cold cathodes have been developed, as shown in fig. 1B-1D, and refer to the patent publications: CN101521135A, and is used in some experimental X-ray imaging systems, see patent publication No.: file of CN 104768467A. The simplest two-stage structure field emission X-ray tube is shown in fig. 1B. An X-ray anode 113 and a cathode 124 of an X-ray generating section based on field emission in a field emission X-ray tube of a two-pole structure are disposed in the vacuum chamber 112. A high voltage power supply 121 supplies power to the X-ray anode 113.

However, when the ordinary carbon nanotube field emission cathode continuously operates for several seconds, thermal runaway occurs and the carbon nanotube field emission cathode is burnt, so the carbon nanotube field emission cathode must operate in a pulse state. Grid control becomes a common configuration for field emission X-ray tubes, as shown in fig. 1C. An X-ray anode 113, a control grid 135 and a cathode 124 of an X-ray generating section based on field emission in an X-ray tube with a control grid are placed in the vacuum chamber 112. A high voltage power supply and gate control circuit 131 supplies power to the X-ray anode 113 and the control gate 135, respectively.

In addition, in order to achieve precise focusing and grid control, some prior art techniques employ complex multipole structures, as shown in fig. 1D, please refer to the literature: J. kim et al, "electrostatic Focusing Lens Module with Large Focusing in Carbon nanotubebemitter-Based X-Ray Sources," IEEEElectron Device Lett., vol.36, No.4, pp.396-398, Apr.2015. An X-ray anode 113, a control grid 135, an electron beam focusing structure 146 and a field emission cathode 124 in an X-ray tube with a control grid and a focusing structure are placed in vacuum chamber 112. The composite control circuit 141 powers the X-ray anode 113, the electron beam focusing structure 146 and the control grid 135, respectively.

The multipole structure not only increases the cost and complexity of the X-ray tube, but also significantly reduces reliability due to the increased number of components. In the prior art, a high-voltage power supply and an X-ray tube are two independent components which are connected through a high-voltage cable. High voltage cables and high voltage connectors are very bulky, greatly affecting the portability and mobility of the system.

[ summary of the invention ]

The invention mainly aims at the problems of too complex structure, complex circuit, poor reliability, poor focusing performance and short service life of the carbon nano tube of the field emission X-ray tube structure, and provides a small-sized X-ray generating device with medium and low power. The device integrates a pulsed high voltage power supply with an X-ray generating component (as shown in fig. 2 and 3). The electron beam is focused by a magnetic field generated by a transformer. The device has the advantages of simple structure, reliable performance, long service life and the like.

The invention relates to a field emission self-focusing pulse X-ray generating device integrated with a high-voltage power supply, which consists of an X-ray generating part and a high-voltage generator, wherein the connection relationship between the X-ray generating part and the high-voltage generator is as follows: the anode of the high voltage generator is connected with the anode of the X-ray generating part, and the cathode of the high voltage generator is connected with the cathode of the X-ray generating part.

Except for a low-voltage power supply and a control circuit, other parts (including an X-ray cathode and an X-ray anode, a transformer and a high-voltage coupling circuit) of the generating device are all arranged in a high-vacuum or ultrahigh-vacuum chamber. The pressure of the vacuum chamber is lower than 1x10^-6Pa。

The X-ray generating part adopts a diode structure based on a carbon nano-tube field emission device and comprises a cathode of the X-ray generating part and an anode of the X-ray generating part. The carbon nanotube field emission device serves as a cathode of the X-ray generating section, and an anode of the X-ray generating section is made of a metal material having a high melting point and a high atomic weight, such as tungsten, gold, molybdenum, or the like.

The high voltage generator comprises a control circuit, a transformer and a high voltage coupling circuit. The high-voltage generator is powered by an external low-voltage direct-current power supply. The transformer adopts a step-up transformer with high turn ratio (the turn ratio is more than 200: 1). The primary coil of the transformer is connected with the control circuit, and the secondary coil of the transformer is respectively connected with the cathode of the X-ray generating part and the anode of the X-ray generating part through the coupling circuit. The primary coil and the secondary coil of the transformer are made of copper or silver wires, the insulating layer is made of a low-vacuum exhaust material, and the common material is polyimide, ceramic or glass. The transformer magnetic core is made of ferrite ceramic material with good high-frequency performance. The coupling circuit is composed of a rectifier diode and a coupling capacitor.

The invention has the advantages and beneficial effects that:

1. the X-ray generating part is integrated with a high-voltage generator, and a high-voltage circuit is completely in vacuum without a high-voltage cable and a high-voltage connector (shown in figure 2); the invention adopts a highly optimized small-sized high-voltage step-up transformer, and the transformer works at the resonance frequency. The step-up ratio of the transformer is the product of the turn ratio and the coil quality factor, so the transformer has small volume and high step-up efficiency, and can meet the requirements of medium and small power X-ray application on voltage and current. The transformer is made of high-temperature resistant materials with low vacuum exhaust, can stably work in a vacuum environment, and meanwhile, the vacuum degree cannot be reduced due to high temperature generated during working. All high-voltage parts in the circuit are in vacuum, and high-voltage insulation is not needed in an external circuit, so that the size and the weight of the whole device are greatly reduced, and the safety and the reliability of the device are improved.

2. The transformer adopts an open magnetic circuit design, and focuses electron beams by utilizing the nonuniformity of magnetic flux (as shown in figure 4); because the transformer adopts the open magnetic circuit design, the position that is close to the magnetic core magnetic flux density is high, and the position of keeping away from the magnetic core, magnetic flux density is low. In the present invention, the cathode of the X-ray generating section is placed at a position where the magnetic flux density is low, and the anode of the X-ray generating section is placed at a position where the magnetic flux density is high. When the electrons leave the cathode of the X-ray generating section, they fly toward the anode of the X-ray generating section by the force of the electric field. Due to the presence of the magnetic field, the electrons move along a helix with the magnetic field lines as the axis. 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 electron beam gradually converges while flying to the anode of the X-ray generating part, achieving the purpose of focusing.

3. The nonlinear characteristic of the cathode volt-ampere characteristic of the X-ray generating part inhibits the generation of low-energy electrons, and the efficiency of the X-ray tube is improved; meanwhile, the maximum current of the transformer limits the cathode current of the maximum X-ray generation part, and the burning of the cathode of the X-ray generation part caused by overcurrent is avoided. The current-voltage characteristic of the cathode of the X-ray generating section is an exponential function. Below the threshold voltage, the cathode of the X-ray generating section has almost no electron emission and therefore no power consumption, and the high voltage generated by the transformer is used to charge the capacitor; when the sum of the output voltage of the transformer and the capacitor voltage exceeds the threshold voltage, the cathode of the X-ray generation part starts to emit electrons to generate X-rays, and when the field emission current exceeds the maximum output current of the transformer, the voltage at two ends of the transformer starts to drop, so that the further increase of the current is inhibited, and the thermal runaway of the cathode of the X-ray generation part is avoided.

[ description of the drawings ]

Fig. 1A is a conventional hot cathode X-ray tube.

Fig. 1B is a field emission X-ray tube of a two-pole structure.

Fig. 1C is a field emission X-ray tube with a control grid.

Fig. 1D is a field emission X-ray tube with a control grid and a focusing structure.

Fig. 2 is a circuit schematic diagram of a first embodiment of the present invention.

Fig. 3 is a cross-sectional view of a first embodiment of the present invention.

Fig. 4 is a schematic diagram of the magnetic field focusing principle.

Fig. 5 is a schematic circuit diagram of a second embodiment of the present invention.

Fig. 6 is a cross-sectional view of a second embodiment of the present invention.

Fig. 7 is a structural diagram of a pot core used in the second embodiment of the present invention.

The numbers in the figures illustrate the following:

111 high voltage power supply and cathode filament power supply; 112 a vacuum chamber; 113 an X-ray anode;

114 a hot cathode based on the X-ray generating portion of the tungsten filament; 121 a high voltage power supply;

124 a cathode based on a field-emitted X-ray generating portion;

131 high voltage power supply and gate control circuitry; 135a control gate;

146 an electron beam focusing structure; 141 a composite control circuit;

201 low voltage power supply and control circuit; 202 a glass vacuum chamber; 203 an anode of the X-ray generating section;

204 a cathode based on a field-emitted X-ray generating portion; 211 a transformer primary coil;

212 a transformer secondary coil;

213 a cylindrical transformer core; 214 a rectifier diode; 215 a coupling capacitance;

421 electron trajectory; 422 magnetic lines of force;

520 a voltage multiplying coupling circuit;

613 can transformer cores; 614 anode of the pyramidal X-ray generating portion.

[ detailed description ] embodiments

The following further describes the embodiments of the present invention with reference to the drawings.

The schematic circuit diagram and the structural cross-sectional diagram of the first embodiment are shown in fig. 2 and 3. The components in this embodiment include a low voltage power supply and control circuit 201, a glass vacuum chamber 202, an anode 203 of an X-ray generating section, a cathode 204 of an X-ray generating section based on field emission, a transformer primary coil 211, a transformer secondary coil 212, a cylindrical transformer core 213, a rectifier diode 214, and a coupling capacitor 215.

The entire device is powered by a low voltage dc power supply of 12 to 24 volts or a battery, and the low voltage power supply and control circuit 201 generates a high frequency signal equal to the resonant frequency of the transformer, in this embodiment 72 kHz. In the low-voltage power supply and control circuit 201, the high-frequency signal controls the switching of the power supply through an H-shaped full bridge circuit formed by semiconductor field effect transistors to generate high-frequency low-voltage heavy current to supply power for the transformer. The low-voltage power supply and control circuit 201 is arranged outside the glass vacuum chamber 202 and is connected with a primary coil of a transformer in the glass vacuum chamber 202 through a lead wire.

The transformer is a boosting transformer with the turn ratio of 500, 5 turns of a primary coil and 2500 turns of a secondary coil, the resonant frequency is 72kHz, and the quality factor is 8. When a 12V power supply is used, the output voltage of the transformer operating at the resonant frequency is 12 × 500 × 8 to 48 kv. The secondary coil of the transformer is connected to the cathode of the X-ray generating section and the anode of the X-ray generating section through a coupling circuit composed of a high-voltage capacitor and a high-voltage diode.

In this embodiment, the cathode 204 of the X-ray generating part based on field emission uses a vertically aligned carbon nanotube array, the diameter of the top of the carbon nanotube is about 5 to 20 nm, and the length is 5 to 10 μm. The pitch is 10 microns. The cathode 204 of the X-ray generating section based on field emission is composed of more than eight thousand carbon nanotubes, and the effective area of the cathode 204 of the X-ray generating section based on field emission is about 0.8 square mm. Pulsed field emission currents in excess of 20 milliamps may be provided. The anode 203 of the X-ray generating section may be a metal having a high atomic weight such as tungsten, molybdenum, or gold, or a metal having a high thermal conductivity such as copper or silver. High atomic weight metal can also be used as a target material to be embedded into a high heat conduction anode so as to improve the X-ray generation amount and improve the heat dissipation performance. The anode 203 of the X-ray generating section is formed in a thin cylindrical shape and placed at the center of the cylindrical transformer core 213, because the magnetic flux density at the center of the cylindrical transformer core 213 is the largest in this embodiment, the best focusing effect can be achieved. The anode 203 of the X-ray generating section should have as small a cross-sectional area as possible to avoid eddy currents causing heating of the anode.

The prototype product of this example determined output voltages in excess of 60 kv in actual measurement by arc length measurement in air. The output power of the transformer is measured to be 120W, and the output current of the transformer is known to be 2 milliamperes, so that the actual use requirement can be met. In actual measurement, the prototype of the present embodiment can continuously generate X-ray pulses for 5 to 200 milliseconds.

The above example is a prototype that has been produced and tested. Some of the components of this embodiment may be implemented in other alternative ways. For example, the carbon nanotubes of the cathode 204 for making the field emission based X-ray generating section may be replaced with other field emission materials, such as graphene, zinc oxide nanowires, silicon nanowires, tungsten nanowires, etc.

The principle of electron beam focusing in the present invention is shown in fig. 4. Lines of flux 422 exhibit variations in the magnetic flux density of the magnetic field produced by the current in the primary and secondary windings of the transformer. The electron trajectory 421 is a spiral line having the magnetic field lines 422 as an axis. The magnetic flux has a greater density at a position close to the anode 203 of the X-ray generating section and a smaller density at a position close to the cathode 204 of the X-ray generating section based on field emission. The electron beam emitted from the cathode 204 of the X-ray generating section based on field emission is accelerated by the electric field between the anode 203 of the X-ray generating section and the cathode 204 of the X-ray generating section based on field emission to fly toward the anode 203 of the X-ray generating section, and under the action of the magnetic field, the electron beam makes a helical motion with the magnetic line of force 422 as an axis, and the diameter of the electron trajectory 421 is gradually reduced, thereby finally realizing the focusing of the electron beam.

The second embodiment adopts the same general structure as the first embodiment.

In this embodiment, the rectifying diode 214 and the coupling capacitor 215 are replaced by a voltage multiplying coupling circuit 520 (shown in fig. 5). The voltage multiplication circuit can multiply the voltage value output by the high-voltage generator, and can improve the energy of the X-ray to more than 100 kilovolts for application requiring higher energy.

In this embodiment, the anode 203 of the X-ray generating portion in the first embodiment is composed of a cone-shaped anode 614 (shown in fig. 6) of the X-ray generating portion. The pointed anode can better focus the electron beam, resulting in a smaller focal spot and improved image resolution.

In this embodiment, the cylindrical transformer core 213 in the first embodiment is replaced with a pot transformer core 613 (shown in fig. 6 and 7). The pot core can reduce magnetic leakage and improve transformer efficiency to increase the output current of the transformer secondary coil 212. Meanwhile, due to the reduction of the leakage magnetic flux, the magnetic flux density between the anode 203 of the X-ray generating section and the cathode 204 of the X-ray generating section based on the field emission is larger than when the cylindrical transformer core is used, and the electron beam focusing ability is enhanced.

Claims

1. A field emission self-focusing pulse X-ray generating device integrated with a high-voltage power supply is composed of an X-ray generating part and a high-voltage generator, and is characterized in that: the connection relationship between them is: the anode of the high-voltage generator is connected with the anode of the X-ray generating part, and the cathode of the high-voltage generator is connected with the cathode of the X-ray generating part;

except for a low-voltage power supply and a control circuit, other parts of the generating device are all arranged in a high-vacuum or ultrahigh-vacuum chamber;

the X-ray generating part adopts a diode structure based on a carbon nano tube field emission device and comprises a cathode of the X-ray generating part and an anode of the X-ray generating part; the carbon nano tube field emission device is used as a cathode of the X-ray generating part, and an anode of the X-ray generating part is made of a metal material with high melting point and high atomic weight;

the high-voltage generator comprises a control circuit, a transformer and a high-voltage coupling circuit; the high-voltage generator is powered by an external low-voltage direct-current power supply; the transformer adopts a step-up transformer with high turn ratio; the transformer primary coil is connected with the control circuit, and the transformer secondary coil is respectively connected with the cathode of the X-ray generating part and the anode of the X-ray generating part through the high-voltage coupling circuit; the primary coil and the secondary coil of the transformer are made of copper or silver wires, and the insulating layer is made of a low-vacuum exhaust material; the transformer magnetic core is made of ferrite ceramic material; the high-voltage coupling circuit consists of a rectifier diode and a coupling capacitor; the anode of the X-ray generating part is made into a thin cylinder shape and is arranged in the center of the cylindrical transformer magnetic core; the magnetic flux density at the center of the cylindrical transformer core is maximum; the anode of the X-ray generating section is small in cross-sectional area to avoid heat generation caused by eddy current.

2. The field emission self-focusing pulse X-ray generating device of the integrated high voltage power supply of claim 1, wherein: the other components include an X-ray cathode and anode, a transformer and a high voltage coupling circuit.

3. The field emission self-focusing pulse X-ray generating device of the integrated high voltage power supply of claim 1, wherein: the pressure of the vacuum chamber is lower than 1x10^-6Pa。

4. The field emission self-focusing pulse X-ray generating device of the integrated high voltage power supply of claim 1, wherein: the metal material with high melting point and high atomic weight is tungsten, gold or molybdenum.

5. The field emission self-focusing pulse X-ray generating device of the integrated high voltage power supply of claim 1, wherein: the transformer adopts a high turn ratio of more than 200: 1.

6. the field emission self-focusing pulse X-ray generating device of the integrated high voltage power supply of claim 1, wherein: the insulating layer is made of polyimide, ceramic or glass.

7. The field emission self-focusing pulse X-ray generating device of the integrated high voltage power supply of claim 1, wherein: the generating device is powered by a low-voltage direct-current power supply or a battery with 12 to 24 volts, and the low-voltage power supply and the control circuit generate a high-frequency signal equal to the resonance frequency of the transformer.

8. The field emission self-focusing pulse X-ray generating device of the integrated high voltage power supply of claim 1, wherein: in the low-voltage power supply and the control circuit, high-frequency signals control the switching of the power supply through an H-shaped full bridge circuit formed by semiconductor field effect transistors to generate high-frequency low-voltage current to supply power for a transformer; the low-voltage power supply and the control circuit are arranged outside the vacuum chamber and are connected with a primary coil of a transformer in the vacuum chamber through a lead.

9. The field emission self-focusing pulse X-ray generating device of the integrated high voltage power supply of claim 1, wherein: the cathode of the X-ray generating part adopts a vertically arranged carbon nano tube array, the diameter of the top end of the carbon nano tube is about 5 to 20 nanometers, and the length of the carbon nano tube is 5 to 10 micrometers; the spacing is 10 microns; the cathode of the X-ray generating part is formed by more than eight thousand carbon nano tubes, and the effective area is about 0.8 square millimeter; providing a pulsed field emission current in excess of 20 milliamps.