JPH0298032A - Electron gun - Google Patents

Abstract

PURPOSE:To reduce an acceleration voltage of an electron beam so as to enhance reliability by forming a cathode with a core material made of a flexible metal having a high melting point coated with a material having excellent thermoelectron emitting performance. CONSTITUTION:The central portion of a cathode 14 is formed with a core material 24 made of a metal having a high melting point such as tungsten, and is further coated or deposited with a material having excellent thermoelectron emitting performance such as lanthanum ana lanthanum hexaboron, thereby obtaining a coating material 19. Therefore, it is unnecessary to extremely heat the cathode, and consequently, deformation, deposition damage or the like of the cathode can be prevented and reliability can be enhanced.

Description

Detailed Description of the Invention [Object of the Invention] (Industrial Application Field) The present invention is directed to an apparatus for separating isotopes such as uranium, in which an electron beam is emitted onto a line used in a heating melting evaporation apparatus for metal raw materials. Concerning an electron gun that fires.

(Prior art) Uranium fuel used as nuclear reactor fuel is loaded into a nuclear reactor after separating and enriching the specific uranium that causes a nuclear reaction from a mixture of uranium isotopes and adjusting it to a predetermined concentration. .

Naturally occurring uranium is a uranium atom consisting of a light nucleus with a mass number of 235 (hereinafter abbreviated as uranium-235).
It contains about 0.7% of uranium, and most of the rest is uranium atoms with a nuclear mass number of 238 (hereinafter abbreviated as uranium-238).
No. 5 is separated and enriched from the above-mentioned natural uranium or nuclear reactor spent fuel, and is usually enriched to about 3 to 4% before being used as a nuclear reactor fuel.

Conventionally, there are gas diffusion methods, centrifugal separation methods, laser methods, etc. to separate uranium-235 from a mixture of isotopes such as uranium-235 and uranium-238, and to increase the concentration to a predetermined concentration level. Separation and concentration operations are performed using differences in physical and physical properties. Among these methods, the laser method is currently attracting attention as a method that is particularly superior in terms of separation performance compared to other methods.

A conventional example of an isotope separation apparatus using a laser method will be described below with reference to FIGS. 3 and 4, taking the separation operation of uranium isotopes as an example. Uranium metal 1 extracted from a depleted fuel body, either natural or used in a nuclear reactor, is loaded into a thermochemically resistant evaporation vessel 2, for example a crucible.

This evaporation container 2 is installed at the inner bottom of a sealed container 3 that is maintained in a nearly vacuum state. Next, an electron beam 5 emitted from the linear electron gun 4 is deflected by a DC magnetic field 6 applied by an external magnetic field coil (not shown) and irradiates the metal uranium 1 in the evaporation container 2. The metallic uranium 1 irradiated by the electron beam 5 is heated to about 2700 K to 3100 K and evaporated, producing a vapor stream 7. Incidentally, the composition of the vapor flow 7 is, for example, when natural uranium is used as metal uranium 1, uranium 235 is 0.7% by weight,
Contains 99.3% uranium-238.

On the other hand, above the evaporation container 2, anode electrodes 8 and cathode electrodes 9 are arranged alternately as band-shaped product recovery electrodes, and a photoreaction part 0 is formed between the electrodes. Photoreactive part 1
A laser beam 11 that selectively ionizes uranium 235 generated by a laser generator 12 is incident in the longitudinal direction of 0, and undergoes a photoreaction with the vapor flow 7. Laser beam 1
The wavelength of laser beam II is adjusted to the resonant ionization wavelength of uranium-235, and only uranium-235 atoms contained in the uranium vapor flow 7 introduced into the photoreaction section 10 resonate with the laser beam II, selectively ionizing the uranium-235 with a certain probability. is ionized. The ionized uranium-235 ions are collected between the positive electrode 8 and the negative electrode 9 by an electric field created by applying a pulsed electrode voltage synchronized with the laser beam 11.
is adsorbed and collected on the surface. Further, the mixed vapor flow of uranium 235 and uranium 238 that passed through the photoreaction section 10 without being ionized is collected by a vapor recovery plate 13 disposed at the outer edge of the photoreaction section 10.
is adsorbed and recovered. The collected and liquefied vapor is returned to the evaporation container 2 or the like by a separate means.

Next, a conventional example of the linear electron gun 4, particularly a direct heating type electron gun, will be explained with reference to FIGS. 5 and 6.

Both ends of the cathode 14 formed in the shape of a wire are connected to a current source (not shown in the figure), whereby the cathode 14 is heated by electricity. The cathode 14 is a thermionic source that emits thermoelectrons from its outer surface, and needs to be heated to a sufficient temperature to obtain the required current density.

For this purpose, the cathode 14 is usually made of a high melting point metal such as tungsten. Further, an anode 16 having an opening 15 is provided on the side of the irradiated object (not shown) with respect to the cathode 14, and accelerates the thermoelectrons emitted from the cathode 14 to generate an electron beam 17. 1 Usually, a beam accelerating voltage source (
(not shown in the figure), a high voltage power source of several KV to several tens of KV is connected and applied. A beam shaping electrode 18 is disposed in a direction opposite to the electron beam generation method of the cathode 14.

The beam shaping electrode 18 is an auxiliary electrode for deflecting the thermoelectrons isotropically emitted from the cathode 14 in the direction of the positive Fi 16, and normally has a potential similar to that of the anode 16.

As a heating method for the filament-shaped cathode 14 made of a high-melting point metal such as tungsten, a method such as Joule heating of the cathode 14 by passing an electric current through the cathode 14 is usually employed. This causes the cathode 14 to reach 2000 K.
The cathode 14 is heated to a high temperature of about 30K (about 10K), and a group of thermoelectrons is conventionally emitted from the surface of the cathode 14.

(Problem to be Solved by the Invention) In the conventional uranium isotope separation apparatus, in order to generate the linear vapor flow 7, the evaporation container 2 containing the metallic uranium 1 has an elongated rectangular shape. ing. Therefore, the electron beam 17 is also required to be linear, and the cathode 14 is also a filament-like long thin wire. However, as mentioned above, it is necessary to maintain the cathode 14 at a high temperature.

In the case of long thin wires, deformation such as thermal elongation is a problem.

Deformation of the cathode 14 causes deformation or diffusion of the electron beam 17, which in turn causes damage to the cathode 14 and the beam shaping electrode 1.
This may lead to accidents such as contact short circuit with the electron beam 17, and makes it extremely difficult to lengthen the electron beam 17.

Furthermore, in order to increase the output of the electron gun, it is necessary to increase the temperature of the cathode 14 to increase the electron beam density, or to increase the beam acceleration voltage. but.

Increasing the temperature of the cathode 14 is not only limited by the above-mentioned reasons, but also causes shortening of the cathode's life and deterioration.

In addition, increasing the beam acceleration voltage induces dielectric breakdown between the electrodes, and it is impossible to increase the beam acceleration voltage above a certain level.

Furthermore, conventional filament-shaped cathodes made of high-melting point metals such as tungsten have to be heated to a considerably high temperature in order to obtain a predetermined thermionic current density.

One possible way to avoid this is to select a material with a small work function as the material for the cathode. However, manufacturing a filament-shaped cathode from a substance with a small work function, that is, an excellent thermionic emission ability, such as lanthanum or lanthanum hexaboride, has problems from the viewpoint of material brittleness.

The present invention has been made to solve the above-mentioned drawbacks, and an object of the present invention is to provide an electron gun that can stably generate a linear high-power electron beam.

[Structure of the invention]

(Means for Solving the Problems) An electron gun according to the present invention includes a wire cathode which is a source of an electron beam for irradiation, and a power source for applying a cathode current to the cathode to heat the cathode and maintain the cathode at a high temperature. and.

An electron gun that has an anode that generates an irradiation electron beam by accelerating thermoelectrons emitted from a cathode that is heated to a high temperature, and an electron beam acceleration power source that holds the anode at a more positive potential than the cathode. , a core material made of a malleable high-melting point metal, and a covering material made of a material with excellent thermionic emission ability that covers the outer surface of the core material.

(Function) In the electron gun having the above configuration, the thermionic emission ability of the filamentary cathode can be improved more than that of the core material. High-melting point metals such as tungsten are certainly 3000
There is no evaporation loss even at high temperatures around K or higher, but the work function of the material is not particularly small, so the filament must be heated to a fairly high temperature in order to obtain the desired thermionic density. .

According to the present invention, the density of thermionic electrons emitted from the filamentary cathode can be increased without raising the temperature of the filamentary cathode. In other words, the electron beam emitted from the electron gun,
High output can be achieved without increasing the accelerating voltage between the cathode and anode.

Furthermore, it becomes easy to operate the electron gun in a cathode temperature range that does not cause thermal expansion or thermal deformation of the filament-shaped cathode that impairs the specified shape of the electron beam, greatly improving the reliability of the electron gun. Can be done.

(Example) An example of the present invention will be described below with reference to FIG. FIG. 1 shows a cross section of a filamentary cathode 14 in an electron gun according to the present invention, and other parts are the same as FIGS. 5 and 6.

Components that are the same as those in the conventional example shown in FIGS. 5 and 6 are given the same reference numerals, and redundant explanation will be omitted.

The center of the cathode 14 is made of a core material 24 made of a high melting point metal such as tungsten, and a coating material 19 is formed by forming a film of a material having excellent thermionic emission ability such as lanthanum or lanthanum hexaboride. As the treatment method, a method such as applying the material 19 to the surface of the core material 24 or performing a vapor deposition treatment is adopted.

As a result, the thermionic emission surface is not the surface of the core material 24,
This becomes the surface of the covering material 19. Conduction electrons (electrons in metal) present in the coating material 19 usually have a low potential Ef corresponding to the phenyl level. Also.

The Fermi level has a potential energy lower than the external potential E0 by the work function φ, and conduction electrons (electrons in the metal) having high energy at a specific high temperature are emitted from the surface of the coating material 19 to the outside world.

Based on the energy phase diagram of conduction electrons in a substance shown in Figure 2, if we calculate the thermionic current density J emitted from the surface of an object assuming that the energy of conduction electrons in the object follows Fermi-Dirac statistics, we can find the following: The temperature T5, that is, the temperature T of the cathode 14 and the material 19 is calculated according to the ω equation.

Here, the elementary charge is e, the electron mass is m, the blank constant is , and the Boltzmann constant is ura. Further, the work function φ of ordinary metals is about 1 eV, and compared to this, lanthanum, lanthanum hexaboride, etc. have a small work function φ and can be said to have excellent thermionic emission ability. Here, the thermionic emission ability is calculated according to specific numerical values, and its effect is evaluated.

Assuming that the work function φ of the metal material constituting the core material 24 is. 1
eV, the work function φ of the thermionic emission material constituting the coating material 19 is set to 0.5 eV, and the temperature T of the cathode 14 is set to 2500. Ratio J/J of the thermionic current density J0 when there is no coating material 19 (corresponding to a conventional cathode structure) and the thermionic current density J when the coating material 19 is formed into a film.

is obtained by the equation 0 as shown in the equation 0.

Conversely, the temperature T when the thermionic current density is set equal in both cases and there is no covering material 19. Assuming that 250 OK, the temperature T when the coating material 19 is formed is 0.
The formula is as follows.

As described above, according to this embodiment, an electron beam source with a high thermionic current density can be provided even at a relatively low temperature.

〔Effect of the invention〕

According to the electron gun according to the present invention, it is possible to improve the electron current density, and it is possible to increase the output of the electron beam emitted by the electron gun. In addition, since there is no need to extremely heat the cathode as in the past, negative effects such as thermal deformation of the cathode and black spots are eliminated, and the structural materials of the electron gun other than the cathode are also exposed to the high-temperature environment. The reliability of the system will also increase.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the cathode of an electron gun according to the present invention, and FIG.
The figure is a graph showing the energy levels of conduction electrons on the outer surface of the cathode and floating electrons in free space in Figure 1, Figure 3 is a perspective view of a conventional isotope separation device, and Figure 4 is the IV in Figure 3. 5 is a perspective view showing the structure of the electron gun shown in FIG. 3, and FIG. 6 is a sectional view taken along the line ■-■ in FIG. 5. 1... Metallic uranium 3... Sealed container 5... Electron beam 7... Vapor flow 8a... Positive electrode extension part 10... Photo reaction part 12... Laser generator 14... Cathode 15...Aperture 17...Electron beam 19...Coating material

Claims (1)

[Scope of Claims] 1. A wire cathode that is a source of an irradiation electron beam, a cathode current supply power source that heats the cathode by applying current to the cathode and maintains the cathode at a high temperature, and the cathode that is heated to a high temperature. In the electron gun, the electron gun includes an anode that generates the irradiation electron beam by accelerating thermoelectrons emitted from the electron beam, and an electron beam acceleration power source that holds the anode at a more positive potential than the cathode. 1. An electron gun comprising: a core material made of a high melting point metal; and a covering material made of a material with excellent thermionic emission ability, which covers the outer surface of the core material. 2. The electron gun according to claim 1, wherein the coating material is deposited on the outer surface of the core material. 3. The electron gun according to claim 2, wherein the core material is made of tungsten, and the coating material is formed by vapor deposition coating of lanthanum on the surface of the core material. 4. The electron gun according to claim 2, wherein the core material is made of tungsten, and the coating material is formed by vapor deposition coating of lanthanum hexaboride on the surface of the core material. .