JP3324052B2 - Isotope separation method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an isotope separation method and apparatus suitable for separating isotopes from gaseous atoms.

[0002]

2. Description of the Related Art In an isotope having an ultrafine structure in a conventional isotope separation apparatus, an apparatus for rapidly sweeping the wavelength of laser light using an electro-optic crystal in order to avoid saturation of excitation by laser light has been proposed. "The Atomic Energy Society of Japan 1991
Spring Annual Meeting "(G32, Proceedings, p. 343).

Further, in an isotope separation apparatus, it is necessary to increase the electron temperature to improve the speed of ion recovery, but in the conventional apparatus, Japanese Patent Application Laid-Open Nos. 3-56125 and 4-187216 disclose. It is known that the electron temperature is increased by a high-frequency electric field as described in US Pat.

[0004]

However, among the above-mentioned prior arts, the one in which the wavelength of laser light is swept at a high speed using an electro-optic crystal requires the use of an electro-optic crystal that can withstand high-power laser light. However, there is a disadvantage that the cost is high.

Further, in the method of recovering ions by raising the electron temperature by a high-frequency electric field, a method is used in which the frequency of the applied high-frequency is set to an electron cyclotron frequency that exists only when a magnetic field is present. However, there is a disadvantage that it cannot be used under the condition that does not exist.

The conventional isotope separation apparatus has a configuration in which a large coil is provided outside the vacuum vessel and a magnetic field is applied to the entire inside of the vacuum vessel, so that the electron temperature can be increased using an electron cyclotron. Met. However, in a recent isotope separation apparatus, a coil is locally provided, and a magnetic field is not always applied to the entire inside of a vacuum vessel. Therefore, it is impossible to use an electron cyclotron in a place where no magnetic field exists.

SUMMARY OF THE INVENTION An object of the present invention is to provide an isotope separation method capable of efficiently performing ionization with a laser beam at low cost and increasing the electron temperature even in the absence of a magnetic field to achieve high ion recovery efficiency. It is to provide a device.

[0008]

In order to solve the above-mentioned object, the present invention irradiates gaseous atoms containing two or more types of isotopes with laser light of two or more types of wavelengths and emits the gaseous atoms in the gaseous atoms. Exciting specific ion isotopes and ionizing them
In the isotope separation method including a first step of forming a plasma and a second step of recovering the ionized specific isotope by an electric field, the recovered specific isotope is
An electrode having a microstructure and forming the electric field
In the first step, the excitation between the levels of the hyperfine structure is performed by applying a high frequency at which the frequency is swept.
In the second step, the electron temperature of the photoionized plasma is increased.

The high frequency applied to the electrode in the first step is a high frequency having two or more frequency components. In the present invention, the frequency is swept during laser beam irradiation.
Further, the frequency of the high frequency applied to the electrode in the second step is set to be lower than the plasma frequency of the electrons of the photoionized plasma.

The present invention also provides a gaseous atom generating means for generating gaseous atoms containing two or more types of isotopes, and a specific isotope in the gaseous atoms by irradiating two or more types of laser light with laser light. Exciting and ionizing the body generates photoionized plasma
In an isotope separation apparatus comprising: a laser beam irradiation unit to be formed ; and an ion recovery unit that recovers the ionized specific isotope by an electric field, the recovered specific isotope is
It has an ultrafine structure and has an electric field for forming the electric field.
High-frequency marking that applies a high frequency that sweeps the frequency to the poles
Forming the photoionized plasma by providing
However, excitation between the levels of the hyperfine structure occurs.
After that, when the above-mentioned ionized specific isotope is recovered,
Ronioite increases the electron temperature of the light-ionized plasma
It is characterized by the following . Further, the high frequency may have two or more frequency components .

[0011]

In order to efficiently perform ionization by laser light at low cost, it is necessary to avoid saturation of excitation by laser light in an isotope having an ultrafine structure. In the present invention, a high frequency is applied to excite between hyperfine structures while irradiating a laser beam for ionization. That is,
As shown in FIG. 3, the hyperfine structure of a certain excitation level has a width of about several GHz at the maximum, but by applying a high frequency equal to the energy difference between the levels of the hyperfine structure, It can excite between the levels of the structure. This makes it possible to avoid excitation saturation while keeping the wavelength of the laser light constant. Further, when there are many levels of the hyperfine structure, it is possible to excite from each level by sweeping the frequency of the high frequency applied during the laser beam irradiation.

Further, it is possible to increase the electron temperature and increase the ion collection efficiency in the absence of a magnetic field by applying a high frequency to the photoionized plasma generated by laser beam irradiation. The lower limit of the frequency at this time is about several MHz, but the upper limit is equal to or lower than the plasma frequency of electrons of the photoionized plasma. By applying such a high frequency superimposed on the DC voltage, the electron temperature of the photoionized plasma can be increased by the high frequency heating, and the amount of ions in the plasma diffused into the sheath region with the increase in the electron temperature is increased. As a result, the ion collection efficiency can be improved.

Since the device used for applying high frequency can be used in common for ionization and the device used for ion recovery, the applied high frequency is continuously applied from the time of laser beam irradiation until the end of ion recovery. Irradiation is possible.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic configuration of an isotope separation device of the present invention, and FIG. 2 is a cross-section taken along line AA of FIG. In order to generate a beam of gaseous atoms containing several or more isotopes in the vacuum vessel 1, for example, a sample 3 placed in a crucible 2 is heated by an electron beam 4. At this time, the sample 3 evaporates and flows upward as an atomic beam 5. The atomic beam 5 is irradiated with a laser beam 6 to excite only specific isotopes in atoms to be ionized. In the ionization process, an example in which ionization is performed in two wavelengths and two stages will be described. Even when the wavelength and the number of steps increase, the basic method is the same as the following.

In the case of ionization at two wavelengths and two stages, two wavelength tunable lasers are used. Here, the tunable lasers are referred to as a first tunable laser 7 and a second tunable laser 8, respectively. Each of the tunable lasers 7 and 8 is excited by an excitation laser 9. Usually, a high-output pulse laser such as an excimer laser or a copper vapor laser is used as the excitation laser 9.

On the other hand, dye lasers are used as the tunable lasers 7 and 8, respectively. In the case of the single mode, the wavelength width of each of the tunable lasers 7 and 8 is about 500 KHz. If ionization is performed with only two wavelengths of laser light without sweeping this wavelength, in the ionization of an isotope having an ultrafine structure,
As shown in FIG. 3 (a), excitation occurs between a specific one of the ground level and the excited level, and at this time,
When excited from the ground level to the excited level, the excitation is saturated, and conversely, stimulated emission from the excited level to the ground level is caused. Further, it cannot excite an atom having an electron at a level of another microstructure.

Therefore, in the present embodiment, as shown in FIG. 3B, electrons are excited between the fine structures of the ground level by high frequencies a, b, and c and excited by laser light. The atoms are always present, and even when excited to an excitation level, high frequencies d, e, and f excite between the fine structures of the excitation level to prevent saturation. Normally, the energy difference between the levels of the hyperfine structure is several tens of MH in wave number notation.
Since the frequency is from z to several GHz, the ionization as described above is achieved by adjusting the frequency of the high frequency to be irradiated at this time to the energy difference between the levels of the hyperfine structure.

As a high-frequency application system at this time, as shown in FIG. 1, a high-frequency oscillator 10 oscillates a high-frequency of a specific wavelength, a high-frequency amplifier 11 amplifies the high-frequency, and a matching unit 12 matches the impedance. Then, a high frequency is applied to the generation region of the photoionized plasma 14 (that is, the laser light irradiation region). Usually, the irradiation time of one pulse of the pulse laser light is about 50 ns.
If the frequency is set to MHz, a high frequency of 10 periods can be applied during laser beam irradiation, so that there is ample room for excitation between levels of the hyperfine structure.

Here, as shown in FIG. 3 (b), when there are many levels of the fine structure and their energy differences are different, the frequency of the applied high frequency is swept or two or more high frequencies are applied. By applying a high frequency (i.e., a high frequency having two or more frequency components) obtained by adding the above, it is possible to excite between the respective levels during one pulse of the laser light.

The laser light 6 from the first tunable laser 7 and the second tunable laser 8 is applied to mirrors 15, 16,
Irradiation is performed on the laser light irradiation area between the recovery electrodes 13 via the light receiving area 17.

Next, the ion recovery process will be described.
First, a DC voltage is applied from a DC power supply 18 through a coil 19 to the photoionized plasma 14 generated between the parallel plate type collection electrodes 13 to collect ions. At this time, the electron temperature immediately after generation of the photoionized plasma 14 is 0.1 to 0.3.
eV, and the ion recovery time takes 50 μs or more. Therefore, in order to shorten the ion collection time, it is known that a method of increasing the electron temperature of the photoionized plasma 14 to increase the amount of ions in the plasma 14 diffused into the sheath is effective.

In this embodiment, a high frequency is applied while being superimposed on a DC voltage. The frequency of the high frequency applied at this time is set to a frequency equal to or lower than the plasma frequency of the electrons of the photoionized plasma 14. The plasma frequency of the electrons is a frequency determined only by the electron density. For example, the frequency is 900 MHz when the plasma density is 10 16 cubic meters. When a high frequency with a frequency lower than this frequency is applied, the high frequency cannot propagate in the plasma, and only the sheath portion fluctuates in potential. At this time, electrons on the plasma surface facing the sheath change due to this potential fluctuation. The electrons are accelerated and heated in the plasma, thereby increasing the electron temperature of the entire plasma. Since this phenomenon exists regardless of the presence or absence of a magnetic field, the electron temperature can be increased without a magnetic field. By increasing the electron temperature in this way, it is possible to shorten the ion collection time.

Next, FIG. 4 is a flowchart of the isotope separation method in the present embodiment, and FIG. 5 is a time chart when a high frequency is applied. First, in step 50 of FIG. 4, in order to increase the ionization efficiency, the above-described high frequency is applied to the recovery electrode 13 while the atomic beam 5 is irradiated with the laser beam 6 (FIG. 5A to FIG. 5A). (c)). This step is the first step. Immediately after the laser irradiation, the photoionized plasma 14 is immediately generated. Therefore, in step 51 of FIG. 4, a high frequency is continuously applied to increase the electron temperature of the plasma (FIGS. 5D to 5F). As described above, in the present embodiment, the high frequency is applied continuously from the time of laser irradiation until the ions are collected.

As described above, according to this embodiment, ionization by laser light can be performed efficiently, and
Even in the absence of a magnetic field, the electron temperature can be increased to increase the ion collection efficiency.

[0025]

As described above, according to the present invention,
The ionization by the laser beam can be performed efficiently, and the electron temperature can be increased even under the condition where no magnetic field exists. In particular, since the electron temperature can be increased without the presence of a magnetic field, it is possible to improve the ion collection efficiency even in a recent isotope separation apparatus in which a coil is locally provided.

In addition, a device for applying a high frequency while irradiating a laser beam and a device for applying a high frequency to the photoionized plasma after the laser beam irradiation can be realized by the same high-frequency applying device. It is very advantageous.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an isotope separation device of the present invention.

FIG. 2 is a sectional view taken along line AA of FIG.

FIG. 3 is a diagram illustrating an excitation / ionization process.

FIG. 4 is a flowchart showing the isotope separation method of the present invention.

FIG. 5 is a time chart of an excitation / ionization process and an ion recovery process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum container 2 Crucible 3 Sample 4 Electron beam 5 Atomic beam 6 Laser beam 7 First tunable laser 8 Second tunable laser 9 Excitation laser 10 High frequency oscillator 11 High frequency amplifier 12 Matching device 13 Recovery electrode 14 Photoionization plasma 15, 16, 17 mirror 18 DC power supply 19 coil

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masahiro Fujima 7-2-1, Omika-cho, Hitachi City, Ibaraki Pref. Energy Research Laboratory, Hitachi, Ltd. (72) Inventor Satoshi Tsuda 7-2 Omika-cho, Hitachi City, Ibaraki Prefecture No. 1 Hitachi, Ltd. Energy Research Laboratory (56) References JP-A-3-196818 (JP, A) JP-A-4-187216 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , (DB name) B01D 59/34

Claims (5)

(57) [Claims] 1. A photoionization plug by ionizing and exciting a particular isotope in the gas atoms by irradiating a laser beam of two or more wavelengths in the gaseous atoms comprising two or more isotopes
In the isotope separation method including a first step of forming a plasma and a second step of recovering the ionized specific isotope by an electric field, the recovered specific isotope is
An electrode having a microstructure and forming the electric field
In the first step, the excitation between the levels of the hyperfine structure is performed by applying a high frequency at which the frequency is swept.
And in the second step, the electron temperature of the photoionized plasma is increased. Wherein two or more of photoionization up by ionizing and exciting a particular isotope in the gaseous atoms by irradiating a laser beam of two or more wavelengths in the gaseous atoms including isotopes
In the isotope separation method including a first step of forming a plasma and a second step of recovering the ionized specific isotope by an electric field, the recovered specific isotope is
It has an ultrafine structure and has an electric field for forming the electric field.
By applying a high frequency having two or more frequency components to the pole , in the first step, excitation between the levels of the hyperfine structure is caused , and in the second step, the electron temperature of the photoionized plasma is reduced. An isotope separation method characterized by raising. 3. The isotope separation method according to claim 1, wherein the frequency of the applied high frequency is lower than the plasma frequency of the electrons of the photoionized plasma. 4. A gaseous atom generating means for generating gaseous atoms containing two or more types of isotopes, and irradiating laser light of two or more types of wavelengths to excite specific isotopes in the gaseous atoms. In an isotope separation apparatus comprising: a laser light irradiation unit that forms a photoionized plasma by ionization ; and an ion collection unit that collects the ionized specific isotope by an electric field, the specific isotope to be recovered is excessively high. Has fine structure
Frequency sweeping the electrodes forming the electric field.
Provide high frequency applying means for applying high frequency to be pulled
By forming the photoionized plasma,
Causes excitation between the levels of the hyperfine structure,
In recovering ionized specific isotopes,
An isotope separation device for increasing the electron temperature of a photoionized plasma. 5. A gaseous atom generating means for generating gaseous atoms containing two or more types of isotopes, and irradiating laser light of two or more types of wavelengths to excite specific isotopes in the gaseous atoms. In an isotope separation apparatus comprising: a laser light irradiation unit that forms a photoionized plasma by ionization ; and an ion collection unit that collects the ionized specific isotope by an electric field, the specific isotope to be recovered is excessively high. Has fine structure
The electrode forming the electric field has two or more components.
High frequency applying means for applying a high frequency having a frequency component of
By providing, in the place where the photoionized plasma is formed
Causes the excitation between the levels of the hyperfine structure.
Therefore, when recovering the ionized specific isotope,
Information, isotope separation apparatus characterized by raising the electron temperature of the light-ionized plasma.