Ion acceleration method and ion acceleration device

Abstract

To provide an ion acceleration method and an ion acceleration device that generate highly monochromatic ions.SOLUTION: There is provided an ion acceleration method comprising: jetting a cluster gas 30 into a vacuum; irradiating the cluster gas 30 with laser light 18 to make the cluster gas 30 into plasma; and ionizing hydrogen 31 included in the cluster gas 30 for acceleration in a traveling direction of the laser light 18, wherein the condensing position of the laser light 18 is set within the cluster gas 30, the laser light 18 has a condensing spot size larger than a cluster 32 in the cluster gas 30 and smaller than a spread range of the cluster gas 30, and the cluster 32 in the cluster gas 30 is a cluster of hydrogen molecules or heavy hydrogen molecules of single kind.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an ion acceleration method and an ion acceleration device for accelerating ions, for example. [Background technology]

[0002] Conventionally, ion generating devices that generate ions have been proposed. For example, in an ion acceleration method for accelerating ions by irradiating a target with laser light and releasing them from the target, a method has been proposed in which the electron density in a front-stage portion irradiated with laser light is set as a critical condition, a target is used in which the electron density in a rear-stage portion following the front-stage portion gradually decreases, the front-stage portion of the target is irradiated with laser light to reduce the group velocity of the laser light incident on the front-stage portion to 0 in the front-stage portion, thereby transmitting the energy of the laser light to electrons, and the electrons to which the energy has been transmitted are gradually accelerated by a rear-stage portion following the front-stage portion, and ions are accelerated by the electrons and released from the rear-stage portion (see Patent Document 1). However, this method has the problem that it is difficult to put the technology into practice. In addition, it is preferable for an ion generating device to obtain monochromatic or quasi-monochromatic ions, but no device that achieves this has been provided. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 4953243 Summary of the Invention [Problem to be solved by the invention]

[0004] In view of the above problems, an object of the present invention is to provide an ion acceleration method and an ion acceleration device that generate ions with high monochromaticity. [Means for solving the problem]

[0005] This invention is an ion acceleration method or ion acceleration device in which a cluster gas is sprayed into a vacuum and a laser is irradiated onto the cluster gas, thereby converting the cluster gas into plasma and ionizing the atoms contained in the cluster gas and accelerating them in the direction of irradiation with the laser, wherein the focusing position of the laser is set within the cluster gas, the focusing spot size of the laser is configured to be larger than the clusters in the cluster gas and smaller than the spreading range of the cluster gas, and the clusters in the cluster gas are clusters of a single type of hydrogen molecule or deuterium molecule. Effect of the Invention

[0006] According to the present invention, it is possible to provide an ion acceleration method and an ion acceleration device that generate ions with high monochromaticity. [Brief description of the drawings]

[0007] [Figure 1] Block diagram of the experimental setup including the laser ion accelerator. [Figure 2A] Simulation results showing the spatial distribution of electrons immediately after passing through a laser. [Figure 2B] Simulation results showing the spatial distribution of proton beam energy immediately after passing through the laser. [Figure 2C] Simulation results showing the proton beam energy spectrum in the direction of laser travel. [Diagram 3] FIG. 13 shows a proton beam energy spectrum resulting from an ion acceleration experiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] FIG. 1 is a block diagram showing the configuration of an experimental device 1. As shown in FIG. The experimental device 1 includes a laser ion accelerator 3 and an ion detector 5 . The laser ion accelerator 3 (ion accelerator) has a laser irradiation device 11, a focusing device 13 installed in the rear stage, and a cluster gas generator 21.

[0009] The laser irradiation device 11 generates a laser beam from a laser light source (not shown) and irradiates it as a pulsed laser beam. For this laser irradiation device 11, a device that emits an ultrashort pulsed laser beam with a high intensity enough to turn the cluster gas 30 into plasma can be used. Specifically, the laser intensity can be set to 1×10 19 W / cm 2 That's it, 3 x 10 21 W / cm 2 In this example, the laser intensity can be about 1×10 20 W / cm 2 The laser intensity may be the intensity of the focused laser light after being focused by the focusing device 13. In this case, the focused laser intensity is set to 1×10 19 W / cm 2 That's it, 3 x 10 21 W / cm 2 For example, the laser focus intensity can be less than about 1×10 20 W / cm 2 It can be said that:

[0010] The pulse frequency of the pulsed laser light can be any frequency. Specifically, it can be 0.1 Hz or more, preferably 10 Hz or more, and more preferably 100 Hz or more. In this embodiment, it is 0.1 Hz, but it is not limited to this.

[0011] As the laser light source in the laser irradiation device 11, a fundamental wave, second harmonic, or third harmonic wave of a titanium sapphire laser, a glass laser, or the like can be used. In the case of a titanium sapphire laser, a laser beam 18 having a fundamental wave wavelength in the range of 650 nm to 1100 nm can be obtained. In the case of a glass laser, a laser beam 18 having a fundamental wave wavelength in the range of 900 nm to 1100 nm can be obtained. The laser beam 18 has a short pulse length and is repeatedly emitted at short intervals. The irradiation (oscillation) timing of the laser beam 18 is preferably controlled in synchronization with the generation of the cluster gas 30.

[0012] The focusing device 13 is an optical system device that focuses the laser light in or near the cluster gas 30. The laser light 18 focused by the focusing device 13 converts the clusters and molecules in the cluster gas 30 into plasma. For example, an aspheric focusing mirror such as an off-axis parabolic mirror can be used for the focusing device 13. The focusing position of the laser light 18 by the focusing device 13 can be within a range of less than 100%, assuming that the spread radius from the center of the cluster gas jet (the center of the cluster gas 30) is 100%, and in this embodiment, it is within a range of 95%. The irradiation direction of the laser light 18 is set at a right angle to the ejection direction of the cluster gas 30 ejected from the cluster gas generator 21, but is not limited to this and can be set at any appropriate angle.

[0013] The cluster gas generator 21 ejects cluster gas into a vacuum. The cluster gas may be a cluster gas generated from a single type of hydrogen molecule or deuterium molecule. In this embodiment, a hydrogen cluster gas made of pure hydrogen (only hydrogen) is ejected. When gas made of pure hydrogen is ejected into a vacuum, the hydrogen solidifies due to a sudden drop in temperature caused by adiabatic expansion when ejected, and the cluster gas 30 (in this case, hydrogen cluster gas) is formed in which minute particulate hydrogen clusters (clusters) 32 are dispersed in a gas made of hydrogen molecules 31. Here, the lower limit of the diameter (particle size) of the hydrogen clusters 32 can be 100 nm or more, and the upper limit can be less than 800 nm. In this embodiment, the diameter (particle size) of the hydrogen clusters 32 is set to 600 nm. In addition, the diameter of the hydrogen clusters 32 is configured to be smaller than the focused spot size that is focused by the focusing device 13 to become a laser irradiation spot. Therefore, the hydrogen clusters 32 are clusters of nanometer size to micrometer size. The density of the hydrogen clusters 32 in the cluster gas 30 can be set to any value, and is not limited to 2.0×10 8 ~2.0×10 10 cm -3 The ratio of the focused spot size to the cluster size, that is, focused spot size / cluster size, can be 1 or more, preferably 10 or more, more preferably 20 or more, and suitably 30 or more. In this embodiment, the focused spot size / cluster size is 33.

[0014] The space from which the gas is ejected is evacuated by a vacuum pump (not shown). Therefore, even when the gas is ejected, a degree of vacuum is maintained to the extent that the cluster gas 30 is stably generated. The ejected cluster gas 30 has a long cylindrical or conical shape extending from the ejection port in the ejection direction.

[0015] An electromagnetic valve (not shown) capable of repeatedly controlling opening and closing operations is provided in the cluster gas generator 21. With this, the cluster gas generator 21 generates a large number of hydrogen molecules (H 2 ) aggregate to form a cluster gas 30 in which hydrogen clusters 32 are dispersed into particles of nanometer to micrometer size, and this cluster gas 30 is sprayed downward in Fig. 1. In particular, the cluster gas generator 21 can form the cluster gas 30 in synchronization with the irradiation of the laser light 18, in response to the laser irradiation device 11 oscillating the laser light 18 as ultrashort pulses at a constant repetition frequency. The temperature of the hydrogen clusters 32, which are the target for laser irradiation (more precisely, the temperature of the cluster gas generator 21 which generates the hydrogen clusters 32), can be set to 20 Kelvin or higher and 50 Kelvin or lower. In this embodiment, the temperature of the hydrogen clusters 32 (the temperature of the cluster gas generator 21) is set to 30 Kelvin.

[0016] When the hydrogen clusters 32 are irradiated with high-intensity laser light 18 , laser plasma acceleration occurs in the hydrogen clusters 32 , and the ions (protons) of the hydrogen atoms are accelerated and generated as an ion beam 50 in the traveling direction of the laser light 18 .

[0017] The generated ion beam 50 has a quasi-monochromatic relative energy spread within the range of plus or minus 50%, more preferably within the range of plus or minus 30%, and even more preferably approximately plus or minus 20%. Specifically, the relative energy spread, assuming the traveling direction of the ion beam 50 to be positive, ranges from 20% (plus 20%) on the traveling direction side to 20% (minus 20%) on the opposite side.

[0018] The ion detector 5 has an ion detector 5a arranged linearly in the direction of propagation of the laser light 18, and an ion detector 5b installed at an angle of 57° from the direction of propagation of the laser light 18 with the center of the cluster gas 30 as its axis.

[0019] In the ion detector 5a, a Thomson parabola 51a, an MCP with a fluorescent screen 52a, and a CCD camera 53a are arranged in this order along the traveling direction of the laser light 18. Similarly, in the ion detector 5b, a Thomson parabola 51b, an MCP with a fluorescent screen 52b, and a CCD camera 53b are arranged in this order in the direction away from the center of the cluster gas 30.

[0020] The ion detector 5a in the laser travel direction can detect the generated ions (hydrogen ions (protons) in this embodiment) and detect the proton beam energy distribution, such as how many MeV the energy peak is at and whether a steep energy peak is obtained. This makes it possible to confirm whether a quasi-monochromatic proton beam is being generated. It can also be confirmed whether impurity ions other than protons are being generated and whether a debris-free state has been achieved.

[0021] In addition, the ion detector 5b, which is installed at an angle in a direction different from the laser propagation direction, can measure the energy distribution of the ion beam generated in a direction other than the propagation direction of the laser 18. In combination with the ion detector 5a, the spatial distribution of the proton beam energy can be detected.

[0022] The laser ion accelerator 3 thus configured is configured to satisfy the following four conditions.

[0023] (1) Condition A: Conditions for generating a high-purity proton beam The target is a hydrogen cluster, which is made up of only one type of proton, specifically hydrogen, making it possible to generate a high-purity proton beam.

[0024] (2) Condition B: Conditions for generating a quasi-monochromatic, high-purity proton beam of several MeV In addition to the above condition A, the following conditions Ba, Bb, and Bc are also satisfied. Condition Ba: The temperature of the cluster gas generator 21 (the temperature of the target cluster gas) is set to 20 Kelvin or more and less than 50 Kelvin (preferably around 30 Kelvin), and hydrogen clusters with diameters of 100 nm or more and less than 800 nm (preferably about 600 nm) are generated. Condition Bb: Focused light intensity 1×10 19 W / cm 2 That's it, 3 x 10 21 W / cm 2 Less than (preferably 1 × 10 20 W / cm 2 ) or so. Condition Bc: The laser focusing position is set near the center of the cluster gas (a position within 95% of the spreading radius from the center of the cluster gas, where the spreading radius is 100%).

[0025] (3) Condition C: No debris is generated The targets are micron-sized clusters, which are smaller than the focused spot size of the laser light 18. This makes it possible to turn the entire target into plasma and prevent the generation of debris.

[0026] (4) Condition D: Repeat condition Ions can be repeatedly generated by irradiating a pulsed laser of any frequency. In particular, by using clusters as targets, it is possible to use an electromagnetic valve that can perform repetition at a higher frequency than the desired repetition frequency (0.1 Hz in this embodiment, but not limited to this) (for example, a high-speed repetition electromagnetic valve that can handle 600 Hz repetition).

[0027] 2A to 2C show the results of a simulation calculated by a computer. In this simulation, we carried out a three-dimensional particle code simulation of the interaction between a high-intensity laser and a hydrogen cluster target. In this simulation, the focused laser intensity was set to 1.2×10 20 W / cm 2 and the laser 50 was irradiated onto the hydrogen cluster 32 having a diameter of 600 nm.

[0028] Figure 2A shows the spatial distribution of electrons immediately after passing through the laser, with the horizontal axis showing the distance in the direction of laser propagation and the vertical axis showing the distance perpendicular to the direction of laser propagation, and the color intensity showing the density of electrons. Near the center of the figure, 81, is the position of the hydrogen cluster before laser irradiation. As shown in this distribution, the electrons (black dots) are strongly deflected and accelerated in the direction of laser propagation due to the strong radiation pressure of the laser caused by the relativistic effect.

[0029] Figure 2B shows the spatial distribution of proton beam energy immediately after passing through the laser, with the horizontal axis showing the distance in the direction of laser travel and the vertical axis showing the distance perpendicular to the direction of laser travel, and the color intensity showing the magnitude of the energy of the hydrogen cluster (MeV). The area near the center of the figure 82 (the position of the hydrogen cluster before laser irradiation) is near 0 MeV, and the dark area on the right side is near 9 MeV. As shown in the figure, a part of the proton beam accelerated in the direction of laser travel is "torn off" by being dragged by the electrons, forming a "quasi-monochromatic proton beam bunch 83."

[0030] Figure 2C shows the proton beam energy spectrum in the direction of laser travel, with the horizontal axis representing the proton beam energy (MeV) and the vertical axis representing the number of protons in arbitrary units. Generally, in the absence of a mechanism for producing monochromatic or quasi-monochromatic beams, the energy spectrum slopes downward to the right, with the number of protons monotonically decreasing in accordance with the thermal energy distribution as the energy value increases from 0 MeV. In contrast, this simulation shows an energy spectrum with a steep peak at approximately 9 MeV, indicating that a quasi-monochromatic proton beam (relative energy spread of approximately ±20%) is being generated.

[0031] 3 is a diagram showing the proton beam energy spectrum resulting from an ion acceleration experiment. The horizontal axis indicates the proton beam energy (MeV), and the vertical axis indicates the number of protons in an arbitrary unit. In this experiment, J-KAREN-P from the Kansai Photon Science Institute was used as the laser irradiation device 11.

[0032] J-KAREN-P is capable of achieving an ultra-high intensity of 1,000 trillion watts at a repetition rate of 0.1 Hz by confining 30 J of laser energy for 30 femtoseconds (1 femto is one quadrillionth of a second).

[0033] In the experiment, the laser focusing intensity was about 1×10 20 W / cm 2 The temperature (target temperature) of the cluster gas generator 21 was set to 30 Kelvin, hydrogen clusters with a diameter of about 600 nm were generated, and the clusters were irradiated with a laser.

[0034] As a result, a quasi-monoenergetic proton beam (relative energy spread of about ±20%) with a peak at around 2 MeV was detected by the ion detector 5a installed in the laser propagation direction. Spectra 71 to 74 in FIG. 3 show the energy of the proton beam detected for each laser shot. It was confirmed that the detected quasi-monoenergetic proton beam (hydrogen ion beam) had a half-width of about 20% and was uniform in energy at several MeV.

[0035] Such a quasi-monoenergetic proton beam was not observed by the ion detector 5b installed at an angle α of 57°. This experimental result is considered to have been generated by the mechanism of quasi-monoenergetic proton beam generation shown in the above simulation results.

[0036] As described above, the laser ion accelerator 3 can generate a highly monochromatic ion beam. Specifically, a highly pure proton beam composed of 100% hydrogen can be generated. Therefore, the generated proton beam is a debris-free proton beam that is not mixed with other substances. By eliminating debris, it is possible to prevent debris from contaminating downstream devices such as the focusing device 13, which could lead to performance degradation or failure.

[0037] The resulting proton beam has an energy of 50 MeV or less, preferably 30 MeV or less, and more preferably 10 MeV or less, and is a highly monochromatic and debris-free proton ion beam, which can be used for various purposes such as resistance diagnosis of nuclear and space materials and neutron generation by nuclear fusion.

[0038] Moreover, a proton beam with uniform energy in the direction of laser propagation can be obtained. In addition, since the size of the target hydrogen clusters is on the order of micrometers and is much smaller than the focused spot size of the laser, the entire target can be converted into plasma, thereby suppressing the generation of debris.

[0039] The ion accelerator in this invention corresponds to the laser ion accelerator 3, and the laser corresponds to the laser light 18, but this invention is not limited to this embodiment and can be made into various other embodiments. Also, the specific configurations and the like given in the above-mentioned embodiment are merely examples, and can be appropriately changed according to the actual product.

[0040] For example, deuterium molecules can be used instead of hydrogen molecules to generate deuterium clusters and generate a deuteron beam. In this case, the laser ion accelerator 3 and the ion detector 5 can have the same configuration, and the cluster gas generator 21 in the laser ion accelerator 3 can be configured to generate deuterium clusters. In this case, too, the same action and effect as in the case of the hydrogen molecules described above can be obtained. [Industrial Applicability]

[0041] The present invention can be used in various industries, such as industries related to resistance diagnosis of atomic and space materials, neutron generation by nuclear fusion, etc. It can also be used in industries other than medical. [Explanation of symbols]

[0042] 3...Laser ion accelerator 11...Laser irradiation device 13...Light collecting device 18...Laser light 21...Cluster gas generator 30…Cluster gas

Claims

1. An ion acceleration method comprising the steps of: ejecting a cluster gas into a vacuum; irradiating the cluster gas with a laser; thereby converting the cluster gas into plasma; ionizing atoms contained in the cluster gas; and accelerating the atoms in the cluster gas in the direction of irradiation with the laser, The focusing position of the laser is set within the cluster gas; The laser has a focused spot size larger than the clusters in the cluster gas and smaller than the spreading range of the cluster gas; The clusters in the cluster gas are clusters of a single type of hydrogen molecule or deuterium molecule. Ion acceleration methods.

2. The focal position of the laser was set within a range of less than 100%, with the radius of spread from the center of the cluster gas being 100%. The method for accelerating ions according to claim 1.

3. The focused intensity of the laser is 1×10 １９ W / cm ２ That's it, 3 x 10 ２１ W / cm ２ is less than 3. The method for accelerating ions according to claim 2.

4. The temperature of the cluster gas generating device that generates the cluster gas is 20 Kelvin or more and 50 Kelvin or less. The method for accelerating ions according to claim 3.

5. The energy of the accelerated ions is 50 MeV or less.

5. The method for accelerating ions according to claim 4.

6. A cluster gas generator for generating cluster gas; A laser irradiation device that irradiates a laser; a focusing device for focusing the laser irradiated from the laser irradiating device and irradiating the clusters contained in the cluster gas generated from the cluster gas generating device; The clusters in the cluster gas are used as targets to be irradiated with the laser, The focusing position of the laser is set within the cluster gas; the focusing device is configured to make the focused spot size of the laser larger than the cluster and smaller than the spreading range of the cluster gas; The clusters in the cluster gas are clusters of a single type of hydrogen molecule or deuterium molecule. Ion accelerator.

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