Patent Application
20120132827
The present invention relates to an ion acceleration method and an ion acceleration apparatus for accelerating a desired ion to high energy for output. The present invention also relates to the structures of an ion beam irradiation apparatus and an ion beam irradiation apparatus for medical use using the same.
Various types of technologies have been known that irradiate a sample with an ion beam consisting of accelerated ions (including protons) for machining, film deposition, analysis, medical practice, etc. Such technologies need a high-energy, high-intensity ion beam stably generated. An apparatus that generates a high-energy ion beam for irradiation typically needs a lot of equipment for the mechanism of accelerating ions to high energy in particular, which makes the entire apparatus large in size. Despite their obvious effectiveness particularly in medical applications and the like, such ion beam irradiation apparatuses are far from being widespread enough.
Under the circumstances, ion beam irradiation apparatuses capable of miniaturization have been known, among which is one using a laser-driven acceleration mechanism. For example, as described in Patent Documents 1 and 2, a laser-driven ion beam irradiation apparatus irradiates a target that can produce a lot of protons or desired ions by high-intensity ultrashort pulsed laser light, thereby vaporizing the target into a plasma. In the plasma, electrons having light mass are initially accelerated to high energy. The accelerated electrons create an electric field, which in turn accelerates heavier protons or ions. The protons or ions form a high-energy beam to irradiate the sample. An electric field for the acceleration in a conventional accelerator has a low upper limit due to restrictions such as the breakdown voltage of material. In contrast, the electric field for the acceleration in the plasma is several orders of magnitude higher and is thus capable of acceleration to high energy in a short distance. As compared to large-sized accelerators and the like that have heretofore been used, the laser-driven ion beam irradiation apparatus can be significantly reduced in overall size. Applications to medical use and various other fields have thus been expected.
For example, in medical applications, an affected area lying in a certain position and a certain depth needs to be exclusively and intensively irradiated with high-energy ions. For that purpose, it is needed to obtain a monochromatic (delta-functioned energy spectrum) high-energy ion beam with high directivity. Efforts are being made to improve the characteristics of a laser-driven ion beam irradiation apparatus so as to reach or exceed those of conventional large-sized accelerators.
Non-Patent Document 1 describes an effective technology which uses a cluster-gas, not an ordinary gas or solid, as the target that serves as a plasma source when irradiated with laser. A cluster-gas is a gas in which particulate aggregates, or clusters, of atoms or molecules suspend. A cluster-gas has properties intermediate between an ordinary gas and solid. The cluster-gas used in Non-Patent Document 1 contains CO2 clusters suspending in He gas. It is shown that high-energy helium (He), carbon (C), and oxygen (O) ions are obtained. The cluster-gas is formed by issuing a jet of the mixed gas from a nozzle into a vacuum for adiabatic expansion.
Such an ion acceleration apparatus (ion beam irradiation apparatus) can provide a high-intensity ion beam with high directivity.
Unlike ordinary gases and solids, a cluster-gas is formed in a temporally and spatially limited region. It has thus been difficult to stably obtain a high-energy ion beam by using a cluster-gas target.
In other words, it has been difficult to provide a high-energy ion beam with high directivity by using a laser-driven acceleration mechanism.
The present invention has been achieved in view of such a problem, and it is an object thereof to provide an invention that solves the foregoing problem.
To solve the foregoing problem, the present invention provides the following configurations.
An ion acceleration method according to the present invention is an ion acceleration method that includes irradiating a cluster-gas with pulsed laser light in a direction generally perpendicular to a jetting direction of a mixed gas to generate a plasma of the cluster-gas so that an atom constituting the cluster-gas is ionized and accelerated, the cluster-gas being formed by jetting the mixed gas including a first component gas and a second component gas from a nozzle into a vacuum to form from the nozzle a columnar shaped cluster-gas in which clusters of molecules of the second component gas suspend in the first component gas. The clusters in the cluster-gas have a density in the range of 2.0×108 to 2.0×1010 cm−3. The pulsed laser light is focused on a position of 80% to 100% rearward of the columnar shaped cluster-gas when seen from an irradiation side.
In the ion acceleration method according to the present invention, the columnar shaped cluster-gas has a transmittance in the range of 5% to 10% to the pulsed laser light.
In the ion acceleration method according to the present invention, a duration of jetting of the mixed gas is 0.01 to 10 ms. The columnar shaped cluster-gas is irradiated with the pulsed laser light in the range of 10% to 20% the duration of jetting since generation of the cluster-gas in terms of generation timing of the cluster-gas that is formed in response to the duration of jetting.
In the ion acceleration method according to the present invention, the first component gas is He, and the second component gas is CO2.
An ion acceleration apparatus according to the present invention is an ion acceleration apparatus for irradiating a cluster-gas with pulsed laser light to generate a plasma of the cluster-gas so that an atom constituting the cluster-gas is ionized and accelerated, the ion acceleration apparatus comprising: a nozzle that jets a mixed gas of a first component gas and a second component gas into a vacuum to form a columnar shaped cluster-gas in which clusters of molecules of the second component gas suspend in the first component gas; a laser light source that emits the pulsed laser light; and a focusing optical system that irradiates the cluster-gas with the pulsed laser light so that the pulsed laser light is focused on a preset focal point. The clusters in the cluster-gas have a density in the range of 2.0×108 to 2.0×1010 cm−3. The focal point is located in a position of 80% to 100% rearward of the columnar shaped cluster-gas when seen from an irradiation side.
In the ion acceleration apparatus according to the present invention, the columnar shaped cluster-gas has a transmittance in the range of 5% to 10% to the pulsed laser light.
In the ion acceleration apparatus according to the present invention, a duration of jetting of the mixed gas is 0.01 to 10 ms. The columnar shaped cluster-gas is irradiated with the pulsed laser light in the range of 10% to 20% the duration of jetting since generation of the cluster-gas in terms of generation timing of the cluster-gas that is formed in response to the duration of jetting.
In the ion acceleration apparatus according to the present invention, the first component gas is He, and the second component gas is CO2.
An ion beam irradiation apparatus according to the present invention includes a configuration that irradiates a sample with an ion accelerated by the ion acceleration apparatus.
An ion beam irradiation apparatus for medical use according to the present invention includes a configuration that irradiates an affected area with an ion accelerated by the ion acceleration apparatus.
With such configurations, the present invention can provide a high-energy ion beam with high directivity by using a laser-driven acceleration mechanism.
An ion acceleration apparatus according to an embodiment of the present invention will be described below.
Laser light (pulsed laser light) 20 is emitted from a laser light source and focused inside a cluster-gas (target) 30. The laser light source may be one for emitting ultrashort pulsed laser light of high intensity that can generate a plasma of the cluster-gas 30 when focused by a focusing optical system 21. The same holds for the configurations described in Patent Documents 1 and 2 and Non-Patent Document 1. Specifically, a glass laser, titanium-sapphire laser, or the like may be used as the laser light source. The focusing optical system 21 may include an aspheric focusing mirror such as an off-axis parabolic mirror. The position of the focal point to be set by the focusing optical system 21 will be described later. The laser light 20 is emitted in a pulse form at a short interval. The irradiation (oscillation) timing is controlled in synchronization with the generation of the cluster-gas 30.
A nozzle 40 is installed in a vacuum. The nozzle 40 is configured so that a jet of gas can be jetted from its top into the vacuum. The gas is a mixed gas of helium (first component gas: He) and carbon dioxide (second component gas: CO2). The gas jetted into the vacuum undergoes adiabatic expansion with a steep cooling, which solidifies CO2. Therefore, a columnar shaped cluster-gas 30 in which CO2 clusters are dispersed in He gas is generated. The space for the gas to be jetted into is evacuated by a vacuum pump (not shown) so that the degree of vacuum, even with the jetting of the gas, is maintained to allow stable generation of the cluster gas. The same holds for Non-Patent Document 1. The gas is not continuously jetted but in a pulsed fashion. The timing of jetting and the irradiation timing of the laser light 20 are therefore controlled in synchronization with each other. As shown to the right in
As shown to the right in
The generation [(b) of
The nozzle 40 is typically turned ON for a duration of approximately 0.01 to 10 ms. The ON and OFF are controlled by an external signal. The cluster-gas 30 is controlled ON so as to be in synchronization with [(c) of
t2) and the output of the laser light 20 (time t3 and t4) are synchronously controlled with consideration given to the foregoing delay time. As described in Non-Patent Document 1, the output of the laser light 20 includes a main pulse which has high intensity and a pre-pulse which has lower intensity than the main pulse and precedes the main pulse. A difference in time between the pre-pulse and the main pulse (difference in time between t4 and t3) is set to approximately 1 to 1000 ps (for example, approximately 150 ps). Both the pre-pulse and the main pulse have a half-width of approximately 3 to 1000 fs (for example, 40 fs). The durations on the timing of the laser light 20 are negligible as compared to the ON duration of the nozzle 40.
As described in Non-Patent Document 1, such a configuration can decompose both He atoms 31 and CO2 clusters 32 in the cluster-gas 30 into a plasma to generate and accelerate electrons. The accelerated electrons create an electromagnetic field structure in the plasma, thereby forming an electric field of high intensity for ion acceleration. As shown in
The inventor has experimentally analyzed the situation where the cluster-gas 30 is irradiated with the laser light 20, and found that the position of the light focal point in particular can be optimized to increase the energy of the output ions. In the cluster-gas 30, the state of plasma formation, the state of electron acceleration, and the state of ion acceleration inside vary as the position of the light focal point changes in the traveling direction of the laser light 20. Consequently, the energy distribution of accelerated ions varies depending on the position of the light focal point.
Assuming that D is the density (cm−3) of CO2 clusters in the cluster gas 30, and r is the radius (cm) of the CO2 clusters, the relationship between the CO2 cluster density D and the CO2 cluster radius r is given by the following equation:
r=(3ρ/4πDS)1/3, (1)
where ρ is the density (cm−3) of CO2 molecules in the cluster-gas 30, and S is the CO2 density (cm−3) in solid CO2.
The values of ρ and S are discussed in Non-Patent Document 1 and A. S. Boldarev, V. A. Gasilov, A. Ya. Faenov, Y. Fukuda, and K. Yamakawa, “Gas-Cluster Targets for FemtosecondLaser Interaction: Modeling and Optimization,” Review of Scientific Instruments, Vol. 77, p. 083112 (2006). For example, with a 60-atm mixed gas (90% of He, 10% of CO2), ρ=1.8×1018 cm−3 and S=2.1×1022 cm−3. From equation (1), r=(2×10−5/D)1/3.
In Non-Patent Document 1, the ion beam generated is subjected to a solid state nuclear track detector (CR-39) for two-dimensional imaging and evaluation of the ion track. The energy distribution of accelerated ions depends on the position of the light focal point. Since it is difficult by using CR-39 to precisely evaluate an optimum position for the generation of high-energy ions, measurements to be described below were examined to determine a condition that can increase ion energy with higher efficiency. Here, a cluster-gas 30 of D=3.0×109 cm−3 (r=0.2 μm) was irradiated with the laser light 20, and the emitted X-rays were measured for intensity while changing the light focal point in the direction of the optical axis of the laser light 20. The X-rays measured had energy of 665.7 eV and 653.7 eV, which correspond to the helium-beta (Heβ) line of a hexavalent oxygen ion (O6+) and the Lyman-alpha (Lyα) line of a heptavalent oxygen ion (O7+), respectively. The intensity ratio between the X-ray line emissions corresponds to the density of plasma generated. Note that such X-rays are characteristic X-rays generated by random collision excitation of high-energy electrons, with no directivity in the generation of X-rays. The X-rays detected here derived from the entire cluster-gas 30.
The probability of emission of high-energy electrons (electrons having energy of 5 MeV and higher) on each single irradiation of the laser light 20 was also determined. The cluster-gas 30 was also optically observed for fine structures without the irradiation of probe light. The result of the optical observation showed the occurrence of pores (bubbles). The probability of occurrence of bubbles was also measured.
High-energy electrons occur with particularly high probabilities in 30% to 80% positions of the orifice from the front side. The probability is near zero at locations off the orifice. Note that the probability distribution is not symmetrical with respect to the center of the orifice, but slightly shifted to the rear.
No bubble structure is observed on the front side of a 10% position of the orifice when seen from the front side. On the other hand, bubble structures are observed even at a location of 100% or so behind the 100% position of the orifice.
With the foregoing configuration, electrons which have light mass are initially accelerated into high-energy electrons. This creates an electromagnetic field structure in the plasma to produce a high electric field (sharp changes in potential). Ions, heavier than electrons, are then accelerated into high-energy ions by the electric field. Bubble structures accommodate a high electric field with no ion-scatterer. If a lot of bubble structures are formed near the end of the region where high-energy electrons are generated, such a location is particularly advantageous for the stable generation of high-energy ions. It is also evident that there occurs a plasma which generates ions to be accelerated.
In view of the foregoing, it can be clearly seen the light focal point is preferably located in the rear part of the cluster gas 30. In particular, the result of
The occurrence of a high accelerating electric field in the direction of irradiation of the laser light 20 also means improved ion directivity. In other words, setting the light focal point to the foregoing position can also provide high directivity.
If the light focal point is set to a 95% position in range, the cluster gas 30 showed a transmittance of 7% to the laser light 20. The transmittance is equivalent to the proportion of the laser light 20 simply transmitted through the cluster gas 30 without being absorbed. Part of the absorbed energy changes into ion energy. Low absorptance (high transmittance) is thus unfavorable because it lowers the energy of electrons and ions.
For efficient ion acceleration, it is important to adjust the position where the magnetic vortex is formed. The light focal point needs to be set so that the magnetic vortex occurs in such a position with respect to a given electron density distribution. Simulations showed that it is effective in view of ion acceleration to locate the magnetic vortex to a position approximately 95% from the irradiation side of the laser light 20. Shifting the position to the front decreases the transmittance of the cluster gas 30 to the laser light 20. Shifting the position to the rear increases the transmittance. The foregoing position of approximately 95% provides a transmittance of 5% to 10%. Such a relationship between the location of the magnetic vortex and the transmittance holds independent of the gas type.
The foregoing discussion has dealt with spatial limitations on the irradiation of the laser light 20. Meanwhile, as shown in
As shown in
If the ON duration is long, the vacuum deteriorates depending on such factors as the configuration of the vacuum chamber and the evacuation speed of the vacuum pump in use. It is evident that the background vacuum deteriorates in the late stage of the period when the nozzle 40 is ON. In other words, the stable formation of the cluster-gas 30 would actually take place in the early stage of the period when the nozzle 40 is ON. For example, it is preferred to perform the irradiation of the laser light 20 in the initial 10% to 20% or so of the period between when the nozzle 40 is turned ON and when the nozzle 40 is turned OFF.
While the foregoing example has dealt with the case where D=3.0×109 cm−3, the density D may be in the range of 2.0×108 to 2.0×1010 cm−3.
The foregoing configuration has dealt with the case of using a cluster-gas that is made of a mixed gas of He and CO2 and in which CO2 clusters are formed. However, a mixed gas of other compositions may be used to form other types of clusters.
The ion acceleration apparatus described above can provide a high-quality ion beam by controlling such factors as the focal position of the laser light (focusing optical system) and the open and close timing of the nozzle that issues a jet of mixed gas. The ion acceleration apparatus can thus be constituted without much change to a conventional laser-driven ion acceleration apparatus. This enables miniaturization of the entire apparatus as compared to acceleration apparatuses of other types, and allows applications to medical use and various other fields.
Ions accelerated by the foregoing ion acceleration apparatus may be used to irradiate samples. Such a configuration can construct an ion beam irradiation apparatus that performs irradiation with various types of ions as an ion beam. Conventional ion beam irradiation apparatuses have been difficult to miniaturize as a whole since their ion-accelerating mechanisms using a cyclotron, a radio frequency cavity, and the like are large in size. In contrast, the ion beam irradiation apparatus can be reduced in overall size since the accelerating mechanism can be miniaturized as described above. The ion beam irradiation apparatus is therefore easy to introduce into various facilities such as medical facilities, and can be particularly suitably used as a medial ion beam irradiation apparatus.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-264833 | Nov 2010 | JP | national |
