Abstract:
A film target for laser-induced particle acceleration includes a first target layer on which a laser is incident; an intermediate layer located behind the first target layer along a propagating direction of the laser, and in which an intended ion beam is generated; and a second target layer located opposite to the first target layer with the intermediate layer interposed therebetween.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application Nos. 10-2013-0168182 filed on Dec. 31, 2013 and 10-2014-0063997, filed on May 27, 2014, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a film target for laser-induced particle acceleration and a method of manufacturing the same, and more particularly, to a film target in which an ion layer is inserted and a method of manufacturing the same. 
     2. Discussion of Related Art 
     Conventionally, research and development of laser-induced particle acceleration using an ultra-short intense laser have been actively conducted. 
     When the ultra-short intense laser is focused on a thin layer, high energy ion beams are generated. The biggest issue is improving the energy characteristic of the ion beam, that is, to increase energy and reduce the energy width in the laser accelerated ion beam generation techniques. 
     Currently, the most remarkable technique is to use a very thin film of tens of nanometers thickness. However, the technique for substantially reducing a preceding pulse of the laser is additionally required, and the energy characteristic of the ion beam resulting from a very thin film are also not continuously improved when the intensity of the laser is increased due to non-uniform transverse spatial distribution of the laser intensity. 
     SUMMARY 
     One aspect of the present invention is directed to a film target for laser-induced particle acceleration in which energy characteristic of an ion bean is improved, and a method of manufacturing the same. 
     Another aspect of the present invention is directed to a film target for laser-induced particle acceleration in which manufacturing thereof is easy and handling thereof is convenient, and a method of manufacturing the same. 
     A further aspect of the present invention is directed to a film target for laser-induced particle acceleration in which the energy characteristic of an accelerated ion beam may be adjusted, and a method of manufacturing the same. 
     One aspect of the present invention provides a film target  100  for laser-induced particle acceleration, including: a first target layer  300  on which a laser  200  is incident; an intermediate layer  400  located behind the first target layer  300  along a propagating direction of the laser  200 , and in which an intended ion beam  600  is generated; and a second target layer  500  located opposite to the first target layer  300  with the intermediate layer  400  interposed therebetween. 
     Another aspect of the present invention provides a method of manufacturing a film target for laser-induced particle acceleration, including: preparing a first target layer  300  on which a laser is incident; forming an intermediate layer  400  behind the first target layer  300  along a propagating direction of the laser to generate an intended ion beam; and forming a second target layer  500  opposite to the first target layer  300  with the intermediate layer  400  interposed therebetween. 
     Another aspect of the invention provides a film target for laser-induced ion beam acceleration, the film target comprising: a first metallic layer comprising a laser-incident surface to which a laser is to be incident; a second metallic layer; and an intermediate layer interposed between the first and second metallic layers, the intermediate layer being of a non-metallic material comprising one or more source elements for ion beams. 
     In the foregoing film target, the first metallic layer may have a thickness measured in a direction perpendicular to the laser-incident surface and greater than that of the second metallic layer. The intermediate layer may contact the first metallic layer on one side, the intermediate layer contacting the second metallic layer on the other side. The non-metallic material may comprise a plastic material. The non-metallic material may comprise either or both of hydrogen (H) and carbon (C). 
     Still another aspect of the invention provides a laser-induced particle acceleration device comprising: a laser source configured to generate a laser beam; and the foregoing film target arranged such that the laser beam is to be incident to the laser-incident surface of the first metallic layer. 
     A further aspect of the invention provides a method of manufacturing a film target for laser-induced ion beam acceleration, the method comprising: providing a first metallic layer; forming an intermediate layer of a non-metallic material over the first metallic layer, the non-metallic material comprising one or more source elements for ion beams; and forming a second metallic layer over the intermediate layer such that the intermediate layer is interposed between the first and second metallic layers, thereby making the foregoing film target. 
     In the foregoing method, the first metallic layer may have a thickness measured in a direction perpendicular to the laser-incident surface and greater than that of the second metallic layer. The intermediate layer may contact the first metallic layer on one side, the intermediate layer contacting the second metallic layer on the other side. The non-metallic material may comprise a plastic material. The non-metallic material may comprise either or both of hydrogen (H) and carbon (C). 
     A further aspect of the invention provides a method of manufacturing the foregoing film target, the method comprising: providing a metallic layer for the second metallic layer of the film target; forming an intermediate layer of a non-metallic material over the metallic layer, the non-metallic material comprising one or more source elements for ion beams; and forming another metallic layer for the first target layer of the film target over the intermediate layer such that the intermediate layer is interposed between the two metallic layers, thereby making the foregoing film target. 
     In the foregoing method, the first metallic layer may have a thickness measured in a direction perpendicular to the laser-incident surface and greater than that of the second metallic layer. The intermediate layer may contact the first metallic layer on one side, the intermediate layer contacting the second metallic layer on the other side. The non-metallic material may comprise a plastic material. The non-metallic material may comprise either or both of hydrogen (H) and carbon (C). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail example embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a view showing a double-layer target including a vacuum layer for laser-induced particle acceleration according to a conventional technique; 
         FIG. 2  is a view showing a film target for laser-induced particle acceleration according to an embodiment of the present invention; 
         FIG. 3  is a view showing a film target for laser-induced particle acceleration according to another embodiment of the present invention; 
         FIGS. 4A and 4B  are graphs showing a gradient in a spatial distribution of an electric field inside the film target and a phase distribution of an ion beam according to changes in a thickness T1 of a first target layer and a thickness T2 of a second target layer; 
         FIG. 5  is a graph showing an energy distribution of the ion beam according to the changes in the thickness T1 of the first target layer and the thickness T2 of the second target layer; 
         FIG. 6  is a graph showing an energy distribution of a proton beam when a single thin film is used; and 
         FIGS. 7A-7C  are schematic cross-sectional views showing a method of manufacturing the film target for laser-induced particle acceleration according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, example embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     In one example, a double-layer target includes a vacuum layer for laser-induced particle generation, as described in  FIG. 1 . A double-layer target  10  includes a first target layer  30  configured to react with a preceding pulse of a laser  20  and generate plasma. The double-layer target  10  further includes a second target layer  50  configured to generate an ion beam  60  by a main pulse of the laser  20 , and a vacuum layer  40  formed between the first target layer  30  and the second target layer  50  to prevent a shock wave by the plasma from being transferred to the second target layer  50 . 
     The double-layer target has an advantage in that the ion beam, of which the energy is high and at the same time the energy width is small, may be generated, even though a high quality laser, in which a preceding pulse required in the existing target is small, is not required. 
     Film Target for Laser-Induced Particle Acceleration 
       FIG. 2  is a view showing a film target for laser-induced particle acceleration according to an embodiment of the present invention. 
     As shown in  FIG. 2 , a film target  100  for laser-induced particle acceleration according to the embodiment of the present invention includes a first target layer  300  configured to react with a laser  200  and adjust energy characteristic of an accelerated ion beam  600 , a second target layer  500 , and an intermediate layer  400  located between the first target layer  300  and the second target layer  500 , and comprising an intended ion species. 
     The laser  200  radiates from left to right in  FIG. 2 , and is focused on a surface of the first target layer  300 . 
     The first target layer  300  and the second target layer  500  may include metal materials of several micrometers thickness, for example, Al, Cu, Ti, Ta, etc. Specifically, it advantageously serves to increase the energy of the ion beam  600  when a material having a higher atomic number is used for the first target layer  300  and the second target layer  500 . 
     The first target layer  300  and the second target layer  500  may be configured to have different thicknesses T1 and T2. Specifically, the thickness T1 of the first target layer  300  may be greater than the thickness T2 of the second target layer  500  in order that the ion beam  600  is smoothly accelerated to the right in  FIG. 2 . 
     The intermediate layer  400  may be a very thin layer of which a thickness T is tens of nanometers, and may be configured of an intended accelerated particle, for example, a material including H, C, etc. Specifically, the intermediate layer  400  may be configured of a plastic-based material. 
     As described above, the film target  100  for laser-induced particle acceleration according to the embodiment of the present invention is insensitive to the influence by the preceding pulse, since the thickness T1 of the first target layer is several micrometers or more, and at the same time the intended ion beam  600  is formed on a center of the entire film target  100 . Therefore, the additional technique for reducing the preceding pulse of the laser  200  is not required compared to the conventional technique. 
     Therefore, the film target for laser-induced particle acceleration according to the embodiment of the present invention may innovatively improve the energy characteristic of the ion beam among laser accelerated ion beam generation techniques. That is, the film target may adjust the energy characteristic of the ion beam and be insensitive to the preceding pulse of the laser as described above, and may be easy to manufacture, as described below. As these effects are difficult to implement by the conventional technique, utilization of the laser accelerated ion beam may be considerably increased in many areas of basic science, nuclear physics, health care, and the like, such as high-speed plasma diagnostics, nuclear reactions, cancer treatment, etc. 
       FIG. 3  is a view showing a film target for laser-induced particle acceleration according to another embodiment of the present invention.  FIGS. 4A and 4B  are graphs showing a gradient in a spatial distribution of an electric field inside the film target and a phase distribution of an ion beam according to changes of a thickness T1 of a first target layer and a thickness T2 of a second target layer, which are generated by focusing an ultra-intense laser pulse. Specifically,  FIG. 4A  is a graph showing a film target configured of copper (Cu) in which the thickness T1 of the first target layer is 1.6 μm, and the thickness T2 of the second target layer is 0.8 μm, and  FIG. 4B  is a graph showing a film target configured of copper (Cu) in which the thickness T1 of the first target layer is 2.0 μm, and the thickness T2 of the second target layer is 0.4 μm.  FIG. 5  is a graph showing an energy distribution of the ion beam according to the changes in the thickness T1 of the first target layer and the thickness T2 of the second target layer. Further,  FIG. 6  is a graph showing an energy distribution of a proton beam when a single thin film is used. 
     A film target for laser-induced particle acceleration according to another embodiment of the present invention is similar to the film target for laser-induced particle acceleration according to an embodiment of the present invention described above except for the thickness T1 of the first target layer and the thickness T2 of a second target layer. Thus, like reference numerals in the drawings denote like elements, and thus the description thereof will not be repeated. 
     As shown in  FIG. 3 , the film target for laser-induced particle acceleration according to another embodiment of the present invention includes a first target layer  300 , a second target layer  500 , and an intermediate layer  400 . 
     In embodiments of the present invention, energy characteristic of an accelerated ion beam  600  may be adjusted by controlling a ratio of a thickness T1 of the first target layer to a thickness T2 of the second target layer. 
     Referring to  FIG. 4A  and  FIG. 5 , in the case where the intermediate layer  400 , in which an intended ion beam  600  is generated, is closely located to a center of the first target layer  300  and the second target layer  500  (T1=1.6 μm, and T2=0.8 μm), when the ion beam  600  is accelerated by an electric field E formed inside a plasma by a laser, since a gradient in the spatial distribution of the electric field E is larger and an acceleration length is longer, an energy width of the ion beam  600  is greater, however, a maximum energy thereof is increased. As shown in  FIG. 4A , the gradient in the spatial distribution of the electric field E is larger in a center area of an X-axis, however, the gradient therein is reduced toward the right. Therefore, since an initial location of the ion beam  600  is concentrated in the middle of the X-axis, and the ion beam  600  is accelerated by an entire electric field E having a positive (+) value, the acceleration length is longer. 
     Referring to  FIG. 4B  and  FIG. 5 , and to the contrary of  FIG. 4A , since the gradient in the spatial distribution of the electric field E is smaller when the intermediate layer  400  is located close to a surface of the second target layer  500  (T1=2.0 μm, and T2=0.4 μm), the maximum energy of the ion beam  600  is smaller, however, the energy width thereof is reduced. As shown in  FIG. 4B , the gradient in the spatial distribution of the electric field E is similar to that of in  FIG. 4A . However,  FIG. 4B  shows that the initial location of the ion beam  600  is more biased to the right on the X-axis. Therefore, since the ion beam  600  is accelerated by a right part of the entire electric field E having a positive (+) value, the acceleration length is reduced. 
     Referring to  FIGS. 5 and 6 , when the intermediate layer  400  is located too close to the surface of the second target layer  500 , rather, the energy characteristic of the generated ion beam  600  becomes the energy characteristic of the ion beam generated in the existing thin film. 
     A Method of Manufacturing the Film Target for Laser-Induced Particle Acceleration 
       FIGS. 7A ,  7 B, and  7 C are schematic cross-sectional views of the process showing a method of manufacturing the film target for laser-induced particle acceleration according to the embodiment of the present invention, and show a process of manufacturing the film target for laser-induced particle acceleration shown in  FIGS. 2 and 3 . 
     As shown in  FIG. 7A , a first target layer  300  having a predetermined thickness T1 is prepared using an easily obtainable metal film. 
     As shown in  FIG. 7B , an intermediate layer  400  is formed by coating the first target layer  300  with a plastic-based material to have a thickness T of tens of nanometers. 
     As shown in  FIG. 7C , a second target layer  500  is formed by coating the intermediate layer  400 , which is formed on the first target layer  300 , to have a thickness T2 of several micrometers, again, using a metal film. 
     According to embodiments of the present invention as described above, there are the following advantages. 
     According to embodiments of the present invention, as different materials are used for the first target layer and the second target layer, the energy characteristic of the accelerated ion beam can be improved. 
     According to embodiments of the present invention, as the first target layer and the second target layer have different thicknesses from each other, the energy characteristic of the accelerated ion beam can be adjusted. 
     According to embodiments of the present invention, as the intended ion beam is generated between the first target layer and the second target layer, it is insensitive to an influence by a preceding pulse of the laser, and it does not require an additional technique for reducing the preceding pulse of the laser. 
     According to embodiments of the present invention, as easily obtainable materials are used, manufacturing thereof is easy. 
     Further, according to embodiments of the present invention, as a target is formed in a film shape, handling thereof is convenient. 
     While the present invention has been particularly described with reference to example embodiments, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention. Therefore, the example embodiments should be considered in a descriptive sense only and not for purposes of limitation. The scope of the invention is defined not by the detailed description of the invention but by the appended claims, and encompasses all modifications and equivalents that fall within the scope of the appended claims.