Patent Publication Number: US-8530852-B2

Title: Micro-cone targets for producing high energy and low divergence particle beams

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/284,736, filed Dec. 23, 2009, the disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     GOVERNMENT FUNDING 
     This invention was made with support under Grant Number DE-FC52-03NA00156, awarded by the U.S. Department of Energy; the United States federal government, therefore, has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates to targets and, more specifically, to micro-cone targets for producing high energy and low divergence particle beams. 
     BACKGROUND 
     Cone-shaped targets appeared in laser target interaction after a series of key steps in the pursuit of fusion. In 1963, applications of fusion were starting to be studied (Basov, N. G. et al, Laser Driven Thermonuclear Reactions, Vol. 2, pp. 1373-1379; Paris et al., Phys. Fluids 7, 981-987; Hora et al. (1970). Conference Digest, 6th Quantum Electronics Conference, Kyoto, pp. 10-11B). Each article herein is expressly incorporated by reference in its entirety. In 1972, the laser implosion concept to produce fusion was conceived, and inertial confinement fusion research was born (John Nuckols et al., Nature, 239, 139, 1972). Some decades later, the concept of fast ignition was introduced as well as the idea of a cone target for fast ignition to allow the laser beam to get far enough into the compressed plasma to produce a fast electron beam that would deliver the ignition spark at the right place (M. Tabak et al., Phys. Plasmal, 1626 (1994); R. Kodama et al., Nature 412, 798-802 (2001)). These concepts have since been expanded. While cone geometries show an increased efficiency (Z. L Chen et al., Phys Rev E 71, 036403 (2005), and shaped flat targets have the ability to shape proton beams (S. C. Wilks et al., Phys Plasma 8, 542 (2001), higher proton beam maximum energies and lower beam divergences are still desired for a variety of laser applications. 
     It would thus be desirable to provide a target of specified dimensions that can produce a proton beam of a higher maximum energy and a lower divergence than current targets and that can produce proton beams that are not limited by the characteristics of the laser. 
     SUMMARY 
     The present invention relates to micro-cone targets for producing high energy and low divergence particle beams. 
     The micro-cone targets are specifically dimensioned such that with specific interaction conditions they can produce and focus particle beams of higher maximum energy and lower angular divergence than, e.g., flat targets. This is particularly relevant to fast ignition, small compact particle beams, medical applications, focused ion and/or electron beam microscopes, and also demonstrates a potential to produce proton beams that are no longer limited by the characteristics of the laser. 
     In one embodiment, a micro-cone target is provided that includes a substantially cone-shaped body having an outer surface, an inner surface, a generally flat and round, open-ended base, and a tip defining an apex. The cone-shaped body tapers along its length from the generally flat and round, open-ended base to the tip defining the apex. In addition, the outer surface and the inner surface connect the base to the tip, and the tip curves inwardly to define an outer surface that is concave, which is bounded by a rim formed at a juncture where the outer surface meets the tip. 
     In another embodiment, a micro-cone target is provided that includes a substantially cone-shaped body including an outer surface, an inner surface, a generally flat and round, open-ended base, and a tip defining an apex. The cone-shaped body tapers along its length from the generally flat and round, open-ended base to the tip defining the apex. In addition, the outer surface and the inner surface connect the base to the tip, and the tip curves inwardly to define an inner surface that is convex and an outer surface that is concave, which is bounded by a rim formed at a juncture where the outer surface meets the tip. The target also is composed of a metal selected from aluminum, titanium, iron, cobalt, nickel, copper, zinc, molybdenum, silver, tantalum, tungsten, platinum, gold or any combination thereof. 
     In yet another embodiment, a method for producing a particle beam from a micro-cone target is provided, which includes projecting a laser through a generally flat and round, open-ended base and onto an inner surface of a substantially cone-shaped body of the micro-cone target. The cone-shaped body further includes an outer surface, and a tip defining an apex, and tapers along its length from the generally flat and round, open-ended base to the tip defining the apex. In addition, the outer surface and the inner surface connect the base to the tip, and the tip curves inwardly to define an outer surface that is concave, which is bounded by a rim formed at a juncture where the outer surface meets the tip. The method further includes emitting a particle beam from the laser from the tip defining the apex of the target. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention. This patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  is a perspective view of a micro-cone target in accordance with embodiments of the invention; 
         FIG. 2  is a cross-sectional view of  FIG. 1  taken along line  2 - 2 ; 
         FIG. 3A  is a cross-sectional view of a standard flat target illustrating a laser beam incident on the target and the resulting divergence of the particles; 
         FIG. 3B  is a cross-sectional view of the micro-cone of  FIG. 2  illustrating a laser beam incident on the micro-cone and the resulting divergence of the particles; 
         FIG. 4A  depicts a proton energy density, in color, for the micro-cone target of  FIG. 1  at 3.10 20  W/cm 2 ; 
         FIG. 4B  depicts a proton energy density, in color, for the flat target of  FIG. 3A  for the same laser intensity; 
         FIG. 5A  depicts an electron energy spectrum for the micro-cone target of  FIG. 1  and the standard flat target of  FIG. 3A  at 3.10 20  W/cm 2 ; 
         FIG. 5B  depicts a proton energy spectrum for the micro-cone target of  FIG. 1  and the standard flat target of  FIG. 3A  at 3.10 20  W/cm 2 ; 
         FIGS. 6A and 6B  depict two-dimensional maps of the divergence of the protons propagating in the same direction as the laser for respectively the micro-cone target of  FIG. 1  and the standard flat target of  FIG. 3A ; and 
         FIG. 7  is a graph illustrating the maximum proton energies for three laser intensities from both the micro-cone target of  FIG. 1  and the standard flat target of  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
       FIGS. 1 and 2  show a micro-cone target  10  in accordance with embodiments of the present invention. The micro-cone target  10  is specifically dimensioned to produce proton beams of a high maximum energy, as compared to maximum energies produced by flat targets discussed further below, and a low divergence, such as less than 25 degree full beam angle. The micro-cone target  10  includes a substantially cone-shaped body  12  that tapers smoothly from a generally flat and round, open-ended base  14  to a tip  16  defining an apex. The tip  16  curves inwardly to define an outer surface  18  that is concave and an inner surface  20  that is convex. The cone-shaped body  12  further includes an outer surface  23  and an inner surface  24 , which taper inwardly from and connect the base  14  to the tip  16 . A rim  26  is formed at the juncture where the outer surface  23  meets the tip  16  and further bounds the outer concave surface  18 . 
     With respect to dimensions, as best shown in  FIG. 2 , the length (L) of the target  10  is about 90 μm and its width (W) at the base  14  is about 90 μm. However, the length (L) of the target  10  can be from about 90 μm to about 1 mm and the width (W) of the target  10  at its base  14  can be from about 50 μm to about 500 μm. In one example, the full inside angle (Θ) of the cone-shaped body  12 , as shown, is desirably kept near 20 degrees. It should be understood, however, that the angle (Θ) can range from about 10 degrees to less than 90 degrees. In another example, the angle (Θ) can range from about 10 degrees to about 30 degrees. Keeping in mind the aforementioned parameters for the cone-shaped target  10  including the length (L), width (W), and the inside angle (Θ) of the tip  16  of the cone-shaped body  12 , a general formula for providing desirable dimensions for the target  10  may be presented as follows: length=((width/2)/sin(angle(Θ))). 
     With continuing reference to  FIG. 2 , the diameter (d) of the inner convex surface  20  is about 10 μm and the diameter (D) of the outer concave surface  18  of the tip  16  is about 30 μm. However, the diameter of the inner convex surface  20  can be from about 5 μm to about 50 μm, and the diameter of the outer convex surface  18  can be from about 20 μm to about 100 μm. The body  12  of the target  10  also is approximately 10 μm thick (t). In one example, the thickness can range from about 1 μm to 40 μm. In another example, the thickness can range from about 5 μm to 40 μm. A thicker body  12  generally provides greater outer dimensions for the cone-shaped target  10 . 
     In addition, the midpoint (M) of the curvature of the outer surface  18  and inner surface  20  is generally perpendicular to a central axis  22  of the target  10 . In terms of radius of curvature, the radius of curvature for the outer concave surface  18 , as shown, is about 82 μm with a 4 μm dip, or drop, in the outer concave surface. In one example, the radius of curvature for the outer surface  18  may be from about 15 μm to 100,000 μm. In another example, the radius of curvature can be from about 82 μm to about 100,000 μm. The radius of curvature of the inner surface  20  follows closely that of the outer surface  18 . In this example, the shape of the inner convex surface  20  is depicted as mirroring that of the outer concave surface  18 . 
     In terms of materials, the target  10  can be formed from a metal including aluminum, titanium, iron, cobalt, nickel, copper, zinc, molybdenum, silver, tantalum, tungsten, platinum, or gold, or any combination thereof, ceramic, plastic, glass, diamond, or any combination thereof, including in layers or in doping. In one example, the target  10  is composed of at least two metals, e.g., aluminum and copper. In another example, the outer surface  23  and outer concave surface  18  may be a different material than the inner surface  24  and inner convex surface  20 . For example, the outer surface  23  and outer concave surface  18  can be composed of aluminum and the inner surface  24  and inner convex surface  20  can be composed of copper. In general, with layering, the outside layer is of a lower Z, i.e., atomic number, than the inside layer. 
     The micro-cone target  10  may be formed or machined in any manner known to those skilled in the art. In one example, vapor deposition is employed to form the target  10 . For example, a wax or resin can be melted into a holder. The wax or resin then can be shaped with a tool having a specified shape of the target  10  to form a mold. In a vapor deposition chamber, a first material can be used to provide a layer, or coating, of a specified thickness on the inside of the mold to form the outer surface  23  and outer concave surface  18  of the target  10 . The first material can be a metal, e.g., aluminum and the thickness can be 1 micron or greater and can be dependent on the size of the mold, for example. A second material can be deposited in the vapor deposition chamber to provide another layer, or coating, of a specified thickness onto the first material to form the interior surface  24  and inner convex surface  20  of the target  10 . The second material can be a metal, e.g., copper. The thickness can be 0.5 micron or greater and can be dependent on the size of the mold, for example. The resulting target  10 , thus, is composed of a 1 micron (or thicker) layer of aluminum on its outside and a 0.5 micron (or thicker) layer of copper on its inside. The target  10  can then be cooled and the wax or resin can be released therefrom using an appropriate solvent, e.g., acetone. 
     With further reference now to  FIGS. 2 ,  3 A, and  3 B, because of the physics occurring in a cone, some criteria need to be met for the micro-cone target  10  to provide its full potential. For example, the cone-shaped target  10  needs to be accurately aligned such that the axis of a laser is co-linear with the central axis  22  of the target  22  as it enters through the base  14 . The laser then hits the inner surface  24  of the target  22  when its diameter is about 3 to 4 times the size of the inner convex surface  20  of the tip  16 . Under low pre-plasma conditions, the laser ‘sees’ a conical shape and then the target  10  micro-focuses the laser light at the tip  16 . At the same time, the laser interacts with the inner surface  24  of the target  10 , creates electrons, and guides them along to the tip  16  where the electron beam gets out. This increases dramatically the electron density in the tip  16 , enables a higher conversion efficiency of laser light into very energetic or hot electrons, and, thus, enhances both electrons and protons. Making use of the inner surface  24  of the target  10  by allowing the laser to spread on it reduces greatly the amount of pre-plasma filling the interior of the target, thus enabling the use of cone-shaped targets  10  for proton acceleration. 
     It is also understood here that the cone-shaped target  10 , not the laser, defines the beam diameter. For example, a smaller cone angle can produce more energetic electrons as compared to a larger angle or more open cone. In addition, the cone-shape provides an increased absorption of the laser light as compared to flat target  26  ( FIG. 3A ), as further discussed below, which makes them more efficient. Also, the multiple bounces of the laser onto the inner surface  24  of the target  10  before it reaches the tip  16  makes the tip  16  a laser imprint free area, enabling more uniform beams. The particle beam also has the potential to be smaller or bigger than the laser best focus by defining the size of the target  10 . 
     Concerning the curvature in the tip  16  of the target  10 , the curvature creates a modification of the divergence of the output particle beam and this can be adjusted by changing the amount of curvature. The net result is a beam with desirable characteristics for fast ignition, laser based accelerators, proton beams for proton radiography of plasmas, isochoric heating shocks, proton therapy, microbeam radiation therapy, positron emission tomography, focused ion beam milling machines, ion beam microscopes and dual beam electron/ion microscopes. For applications such as proton therapy, ion milling machines and microscopes, a micro magnetic device can separate the electron and or proton beam from the x-rays. 
     A 2-D collisionless Particle-In-Cell (PIC) was utilized for the cone-shaped target  10  so as to run simulations and assess the electromagnetic fields structures and proton beam characteristics in comparison with that of flat target  26 . Several intensities were run to span the range available to short pulse lasers. 
     For the simulations, the simulations box is 150 μm long to capture the emitted particles. The incident laser pulse has a 1 μm wavelength, a pulse duration of 40 fs and a transverse spot size of 21 μm FWHM at 3×10 18  W/cm 2  with a Gaussian temporal and transverse spatial profile. The pulse is injected to the left of a 120×150 μm box. The laser interacts with the target at normal incidence, with its polarization plane in the simulation plane. The peak of the pulse enters the box 40 fs after the beginning of the calculation. The initial target density is 40 times higher than the relativistic critical density, a 0 n c , where a 0  is the normalized laser amplitude and n c  is the critical density (n c =1.1×10 21 /λ, (μm) 2  cm −3 , λ is the laser wavelength). The plasma, composed of aluminum ions and electrons, is initially fully ionized. The mesh size is Δx=Δy=40 nm with 40 deuteron ions and 40 electrons per cell. The time step equals 0.132 fs. The preplasma used in the simulation fills the interior of the cone-shaped target  10  and has a density 1% to n c  over 50 microns with a characteristic length of 1 μm. 
       FIG. 4A  shows the 2D proton energy density for a 10 μm thick curved-tip  16  cone-shaped target  10  in a high intensity case at 3.10 20  W/cm 2 .  FIG. 4B  shows the same 2D proton energy density for a 10 μm flat target  26  in the same high intensity case. Based thereon, the protons are much more confined in the cone-shaped target  10  than in the flat target  26  where they tend to diffuse laterally. Indeed, the particles from the target  10  are much more collimated than for the flat target  26 , thus the density of particles on axis is higher. 
       FIGS. 5A and 5B  confirms that the cone-shaped target  10  is a more efficient structure.  FIG. 5A  shows the electron energy spectrum for the micro-cone target of  FIG. 1  and the standard flat target of  FIG. 3A  at 3.10 20  W/cm 2 ; and  FIG. 5B  shows the proton energy spectrum for the micro-cone target of  FIG. 1  and the standard flat target of  FIG. 3A  at 3.10 20  W/cm 2 . Both electrons ( FIG. 5A ) and protons ( FIG. 5B ) are accelerated to higher energies in a higher number for the cone-shaped target  10 . 
       FIGS. 6A and 6B  show the divergence of the proton beam from respectively the cone-shaped target  10  and the flat target  26  at t=924 fs. This clearly shows the ability to control the divergence. And in  FIG. 7 , a scan of different intensities representing the range of intensities available with short pulse lasers shows that a higher intensity (3.10 20  W/cm 2 ) enhances the increased maximum energy of the protons compared to lower intensities. 
     Accordingly, the micro-cone target  10  of the present invention produces proton beams of a desirable high maximum energy and controllable, desirably lower divergence. And such target  10  is not limited by the size or quality of the laser focal spot, the contrast of the laser pulse, or the f number of the focusing optic. Indeed, the target  10  defines the proton beams characteristics. 
     While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.