Abstract:
A device is provided for establishing the uniformity of wall thickness for a plurality of hollow spherical shells suspended in a liquid. Specifically, the device imposes a variable angular acceleration on each shell in order to establish a uniform wall thickness for each shell. The device includes a container for receiving the liquid and the suspended shells. Further, the device includes a motor for moving the liquid to impose a variable angular acceleration on each shell. Also, the device includes an element for polymerizing each shell after each shell&#39;s wall thickness has become substantially uniform.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains generally to devices and methods for forming hollow spherical shells for inertial fusion programs. More specifically, the present invention pertains to devices and methods for controlling the dimensions of the shells. The present invention is particularly, but not exclusively, useful as a device and method for controlling the uniformity of wall thickness for a plurality of hollow spherical shells formed with central cores. 
       BACKGROUND OF THE INVENTION 
       [0002]    Inertial fusion programs require spherical shell targets that have a precisely controlled wall thickness. Stated differently, there needs to be concentricity between the inner wall and outer wall of the shell. Heretofore, an approach for attaining the requisite wall thickness and concentricity has involved gently tumbling the shells in a fluid-filled container. While this approach has achieved modest success, it still does not achieve a uniformity for wall thickness that is always within the acceptable limits of ±1% variation. Specifically, the tumbling approach has resulted in a good sphericity for the shells, but there has been relatively poor uniformity in the wall thickness. For instance, typical variations in wall thickness are reported to be about ±5%, with values in the range of ±1% regime being rare. 
         [0003]    With a review of some basic physical notions, it can be appreciated that in order to produce polymerized divinyl benzene (DVB) shells having a uniform wall thickness, forces must somehow be applied on the shells that will achieve this result. Preferably, these forces will be applied on the shell prior to (and during) the polymerization process. Importantly, if the shells are caused to move in a controlled manner, force(s) will act on the shells that are proportional to the acceleration of particles in the shell, and in the direction of the force(s). Stated differently, forces can be exerted on the shells in accordance with Newton&#39;s Second Law, F=ma. 
         [0004]    With the above in mind, it is also to be appreciated that acceleration “a”is defined as the time rate of change of a particle&#39;s velocity “v” 
         [0000]    
       
         
           
             
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         [0000]    Further, in vector notation, velocity has both a magnitude (speed) and a direction. Thus, an acceleration will result whenever either the magnitude, or the direction of the velocity of a particle is changed. In accordance with Newton&#39;s Second Law, (F=ma), whenever a non-zero resultant force acts on a particle, the particle will be accelerated. It follows that a particle experiences the application of a force whenever it is being accelerated (+a) or decelerated (−a). 
         [0005]    In light of the above, it is an object of the present invention to provide a device and method for imposing forces (which, in turn, cause angular accelerations) on DVB shells to control the uniformity of their wall thickness. Another object of the invention is to provide a device and method for performing a large-scale production process which routinely provides a high yield of shells that exhibit wall thickness variations which are substantially less than +/−1%. Still another object of the invention is to provide a device and method for optimizing the motion of a shell-suspending fluid during the process of polymerizing the shells. Still another object of the invention is to provide a device and method for imposing a variable angular acceleration on each shell to establish a wall for each shell having a substantially uniform thickness. Yet another object of the present invention is to provide devices and methods for forming polymerized, hollow spherical shells that are easy to implement, simple to perform, and cost effective. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with the present invention, a device is provided that will control the uniformity of the wall thickness that is established for hollow spherical shells, as the shells are formed during a polymerization process. Specifically, during the polymerization process, the device imposes a variable angular acceleration on each shell to establish the shell wall with a substantially uniform thickness. As envisioned for the present invention, the hollow spherical shells will initially be suspended in a liquid and held in a reservoir before they are polymerized. The shells then remain suspended in the liquid as they are transferred from the reservoir to a container. Once the liquid and shells are in the container, the container is then moved by a motor to impose the variable angular acceleration on the shells that is required to achieve substantial uniform wall thickness. After each shell&#39;s wall is established with a substantially uniform thickness, the shell is polymerized. 
         [0007]    In one embodiment of the present invention, the container of the device is a tube that is formed with a lumen that defines a longitudinal axis. Preferably, the lumen has a radius that varies somewhat sinusoidally along its axial direction between a minimum radius of r 1  and a maximum radius of r 2 . Thus, at each location where the radius is r 1 , a bottleneck is formed in the tube. Between each pair of adjacent bottlenecks, there is a compartment. With this structure, a plurality of compartments are formed, and each compartment will have a substantially same predetermined volume. With this in mind, the device further comprises a pump or injector for sequentially introducing the liquid and suspended shells from the reservoir into the tube as a bolus having the same predetermined volume as each shell compartment. 
         [0008]    For the operation of this embodiment of the present invention, the pump first introduces the liquid with suspended shells into the tube. The motor then rotates the tube about the tube&#39;s longitudinal axis to impose a variable angular acceleration on the shells in the lumen. Specifically, the tube is rotated with an angular velocity that is varied in a range between Ω 1  and Ω 2 . Furthermore, Ω 1  will be greater than zero (0&lt;Ω 1 &lt;Ω 2 ). Thus, as the motor increases and decreases the angular velocity of the tube within the range between Ω 1  and Ω 2 , there are consequent changes in the direction of the angular acceleration “α” (i.e. ±α). For purposes of the present invention, these angular acceleration changes (±α) occur approximately every second or less. While the magnitude of the shells&#39; angular velocity is varied, the direction of the shells&#39; angular velocity is held constant. As a consequence, any forces arising from a density mismatch between the shells and their surrounding fluid are continuously changing in direction relative to each shell; the reason being that the effects of gravity on the shell are obviated and the core in the shell is centered. 
         [0009]    In another embodiment of the present invention, the container of the device is a vessel for holding the liquid with the shells in the liquid suspended. For this embodiment, the vessel has an open end and a closed end, and the vessel defines a vessel axis that extends between these two ends. Further, this embodiment includes an arm having first and second ends. One end of the arm is attached to a pivot point and the arm extends from this pivot point to a distal end. The arm also defines a pivot axis that passes through the pivot point. The vessel is then positioned on the distal end of the arm, with the vessel axis distanced from, and parallel to, the pivot axis. 
         [0010]    During operation of this embodiment of the present invention, a batch of the shells is delivered into the vessel, through its open end, with the shells suspended in the liquid. The vessel is then rotated about the vessel axis with an angular velocity (+Ω). At the same time, the arm is rotated about the arm&#39;s pivot axis in an opposite direction, but with a substantially same angular velocity (−Ω). As a result of the rotation of the vessel about the vessel axis, and the simultaneous rotation of the vessel axis about the pivot arm axis, the motor continuously varies the direction of motion of the suspending fluid relative to the shells. More importantly, velocity gradients within the fluid surrounding each shell impose a variable angular acceleration on the shells. Specifically, this variable angular acceleration results because, although the magnitude of the shells&#39; angular velocity is held constant, the direction of their angular velocity vectors is continuously changing. 
         [0011]    In each embodiment of the present invention, the variable acceleration is imposed on the shells to center each shell&#39;s core. The force responsible for centration arises as a consequence of the transfer of angular momentum from the outer surface of a shell to its inner surface. The result is to make each shell&#39;s wall thickness substantially uniform. Once the uniform wall thicknesses are established, the element for polymerizing each shell is activated to heat or otherwise fix each shell&#39;s wall. For the tube embodiment, the polymerizing action may be sequentially administered on shells at a downstream position in the tube. For the vessel embodiment, polymerization is typically accomplished as a batch process in which all shells in the vessel are polymerized at the same time, after uniform wall thicknesses are established. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0013]      FIG. 1  is a schematic drawing of an embodiment of a continuous process device for controlling the uniformity of wall thickness for hollow spherical shells in accordance with the present invention; 
           [0014]      FIG. 2  is a cross sectional view of the tube shown in  FIG. 1 , with the cross section passing through the axis of the tube; 
           [0015]      FIG. 3  is a schematic drawing of an embodiment of a batch process device for controlling the uniformity of wall thickness for hollow spherical shells in accordance with the present invention; and 
           [0016]      FIG. 4  is an overhead view of the device shown in  FIG. 3 , depicting sequential positions of the vessel in phantom. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring initially to  FIG. 1 , a device for controlling the uniformity of wall thickness for hollow spherical shells is shown, and is generally designated  10 . As shown in  FIG. 1 , the device  10  includes a reservoir  12  in which a liquid  14  is held. For purposes of the present invention, the hollow spherical shells  16  are suspended in the liquid  14  within the reservoir  12 . Preferably, the liquid  14  is viscous, non-miscible, and has a specific gravity which is substantially identical to that of the shells  16 . Further, the shells  16  are preferably formed from divinyl benzene (DVB). Referring briefly to  FIG. 2 , it can be seen that each shell  16  is substantially hollow and includes a core  18 . As a result, each shell  16  has an outer surface  20  and an inner surface  22  that defines each shell&#39;s thickness  24 . For purposes of the present invention, the shells  16  can be manufactured in any manner well known in the pertinent art, such as by an extruding process. 
         [0018]    As shown in  FIG. 1 , the device  10  further includes a pump or injection device  26  that is in fluid communication with the reservoir  12 . Further, the device  10  includes a tube  28  that is connected downstream of the reservoir  12  for receiving the liquid  14  and shells  16  from the pump  26 . For purposes of the present invention, the tube  28  is substantially cylindrical and defines a longitudinal axis  30 . As shown in  FIG. 2 , the tube  28  includes a lumen  32  that has a periodically (e.g. sinusoidally) varying radius  34 . Specifically, the radius  34  of the lumen  32  varies between a minimum radius of r 1  and a maximum radius of r 2 . As a result, the tube  28  forms a plurality of bottlenecks  36  at positions where the radius  34  is r 1 . As shown, each bottleneck  36  is distanced from the adjacent bottleneck  36  by a length, L. As further shown, each pair of adjacent bottlenecks  36  bounds a shell compartment  38  such that all shell compartments  38  have the substantially same volume. 
         [0019]    Referring back to  FIG. 1 , the device  10  also includes a motor  40  that is connected to the tube  28  via a drive belt or coupling  42 . As a result, the motor  40  is able to rotate the tube  28  about the tube&#39;s longitudinal axis  30  with an angular velocity Ω. As further shown, the device  10  includes a heater or other polymerizing unit  44  such as a polymerization agent injector. For purposes of the present invention, the polymerizing unit  44  is provided to solidify or set each shell  16  after a uniform wall thickness  24  has been attained. 
         [0020]    During operation of the device  10  shown in  FIGS. 1 and 2 , the shells  16  are first suspended in the liquid  14  and are stored in the reservoir  12 . Because the shells  16  and the liquid  14  have the same specific gravity, the shells  16  may be suspended in the liquid  14  by merely mixing the two components. Thereafter, the pump  26  sequentially injects volumes of liquid  14  with shells  16  into the tube  28 . Preferably, the pump  26  sequentially injects a bolus of the liquid  14  containing one shell  16 . As envisioned for the present invention, this bolus will have substantially the same volume as each shell compartment  38 . As a result, one shell  16  is positioned within each shell compartment  38  during operation of the device  10 . 
         [0021]    As the liquid  14  and shells  16  are received within the tube  28 , the motor  40  rotates the tube  28  about the tube&#39;s longitudinal axis  30 . Specifically, for this rotation, the motor  40  selectively varies the angular velocity of the tube  28  in a range between Ω 1  and Ω 2 . More specifically, the angular velocity Ω 1  is greater than zero, and the angular velocity Ω 2  is greater than Ω 1 . Thus, as the motor  40  changes the magnitude of the angular velocity of the tube  28  (i.e. the rotation velocity Ω of the tube  28  increases or decreases) the angular acceleration periodically reverses direction. Preferably, the cycle of “speed-up” and “slow-down” is done approximately once per second or faster. As the tube  28  is rotated, shear forces between the tube  28  and concentric layers of the liquid  14  are translated inward through the liquid  14  to the shells  16 . As a result of the variable angular acceleration imposed on the shells  16 , the shells  16  are centered on the longitudinal axis  30 , and the cores  18  are centered within the shells  16 . While the magnitude of the shells&#39; angular velocity is varied, the direction of the shells&#39; angular velocity is held constant to obviate the effects of gravity on the shell  16  as the core  18  in the shell  16  is centered. A further effect of varying the angular velocity of the tube is to cause each shell to become centered longitudinally within its shell compartment  38  during the time intervals between successive injections by the pump  26 . 
         [0022]    The process of intermittently centering the shells  16  on the longitudinal axis  30  and continuously centering the cores  18  within the shells  16  occurs as the shells  16  move downstream from shell compartment  38  to shell compartment  38 . Once the wall thickness  24  of a shell  16  is sufficiently uniform, the shell  16  is polymerized by the application of heat or a polymerization agent from the polymerizing unit  44 . Thereafter, the shell  16  is ejected from the tube  28 . 
         [0023]    Turning now to  FIG. 3 , another embodiment of the device is illustrated and is generally designated  10 ′. In this embodiment, the device  10 ′ includes a vessel  46  which holds a batch of the shells  16  suspended in the liquid  14 . As shown, the vessel  46  includes an open end  48  and a closed end  50  and defines a vessel axis  52  that extends between the ends  48 ,  50 . Further, the vessel  46  includes an interior wall  54  that extends between the open end  48  and a point  53  on the vessel axis  52  at the closed end  50  of the vessel  46 . Preferably, the interior wall  54  of the vessel  46  is defined by a decreasing radius of curvature in a direction from the point  53  at the closed end  50  to the open end  48 . 
         [0024]    Still referring to  FIG. 3 , the device  10 ′ further includes a pivot arm  56 . As shown, the pivot arm  56  extends from a proximal end  58  to a distal end  60 . The proximal end  58  defines a pivot axis  62  about which the arm  56  may be rotated. For purposes of the present invention, the vessel  46  is mounted at the distal end  60  of the pivot arm  56  for rotation about the vessel axis  52 . As shown, the pivot axis  62  is distanced from the vessel axis  52  and is substantially parallel to the vessel axis  52 . As a result of this arrangement, the vessel  46  may be rotated about the vessel axis  52  while, simultaneously, the vessel axis  52  is rotated about the pivot axis  62 . 
         [0025]    For purposes of the present invention, the device  10 ′ further includes a motor  64  for rotating the vessel  46  about the vessel axis  52 . The motor  64  can also be used for pivoting the arm  56  about the pivot axis  62  such that the vessel axis  52  rotates about the pivot axis  62 . In addition to the above disclosed structure, the device  10 ′ includes a polymerization unit  44  such as a heater or a polymerization agent injector to polymerize the shells  16  after their wall thickness has become substantially uniform. 
         [0026]    Referring now to  FIG. 4 , the operation of the device  10 ′ of  FIG. 3  is depicted. In  FIG. 4 , the shells  16  are suspended in the liquid  14  and held in the vessel  46 . The motor  64  is connected via a drive belt  65  to rotate the vessel  46  about the vessel axis  52  in a direction  66  (illustrated as being counterclockwise). This rotation is with a substantially constant angular velocity +Ω. At the same time, the motor  64  pivots the arm  56  such that the vessel axis  52  rotates about the arm&#39;s pivot axis  62  in an opposite direction  68  (illustrated as being clockwise). Importantly, this counter-rotation is done with a substantially constant angular velocity −Ω. As a result of the rotation of the vessel  46  about the vessel axis  52  (+Ω) and the counter-rotation of the vessel axis  52  about the pivot arm axis  62  (−Ω), the shells  16  are enveloped in a suspending liquid  14  wherein the liquid&#39;s angular velocity varies as a function of depth beneath its surface. This, in turn, causes each shell  16  to experience a constantly varying angular acceleration. Specifically, this varying angular acceleration results because, the magnitude of the shells&#39; angular velocity is held constant while the direction of their angular velocity is continually varied. As a result of the variable angular acceleration imposed on each shell  16 , the core  18  in each shell  16  experiences a force which drives the core  18  toward the center of the shell  16 , thereby creating a situation wherein the shell&#39;s wall thickness  24  becomes substantially uniform. Once each shell  16  attains a sufficiently uniform wall thickness  24 , the polymerizing unit  44  (shown in  FIG. 3 ) polymerizes the shells  16  through the application of heat or by adding a polymerizing agent to the liquid  14 . After the shells  16  are polymerized, they may be removed from the vessel  46  and the process repeated for another batch of shells  16 . 
         [0027]    While the particular Controlling Wall Thickness Uniformity in Divinyl Benzene Shells as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.