Patent Number: 
Section: description

Referring initially to FIG. 1, a separation device in accordance with the present invention is shown and generally designated 10. As shown, the separation device 10 includes a substantially cylindrical wall 12 that surrounds a chamber 14 and extends from a closed end 16 to an open end 17. As shown in FIG. 2, the cylindrical wall 12 defines a longitudinal axis 18. In the preferred embodiment of the present invention, the wall 12 is formed with a plurality of holes 20a-d to allow a working gas to be introduced into the chamber 14. Although four holes 20a-d are shown, it is to be appreciated that the size, shape and number of holes 20a-d shown for introducing a working gas into the chamber 14 is merely exemplary. Referring now with cross reference to FIGS. 1 and 2, it can be seen that coils 22a-d are mounted on the outside of the wall 12. For the present invention, a current source (not shown) can be used to pass an electrical current through the coils 22a-d to generate a magnetic field, B, in the chamber 14. For the present invention, a magnetic field that is oriented substantially parallel to the longitudinal axis 18, and is substantially uniform in strength throughout the chamber 14 is preferably established. For some applications, small magnetic mirrors (not shown) can be established near the ends 16, 17 of the wall 12 to axially confine the plasma within the chamber 14. Although four coils 22a-d are shown mounted on the outside of the wall 12 to generate a uniform magnetic field in the chamber 14, it is to be appreciated that these coils 22a-d are merely exemplary, and that the size, shape and number of coils 22a-d can be varied in accordance with the present invention. Furthermore, it is to be appreciated that other methods known in the pertinent art for establishing a uniform, axially aligned magnetic field in the chamber 14 can be used for the present invention. In accordance with the present invention, as shown in FIGS. 1 and 2, a plurality of ring electrodes 24a-c, are positioned in the chamber 14 near the closed end 16 of the wall 12. As shown, each ring electrode 24a-c is preferably positioned to be concentrically centered on the longitudinal axis 18. For the present invention, the ring electrodes 24a-c can be staggered axially to reduce sputtering of the electrodes 24a-c by the ion beam exiting the chamber 14 (note: axially staggered electrodes not shown). With this combination of structure, a voltage source (not shown) can be connected to the electrodes 24a-c to establish a radially oriented electric field, E, in the chamber 14. Referring still to FIG. 2, it can be seen that an elongated central electrode 28 is positioned in the chamber 14 and oriented substantially along the longitudinal axis 18. Preferably, as shown, the outer surface of the central electrode 28 is formed with a plurality of disk-shaped projections 30 that extend radially outward from the longitudinal axis 18. In accordance with the present invention, the projections 30 are provided to minimize loss of the central electrode 28 to the plasma in the chamber 14 due to sputtering of the central electrode 28. Although disk-shaped projections 30 are shown, it is to be appreciated that any surface feature known in the pertinent art, such as a beehive configuration (not shown), that effectively minimizes sputter loss can be used on the surface of the central electrode 28 in conjunction with the present invention. In some embodiments, a voltage source (not shown) can be connected to apply a voltage between the wall 12 and the central electrode 28 to establish part or all of the required radially oriented electric field in the chamber 14. Thus, a radially oriented electric field is established by either the ring electrodes 24a-c, the wall 12 and central electrode 28, or both. Importantly, the radially oriented electric field is directed inwardly from the wall 12 towards the central electrode 28, and accordingly, the central electrode 28 functions as a cathode while the wall 12 functions as an anode. As shown, the central electrode 28 is distanced from the wall 12 by a distance 29. As further shown in FIG. 2, the central electrode 28 is preferably formed with a gas-box 32 for the purpose of recycling working gas that has accumulated at the central electrode 28. As shown, channels 34 are formed in the central electrode 28 to allow working gas to pass into the gas-box 32 from the chamber 14. Once inside the gas-box 32, the working gas is able to travel in the direction of arrow 36 and into a duct 38 that is located outside of the wall 12. In the preferred embodiment of the present invention, the duct 38 is routed along the outside of the wall 12 to deliver working gas from the gas-box 32 to the holes 20a-d in the wall 12 for subsequent reintroduction into the chamber 14. Also shown, a control valve 40 is preferably installed along the duct 38 to selectively meter the working gas through the duct 38. Although only one duct 38 is shown to deliver working gas from the gas-box 32 to the holes 20a-d, it is to be appreciated that any number of ducts 38 can be provided to deliver working gas from the gas-box 32 to the holes 20a-d.  In the preferred embodiment of the present invention, the chemical mixture requiring separation is formed into tiles 42 and mounted on the inside of the wall 12 facing the chamber 14, as shown in FIGS. 2 and 3. In an alternative embodiment of the present invention (not shown), the wall 12 can be made of the chemical mixture requiring separation. In accordance with the present invention, the chemical mixture can be a mixture, aggregate or alloy of two or more constituents. Each constituent, in turn, can be a chemical element, isotope or chemical compound. One chemical mixture that is particularly applicable for the present invention is a metallic alloy of Zirconium and Hafnium. The operation of the present invention can best be appreciated with reference to FIGS. 2 and 3. Once the constituents of the chemical mixture are known, the working gas can be selected. Specifically, the chemical mixture will be separated into constituent(s) having relatively low mass to charge ratios in the plasma and constituent(s) having relatively high mass to charge ratios in the plasma. Preferably, the working gas is selected to have a mass to charge ratio in the plasma that is between the low mass to charge ratio constituent and the high mass to charge ratio constituent. Further, a noble element is preferably used as the working gas. For the case where the chemical mixture requiring separation is a metallic alloy of Zirconium and Hafnium, the working gas is preferably Xenon. To create a plasma in the chamber 14, the chamber 14 is first evacuated and then filled with the working gas using the holes 20a-d in the wall 12. Next, a plasma is created from the working gas in the chamber 14 by energizing the ring electrodes 24a-c. Upon obtaining a plasma in the chamber 14, the strengths of the electric and magnetic fields are adjusted to control the trajectories of the ions to effect separation of the chemical mixture. In one embodiment, the electric and magnetic fields are adjusted to cause the Larmor diameter of the working gas to be slightly smaller than the distance 29 between the central electrode 28 and the anode (i.e. the tiles 42). Generally, a relatively large electric field is required to initially create a plasma from the working gas and a smaller electric field necessary to establish the proper ion trajectories. Once the strengths of the electric and magnetic fields have been properly adjusted, molecules/atoms of the working gas that are ionized near the wall 12 are directed on trajectories (shown by exemplary arrow 44) toward the central electrode 28. Specifically, the strengths of the magnetic and electric fields are established such that the Larmor diameter of the working gas ions in the fields is somewhat smaller than the distance 29 between the wall 12 and the central electrode 28. Near the central electrode 28, a portion of the working gas ions that were directed toward the central electrode 28 (i.e. arrow 44) will undergo electron exchange reactions with neutrals atoms that are present there, creating fast neutrals that are directed on trajectories (shown by exemplary arrow 46) towards the tiles 42. Another portion of the working gas ions that were directed toward the central electrode 28 (i.e. arrow 44) will strike the central electrode 28, neutralize, and contribute to the neutral gas pressure in the gas-box 32. It is to be appreciated that the fast neutrals that are produced have sufficient energy to strike the chemical mixture near the wall 12 and sputter the chemical mixture into the chamber 14 where a plasma has been established from the working gas. In the plasma, the sputtered chemical mixture is dissociated into its constituents, and the constituents are ionized. Due to the strengths and orientations of the electric and magnetic fields in the chamber 14, ionized constituents having a relatively high mass to charge ratio are placed on trajectories that are directed towards the central electrode 28 (i.e. orbital trajectories of large radius, shown by exemplary arrow 48). Specifically, the strengths of the magnetic and electric fields are established such that the Larmor diameter of the high mass to charge ratio ions in the fields is larger than the distance 29 between the wall 12 and the central electrode 28. Upon striking the central electrode 28, these ions are captured. On the other hand, ions having a relatively low mass to charge ratio are placed on small radius, orbital trajectories (shown by exemplary arrow 50). Specifically, the strengths of the magnetic and electric fields are established such that the Larmor diameter of the small mass to charge ratio ions in the crossed electric and magnetic fields is smaller than the diameter of the chamber 14. As such, these ions are directed out of the chamber 14 through the open end 17. The result is an essentially pure ion beam containing almost exclusively ions of relatively low mass to charge ratio exiting from the open end 17 of the wall 12. During the operation of the present invention, neutrals of the working gas are accumulated near the central electrode 28 and depleted near the wall 12. Denoting the direct ion current to the cathode as I and the charge exchange flux as xcex930, energetic ions that reach the cathode due to direct loss (particle flow I/e) and energetic neutrals that are produced due to charge exchange of these ions with neutral gas near the cathode (xc2xdxcex930) both sputter the cathode and create a flow of neutrals from the cathode, n0cV0. Here n0c is neutral density of the sputtered atoms near the cathode and V0 is average velocity of the sputtered ions. It is further assumed that about one-half (xc2xd) of the charge exchange neutrals can reach the cathode and about one-half (xc2xd) can reach the anode. The dependence of this partition on attenuation of the beam has been neglected. The ratio xcex2 can be defined as: Ratio xcex2=xcex930/(I/e). The flux of fast neutrals that end up on the cathode, are neutralized, and contribute to the pressure in the gas-box 32, can be expressed as a function of ion current: Gc=(I/e)(1+xcex2/2). The flux of fast neutrals in the direction of the anode is: Ga=(I/e)xcex2/2  which is smaller than the gas flow from the anode: n0a less than V0 greater than S=(I/e)(1+xcex2). Here n0a is the neutral density of the sputtered atoms near the anode. Accordingly, to sustain the discharge, a bypass for the gas flow can be provided. Assuming Knudsen flow in the bypass duct 38: Ga=Sduct(D/L) less than V0 greater than (n0cxe2x88x92n0a). Here S=xcfx80D2/4, D is duct 38 diameter and L is duct 38 length. As indicated above, the gas conductance in the duct 38 can be controlled by a valve, and, hence, the ratio n0a/n0c can be varied if necessary. Table 1 below provides exemplary parameters for a small-scale device to separate a Zirconium-Hafnium alloy. While the particular methods and devices as herein shown and disclosed in detail are fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that they are 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.