Patent Publication Number: US-6217776-B1

Title: Centrifugal filter for multi-species plasma

Description:
This application is a continuation-in-part of application Ser. No. 09/192,945, filed Nov. 16, 1998, now U.S. Pat. No. 6,096,220. The contents of Application Serial No. 09/192,945 now U.S. Pat. No. 6,096,220 are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to devices and methods for separating high-mass particles from low-mass particles in a multi-species plasma. More particularly, the present invention pertains to devices and methods for generating a magnetic field which, when crossed with a radially directed electric field, will improve the efficacy of the crossed fields for separating particles in a multi-species plasma and allow for a greater throughput. The present invention is particularly, but not exclusively, useful for a plasma mass filter which confines low-mass particles, but not high-mass particles, to orbits within a definable plasma passageway. 
     BACKGROUND OF THE INVENTION 
     In accordance with well known physical principles, whenever a charged particle is placed in an environment wherein a magnetic field is crossed with an electric field (i.e. the magnetic field is perpendicular to the electric field), the charged particle will be forced to move in a direction that is perpendicular to the plane of the crossed fields. For configurations wherein the electric field is radially oriented perpendicular to a central axis, and the magnetic field is oriented parallel to the central axis, the charged particle will be forced to move along circular paths around the central axis. This circular motion, however, generates centrifugal forces on the charged particle that will cause the particle to also move outwardly and away from the central axis. 
     In addition to the phenomenon described above, it is also known that charged particles will tend to travel through a magnetic field in a direction that is generally parallel to the magnetic flux lines. Thus, for the situation described above wherein the magnetic flux lines are oriented substantially parallel to a central axis of rotation, the magnetic flux lines will generally oppose the centrifugal force that is exerted on a charged particle as the particle rotates about the axis of rotation. It happens, however, that this opposing force is generally proportional to the magnitude of the magnetic field, with a lower magnitude magnetic field giving less opposition to the movement of the particle than a higher magnitude magnetic field. 
     Because the magnitude of a centrifugal force acting on a charged particle is a function of the mass of the particle, it follows that, for a given condition (i.e. for given crossed electric and magnetic fields), high-mass particles will experience higher centrifugal forces than will low-mass particles. Indeed, plasma centrifuges which are used for the purpose of separating charged particles from each other according to their respective masses (e.g. multi-species plasmas) rely on this fact. Centrifuges, however, also rely on a condition wherein the density of the plasma in the centrifuge chamber is above its so-called “collisional density” and on the fact that the electric field is directed away from the axis of rotation. In comparison with a plasma centrifuge, for a condition wherein the density of the plasma is maintained below the “collisional density” and wherein the electric field is directed toward the axis of rotation, a much different result is obtained. 
     It can be mathematically shown that when using a cylindrical shaped chamber which has a wall that is located at a distance “a” from the central longitudinal axis of the chamber; with a magnetic field, B z , oriented in a direction substantially parallel to the longitudinal axis of the chamber; and with an electric field established with a positive potential “V ctr ” on the longitudinal axis and a substantially zero potential on the wall, where “e” is the electric charge on the ion, an expression pertains wherein: M c =ea 2 (B z ) 2 /8V ctr . In this expression, M c  is an effective cut-off mass which differentiates between high-mass particles and low-mass particles. For environments inside a plasma chamber wherein the mass of a multi-species plasma is maintained below its “collisional density,” M c  can be established such that the high-mass particles in a multi-species plasma (i.e. those particles which have a mass greater than the cut-off mass) will be ejected into the wall of the chamber as the plasma transits the chamber. Low-mass particles, on the other hand, will not be ejected during their transit of the chamber. 
     Recall that the movement of charged particles in a direction which is across or perpendicular to the magnetic flux lines will be generally opposed by the magnetic field. Further, this opposition will be generally proportion to the magnitude of the magnetic field. Like other magnetic field environments, this opposition also pertains to the specific situation for a plasma rotating around an axis and in an environment wherein the electric field is directed to extract ions resulting in a cut-off mass of M c =ea 2 (B z ) 2 /8V ctr . Thus, by decreasing the magnitude of the magnetic field near the central axis of rotation in a cylindrical shaped plasma chamber, there will be decreased resistance to the outwardly radial movement of rotating charged particles away from the central axis. At the same time, because low-mass charged particles will experience lower centrifugal forces than will the high-mass particles, the low-mass particles will react more slowly and, therefore, will be more likely to remain nearer the central axis. Consequently, these trends will facilitate the movement of high-mass charged particles away from the central axis and into the region of the plasma chamber where the expression, M c =ea 2 (B z ) 2 /8V ctr  becomes more effectively operable. Importantly, with an increased efficacy in the separating of particles, there is also the ability to increase throughput. 
     In light of the above, it is an object of the present invention to provide a centrifugal mass filter which, for given crossed magnetic and electric fields, will facilitate the movement of both high-mass and low-mass charged particles into a region where they can be effectively separated from each other. It is another object of the present invention to provide a centrifugal mass filter which more predictably confines low-mass particles in the chamber, and more predictably ejects high-mass particles from the chamber, during their respective transit through the chamber. Yet another object of the present invention is to provide a centrifugal mass filter which will effectively process increased throughput. It is another object of the present invention to provide a centrifugal mass filter which is relatively easy to manufacture, is easy to operate and is comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     A centrifugal filter for separating low-mass particles from high-mass particles in a rotating multi-species plasma includes a first annular shaped conductor and a second annular shaped conductor. For the present invention, both of these annular shaped conductors are aligned and oriented along a central longitudinal axis in a coaxial configuration. Thus, they are also coaxially oriented relative to each other. A substantially cylindrical shaped container is also aligned along the central axis with the wall of the container positioned between the conductors. Specifically, one of the annular shaped conductors (the outer conductor) is mounted on the outer surface of the container wall, while the other conductor (the inner conductor) is positioned around and adjacent to the central axis. More specifically, the inner conductor is distanced from the inner surface of the container wall and is located in a plasma passageway that is established between the inner surface of the container wall and the central axis. The portion of this plasma passageway that is located between the inner conductor and the central axis is used to receive a multi-species plasma into the container and is hereinafter referred to as a central passageway. 
     As intended for the present invention, the outer annular shaped conductor and the inner annular shaped conductor respectively generate magnetic field components, B z1  and B z2 . Specifically, these components are generated such that B z1  and B z2  are additive. In the plasma passageway between the inner surface of the container wall and the inner conductor, the magnitude of the magnetic field is at its maximum and is such that B z1 +B z2 =B z . On the other hand, the magnetic field components B z1  and B z2  oppose each other in the central passageway between the inner conductor and the central axis. In the central passageway the magnetic field components B z1  and B z2  such that B z1 +B z2 ≅0 along the central longitudinal axis or is, at least, minimal. The result is an increased magnetic field in the plasma passageway between the inner surface of the container wall and the inner conductor and a decreased magnetic field in the central passageway. As intended for the present invention, this configuration for the magnetic field creates a condition in which the efficacy of the filter is improved by facilitating the movement of charged particles from the central axis into the plasma passageway. More specifically, this condition favors the movement of high-mass particles and allows them to concentrate in the passageway where they can be more predictably separated from the low-mass particles. A consequence of this is that the filter can handle a greater throughput. 
     Preferably, for the present invention the outer conductor for generating the magnetic field component (B z1 ) is a magnetic coil that is mounted on the outer surface of the wall. The inner conductor, which is used for generating the magnetic field component (B z2 ), is preferably a plurality of magnetic loops which encircle the longitudinal axis and are located in the passageway at a distance from the inner surface of the wall. The present invention also includes means, such as concentric ring electrodes, which are mounted at one end of the passageway for establishing an electric field, E r , that is oriented substantially perpendicular to the magnetic field (B z ). 
     For the operation of the centrifugal filter of the present invention, the container passageway is dimensioned such that the inner surface of the container wall is at a distance “a” from the central longitudinal axis. Additionally, the magnetic field in the passageway is oriented substantially parallel to the central longitudinal axis, and has a magnitude which varies between a maximum, B z  in the passageway, to a minimum of approximately zero along the central axis. Further, the electric field (E r ) is established in the passageway to be substantially perpendicular to the magnetic field (B z ). Importantly, E r  increases linearly with the radius and is determined by a positive potential on the central longitudinal axis equal to “V ctr ”, and a substantially zero potential at the inner surface of the container wall. With this configuration, when a rotating multi-species plasma is injected into the central passageway, high-mass particles in the plasma which have a mass (M 2 ) that is greater than a predetermined cut-off mass (M c ) will tend to concentrate farther from the central axis than will low-mass particles which have a mass (M 1 ) that is less than M c . The high-mass particles can then be more predictably ejected into the inner wall of the container where they can be subsequently collected. On the other hand, low-mass particles which have a mass (M 1 ) that is less than M c  will not be ejected from the passage away and, instead, will transit through the container. For the present invention M 1 &lt;M c &lt;M 2 , where: M c =ea 2 (B z ) 2 /8V ctr . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a perspective view of the centrifugal mass filter of the present invention with portions broken away for clarity; and 
     FIG. 2 is a cross sectional view of a portion of the centrifugal mass filter as seen along the line  2 — 2  in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1 a centrifugal mass filter in accordance with the present invention is shown and generally designated  10 . As shown, the filter  10  preferably includes a cylindrical shaped container  11  with a wall  12  having an inner surface  14  and an outer surface  16 . The container  11  has a substantially open end  18  and a substantially open end  20  and is oriented on a central axis  22 . 
     FIG. 1 also shows that an outer conductor  24  comprising a plurality of annular coils  26  (of which the coils  26   a ,  26   b ,  26   c  and  26   d  are representative) is mounted on the outer surface  16  of the container wall  12 . For the particular embodiment of the centrifugal mass filter  10  shown in FIG. 1, there is also an electrode  28  which comprises a plurality of concentric rings that are positioned at the end  18  of container  11  around the central axis  22 . For the filter  10  of the present invention, this electrode  28  is used to establish a positive potential, V ctr , on the axis  22 . It will be appreciated, however, that the electrode  28  or, alternatively, a spiral electrode (not shown), can be positioned at either end  18  or end  20  (or both) of container wall  12  for this purpose. Importantly, the electrical potential at the container wall  12  will be approximately zero so that a radially oriented electrical field, E r , is established between the central axis  22  and the container wall  12  substantially as shown in FIG.  2 . 
     FIG. 1 shows that the filter  10  of the present invention includes an inner conductor  30  which comprises a plurality of coils  32  (of which the coils  32   a ,  32   b  and  32   c  are representative). In both FIG.  1  and FIG. 2 it will be seen that the inner conductor  30  surrounds a central passageway  33  inside the container  11 . With this structure, the filter  10  is configured to establish a plasma passageway  34  which extends from end  18  to end  20  between the inner surface  14  of the container wall  12  and the central axis  22 . Note the central passageway  33  is a portion of the larger plasma passageway  34 . 
     In accordance with earlier disclosure, it will be appreciated that the radially oriented electric field E r  is established in this passageway  34  and is oriented substantially perpendicular to the central axis  22  (see FIG.  2 ). Also, a magnetic field B z  is established in the passageway  34  by the concerted effects of both the outer conductor  24  and the inner conductor  30  which will be oriented substantially parallel to the central axis  22 . The magnetic field Bz will, therefore, be crossed with the electric field E r  in the passageway  34 . 
     For the present invention it is preferred that the outer conductor  24  and the inner conductor  30  generate respective magnetic field component B z1  and B z2 , which are additive in the passageway  34 . More specifically, as substantially shown in FIG. 2, these components are additive between the inner surface  14  of the container wall  12  and the inner conductor  30  such that B z1 +B z2 =B z . On the other hand, as also shown in FIG. 2, these components are additive such that B z1 +B z2 ≅0 or is, at least, minimal in the central passageway  33  near the central axis  22 . The particular configuration of the magnetic field can, to some extent, be determined by the use of casings  35  (see coil  32   c ) which can be placed around each of the annular coils  32 . The result of all this for the magnetic field in the passageway  34  is best exemplified by the magnetic flux lines  36  shown in FIG. 2 (of which the flux lines  36   a ,  36   b ,  36   c  and  36   d  are representative). 
     In the operation of the filter  10  of the present invention, an electrical field, E r , is established in the passageway  34  with positive potential, V ctr , on the central axis  22 , and a substantially zero potential at the container wall  12 . Further, a magnetic field, B z , is established in the passageway  34 . Specifically, the magnetic field, B z , that is generated by the combined outputs of outer conductor  24  and inner conductor  30 , and is oriented in the passageway such that E r  and B z  are crossed with each other. The magnitude of the magnetic field, B z , and the magnitude of the positive potential, V ctr , for the electric field, E r , are then established such that M c =ea 2 (B z ) 2 /8V ctr , where “a” is effectively the radial distance from the central axis  22  to the container wall  12  and “e” is the ion charge. A rotating multi-species plasma  38  is then injected into the central passageway  33  through the end  18 . 
     Typically, as envisioned for the present invention, the multi-species plasma  38  will include various types of specific elements which can be generally classified as either low-mass particles  40 , having a representative mass, M 1 , or high-mass particles  42 , having a representative mass, M 2 . Importantly, M c  is established so that M 1 &lt;M c &lt;M 2 . The consequence of establishing M c =ea 2 (B z ) 2 /8V ctr  is that the low-mass particles  40  (M 1 ) will be confined within the passageway  34  during their transit through the filter  10 , while the high-mass particles  42  (M 2 ) will be ejected into the container wall  12  before they can completely transit the filter  10 . Further, the configuration of the magnetic field that is created by the combined outputs of the outer conductor  24  (B z1 ) and inner conductor  30  (B z2 ), wherein the magnitude of the magnetic field varies from B z  in the passageway  34  down to approximately zero on the central axis  22 , facilitates the separation of high-mass particles  42  from the low-mass particles  40 . Specifically, due to the configuration of the magnetic field, the high-mass particles  42  tend to concentrate in the passageway  34  at a distance from the central axis  22  where the expression M c =ea 2 (B z ) 2 /8V ctr  is most effective. The beneficial consequence of this is that the filter  10  is able to increase it throughput over what would otherwise be realizable. 
     While the particular Centrifugal Filter for Multi-species Plasma 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.