Abstract:
In a system for isotope separation, method and apparatus empolying crossed-field MHD particle accelerating techniques for collecting ions of a desired isotope after selective ionization of that isotope. In the plasma of electrons and selectively ionized atoms which results from selective isotope ionization, the ions are collected by inducing a circulation of the plasma electrons to provide a JXB relationship for MHD acceleration of the ions. The electron circulation enhances penetration of the plasma by the accelerating electric field and avoids the generation of large Hall voltages within the system as well as reducing the required magnitude of electrode currents.

Description:
FIELD OF THE INVENTION 
     This invention relates to method and apparatus for the separation of particles in a plasma and in particular to the collection of selectively ionized isotopes for isotope enrichment. 
     BACKGROUND OF THE INVENTION 
     Nearly all fission reactions utilizing the uranium isotope, U 235 , require a concentration of the U 235  isotope greater than in the naturally occurring state. The process of enrichment whereby the concentration of U 235  in natural or depleted uranium is raised to a desired level has been achieved in the past by many techniques which generally operate to separate U 235  from the other uranium isotopes, chiefly U 238 , on the basis of its slight chemical or mass difference. Enrichment according to these techniques often requires cascaded processing using a sequence of repeated applications of the same steps, each step providing a slight increase in the concentration of the desired U 235  isotope. 
     A promising new technique for efficient isotope enrichment operates by exposing a vapor of uranium to pulsed laser radiation to produce a plasma of selectively ionized U 235  atoms and to permit separation of U 235  ions based on their electrical charge. Typically, separation of the ions resulting from selective ionization is achieved by accelerating them out of the vapor toward a collecting surface through pulsed application of crossed-field MHD acceleration forces to the plasma just after its creation with each pulse of laser radiation. In using this process, it has been noted that the plasma, a conducting medium, has associated with it a skin depth effect which impedes the penetration and correspondingly the effectiveness of the electrical field component of the crossed-field MHD acceleration force. Additionally, the electrons in the plasma being of much lower mass and more easily accelerated, will contribute a substantial electron current distinct from the ion current of desired U 235  atoms. The large electron current may result in electrode degradation and will lead to the generation of substantial Hall voltages which permitted to short circuit will impair the enrichment process efficiency. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, an efficient crossed-field MHD acceleration system is disclosed for separating ions from a plasma with reduced effect from the usual impairing factors associated with the plasma. In implementing this system a plurality of ion separation chambers are provided to receive a radially expanding flow of uranium vapor which is selectively ionized to create a plasma within each chamber. The chambers are operative to apply to the plasma orthogonal electric and magnetic fields to impart a motion to the ions which drives them to separate collecting surfaces out of the expanding flow. The fields are applied in a configuration which reduces skin effect problems, provides a high driving point impedance for the electric field with a consequent reduction in current and avoids the generation of extraneous Hall voltage problems. 
     In accordance with a specific preferred embodiment of the invention for uranium enrichment each chamber typically comprises a planar first electrode placed plane parallel to the plasma flow. A second electrode in an extruded U shape surrounds the first electrode on three sides with the open side facing the expanding flow. A plurality of chambers are placed side-by-side in an arc surrounding the vapor flow, and a beam of laser energy is directed between the electrodes in each chamber to ionize the U 235  uranium isotope in the vapor. An axial or lengthwise magnetic field is applied to the region between the electrodes and an electric field is periodically applied between the first and second electrodes for a brief interval. Under the influence of the momentary electric field and steady D.C. magnetic field, the electrons are accelerated to assume a trajectory circulating about the first electrode while the ions, having a mass many orders of magnitude greater than that of the electron, are accelerated toward one of the electrodes where they are collected. The radius of gyration for the uranium ion in the magnetic field is very much larger than the radius of gyration for the electron and its trajectory appears almost as a straight line while the path of the electrons can be made to close upon itself within the chamber. 
     Because the electrons are induced to orbit rather than complete an electric circuit between the electrodes, the driving point impedance for the two electrodes is substantially increased which not only reduces the current that is supplied to the electrodes, but also increases the skin depth of the plasma to improve the penetration of the pulsed electric field between the electrodes. Because the electrons are circulating on closed paths within each chamber, Hall voltages associated with the electron current in the magnetic field alternate in direction between each electrode and do not accumulate to a large potential which could impair the operation of the enrichment process. 
     The unaccelerated particles in the vapor flow continue on a trajectory towards a separate collection plate where they may be periodically removed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the present invention will be more fully understood by reference to the detailed description of a preferred embodiment presented below for purposes of illustration, and not by way of limitation, and to the accompanying drawings of which: 
     FIG. 1 is an energy level diagram useful in explaining the functioning of the enrichment system with which the method and apparatus of the invention is associated; 
     FIG. 2 is a diagram of enrichment apparatus for use in the invention; 
     FIG. 3 is a sectional view of a portion of the apparatus of FIG. 2, illustrating details of the ion separation chambers according to the present invention and 
     FIG. 4 is an expanded view of one chamber in the diagram of FIG. 3 showing details useful in explaining the operation of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention contemplates a technique for separating charged particles of elemental, molecular or other type from a generally neutral environment having particles of at least both charge polarities with different mass-charge ratios. The technique contemplates establishing significantly different trajectories for the particles of each charge polarity. 
     The present invention may best be understood by reference to the enrichment system with which it is preferably associated in order to separate and collect the U 235  isotope of uranium from the other constituents of the naturally occurring or depleted element. In accomplishing this enrichment, a vapor of uranium is generated and passed through a periodically applied radiant energy in a beam of laser radiation having one or more frequencies of radiation to produce selective ionization of the desired U 235  isotope. Typical energy steps are illustrated in FIG. 1 for selective ionization of the U 235  isotope by employing two energy steps 12 and 14 which respectively produce selective excitation of U 235  atoms to an intermediate energy level 16 and subsequent ionization of the excited atoms from the level 16 to above the ionization energy level 18. 
     As a result of the technique briefly illustrated by reference to FIG. 1, there will result a plasma containing, in addition to neutral atoms, a plurality of electrons and corresponding ions of which a larger proportion are of the U 235  isotope by comparison to the proportion in the neutral state. The charge carried by the ionized U 235  atoms provides them with a unique characteristic whereby they may be separated from the remainder of the plasma. In understanding the method and apparatus for accomplishing this separation, it will be beneficial to describe the overall enrichment system with which it is associated. A prior enrichment system of this type is shown in French Pat. No. 71.14007 granted Jan. 10, 1972 (Publication No. 2.094.967), incorporated herein by reference. A similar system modified in accordance with this invention is shown in FIG. 2. 
     In FIG. 2 a laser system 30 includes a lasing medium 32 which may typically be a solution of a lasing dye. Lasing medium 32 is pumped or excited to a lasing condition by an excitation source 34 which may typically be a flash lamp or another laser. The frequency of radiation from the lasing medium 32 is controlled by a tuning system 36 and a timing system 38 initiates each pulse of an output beam 40. One or more stages of amplification may be used to intensify beam 40 as desired. The lasing system 30 may comprise a &#34;Dial-A-Line&#34; laser of the Avco Everett Research Laboratory, Everett, Mass., as generally disclosed in U.S. Pat. No. 3,684,979. The duration of the beam 40 is determined by properties of the laser system and lasing medium as is well known in the art. Typical laser pulse duration is a substantial fraction of a microsecond. 
     The beam 40 is combined with a beam 42 from a second laser system 44. A dichroic mirror 46 operates in a conventional mode to combine the beams 40 and 42 into a composite beam 48. 
     The timing system 38 in the laser system 30 and corresponding timing system in the laser system 44 are operated in unison by a timer 50 which also provides other activation signals in the system as will be described below. 
     The beam 48 enters a chamber 52 through a window 54 which may be of quartz crystal and which extends beyond chamber 52 on a pipe 56 to reduce contamination from vapor within chamber 52. Beam 48 traverses the length of the chamber 52 exciting through a similar window 58 and pipe 60 for subsequent use in one or more additional chambers through a window 62 on a pipe 64 to provide as complete utilization of the energy in the beam 48 as is possible. The chamber 52 is maintained at a low pressure through a vacuum system 66 and conduit 68 to prevent interference with the process of selective ionization and collection in chamber 52 from chemical reaction or particle collision with extraneous elements. 
     Within chamber 52 a uranium vapor source 70 is operative to provide a radially expanding uranium vapor flow. The vapor expands into a uranium ion collector 72 where it is periodically irradiated with the laser beam 48 from the respective laser sources to produce selective ionization of the desired U 235  isotope in the vapor. Subsequent to ionization, the timer 50 activates a voltage pulse source 74 to produce within ion separator 72 a short duration electric field. Respective magnetic field coils 78, 80 and 82 are energized by a magnetic field source 84 to provide a D.C. magnetic field parallel to the beam of laser radiation 48 throughout the ion collector 72 for interaction with the electric field pulse according to crossed-field MHD accelerator principles. 
     In FIG. 3, internal structure of the chamber 52 is more clearly shown. The vapor source 70 is indicated as including a crucible 86 having cooling ducts 88 through the walls thereof to communicate with a cooling and circulating system 90 to prevent excessive temperature in the crucible 86. The crucible contains a mass of uranium metal 92 in a trough running substantially the length of the chamber 52 parallel to the beam 48. 
     A line 94 lengthwise along the surface of the uranium mass 92 is irradiated with a beam 96 of electrons from a beam source 98 which typically comprises a long, heated filament 100 as electron source and collimating accelerator plates 102. The beam is focused toward line 94 by magnetic field 104 from coils 78, 80 and 82. The intensity of the beam 96 is adjusted to provide local heating of the line 94 such that the uranium is vaporized in the region of the line to produce a radial expansion of uranium vapor directed away from the line toward the uranium ion collector 72 over a wide angle. The expanding uranium vapor establishes in general a locally unique particle direction. At least a portion of the uranium mass 92 will typically be maintained in the liquid state by heat from beam 96 and thus replenish the vaporized uranium. 
     The ion collector 72 is shown to comprise a plurality of separation chambers 106 arranged in an arc about the line 94 to receive the expanding vapor flow. A plurality of electrodes 108 emanate toward line 94 from a cylindrical arc 110 to define each chamber 106. These electrodes 108 are in the local plane of the vapor flow and extend into the page of FIG. 3 along with the arc 110. A further set of electrodes 112 are placed between each electrode 108 running parallel to the local flow direction and extending substantially the length of the chamber 52 with electrodes 108. The electrodes 112 are placed to permit circulation of electron current around the electrodes 112 in each chamber 106. Electrodes 108 and 112 are typically supported at their ends. The electrodes 108 are connected in common to one output line of the pulse source 74 to receive a voltage pulse, typically in the range of 30 to 100 volts, while the electrodes 112 are tied in common and connected to the other output line of pulse source 74. The source 74 is activated by the timer system 50 for a short duration typically on the order of 2 microseconds or less as will be described below. 
     To provide illumination of each chamber 106 the laser beam 48 is directed by mirror systems 114 and 116 shown in FIG. 2 such that the beam proceeds, by way of multiple reflections, through each chamber 106 of the ion collector 76. Alternatively, a plurality of laser systems may be utilized to provide a separate beam 48 for each chamber 106. 
     By reference now to FIG. 4, the details of operation of the ion collector 76 may be most clearly understood from a single one of the chambers 106 with the central electrode 112 surrounded by the side electrodes 108. After the laser beam is applied to the area between electrodes 108, there will exist a substantial number of ions 120 which will include the U 235  isotope of uranium in a substantially higher proportion than in the neutral state. There will also be a corresponding number of electrons 122 produced from the ionization process. Many neutral atoms 124 will also exist in the region between the electrodes 108 and surrounding the electrode 112. The entire environment in that region can be considered a plasma. 
     Directly subsequent to the ionization produced by simultaneous application of the laser beams by the timer 50, the pulse source 74 connects a voltage between the electrodes 108 and 112 for a brief interval. The magnetic field 104 is produced in the region between the electrodes 108 continuously so that the application of voltage between the electrodes 108 and 112 will cause a brief acceleration of the electrons 122 toward the positive electrode 108. The electrons will, however, be deflected by the magnetic field 104 and ultimately constrained to follow an orbit 126 about the electrode 112 on equipotential lines between the electrodes 108 and 112. The application of the voltage between the electrodes 108 and 112 also induces motion of the ions 120 toward the negative electrode 112. The strength of the magnetic field 104, typically 200 gauss, and the electric field between the electrodes 104 and 108 is selected such that the resulting radius of gyration for the electrons 122 is relatively small and orbital circulation of the electrons about the electrode 112 may be achieved, but such that the radius of gyration of the ions 120, due to their substantially greater mass, or greater mass to charge ratio is very large and the trajectory for the ions 120 will be substantially a straight line toward the collection electrode 112. Similar operation may be achieved by reversing the polarities of the crossed-field. The size of chamber 106 preferably minimizes ion 120 collisions with other particles. 
     The duration for the electric field between the electrodes 108 and 112 is typically selected to be less than a charge exchange time for an ion 120 and a neutral atom 124. The resulting ionization of a previously neutral atom 124 from the charge exchange reaction will thus occur subsequent to the application of the electric field and the undesired atom 124 will not be attracted toward the collection electrode 112 but will continue in the direction of radial expansion of the uranium vapor toward a rear plate 128 on the arc 110. 
     It can now be appreciated that the induced circulation of the electrons 122 in orbits 126 about the electrode 112 substantially inhibits their flow in the circuit of the electrodes 108 and 112 and source 74 so that a relatively high potential can be maintained across the electrodes without a substantially high current flow. This relationship creates a high driving point impedance for the source 74. The high impedance and relatively low current flow also helps to retard electrode degradation from particle exchange at the electrode surfaces. This high impedance further reduces or eliminates skin depth difficulties which would retard penetration of the electric field between the electrodes 108 and 112 particularly in view of the relatively short duration of the electric field. 
     It can also be appreciated that the structure indicated in FIG. 5 provides in the electron orbits 126 a current density j to satisfy JXB relationship which gives an alternative description of the forces which separate the charged ions. Since the circulation of electrons in orbits 126 provides an opposite current flow on each side of the electrode 112, the Hall voltages associated with the JXB relationship are in opposing directions between the electrodes 108 such that despite the provision of many electrodes 108 and 112 in the alternating sequence shown in FIG. 3 there is no cumulative generation of a large Hall voltage over the stack of electrodes. Such Hall voltages could distort the current flow pattern and cause voltage arcing problems. 
     Various additional considerations may enter into the fabrication of the above indicated structure. In particular it may be desirable to include a return path for the magnetic field 104 outside of the coils 78, 80 and 82 by providing high permeability discs on the end of chamber 52 and bars of similar characteristics connecting the discs outside of the coils. The material for the chamber 52 vapor source 70, and ion collector 72 are also preferable, of a material which will not substantially affect the magnetic field 104. It may also be desired to provide collection plates for the ions and neutral particles distinct from the electrodes. A source of electrons such as filament 130 in FIG. 4 may be provided to offset the slight flow of electrons in the circuit of the pulsed electric field or the molten uranium 92 may be used as such a source. 
     Having described above a preferred embodiment for the present invention, it will occur to those skilled in the art that further alternatives and modifications to the disclosed system can be devised within the spirit of the invention. Accordingly, it is intended to limit the scope of the invention only as indicated in the following claims.