Patent Number: 042773067
Section: description

Generally, the present invention is directed to methods for the control of impurity flow in toroidal plasma systems by providing a magnetic confinement system having outer flux lines which are axisymmetrically bound with respect to the plasma. In this connection, a confining field is provided which has outer flux lines bound in a zone adjacent the plasma and displaced from the plasma in a direction along the major toroidal axis opposite the direction of positive ion toroidal drift. This may be accomplished by providing a relocatively high flux value in the plasma confinement zone, and a relatively low flux value in the adjacent flux binding zone. By further providing a particle sink such as a vacuum pumping zone in the zone adjacent the plasma, in which outer magnetic flux lines of the plasma are bound to vacuum chamber walls, a plasma diffusion pump may be provided which is adapted to control impurities, but which does not require complicated poloidal divertor structures using internal coils. Apparatus which may be adapted for performance of the present invention comprises the elements of toroidal plasma confinement systems, preferably of Tokamak design and more preferably of Tokamak design and having a noncircular plasma cross section which is elongated in a direction along the major toroidal axis. Such tokamak systems for the containment of high-temperature plasmas comprise means for providing a strong, toroidal magnetic field in which the plasma ring is to be embedded, and which is generally provided by electrical current in one or more conductive coils encircling the minor toroidal axis. Such systems also comprise means for providing a toroidal electric field to maintain a toroidal current flowing in the plasma, and this plasma current in turn generates a magnetic field component which is poloidal. The combination of the poloidal magnetic field with the toroidal magnetic field with the toroidal magnetic field produces resultant magnetic field lines that lie on closed, nested magnetic surfaces, and the plasma is subjected to confining, constricting forces generated by the current flowing in it. The toroidal confinement systems may also include various means to generate, heat or otherwise control the plasma, such as neutral beam injection systems. Examples of apparatus which may be utilized in connection with the present invention include the Doublet III apparatus of General Atomic Company and the ISX tokamak system of Oak Ridge National Laboratory. The invention will now be more particularly described with specific reference to the toroidal plasma confinement system 10 illustrated in FIGS. 1 and 2 of the drawings. The plasma generation and confinement apparatus 10 may be a toroidal fusion reactor for producing high energy neutrons by nuclear reaction occasioned by the fusion of deuterium and tritium nuclei, or may utilize the light hydrogen isotope in provision of a high temperature plasma for study of plasmas or any other use to which hydrogen plasmas may be put. The apparatus 10 has a large toroidal reaction chamber 12 for plasma generation and confinement. A plasma may be created in the vacuum chamber 12 by an appropriate poloidal field, established by E-coils 14. When the E-coils are energized, they produce a time varying magnetic flux linking the chamber 12. The electric field induced by this flux variation initiates and maintains the toroidal discharge current required for plasma confinement and ohmic heating. F-coils 16a, b control the magnetic configuration and position of a plasma discharge in a predetermined manner. The F-coil system establishes the magnetic boundary conditions for the plasma 40 in the upper zone of vacuum chamber 12 and may be varied to control the position and other parameters of the plasma 40. Also provided around the chamber 12 are toroidal B-coils 18, which establish an azimuthal magnetic field for stable plasma confinement. The F-coils 16a, b are programmed to provide a higher flux .psi. value in the upper chamber, such that the outer magnetic lines of flux intersect with the vacuum chamber walls in the lower zone of the vacuum chamber adjacent the plasma, and such that a "D" shaped plasma is provided in the upper zone of chamber 12. The .psi. flux values and outer, bound flux lines 44 are shown in more detail in FIG. 2, where the plasma zone comprising the predominant amount of the plasma [e.g., at least 90% of the plasma mass] is shown as shaded plasma region 40. As indicated, the .psi. value of the upper portion 46 of the vacuum chamber 12 is greater than the .psi. value of the lower portion 48 of the chamber, such that the outermost magnetic flux lines (indicated by lines 44) adjacent the walls of the chamber 12 in the upper portion 46, are directed to intersect the chamber walls in the lower portion 48. Of course, inner flux lines in the upper chamber are closed, or continuous, such that charged particles of the plasma 40 which are influenced by electromagnetic forces to follow the flux lines will experience force tending to confine the plasma in the zone 40. However, ionized particles adjacent the chamber 12 walls which tend to follow the outer flux lines, will experience a charge particle path which intersects with the wall in the lower portion 48 of the chamber 12. The plasma conditions are initiated at relatively low pressures. Hence, the chamber 12 is constantly pumped out by vacuum pumps through ports 30 located at the bottom of the vacuum chamber 12 and corresponding conduits 32. At the high temperatures thus produced in the reaction region containing the plasma, the dueterium and tritium nuclei may undergo fusion, producing helium nuclei and high energy neutrons. Such neutrons at energies of about 14 MeV may penetrate the first wall 22 and pass into a blanket 24 surrounding the chamber 12. The blanket 24, formed in part of carbon and lithium, is used for extracting the energy from the neutrons, raising the temperature of the blanket 24. Helium gas may be circulated through the blanket 24 from a conduit 26. Cool helium is introduced into the conduit 26, and heated helium is withdrawn from a conduit 28. The helium provides a safe, yet effective, heat transfer function, carrying heat from the reactor to an external heat exchanger, and recirculated through conduit 26. A radiation shield 34 may be provided to limit the escape of harmful radiation. By providing a particle sink such as a vacuum pump system in the lower chamber zone, impurity particles adjacent the chamber walls may thus be collected at the lower zone and removed from the system. By providing a hydrogen source in the upper chamber zone, such as the neutral beam system input, the net impurity flow into the lower zone is enhanced. The illustrated apparatus 10 may be regarded as an axisymmetrical plasma diffusion pump. Magnetic flux lines 44 are bound between two walls (i.e., intersect the lower walls of the vacuum chamber at two places). Hydrogen gas is introduced in the middle and r-f power from a suitable source (not shown) is applied to ionize the gas. Resulting plasma flows along the outer flux lines towards the wall in the lower portion of the chamber at sound velocity. Because the impurity atoms produced at the wall are ionized in a much shorter distance than the distance to the main plasma, the plasma flow will push the impurity ions back towards the wall by collisional and/or electrostatic interaction. In the case of collisional pumping, the conditions that the impurity ions are ionized in a short distance may be represented by ##EQU1## The condition that the collisions between protons and impurity ions are sufficiently frequent to produce collisional pumping may be represented as: ##EQU2## where v.sub.o is the velocity of sputtered atom, n is the plasma density, &lt;.sigma.v&gt; is the ionization probability, a is the distance from the wall to the main plasma, .nu..sub.iz is the proton-impurity collision frequency, T.sub.i is the plasma ion temperature, m.sub.p is the proton mass, v.sub.s is the sound velocity of the plasma, and l is the distance along a line of magnetic induction from the main plasma to the intersection of that line of magnetic induction with the vacuum chamber wall. By utilizing the following relationship of .nu..sub.iz ; EQU .nu..sub.iz .apprxeq.nZ.sup.2 10.sup.-12 (T.sub.i /e).sup.-3/2 sec.sup.-1 ( 3) Equation (2) may be rewritten as: EQU nl&gt;2.times.10.sup.16 (T.sub.i /e).sup.2 (T.sub.i /T.sub.e).sup.1/2 Z.sup.-2 (4) where T.sub.e is the electron temperature and Z is the charge of impurity ions. The ion temperature T.sub.i may be estimated from the energy balance, i.e., EQU .nu..sub.ei (T.sub.e -T.sub.i)=2T.sub.i v.sub.s /l (5) or ##EQU3## There are two regimes depending on plasma density, as follows: ##EQU4## For the high density case, where ln is greater than 4.times.10.sup.17 (T.sub.e /e).sup.2, the electron temperature T.sub.e is approximately equal to the plasma ion temperature T.sub.i, and Equation (4) is satisfied for Z less than 5. For the low density case where ln is less than 4.times.10.sup.17 (T.sub.e /e).sup.2, the ion temperature T.sub.i may be represented as: ##EQU5## By combining Equation (8) with Equation (4), EQU nl&lt;3.times.10.sup.18 (T.sub.e /e).sup.2 Z.sup.4/3 (9) This condition is automatically satisfied. The lower limit of the density is given by Equation (4) by utilizing the lowest value for the ion temperature and by Equation (7). For an electron temperature equal to ionization potentials, Equation (1) becomes EQU n&gt;&gt;2.times.10.sup.12 (v.sub.o /a) (10) By assuming the sound velocity v.sub.o to be approximately equal to 1.times.10.sup.3 meters per second, we have EQU n&gt;&gt;2.times.10.sup.15 a.sup.-1 m.sup.-3 (11) As indicated, the pumping interaction may be collisional and/or electrostatic, and in a substantially collisionless system, the pumping is done by electrostatic potential. When the hydrogen plasma flows towards the lower vacuum chamber wall at sound velocity and where the electron temperature T.sub.e is very much greater than the plasma ion temperature T.sub.i (T.sub.e &gt;&gt;T.sub.i) there is an electrostatic potential accelerating the protons. The potential is of the order of about T.sub.e /e. The impurity atoms, after becoming ionized, are repelled by the potential barrier, if the electron temperature T.sub.e is greater than W.sub.z /Z, where W.sub.z is the kinetic energy of the impurity atoms. The condition on the plasma density in this case is that the plasma flow be substantially unaffected by the impurity flux, that is, that the proton density shall be much higher than the impurity density. The impurity density n.sub.z may be estimated from the sputtering yield, by the following relationship: ##EQU6## where .alpha. is the sputtering yield, n.sub.h is the plasma density of hot plasma, .tau..sub.h is the particle confinement time of hot plasma, and .delta. is the ratio of volume of hot plasma, and volume of divertor space. For typical plasma parameters of the illustrated apparatus 10, n.sub.h =10.sup.20 m.sup.-3, .tau..sub.h =0.5 sec, a=0.5 m, .delta.=1, .alpha.=0.1, and v.sub.o =10.sup.3 m/sec. Accordingly, from Equation (12) the impurity density n.sub.z under such conditions is about 1.times.10.sup.16 m.sup.-3. The condition for ionization as set forth in Equation (1) is: ##EQU7## Accordingly, it will be appreciated that these two conditions are not very different. The power P required to maintain the divertor plasma may be represented as: ##EQU8## where R is the major radius. By substituting a plasma length which is approximately equal to Rq (q is the safety factor), the power P may be represented as: ##EQU9## For typical plasma parameters of the illustrated embodiment, n=2.times.10.sup.17, q=3, T.sub.e =20 eV, a=0.5 m, and v.sub.s =4.5.times.10.sup.4 m/sec, the power P may be seen from Equation (15) to be about 30 kW. Turning back to the drawings of FIGS. 1 and 2, a D-shaped plasma 40 is produced in the top half of the chamber 12. The flux surfaces 44 that are not closed in the chamber are made to intersect with the wall in the bottom half. As indicated previously, pure hydrogen gas may be supplied to the upper portion of the vacuum chamber containing the plasma 40 and the vacuum pump ports are in the bottom half. However, a separate gas supply may not be needed, if sufficient gas is introduced to the plasma 40 by means of the neutral beam injector utilized for heating of the plasma 40. A 50 kW r-f power system is applied to maintain the low temperature plasma in the bottom half of the chamber 12. The low temperature plasma has a density of about 10.sup.11 cm.sup.-3 and temperature of about 20 eV. The r-f heating system may be lower hybrid resonance type operating at a frequency of about 1 GHz. The pumping speed of the bottom half of the chamber around the torus of the illustrated embodiment is approximately 10.sup.9 cc/sec. Therefore, the neutral gas density is of the order of 10.sup.-5 Torr. Coating of the wall with titanium may be used to increase the pumping speed. The density and the temperature of the pumping plasma in the illustrated embodiment generally corresponds to the temperature and density of plasma present near the wall or behind the limiter of typical tokamak plasma systems. The heating power utilized to maintain the pumping plasma is a very small fraction of the power throughput of the tokamak plasma 40 in the upper chamber. As is the case of conventional divertors, the design constraint is mainly due to pumping speed. While the method has been particularly described with respect to utilization with Doublet III apparatus, the method may also be used with other apparatus such as the ISX apparatus of ORNL. Furthermore, while the method has been particularly described with respect to a specific operational embodiment, it will be appreciated that various modifications, adaptations and variations will become apparent from the present disclosure and are considered to be within the spirit and scope of the present invention as defined by the following claims. Various of the features of the invention are set forth in the following claims.