Patent Number: 
Section: description

A toroidal vessel system 1 containing a magnetically confined DT fusion plasma and a liquid metal first wall is depicted in FIG. 1. The system 1 is axisymmetric about centerline 5, whereas the complete structure of system 1 is obtained by rotating the cross-section shown 360xc2x0 about centerline 5. The toroidal field coils 8 are spaced around the toroid 10 so as to magnetically contain the plasma 6 within the toroidal chamber 9. The toroidal field coils 8 are positioned external to the toroidal vessel 10. Liquid metal, preferably lithium, enters the toroidal vessel chamber 9 as a continuous stream through apertures 14 and 15 located at the top of the toroidal vessel 10 forming two axisymmetric streams of liquid metal 12 and 13. The thick liquid metal streams 12 and 13 continuously flow down each side of the inner wall 7 of the toroidal vessel 10 and exit through a common exit aperture 16 at the base of the toroidal vessel 10. A pump 18 pumps the hot liquid lithium 20 from the base of the toroid 10 to a heat extraction and power conversation unit 22. As the cooled liquid lithium 24 flows back to the top of the toroid for reuse, it is divided into two streams 42 and 43. Two electrodes 26 and 27 separated by an insulator 28 are positioned at the top interior of the toroid 10. The electrodes 26 and 27 are positioned at the edge of the openings 14 and 15 allowing the liquid lithium stream to enter the toroid 10 and to establish electrical contact between the respective liquid lithium stream and its respective electrode. When energized by a direct current source 11 the electrodes 26 and 27 provide a poloidal current which flows in the lithium layers 12 and 13. The current source 11 is connected to the electrodes 26 and 27 so that the poloidal current flows in the same direction as the current in the toroidal field coils 8 which surround the toroid 10. An alternate embodiment is depicted in FIG. 2. were instead of the heat extraction and power conversion unit 22 being outside of the toroidal field coil 8 the unit is placed inside the field coil 8. FIG. 3A depicts a top view of the toroid along section line axe2x80x94a of FIG. 1. In this view the toroidal field coils 8 are missing. The figure illustrates the symmetry and continuity of the entrance apertures or ducts 14 and 15, the electrodes 26 and 27 and the insulator 28. The axis of symmetry is the toroidal axis 5. FIG. 3B depicts a bottom view of the toroid along section line bxe2x80x94b of FIG. 1. Also, in this view the toroidal field magnets 8 are missing. The symmetry and continuity of the exit aperture or duct 16 about the toroidal axis 5 is shown. FIG. 4 depicts a portion of the liquid metal stream and a force diagram resulting from the interaction of the poloidal current in the liquid lithium layer, first wall, and the magnetic field generated in the toroid by the toroidal field coil. This interaction results in electromagnetic xe2x80x9cJxc3x97Bxe2x80x9d forces which push the liquid lithium stream 12 against the toroidal chamber wall, thus, keeping the stream away from the plasma. Under the combined influence of gravity and electromagnetic forces, the stream moves along the chamber wall to the base of the chamber where it exits through exit apertures 16. FIG. 5 depicts an alternate embodiment 40 where the first stream 42 and the second stream 44 enter the toroid at the upper end of the toroid Each stream is electrically coupled to a specific electrode 46 and 48 respectively. The electrodes 46 and 48 are insulated from each other by an insulator 50. At the base of the toroid, each stream is electrically coupled to another electrode 52 and 54. As at the top of the toroid, the two electrodes 52 and 54 are electrically insulated from each other by an insulator 56. The paired electrodes, 48 and 52, and, 46 and 54, are each connected to individual current sources 58 and 60 respectively. Electrodes 48 and 52 which are in contact with the liquid lithium conduct the current supplied by the external direct-current power sources 58 and 60 to produce the lithium-confining poloidal currents. As in the prior embodiment, when each liquid lithium stream 44 or 46 reaches the base of the toroid it flows through orifices 61 and 62 respectively into a common pipe 63 and is pumped by a pump 64 to a heat extraction and power conversion unit 66. The insulating structure 50, also, serves as a site for external access to the plasma for diagnostics, for DT fueling and exhaust, and for plasma current profile control. This invention can also incorporate passive electrical conducting solid structures 30 and 32 mounted on the chamber walls and immersed in the flowing liquid lithium layers 12 and 13 FIG. 1 and FIG. 4. The structures produce eddy currents in the flowing liquid metal causing the poloidal currents within the liquid metal to interact with the applied magnetic to produce xe2x80x9cJxc3x97Bxe2x80x9d forces which limit the flow velocity of the descending liquid lithium to the desired design values. These structures are only optional and are not a required item. In an alternate embodiment to reduce neutron streaming, FIG. 6, the electrodes and insulator 70, 72, and 74 are skewed to that they are no longer approximately perpendicular to the inner surface 9 of the toroid 10. A further embodiment provides for a lower surface vapor pressure of the plasma-facing liquid lithium layer. This is achieved by providing for a liquid lithium sublayer relative to the outer layer which is exposed to the plasma, FIG. 7. To achieve this dual layer approach, relatively cool inner layers 82 are injected axially symmetrically at the top of the toroid 10 (only one side of the symmetrical toroid liquid lithium system is shown). The inner layer 82 rides on the hotter outer layer 84 (only one side shown). The two layers 82 and 84 will not mix or interchange because liquid metal flowing in a strongly magnetic field is laminar not turbulent. The relatively cool inner layer 82 will descend more rapidly than the hot outer layer 84. When the inner layer reaches the base of the toroid 86, it is pumped, by pump 87, via the return pipe 88 back to the top of the toroid and reinjected, through port 90 as the warm outer layer 84. The outer liquid lithium layer 84 descends along the inner wall of the toroid 10 and exits at the base of the toroid through port 94 as hot liquid lithium. The hot liquid lithium 96 is pumped, by pump 95, to a heat extraction and power conversion device 98 which cools the liquid lithium. This cooled liquid lithium 100 is then injected as the inner layer 82 at the top of the toroid through port 102. When the hot layer 96 is cooled to just above the lithium melting point, 181xc2x0 C., the vapor pressure is in the neighborhood of 10xe2x88x9210 torr. If the inner layer 82 exits the toroid at a temperature of 271xc2x0 C. after absorbing about 16% of the fusion heating from alpha heating, this corresponds to a surface vapor pressure of 10xe2x88x927 torr. After reinjection as the outer sublayer 84, the lithium leaves the base of the toroid at approximately 745xc2x0 C. which is then used to generate power. The outer sublayer 84 is more closely coupled to the wall of the toroid and descends more slowly than the inner layer 82. Laminar flow ensures that the hot exiting liquid lithium (with 1 torr vapor pressure) will never directly face the plasma. In order to both moderate i.e., slow down, the 14 Mev neutrons and adequately breed tritium, natural liquid lithium blanket designs for pure DT fusion reactors must be typically on the order of 1 meter in thickness. Furthermore the need to be of low atomic number (low-Z) walls for magnetic fusion so that if any impurities enter the hydrogenic Z=1 DT plasma from the wall, such impurities would not greatly increase radiative losses. Pure lithium is good in this respect as it has a low atomic number Z=3. On the other hand, the salt of liquid lithium and beryllium, known as xe2x80x9cFLIBExe2x80x9d, should not be used as a first wall for magnetic fusion since it contains fluorine, which is expected to greatly increase plasma radiative losses and thus degrade energy confinement Liquid Lithium""s vapor pressure is extremely low, which is a requirement for using liquid walls with magnetic fusion. Examined references state lithium""s 10xe2x88x9210 Torr vapor pressure at selected temperatures and vapor pressure formulae accurate for temperatures above 700xc2x0 C. These formulae have been used to calculate the following table: Note for comparison that typical vacuum xe2x80x9cbase pressuresxe2x80x9d on the TFTR varied from sightly below 10xe2x88x928 Torr to several times 10xe2x88x928 Torr, and when the xe2x80x9cbase pressurexe2x80x9d was 10xe2x88x927 Torr or higher, some type of surface xe2x80x9ccleanupxe2x80x9d operation was usually attempted. Based on this TFTR experience, one might expect that a tokamak reactor plasma would tolerate a liquid lithium first wall whose surface vapor pressure does not exceed 10xe2x88x928 Torr. However, it is conceivable that a fusion reactor plasma might tolerate a much higher lithium surface vapor pressure after the plasma is initially establish. Moreover, rather than degrade performance, it was found that deliberately introduced lithium impurities in the plasma edge region actually seemed to improve plasma performance in TFTR experiments. Studies of lithium/helium LMMHD Power conversion have identified 800xc2x0 C. as a maximum design temperature above which systems performance does not significantly improve. Materials such as vanadium are compatible with liquid lithium up to this maximum temperature, but not significantly beyond. However, most fusion design studies considering liquid lithium in blankets have chosen a lower maximum design temperature. Liquid Lithium density varies linearly from 515 kg/m3 at 200xc2x0 C. to 454 kg/m3 at 800xc2x0 C. with a very high heat capacity, making it useful in heat transfer applications. It""s heat capacity is 1.01xc2x10.04 cal/(gmxc2x0 C.) over the temperature range 181xc2x0 C.-800xc2x0 C. Thus, 1.23 Gigajoules/m3 heat raises the temperature from 200xc2x0 C. to 800xc2x0 C. Furthermore, lithium melts at about 180.54xc2x0 C. and requires 103.2 cal/gm xe2x80x9cheat of fusionxe2x80x9d to melt. Thus, melting lithium requires about 0.22 Gigajoules/m of heat. Indeed, if the heat needed to raise the temperature from room temperature is included, i.e., from 25xc2x0 C. to 185xc2x0 C.00, a total of about 0.57 Gigajoules/m3 of heat to completely melt the lithium is needed. Furthermore, liquid lithium""s thermal conductivity varies linearly from 43 w/(m xc2x0 K) at 200xc2x0 C. to 55 w/(m xc2x0 0K) at 800xc2x0 C. Combining this with lithium""s density and heat capacity results in a heat diffusion constant ranging from 0.2 cm2/sec at 200xc2x0 C. to 0.3 cm2/sec at 800xc2x0 C. It has been determined that liquid lithium has the following electrical resistivity: Note that in order to electromagnetically hold a liquid lithium layer against the wall (or the ceiling) of a chamber, it is necessary that the component of the Jxc3x97B product directed towards that chamber wall must exceed the component of the lithium""s weight density directed away from the chamber wall, i.e., xcfx81gcosxcex8, where xcex8 is the angle between the downward vertical and the local inward normal to the wall, xcfx81 is the lithium density, and g is the gravitational acceleration, 9.8 m/sec2. Using the maximum possible magnitude of xcfx81gcosxcex8 shows that J greater than 5047/B, where B is in Tesla, is always adequate. Thus, for example, if a toroidal field within the liquid lithium layers obeys B greater than 5.1 Tesla then a liquid lithium current density of J greater than 1 kA/m2 guarantees the liquid lithium will be forced against the chamber ceiling and walls, thus keeping it away from the plasma. Electrical power dissipation by this confining current within the liquid lithium will be negligible compared to the fusion power, i.e., xcex7J2 greater than 0.35 watts/m3. For example, an ITER-size DT reactor is contemplated with a plasma having a major radius R=8, a minor radius a=3 meters, a plasma half-height b=5 meters and the plasma is surrounded with a close-fitting liquid lithium blanket with a thickness 1 meter. Then the total volume of liquid lithium is approximately 1300 m3, the total mass of the liquid lithium is about 650,000kg, the total current driven through the lithium must exceed 50 kA, and so the associated dc voltage needed to drive this current exceeds 8.8 millivolts. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical applications and should enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.