Patent Number: 047740485
Section: description

DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. As illustrated in FIG. 1, a typical tokamak reactor uses TF coils 20 which are D-shaped superconducting magnets. For purposes of illustration, the parameters of the proposed TFCX Lite-R1 reactor with D-shaped TF coils will be used as a model. The parameters for Lite-R1 are listed below in Table 2. It will become readily apparent to those skilled in the art that the present invention may also be used in a reactor having other typical TF coil designs, such as bean-shaped or window frame designs. TABLE 2 ______________________________________ Plasma Parameters for the TFCX LITE-R1 Device ______________________________________ Bo = 6.1 T Moderate Ip = 8.11 MA Ro = 2.44 m a = .84 q = 3.5 = 5% V-S = 12.6 Fusion power = 600 MW ______________________________________ The OH coil 22 for Lite-R1 is in the central bore and requires 30 MA of current to produce 12.6 volt-sec. (A "volt-sec" here is defined as the voltage produced by the OH coils in the plasma multiplied by the duration of the pulse time.) The flux of the OH solenoide 22 is shown in FIG. 2. Some flux lines 28 are passing through the plasma 24. This reduces the effect of volt-sec and also distorts the plasma 24. Therefore, additional shaping coils 26 have to be added to compensate the effects of flux leakage. The ideal OH coil, which gives the least flux leakage, is an infinitely long solenoid, but it is not practical to use. Therefore, according to the invention a solenoid is shaped into a vertically standing toroidal-like coil 30 as illustrated in FIG. 3. OH coil 30 is disposed around a TF coil 32. Most of the flux lines 34 are confined inside the OH coil 30 bore and the leakage is very small as illustrated in FIG. 4. The advantages of this preferred embodiment are the increase in volt-sec and complete decoupling of OH and PF coils. FIG. 5 illustrates another preferred embodiment of the present invention. The central bore of a vertically-standing OH coil is removed and OH coil 50 now resembles a horseshoe. Horseshoe-shaped OH coil 50 is disposed around a TF coil 52. The flux lines 56 generated by horseshoe-shaped OH coil 50 are no longer straight but bulge out radially at the mid-plane, as illustrated in FIG. 6. However, flux lines 56 are not passing through the plasma 58, therefore, the effective volt-sec is not reduced. The central bore space 54 now becomes available for other purposes. One drawback for the two type of coils described above is that the minor radius of the coil is large which results in a bulky coil. To solve this problem, a preferred embodiment utilizes a multi-toroidal coil arrangement as illustrated in FIG. 7. The OH coil is no longer a single continuous winding in the poloidal direction. A plurality of vertically-standing toroidal-type coils 70 are disposed around PF coils 72. The size of each individual coil 70 is substantially reduced from the size of the OH coils embodied in FIGS. 3 and 5. FIG. 8 illustrates another preferred embodiment of the present invention which comprises a multi-toroidal coil arrangement. OH coils 80 are wound around the TF coils 82, such that OH coils 80 envelope TF coils 82. The magnetic flux produced by this OH coil 80 is parallel to the current path in the TF coil winding, as shown in FIG. 4. The only space occupied by OH coil 80 is the conductor. The central core has now become vacant and can be used for TF coils, a bucking cylinder, or other suitable purposes. The TF coils 82 and OH coils 80 now form a single unit. As discussed above, the plasma confining PF is conventionally generated by continuous PF coils which are external to the TF coils. An internal PF coil design, however, requires much less current than an external coil, since internal coils are in close proximity to the plasma. The major drawback is that the continuous internal PF coils and TF coils are then interlocked. Illustrated in FIG. 9 is a modular saddle coil system which may be used as the PF coils. This preferred embodiment of the present invention is adopted for providing a modular tokamak design while providing PF coils which are internal to the TF coils. The PF coil system is comprised of divertor-like and equilibrium field (EF) coils. Typically, these coils carry current opposite to each other. Saddle coil 93 is used as the divertor coil. Saddle coil 93 is comprised of internal coil 96 and a return coil 94 which are connected to each other by conductor 97. Return coil 94 is disposed at a distance from TF coil 95 and serves as a return conductor by carrying current in the opposite direction to that of internal coil 96. In a similar manner, saddle coil 91 serves as the EF coil and is comprised of internal coil 90 connecting conductor 98 and return coil 92. Since the return conductors of saddle coils 91 and 93 are far away from the plasma and have a very small net current, the effect on the plasma is minimal. FIG. 10(a) shows the shaping of plasma 100 using an all internal PF coil system 102. FIG. 10(b) shows the shaping of plasma 100 using saddle coils 91 and 93, each having internal PF coils with return conductors. As illustrated in FIG. 9, a trimming coil 99 disposed outside the TF coil may be used with PF coils 91 and 93 to compensate for the non-vanished effects of the return conductors. It will be readily apparent to those skilled in the art that although the preferred embodiment of the invention has been described with reference to saddle coils, any suitable loop coil arrangement, such as a window frame coil, may also be used. A modular magnetic sector can be made from combined OH and PF units of the embodiments of FIGS. 7 and 8 and a PF loop of the embodiment of FIG. 9. The OH coils embodied in FIG. 8, can provide the same amount of volt-sec as a straight solenoid in the central core, with much less current and field. The superconductor can be operated at a much higher current density and the space requirement is minimal. The embodiment of FIG. 9 provides a modular PF coil system with the PF coils disposed near the plasma. A module may contain one, or as many as half of the total of the OH-TF coil sets. There should be at least two 180.degree. sectors, such that the system can be separated for maintenance. However, a two 180.degree. sector system would still be too heavy and bulky to maneuver. A module constructed from a single OH-TF coil set, would result in too many modules. Preferably, the modular unit would contain two OH-TF coils as illustrated in FIG. 11. Each module 110 contains a vacuum vessel 111, divertor coils 114, equilibrium field coils 112, TF coils 116, and OH coils 118. The entire sector 110 may be housed in a vacuum can. A section of blankets may also be inserted into the inboard space. The tokamak reactor may be assembled from these modules as illustrated in FIG. 12. When one module needs repair, it can be removed to a hot cell for service. A spare can be rolled into its place. This will greatly enhance the availability of the reactor. The comparison of key values of the conventional external PF and OH system and the modular system is given below in Table 3. TABLE 3 ______________________________________ Comparison of External/Internal PF Systems Conventional Modular PF Design Design ______________________________________ Total PF Current 59.7 MA 29.9 MA Total OH Current 82.5 MA-turns 21 MA-turns Current Density 4.0 kA/cm.sup.2 4 kA/cm.sup.2 OH Conductor Width 30 cm 1.5 cm Stored Energy in PF System 8.30 .times. 10.sup.8 j 2.6 .times. 10.sup.8 j Outplane Force 16 MN 7.8 MN Maximum Vertical Field 2.0 T 1.5 T on TF Coil ______________________________________ The total current in the PF is reduced by a factor of two. The OH current is reduced by a factor of four. The OH coil thickness for the conventional design is 30 cm, whereas, for the modular design it is only 1.5 cm. The stored energy is near a factor of four less for the modular design than for the conventional design. The outplane force in the modular system is only half of the external case. The foregoing description of the preferred embodiments of this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to precise the forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments where chosen and described in order to better explain the principle of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with other modifications as are suited to the particular use contemplated. It is intended that that scope of the invention be defined by the claims depended hereto.