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Fabrication of the MFTF magnet windings
Abstract
The Lawrence Livermore Laboratory (LLL) is currently in the construction stage of the Mirror Fusion Test Facility (MFTF). MFTF will be the next large mirror fusion experiment and employs a large set of superconducting Yin-Yang coils. These coils contain 54,430 kg of stabilized NbTi conductor and will generate a peak field of 7.68 T with a stored energy of 409 MJ. This paper presents details of the design of these coils and the status of the fabrication.
Three concepts of high temperature superconductor cables carrying kA currents (RACC, CORC and TSTC) are investigated, optimized and evaluated in the scope of their applicability as conductor in fusion magnets. The magnetic field and temperature dependence of the cables is measured; the thermal expansion and conductivity of structure, insulation and filling materials are investigated. High temperature superconductor winding packs for fusion magnets are calculated and compared with corresponding low temperature superconductor cases.
Lap joint resistance as a function of current and magnetic field has been measured across 12 Tesla Coil conductor cable terminations. The terminations were at the ends of a 2.2 m Nb 3 Sn hairpin of internally cooled and cabled superconductor (ICCS), and were soft soldered to NbTi bus bars. The resulting lap joints were each 15 cm long with a contact area of 19.8 cm<sup>2</sup>. The maximum measured lap joint voltage drop was 75 μV at 21 kA and 4.2 K, with the cable center at zero magnetic field. This represents an upper bound on all measured voltage drops, including those with the cable center at 10 and 12 T, and corresponds to a maximum heat flux of 0.01 w/ cm<sup>2</sup>to pool boiling helium. The results imply that the 12 Tesla Coil, with four lap joints, would dissipate less than 7 watts in the steady-state at 21 kA. The zero field results and results at cross fields of 10 and 12 T are discussed. Manufacturing and experimental parameters relevant to this study are also considered.
Mirror experiments have led the way in applying superconductivity to fusion research because of unique requirements for high and steady magnetic fields. The first significant applications were Baseball II at LLL and IMP at ORNL, which used multifilamentary niobium-titanium and niobium-tin tape, respectively. Now the USSR at Kurchatov is building a smaller baseball coil with a 6.5 mm square multifilamentary niobium-titanium superconductor similar to the Baseball II conductor. However, the largest advance in fusion magnets will be used in the Mirror Fusion Test Facility (MFTF) now under construction at LLL. Improvements in the technology of the previous LLL experiment, Baseball II, have been made using new conductor joining techniques, a ventilated wrap-around copper stabilizer, and stronger structural welding methods. The MFTF coil winding is proceeding on a separate former to allow parallel construction of the main structure. Not only does this shorten the project schedule to equal that of other conventional constructions, but a second vacuum barrier is created between the magnet helium and the plasma environment for reliable operation. In the future, LLL envisions a superconducting version of the Tandem Mirror Experiment and a possible hybrid reactor leading to economical fusion power.
The Lawrence Livermore Laboratory has put forward proposals for building a large mirror fusion experiment called MX (Mirror Experiment). This machine is designed to advance both the physics of mirror systems and the technologies which will be required on future machines such as FERF (Fusion Engineering Research Facility) and reactors. One such technology to benefit is superconductivity, since the confining field will be generated by two large NbTi Yin-Yang shaped coils. The maximum field at the conductor is 7.5 T and the total stored energy is 500 MJ. The paper gives details of the magnet system conceptual design including the design philosophy of the superconductor and the structure to restrain the very large electromagnetic forces.
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