User:Phoenig/sandbox

Mitchell O. Hoenig was a research engineer of superconducting magnet development in the Fusion Technology and Engineering Division of the MIT Plasma Fusion Center( date). His conception and development of the Cable-In –Conduit Conductor (CICC) concept led to it becoming the primary choice of conductor type in most high field superconducting magnet applications, including the International Thermonuclear Experimental Reactor (ITER) in France.http://www.iter.org/mach/magnets Hoenig moved to MIT in the early 1970’s and worked with Bruce Montgomery, and Yukikazu Iwasa  to improve the stability of high-current superconductors. Superconductivity of a material is easily disrupted (quenched) by thermal energy, which is the reason most materials capable of becoming superconducting must be kept very cold. High external magnetic fields or high current density also can destroy superconductivity. Before 1975 superconducting material was bound with copper which functioned to siphon off heat to the surrounding liquid Helium. This heat transfer mechanism required the liquid helium to be in a “high-speed turbulent flow” .(6,9) The high flow  caused large pressure drops, required high pumping power, and was expensive and technically challenging. Hoenig reduced the pressure drop and the pumping power while preserving the superconductivity by dividing the superconducting wire into many fine strands. These strands exponentially increased the surface area for cooling. The large cooled surface then provided the heat transfer required for superconducting stability even when the helium flowed at low pressure. The strands and the helium were held within a conduit, which also served to protect the fragile superconductor. Hoenig called this the cable-in-conduit conductor (CICC). Supercritical helium was used in the CICC instead of liquid helium. This change was motivated by the heat transfer experiments done by Henry Kolm (connect) in the 1960s at MIT (?). The experiments demonstrated that helium in a supercritical phase is an excellent heat transfer fluid. It provided more thermal stability than liquid helium and therefore permited higher current density. It behaves like a very dense gas, or a very compressible fluid, incapable of bubble formation or cavitation, and had a low viscosity. The supercritical helium was placed within the conduit, in the spaces between the superconducting strands. The superconducting material used to make the strands within the conduit was an alloy made of Niobium and Tin (Nb3Sn) or Niobium-Titanium (NbTi). Both alloys are brittle and thus a strong conduit was needed to protect them. An iron-nickel base alloy, that was developed jointly by Ronald G. Ballinger, MIT, (?) and the International Nickel Company, was used. It was called Incoloy 908 and was found to be very stable with large temperature shifts, able to withstand cryogenic temperatures, and had very low coefficient of thermal expansion similar to  that of  the superconducting strands. (?)    The CICC coils were designed to be used in the demanding environment associated with the operation of  magnetically confined thermonuclear plasma. The power of the magnetic field required to confine the plasma is extreme. The CICCs were found to be able to support high electrical currents and thereby produce large magnetic fields while maintaining energy losses at a minimum. The magnet coils constructed with CICCs have been characterized by good structural integrity, high electro-thermal stability, and strong resistance to high voltages.(?) The CICC conductor design was accepted as the baseline for the ITER magnet design. Following the death of Hoenig in 1991 the project continued under the leadership of Joseph J. Minervini. In 2000, a 150-ton magnet, using CICC, was successfully tested in Japan. It produced a magnetic field of 13 tesla (about 260 thousand times more powerful than the earth’s magnetic field) with a stored energy of 640 megajoules at a current of 46,000 amperes (about three thousand times the current handled by typical household wiring). The purpose of this large magnet was to demonstrate superconducting performance parameters and manufacturing methods for the larger magnets planned for the International Thermonuclear Experimental Reactor (ITER). ITER goals include demonstrating the feasibility of nuclear fusion as an energy source. The ITER magnets will provide the magnetic fields needed to initiate and sustain the plasma necessary for the fusion reaction.