Eddy current and thermal analysis of the TF model coil during safety discharges

The Toroidal Field Model Coil (TFMC) is being manufactured by European industry since 1997 in the frame of the International Thermonuclear Experimental Reactor (ITER) project. It is representative of the toroidal field system of a large tokamak. About 800 m of Nb3Sn cable-in-conduit has been used to build this coil which will be tested at Karlsruhe (Germany) in the TOSKA test stand facility. The TFMC can achieve magnetic field values between 7.8 T alone and 9 T with the adjacent LCT coil of TOSKA. The 10 pancakes of the TFMC are inserted between stainless steel radial plates, and the whole winding pack is enclosed in a thick stainless steel case. These passive conductive structures are expected to experience severe eddy currents during the last testing phase of the TFMC, when triggering rapid discharges to study the electromagnetic and thermohydraulic transient behavior of the coil. Using a finite element code, CORFOU, the safety rapid-discharge from current levels up to 80 kA in the 98 turns (time constant 4 s), and another even faster discharge (time constant 58 ms), aimed at loading the coil to 10 kV, are modeled. The eddy currents induced in the passive structures are computed with the thin shell approach and validated with a simple analytical model. The Joule power, the total Joule heating and the temperature increase in the radial plates are deduced. The thermal analysis is carried out with a 1D model (finite difference method): the heat propagation through insulation to the cable and to the helium is studied. The possibility of a quench of the whole magnet is discussed.