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The electrical behavior of the ITER central solenoid model coil (CSMC) exposed to the voltage rise occurring during a safety discharge initiated by a counter acting current switch has been studied. A detailed network model has been set up to determine the transient overvoltages inside the windings of the CSMC when the coil is subjected to transient voltages. The analysis takes into account the frequency-dependent resistance of the conductor in case of high-frequency oscillations, and considers the influence of the extensive instrumentation cables of the coil. The model's accuracy is demonstrated over a frequency range up to 30 Khz. The total inductance and capacitance of the coil model are in very good agreement with previously obtained measurements and design values. The discharge circuit has also been modeled in order to accurately simulate the discharge process. It was found that the terminal voltage generated during a safety discharge causes transient oscillations inside the windings. However, they do not cause overvoltages exceeding the maximum acceptable insulation stress. The influence of several parameters of the discharge circuit on the rise time and shape of the resulting terminal voltage was investigated. Controlling these values might be a measure to prevent excessive internal oscillations for larger coils with lower natural frequencies than the CSMC.

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In a phase II experiment on the International Thermonuclear Experimental Reactor (ITER) toroidal field model coil (TFMC) the operation limits of its 80 kA Nb 3 Sn conductor were explored.To increase the magnetic field on the conductor, the TFMC was tested in the presence of another large coil: the Euratom LCT coil. Under these conditions the maximum field reached on the conductor was around 10 T.This exploration has been performed at constant current, by progressively increasing the coil temperature and monitoring the coil voltage drop in the current sharing regime.Such an operation was made possible thanks to the very high stability of the conductor.The aim of these tests was to compare the critical properties of the conductor with expectations and to assess the ITER TF conductor design. These expectations are based on the documented critical field and temperature dependent properties of the 720 superconducting strands which compose the conductor.In addition the conductor properties are highly dependent on the strain, due to the compression appearing on Nb 3 Sn during the heat treatment of the pancakes and related to the difference in thermal compression between Nb 3 Sn and the stainless steel jacket. No precise model exists to predict this strain, which is therefore the main information which is expected from these tests.The method to deduce this strain from the different tests is presented, including a thermohydraulic analysis to identify the temperature of the critical point and a careful estimation of the field map across the conductor.The measured strain has been estimated in the range −0.75% to −0.79%. This information will be taken into account for ITER design and some adjustment of the ITER conductor design is under examination.