Magnetic Determination of the Current Center Line for the Superconducting ITER Toroidal Field Coils: Results on a Double-Pancake Prototype

In the context of the ITER project, the winding geometry of the large-scale magnetic field coils has to be validated. A geometrical survey during manufacturing is combined with a magnetic survey of finished assemblies. We are developing a fluxmeter system, which is capable of sampling magnetic flux density values (or gradients thereof) at many locations distributed around the assembly. The measurement method uses low-frequency (300 mHz-1.2 Hz) ac excitation of the coil under study and acquires induced voltage values sensed by magnetic flux pickup coils. The sampling strategy leverages on the use of calibrated arrays of coils mounted on rigid supports. The supports, whose positions in space are measured with a laser tracker, carry 48 distinct coils and are placed at several locations around the perimeter of the magnet under study. Maps of induced voltage values, corresponding to time-varying magnetic flux, are recorded. In this contribution, we present experimental results obtained on the first available European double-pancake prototype (12 turns) for the toroidal field coils and compare the results with feedforward computation using magnetostatics. At 300 mHz, the global relative agreement between measured and computed signal amplitude is in the % range. It is necessary to take into account the knowledge of the actual shape of the 12-turn current filament.