Abstract: The manipulation of massive components under tight tolerances presents a formidable challenge. Whilst reference to millimetres can seem commonplace, the realization that these targets must be satisfied over long distances and with very large components, such ITER’s 17 meter high, 360 tonne D-shaped Toroidal Field magnets, places this alignment challenge firmly in the world of precision engineering.
The need for such an alignment precision for ITER’s magnets comes from physics. The magnets produce the magnetic field to confine the plasma and this needs to be as close to perfectly symmetric as possible. ITER is equipped with a set of error field correction coils to correct for such inaccuracies, but their capabilities are limited. Limitations on the number and current capability of these coils restrict corrections to the first three toroidal harmonics and with a limited magnitude. It follows thus that asymmetries introduced during the manufacturing and assembly process must be kept as low as possible and mandatorily within the correction coils’ capabilities.
The impact of component and manufacturing inaccuracies are assessed via a series of large scale Monte Carlo simulations. These simulations propagate uncertainties from assembly tolerances to physics limits. Taking an iterative approach, we show how multiple Monte Carlo simulations are used to tune assembly tolerances applied to each component. The result is a coherent assembly strategy for the Toroidal Field coils, Poloidal Field coils, the Central Solenoid and the first wall. We demonstrate how trade-offs may be made between tolerances placed on various components. One such trade-off links the TF coils to the first wall. Here, an increased alignment tolerance is applied to the TF coils to relax the first wall alignment tolerance necessary to limit start-up heat loads.
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