3D Analysis of magnetic field lines to assess the impact of stray fields at breakdown in ITER

Plasma initiation in ITER will be critical, since low electric field will be available, and a reduction of field lines connection length may occur due to the presence of stray fields. A number of studies have been performed to assess the impact of different stray field sources, but they mostly use equivalent 2D models to assess effects of intrinsically 3D fields, or were performed on other machines run in "ITER-relevant" modes. Magnet tolerances will provide local contributions to stray field at breakdown in the order of some mT. The toroidally averaged figures are much smaller (in the order of fractions of mT), but the impact of localized field map deformations on the connection lengths may provide deeper understanding of ITER behavior at breakdown. In this paper, starting from a waveform scenario optimized using an axisymmetric model for nominal coils and vessel, a full 3D numerical model is used to analyze the contribution to the magnetic field in the breakdown region generated by tolerances on the toroidal field coils, and the effects of stray fields on the connection lengths are analyzed.