Vacuum Estimation of Error Field Correction on ASDEX Upgrade

A study of the vacuum magnetic field produced by the Poloidal Field (PF) coil system of ASDEX Upgrade is presented. In the model both coils and their power-supply feedthroughsare considered. The latter contribute to the error fields (EFs) that have already been observed and reported in [1, 2]. The effect on the Bfield due to PF coils, assumed to be perfectly axisym-metric, is addressed by analytical formulas, while a 1-D model is applied to feedthroughs [3].Figure 1:Bn(values in T) onq=2 sur-face, before (above) and after (below)correction.Figure 2: Virtual shell (blue) and activesaddle coils (yellow).The model is applied to a reference shot (#35352) for the computation of the normal component ofB(i.e.Bn) projected on theq=2 rational surface. The resulting EF patternis located near the feedthroughs region, with a maximumvalue close to 0.25 mT. ASDEX Upgrade is equipped with aset of 16 in-vessel saddle coils [4], that can be used to minimize the effect of EFs on plasma discharges. A preliminary correction strategy of the non-axisymmetric field generated by feedthroughs is based on a Virtual Shell Approach, that is made up by 8 upper and 8 lower saddle loops built ontheq=2 surface. The matrixM16×16of the mutual inductances between active coils and virtual shell is computed nu-merically, such thatI=M-1???, where???is the vector of the fluxes linked by the virtual shell loops and Istores the unknown currents needed for compensating those fluxes. A more detailed analysis is foreseen to include further possible sources of EFs(e.g. passive conductive structures, ferromagnetic tiles, tilting of PF coils). Moreover, MARS-F[5] can be used for calculations of plasma response to a prescribedm, nharmonic of the EFdistribution, and a new correction strategy may be applied for reducing the resulting EF pattern.