Simulation of coil with core


I’m new to use Ngsolve and I would like to simulate a coil with core to calculated force in 3D.

I began by taking the file and modify it to simulate the coil and core with linear materials for the moment.

I obtain the result in copy that is fieldlines that are not concentric and I ask me why because all my problem is symetric with Z axis.

Maybe somebody could help me to understand where is my mistake?