field distribution (spiral dee)

SOS.
Look at the attached picture. This is a spiral dee of a cyclotron with a central stem in the middle. The aim is to get the essential field distribution . The question is - why does not the stem affect the field distribution? (If we delete the stem, the image remains the same(and otherwise), but it shouldn't !) What do I do to make the software 'notice' that the capacity is not empty and there's this stem inside.

ps: The max. of distribution must be where there's no stem and the min. is in the center of the stem..
Thanks..