How to Use Artificial Gravity in Spacecraft?

*Setting: a space station in orbit of Earth*<br>*Context: the space station is a training facility that the astronauts will stay in for two weeks before taking a months long trip to Mars.  The space station is also a  training facility for scientists, engineers and medical researchers.  It is 12 km long and 8 km wide.*<br>*Question:  The space station spins to create artificial gravity.  My concern is that the farther out from the center you go, the stronger the artificial gravity is, and the closer you get to the center, the weaker the gravity is.  If you're in the center of the space station, you'll be in microgravity.  What can be put in the center of the space station to mitigate the differences in gravity between the center and the rim?*