The lower pole piece can be seen in position in the middle of the magnet.
The six racetrack coils are wound around a pole piece made of iron in the center and aluminum at each end.
WD is the distance between the sample surface and the lower surface of the pole piece.
British Dictionary definitions for pole piece
(electrical engineering) a piece of ferromagnetic material forming an extension of the magnetic circuit in an electric motor, etc, used to concentrate the magnetic field where it will be most effective