  We present a thorough analysis of a Zeeman slower for sodium atoms made of
permanent magnets in a Halbach configuration. Due to the orientation of the
magnetic field, the polarisation of the slowing laser beam cannot be purely
circular leading to optical leakages into dark states. To circumvent this
effect, we propose an atomic state preparation stage able to significantly
increase the performances of the Zeeman slower. After a careful theoretical
analysis of the problem, we experimentally implement an optical pumping stage
leading to an increase of the magneto-optical trap loading rate by 3.5. Such
method is easy to set up and could be extended to other Zeeman slower
architectures.
