  The efficiency of evaporative cooling, which is used for the creation of a
Bose Einstein condensate, depends strongly on the number of particles at the
start of the evaporation. A high efficiency can be reached by filling the
magneto-optical trap with a large number of atoms and subsequently transferring
these atoms to the magnetic trap as efficiently as possible. In our case (for
sodium) this efficiency is limited to 1/3, because the magnetic substates of
the F = 1 state, which is used in the trapping process, are equally populated.
This limit can be overcome by spin-polarizing the sample before the transfer.
For sodium atoms, however, the improvement is very small when it is done in a
small magnetic field due to the large number of optical transitions in
combination with the high optical density. In this article we describe
spin-polarizing sodium atoms in a high magnetic field. The transfer efficiency
is increased by a factor of 2. The high magnetic field makes the process also
more robust against variations in the magnetic field, the laser frequency and
the polarization of the laser beam.
