  Zeeman deceleration is an experimental technique in which inhomogeneous,
time-dependent magnetic fields generated inside an array of solenoid coils are
used to manipulate the velocity of a supersonic beam. A 12-stage Zeeman
decelerator has been built and characterized using hydrogen atoms as a test
system. The instrument has several original features including the possibility
to replace each deceleration coil individually. In this article, we give a
detailed description of the experimental setup, and illustrate its performance.
We demonstrate that the overall acceptance in a Zeeman decelerator can be
significantly increased with only minor changes to the setup itself. This is
achieved by applying a rather low, anti-parallel magnetic field in one of the
solenoid coils that forms a temporally varying quadrupole field, and improves
particle confinement in the transverse direction. The results are reproduced by
three-dimensional numerical particle trajectory simulations thus allowing for a
rigorous analysis of the experimental data. The findings suggest the use of a
modified coil configuration to improve transverse focusing during the
deceleration process.
