  We study dipolar relaxation in both ultra-cold thermal and Bose-condensed
chromium atom gases. We show three different ways to control dipolar
relaxation, making use of either a static magnetic field, an oscillatory
magnetic field, or an optical lattice to reduce the dimensionality of the gas
from 3D to 2D. Although dipolar relaxation generally increases as a function of
a static magnetic field intensity, we find a range of non-zero magnetic field
intensities where dipolar relaxation is strongly reduced. We use this resonant
reduction to accurately determine the S=6 scattering length of chromium atoms:
$a_6 = 103 \pm 4 a_0$. We compare this new measurement to another new
determination of $a_6$, which we perform by analysing the precise spectroscopy
of a Feshbach resonance in d-wave collisions, yielding $a_6 = 102.5 \pm 0.4
a_0$. These two measurements provide by far the most precise determination of
$a_6$ to date. We then show that, although dipolar interactions are long-range
interactions, dipolar relaxation only involves the incoming partial wave $l=0$
for large enough magnetic field intensities, which has interesting consequences
on the stability of dipolar Fermi gases. We then study ultra-cold chromium
gases in a 1D optical lattice resulting in a collection of independent 2D
gases. We show that dipolar relaxation is modified when the atoms collide in
reduced dimensionality at low magnetic field intensities, and that the
corresponding dipolar relaxation rate parameter is reduced by a factor up to 7
compared to the 3D case. Finally, we study dipolar relaxation in presence of
radio-frequency (rf) oscillating magnetic fields, and we show that both the
output channel energy and the transition amplitude can be controlled by means
of rf frequency and Rabi frequency.
