  Magnetron sputtering discharges feature complex magnetic field configurations
to confine the electrons close to the cathode surface. This magnetic field
configuration gives rise to a strong electron drift in azimuthal direction,
with typical drift velocities on the order of \SI{100}{\kilo\meter\per\second}.
In high power impulse magnetron sputtering (HiPIMS) plasmas, the ions have also
been observed to follow the movement of electrons with velocities of a few
\si{\kilo\meter\per\second}, despite being unmagnetized. In this work, we
report on measurements of the azimuthal ion velocity using spatially resolved
optical emission spectroscopy, allowing for a more direct measurement compared
to experiments performed using mass spectrometry. The azimuthal ion velocities
increase with target distance, peaking at about
\SI{1.55}{\kilo\meter\per\second} for argon ions and
\SI{1.25}{\kilo\meter\per\second} for titanium ions. Titanium neutrals are also
found to follow the azimuthal ion movement which is explained with resonant
charge exchange collisions. The experiments are then compared to a simple
test-particle simulation of the titanium ion movement, yielding good agreement
to the experiments when only considering the momentum transfer from electrons
to ions via Coulomb collisions as the only source of acceleration in azimuthal
direction. Based on these results, we propose this momentum transfer as the
primary source for ion acceleration in azimuthal direction.
