  HiPIMS plasmas generate energetic metal ions at the substrate as a major
difference to conventional direct current magnetron sputtering. The origin of
these ions is still an open issue, which is unraveled by using two fast
diagnostics: time resolved mass spectrometry with a temporal resolution of 2
$\mu$s and phase resolved optical emission spectroscopy with a temporal
resolution of 1 $\mu$s. A power scan from dcMS-like to HiPIMS plasmas was
performed, with a 2-inch magnetron and a titanium target as sputter source and
argon as working gas. Clear differences in the transport as well in the
energetic properties of Ar$^{+}$, Ar$^{2+}$, Ti$^+$ and Ti$^{2+}$ were
observed. For discharges with highest peak power densities a high energetic
group of Ti$^+$ and Ti$^{2+}$ could be identified. A cold group of ions is
always present. It is found that hot ions are observed only, when the plasma
enters the spokes regime, which can be monitored by oscillations in the
IV-characteristics in the MHz range that are picked up by the used VI-probes.
These oscillations are correlated with the spokes phenomenon and are explained
as an amplification of the Hall current inside the spokes as hot ionization
zones. To explain the presence of energetic ions, we propose a double layer
(DL) confining the hot plasma inside a spoke: if an atom becomes ionized inside
the spokes region it is accelerated because of the DL to higher energies
whereas its energy remains unchanged if it is ionized outside. In applying this
DL model to our measurements the observed phenomena as well as several
measurements from other groups can be explained. Only if spokes and a double
layer are present the confined particles can gain enough energy to leave the
magnetic trap. We conclude from our findings that the spoke phenomenon
represents the essence of HiPIMS plasmas, explaining their good performance for
material synthesis applications.
