  A constant supply of oxygen has been assumed to be necessary for the growth
of titanium nanoparticles by sputtering. This oxygen supply can arise from a
high background pressure in the vacuum system or from a purposely supplied gas.
The supply of oxygen makes it difficult to grow metallic nanoparticles of
titanium and can cause process problems by reacting with the target. We here
report that growth of titanium nanoparticles in the metallic hexagonal titanium
({\alpha}Ti) phase is possible using a pulsed hollow cathode sputter plasma and
adding a high partial pressure of helium to the process instead of trace
amounts of oxygen. The helium cools the process gas in which the nanoparticles
nucleate. This is important both for the first dimer formation and the
continued growth to a thermodynamically stable size. The parameter region where
the synthesis of nanoparticles is possible is mapped out experimentally and the
theory of the physical processes behind this process window is outlined. A
pressure limit below which no nanoparticles were produced was found at 200 Pa,
and could be attributed to a low dimer formation rate, mainly caused by a more
rapid dilution of the growth material. Nanoparticle production also disappeared
at argon gas flows above 25 sccm. In this case the main reason was identified
as a gas temperature increase within the nucleation zone, giving a too high
evaporation rate from nanoparticles (clusters) in the stage of growth from
dimers to stable nuclei. These two mechanisms are in depth explored in a
companion paper [1]. A process stability limit was also found at low argon gas
partial pressures, and could be attributed to a transition from a hollow
cathode discharge to a glow discharge.
