  Titan is unique in the solar system as it hosts a dense atmosphere mainly
made of N$_2$ and CH$_4$. Cassini-Huygens revealed the presence of an intense
atmospheric photochemistry initiated by the photo-dissociation and ionization
of N$_2$ and CH$_4$. In the upper atmosphere, Cassini detected signatures
compatible with the presence of heavily charged molecules, precursors for the
solid core of the aerosols. However, the processes coupling ion chemistry and
aerosol formation and growth are still mostly unknown. In this study, we
investigated the cation chemistry responsible for an efficient organic growth
that we observe in Titan's upper atmosphere, simulated using the PAMPRE plasma
reactor. Cation precursors were measured by in situ ion mass spectrometry in a
cold plasma and compared with INMS observations taken during the T40 flyby. A
series of positive ion measurements were performed in three CH$_4$ mixing
ratios (1%, 5% and 10%) showing a variability in ion population. Low CH$_4$
concentrations result in an abundance of amine cations such as NH$_4^+$ whereas
aliphatic compounds dominate at higher methane concentrations. In conditions of
favored tholin production, the presence of C2 compounds such as HCNH$^+$ and
C$_2$H$_5^+$ is found to be consistent with copolymeric growth structures seen
in tholin material. The observed abundance of these two ions particularly in
conditions with lower CH$_4$ amounts is consistent with modeling work
simulating aerosol growth in Titan's ionosphere, which includes mass exchange
primarily between HCNH$^+$ and C$_2$H$_5^+$ and negatively charged particles.
These results also confirm the prevalent role of C2 cations as precursors to
molecular growth and subsequent mass transfer to the charged aerosol particles
as the CH$_4$ abundance decreases towards lower altitudes.
