  Investigating the magnetic field structure in the innermost regions of
relativistic jets is fundamental to shed light on the crucial physical
processes giving rise to the jet formation, as well as to its extraordinary
radiation output up to gamma-ray energies. We study the magnetic field
structure of the quasar CTA 102 with 3 and 7 mm-VLBI polarimetric observations,
reaching an unprecedented resolution (~50 microarcsec). We also investigate the
variability and physical process occurring in the source during the observing
period which coincides with a very active state of the source till
high-energies. The Faraday rotation analysis between 3 and 7mm shows a gradient
in rotation measure with a maximum value of ~6X10^4 rad/m^2 and intrinsic
electric vector position angles (EVPAs) oriented around the centroid of the
core, suggesting the presence of large-scale helical magnetic fields. Such a
magnetic field structure is also visible in 7 mm images when a new superluminal
component is crossing the core region. The 7mm EVPAs orientation is different
when the component is exiting the core or crossing a stationary feature at ~0.1
mas. The interaction between the superluminal component and a recollimation
shock at ~0.1 mas could have triggered the multi-wavelengths flares. The
variability Doppler factor associated with such interaction is large enough to
explain the high energy emission, as we infer from the analysis of gamma-ray
and X-ray data, and it is in agreement with the Doppler factor obtained to
explain the extraordinary optical flare by Raiteri et al.(2017).
