  The NIKA2 polarization channel at 260 GHz (1.15 mm) has been proposed
primarily to observe galactic star-forming regions and probe the critical
scales between 0.01-0.05 pc at which magnetic field lines may channel the
matter of interstellar filaments into growing dense cores. The NIKA2
polarimeter consists of a room temperature continuously rotating multi-mesh HWP
and a cold polarizer that separates the two orthogonal polarizations onto two
260 GHz KIDs arrays. We describe in this paper the preliminary results obtained
during the most recent commissioning campaign performed in December 2018. We
concentrate here on the analysis of the extended sources, while the observation
of compact sources is presented in a companion paper [12]. We present
preliminary NIKA2 polarization maps of the Crab nebula. We find that the
integrated polarization intensity flux measured by NIKA2 is consistent with
expectations.In terms of polarization angle, we are still limited by systematic
uncertainties that will be further investigated in the forthcoming
commissioning campaigns.
