  Gamma-ray polarimetry is a new powerful tool to study the processes
responsible for the emission from astrophysical sources and the environments in
which this emission takes place. Few successful polarimetric measurements have
however been performed thus far in the gamma-ray energy band due to the
difficulties involved. POLAR is a dedicated polarimeter designed to perform
high precision measurements of the polarization of the emission from gamma-ray
burst in the 50-500 keV energy range. This new polarimeter is expected to
detect approximately 50 gamma-ray bursts per year while performing high
precision polarization measurements on approximately 10 bursts per year. The
instrument was launched into lower earth orbit as part of the second Chinese
space lab, the Tiangong-2, on September 15th 2016 and has been taking data
successfully since being switched on one week after. The instrument uses a
segmented scintillator array consisting of 1600 plastic scintillator bars, read
out by 25 flat-panel multi-anode photomultipliers, to measure the Compton
scattering angles of incoming photons. The small segmentation and relatively
large uniform effective area allow the instrument to measure the polarization
of a large number of transient events, such as gamma-ray bursts, with an
unprecedented precision during its two year life-time. The final flight model
underwent detailed calibration prior to launch as well as intensive space
qualification tests, a summary of which will be presented in this paper. The
instrument design will be discussed first followed by an overview of the
on-ground tests, finally the in-orbit behavior as measured during the first
weeks of the mission will be presented.
