  A cylindrical GEM detector is under development, to serve as an upgraded
inner tracker at the BESIII spectrometer. It will consist of three layers of
cylindrically-shaped triple GEMs surrounding the interaction point. The
experiment is taking data at the e+e- collider BEPCII in Beijing (China) and
the GEM tracker will be installed in 2018. Tests on the performances of triple
GEMs in strong magnetic field have been run by means of the muon beam available
in the H4 line of SPS (CERN) with both planar chambers and the first
cylindrical prototype. Efficiencies and resolutions have been evaluated using
different gains, gas mixtures, with and without magnetic field. The obtained
efficiency is 97-98% on single coordinate view, in many operational
arrangements. The spatial resolution for planar GEMs has been evaluated with
two different algorithms for the position determination: the charge centroid
and the micro time projection chamber (mu-TPC) methods. The two modes are
complementary and are able to cope with the asymmetry of the electron avalanche
when running in magnetic field, and with non-orthogonal incident tracks. With
the charge centroid, a resolution lower than 100 micron has been reached
without magnetic field and lower than 200 micron with a magnetic field up to 1
T. The mu-TPC mode showed to be able to improve those results. In the first
beam test with the cylindrical prototype, the detector had a very good
stability under different voltage configurations and particle intensities. The
resolution evaluation is in progress.
