  A large Time Projection Chamber (TPC) is the main device for tracking and
charged-particle identification in the ALICE experiment at the CERN LHC. After
the second long shutdown in 2019-2020, the LHC will deliver Pb beams colliding
at an interaction rate up to 50 kHz, which is about a factor of 100 above the
present read-out rate of the TPC. To fully exploit the LHC potential the TPC
will be upgraded based on the Gas Electron Multiplier (GEM) technology.
  A prototype of an ALICE TPC Outer Read-Out Chamber (OROC) was equipped with
twelve large-size GEM foils as amplification stage to demonstrate the
feasibility of replacing the current Multi Wire Proportional Chambers with the
new technology. With a total area of $\sim$0.76 m$^2$ it is the largest
GEM-based detector built to date. The GEM OROC was installed within a test
field cage and commissioned with radioactive sources.
