A dedicated beam tests of the full-scale prototype of gems for cms in a strong magnetic field

In the high-eta (1.6 - 2.4) region of the CMS endcap, Gas Electron Multipliers (GEM) present an interesting option for a future upgrade of the forward region of the CMS muon system. Large GEM detectors are challenging due to technological issues; in the view of the CMS upgrade we have designed and built the largest full-size triple GEM-based muon detector to-date. This prototype meets the stringent requirements of the hostile forward environment of CMS at high-luminosity LHC. Dedicated test beam measurements have been performed at the SPS in June 2011 to study efficiency, space resolution, and timing performance with different inter-electrode gap configurations and gas mixtures and in a strong magnetic field of 3T (as at CMS). Preliminary results of these experimental tests will be presented