  The high-luminosity phase of the Large Hadron Collider (HL-LHC) will result
in ten times higher particle background than measured during the first phase of
LHC operation. In order to fully exploit the highly-demanding operating
conditions during HL-LHC, the Compact Muon Solenoid (CMS) Collaboration will
use Gas Electron Multiplier (GEM) detector technology. The technology will be
integrated into the innermost region of the forward muon spectrometer of CMS as
an additional muon station called GE11. The primary purpose of this auxiliary
station is to help in muon reconstruction and to control level-1 muon trigger
rates in the pseudo-rapidity region $1.6 <|\eta|<2.2$. The new station will
contain trapezoidal-shaped GEM detectors called GE11 chambers. The design of
these chambers is finalized, and the installation is in progress during the
Long Shutdown phase two (LS-2) that started in 2019. Several full-size
prototypes were built and operated successfully in various test beams at CERN.
We describe performance measurements such as gain, efficiency, and time
resolution of these prototype chambers, developed after years of R&D, and
summarize their behavior in different gas compositions as a function of the
applied voltage.
