  We present design and construction of a large low-mass Triple-GEM detector
prototype for forward tracking at a future Electron-Ion Collider. In this
environment, multiple scattering of forward and backward tracks must be
minimized so that electron tracks can be cleanly matched to calorimeter
clusters and so that hadron tracks can efficiently seed RICH ring
reconstruction for particle identification. Consequently, the material budget
for the forward tracking detectors is critical. The construction of the
detector builds on the mechanical foil stretching and assembly technique
pioneered by CMS for the muon endcap GEM upgrade. As an innovation, this
detector implements drift and readout electrodes on thin large foils instead of
on PCBs. These foils get stretched mechanically together with three GEM foils
in a single stack. This reduces the radiation length of the total detector
material in the active area by a factor seven from over 4% to below 0.6%. It
also aims at improving the uniformity of drift and induction gap sizes across
the detector and consequently signal response uniformity. Thin outer frames
custom-made from carbon-fiber composite material take up the tension from the
stretched foil stack and provide detector rigidity while keeping the detector
mass low. The gas volume is closed with thin aluminized polyimide foils. The
trapezoidal detector covers an azimuthal angle of 30.1 degrees and a radius
from 8 cm to 90 cm. It is read out with radial zigzag strips with pitches of
1.37 mrad at the outer radius and 4.14 mrad at the inner radius that reduce the
number of required electronics channels and associated cost while maintaining
good spatial resolution. All front-end readout electronics is located away from
the active area at the outer radius of the trapezoid.
