Characterization of the second generation prototype chamber for ME0 in the endcap muon system for the CMS phase-II upgrade

The CMS experiment is a general-purpose detector installed in the Large Hadron Collider (LHC) at CERN. During the High Luminosity LHC phase, the luminosity is expected to increase by a factor of 10 compared to the LHC design value. The forward region of the CMS muon system will be equipped with 3 additional triple GEM-based muon stations, where GEM stands for Gas Electron Multiplier. The three stations, in order of distance from the interaction point, are called ME0, GE1/1, and GE2/1. The ME0 station, where ME stands for Muon Endcap, is located just behind the new endcap calorimeter, where the background particle flux can reach up to 150 kHz/cm$^{2}$. Recent studies of rate capability and gain drop resulted in a design change in the segmentation of the high voltage (HV) distribution for the GEM foils. The second-generation ME0 prototype has radial segments in contrast to the (approximate) segmentation in pseudorapidity employed for GE1/1 and GE2/1. In this report, we describe the mechanical design of the second-generation prototype, the simulation of the background counting rate in the HV segments, and the measurement of the energy spectrum, the effective gain, and the response uniformity.