Optimization of the Gas Flow in a GEM Tracker with COMSOL

In this paper we report results of a fluid-dynamics performance study of Gas Electron Multiplier (GEM) detector. The GEM technology has been proven to tolerate a rate larger than 50 MHz/cm2 without noticeable aging and to provide the sub-millimeter resolution on working chambers up to 45x45 cm2. A new GEM based tracker is under development for the upgrade of the Hall A equipment at Jefferson Lab. The chambers of the tracker have been designed in a modular way: each chamber consists of 3 adjacent GEM modules, with an active area of 40x50 cm2 each. We optimized the gas flow inside the GEM module volume, a mixture of Ar/CO2 (70/30), using a COMSOL code. Our simulation includes design of the inlet-outlet pipes, maximization of the uniformity of the gas flux and minimization of the zones where such flux is too low; the result for the selected configuration is represented in figure 1. We have optimized the procedures for the assembly of the GEM foils and designed a mechanical system (TENDIGEM figure 2) that will be used to stretch the GEM foils at the proper tension (few kg/cm); the TENDIGEM is based on the original design developed at the LNF.INGLESE