# Current Status and Future Developments of Micromegas Detectors for Physics and Applications
## 4 Micromegas in Atlas for HL-LHC Upgrade  
Micromegas detectors will be used in the upgrade of the ATLAS detector for the High-Luminosity Large Hadron Collider (HL-LHC) era, where detectors will face an increased particle flux and detector irradiation during operation. Micromegas is a well-suited detector technology for large-area applications with higher particle fluxes due to its rapid evacuation of positive ions. The existing forward inner part of the ATLAS Muon Spectrometer [53], the Small Wheels (SW), are expected to receive irradiation up to 15 kHz cm\({}^{-2}\), an overly high rate for the existing detectors. An upgrade, called the New Small Wheels (NSW) [15], is foreseen to replace the SWs in 2021-2022. The purpose of the NSW, placed behind the calorimeters and in front of the magnetic toroid, is to track muons and to provide a Level 1 trigger in the endcaps. Micromegas modules (MM) will be installed in the New Small Wheels as precision measurement detector, together with Small Thin Gap Chambers (sTGC) [54; 55], used as reference detectors for the L1 trigger, both providing trigger and tracking information. Each NSW consists of 16 wedge-shaped sectors with each sector consisting of 16 detection layers (eight stGC and eight MM) with the stacking order sTGC-MM-MM-sTGC in the z-direction (see Figure 10). For the Micromegas modules part, each of the eight small and eight large sectors are divided in two, leading to 4 types of modules: two inner (small and large) and two outer (small and large). Four consortia are in charge of module construction: Italy (SM1), Germany (SM2), France (LM1), and Greece/Russia (LM2). Each consortium is then committed to provide 32 modules, enclosing four readout planes of 2 to 3 m\({}^{2}\), since two modules of each type are needed to built one large and one small sector. Figure 10: NSW layout with eight large sectors on the HO side (**left**) and detail of Micromegas module stacking in one sector (**right**). To reconstruct the muon momentum with 15% resolution for an energy of 1 TeV, the main specifications of the Micromegas detectors are:  
* spatial resolution in the radial (\(\eta\)) direction, for all track angles: 100 \(\upmu\)m;
* spatial resolution on the second coordinate (\(\Phi\)): a few millimeters;
* track selection: angular resolution of 1 mrad;
* rate capability up to 15 kHz/cm\({}^{2}\); and
* no radiation aging for a period of 15 years of exploitation. As a consequence, this implies a new set of constraints on the Micromegas detectors:
* a strip shape and absolute position knowledge of the order or below 40 \(\upmu\)m;
* an absolute knowledge of all strip positions within Micromegas detectors of the order or below 60 \(\upmu\)m;
* an absolute external mechanical frame accessible at the level of 40 \(\upmu\)m; and
* a flatness detector over its full area of the order of or below \(\sim\)110 \(\upmu\)m.  
Given the constraints and physics requirements on the measurement precision for these detectors, module construction requires very strict conditions of temperature, humidity, and cleanliness. For instance, a new detector construction facility, CICLAD (Conception, Integration, and Characterization of Large Area Detectors), was set up at CEA Saclay with financial help from the French Ile de France region [56]. A Micromegas module encloses four gas gaps where a Micromegas structure is supported by five composite panels: three drift and two readout (RO). Drift panels hold the drift electrode and the mesh, glued on a mesh frame attached to the drift panel, defining a drift gap of 5 mm. Readout strips and pillars constitute both sides of the two RO panels: \(\eta\) panel, with strips in the precision direction (\(\eta\)) and stereo panel, with an angle of \(\pm\)1.5\({}^{\circ}\) between both sides to measure the azimuth angle (\(\Phi\)). Modules are assembled by stacking panels using a dedicated tooling for the alignment.