# Current Status and Future Developments of Micromegas Detectors for Physics and Applications
## 3 Micromegas as a Tracker in CLAS12 and COMPASS
### Compass  
Micromegas are used as large-area tracking detectors in the COMPASS experiment at CERN [42]. The COMPASS experiment is a fixed target experiment with 200 GeV muon or pion beams from the SPS accelerator [43]. It was designed to perform hadron structure and spectroscopy studies. During recent years, the experiments focused particularly on spin structure investigations using the Drell-Yan process [44] and on the proton (GPD) measurements based on Deep Virtual Compton Scattering (DVCS) processes [45]. Micromegas are used as a small angle tracker for particles in the beam direction.  
The 4-um thin nickel mesh of the first detectors was stretched and glued to a replaceable frame. Twelve Micromegas detectors were installed, with an active area of 40 \(\times\) 40 cm\({}^{2}\) covered by a strip readout with a pitch of 360 um on the central part and 420 um on the sides. The tracker reached a spatial resolution of less than 100 um and operated under a maximal particle rate of 90 kHz per strip [46; 47]. Micromegas are ideal detectors for these conditions, as they have a low material budget to minimize additional particle scattering compared to solid-state tracking detectors.  
COMPASS Micromegas uses a gas mixture of neon (80%)/ethane (10%)/CF\({}_{4}\) (10%). The addition of CF\({}_{4}\) increases the drift velocity, reducing the collection time of all charges, and, per definition, the Time-Over-Threshold (TOT). A short TOT is important to reduce the detector occupancy and to improve the operation in higher particle fluxes [46]. However,  
Figure 8: The CLAS12 Micromegas detectors. (**a**) Cut view of the CLAS12 Micromegas Vertex Tracker. The inner silicon detectors are not shown. The red dot highlights the position of the target. (**b**) Photograph of a full stack of six BMT layers mounted on the support structure and with the gas plumbing connected. (**c**) The CLAS12 Forward Tagger Tracker.  
the proportion of CF\({}_{4}\) was reduced to 5% in hadron beam conditions in order to reduce the too high sparking rate, which appears in such conditions.  
An upgraded version of the COMPASS experiment was launched in 2012 [48]. New Micromegas produced with the bulk technology and with a pixelated readout were developed, read by APV-based electronic cards [49]. Spark rates in hadron beam conditions were a limiting factor for the initial Micromegas detectors, leading to a reduction of the gain in such conditions. The sparking rate was strongly reduced by a factor larger than ten on the new detectors by adding a GEM foil [18] 2 mm above the micromesh, which acts as a preamplification stage [50].  
The bulk Micromegas improved the robustness, and the detectors were designed for a high gas tightness. A 5-cm wide pixelated readout covered the central area of the planes where the beam passes through (see Figure 9) and which was not active on the former detectors. These planes are composed of 640 2.5 \(\times\) 0.4 mm\({}^{2}\) pixels in the center surrounded by 640 larger 6.25 \(\times\) 0.4 mm\({}^{2}\) pixels. Such a solution was chosen to reduce the particle-flux per readout channel, with a maximum rate of 200 kHz/channel [51]. Very good performance was obtained with these new detectors with a spatial resolutions of 70 um and a time resolution lower than 10 ns in nominal beam conditions [52].  
Figure 9: Scheme of the Compass pixelated Micromegas plane. The left figure shows the detail of the pixelated area in the center of the plane. On the right figure, the red lines are the readout strips on the top side of the board, and the blue ones are the strips used to extract the pixel signals on the bottom side.