# Current Status and Future Developments of Micromegas Detectors for Physics and Applications
## 6 Micromegas Readout of TPCs
### ILC-TPC  
The ILC is a project for an e\({}^{+}\)e\({}^{-}\) collider at a center-of-mass energy of 250 GeV, extendable to 500 GeV and 1 TeV [76] One of the two detector concepts (ILD) for ILC assigns the intermediate tracking to a TPC. The main requirements are the following: a track separation of <3 mm; a momentum resolution ten times better than at LEP, translating into a point resolution of 100 \(\upmu\)m per pad-row at all drift distances; a low ion back-flow in the drift region; a working gas with low hydrogen content to minimize the sensitivity to the neutron background; and operation under a magnetic field of \(\sim\)3.5 T [76]. Micromegas are ideal for this use, as they are unaffected by magnetic fields and have a robust design with a low ion back-flow and high gain and resolution. Several readout technologies for a cylindrical TPC with a radius of 1.8 m are considered for ILD: resistive Micromegas, GEMs and GridPix. They are tested at the DESY II test-beam facility in a 1 T magnet [77]. The facility includes a field cage with a diameter of 72 cm and a length of 60 cm. The resistive Micromegas option evolved throughout 12 test campaigns that had been conducted since 2008. A single module had 24 rows of 72 pads with the dimensions 3 mm \(\times\) 7 mm. The readout, based on the AFTER chip developed for T2K, was integrated in 2011-2013 into front-end cards parallel to the module, allowing the connection of 2000 channels with a HV cable for Micromegas, a low-voltage cable for supplying the electronics, and a double optical fibre for the extraction of the data and for setting the configuration parameters. An end-plate paved by seven modules was operated in 2014-2015, with a two-phase CO\({}_{2}\) cooling (Figure 17). The charge spreading was obtained with a continuous resistive-capacitive network made of a resistive overlay on top of an insulating film. In 2018, a new scheme was adopted: the Encapsulated Resistive Anode Micromegas (ERAM), where the mesh and the surrounding mechanics are grounded, while the resistive layer is set at a positive high voltage. This presents the advantage of minimizing the distortions and provides more flexibility in the high voltage setting of the modules, as the equipotentiality of the end-plate being ensured by the grounding of the mesh. The required performances are met: a 70-\(\upmu\)m resolution at zero drift distance, which extrapolates to 100 \(\upmu\)m at 2 m of drift in a magnetic field of 3.5 T and a 5% resolution on the dE/dx for a track crossing the TPC in the transverse plane. The ion back-flow must be very well controlled to preserve the performances. Ions are copiously produced in the amplification gap. A fraction of them will flow back to the drift space and add to the primary ions produced by the charged tracks and converted X-rays. Ions drifting in the electric field of a TPC (a few 100 V/cm) are very slow, on the order of a m/s. As a consequence, this positive charge accumulates and gives rise to a  
Figure 16: The MINOS TPC. space charge. As the region where this space charge builds up is limited, and as the density is non-uniform, this produces transverse E field components that give rise to distortions enhanced by E\(\times\)B effects. There is a natural ion back-flow suppression in Micromegas, due to the electric field configuration: as the field in the amplification region is much larger, by a factor of 100 to 1000 than in the drift region, the field lines are squeezed by a factor equal to the field ratio into a small tube when passing through the holes of the mesh. The ionization electrons undergo diffusion in the gas during the avalanche, escaping the small tube. The ions produced in the avalanche are drifted in the opposite direction, along field lines which, for most of them, return to the mesh where the ions are neutralized. Typical ion back-flow suppression factors are \(10^{2}\) to \(2\times 10^{3}\)[62], whereas the ion back-flow in wire chambers is typically 10-30%.