# Current Status and Future Developments of Micromegas Detectors for Physics and Applications
## 6 Micromegas Readout of TPCs
### Micromegas Readout TPCs for Long Baseline Neutrino Experiments  
The first worldwide large scale TPCs equipped with a Micromegas readout are those of the near detector of the Tokai to Kamioka (T2K) long baseline neutrino experiment. T2K is an experiment in Japan to study neutrino oscillation from artificially emitted neutrinos. The goal of this experiment is the measurement of the mixing parameters between different lepton flavors, which may lead to new insights of leptonic CP-violation [67]. The composition of a neutrino beam is measured at 280 m from the target in the Near detector (ND280) and again at a distance of 295 km in the Super Kamiokande detector at Kamioka. The ND280 Near Detector measures the neutrino differential rate before the oscillation, providing a measurement of neutrino flux and a neutrino-nucleus interaction cross-section. The neutrinos perform Charged Current Quasi-Elastic (CCQE) [68] interactions in the scintillator targets composing the Fine Grained Detectors (FGD). The momentum of the charged lepton produced by these interactions is then measured in the TPCs under a 0.2-T magnetic field. In order to constrain the uncertainty on the neutrino differential rate at about 4-5%, the TPCs must provide good particle identification performances for muons versus electrons with a constraint of about 10% on the dE/dx and on the lepton momentum resolutions. The later requirement translates in a requirement of 0.7 mm spatial resolution for the track curvature measurement. The TPCs and their performances are described in [69]. The TPCs are filled with the so-called \"T2K gas\" mixture, argon (95%)/CF\({}_{4}\) (3%)/iC\({}_{4}\)H\({}_{10}\) (2%), which combines low ( 300 \(\upmu\)m/\(\sqrt{\text{cm}}\) ) transverse and longitudinal diffusion and a saturated electron drift velocity of 7.8 cm/\(\upmu\)s for a drift electric field of 270 V/cm. The 1726 pads of 70 mm\({}^{2}\) segmenting the 34 \(\times\) 36 cm\({}^{2}\) of the bulk-Micromegas modules were scanned with a \({}^{55}\)Fe X-ray source on a dedicated test bench to measure the gain and energy resolution at 5.9 keV. The results show an excellent uniformity of response at a level of 3% [70]. Up to now, after 10 years of operation, all the 72 bulk-Micromegas modules have been performing smoothly with a very low level of failures (less than 0.1 per mil dead channels) and without any degradation of the performances, proving the reliability and robustness of the technology. The TPCs fulfill the required performances: an 8% dE/dx resolution and a track reconstruction with a resolution better than 600 \(\upmu\)m.  
For the phase-2 of the T2K experiment, two new TPCs, called High-Angle TPCs, are under construction and will be installed in the ND280 detector in 2022. These new TPCs will be equipped with larger Micromegas modules (34 \(\times\) 42 cm\({}^{2}\)) but with larger 110 mm\({}^{2}\) pads, using a new resistive anode scheme called ERAM (Encapsulated Resistive Anode Micromegas) described in Figure 14a first introduced and extensively studied by the ILC-TPC collaboration [71]. A low resistivity Diamond-Like Carbon (DLC) layer that is a few tens of nanometers thick is deposited on top of a 50 \(\upmu\)m APICAL insulated foil glued on top of the segmented anode Printed Circuit Board. The avalanche charge is spread over few pads due to the RC coupling to improve the spatial resolution. R is the DLC surface resistivity, and C is the capacitance between the DLC layer and the anode pads). The lower the RC constant, the larger this spreading is.