Micromegas TPC for future colliders

Transparents - Groupe ILCA MPGD (Micro-Pattern Gas Detector) TPC is a leading candidate for the main tracker of the future electron-positron International Linear Collider experiment, for which excellent 2-track separation and transverse spatial resolution are needed. Since more than five years a collaboration led by DAPNIA-CEA is studying a MICROMEGAS (MICRO-MEsh GAS detector) read-out for this TPC. First, in collaboration with LAL-Orsay, many studies have been achieved, with various small Micromegas devices and electron sources (X-ray sources and gun, beta, photo-electrons), on gas properties (drift velocity, diffusion, gain), ion feedback, aging, behaviour in the magnetic field, etc... Among other goodies, a natural suppression of the positive ion feedback at the level of a few per mil has been demonstrated; this is a key point in view of the operation in the high-background environment of the ILC, where the space charge would lead to large electric field distortions. Then a collaboration between Saclay, LAL-Orsay and LBL-Berkeley has built in 2003 a large TPC prototype (50 cm in diameter and length, 1000 channels) in order to detect and reconstruct cosmic ray tracks in a 2T superconducting magnetic field. Analysis is near completion. Finally, in 2005, a large collaboration between laboratories from France, Germany, Canada, Japan and Philippines, has exposed two medium-size TPCs, with a few hundred channels, to a 4GeV/c pion beam at KEK, in two steps: first with a classical Micromegas read-out, then with a Micromegas and a resistive foil (developed by Carleton University, Ottawa) on the anode, in order to spread the signal on the pad plane and benefit from the potentially excellent spatial resolution. A very preliminary result from the last data taken in October 2005 gives a transverse resolution of approximately 40_m for zero-drift distance, so the point resolution for a 6 mm long pad at 4T-magnetic field is expected to be better than 100mm for the maximum drift distance of 250cm