Micromegas TPC studies at high magnetic fields using the charge dispersion signal

The International Linear Collider (ILC) Time Projection Chamber (TPC) transverse space-point resolution goal is 100μm for all tracks including stiff 90∘90∘ tracks with the full ∼2m drift. A Micro-Pattern Gas Detector (MPGD) readout TPC can achieve the target resolution with existing techniques using 1 mm or narrower pads at the expense of increased detector cost and complexity. The new MPGD readout technique of charge dispersion can achieve good resolution without resorting to narrow pads. This has been demonstrated previously for 2mm×6mm pads with GEMs and Micromegas in cosmic ray tests and in a KEK beam test in a 1 T magnet. We have recently tested a Micromegas-TPC using the charge dispersion readout concept in a high field super-conducting magnet at DESY. The measured Micromegas gain was found to be constant within 0.5% for magnetic fields up to 5 T. With the strong suppression of transverse diffusion at high magnetic fields, we measure a flat 50μm resolution at 5 T over the full 15 cm drift length of our prototype TPC.