Studies with a GEM-TPC prototype for the ILC: Dependencies of spatial resolution for short drift distances in a 4 T magnetic field

Time Projection Chambers (TPCs) with a micro pattern gas amplification stage are best suited for high precision tracking at a future linear collider. Among the major advantages of this technology are the vanishingly small distortions due to E x B effects. In order to prove the functionality of this concept, and to study the behavior of such TPCs in high magnetic fields, a small cylindrical TPC prototype has been equipped with two Gas Electron Multipliers (GEMs) and a micro pad readout plane. We discuss results of a test of this setup in high magnetic fields at DESY, where the prototype was placed into a solenoidal magnet with a field of up to 5.5 T. The studies include measurements of transverse and longitudinal spatial resolution using cosmic rays. Special interest is given to dependencies on drift distance, pad geometry, sampling rate, and on various cuts on angles and chi(2). We also briefly describe a new method of data analysis and show that the spatial resolution can be improved by taking only a smaller fraction of all data points. With the used gas mixture of Ar:CH4:CO2-93:5:2, transverse spatial resolutions down to 100 mu m can be achieved.