Charge transfer of GEM structures in high magnetic fields

We report on measurements of a triple gas electron multiplier (GEM) structure in high magnetic fields up to 5 T which were performed in the framework of the R&D work for a Time projection chamber at a future Linear Collider. The determination of charge transfer is performed using a triple GEM structure installed into a small test chamber which is irradiated with an 55Fe source. The measurements are parametrised using a functional dependence on the electric set-up which was motivated by detailed numerical simulations of a GEM using the programs MAXWELL and GARFIELD. This parametrisation of a single GEM foil is extended to a model which describes the performance of the triple GEM structure and allows to predict the parameter set-up leading to minimum ion feedback. Applying this set-up, ion feedback of only two permille is achieved. Additionally, the width of the charge cloud of individual 55Fe photons is measured using a fine segmented strip readout after the triple GEM structure. Charge widths between 0.5 mm RMS with no magnetic field and 0.3 mm at 4 T are observed.