Charge Transfer of GEM Structures in High Magnetic Fields

We report on measurements of a triple GEM (Gas Electron Multiplier) 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 irraditated with a 55 Fe source. The measurements are parametrised using a functional depen- dence on the electric setup 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 setup leading to minimum ion backdrift. Applying this setup, ion backdrift of only 2.5 permille is achieved. Also the use of MHSPs (Micro Hole Strip Plates) for ion backdrift reduction is investigated. Setting an optimised negative strip voltage, a suppression factor of approximately 4 is reached at 4 T magnetic field. Additionally, the width of the charge cloud of individual 55 Fe photons is measured using a fine segmented strip readout after the triple GEM structure. Charge widths between 0.2 and 0.3 mm RMS are observed, which appear to be dominated by the diffusion in the space between the individual GEMs. This charge broadening is partly suppressed at high magnetic fields.