Ion backdrift minimisation in a GEM-based TPC readout

During the last few years, the worldwide high energy physics community has reached a consensus that the next large particle collider which will be built after the LHC (Large Hadron Collider) will be the electron-positron linear collider ILC (International Linear Collider). It will allow accelerating electrons and positrons to energies of up to 1 TeV without the great losses due to synchrotron radiation specific to accelerator rings. The linear electron-positron accelerator will complement the LHC: The hadron collider can reach the highest energies and luminosities, which makes it a machine for the discovery of new particles. A lepton collider, on the other hand, provides clean and well defined events necessary for precision measurements. Reaching the aspired accuracy over a large energy range poses a challenge to its detector. Especially the track momentum resolution, vertex reconstruction, energy flow measurement, and hermeticity have to be considerably improved compared to the LEP (Large Electron Positron Collider) detectors.One of the preferred choices for the main tracking detector of the linear collider is a time projection chamber (TPC) operated in a magnetic field of 4 T. TPCs have been successfully employed in many experiments before, for example in ALEPH and DELPHI at LEP. In these detectors, anode wires provided the gas amplification in the TPC's readout.For the tracker of the ILC detector, the use of micro-pattern devices, for example Gas Electron Multipliers (GEMs), is studied as an alternative to wires. GEM foils promise a higher spatial resolution and an intrinsic suppression of the ions produced during gas amplification. Ions drifting back into the TPC's sensitive volume would lead to a space charge compromising the homogeneity of the electric field.In the scope of the present thesis, a model of the charge transfer through multi-GEM stacks has been developed. The model, which is based on the parametrisation of measured transfer coefficients, allows an optimisation of the electrostatic parameters of the GEM readout in order to reach a minimum ion backdrift to the TPC´s drift volume. The measurements presented include the first systematic study of the operation of GEMs in magnetic fields up to 5 T with an orientation perpendicular to the foil surface. These conditions will be encountered in the tracker of the ILC detector.The results show that there is no significant drop in the charge transfer efficiency of the structures due to the high magnetic field. Using optimised settings, an ion backdrift ratio of only 2.4 per mill has been reached with a triple GEM structure in a 4 T field. It has been demonstrated that this value is independent of the effective gain within a large range. Additional studies show the effect of the minimised ion backdrift on the distortion of tracks from cosmic muons in a TPC.