  A fundamental limit to the application of Time Projection Chambers (TPCs) in
high-rate experiments is the accumulation of slowly drifting ions in the active
gas volume, which compromises the homogeneity of the drift field and hence the
detector resolution. Conventionally, this problem is overcome by the use of
ion-gating structures. This method, however, introduces large dead times and
restricts trigger rates to a few hundred per second. The ion gate can be
eliminated from the setup by the use of Gas Electron Multiplier (GEM) foils for
gas amplification, which intrinsically suppress the backflow of ions. This
makes the continuous operation of a TPC at high rates feasible.
  In this work, Monte Carlo simulations of the buildup of ion space charge in a
GEM-based TPC and the correction of the resulting drift distortions are
discussed, based on realistic numbers for the ion backflow in a triple-GEM
amplification stack. A TPC in the future PANDA experiment at FAIR, in which
antiproton-proton interaction rates up to 2 x 10^7 per second will be reached,
serves as an example for the experimental environment. The simulations show
that space charge densities up to 65 fC per cubic cm are reached, leading to
electron drift distortions of up to 10 mm. The application of a laser
calibration system to correct these distortions is investigated. Based on full
simulations of the detector physics and response, we show that it is possible
to correct for the drift distortions and to maintain the good momentum
resolution of the GEM-TPC.
