  Simulation of the CERN crystal-extraction experiments at the SPS, including
the real crystal geometry and SPS parameters, as well as multiple turns and
passes, has been performed. Two crystals with different geometry have been
tried. We argue that in both cases the extraction experiment suffered from the
crystal edge imperfection. For the experimental conditions reproduced, the
efficiency of the first (twisted) crystal was found to be 12-18% (peak) in the
angular range of 140-260 microrad (FWHM), depending on the vertical beam
position at the crystal location. For the second (U-shaped) crystal the peak
efficiency was much the same, ~19%, while the angular scan FWHM reduced to 70
microrad. These results, as well as the beam profiles, are in agreement with
measurements. Finally we suggest some ways to advance this experiment.
