  Using a computer model for the crystal extraction, we investigate the
bent-crystal parameters optimal for the extraction experiment. The optimal
crystal curvature is found to be near 1 GeV/cm (for pv/R), i.e. a factor of
2--3 higher than for the crystal application in beam lines. An influence of the
accelerator optics on extraction is discussed. A possibility of using the
high-Z crystals for extraction is considered. The simulations for the ongoing
experiments at the CERN-SPS and the Fermilab Tevatron, and for the proposed
extraction at LHC, are presented.
