  It has been shown that the use of a bent crystal with a variable curvature
radius makes it possible to reduce the fraction of dechanneled particles by an
order of magnitude. This effect enables the strong reduction of the particle
density at the edge of a collimator or the partition of a septum upon the
multiturn extraction of a beam from a ring accelerator. In particular, the beam
extraction efficiency at the U-70 synchrotron and Large Hadron Collider can be
increased to 95 and 99.65%, respectively.
