Use of a Bent Crystal with a Decreasing Curvature to Increase the   Efficiency of the Extraction and Collimation of a Beam in an Accelerator
  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. 
I. A. Yazynin, V. A. Maisheev, and Yu. A. Chesnokov
physics.acc-ph