Crystal slow extraction of positrons from the Frascati da φ NE collider

The slow high-efficiency extraction from a ring positron accelerator (SHERPA) project's aim is to develop an efficient technique to extract a positron beam from one of the accelerator rings composing the DAφNE complex at the Frascati National Laboratory of INFN, setting up a new beam line able to deliver positron spills of O(ms) length, excellent beam energy spread and emittance. The most common approach to slowly extract from a ring is to increase betatron oscillations approaching the third order tune resonance to gradually eject particles from the circulating beam. SHERPA proposes a paradigm change for lepton machines using coherent processes in bent crystals to kick out positrons from the ring, a cheaper and less complex alternative. A description of this innovative nonresonant extraction technique is reported in this manuscript, including its performance preliminary estimation.