  The SHERPA project aim is to develop an efficient technique to extract a
positron beam from one of the accelerator rings composing the DAFNE 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 a tune resonance in order to
gradually eject particles from the circulating beam. SHERPA proposes a paradigm
change using coherent processes in bent crystals to kick out positrons from the
ring, a cheaper and less complex alternative. A description of this innovative
non-resonant extraction technique is reported in this manuscript, including its
performance preliminary estimation.
