The SHERPA project: Bent crystal-assisted beam extraction simulations

In this article, we report on the SHERPA project, aiming at developing an efficient technique to extract a positron beam from one of the rings of the LNF DAΦNE collider, creating O(ms) long pulses. The most common slow extraction method is the resonant technique: after having created an unstable region in phase space, particles are gradually extracted from the circulating beam using a combination of electrostatic and magnetic septa. Instead, SHERPA proposes to use coherent processes in bent crystals, a cheaper and less complex alternative. This non-resonant technique, already used in hadron accelerators, will provide continuous multi-turn extraction with high efficiency. Various Geant4 simulations were carried out to study the channeling properties of positrons below GeV, in preparation for the first beam tests of the crystals produced for SHERPA. For positrons in this energy region, no experimental data exist on crystal channeling. Validation of the Geant4 simulations was obtained using a combination of analytical theoretical equations and other Monte Carlo simulations.