Status of the project for a positron laboratory at ELI-NP

We report on the plans of a positron laboratory to be built at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility. The slow e+ source is based on e−–e+ pair production in a converter made of tungsten foils by a brilliant gamma beam (Eγ<3.5 MeV, Iγ = 2.4×1010 γ s-1) which will be generated by Compton back–scattering of photons from a high power laser on electrons from a LINAC. Numerical simulations of γ–ray interactions with the designed converter showed that, if the tungsten foils are used for moderation of the created fast e+, a slow e+ beam of an intensity of ~1×106 s-1 can be obtained. Circular polarization of the γ-beam is proposed to be one of the ELI-NP's upgrades. The slow e+ will be extracted perpendicularly to the γ–beam and will have ~ 30% transversal polarization degree. By using a transmission re-moderation stage and electrostatically turning the re–moderated beam by 90°, the transversal polarization will be changed to longitudinal. Longitudinal polarization can then be preserved in the longitudinal magnetic guiding field. The heat load to the converter was estimated at < 2 mW which implies that e+ moderation by frozen neon is applicable.