The Gravitational Behaviour of Antihydrogen at Rest experiment - GBAR - is designed to perform a direct measurement of the weak equivalence principle on antimatter by measuring the acceleration (gbar) of antihydrogen atoms in free fall. Its originality is to produce Hbar+ ions and use sympathetic cooling to achieve µK temperature. Hbar+ ions are produced by the reactions : pbar + Ps -> Hbar + e-, and Hbar + Ps -> Hbar+ + e-, where pbar is an antiproton, Ps stands for positronium (the bound-state of a positron and an electron), Hbar is the antihydrogen and Hbar+ the antiion associated. To produce enough Ps atoms, 2x10^10 positrons must be impinged on a porous SiO2 target within 100ns. Such an intense flux requires the accumulation (collection and cooling) of the positrons in a particle trap. This thesis describes the injector being commissioned at CEA Saclay for GBAR. It consists of a Penning-Malmberg trap (moved from RIKEN) fed by a slow positron beam. A 4.3MeV linear accelerator shooting electrons on a tungsten target produces the pulsed positron beam, which is moderated by a multi-grid tungsten moderator. The slow positron flux is 10^4 e+/pulse, or 2x10^6 e+/s at 200Hz. This work presents the first ever accumulation of low-energy positrons produced by an accelerator (rather than a radioactive source) and their cooling by a prepared reservoir of 2x10^10 cold electrons.
