Optimisation of positron accumulation in the GBAR experiment and study of space propulsion based on antimatter.

The goal of the GBAR experiment is to determine the effect of gravity on antihydrogen atoms. The antihydrogen atoms are created by neutralising antihydrogen ions using laser pulses. The antihydrogen ions are produced after two positrons captures by antiprotons flying through a positronium cloud. In this scheme to produce one single antihydrogen atom 10e10 positrons have to be beamed on a nanoporous silica to yield the positronium cloud. The positrons are produced by a 9 MeV LINAC accelerating electrons into a tungsten target equipped with a mesh moderator. In this thesis we have studied and optimised the accumulation and trapping of positrons in two subsequent trapping devices. The LINAC based source providing 3e7 positrons per second, the particles have to be accumulated. They are first accumulated into a Buffer Gas Trap (BGT), a Penning trap, divided in 3 stages, with N2 and CO2, leading to inelastic collisions which insure the trapping and the cooling of the positrons. The positrons are then slowed in the first stage and accumulated in the second stage for 100 ms with a trapping rate of about 1.7e6 positrons per second, then they are transferred into the BGT's third stage. This accumulation and transfer procedure is repeated 10 times to finally provide a bunch of 1.5e7 positrons every 1.1s (a loss happens during this stacking operation and 100 ms are added for a final radial compression using the Rotating Wall technique, the trapping efficiency is then 5%). This new bunch is then ready to be sent and re-trapped into the High Field Trap. The High Field Trap is a 5 T multi-ring Penning trap allowing to trap large amounts of charged particle for hours. We first tested this trap with electrons by trapping about 5e9 of them. The experiments on the electrons lead to the conclusion that a better alignment of the electrodes with respect to the magnetic field still needs to be performed. However, an acceptable situation has been found allowing to re-trap the positrons with 66% efficiency. Then, accumulating the positrons bunches coming from the BGT, it was possible to accumulate 1e9 positrons in 1100. This is a really promising result for the GBAR experiment. For the future, it is about to do 10 times more, 10 times faster to collect the desired amount of positrons each time the ELENA decelerator provides a bunch of antiprotons (every 100 s). We also studied how it could be possible to use antimatter to propel a rocket. Indeed, the energy resulting from the antimatter-matter annihilation reaction has properties defying any other propellant. In our study, we focused on the proton-antiproton annihilation reaction in a high magnetic field in order to have the annihilation products aligned with the direction of the thrust. The theoretical model is named the beam cored engine. A simulator has been developed using GEANT4 to evaluate some parameters such the intensity of the field. According to our simulation, it is then possible to get a rocket with a specific impulse of about 0.5 c/g i.e., 1.5e7 s (with c the speed of light and g the earth's gravitational acceleration), which is outsized if it is compared to the most modern rocket (434 s for Vulcain, propelling Ariane 5). However, this model assumes the capability to produce and store a macroscopic number of antiprotons, which might be an insurmountable showstopper. Also, with this model, a large amount of gamma rays are produced and a solution to evacuate their energy has to be found.