Etude et réalisation d’un faisceau de positons lents

This research thesis first proposes a presentation of the GBAR project (Gravitational Behaviour of Anti-hydrogen at Rest) within which this research took place, and which aims at performing the first direct test of the Weak Equivalence Principle on anti-matter by studying the free fall of anti-hydrogen atoms in the Earth gravitational field. The author presents different aspects of this project: scientific objective, experiment principle and structure, detailed structure (positron beam, positron trap, positron/positronium conversion, anti-proton beam, trapping, slowing down and neutralisation of anti-hydrogen ions). The author then reports the design of the positron beam: study of source technology, studies related to the fast positron source, design of the low positron line (approach, functions, simulations, technology). The two last chapters report the construction and the characterization of the slow-positron line.

N. Ruiz, Université Pierre et Marie Curie, Paris, France (2011). pdf

Piégeage de positons dans un piège de Penning Malmberg, en vue de leur accumulation avec un faisceau pulsé

The weak equivalence principle, a fundament of Einstein general relativity, states that gravitational mass and inertial mass are equal whatever the body. This equivalence principle has never been directly tested with antimatter. The GBAR (Gravitational Behaviour of Antimatter at Rest) experiment intends to test it by measuring the acceleration of ultra cold anti-hydrogens in free fall. The production of such anti-atoms requires a pulse of about 1010 positrons in a few tens of nanoseconds. This thesis focuses on the development of a new accumulation technique of positrons in a Penning-Malmberg trap in order to create this pulse. This new method is an improvement of the accumulation technique of Oshima et al.. This technique requires a non-neutral electron plasma to cool down positrons in the trap in order to confine them. A continuous beam delivers positrons and the trapping efficiency is about 0.4%. The new method needs a positron pulsed beam and the method efficiency is estimated at 80%. A part of this thesis was performed at Riken (Tokyo) on the trap of Oshima et al. to study the behavior of non-neutral plasmas in this type of trap and the first accumulation method. A theoretical model was developed to simulate the positron trapping efficiency. The description and the systematic study of the new accumulation technique with a pulsed positron beam are presented. They includes notably the optimization through simulation of the electromagnetic configuration of the trap and of the parameters of the used non-neutral plasmas.

P. Dupré, Université Pierre et Marie Curie, Paris, France (2011). pdf

Scattering of ultracold atoms from material surfaces is characterized by the reflection of the atomic matter wave from the attractive Casimir-Polder potential. In this thesis, the reflection probability is computed, taking into account the optical response of the material medium. This reflection probability is shown to be enhanced for slow atoms and weak potentials. A Liouville transformation of the Schrödinger equation is used show that scattering from a potential well can be reinterpreted as a collision with a potential barrier. This approach highlights the link between quantum reflection and the breakdown of the semiclassical approximation and clarifies the dependence of the reflection probability on the energy and potential strength. Our results have implications for the GBAR project at CERN, which will time the free fall of a cold antihydrogen atom onto a detector. We analyze the effect of quantum reflection on the GBAR detection scheme and propose to use quantum reflection to improve the accuracy of equivalence principle tests with antimatter.

G. Dufour, Université Paris VI, Pierre et Marie Curie, France (2015). pdf

The future CERN experiment called GBAR intends to measure the gravitational acceleration of antimatter on Earth using cold (neV) antihydrogen atoms undergoing a free fall. The experiment scheme first needs to cool antihydrogen positive ions, obtained thanks to two consecutive reactions occurring when an antiproton beam collides with a dense positronium cloud. The present thesis studies these two reactions in order to optimise the production of the anti-ions. The total cross sections of both reactions have been computed in the framework of a perturbation theory model (Continuum Distorted Wave ñ Final State), in the range 0 to 30 keV antiproton kinetic energy; several excited states of positronium have been investigated. These cross sections have then been integrated to a simulation of the interaction zone where antiprotons collide with positronium; the aim is to find the optimal experimental parameters for GBAR. The results suggest that the 2P, 3D or, to a lower extend, 1S states of positronium should be used, respectively with 2, less than 1 or 6 keV antiprotons. The importance of using short pulses of antiprotons has been underlined; the positronium will have to be confined in a tube of 20 mm length and 1 mm diameter.In the prospect of exciting the 1S-3D two-photon transition in positronium at 410 nm, a pulsed laser system had already been designed. It consists in the frequency doubling of an 820 nm pulsed titanium-sapphire laser. The last part of the thesis has been dedicated to the realisation of this laser system, which delivers short pulses (9 ns) of 4 mJ energy at 820 nm.

P. Comini, Université Paris VI, Pierre et Marie Curie, France (2014). pdf

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 H
and H^{+} ions and use
sympathetic cooling to achieve µK temperature. H and
H^{+} ions are produced by the reactions :
p + Ps →
H and H + e^{-}, and H +
Ps → H and H^{+} +
e^{-}, where pbar is an antiproton, Ps stands for positronium
(the bound-state of a positron and an electron), H and
H is the antihydrogen and H and
H^{+} 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.

P. Grandemange, Université Paris Sud, Paris XI, France (2013). pdf