Optimisation of the Gbar experiment for the production of antihydrogen ions

The aim of the Gbar experiment (Gravitational Behavior of Antihydrogen at Rest) at CERN is to produce a large number of antihydrogen atoms to measure their acceleration in Earth's gravitational field. The principle relies on the production of antihydrogen ions through two successive charge exchange reactions that occur when a beam of antiprotons passes through a positronium cloud. In 2022, Gbar demonstrated its operational scheme by producing antihydrogen atoms through the first charge exchange reaction. The current focus is on optimizing and improving various elements of the experiment to achieve the production of anti-H+, particularly the positron line leading to the creation of the positronium cloud. The challenge is to increase the number of positrons trapped in the second electromagnetic trap of the line, and then to transport them efficiently to the reaction chamber where they are converted into positronium.
The thesis work will involve operating, diagnosing, and optimizing the two electromagnetic traps of the line, as well as the positron acceleration and focusing devices to yield a sufficient number of positroniums and then the production of antihydrogen ions. The student will also participate in the measurement campaign for studying the mater counterpart of the second charge exchange reaction, relying upon a beam of H- ion instead of the beam of antiprotons.

Simulation and characterization of very high intensity ion sources

Light ion accelerators (such as protons and deuterons) at very high intensity (typically exceeding 50 mA) have numerous applications in various fields of physics. From the IFMIF accelerator project, to characterize future materials for fusion reactors, to IPHI-Neutrons, to produce images through neutron radiography, CEA is involved in many projects that require the design and construction of very high-intensity ion sources. The increasing demand for intensity and beam quality from these ion sources requires a better understanding and prediction of their operation.
Ion sources are composed of a plasma chamber inserted into a magnetic coil, in which a gas heated by an RF wave is injected. The produced ions are extracted from the chamber using an electric field applied to extraction electrodes. Their operation depends on a large number of parameters. Determining an ideal set of parameters is very complex to achieve, and no software currently exists to reliably predict its proper functioning.
CEA has been working for several years on the design of a test bench, BETSI, to test and optimize various ion sources for future accelerator projects. Experimental campaigns have been conducted in the past on this test bench to systematically test sets of parameters.
In the context of this thesis, we propose to develop a simulation code that takes into account all the parameters that we can qualify on BETSI (from past experiments or new ones). We will be able then to use the code to propose new sources for upcoming accelerator projects.

Implementation of a novel injector concept to boost the accelerated charge in laser-driven electron accelerators to enable their use for scientific and technological applications

Ultra-short, high-energy (up to few GeVs) electron beams can be accelerated over just a few centimeters by making an ultra-intense laser interact with a gas-jet, with a technique called “Laser Wakefield Acceleration” (LWFA). Thanks to their small size and the ultra-short duration of the accelerated electron beams, these devices are potentially interesting for a variety of scientific and technological applications. However, LWFA accelerators do not usually provide enough charge for most of the envisaged applications, in particular if a high beam quality and a high electron energy are also required. The goal of this thesis is to implement a novel LWFA injector concept in several state-of-the-art laser facilities, in France and abroad. This injector concept, recently conceived in our group, consists in a solid target coupled with a gas-jet, and should be able to accelerate a substantially higher amount of charge with respect to conventional strategies, while preserving at the same time the quality of the beam. The proposed activity is mainly experimental, but with the possibility to be involved in the large-scale numerical simulation activities that are needed to design an experiment and to interpret its results. The PhD student will have the opportunity to be part of a dynamic team with strong national and international collaborations. They will also acquire the necessary skills to participate in laser-plasma interaction experiments in international facilities. Finally, they’ll have the possibility to be involved in the numerical activities of the group, carried out on the most powerful supercomputers in the world with a state-of-the-art Particle-In-Cell code (WarpX, Gordon Bell prize in 2022).

Design and manufacturing of a 4D-emittancemeter for characterization of very high current ion sources

Ion accelerators, including protons, at very high intensity (> 50 mA), find numerous applications in various fields of nuclear physics or material characterization for medical, nuclear, and other applications. The Department of Accelerators, Cryogenics, and Magnetism (DACM) at CEA-Saclay specializes in the design and realization of very high-intensity ion sources.
With the increase in beam current, these sources become increasingly complex. Therefore, mastering the quality of the beam becomes critical to limit power deposition and the activation of accelerator elements. To better understand and describe this beam, it is necessary to determine its 4D emittance, which includes both the geometric shape of the beam and its trajectory. The device used for this measurement is called a 4D emittance meter.
Such a device based on a scintillator has already been designed and tested. This scintillator converts a portion of the beam into an image, which is then captured by a camera. Unfortunately, while this technology is functional at high energy, it is not suitable at the source outlet, at low energy, as the scintillation layers are quickly damaged by the ion flux.
The charge reading method proposed in this thesis is novel and benefits from the synergy between particle detector research for high-energy physics and proton source research. Instead of using a camera for reading, the idea is to measure, from a PCB placed directly in the beam, the current carried by the ions. This method allows reading this current at several thousand positions to obtain the 4D emittance. The fast acquisition system will also allow observing the temporal variation of the emittance during the start-up and shut-down phases of the source.
This device will be used for analyzing the beam generated by the ALISES sources developped by the laboratory.

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