Compact source of electrons-positrons/muons-antimuons pairs

### Context
The context of this PhD thesis deals with laser plasma electron accelerators (LPA), which can be obtained by focusing a high-power laser into a gas medium. At focus, the laser field is so intense that it quasi-instantly ionizes matter into an undersense plasma, in which it can propagate. During laser propagation, the ponderomotive laser pressure expels plasma electrons from its path, forming a cavity void of electrons in its wake. This cavity, called ‘bubble’, can sustain accelerating fields (100GV/m) that are roughly three orders of magnitude larger than what can be provided by Radiofrequency cavities, which equip the current generation of conventional accelerators. These accelerating structures can trap some plasma electrons and accelerate them at relativistic energies (few GeVs) over distances of a few centimeters. This offers the prospect of producing much more compact and affordable accelerators, with the following goals: (i) democratizing their usage for existing applications currently reserved to only a few installations in the world (ii) enabling new applications in strategic sectors (fundamental research, industry, medicine, defense).

Among the applications for which a strong international competition exist we remark:

> The usage of these accelerators to provide the first high-energy (100 MeV) electron radiotherapy machine for medical treatmes

> The usage of these accelerators as a building block of a future large scale TeV electron/positron collider for high-energy physics

> The usage of these accelerators to develop a compact and mobile relativistic muon source to perform active muon tomography. Such a tool would be a major asset for industrial applications (e.g., safety diagnostic of nuclear reactors), and for defense applications (non-proliferation). It is worth to mention that in these two sectors the american agency DARPA has already funded an ambitious program ( Muons for Science and Security, MuS2) in 2022, with the aim of providing a first conceptual report of a relativistic moun source based on a plasma accelerator (cf. https://www.darpa.mil/news-events/2022-07-22).

### Challenges:

In order to enable the aforementioned applications, strong limitations of current laser-plasma accelerators need to be addressed. An important limitation is the low amount of charge at high-energies (100 MeV – few GeV) provided by these accelerators. The main reason behind the low accelerated charge is the fact that present-day injection techniques are based on the injection of electrons from the gas, whose density is very low. In order to address this limitation, we have recently proposed a new injection concept based on a remarkable physical system called “plasma-mirror”. This concept relies on the use of a hybrid solid-gas target. When impinging on such a target, the high-power laser fully ionizes the solid and the gas. The solid part is so dense that it can reflect the incident laser, forming a so-called ‘plasma mirror’. In the gas part, the laser propagates and drives a LPA. Upon reflection on the plasma mirror, ultra-dense electron bunches can be highly-precisely injected into the bubble of the LPA formed by the reflected laser field. As the solid offers orders of magnitude more charge than the gas medium and as charge is injected from a highly-localized region from the plasma (plane), it has the potential to level up the injected charge in LPAs while keeping a high electron beam quality.

The PHI group is an international leader in the study and control of these systems. In collaboration with LOA, by using a 100TW-class laser, we have demonstrated that this new concept allows for a significant increase of the accelerated charge while preserving the quality of the beam.

### Goals

The first objective of this PhD thesis will be to develop a multi-GeV laser-plasma accelerator based on a plasma-mirror injection on Petawatt-class laser installations like the APOLLON laser facility. With a Petawatt-class laser this accelerator should produce electrons beams at 4 GeV with a total charge of hundreds of pC and a few % energy spread. Such a beam quality would represent a substantial progress in the domain.

The second objective will be to send this electron beam into a high-Z converter in order to generate muons/anti-muons pairs. Our estimations show that we could obtain roughly 10^4 relativistic muons per shot, which would allow for the radiography of a high-Z material in a few minutes.

This PhD subject foresees:
> Theoretical/numerical modeling activities based on our exascale code WarpX (to model the laser-plasma accelerator) and on the Geant4 code (for the modeling of the high-Z converter).

> Experimental activities (high-intensity laser-plasma interaction, detection of relativistic muons)

The project involves several partner laboratories:

> The Laboratoire d’Optique Appliquée for the laser-plasma acceleration activities (A. Leblanc)

> The Lawrence Berkeley National Lab for code development activities (WarpX, J.L Vay)

> The CEA-IRFU for the detection part (micromegas technology, O. Limousin)

For the experimental part, we will use several laser facilities:

> The UHI100 laser installation for the setup and testing of the laser-plasma accelerator at reduced power

> The APOLLON installation for the setup and testing of the plasma accelerator with a PW-class laser. A first experience implementing the concept of a plasma-mirror injector at the PW-level is scheduled for May 2024 in the framework of a collaboration between CEA and LOA. Following this experiment, we will perform a second experiment (2025-2026) to generate muons on APOLLON or other laser facilities in Europe (e.g., the ELI installations).

The impact of intrinsic and of extrinsic defects on the dynamic Ron and off-state leakage current of lateral GaN power devices

The intentional doping of lateral GaN power high electron mobility transistors (HEMTs) with carbon (C) impurities is a common technique to reduce buffer conductivity and increase breakdown voltage. However, this comes at the cost of increased intrinsic defects together with degraded dynamic on-resistance (Ron) and current-collapse effects.
The aim of this project is compare the performance of HEMTs devices containing different quantities of extrinsic defects (such as C atoms) and intrinsic defects (such as dislocations), as a function of growths conditions to guide toward optimized buffer structure with good dynamic Ron and low vertical leakage simultaneously.

Electrical polarisation mapping in ferroelectric devices at the nanoscale

Ferroelectric materials, with their high dielectric constant and spontaneous polarisation, are the subject of intense research in microelectronics. Polarisation is an essential parameter for these materials while its characterization remains mainly limited to the macroscopic scale by conventional electrical methods. To deepen the understanding of these materials, particularly in thin layers, and built new devices, local measurements are essential. This thesis project aims to develop a new methodology to directly map polarisation in devices at nanoscale. By combining the expertise of SPEC in thin film growth and of C2N in nanostructuration and electric measurements, we will elaborate and design a particular geometry of nanostructures allowing the use of operando electronic holography (collaboration with CEMES-CNRS, ANR POLARYS) to quantitatively map the local electrical potential in nanodevices upon application of a voltage.

Virtual neutron scattering experiments from the moderation to the neutron detection.

The French neutron scattering community is proposing to build a new High-Current Accelerator-driven Neutron Source (HiCANS). Such a source would use a low-energy proton accelerator, a few tens of MeV, to produce thermal and cold neutrons and power an instrumental suite of around ten spectrometers. The aim of the thesis project is to build a multi-scale description of the operation of a neutron scattering spectrometer, ranging from the description of microscopic neutron moderation processes and neutron interactions with atomic structure and sample dynamics, to the propagation of neutrons through advanced optical elements and the production of background by secondary particles. The ultimate aim is to be able to carry out virtual neutron scattering experiments and accurately predict instruments performances on the future ICONE source.

Exploring chemotaxis in magnetotactic bacteria

Magnetotactic bacteria (MTB) are a diverse group of bacteria characterized by their capacity to biomineralize magnetite nanoparticles called magnetosomes. The latter allow MTB to passively align along magnetic field lines. This feature makes MTB of great interest to develop magnetic-guided microrobots used for medical applications such as targeted drug delivery. To make the latter efficient, it is not only essential to understand MTB magnetic behavior but also how MTB react to diverse chemical stimuli.
The aim of this internship is to broaden our understanding of chemotaxis in MTB. Several MTB species can be grown in the lab and will be investigated during the thesis. Typically, tethered cells and motility assays involving the use of microfluidics, microscopy and image analysis approaches will be developed to investigate, on a single-cell and population level, the chemotaxis responses of the strains to different chemical stimuli. These responses will be studied with bacteria grown in different growth conditions and in the absence or presence of a magnetic field using a custom-made magnetic microscope. Altogether, the data generated will give first insight into how MTB give an integrated response to chemical and magnetic stimuli and will therefore open new routes for the further development of targeted drug delivery.

High-throughput experimentation applied to battery materials

High throughput screening, which has been used for many years in the pharmaceutical field, is emerging as an effective method for accelerating materials discovery and as a new tool for elucidating composition-structure-functional property relationships. It is based on the rapid combinatorial synthesis of a large number of samples of different compositions, combined with rapid and automated physico-chemical characterisation using a variety of techniques. It is usefully complemented by appropriate data processing.
Such a methodology, adapted to lithium battery materials, has recently been developed at CEA Tech. It is based, on the one hand, on the combinatorial synthesis of materials synthesised in the form of thin films by magnetron cathode co-sputtering and, on the other hand, on the mapping of the thickness (profilometry), elemental composition (EDS, LIBS), structure (µ-DRX, Raman) and electr(ochim)ical properties of libraries of materials (~100) deposited on a wafer. In the first phase, the main tools were established through the study of Li(Si,P)ON amorphous solid electrolytes for solid state batteries.
The aim of this thesis is to further develop the method so as to enable the study of new classes of battery materials: crystalline electrolytes or glass-ceramics for Li or Na, oxide, sulphides or metal alloys electrode materials. In particular, this will involve taking advantage of our new equipment for mapping physical-chemical properties (X-ray µ-diffraction, Laser-Induced Breakdown Spectroscopy) and establishing a methodology for manufacturing and characterising libraries of thin-film all-solid-state batteries. This tool will be used to establish correlations between process parameters, composition, structure, and electrochemical properties of systems of interest. Part of this work may also involve data processing and programming the characterisation tools.
This work will be carried out in collaboration with researchers from the ICMCB and the CENBG

Understanding of corrosion mechanisms and means of mitigating corrosion in a NaCl-ThCl4-UCl3 salt. Application to future molten salt fuel and coolant reactors

The molten salt reactor concept is based on dissolving the fuel in a molten salt. This liquid fuel concept is highly innovative and in many respects represents a break with current reactors, which are all based on the use of a solid fuel and a fluid coolant. Recently, the emergence of American start-ups proposing this innovative concept and the major effort made in China have revived interest worldwide in studying this technology, which offers a number of advantages, both real and potential, over the use of solid fuel, particularly in terms of incineration and intrinsic safety. To build a feasibility demonstrator for this breakthrough concept, extensive research is needed to acquire data and justify the behaviour of the containment barriers, primarily the metal barrier in contact with the salt. In the case of molten salt reactors, the structural materials, nickel-based alloys, are chosen to optimise their behaviour in terms of corrosion and high temperature. Corrosion of the materials is one of the critical points to be overcome when building this reactor. A detailed understanding of the corrosion mechanisms of the alloy chosen as the structural material, on the one hand, and of the chemistry of the ternary salt NaCl-ThCl4-UCl3 envisaged, on the other hand, are necessary to predict the material corrosion rate over the lifetime of the demonstrator. These studies will enable several corrosion mitigation methods to be developed. Each of these processes will be tested and evaluated under nominal conditions and then aggravated.
The first part will be devoted to understanding the corrosion mechanisms of the alloy and the chemistry of the NaCl-ThCl4-UCl3 salt. To this end, tests will be carried out at the IPN in Orsay and the corrosion mechanisms and chemistry studies will be established using electrochemical techniques and microstructural characterisation of corroded samples (thermogravimetry, SEM, TEM, XPS, Raman, GD-OES, etc.). Secondly, material protection tests using different types of salt redox control will be carried out and then tested in nominal and aggravated environments.
This approach will make it possible to meet a major and ambitious corrosion control challenge for an innovative energy process.

Study of the behaviour of volatile fission products in nuclear fuels under temperature transients

The quantity of radioactive fission products, which can be released out of irradiated nuclear fuel in accidental conditions, is a key data for the design of safety components of a Nuclear power plant. This release was largely studied in the case of severe accidents but less is known for temperature below 1400°C. The aim of the Ph.D. Work will consist in using new in-situ characterization tools that are now available on MERARG furnace at the IRESNE institute in CEA-Cadarache in order to improve the characterization of volatiles PFs in this temperature range. The Ph.D. Student will have to design and run thermal treatment on irradiated nuclear fuel making use of in and ex-situ gamma spectrometry, in-situ optical sighting device and measurement of the PO2 of the furnace atmosphere. This work will be performed with the support of experimental team at IRESNE. The Ph.D. Student will also participate to the interpretation of the results in close collaboration with experimental and modeling teams at IRESNE. In order to test the proposed mechanism, the Ph.D. Student can also perform separate-effects experiments on simulating materials in collaboration with team at university. This work will be valued both in industrial context with technical notes and in academic context with paper in journals and attending scientific conferences.

Modeling and Optimization of 2D Material-Based Field-Effect Transistors: From Multi-Physics Simulations to Atomic-Scale Insights

Field-effect transistors employing 2D materials are emerging as promising candidates due to their superior mobility and atomic thinness. Nonetheless, this technology faces multiple challenges, including minimizing contact resistances, controlling variability, and optimizing short-channel transistors (< 10 nm). At CEA-Leti, a concerted experimental and computational effort is underway to address these issues and propel the development of 2D material-based technologies.

This doctoral research project is situated within this context, aiming to harness multi-physics simulations to evaluate and enhance the performance of 2D material-based FETs by exploring the interplay between technological parameters and device performance. The flexibility in choosing materials and geometric configurations opens the door to pioneering research directions. A pivotal aspect of this work will involve coupling Technology Computer-Aided Design (TCAD) simulations with ab initio methods to achieve a comprehensive understanding of the devices' structural and electronic behaviors at the atomic level.

The project benefits from access to state-of-the-art computational resources and software (Sentaurus, VASP, GPAW, etc.), supported by CEA-Leti's expertise in simulation methodologies and close collaboration with experimental teams. This doctoral endeavor offers a unique opportunity to develop a wide-ranging skill set in electronic device simulation, contributing to the scientific community through presentations at leading international conferences and publications in esteemed journals.

Protection by self-decontaminating coatings against biocontamination of surfaces

The proposed PROBIO-ES project falls within the scope of the priority defense theme « biologie, santé, NRBC », and in particular the sub-themes of protection and decontamination. Its aim is to develop self-decontaminating surfaces for a number of terrestrial and space applications. The project has been shortlisted by CNES for the award of a 1/2 thesis grant. In the context of manned spaceflights to distant destinations such as low Earth orbit, the Moon, and potentially Mars, biological contamination poses a significant threat to the health of the crew and the preservation of space equipment. The microflora carried by the crew in enclosed habitats is an unavoidable concern, heightened by prolonged periods of isolation and dependence on closed-loop life support systems. Beyond risks to astronaut health, biocontamination can damage critical equipment aboard spacecraft. Microorganisms exposed to the space environment can develop resistance and mutate, transforming benign microbes into pathogens. To mitigate these risks, effective measures such as filtration systems and self-decontaminating surfaces limiting bacterial proliferation must be implemented. The MATISS experiment (2016-2024) explored the use of hydrophobic coatings to reduce biocontamination aboard the ISS, but improvements are needed. This collaborative thesis between SyMMES and CEA-Leti in Grenoble aims to develop durable antimicrobial layers without harmful substances, using a new method of deposition through cold atmospheric plasma, suitable for large surfaces. The PROBIO-ES project is therefore fully in line with the « biologie, santé, NRBC » thematic priorities of AID 2024 call for projects.

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