Kinetic description of laser-plasma interaction relevant to inertial confinement fusion

Many applications, such as inertial confinement fusion, require an understanding of the physical mechanisms involved when high-energy laser beams propagate in a plasma. In particular, in the case of fusion, the aim is to quantify the deposition of laser energy on a cryogenic deuterium-tritium target, and the efficiency with which this target can be compressed to trigger fusion reactions. However, during their propagation, laser beams create a plasma wave that grows at the expense of the incident laser energy. However, the growth of this wave is not infinite and stops when the wave breaks up. This is accompanied by the production of hot electrons, which can preheat the target and hinder its compression. The breaking of a plasma wave is a physical phenomenon of the kinetic type, which can only be correctly described by calculating the velocity distribution of the electrons in the plasma. The aim of this thesis is to study wave breaking both theoretically and numerically, using Vlasov-type kinetic codes. One of the main difficulties lies in the discontinuity of the distribution functions to be described. In addition, it is necessary to describe the surge from its linear phase to the non-linear regime, enabling the creation of hot electrons to be quantified. The ultimate goal of the thesis is to produce models that are simple enough to run on the CEA's dimensioning codes.

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.

Study of low-frequency radiation produced by particle acceleration at ultra-high laser intensity in relativistic plasmas

Today, petawatt laser sources deliver optical pulses lasting a few tens of femtoseconds with an intensity larger than 1020 W/cm2. When such a light beam interacts with a gas or a solid target, the electrons accelerated by the laser ponderomotive force become relativistic and acquire high energies, in excess of the GeV. These laser systems also produce various radiations such as hard X photons or electron-positron pairs by quantum conversion of gamma photons. As laser technology is advancing rapidly, these light sources have increasingly compact dimensions and they nowadays complement many international laboratories hosting synchrotrons or conventional particle accelerators.
If this extreme light makes it possible to generate radiation in the highest frequencies regions of the electromagnetic spectrum, it also fosters, through the production mechanisms of plasma waves and particle acceleration, conversion processes towards much lower frequencies belonging to the gigahertz and terahertz (THz) ranges.

Having high-power transmitters operating in this frequency band is attracting more and more interest in Europe, overseas and in Asia. On the one hand, the generation of intense electromagnetic pulses with GHz-THz frequencies is harmful for any electronic device close to the laser-plasma interaction zone and the diagnostics used on large-scale laser facilities like, e.g., the PETAL/LMJ laser in the Aquitaine region. It is therefore necessary to understand their nature to better circumvent them. On the other hand, the waves operating in this field not only make it possible to probe the molecular motions of complex chemical species, but they also offer new perspectives in medical imaging for cancer detection, in astrophysics for the evaluation of ages of the universe, in security as well as environmental monitoring. The processes responsible for this violent electromagnetic field emission, if properly controlled, can lead to the production of enormous magnetic fields in excess of 1000 Tesla, which presents exciting new opportunities for many applications such as particle guiding, atomic physics, magnetohydrodynamics, or modifying certain properties of condensed matter in strong field.

The objective of this thesis is to study the physics of the generation of such giant electromagnetic pulses by ultrashort laser pulses interacting with dense media, to build a model based on the different THz/GHz laser-pulse conversion mechanisms, and validate this model by using dedicated experimental data. The proposed work is mainly oriented towards an activity of analytical modeling and numerical simulation.

The doctoral student will be invited to deal with this problem theoretically and numerically using a particle code, whose Maxwell solver will be adapted to describe radiation coming from different energy groups of electron/ion populations. A module calculating online the field radiated by each particle population in the far field will be implemented. Particular attention will be given to the radiation associated with the acceleration of electrons and ions on femto- and picosecond time scales by dense relativistic plasmas and their respective roles in target charging models available in the literature. This field of physics requires a new theoretical and numerical modeling work, at the crossroads of extreme nonlinear optics and the physics of relativistic plasmas. Theory-experiment confrontations are planned within the framework of experiments carried out on site at CELIA facilities and experiments carried out in collaboration with US laboratories (LLE/Rochester). The thesis will be prepared at CELIA laboratory on the campus of Bordeaux university.

Gyrokinetic study of turbulent transport bifurcations in tokamak plasmas: impact of plasma-neutral interactions

Turbulence and associated transport degrade the confinement of tokamak plasmas, reducing the expected performance in terms of energy gain. Experimentally, several regimes of improved confinement have been observed, notably those where turbulent transport is strongly reduced at the plasma edge. These external transport barriers lead to strong density and/or temperature gradients that maximize the energy content of the confined plasma. These spontaneous bifurcations result from the self-organization of turbulence under the forcing of different sources, of particles and heat. Their mechanism is poorly understood, not least because of the topological complexity of this outer region and the wealth of probable processes at play. These regimes represent a major opportunity for achieving the best performance in ITER plasmas. It is therefore crucial to gain a better understanding of them, so as to be able to predict their transition thresholds and, if possible, control them.
The proposed PhD thesis falls within this framework. It is based on state-of-the-art numerical modeling of fusion plasmas, the five dimensional (in phase space) gyrokinetic approach. Recent developments have made it possible to treat self-consistently both transport of matter and heat in this peripheral region. What remains to be done is to implement a source of neutral particles which, through ionization, will constitute the plasma's density forcing. We already know, thanks in particular to reduced models, that this dynamic source plays a crucial role in self-organization processes. The aim of this thesis work is to couple a reduced fluid model of neutrals to kinetically described electrons and ions, and to study their impact on turbulent transport and self-organization using high-performance computing (HPC) simulations with the GYSELA code.

Wall conditioning of a long pulse, tungsten tokamak: from WEST to ITER

Research on controlled thermonuclear fusion as a new source of energy is carried out in devices called tokamaks, where matter is brought to high temperature (plasma state) and confined by magnetic fields. Interactions between the plasma and the walls of the vacuum chamber of tokamaks releases impurities, which can affect plasma performance. Different conditioning methods are used to control the surface state of the vacuum chamber, and thus impurity fluxes. These mainly use low-temperature plasmas (glow or radio-frequency discharges) in hydrogen or helium, but also deposition of thin layers of boron, because of its ability to trap by chemical affinity impurities like oxygen. With the advent of metallic plasma facing components and the extension of plasma duration in superconducting tokamaks, like ITER and WEST, operated in the Institute of Research on Magnetic Fusion (CEA Cadarache, France), innovative wall conditioning techniques to maintain optimal surface state and performances are under development. The aim of this thesis is to characterize and evaluate in WEST the relevance for ITER of different methods of boron injection, both a priori and in real time. The work will consist on the one hand to participate in experiments on WEST and to analyze experimental data (location and lifetime of boron deposits, effect on plasma performance). In order to understand the transport of boron, the candidate will work with plasma boundary numerical models (SOLEDGE, EIRENE, DIS). This work, combining experiments and simulations, will consolidate the understanding of the physics of wall conditioning in a metallic environment and predicting consequences for ITER and future fusion devices.

Anomaly Detection Machine learning Methods for Experimental Plasma Fusion Data Quality - Application to WEST Data

Fusion plasmas in tokamaks have complex non-linear dynamics. In the WEST Tokamak, of the same family as the ITER project, a large amount of heterogeneous experimental fusion data is collected. Ensuring the integrity and quality of this data in real time is essential for the stable and safe operation of the Tokamak. Continuous monitoring and validation are essential, as any disturbance or anomaly can significantly affect our ability to ensure plasma stability, control performance and even lifetime. The detection of unusual patterns or events within the collected data can provide valuable insights and help identify potentially abnormal behavior in plasma operations.
This Ph.D. research aims to study and develop anomaly detection system for WEST -- prefiguring what could be installed on ITER -- by integrate machine learning algorithms, statistical methodologies and signal processing techniques to validate various diagnostic signals in Tokamak operations, including density, interferometry, radiative power and magnetic data.
The expected outcomes are:
– The development of dedicated machine-learning algorithms capable of detecting anomalies in selected time series data from WEST Tokamak.
– The fine-tuning of an operational autonomous system able to ensure data quality in Tokamak reactors, integrated into the WEST AI platform.
– The constitution of a comprehensive database.
– The validation of a data quality framework built for the specific needs of plasma fusion research.

Turbulence in the edge plasma of tokamaks in regimes of stiff reactive coupling with neutrals

The strategy to manage the extreme heat fluxes to the wall of magnetic fusion reactors relies on the dissipation of the plasma’s energy through interaction with neutral gas present in the edge of the plasma mainly due to the recombination of the plasma in contact with solid materials. The physics at play consists in a balance between plasma transport, dominated by turbulence, and atomic and molecular reactions. The modelling of this extremely non-linear phenomenology is mandatory for the design and operational space definition of future devices like ITER. It requires the use of numerical codes treating self-consistently the related mechanisms, which has not been done to date. IRFM and AMU have co-developed such numerical tool, the SOLEDGE3X-EIRENE code package, which offers the capability to model self-consistently turbulent transport and neutral particles dynamics in 3D realistic geometry. First studies demonstrated that the inclusion of plasma-neutrals interactions in simulations significantly change the self-organization of turbulence and the resulting transport. They also highlighted several specific challenges related to the appearance of long time scales in the system. This PhD project aims at pursuing this work to extend it to regimes of tight coupling between the plasma and neutrals, which are the regimes of interest for future reactors. The work will rely on numerical simulations to be run on world-class High Performance Computers. Their outputs will be analyzed in order to understand the underlying phenomenology and compare it with experimental trends. Depending on the taste and capabilities of the successful candidate, it could also include a numerical (improvement of the code) or an experimental (dedicated experiments on the WEST tokamak or European partner devices) arm.

Design and realization of a high-temperature optical neutron detector. Application to an experimental program in the JOYO reactor

As part of the development of fourth-generation sodium-cooled fast reactors, the CEA/IRESNE Dosimetry and Instrumentation Laboratory is working on innovative neutron measurement systems capable of operating at temperatures of the order of 600°C, and insensitive to the parasitic phenomena that occur under these conditions. Recently, a new type of optical signal neutron detector (ODN) has been developed at the laboratory. Despite a more complex signal interpretation, this instrument has several advantage: it can be miniaturized and it is intrinsically insensitive to problems of partial discharges and leakage currents that occur in ionization chambers at high temperature.
We propose to pursue the theoretical and experimental development of ODNs to adapt them to high temperatures. The PhD student will further develop the modelling tools already available in the laboratory for simulating the detector response. The work will investigate heavy ion-noble gas interaction cross sections, also a radiative collisional model to predict emission spectra and their temporal dynamics. Part of the work will involve dimensioning a high-temperature prototype and testing it in the JSI TRIGA reactor. Ultimately, the detector will be qualified in the JOYO research reactor as part of a broader experimental program.

Large-scale numerical modeling and optimization of a novel injector for 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 first goal of this thesis is to understand the basic physics of a novel LWFA injector concept recently conceived in our group. This injector consists of 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. Large scale numerical simulation campaigns and machine learning techniques will be used to optimize the properties of the accelerated electrons. The interaction of these electron beams with various samples will be simulated with Monte Carlo code to assess their potential for applications such as Muon Tomography and radiobiology/radiotherapy. The proposed activity is essentially numerical, but with the possibility to be involved in the experimental activities of the team.

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 will acquire the required skills to contribute to the development of a complex software written in modern C++ and designed to run efficiently on the most powerful supercomputers in the world: the state-of-the-art Particle-In-Cell code WarpX (prix Gordon Bell en 2022). The development activity will be carried out in collaboration with the team led by Dr. J.-L. Vay at LBNL (US), where the candidate could have the opportunity to spend a few months during the thesis.

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).

Top