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).

Stability of ablation flows in inertial confinement fusion: transient growth

Inertial confinement fusion (ICF) aims at producing energy from thermonuclear fusion reactions between low atomic-number elements. A possible approach for reaching the high densities and temperatures needed for triggering these reactions, consists in imploding a spherical capsule, filled with a mixture of fusible elements, by means of a high energy density irradiation. This irradiation induces a violent vaporization – ablation – of the capsule outer shell that drives the implosion. The finite duration of these implosions emphasize the need for investigating possible perturbation transient growth that may dominate the flow over short-time horizons. For this project, we wish to investigate such transient growth in strongly accelerated self-similar ablation flows, with planar of spherical symmetry, which are relevant to the main stage of an implosion. This work will be carried out using a direct-adjoint method of non-modal stability theory, previously devised for weakly accelerated self-similar ablation flows in planar symmetry, that will have to be adapted to handle strongly accelerated configurations. Results could be used to setup, in a more realistic setting, `multi-physics' simulations of capsule implosions.

Relativistic laboratory astrophysics

This PhD project is concerned with the numerical and theoretical modeling of the ultra-relativistic plasmas encountered in a variety of astrophysical environments such as gamma-ray bursts or pulsar wind nebulae, as well as in future laboratory experiments on extreme laser-plasma, beam-plasma or gamma-plasma interactions. The latter experiments are envisioned at the multi-petawatt laser facilities currently under development worldwide (e.g. the European ELI project), or at next-generation high-energy particle accelerators (e.g. the SLAC/FACET-II facility).
The plasma systems under scrutiny have in common a strong coupling between energetic particles, photons and quantum electrodynamic effects. They will be simulated numerically using a particle-in-cell (PIC) code developed at CEA/DAM over the past years. Besides the collective effects characteristic of plasmas, this code describes a number of gamma-ray photon emission and electron-positron pair creation processes. The purpose of this PhD project is to treat additional photon-particle and photon-photon interaction processes, and then to examine thoroughly their impact and interplay in various experimental and astrophysical configurations.

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