Plasma Mirrors Towards Extreme Intensity Light Sources and High-Quality Compact Electron Accelerators
The research programs conducted at the Lasers Interactions and Dynamics Laboratory of the French Atomic Energy Commission (CEA) aim to understand the fundamental processes involved in light-matter interactions and their applications. As part of the CEA-LIDYL, the Physics at High Intensity (PHI) group conducts studies of laser-matter interactions at extreme intensities, for which matter turns into an ultra-relativistic plasma. Using theory, simulations and experiments, researchers develop and test new concepts to control the laser-plasma interaction with the aim to produce novel relativistic electron and X-UV attosecond light sources, with potential applications to fundamental research, medicine and industry.
In collaboration with the Lawrence Berkeley National Laboratory, the group is a core developer of the exascale Particle-In-Cell (PIC) codes WarpX/PICSAR for the high-fidelity modelling of laser-matter interactions. It also pioneered the study and control of remarkable optical components called ‘plasma mirrors’, which can be obtained upon focusing a high-power laser with high-contrast on an initially solid target. In the past five years, the PHI group has developed two concepts exploiting plasma mirrors to manipulate extreme light for pushing the frontiers of high-field science. The first concept uses relativistic plasma mirrors to amplify the intensity of existing lasers by orders of magnitude and probe novel regimes of Strong-Field Quantum Electrodynamics (SF-QED). The second uses plasma mirrors as high-charge injectors to level up the charge produced in laser-plasma accelerators (LPAs) to enable their use for medical studies, industrial applications and fundamental research (collider design, electron-laser collisions for SF-QED studies).
In this context, the PhD candidate will first improve our simulation tool WarpX to speed-up plasma mirror simulations. They will then use WarpX to optimize the use of plasma mirrors as intensity boosters for the study of SF-QED. In collaboration with Brigitte Cros's team at CNRS and within the framework of novel collider designs based on Laser-Plasma Accelerators (LPAs), the PhD candidate will finally investigate and optimize the use of plasma mirrors as optical components for the coupling of multiple LPA stages. This will be crucial for developing compact acceleration schemes that can be scaled to produce high-energy, high-quality electron beams.
Quantum simulation of atomic nulei
Atomic nuclei constitute strongly correlated quantum many-body systems governed by the strong interaction of QCD. The nuclear shell model, which diagonalizes the Hamiltonian in a basis whose dimension grows exponentially with the number of nucleons, represents a well-established approach for describing their structure. However, this combinatorial explosion confines classical high-performance computing to a restricted fraction of the nuclear chart.
Quantum computers offer a promising alternative through their natural ability to manipulate exponentially large Hilbert spaces. Although we remain in the NISQ era with its noisy qubits, they could revolutionize shell model applications.
This thesis aims to develop a comprehensive approach for quantum simulation of complex nuclear systems. A crucial first milestone involves creating a software interface that integrates nuclear structure data (nucleonic orbitals, nuclear interactions) with quantum computing platforms, thereby facilitating future applications in nuclear physics.
The project explores two classes of algorithms: variational and non-variational approaches. For the former, the expressivity of quantum ansätze will be systematically analyzed, particularly in the context of symmetry breaking and restoration. Variational Quantum Eigensolvers (VQE), especially promising for Hamiltonian-based systems, will be implemented with emphasis on the ADAPT-VQE technique tailored to the nuclear many-body problem.
A major challenge lies in accessing excited states, which are as crucial as the ground state in nuclear structure, while VQE primarily focuses on the latter. The thesis will therefore develop quantum algorithms dedicated to excited states, testing various methods: Hilbert space expansion (Quantum Krylov), response function techniques (quantum equations of motion), and phase estimation-based methods. The ultimate objective is to identify the most suitable approaches in terms of scalability and noise resilience for applications with realistic nuclear Hamiltonians.
Description of collective phenomena in atomic nuclei beyond Time-Dependent Density Functional
Context :
Predicting the organization and dynamics of neutrons and protons within atomic nuclei is a significant
scientific challenge, crucial for designing future nuclear technologies and addressing fundamental questions
such as the origin of heavy atoms in our universe. In this context, CEA, DAM, DIF develops theoretical
approaches to simulate the dynamics of the elementary constituents of atomic nuclei. The equations of
motion, derived within the framework of quantum mechanics, are solved on our supercomputers. The 2010s
saw the rise of the time-dependent density functional theory (TDDFT) approach for tackling this problem.
While TDDFT has provided groundbreaking insights into phenomena such as giant resonances observed in
atomic nuclei and nuclear fission, this approximation has intrinsic limitations.
Objectives :
This PhD project aims to develop and explore a novel theoretical approach to describe the collective motion
of protons and neutrons within the atomic nucleus. The goal is to generalize the TDDFT framework to
improve the prediction of certain nuclear reaction properties, such as the energy distribution among the
fragments resulting from nuclear fission. Building on initial work in this direction, the PhD candidate will
derive the equations of motion for this new approach and implement them as an optimized C++ library
designed to leverage the computational power of CEA's supercomputers. The final objective will be to assess
how this new framework enhances predictions of phenomena such as the damping of giant resonances in
atomic nuclei and the formation of fragments during nuclear fission.
Microscopic description of fission fragment properties at scission
Fission is one of the most difficult nuclear reactions to describe, reflecting the diversity of dynamic aspects of the N-body problem. During this process, the nucleus explores extreme deformation states leading to the formation of two fragments. While the number of degrees of freedom (DOF) involved is extremely large, the mean-field approximation is a good starting point that drastically reduces the DOF, with elongation and asymmetry being unavoidable. This reduction introduces discontinuities in the successive generation of states through which the nucleus transits, since continuity in energy does not ensure the continuity of states resulting from a variational principle. Recently, a new method based on constraints associated with wave function overlaps has been implemented to ensure this continuity up to and beyond the scission (Coulomb valley). This continuity is crucial for describing the dynamics of the process.
The objective of the proposed thesis is to carry out for the first time a two-dimensional implementation of this new approach in order to take into account the whole collectivity generated by elongation and asymmetry DOF. The theoretical and numerical developments will be done within the framework of the time-dependent generator coordinate method. This type of approach contains a first static step, which consists of generating potential energy surfaces (PES) obtained by constrained Hartree-Fock-Bogoliubov calculations, and a second dynamic step, which describes the dynamic propagation of a wave packet on these surfaces by solving the time-dependent Schrödinger equation. It is from this second step that the observables are generally extracted.
As part of this thesis, the PhD student will:
- as a first step, construct continuous two-dimensional PESs for the adiabatic and excited states. This will involve the three algorithms Link, drop and Deflation
- secondly, extract observables that are accessible using this type of approach: yields, the energy balance at scission, fragment deformation and the average number of emitted neutrons. In particular, we want to study the impact of intrinsic excitations on the fission observables, which are essentially manifested in the descent from the saddle point to the scission.
Finally, these results will be compared with experimental data, in actinides and pre-actinides of interest. In particular, the recent very precise measurements obtained by the SOFIA experiments for moderate to very exotic nuclei should help to test the precision and predictivity of our approaches, and guide future developments of N-body approaches and nuclear interaction in fission.
Control of trapped electron mode turbulence with an electron cyclotron resonant source
The performance of a tokamak plasma largely depends on to the level of turbulent transport. Trapped electron modes are one of the main instabilities responsible for turbulence in tokamaks. On the other hand, electron cyclotron resonance heating is a generic heating system for tokamaks. Both physical processes rely on resonant interactions with electrons. Non-linear interaction between the resonant processes is theoretically possible. This thesis aims to evaluate the possibility of exploiting this non-linear interaction to stabilize the trapped electron modes instability within tokamak plasmas, using a heating source present on many tokamaks, including ITER. This control technique could improve the performance of certain tokamaks without any extra cost.
The thesis will be based on a theoretical understanding of the two processes studied, will require the use of the gyrokinetic code GYSELA to model the non-linear interactions between resonant processes, and will include an experimental aspect to validate the identified turbulence control mechanism.
Modelling spin shuttling in Si and Ge spin qubits
Silicon and Germanium spin qubits have made outstanding progress in the past few years. In these devices, the elementary information is stored as a coherent superposition of the spin states of an electron or hole confined in a quantum dot embedded in a Si/SiO2 or SiGe heterostructure. These spins can be manipulated electrically and are entangled through exchange interactions, allowing for a variety of one- and two-qubit gates required for quantum computing and simulation. Grenoble is promoting original spin qubit platforms based on Si and Ge, and holds various records in spin lifetimes and spin-photon interactions. At CEA/IRIG, we support the progress of these quantum technologies with state-of-the-art modelling. We are, in particular, developing the TB_Sim code, able to describe very realistic qubit structures down to the atomic scale if needed.
Spin shuttling has emerged recently as a resource for spin manipulation and transport. A carrier and its spin can indeed be moved (shuttled) coherently between quantum dots, allowing for the transport of quantum information on long ranges and for the coupling between distant spins. The shuttling dynamics is however complex owing to the spin-orbit interactions that couple the motion of the carrier to its spin. This calls for a comprehensive understanding of these interactions and of their effects on the evolution and coherence of the spin. The aim of this PhD is to model shuttling between Si/Ge spin qubits using a combination of analytical and numerical (TB_Sim) techniques. The project will address spin manipulation, transport and entanglement in arrays of spin qubits, as well as the response to noise and disorder (decoherence). The PhD candidate will have the opportunity to interact with a lively community of experimentalists working on spin qubits at CEA and CNRS.
Can we predict the weather or the climate?
According to everyone's experience, predicting the weather reliably for more than a few days seems an impossible task for our best weather agencies. Yet, we all know of examples of “weather sayings” that allow wise old persons to predict tomorrow’s weather without solving the equations of motion, and sometimes better than the official forecast. On a longer scale, climate model have been able to predict the variation of mean Earth temperature due to CO2 emission over a period of 50 year rather accurately.
In the late 50’ and 60’s, Lewis Fry Richardson, then Edward Lorenz set up the basis on the resolution of this puzzle, using observations, phenomenological arguments and low order models.
Present progress in mathematics, physics of turbulence, and observational data now allow to go beyond intuition, and test the validity of the butterfly effect in the atmosphere and climate. For this, we will use new theoretical and mathematical tools and new numerical simulations based on projection of equations of motion onto an exponential grid allowing to achieve realistic/geophysical values of parameters, at a moderate computational and storage cost.
The goal of this PhD is to implement the new tools on real observations of weather maps, to try and detect the butterfly effect on real data. On a longer time scale,, the goal will be to investigate the “statistical universality” hypothesis, to understand if and how the butterfly effect leads to universal statistics that can be used for climate predictions, and whether we can hope to build new “weather sayings” using machine learning, allowing to predict climate or weather without solving the equations.
Building a new effective nuclear interaction model and propagating statistical errors
At the very heart of any « many-body » method used to describe the fundamental properties of an atomic nucleus, we find the effective nucleon-nucleon interaction. Such an interaction should be capable of taking into account the nuclear medium effects. In order to obtain it, one has to use a specific fitting protocol that takes into account a variety of nuclear observables such as radii, masses, the centroids of the giant resonances or the properties of the nuclear equation of state around the saturation density.
A well-known model of the strong interaction is the Gogny model. It is a linear combination of coupling constants and operators, plus a radial form factor of the Gaussian type [1]. The coupling constants are determined via a fitting protocol that typically uses the properties of spherical nuclei such as 40-48Ca, 56Ni, 120Sn and 208Pb.
The primary goal of this thesis is to develop a consistent fitting protocol for a generic Gogny interaction in order to access some basic statistical information, such as the covariance matrix and the uncertainties on the coupling constants, in order to be able to perform a full statistical error propagation on some selected nuclear observables calculated with such an interaction [2].
After having analysed the relations between the model parameters and identified their relative importance on how well observables are reproduced, the PhD candidate will explore the possibility of modifying some terms of the interaction itself such as the inclusion of a real three-body term or beyond mean-field effects.
The PhD candidate will work within a nuclear physics group at CEA/IRESNE Cadarache. The work will be done in close collaboration with CEA/DIF. Employment perspectives are in academic research and nuclear R&D labs.
[1] D. Davesne et al. "Infinite matter properties and zero-range limit of non-relativistic finite-range interactions." Annals of Physics 375 (2016): 288-312.
[2] T. Haverinen and M. Kortelainen. "Uncertainty propagation within the UNEDF models." Journal of Physics G: Nuclear and Particle Physics 44.4 (2017): 044008.
Microscopic nuclear structure models to study de-excitation process in nuclear fission
The FIFRELIN code is being developed at CEA/IRESNE Cadarache in order to provide a detailed description of the fission process and to calculate all relevant fission observables accurately. The code heavily resides on the detailed knowledge of the underlying structure of the nuclei involved in the post-fission de-excitation process. When possible, the code relies on nuclear structure databases such as RIPL-3 that provide valuable information on nuclear level schemes, branching ratios and other critical nuclear properties. Unfortunately, not all these quantities have been measured, nuclear models are therefore used instead.
The development of state-of-the-art nuclear models is the task of the newly-formed nuclear theory group at Cadarache, whose main expertise is the implementation of nuclear many-body solvers based on effective nucleon-nucleon interactions.
The goal of this thesis is to quantify the impact of the E1/M1 and E2/M2 strength functions on fission observables. Currently, this quantity is estimated using simple models such as the generalized Lorentzian. The doctoral student will be tasked with replacing these models by fully microscopic ones based on effective nucleon-nucleon interaction via QRPA-type techniques. A preliminary study shows that the use of macroscopic (generalized Lorentzian) or microscopic (QRPA) has a non-negligible impact on fission observables.
Professional perspectives for the student include academic research as well as theoretical and applied nuclear R&D.
Near-threshold phenomena in nuclear structure and reactions
It is proposed to study the salient effects of coupling between discrete and continuous states near various particle emission thresholds using the shell model in the complex energy plane. This model provides the unitary formulation of a standard shell model within the framework of the open quantum system for the description of well bound, weakly bound and unbound nuclear states.
Recent studies have demonstrated the importance of the residual correlation energy of coupling to the states of the continuum for the understanding of eigenstates, their energy and decay modes, in the vicinity of the reaction channels. This residual energy has not yet been studied in detail. The studies of this thesis will deepen our understanding of the structural effects induced by coupling to the continuum and will provide support for experimental studies at GANIL and elsewhere.