Master Intership

PhD Thesis

Laser-plasma electron acceleration offers a unique way to produce highly energetic and ultra-short electron bunches, on very short distances. It have risen much interest since the first, pioneering, experiments in the early 2000s (Malka 2002; Faure et al. 2004). The interaction between an intense laser pulse and a target material is responsible of the whole extraction, selection and acceleration process, which makes primordial the understanding of the role of the involved parameters, such as the target density, shape and profile, laser duration, phase and intensity.
Among the research fields which laser-driven particle sources are relevant for, radiation biology opens to the exploration of fundamental aspects of radiation toxicity on living matter, that will be accessible only with a radiation source as short as the physical dose deposition time (Bayart et al. 2019; Favaudon et al. 2000; 2014). In order to make laser-driven electron sources interesting and compatible with radiobiology applications, a number characteristics should be addressed, such as the total charge per accelerated bunch, the spectral features, the stability and the duration. The required improvements demand a deep understanding of the acceleration mechanisms, the design of novel acceleration strategies and schemes.
Throughout the thesis activity, high potential topics for fundamental and applied science will be addressed, in the field of laser-created plasmas, particle acceleration, particle detection or dosimetry and engineering of experimental systems towards applications.

keywords :Ultra-intense lasers, Laser-plasma interaction, Laser-driven electron acceleration, Numerical simulations, Ultra-high dose-rate

Contact : Alessandro Flacco / Cédric Thaury

To unlock the mysteries of the subatomic world and to study matter’s fundamental components and the forces between them, physicists use particle colliders to smash highly energetic particle beams into one another. But as we push the particle energy of these colliders even higher, conventional accelerator techniques are attaining their limits and new concepts are emerging. The use of an ionized gas —or plasma— circumvents the most significant barrier of conventional techniques by increasing the energy gained per unit length by several orders of magnitude. One strategy being studied in the research field of plasma accelerators consists in using a particle beam (typically made of electrons or positrons), « the driver », to excite a high-amplitude plasma wave [1], that can then be used to accelerate the main particle beam [2]. One of the key advantages of this strategy is the good energy efficiency [2], which makes it more appropriate for high energy physics (HEP) applications, where energy efficiency is critical.

While plasma accelerators made considerable progress to manage to accelerate electrons, two major questions remain open for beam-driven plasma accelerators. One the one hand, HEP requires extremely bright beams to reach the necessary luminosity, which requires two ingredients: being capable of generating ultralow-emittrance and ultrabright beams, which can have many applications beyond HEP, and preserving this quality during acceleration in the plasma. On the other hand, research on the acceleration of the electron antimatter counterpart, the positron, has only recently yielded results [3]. This research is however extremely important as it is mandatory to accelerate both electrons and positrons to apply plasma accelerators to HEP and particle colliders.

While the recent experimental results [3] have attracted a lot of attention and have opened new opportunities, theoretical and numerical studies are now necessary to tackle some of the most important remaining challenges in our field of research. The PhD student will work at LOA and will use a Particle-In-Cell code to simulate the interaction between the particle beams and the plasma. New tools and numerical schemes will be developed and implemented in the Particle-In-Cell code to improve the modeling of this physics. The work will aim at addressing, both theoretically and numerically, how we can generate high brightness beams in a plasma accelerator and preserve its quality, as well as studying new schemes to accelerate positrons in plasma. During the PhD, the student will also work on experiments carried out using the Salle Jaune high power laser system at LOA, where the beam-driven plasma accelerator is powered by laser-accelerated electron beams, and on experiments carried out at the FACET-II accelerator facility (SLAC, US) where positron beams will become available in the time frame of the PhD thesis.

Contact : Sebastien Corde

Our group has recently demonstrated a new scheme to achieve backward lasing from air plasma using circularly polarized 800 nm femtosecond pulses [2-4], which is widely available especially for high energy pulses. Up to now, there exist several important fundamental questions concerning this new scheme of backward lasing. For example, the presence of oxygen molecules is found to decrease the lasing efficiency significantly and the physical mechanism for this detrimental role is unclear. At the same time, the pulsed backward emission has not been characterized in the temporal domain and the dynamics of this lasing process is largely unknown. As to its applications, it is still at an early stage.

The student will participate in a series of research activities in order to clarify the fundamental physical mechanisms involved in the lasing actions of neutral nitrogen, to characterize this transit lasing process in the temporal domain. Another aspect of his/her research is to search for the optimal operational conditions for the backward nitrogen laser and improve its properties such as pulse energy and divergence. Several schemes have been envisaged at this moment.

Contact : Aurélien HOUARD

Quand une impulsion laser intense de durée femtoseconde se propage dans l’air, elle donne lieu à la filamentation, un processus spectaculaire où le faisceau se contracte spatialement pour former un mince canal de lumière dans lequel l’intensité est maintenue à ~1015 W /cm2. La filamentation s’accompagne de la formation d’une longue colonne de plasma de courte durée de vie générée dans le sillage de l’impulsion laser. Cette colonne présente notamment la capacité d’initier et de guider des arcs électriques de plusieurs mètres avec une grande reproductibilité.
Ces dernières années plusieurs applications basées sur les filaments de plasma ont été proposées telles que le paratonnerre laser et l’antenne virtuelle radiofréquence.
Pour rendre possibles ces applications, il est nécessaire de caractériser et d’optimiser les paramètres du plasma produit par le laser femtoseconde ainsi que l’arc électrique guidé. Pour ce faire, différentes techniques de spectroscopie résolue en temps, d’interférométrie et d’imagerie [4] seront mises en oeuvre dans le cadre de ce stage. Elles seront testées sur des expériences de guidage de décharges électriques en laboratoire dans le cadre du développement d’une antenne plasma et d’une application de paratonnerre laser.

Contact : Aurélien HOUARD

Our team studies laser-plasma interaction with extreme laser pulse parameters. The laser pulse is composed of a few optical cycles so that the electromagnetic field oscillates only a few times in the pulse. This unique property brings us to an interesting regime that has not been studied much until now and in which the absolute phase of the laser (also called the carrier envelope phase, CEP) plays an important role.

The research is focused on several aspects: (i) making an ultra-compact electron accelerator relying on laser-plasma interaction, (ii) using that interaction to produce ultrashort X-UV radiation. In practice, our team performs laser-plasma interaction experiments in gas targets (for particle acceleration in underdense plasmas) but also on solid targets (for harmonic generation in overdense targets) using an ultra-intense high repetition rate laser. The obtained particle and radiation beams could have a very large impact because they could permit to probe matter at unprecedented time scales. They could permit the direct observation of atomic motion in a molecule for instance, or in the lattice of a solid state sample.

In this PhD work, the student will focus on the theory and numerical aspect of the research and study the physics of laser-plasma interaction in this new regime. He or she will use numerical simulations relying on the PIC (Particle In Cell) method and also theory for modeling and understanding the complex coupling of nonlinear effects occurring during the interaction. Effects such as plasma dispersion, ionization, relativistic self-focusing and their effect on the few cycle laser pulse will be studied. During the PhD, the student will provide theory and simulation support for providing guidance and interpretation to the experiments, both for underdense and overdense plasmas. In addition to providing this support, he or she will study a vast parameter space in order to be able to define the ideal laser source for driving a high repetition rate laser-plasma accelerator: laser wavelength, pulse duration, energy…The student will also model the X-ray radiation that such a source could produce.

Contact : Jérôme FAURE

Quand une impulsion laser intense de durée femtoseconde se propage dans l’air ou dans l’eau, l’apparition de nombreux effets d’optique non-linéaire donne lieu à la filamentation, un processus spectaculaire où une partie de l’énergie du faisceau se contracte pour former un long canal dans lequel l’intensité est maintenue à ~10^15 W /cm2. Ces filaments permettent d’envisager des applications telles que le guidage de faisceaux laser énergétiques ou de micro-ondes, le contrôle d’écoulements hydrodynamiques en régime supersonique, la génération de rayonnement laser UV ou d’impulsions THz à distance ou enfin le paratonnerre laser [1-3].
Une des difficultés liée à l’utilisation des nouvelles sources laser de très haute puissance est que le processus de filamentation devient fortement imprédictible. En effet, lorsque la puissance crête du faisceau dépasse la centaine de Gigawatt, celui-ci donne naissance à une multitude de filaments qui se développent par un mécanisme d’instabilité modulationnelle. L’objectif de cette thèse sera de tester expérimentalement plusieurs méthodes de mise en forme d’impulsions (optique adaptative, lames de phase, interféromètre pour la génération de trains d’impulsions..) permettant de contrôler l’apparition des filaments, de les organiser spatialement et d’optimiser les mécanismes d’ionisation. Les expériences seront réalisées au LOA sur les installations laser du groupe Filamentation et Interaction Laser Matière (F-ILM).

Contact : Aurélien HOUARD

When a charged particle beam propagates through a plasma or a conductor, return currents by the background plasma electrons are established. The counter-streaming system of beam electrons and of plasma electrons is unstable to electromagnetic perturbations, which leads to the formation of self-generated electromagnetic fields and of beam filaments whose typical size corresponds to the skin depth of the background plasma [1]. As a result of the electromagnetic instability, the particles in the beam experience large electromagnetic fields and thus emit bright burst of gamma rays. On the one hand, the physics of filamentation, magnetic field amplification, and gamma-ray burst emission has strong fundamental implications for astrophysics, as relativistic plasma instabilities are at the heart of many high-energy astrophysical environments and the mechanisms underlying the observed particle acceleration and gamma-ray emission (e.g. in GRBs) are not yet fully understood. On the other hand, the generation of gamma-ray sources in the laboratory is important for a variety of applications, from medical imaging and therapy to nuclear inspection, and electromagnetic filamentation instabilities in high-density plasmas could become a novel mechanism for bright gamma-ray sources [2].

When the density of the plasma is very high, which is the case when using a conductor or ionized solid medium, additional physical effects come into play, in particular the collisions of the plasma electrons with the ions. During the PhD, the student will use a Particle-In-Cell code to simulate the interaction between the particle beams and the plasma, and the growth of the electromagnetic instability. He will implement a new model to account for the collisions in the Particle-In-Cell simulations when the temperature of the plasma stays below the Fermi temperature, and therefore to reproduce the expected conductivity of the material. The work will aim at addressing, both theoretically and numerically, how the collisions affect the dynamics of the instability and the generation of gamma rays. Then, the student will study more generally the different modes of instability in different regimes of plasma density (from gas to solid density) to understand the conditions that optimize the generation of gamma rays, and how the gamma rays provide information about the growth of the instability. He will help in the modeling of experiments that aim at demonstrating for the first time the filamentation of high-energy particle beams in high-density plasmas, and the associated emission of bright gamma rays. These experiments will be conducted on the “Salle Jaune” high-power laser facility at LOA and using external beam time at the FACET-II accelerator facility (SLAC, US). Finally, he will contribute to the development of new modules in the Particle-In-Cell code, to describe additional physical effects occurring at high particle energies and to allow for a faster computation in the modeling of the dynamics of particle beams in plasma.

Contact : Sebastien Corde

Postdoc positions

Contexte : L’apparition des systèmes laser ultra-courts de haute puissance à la fin des années 90, et les avancées technologiques récentes dans les amplificateurs pompés par diodes, permettent aujourd’hui d’envisager à moyen terme le développement d’applications inédites des lasers de durée femtoseconde qui ont fait l’objet du prix Nobel de physique en 2018.

En 2017 le LOA et l’ONERA ont démontré en soufflerie supersonique qu’en effectuant ce dépôt d’énergie filamentaire en amont d’une onde de choc créée par une ogive, on pouvait réduire de façon transitoire la traînée de l’ogive de 30 à 50 %. Cette expérience constitue la première validation expérimentale du concept de perche laser aéronautique femtoseconde [1]. Les applications de cette perche laser sont nombreuses : réduction de trainée d’engins supersoniques, contrôle de trajectoire, contrôle de stabilité de régimes de vol. Ce système peut également être envisagé pour la réduction des nuisances sonores et en particulier du bang sonique, pour les futurs systèmes de transport supersoniques. Ce projet, principalement de nature expérimentale, s’inscrit dans la continuité de ces travaux sur le concept de perche laser. 

Le candidat sera amené à analyser les mécanismes de dépôt d’énergie laser dans l’air par filamentation à l’aide de méthodes de spectroscopie plasma et d’interférométrie. Il cherchera ensuite à mettre en évidence l’influence de la cadence du laser femtoseconde sur le filament de plasma et sur ses effets hydrodynamiques. Ce travail expérimental se fera au sein de l’équipe ILM. Ces résultats pourront ensuite être analysés avec l’ONERA et comparés à des simulations numériques du plasma. Dans un second temps, le postdoctorant pourra participer à une campagne de tests en soufflerie supersonique avec le nouveau système laser haute cadence et haute énergie du LOA qui sera disponible en 2021.

Le candidat devra avoir de solides connaissances en physique des plasmas, en diagnostics optiques ou plasma, et des notions d’optique.

Salaire net mensuel : entre 2100 et 2300 euros suivant l’expérience du candidat Durée du contrat : un ou deux ans.

Les travaux étant réalisés dans le cadre d’un contrat de la DGA, le candidat devra être issu de l’Union européenne ou de la Suisse.

Nom du responsable : Aurélien HOUARD

Contexte

L’apparition des systèmes laser ultra-courts de haute puissance à la fin des années 90, et les avancées technologiques récentes dans les amplificateurs pompés par diodes, permettent aujourd’hui d’envisager à moyen terme le développement d’applications inédites des lasers de durée femtoseconde qui ont fait l’objet du prix Nobel de physique en 2018.

Le présent projet consiste à étudier l’utilisation de filaments laser femtoseconde pour produire une antenne plasma « virtuelle » émettant dans la gamme RF [2]. Pour ce faire, il sera nécessaire d’enrichir la colonne de plasma initialement créée par l’impulsion laser femtoseconde à l’aide d’un générateur haute-tension [2,3] ou d’une source micro-onde de puissance [4]. Les deux méthodes seront testées expérimentalement dans les locaux LOA et l’antenne plasma sera caractérisée à l’aide de divers diagnostics (caméra rapide, interférométrie, mesure de rayonnement..).

Profile du candidat

Le candidat devra avoir de solides connaissances en physique des plasmas, en diagnostics optiques ou plasma, et des notions d’optique.

Salaire net mensuel : entre 2100 et 2700 euros suivant l’expérience du candidat Durée du contrat : un à deux ans.

Les travaux étant réalisés dans le cadre d’un contrat de la DGA, le candidat devra être issu de l’Union européenne ou de la Suisse.

Contact : Aurélien Houard