Master Internship

As an ionized media, a plasma is not subject to electrical breakdown and can therefore withstand electric fields of extremely large amplitude. Laser-plasma accelerators are based on this property. An intense laser pulse is focused in a gas cloud that is instantaneously turned into a plasma. The laser then expels electrons out of the propagation axis and generates an ion cavity in its wake. The electric fields in this cavity are 3 to 4 orders of magnitude larger than those in conventional accelerators. Electrons trapped in the cavity can thus be accelerated up to energies of a few gigaelectronvolts in just a couple of centimeters, while it would require hundreds of meters with a radio-frequency accelerator.

The maximum energy is mainly limited by the dephasing between the electron beam and the accelerator field.This phenomenon arises from the difference in speed between the laser driver and the electron beam, which makes that the electron beam exits the accelerating field after a few centimeters of acceleration. This limitation could, in principle, be suppressed by controlling the velocity of the ion cavity, in order to lock it on the laser velocity. In practice, this implies that the laser pulse should propagate in a vacuum at a superluminal speed (> c). Different techniques make it possible to obtain this type of propagation, but they are not compatible with ultra-high intensity laser pulses, yet.

The objective of the internship is to demonstrate a technique that we have recently proposed and that allows producing superluminal pulses of arbitrary intensities. The student will use an interferometer to measure the speed of propagation. It will also test the effect of different parameters on this speed. This work could be completed by a theoretical study.

The internship could be pursued into a PhD during which the superluminal laser beam will be used to accelerate electrons. The student will also study the physics of this new regime of laser-plasma interaction that shouldallow increasing the electron energy by at least one order of magnitude, compared to the state of the art.

Ce stage pourra-t-il se prolonger en thèse? Possibility of a PhD? : YES

Si oui, financement de thèse envisagé/ financial support for the PhD: IPP grant

Contact : Cedric Thaury

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 (voir photo ci-dessous [1]).

Ces dernières années plusieurs applications basées sur les filaments de plasma ont été proposées telles que le paratonnerre laser [2] et l’antenne virtuelle radiofréquence [3].

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 œuvre 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.

Le candidat devra avoir des connaissances de base en optique ou en physique des plasmas, un bon niveau d’anglais et présenter de solides références scolaires.

Ce stage sera rémunéré et pourra donner lieu à une prolongation en thèse.

[1] B. Forestier, et al., “Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament”, AIP Advances 2, 012151-13 (2012)

[2] J. Kasparian et al. Science 301, 61 (2003)

[3] Y. Brelet, et al., Radiofrequency plasma antenna generated by femtosecond laser filaments in air”, Applied Physics Letters 101, 264106 (2012)

[4] Improving supersonic flights with femtosecond laser filamentation, P.-Q. Elias, et al., Science Advances 4, eaau5239 (2018)

Ce stage pourra-t-il se prolonger en thèse ? Possibility of a PhD ? : OUI

Rémunération du stage/ financial support for the internship : OUI

Financement de thèse envisagé / financial support for the PhD : Ecole doctorale IP Paris

Type de stage et/ou de thèse (expérience/théorie/simulations) : Expérience

Contact : Houard Aurelien

PhD Thesis

Context:
Non-destructive testing (NDT) is used to detect and size the object defects as well as to follow their evolution, with challenges in terms of safety (avoiding critical accidents due to breakage), performance (identify the repairs to be made, plan the maintenance phases) and quality (checking the quality of welds for example). The fields of application are very diverse: energy (nuclear, oil, gas, hydraulic, wind, etc.); land transport (particularly rail); Marine ; aeronautics; aerospace; defense and security… Among the various NDT processes, X-ray inspection is a very diagnostic sought after because it offers the best performance, particularly in terms of spatial resolution for large  thicknesses of materials. The LOA and its partner SourceLAB (spin-off of the LOA) have been leading since 2015 a unique project in the world to develop a laser-plasma accelerator for non-destructive testing of dense and thick material. This project, supported by The objective of the DGA is to put the first LOA into operation high energy X-ray tomography demonstrator to the world. It’s about a disruptive technological innovation for the NDT market. The sources currently in use (conventional particle accelerators and sources radioactive) remain limited in terms of spatial resolution and X-energy. The interest of laser-plasma sources is multiple: (i) sub-millimeter resolution for the identification of small defects, (ii) versatility and production potential various radiations (X, electrons, neutrons) from the same machine, allowing identification of specific elements in images.

Thesis subject:
The general objective of the thesis is to develop the digital twin of the X-ray experiment carried out at the LOA in order to optimize it and demonstrate its relevance for NDT. The candidate will be supervised by a research engineer specializing in digital codes and a researcher CNRS. He will be part of a team dedicated to this project, which consists of a laser engineer, a research engineer, a fixed-term engineer and a post-doc working on experiments. The work will also be carried out in close collaboration with the startup Sourcelab. The first objective will be to completely simulate the experiment using a Particle In Cell (PIC) code for modeling laser-plasma acceleration, then a Monte-Carlo code to model the electron-matter interaction and finally the X-matter interaction in the X-ray object and the detector. Secondly, the goal will be to conduct a digital exploration and algorithmic to improve the processing of X images and increase their quality. This optimization will first be done on two-dimensional images, then on three-dimensional reconstructions by identifying adapted tomography algorithms.

Required profile :
The candidate must have general knowledge of optics and physics and a strong taste for simulation. Mastery of one or more languages of programming would be a plus.
He must be autonomous, have a good analytical mind and a good ability editorial.

Contact: cedric.thaury@ensta.fr

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

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 : Houard Aurelien

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 : Houard Aurelien

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 : Houard Aurelien

Postdoc positions

Context :

Laboratoire d’Optique Appliquée (LOA) is developing a new 3D X-ray imaging technique, called plenoptic, based on the combination of main optic and wavefront sensor. Three systems have been built. The first one was a demonstrator running at PETRA III synchrotron in Germany at an energy of 11 keV. It is now disassembled after producing excellent results. Another camera is set and aims at imaging living biological cells. It is working around 400 eV. The last system is targeting small animal imaging with X-rays of energy around 17 keV. Both systems are tabletop.

Both systems still need to be fully tested and improved. The low energy camera is running at LOA, near Paris, while the high energy system is set at Imagine Optic company in Bordeaux, France. LOA’s team is in charge of running and improving both systems through a collaboration agreement.

Today, we are using three software: those specifically developed for the so-called focused ad unfocused plenoptic cameras and a homemade software integrating the two geometries. These softwares are too slow and complex for generating a 3D image.

Topic of the post-doctoral fellowship:

A post-doctoral position is open in FLUX group for 18 months with possibility of extension. During this period, the post-doc will be in charge of analyzing ability of machine vision software to generate efficiently a 3D image from the raw plenoptic data. Disparity as well as machine learning are two options we consider. The candidate will have to choose one or different techniques, implement it/them and then test it/them on real X-ray plenoptic images.

Experience :

Candidate should have developed strong computational skills related to machine vision. Good knowledge on PYTHON is a plus but not required. The candidate should be rigorous and have a proficiency in working in team. English is the work language.

Contact : philippe.zeitoun@ensta.fr

Context :

Laboratoire d’Optique Appliquée (LOA) is developing a new 3D X-ray imaging technique, called plenoptic, based on the combination of main optic and wavefront sensor. Three systems have been built. The first one was a demonstrator running at PETRA III synchrotron in Germany at an energy of 11 keV. It is now disassembled after producing excellent results. Another camera is set and aims at imaging living biological cells. It is working around 400 eV. The last system is targeting small animal imaging with X-rays of energy around 17 keV. Both systems are tabletop.

Both systems still need to be fully tested and improved. The low energy camera is running at LOA, near Paris, while the high energy system is set at Imagine Optic company in Bordeaux, France. LOA’s team is in charge of running and improving both systems through a collaboration agreement.

Topic of the post-doctoral fellowship :

A post-doctoral position is open in FLUX group for 18 months with possibility of extension. During this period, the post-doc will be in charge of finishing to set the two systems, test them and then optimizing them. On both cases, the main objective consists in generating 3D images of adequate samples in a single exposure. Known samples like USAF 1951 will be used first; we will move later to biological samples provided by our collaborators. The measurement of the dose delivered to generate a 3D image will be performed at every step. Most interesting samples will be imaged by X-ray computed tomography and results will be compared to those obtained by X-ray plenoptic.

Experience :

Candidate should have a strong background on experiment in optics or using complex optical systems. Knowledge on X-rays, Optical design software (ZEEMAX, OSLO etc) or PYTHON is a plus but not required. The candidate should be rigorous and have a proficiency in working in team. English is the work language.

Contact : philippe.zeitoun@ensta.fr

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 : Houard Aurelien

Context :

Non-destructive testing (NDT) allows to detect and measure defects of the object as well as study their evolution, potential danger in terms of security (avoiding critical accidents caused by the rupture), performance (identification of repair actions to be made during the maintenance) and quality (for example, controlling quality of welding). Fields of application are very diverse: engineering (nuclear, fuel, gas, hydraulic, wind, etc.); land transport (in particular rail transport); the navy; aeronautics; aerospace; defense and security… Among various NDT processes, X-ray based inspection has strong advantage because it offers the best performance, particularly in terms of spatial resolution for large thicknesses of materials. LOA and its partner SourceLAB (a LOA spin-off) have been running a worldunique project since 2015 to develop a laser-plasma gas accelerator for the nondestructive testing of dense and thick matter. This project, supported by the DGA, aims to put the world’s first high-energy X-ray tomography demonstrator into operation at LOA. This is a breakthrough technological innovation for the NDT market. The sources currently used (conventional particle accelerators and
radioactive sources) remain limited in terms of spatial resolution and X-ray energy. The interest of laser-plasma sources is multiple: (i) sub-millimeter resolution for identification of small defects, (ii) versatility and potential for production of various radiations (X-rays, electrons, neutrons) from the same machine, allowing identification of specific elements in the images.

Subject of the post-doctorat :

The general goal of the post-doctoral fellow is to develop the capacities and optimize the performance of the X-ray source in terms of stability, resolution and signal-to-noise ratio and to demonstrate its relevance for NDT. The candidate will be supervised by a research engineer and a CNRS researcher. He will be a part of a team dedicated to this project, which includes in addition to the post-doc, a laser engineer, a CDD engineer who will work on the experiments and a doctoral student who will take care of the numerical part. The first objectives will be the qualification of the X-ray source, which has just been put into operation, and its optimization at 10 Hz for a sub-1mm resolution. Secondly, the goal will be to perform the tomography of a thick part with submm resolution, as well as to experimentally optimize the X-ray source to approach a sub-100 micron resolution, based on the results, obtained using numerical modeling. The final goal will be to perform the tomography of objects of interest with the optimized system. The thickness of the considered parts will depend on the results obtained previously.

Required profile:

The candidate is expected to have strong skills in experimental physics and use of complex systems. He/she should have experience in using ultra-intense lasers for laser-matter interaction experiments. Skills in electron accelerators, associated X-ray sources, or X-ray imaging would be a plus. He/she is expected to have a taste ffor teamwork, experimental research and development of instrumentation.

Contact : cedric.thaury@ensta.fr

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