PhD Position: “Study of low-frequency radiation produced by particle acceleration at ultra-high laser intensity in relativistic plasmas” at CELIA, Bordeaux University, France

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 frequency 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 in 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 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 PhD 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).

Progress of the thesis: Thesis’s first year will consist in acquiring the bibliographical knowledge
necessary for the proposed work. The student will be invited to familiarize him/herself with the physics
of relativistic plasmas, particle acceleration by ultrahigh intensity lasers and the production of
electromagnetic radiation by a beam of particles accelerated in a plasma. Original scenarios and
devices optimizing the production of radiation will be proposed. The student will invest in a kinetic,
"particle-in-cell" code available at CELIA and will be guided to adapt routines to map and manipulate
populations of electrons and accelerated ions according to their energy range. The second year of
the thesis will aim to test the interaction schemes proposed by the student from simulations using
this code on the massively parallel supercomputers of the TGCC and the CINES. The third year of
the thesis may be devoted to comparing the theoretical and numerical results acquired by the student
with experimental data already published or obtained within the framework of collaborations. This last
year will also be that of writing the thesis.

The candidate must have advanced training in plasma physics and/or scientific computing, with an
ability to handle simulation codes or for programming (Python, Fortran, C++).

Thesis supervisors: Luc Bergé & Emmanuel D’Humières – Centre Lasers Intenses et Applications,
Université de Bordeaux, CNRS, CEA, 33405 Talence, France
PhD School : Ecole Doctorale Sciences Physiques et de l’Ingénieur de l’Université de Bordeaux
(Ecole doctorale n°209)

Contact: Luc Bergé – luc.berge@cea.fr or luc.berge@u-bordeaux.fr – +33 (0)5 40 00 33 66 – Centre
Lasers Intenses et Applications, Université de Bordeaux, CNRS, CEA, 33405 Talence, France
Emmanuel D’Humières – emmanuel.dhumieres@u-bordeaux.fr – +33 (0)5 40 00 37 77 – Centre
Lasers Intenses et Applications, Université de Bordeaux, CNRS, CEA, 33405 Talence, France