19 Décembre – Thesis defense - Quentin Moreno-Gelos

14 h Amphi 1 - building A9 (University of Bordeaux / Talence campus)

Non-relativistic collisionless shocks in laboratory astrophysics.

Collisionless shocks are ubiquitous in the Universe, especially in the supernova remnants, and are formed via various plasma instabilities mainly depending on the speed and magnetization of plasma flows. The description of such shocks requires a kinetic approach, both analytical and numerical.
In this thesis, we have studied, through Particle-In-Cell (PIC) simulations, the underlying processes by which instabilities compete with each other.
We have shown that the reduction of the ion-to-electron mass ratio, often used in numerical simulations to accelerate the dynamics of shocks, can have strong consequences on the energy transfer between particles during the non-linear phase of instabilities.
These instabilities, like the ionic acoustic instability (IAI) lead under certain conditions to the formation of electrostatic shocks, which can give rise to phase space holes formation, propagating in the downstream shock region, and accelerating the shock.
The addition of an external magnetic field leads to different shock mediation, which can vary between the IAI to the slow or fast magneto-sonic waves as a function of the obliquity between the magnetic field and the shock normal.
Furthermore, we have shown that the orientation of the magnetic field makes it possible to choose between a convex or concave dispersion of the plasma waves leading to the creation of precursor waves in the upstream or downstream shock  regions.
These magnetized shocks are correctly represented by the magnetohydrodynamic (MHD) model as long as they remain laminar and their potential in the downstream region is not large enough to reflect the particles of the upstream medium.
We have shown that even for sub-critical shocks, a fraction of reflected ions, which cannot be modeled by the MHD, is sufficient for the growth of solitary waves upstream of the shock, leading to the acceleration of the latter, but not to a process of 'self-reformation' as for super-critical shocks.
Although spatio-temporal scales are very different, scaling laws make possible the study of such phenomena in the laboratory. Our numerical studies have been done in the context of shock tubes that can be experimentally tested.
As such, we propose in this thesis an experiment on the creation of magnetic islands, formed by the interaction of plasmas generated by the irradiation of laser targets bathed in an external magnetic field, leading to the formation of such shocks.
Finally, we experimentally and numerically demonstrated the formation of collisionless electromagnetic shocks through the Weibel instability stimulated by theBiermann Battery instability, and leading to particle acceleration by the Fermi mechanism.
This new type of experiment could explain the origin of cosmic radiation from supernova remnants.

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