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Classical Molecular Dynamics simulation study of non equilibrium correlated plasmas

par Elodie PICO - publié le

Directeurs de thèse : Annette CALISTI
Coordonnées : annette.calisti@univ-­‐,

Sandrine FERRI
Coordonnées : sandrine.ferri@univ-­‐,

Description du sujet :

Transitions induced by intense fs X-­‐ray pulses drive the system highly out of equilibrium resulting in very complex matter heating and sample damage : cold solid matter transforms to warm dense matter (WDM) and then to a dense strongly coupled
plasma (DSCP).
When matter is exposed to extreme conditions the electromagnetic interaction (in particular the Coulomb force) between charged particles results in a collective behavior of interacting electrons and ions. At low temperatures and high densities, the Coulomb energy may exceed the thermal energy and the particles are
strongly correlated resulting in a geometrical ordering that impacts directly on the atomic structure. E.g., as particle density increases, mean distances between the
particles become smaller than the mean atom radius thereby shifting the bound states towards the continuum. Consequently the number of bound states becomes finite and correlated with continuum lowering, line shifts and line broadening. The partition function and therefore all thermodynamic properties of the system are also modified. Moreover, at low temperature, when the Fermi energy becomes of the same order as the thermal energy, electron degeneracy effects have to be taken into account.

In this context, a classical molecular dynamics (MD) code, the BINGO-­‐TCP code, has been developed recently to simulate neutral multi-­‐component (various charge
state ions and electrons) plasmas. Our simulations involve the mechanism of collisional ionization recombination necessary to simulate plasmas with a definite temperature and equilibrated populations of ions of various charge states. As
the particle environment is explicitly described, this method gives access to different quantities of interest (electric microfields, structure factors, etc.) and their statistical static and dynamics properties.

Preliminary calculations of dynamical structure factor [1] in warm dense beryllium experiment conditions [2], and the investigations of plasma effects on the ionization potential depression in stationary [3] and non-­‐equilibrium [4] regimes for aluminum plasmas [5] are in good agreement with results proposed by models found elsewhere.This contributes to the validation of the method.

At present an overall theoretical consistency in the description of the matter under extreme conditions has not yet been obtained.Even one of the “simplest” phenomena, the plasma edge shift (continuum shift, plasma potential depression) is under continuous discussion in the literature up to present days.

The proposed thesis concerns a study of the structural (many X-­‐ray diffraction experiments are being developed for this purpose) and dynamical properties of these
transient non-­‐ equilibrium correlated plasmas. To do this, improvements will be made to the simulation code, particularly the ability to take into account the effects of
electron degeneracy in simulations through a classical pseudo-­‐potential will be investigated. Moreover, the charge of a given ion results on many collisional and raditiave processes. Usually, collisional-­‐ radiative models (CR) are used to describe the population of the inner shell states of that given ion. Here, a Monte Carlo statistical
method can be used to account for radiation in the BINGO-­‐TCP code. Development of alternative methods such as CR models or Kinetic Monte Carlo approach [6] will be done in order to implement properly such radiative effects in our simulation.

[1] A. Calisti, S. Ferri, M. Marciante, B. Talin, Warm Dense Matter through Classical Molecular Dynamics, High Energy Density Physics, Vol. 13, 1-­‐8 (2014).
[2] S.H. Glenzer et al., Observations of Plasmons in Warm Dense Matter, Phys. Rev. lett. 98, 065002 (2007) ; H.J. Lee et al., X-­‐Ray Thomson-­‐Scattering Measurements of density and Temperature in Shock-­‐Compressed Beryllium, Phys. Rev. Lett. 102,
115001 (2009).
[3] A. Calisti, S. Ferri, B. Talin, Ionization potential depression in hot dense plasmas through a pure classical model, Contri. Plasma Phys. No 5, 360 (2015).
[4]A. Calisti, S. Ferri, B. Talin, Ionization potential depression for non equilibrated aluminum plasmas, J. Phys. B : At Mol. Opt. Phys. 48, 224003 (2015).
[5] O. Ciricosta et al., Direct Measurements of the Ionization Potential Depression in a
Dense Plasma, Phys. Rev. Lett. 105, 065002 (2012) ; D. J. Hoarty et al., Observations of the Effect of Ionization-­‐Potential Depression in Hot Dense Plasma,
Phys. Rev. Lett. 110, 265003 (2013).
[6] O. Peyrusse, Collisional-­‐Radiative modeling and interaction of monochromatic X-­ -rays with matter, to be published in Modern Methods in Collisional-­‐Radiative modeling of Plasmas, ed. Y. Ralchenko, Springr (2016)