Heat transport in hot plasmas

The question of heat transport is fundamental to understand the radiative or hydrodynamic properties of hot plasmas. Thanks to the development of numerous plasma diagnostics, LULI plays a major role in the elucidation of these complex phenomena.

The question of heat transport is fundamental not only in the framework of CFI but also in astrophysics. In the framework of the CFI in indirect attack, the experiments carried out on the National Ignition Facility (NIF) have shown an energy deficit compared to the simulations of about 200 kJ on the 1.6 MJ of initial laser energy. A possible explanation for this energy deficit is the better consideration of heat transport in the hydrodynamic codes. The laser energy is deposited where the critical density is found and then transported by the electrons towards the cavity walls. The classical heat transport models (Spitzer type) fail to reproduce the experimental data obtained. These same models fail in these high energy density regimes and are used in astrophysics.

The influence of magnetic fields, self-generated or pre-imposed, on heat transport can also be studied.

Fast particles (alpha particles, protons) are another vector of heat transport. Heat deposition by fast particles is fundamental in CFI; indeed, ignition can only be achieved if the alpha particles redeposit their energy in the hot spot (and not in its environment). The stopping power of fast particles in dense and hot matter is therefore a primary subject of study. In this regime, the experiments are still few and are still too imprecise to discriminate the various models of stopping power. APOLLON, by allowing the acceleration of particles at relativistic velocities, will allow to extend the current studies to very high ionic velocities. The team of J. Fuchs will lead the studies on this subject.

The existence of a non-negligible fraction of supra-thermal electrons (generated by parametric instabilities) can also, in certain cases, prevent ignition. Measures to avoid this have been implemented but a precise characterization of the electron distribution remains of interest, especially in the context of shock ignition experiments. Experiments, using spectroscopy tools and analyzed with the MARIA code of F. Rosmej, have been successfully carried out by the PAPD team on the LULI2000, PALS and ELFIE lasers.

The electron-ion equilibrium time: The electrons, having a lower mass than the ions, respond first to the laser excitation. In the first moments of the laser irradiation, the temperature of the electrons is then higher than that of the ions. The heat is then transported from the electrons to the ions by collisions. The ion/ion and electron/electron thermal equilibria are quickly established while the electron/ion thermal equilibrium is reached later. These "non-equilibrium" conditions can give the matter atypical properties. The determination of the electron-ion relaxation rate in this complex domain of hot dense matter is therefore an active subject of study. The LULI, with its CPA laser facilities (pico2000 and APOLLON), capable of heating matter before any hydrodynamic expansion, will effectively contribute to these international efforts.

LRadiative transport: The knowledge of the opacity of plasmas at densities several times the density of the solid and temperatures ranging from the hundred eV to the keV remains a theoretical and experimental challenge. In stellar physics, the absorption coefficients of the constituent elements of stars condition the heat transport from their core to their surface. In the case of the Sun, the question of the opacity of iron remains a mystery. Experiments carried out at Sandia National Laboratory (USA) have shown that the opacity of iron under solar conditions is still very poorly understood and that experimental data in this field were needed.
What we have done at LULI: LULI has developed - in collaboration with the CEA (DRF and/or DAM) - two platforms dedicated to opacity measurements, in cavity and in open geometry, on LULI2000. The first of these two platforms is dedicated to the study of X and XUV opacities in the vicinity of thermodynamic equilibrium. The radiative heating - uniform but moderate (of the order of a few tens of eV) - of elements of intermediate Z is ensured thanks to an innovative double cavity. After a first demonstration for nickel plasmas, the platform will soon be used to collect new experimental data in order to answer, for example, the problem posed by the opacity of iron in the conditions of the Sun and the role of ablator dopants in fusion microballoons.
The second platform, developed on the ELFIE installation and then on PICO2000, is devoted to the study of non-equilibrium plasmas, hot - of the order of keV - and at solid density, for astrophysics and CFI.