Vincent Morel

Global rate coefficients for ionization and recombination of carbon, nitrogen, oxygen, and argon

The flow field modeling of planetary entry plasmas, laser-induced plasmas, inductively coupled plasmas, arcjets, etc., requires to use Navier-Stokes codes. The kinetic mechanisms implemented in these codes involve global (effective) rate coefficients. These rate coefficients result from the excited states coupling during a quasi-steady state. In order to obtain these global rate coefficients over a wide electron temperature (Te) range for ionization and recombination of carbon, nitrogen, oxygen, and argon, the behavior of their excited states is investigated using a zero-dimensional (time-dependent) code. The population number densities of these electronic states are considered as independent species. Their relaxation is studied within the range 3000  K≤Te≤20 000  K and leads to the determination of the ionization (ki) and recombination (kr) global rate coefficients. Comparisons with existing data are performed. Finally, the ratio ki/kr is compared with the Saha equilibrium constant. This ratio increases more rapidly than the equilibrium constant for Te>15 000  K.The flow field modeling of planetary entry plasmas, laser-induced plasmas, inductively coupled plasmas, arcjets, etc., requires to use Navier-Stokes codes. The kinetic mechanisms implemented in these codes involve global (effective) rate coefficients. These rate coefficients result from the excited states coupling during a quasi-steady state. In order to obtain these global rate coefficients over a wide electron temperature (Te) range for ionization and recombination of carbon, nitrogen, oxygen, and argon, the behavior of their excited states is investigated using a zero-dimensional (time-dependent) code. The population number densities of these electronic states are considered as independent species. Their relaxation is studied within the range 3000  K≤Te≤20 000  K and leads to the determination of the ionization (ki) and recombination (kr) global rate coefficients. Comparisons with existing data are performed. Finally, the ratio ki/kr is compared with the Saha equilibrium constant. This ratio increases ...

Electron properties experimental determination of a nascent nanosecond aluminum plasma

In order to validate a collisional-radiative model elaborated in the purpose of describing the behavior of the plasma produced during a LIBS (Laser Induced Breakdown Spectroscopy) experiment, the measurement of the electron temperature and density is performed during the rst tens of nanoseconds by using optical emission spectroscopy. Synthetic spectra are calculated involving lines, Bremsstrahlung and radiative recombination and then compared with experiments. In the case the energy ux density reaches the maximum value 10 14 W m 2 , the electron temperature and density reach 16000 K and 3 10 25 m 3 respectively.

Physico-Chemistry of Planetary Atmospheric Entry Plasmas

This contribution deals with the description of the physico-chemistry in the reactive flow produced near the surface of a body entering a planetary atmosphere in hypersonic regime. A shock layer is formed in which a boundary layer is developed where energy is released to the body surface. N2 is chosen as a test-case of Earth atmospheric reentry to illustrate the main characteristics of the flow. The vibrational and electronic specific Collisional-Radiative model CoRaM-N2 is described and implemented in a one-dimensional Euler flow solver. The well-known FIRE II flight experiment conditions are used to illustrate the behaviour of the different ground and excited states of atomic and molecular species. The results show that the three successive phases occur: vibrational excitation, dissociation and ionisation. Radiation plays a minor role. According to the upstream thermodynamic conditions, the boundary layer edge can be in local thermodynamic equilibrium or not. The different strategies using thermal protection systems are discussed to reduce the damaging of the entering body.

State-to-state modeling of non equilibrium low-temperature atomic plasmas

The most relevant approach leading to a thorough understanding of the behavior of non equilibrium atomic plasmas is to elaborate state-to-state models in which the mass conservation equation is applied directly to atoms or ions on their excited states. The present communication reports the elaboration of such models and the results obtained. Two situations close to each other are considered. First, the plasmas produced behind shock fronts obtained in ground test facilities (shock tubes) or during planetary atmospheric entries of spacecrafts are discussed. We focused our attention on the nitrogen case for which a complete implementation of the CoRaM-N2 collisional-radiative model has been performed in a steady one-dimensional computation code based on the Rankine-Hugoniot assumptions. Second, the plasmas produced by the interaction between an ultra short laser pulse and a tungsten sample are discussed in the framework of the elaboration of the Laser-Induced Breakdown Spectroscopy (LIBS) technique. In the p...

Achievement of local thermodynamic equilibrium for ns laser-induced plasmas on aluminium sample at different wavelengths

The Collisional-Radiative model CoRaM-Al is elaborated and implemented in a 0D numerical approach in the purpose of describing the formation of the plasma resulting from the interaction between a τ = 4 ns Nd:YAG laser pulse and an aluminium sample in vacuum. The influence of the four harmonics at 266, 355, 532 and 1064 nm on the behavior of the nascent plasma is studied. In each case, the fluence is set equal to the threshold above which a phase explosion takes place (fluence of 7.7, 7.4, 6.8, 5.1 J cm−2 in order of increasing wavelength). The model takes into account free electrons and excited states of Al, Al+, Al2+ and Al3+. Both groups of particles are characterized by their translation temperature in thermal non-equilibrium. Besides, each population density is assumed to be in chemical non-equilibrium and to behave freely through the involved seven elementary processes (electron impact induced excitation and ionization, elastic collisions, multiphoton ionization, inverse laser Bremsstrahlung, direct thermal Bremsstrahlung and spontaneous emission). Atoms passing from the sample to the gas are described by considering classical vaporization phenomena (governed by the Hertz-Knudsen law) so that the surface temperature is limited to values less than the critical point (Tc = 6700 K). The relative role of the elementary processes is discussed and the time-evolution of the excitation of the species is analyzed for the four considered wavelengths. This study allows to determine the different excitation temperatures as well as their evolution in time. Thus the conditions required for the achievement of the Local Thermodynamic Equilibrium can be precisely described.

Temporal description of aluminum laser-induced plasmas by means of a collisional-radiative model

A 0D numerical approach including a Collisional-Radiative model is elaborated in the purpose of describing the behavior of the nascent plasma resulting from the interaction between a laser pulse (λ = 532 nm, τ = 4 ns and F = 6.5 J cm−2) with an aluminum sample. The species considered are Al, Al+, Al2+ and Al3+ on their different excited states and free electrons. Both groups of particles are characterized by their translation temperature in thermal non-equilibrium state. Besides, each population density is assumed to be in chemical non-equilibrium and behaves freely through the seven involved elementary processes (electron impact induced excitation and ionization, elastic collisions, multi-photon ionization, inverse laser Bremsstrahlung, direct electron Bremsstrahlung and spontaneous emission). Atoms passing from sample to gas phase are described by considering classical vaporization phenomena so that the surface temperature is limited to values less than the critical point. The relative role of the elementary processes is discussed and the time-evolution of the excitation of the species is analyzed.

CoRaM-Al: a Collisional-Radiative model dedicated to aluminum laser-induced plasma

A nonlinear time-dependent Collisional-Radiative (CR) model is elaborated in the purpose of understanding the behavior of the plasma created during a Laser Induced Breakdown Spectroscopy (LIBS) experiment when a nanosecond pulse (4 ns at 532 nm and 65 mJ) is focused on an aluminum sample with uence approximatively equal to 2 10 W m . The underlying interaction is modeled from the moment of the beginning of the sample vaporization until the nal recombination phase. The plasma is assumed in thermal and chemical non equilibrium: (1) electrons and heavy particles (aluminum ions to the Al ion and atoms) are in Maxwell-Boltzmann equilibrium with di erent temperatures and (2) the heavy particles are neither in Boltzmann nor in Saha equilibrium with electrons. The electronic excited states are considered independent and behave freely. The whole is coupled by collisional and radiative elementary processes: by this way, it is possible to follow in time the dynamic of the plasma and to observe the possible departure from equilibrium. We show that the plasma equilibrium is obtained with di culty even if the electron density is high owing to the persistence of the initial strong non equilibrium.