NEW! On this page will be posted all the upcoming seminars organised by the PLAS@PAR community
Dr Katerina Falk, Equation of state and electron transport measurements of warm dense Carbon
Institute of Physics of the ASCR, ELI-Beamlines, Prague
March 3rd, UPMC - Jussieu, LPP/LERMA (24-34, 5th floor) - 10 am
Warm dense matter (WDM) is a unique state of dense plasmas common in many astrophysical objects including large gaseous planets, brown dwarfs, crusts of old stars and others. It is readily created when laser radiation heats up solid targets and during implosion of deuterium-tritium-fuel pellets during inertial confinement fusion (ICF) implosions. At moderately high temperatures at 1-100 eV, solid densities, and pressures above 1 Mbar the plasma is created both by collisional and pressure ionization resulting in a system where ions are strongly correlated and the electron population is partially or fully degenerate. Such states are challenging to study both theoretically and experimentally. A detailed knowledge of the equation of state (EOS) of light elements such as carbon is however essential to understanding of many processes in the formation and structure of these massive astrophysical objects.
Thus, a novel technique of x-ray Thomson scattering (XRTS) was developed to provide an active probing of such dense plasma states. The talk will present a new platform developed for the OMEGA laser facility to obtain a full EOS measurement of off-Hugoniot states in the WDM regime that does not rely on any theoretical models. Spatially and spectrally resolved XRTS was used to obtain accurate temperature measurement. Additional diagnostics including x-ray radiography, velocity interferometry and streaked optical pyrometry provided complementary measurements of density and pressure. This platform was used to study shock-released diamond and graphite at pressures between 1 and 10 Mbar and temperatures between 5 and 15 eV. The platform was also modified to obtain the first x-ray Thomson scattering data from shocked low density CH foams reaching five times compression and temperatures of 20–30 eV. Data analysis with the support of ATOMIC, RAGE and PETE codes confirmed that a significant preheat by thermal electrons transporting through warm dense matter was changing the jump conditions for the shockwave. Such conditions are relevant to convection and electric conductivity in ICF and white dwarfs.
Dr Setthivoine You, A field theory approach to plasma self-organization
University of Washington, USA
September 20th, UPMC - Jussieu, LPP/LERMA (24-34, 5th floor) - 3:30 pm
Self-organization is concerned with the spontaneous emergence of large-scale structures in physical systems. A fundamental conjecture, borrowed from the mathematics of topology, is the invariance of a global property during the process of self-organization: for example, a system relaxes to reduce energy constrained by a constant value of the helicity of the canonical momentum. We present a unifying field-theory framework that reformulates the single-particle, kinetic and fluid equations governing plasma dynamics as a single set of generalized Maxwell’s equations and Ohm’s law for canonical force-fields . The new Lagrangian includes terms representing the coupling between the motion of particle distributions, between distributions and electromagnetic fields, with relativistic contributions. The formulation shows that the concepts of self-organization and canonical helicity transport are applicable across single-particle, kinetic, and fluid regimes, at classical and relativistic scales. The framework shows that a species’ canonical helicity is well conserved compared to the species’ energy in shallow density gradients but not in steep density gradients (in the simplest case of an isolated, dissipative system). These results suggest that in the edge of multi-species, collisionless, kinetic plasmas, magnetic helicity can couple to ion canonical helicity, spontaneously generating flowing structures when density gradients are of the order of the ion skin depth. This field theory approach to helicity and energy evolution suggests that electrical engineering methods used for analyzing magnetostatic configurations can be extended to flowing magnetized (or non-magnetized) plasmas and flowing neutral fluids with finite vorticity. The driving circuits can be any combination of gravitational, pressure, kinetic or electrical supplies since these power supplies are simply enthalpy sources for a canonical Maxwell cicuit. The recently operational Mochi project at the University of Washington thus produces a laboratory astrophysical jet to investigate the interaction between flows and magnetic fields with concepts of generalized helicity transport. This work is supported by by US DOE Early Career Award DE-SC0010340.  S. You, Phys. Plasmas, 23, 072108 (2016).
Dr Alex Raga, Time variable jets
Instituto de Scienças Nucleares, UNAM, Mexico
June 8th, UPMC - Jussieu, salle 509 LPP/LERMA (24-34, 5e étage) - 15h00-17h00
One of the clear contributions of astrophysics to the theory of supersonic flows has been the study of jets from variable sources. This is a direct result of the fact that laboratory experiments (at least in the past) have focussed on steady jets, observations of astrophysical jets mostly show transients that result from the unsteadiness of the ejection mechanism. In the case of a hypersonic jet, the effect of an ejection velocity variability is to produce "internal working surfaces" that travel along the jet beam. These working surfaces are two-shock structures, separated by a high pressure region that ejects material sideways. A variable jet with internal working surfaces then naturally resembles the "aligned bow shock structures" seen in jets from young stars. The problem of a hypersonic, variable jet breaks into three main parts:
- the free streaming solution for the continuous regions of the jet
- the times and positions at which internal working surfaces are formed
- the equation of motion of the working surfaces (and its solution).
These three points will be developed, and the present limits of the analytic theory will be discussed.
Pr Richard Engeln, Reaction dynamics in a magnetized hydrogen plasma unraveled by optical spectroscopic techniques
Eindhoven University of Technology (TU/e)
March 23rd, 10h30, LPP, Palaiseau, salle CPHT Aile 0 ex LiX with visio-conference in Jussieu
Reaction dynamics in a magnetized hydrogen plasma unraveled by optical spectroscopic techniques
The development of efficient sources of reactive hydrogen radicals is important in many research fields and applications. For instance, atomic hydrogen radicals serve as primary reactive particles for surface modification or thin film deposition. For fusion plasma heating, one of the main research challenges is to develop efficient negative ion sources. A promising route is via dissociative attachment of ro-vibrationally excited hydrogen molecules H2r,v. The ro-vibrationally excited molecules are important precursors in volume reactions leading to excited hydrogen atoms. During my presentation I will discuss the importance of several molecular activated recombination (MAR) processes in expanding thermal hydrogen plasmas, and especially, the formation of excited atoms via the mutual neutralization process of H- and H2+.
The picture shows the expanding hydrogen plasma (flowing from left to right) weakly magnetized with a magnetic field (14 mT) that is applied parallel to the expansion axis.
Pr Edward Grant, Arrested relaxation in an isolated molecular ultracold plasma
Department of Physics & Astronomy / Department of Chemistry, University of British Columbia, Vancouver
February 15 - UPMC - Jussieu, salle 509 LPP/LERMA (24-34, 5e étage) - 11h-12h30
We will describe particular conditions under which an ultracold plasma evolves from a molecular Rydberg gas of nitric oxide, adiabatically sequesters energy in a reservoir of mass transport, and relaxes to form a spatially correlated strongly coupled plasma. Short-time electron spectroscopy provides evidence for complete ionization. The long lifetime of the system, particularly its stability with respect to recombination and neutral dissociation, suggest a robust process of self-organization to reach a state of arrested relaxation, far from thermal equilibrium.
Pr Vasco Guerra, Modelling heterogeneous molecule formation
Instituto Superior Técnico, Universidade de Lisboa, Portugal
February 8 - LPP, Palaiseau, salle CPHT Aile 0 ex LiX + Jussieu salle 509 LPP/LERMA (visio) - 10h30-12h00
This lecture addresses the study of molecule formation on surfaces using a mesoscopic description. In particular, different variants of a stochastic dynamical Monte Carlo approach are developed and compared with the deterministic approach based on reaction-rate equations. These include a null event algorithm, the n-fold way / BKL algorithm and an “hybrid” variant of the latter. NO2 formation by NO oxidation on Pyrex and O recombination on silica with the formation of O2 are taken as case studies. The influence of the grid size on the CPU calculation time and the accuracy of the results are analysed. It is shown that the dynamical Monte Carlo schemes are flexible, simple to implement, describe easily elementary processes that are not straightforward to include in deterministic simulations, can run very efficiently if appropriately chosen and give highly reliable results. Moreover, the present approach provides a relatively simple procedure to describe fully coupled surface and gas phase chemistries.