PLAS@PAR Thematic Days

Workshop Dynamo Action: from the lab to the stars

September 10, 2019

Sorbonne University, Pierre et Marie Campus, Centre de conférence (Tower 44 - first floor - ROOM 108)

Contact: Ludovic Petitdemange ludovic.petitdemange@upmc.fr

REGISTRATION IS OPEN HERE

 "How could a rotating body such as the Sun become a magnet? This was the title of a famous ‘brief communication’ from Sir Joseph Larmor in 1919 to the British Association for the Advancement of Science; and the question was certainly a natural one since the origin of the magnetic field of the Sun was at that time a total mystery. Curiously, the magnetic field of the Earth did not then excite similar inquiry, because it was still believed that this could be explained in terms of permanent magnetisation. However it has now long been known that the temperature of the Earth’s interior is well above the critical temperature (the Curie point) at which ferromagnetic materials lose their ‘permanent’ magnetisation and that some other explanation for the Earth’s magnetic field must be found. The fact that a large fraction of the Earth’s interior is now known (by inference from seismological observations) to be in a liquid state is profoundly relevant to the problem.

Not only the Earth and the Sun; it is probably safe to state that a magnetic field is a normal accompaniment of any cosmic body that is both fluid (wholly or in part) and rotating. There appears to be a sort of universal validity about this statement which applies quite irrespective of the length-scales considered.'' Moffatt's book, 1978

As you can see, since the beginning of dynamo theory, the mechanism responsible for magnetic activity, physicists interested in very different objects have tried to connect their experiences to better understand the magnetic fields present at all length-scales. The simple idea is that magnetic fields can result from some fundamental processes that are universal. In this spirit and because of the presence of established experts on dynamo action in the Paris region, the workshop "Dynamo Action: from the lab to the stars (in the Paris region)" will be the opportunity for a multidisciplinary approach.

 Full program: coming soon

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Workshop on Mathematics and Plasma Physics interfaces

February 21^st, 2019

Sorbonne University, Pierre et Marie Campus, (Tower 24/34 - 5^st floor, LPP - Room 509)

Program:

9h30-10h: Pierre-Henri Tournier (LJLL) + discussion (10min)

FreeFem ++, a DSL for Finite Elements in Scientific Computing

Presentation of the Freefem ++ language kernel, which highlights the advantages of creating a user DSL and explains the main lines of its implementation in C ++: the virtual machine, the management of language types, etc.

10h-10h15: Coffee break

10h15-10h45: Fabrice Debbasch (LERMA) + discussion (10min)
Quantum Walks as models of relativistic electrons interacting with gauge fields

Quantum walks (QWs) have been first considered by Feynman in the 1940's and later introduced systematically in the 1990's in the context of quantum information. These discrete automata are a universal quantum computation tool and their first experimental realisation is less than 10 years old (2009). I will present some of the research conducted at the LERMA since 2012. I will explain that QWs can be viewed as models of Dirac fermions (electrons etc.) interacting with gauge fields like EM fields and gravitation and that QWs can be used to build self-consistent many body theories. These results pave the way towards new numerical simulations and laboratory experiments modelling astrophysical and cosmological relativistic quantum plasmas through QWs.

10h45-11h15: Véronique Cayatte (LUTH) + discussion (10min)
Extragalactic jets and singular solutions

The astrophysical jets are phenomena of magnetized plasma ejection related to the accretion of a disk on a central object that can be a young star, a neutron star in a binary system, a black hole resulting from death of a star or more massive in the center of a galaxy. These jets are characterized by their size, power and plasma speed.
To model plasma ejection from the magnetosphere surrounding the compact object, self-similar co-latitude and semi-analytical solutions have been studied for thirty years. The most recent model applies to relativistic jets because it has been developed in Kerr metric and includes a coherent treatment of the cylinder of light. It makes it possible to model the ejection in the jet on a large scale and the flow of e- / e + pairs on the horizon of the black hole. Without having explored the whole of the parameter space, it already appears that some solutions are singular or non-physical. Another problem that will be presented is the presence or absence of characteristic surfaces.

11h15-11h30 : Coffee break

11h30-12h00: Frédéric Nataf (LJLL)

Domain Decomposition Methods: Algorithms, Libraries, and Results

12h00-12h30: Marc Massot / Anne Bourdon (CMAP-LPP, École Polytechnique)
Fluid modeling and simulation of plasma flows out of equilibrium

Mathematical modeling, new numerical methods and HPC with applications to plasma discharges, solar physics and electric propulsion.