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Fluid-kinetic description of Plasmas

Formation:

Training module for doctoral students, offered regularly by the École Doctorale d'Astronomie & Astrophysique d'Île-de-France (ED127). Generally given in English.

beam

objective:

Astrophysical plasmas, like those used in laboratory physics, are often modeled by "fluid" theories, i.e. based on equations linking macroscopic parameters (density, fluid velocity, pressure, etc.). This is particularly true of MHD (MagnetoHydroDynamics), the basic fluid model for describing magnetized plasmas, which are widespread throughout the universe. The use of these fluid models is easily justified for collisional plasmas, but much less so when these are weakly collisional or collisionless. This is the case, for example, in the solar corona, solar wind or planetary magnetospheres, where the mean free path between two successive collisions of an ion or electron is of the order of a million or ten million km. Under such conditions, the plasma must be described by a "kinetic" model. Unfortunately, "kinetic" models, which describe in detail the evolution of the particle velocity distribution function, are much heavier (for the same physical system) in terms of computing resources. As a result, "kinetic" modeling of a collisionless plasma is often impossible at scales much larger than the so-called kinetic scales, such as the ion radius of gyration. Fluid models are still an essential tool for describing collisionless plasmas at these scales.

Since “fluid” models remain an essential tool for describing collisionless plasmas on large scales, it is instructive to gain a glimpse of their limitations by comparing the two models in simple cases.

Two codes for two-dimensional systems will be used during the course. A “fluid” code based on MHD equations and a so-called “hybrid” code in which plasma ions are treated as particles and electrons as a fluid. Since the 2021 course (which exceptionally took place 100% on-line), we are asking participants to install and run the codes on their own laptops.

Training philosophy:

This training program is not designed to teach participants how to write the two types of code, or how to use them for a particular astrophysical application. It is designed more in the spirit of “research”, where participants manipulate the two codes on a variety of elementary examples proposed to them. The aim is to familiarize participants with the possibilities and limitations of the MHD and hybrid codes. This simulation activity is complemented by lectures and a seminar showing astrophysical research results where the “fluid-kinetic” duality arises crucially, and where equivalents to the elementary problems studied in the practical sessions can be found. Each lab is followed by a debriefing session, which is much appreciated by the participants.

Level required:

You don't need to have a good level in plasma physics or numerical simulations. However, a certain interest in both is naturally recommended to get the most out of the course. In keeping with the spirit of the course, interaction between participants, even at heterogeneous levels, and personal initiative are just as essential to getting the most out of the week as interaction with the supervisors.

Contact : Filippo Pantellini (LESIA, Observatoire de Paris), email : Filippo.Pantellini @ obspm.fr

Encadrants : Nicolas Aunai (LPP), Gérard Belmont (LPP), Filippo Pantellini (LESIA), Roch Smets (LPP)