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Marie-Christine Firpo
CNRS research director.
Member of the LPP’s fusion group.
Co-editor of EPL : A letters journal exploring the frontiers of Physics.
Member of the board of the Nonlinear Physics division of Société Française de Physique.
Membre du bureau de la section 04 du comité national de la recherche scientifique.
Invited speaker at the International Conference on Fundamental Plasma Physics (ICFPP), March 25-27, 2025, the University of Texas at Austin.
Journal of Plasma Physics, Volume 90, Issue 6, December 2024, 175900601.
In magnetic confinement fusion devices close to axisymmetry, such as tokamaks, a key element is the winding profile of the magnetic field lines, or its inverse, the safety profile q = qB. A corresponding profile, qJ, can be defined for the current density field lines. Ampère’s law relates any mode of current perturbation δ Jm,n with a mode of magnetic perturbation δ Bm,n. It is shown that the knowledge of the pair (qB, qJ) allows then to characterize the resonant, or non-resonant, nature of the modes for both the magnetic and current density field lines. The expression of qJ in flux coordinate is derived. Including this calculation in the real-time Grad-Shafranov equilibrium reconstruction codes would yield a comprehensive view of the magnetics.
The monitoring of the pair (qB, qJ) would then allow investigating the role played by the resonant modes for the current density, that are current filamentary modes, in the plasma small-scale turbulence. By driving the magnetic and current density profiles apart so that the images of qB and qJ are disjoint, these filamentary modes would not impact the magnetic field topology, being not associated to magnetic islands but to non-resonant magnetic modes. It remains to be explored to which extent such a configuration, where the spectrum of tiny current density filaments produces a spectrum of magnetic modes that has practically no effect on heat transport, is beneficial.
Breaking up-down symmetry with magnetic perturbations in tokamak plasmas : Increase of axisymmetric steady-state velocities, H. Oueslati and M.-C. Firpo,
Physics of Plasmas (2020).
Supersolidity-like features from the classical side Coexisting crystal and liquid-like properties in a 2D long-range self-consistent model
A two-dimensional class of mean-field models serving as a minimal frame to study long-range interaction in two space dimensions is considered. In the case of an anisotropic mixed attractive-repulsive interaction, an initially spatially homogeneous cold fluid is dynamically unstable and evolves towards a quasi-stationary state in which the less energetic particles get trapped into clusters forming a Bravais-like lattice, mimicking a crystalline state. Superimposed to this, one observes in symplectic numerical simulations a flux of slightly more energetic particles channeling through this crystalline background. The resultant system combines the rigidity features of a solid, as particles from a displaced core are shown to snap back into place after a transient, and the dynamical diffusive features of a liquid for the fraction of channeling and free particles. The combination of solid and liquid properties is numerically observed here within the classical context. The quantum transposition of the model may be experimentally reached using the latest ultracold atoms techniques to generate long-range interactions.
– Invited talk, ICPP2016, Taïwan.
– Review Editor in Interdisciplinary Physics, part of the journal Frontiers in Physics.
– Member of the editorial board of the journal Discontinuity, Nonlinearity, and Complexity https://lhscientificpublishing.com/....
– Wahb Ettoumi’s thesis award : Prix Le Monde de la recherche universitaire 2014 https://www.polytechnique.edu/fr/wa... ;
– Recent works :
Magnetic reconnection : the role of chaotic magnetic field lines
Impact of the Eulerian chaos of magnetic field lines in magnetic reconnection
http://aip.scitation.org/doi/10.106...
Evidence and relevance of spatially chaotic magnetic field lines in MCF devices
https://doi.org/10.1088/1361-6587/aa570d
Entropy–complexity analysis in some globally-coupled systems, Physica A 460, 162-173 (2016) http://www.sciencedirect.com/scienc....
Microtearing turbulence : magnetic braiding and disruption limit, Phys. Plasmas 22, 122511 (2015) http://scitation.aip.org/content/ai....
A realistic reduced model involving a large poloidal spectrum of microtearing modes is used to probe the existence of some stochasticity of magnetic field lines. Stochasticity is shown to occur even for the low values of the magnetic perturbation delta_B/B devoted to magnetic turbulence that have been experimentally measured. Because the diffusion coefficient may strongly depend on the radial (or magnetic-flux) coordinate, being very low near some resonant surfaces, and because its evaluation implicitly makes a normal diffusion hypothesis, one turns to another indicator appropriate to diagnose the confinement : the mean residence time of magnetic field lines. Their computation in the microturbulence frame points to the existence of a disruption limit, namely of a critical order of magnitude of delta_B/B above which stochasticity is no longer benign yet leads to a macroscopic loss of confinement in some tens to hundred of electron toroidal excursions. Since the level of magnetic turbulence delta_B/B has been measured to grow with the plasma electron density this would also be a density limit.
Some results (partial)
On the issue of confinement in magnetic confinement devices
Some related publications :
– L. Nasi and M.-C. Firpo, Enhanced confinement with increased extent of the low magnetic shear region in tokamak plasmas, Plasma Physics and Controlled Fusion 51 045006, 2009.http://iopscience.iop.org/0741-3335...
– M.-C. Firpo and D. Constantinescu, Study of the interplay between magnetic shear and resonances
using Hamiltonian models for the magnetic field lines, Physics of Plasmas 18 032506, 2011.http://pop.aip.org/resource/1/phpae...
– D. Constantinescu and M.-C. Firpo, Modifying locally the safety profile to improve the confinement of magnetic field lines in tokamak plasmas, Nucl. Fusion 52 054006, 2012. http://iopscience.iop.org/0029-5515...
Early nonlinear regime of the m=1 internal mode
Some related publications :
– M.-C. Firpo and B. Coppi, Dynamical Analysis of the Nonlinear Growth of the m=n=1 Resistive Internal Mode, Phys. Rev. Lett. 90, 095003 (2003). http://prl.aps.org/abstract/PRL/v90...
– M.-C. Firpo, Onset of the nonlinear regime and finite-resistivity effects for the resistive kink instability, Phys. Plasmas. 11, 970 (2004).http://pop.aip.org/resource/1/phpae...
A statistical mechanics viewpoint on nonlinear Landau damping
The question of the long-time fate of the electric field in the case of initial conditions subject to Landau damping has been much debated. Under the ergodic hypothesis, this may be answered through an equilibrium statistical mechanics approach of the finite-dimensional wave-particle Hamiltonian model.
In the case of damping of a single Langmuir wave, O’Neil’s threshold, that separates nonzero time-asymptotic wave amplitude states from zero ones, is associated to a second order phase transition [2]. In the general case of a continuum of waves, in which the ergodic hypothesis is the most likely to be satisfied, the wave spectrum is predicted to collapse towards small wavelengths together with the escape of initially resonant particles towards low bulk plasma thermal speeds. This study [3] reveals the fundamental discrepancy between the long-time dynamics of single waves, that can support finite amplitude steady states, and of wave spectra, that cannot.
[1] Y. Elskens and D. F. Escande, Microscopic Dynamics of Plasmas and Chaos (IOP, Bristol, 2002).
[2] M.-C. Firpo and Y. Elskens,
Phase Transition in the Collisionless Damping Regime for Wave-Particle Interaction, Phys. Rev. Lett. 84, 3318 (2000).http://prl.aps.org/abstract/PRL/v84...
[3] M.-C. Firpo, F. Leyvraz and G. Attuel, Equilibrium statistical mechanics for single waves and wave spectra in Langmuir wave-particle interaction, Phys. Plasmas 13, 122302 (2006).http://pop.aip.org/resource/1/phpae...
Beam-plasma interactions for fast ignition applications
Linear theory of the beam/return plasma equilibrium setting, done without any restriction on the wave vector orientation, reveals that filamentation is mainly triggered by an oblique mode belonging to the two-stream/filamentation branch.
