Energy transformation mediated by electromagnetically-driven instabilities - from astrophysics to industrial applications – MagnetDrive

The ANR is based on a close combination of innovative laboratory experiments and associated numerical modelling.

1) GAMIN, KEPLER and thermoelectricity experiments:

A column of liquid metal is subjected to strong axial currents and intense magnetic fields, allowing the triggering of the Tayler instability, the MRI instability or the generation of MHD turbulence;

In some cases, the metal-metal (annular) interface is studied with current injection and an external field when the experiment is filled with two different liquid metals.

Instrumentation: in these experiments, capacitive sensors and ultrasonic Doppler velocimetry are used to access the velocity and position of any interface. Fluid velocity is measured using potential and Doppler probes, the magnetic field is measured using Hall-effect probes, and temperature and heat flux measurements are made.

2) Thermo-acoustics: experimental study of the coupling between sound waves and thermal gradients leading to efficient heat transport in the absence of gravity, essentially measurements of flow and temperature variations. Coupled with flow measurements using the Schlieren effect.

3) MICRO-Gravity: capillary wave campaigns in micro-gravity to isolate wave non-linearities and surface turbulence.

4) VORTEX/ONDES: dispersion analysis and spatio-temporal diagnostics (2D k-f spectra) of vortex-guided Kelvin waves.

5) MODELLING :

a) Using the Parody-JA-SHTNS code, the radiative layers of rapidly rotating stars are modelled, enabling their magnetic fields and slowdown to be assessed and studied.

(b) geodynamo with stable stratification and variable conductivity,

(c) Europa's ocean coupled to the Jovian field (magnetic-induced signatures),

(d) post-processing of energy balances and scaling laws.

Astrophysics & geosciences

1) Radiative stars: The ANR project has led to a better understanding of how the presence of a magnetic field generates a violent instability (the Tayler instability) that massively converts magnetic energy into turbulent motion, which explains the spectacular slowdown observed in certain stars.

Typically :

(i) spin-down by dynamo action (Science 2023) ;

(ii) Tayler-Spruit in radiative layers (A&A 2024);

(iii) dynamo-triggered subcritical transition (PRFluids 2023).

2) Moons: Here again, we are studying how the magnetic energy supplied by Jupiter's field is converted into kinetic energy, enabling the emergence of deep ocean currents that modify the heat transport and geology of Jupiter's moons. Articles:

(i) Europa's magnetic equatorial jet (Nature Astronomy 2019)

(ii) New constraint on the thickness of Europa's ice via magnetic ocean signatures (Icarus 2025).

3) Planets: A better understanding of the dynamics of planetary magnetic fields:

(i) reversals in a stable layer model (submitted 2025); (ii) enhanced dynamo growth in non-homogeneous conductivity environments (PRE 2021).

4) Vortices/waves: experimental evidence of Kelvin waves along vortex axes (submitted 2025), relevance for the internal dynamics of tornadoes and vortex flows.

5) The Kepler experiment has made it possible to reproduce the mechanism of angular momentum transport in accretion disks around black holes and protostars, considered to be one of the cornerstones of star formation.

(i) Presentation of the Kepler experiment and 2D turbulence regimes (JFM 2021)

(ii) Experimental modelling of an accretion disc and observation of the Kraichnan transport regime predicted in 1962 (PRL 2022)

Energy & industry

6) Liquid-liquid thermoelectricity: demonstration/quantification at the Ga/Hg interface (PNAS 2024), paving the way for several types of new technologies (start-up under development)

7) Thermoacoustics: cooling by baroclinic streaming (PR Applied 2021).

8) Liquid metal batteries (GAMIN): experimental demonstration of Tayler instability in a liquid metal column under strong axial current (lab results, thesis defended, article in preparation); transposable LMB interface sensors and attenuation strategies.

9) Micro-gravity: capillary wave turbulence (EPL 2020) as a test bench for relevant non-linear transfers for free surfaces without gravity.

Synthesis/positioning

- Laboratory review of MHD disks (C. R. Physique 2024) to situate our experiments in the astro-laboratory panorama.

- Coherent series of articles 2019-2025 linking transitions/instabilities, transports, and observable signatures from the laboratory (liquid metals, thermo) to natural systems (stars, geodynamo, Europe).

1. instability control for LMB: mapping the Tayler threshold/saturation in industrial geometries; optimising, obtaining current spectra, effect of transverse magnetic fields; integration of interface sensors for robust operation.

2. energy conversion: metal-to-metal thermoelectric architectures (interfacial Seebeck effect) and thermoacoustics for cooling and recovery; coupling with calorimetric diagnostics. Development of a prototype as part of a start-up.

3 Astro/Geo: linking the scaling laws obtained to radiative star models, geomagnetic reversals and data inversions for Europa (space missions).

4. vortex/wave: extend Kelvin wave physics to more realistic regimes (ambient turbulence, strong non-linearities, vortex array), including meteorological relevance (tornadoes) and engineering (controlled mixing).

5 Cross-cutting: closed (ohmic/viscous) energy balances and efficiency bounds for magnetic→kinetic conversion, transferable from experiments to models.

The conversion of electromagnetic energy into kinetic energy by an electrically conducting fluid is a ubiquitous phenomenon that can be found in several natural systems as well as industrial applications.
In nature, the accretion of matter around black holes and proto-stars is a typical example in which a tremendous amount of kinetic energy is produced from strong magnetic fields. Similarly, many industrial systems involve a liquid metal subject to electrical currents or external magnetic fields, among which we can cite electromagnetic driving of liquid sodium or production of aluminum by electrolysis.
Despite the importance of these applications, several aspects of the dynamics of electromagnetically driven (EMD) flows still remain poorly understood. One major problem is to identify the mechanisms that limit the maximum efficiency of magnetic to kinetic energy transformation in the presence of turbulence. Interestingly, this bound on the efficiency almost always results from unexpected flow instabilities occurring as soon as the size of the system or the magnitude of the driving becomes large enough. The origin itself of such flow instabilities is not fully understood.

The aim of this research project is twofold: first, we want to elucidate some of these major aspects through original laboratory MHD experiments, connected by a simple question: why, how and under which circumstances do instabilities mediate energy conversion in electromagnetically-driven flows? We therefore aim at coordinating theoretical, numerical and experimental efforts to identify general mechanisms involved in such instabilities and substantially expand our comprehension of the dynamics of electromagnetically driven flows
Second, this project strongly connects to industrial and astrophysical applications, such as interstellar turbulence and liquid metal batteries. One purpose of the project is precisely to bridge the gap between fundamental physics and industrial motivations. To complete these tasks successfully, our work program is divided into two different parts with their own interests and purposes, but strongly connected through the general questions raised above.