진학프로석·박사 채용 접수 플랫폼

Project at the Laboratoire de Physique, on the Monod campus of the École Normale Supérieure de Lyon.
All experimental methods are available and currently fully operational in the laboratory.
Collaborative Environment:
Centrifugal instabilities are not limited to magnetized plasma columns in the laboratory but also occur in astrophysical objects, such as the plasma disk magnetospheres of gas giants. The cases of Saturn and Jupiter have recently attracted significant attention, and the understanding and parameterization of turbulent transport in this astrophysical context remain open questions. This thesis will be part of a joint research program with colleagues in Toulouse (Laplace and IRAP) and Marseille (PIIM), and the PhD student will be involved in several collaborative projects.
During the second year of the thesis project, the work will be carried out in close collaboration with our colleagues at the Laplace laboratory, where numerical and analytical models are being developed to predict plasma potential control and flux entrainment from polarized electrodes.
During the third year of the thesis project, the work will benefit from the complementary expertise of the PIIM and IRAP groups in theoretical modeling and numerical simulations of centrifugal instabilities, in the context of both laboratory devices and gas giants.

General Scientific Background.
In plasma columns confined by an axial magnetic field, the presence of radial gradients in pressure and potential (or equivalently, a radial electric field) perpendicular to the magnetic field generates intense azimuthal flows. This canonical configuration can trigger centrifugal instabilities, Kelvin-Helmholtz instabilities, or drift wave instabilities. Their nonlinear evolution can lead to the emergence of large-scale coherent structures or turbulence, resulting in significant radial plasma transport—and thus degradation of plasma confinement. Understanding this turbulent transport is crucial for both fundamental research and applications.

Theoretical models of centrifugal instabilities have so far been limited to regimes where the phase velocity of the instability is low compared to the ion cyclotron frequency. However, this low-frequency approximation does not apply to most experiments, where the phase velocity is of the same order of magnitude as the ion cyclotron frequency. Recent work extends linear stability analysis beyond this approximation, neglecting collisions and gyroviscous effects. Nevertheless, this model fails to reproduce certain experimental features, such as the emergence of a dominant mode with a small number of azimuthal modes (typically 1 or 2), and does not allow for accurate estimation of turbulent transport, which is essential for predicting equilibrium plasma parameter profiles.

Objectives and Provisional Schedule
This PhD project aims to experimentally characterize centrifugal instabilities and associated turbulent transport in the Von Kármán plasma experiment (a 1-meter-long, 20-cm-diameter plasma column that has been operational for several years). The focus will be on two dimensionless parameters:
i) the ratio of the plasma rotation frequency to the ion cyclotron frequency (characterizing the influence of ion magnetization);
ii) the ratio of the plasma rotation frequency to the ion-neutral collision frequency (characterizing the influence of neutral friction on plasma ions).

The first year will be dedicated to developing and characterizing plasma rotation control techniques to achieve high-speed, solid-body rotation profiles, enabling the isolation of centrifugal instabilities. These techniques will rely on biased emissive cathodes to control the plasma potential profile and drive strong azimuthal flows, as demonstrated in a previous thesis. The innovation lies in the use of highly emissive cathodes (up to 20 A) and a new microwave-based plasma ionization source.

The second year will focus on a precise experimental description of plasma rotation speeds and equilibrium parameter profiles, including the characterization of a strong electric field at the edge of the plasma column, known as the anodic sheath. The experimental data will be compared with models developed in collaboration with the LAPLACE team in Toulouse.

The third year will concentrate on the influence of ion magnetization and collisions on the characteristics of centrifugal instabilities. The dynamics of large-scale structures will be probed using high-speed imaging of spontaneous plasma light emission, while spatially and temporally resolved plasma parameter fluctuations and turbulent transport will be measured using advanced electrostatic probes. Scaling laws for the evolution of turbulent diffusivities as a function of ion magnetization and neutral friction will be derived from these measurements.