Typologie d'emplois: Thesis

Organism: Aix-Marseille University

Laboratory: PIIM UMR 7345

Location: Campus Saint-Jérôme

Supervisor: Alexandre Escarguel

Funding : AMIDEX Aix-Marseille University project “Table Top Accretion Disks”

e-mail: alexandre.escarguel@univ-amu.fr 

Analysis and control of ExB magnetized plasma column self-organization in the frame of astrophysical accretion mechanisms’ study

This PhD proposes an innovative way to use a laboratory experiment to study Keplerian rotating discs in astrophysics. Indeed, stellar accretion disks are complex systems whose dynamics cover a large number of research fields. They are indeed made of dust, neutral gas and plasmas orbiting around young or rising stars and seed planet formation [1]. In link with the observation capabilities of modern instruments such as James Webb space telescope [2], intense efforts are nowadays undertaken to explain accretion mechanisms and disk formation. How a Keplerian rotation can lead to matter transport toward the center is still a matter of debate, since collisional diffusion is negligible in these systems, and Keplerian discs are stable with respect to classical hydrodynamics instabilities. How instabilities and transport occur, can be elucidated by setting up dedicated experimental devices. Laboratory plasma experiments with controlled plasma rotation is an innovative way to explore such scientific questions.

Mistral is a cold magnetized plasma experiment with a constant magnetic field [3, 4, 5, 6]. It is a canonical experiment to study various kind of instabilities of weakly magnetized ExB plasmas, such as centrifugal instabilities [7]. In the presence of a magnetic field B perpendicular to an electric field E, charged particles drift in the ExB direction. Combined with plasma inhomogeneities, this drift is favorable to the apparition of instabilities that considerably increase the transport across the magnetic field B (« anomalous transport »). These cross-field configurations are exploited in numerous applications.

Rotating plasma are easily obtained in the Mistral experiment, but there is a lack of control of the azimuthal differential rotation. This can be done by a fine control of the radial electric field profile in the plasma. Indeed, the prediction of the rotation properties and the control of the flow profiles is still an open problem regarding rotating plasmas in such devices. Previous works in our SoPlasma network (https://gitlab.com/soplasma/soplasma), including the Mistral device, have shown that rotation of plasmas can be challenging to control. Recent progress however provides a path to achieve this important objective by the use of concentric cold or hot cathodes [8, 9] . Another possible way to control the plasma column rotation specific to Mistral is to control the energetic ionizing electrons in Mistral by independent concentric grids.

Investigation of the plasma self-organization and experimental control of its rotation is the main objective of this PhD proposition. First, PhD student will study the stability/turbulent areas in the parameter space (plasma pressure and boundary conditions) by experimental acquisition of plasma parameters with Langmuir probes, fast camera and optical tomography. Second, new innovative experimental configurations will be studied to better control the plasma azimuthal differential rotation: concentric grids controlling ionizing electrons injection and cold/hot cathodes placed at the end of the plasma column.

Finally, a comparison of experimental results with theory will allow a better understanding of ExB plasma rotation physics to ultimately propose an experimental setup with a Keplerian plasma rotation. The importance of this part will be modulated, according to the student’s motivation for theoretical work.

The project, being within the framework of AMIDEX (“Excellence initiative” of Aix-Marseille University) project “Table Top Accretion Disks”, is 100% funded.

References

[1] G. R. J. Lesur, J. Plasma Phys. 87 205870101 (2021) [2] Burrows et al, Astrophys. J 473, 437 (1996), https://jwst.nasa.gov

[3] N. Claire, A. Escarguel, C. Rebont, F. Doveil, Phys.Plasma 25, 061203 (2018)

[4] A. Escarguel, Eur. Phys. J. D, 56, 209-214 (2010).

[5] Th. Pierre, A. Escarguel, D. Guyomarc’h, R. Barni, C. Riccardi, Phys. Rev. Lett., 92, 065004 (2004).

[6] S. Aggarwal, Y. Camenen, A. Escarguel, and A. Poye, Journal Plasma Phys., 89(3), 905890310 (2023).

[7] R. Gueroult et al, Phys. Plasmas 082102 (2017)

[8] B. Trotabas and R. Gueroult, Plasma Sources Sci. Technol. 31, 025001 (2022)

[9] V.Désangles et al, J. Plasma Phys. 87, 905870308 (2021) and Désangles, Ph.D. thesis, Ecole Normale Supérieure de Lyon, France (2018)

 

Laboratory : PIIM/Turbulence Plasma team

Supervisors : Alexandre Escarguel and Laurence Cherigier-Kovacic

Tel: 06 42 54 87 97

Email : alexandre.escarguel@univ-amu.fr ; laurence.kovacic@univ-amu.fr

Funding : Aix-Marseilles University doctoral school of physics and matter science 352

Subject : Optical electric field diagnostic in a magnetized plasma by Lyman-alpha stimulated emission (EFILE) 

Subject description :

Project framework and experiment description:

Plasma, result from the partial or total ionization of neutral gases. Coupling between fields (electric, magnetic) and charged particles leads to collective effects and turbulence, specific to these media. Their behavior and their fields of application depend on the ion temperature: cold plasmas are used in industry for surface treatment (etching of circuits, deposits, production of reactive species, etc.); hot plasmas are produced in tokamaks (Tore-Supra, ITER…) in order to produce energy from controlled fusion. In both cases, it is essential to determine the fundamental parameters associated with the charged species present in the plasmas.

The Turbulence Plasma team has developed an optical diagnostic (EFILE) for direct measurement of an electric field in vacuum or in plasma [1, 2]. This diagnostic is based on the emission of the Lyman-α line by a hydrogen probe beam in the 2s state submitted to an electric field. As a result of the 2s-2p coupling created by the field, atoms in the 2s (metastable) level is transferred to the 2p level, which then rapidly de-excites to the ground level. The intensity of the electric field-induced Lyman-α emission is proportional to the square of the field amplitude. This diagnostic was experimentally validated in a simple cylindrical configuration, in vacuum and in a non-magnetized plasma.

Objective and description of the subject:

The objective of the thesis is to measure the electric field in a magnetized plasma. The EFILE diagnostic is being implemented on the MISTRAL machine of the Turbulence Plasma team of the PIIM laboratory. The MISTRAL machine [3, 4] produces a cold plasma column in a linear magnetic field, over a wide range of parameters. It is a fundamental research machine whose linear configuration simplifies the study of instabilities in a magnetized plasma (compared to tokamaks where the curvature of the magnetic field induces more complex phenomena). Mistral is the ideal device to validate the EFILE diagnostic which is a unique way of measurement of the electric field in a direct and non-intrusive way.

Present work is focused on the study of the influence of the magnetic field on the diagnostic and the measurement of an electric field at different points along the radius of the machine. These two aspects are studied in vacuum or in a plasma, independently of each other, in order to fully understand the capabilities of the diagnostic.

The selected PhD student will proceed to the measurement of the electric field which, together with the magnetic field, is responsible for rotating non linear instabilities of the plasma column [5, 6].

The general objective of this work is to develop a diagnostic for the absolute measurement of a static or oscillating electric field that can be transferred to different systems and applied to various current research problems in plasma physics. Within this framework, the diagnostic will be applied by the PhD student to the study of plasma sheaths, a crossdisciplinary issue involving cold plasmas, hot plasmas, applied mathematics, theories, simulations, and experiments [4].

This project is granted by the FR-FCM (Federation de Recherche sur le Fusion Contrôlée par Confinement Magnétique).

 

References

[[1] L. Chérigier-Kovacic, P. Ström, A. Lejeune and F. Doveil, Review of Scientific Instruments 86, 063504 (2015); doi: 10.1063/1.4922856

[2] L. Chérigier-Kovacic, Static and RF electric field direct measurement based on Lyman-a emissionfrom a hydrogen probe beam ; Invited talk @ XXXIV ICPIG conference, July 14-19 2019, Sapporo, Japan.

[3] A. Escarguel, ExB workshop, nov 2018, Princeton Plasma Physic Lab, USA.

[4] Atelier Gaine Plasma 4-6 novembre 2024, Marseille, https://gaine2024.sciencesconf.org/?lang=fr (consulté le 30 novembre 2024).

This PhD proposes an innovative way to use a laboratory experiment to study Keplerian rotating discs in astrophysics. Indeed, stella@r accretion disks are complex systems whose dynamics cover a large number of research fields. They are indeed made of dust, neutral gas and plasmas orbiting around young or rising stars and seed planet formation [1]. In link with the observation capabilities of modern instruments such as James Webb space telescope [2], intense efforts are nowadays undertaken to explain accretion mechanisms and disk formation. How a Keplerian rotation can lead to matter transport toward the center is still a matter of debate, since collisional diffusion is negligible in these systems, and Keplerian discs are stable with respect to classical hydrodynamics instabilities. How instabilities and transport occur, can be elucidated by setting up dedicated experimental devices. Laboratory plasma experiments with controlled plasma rotation is an innovative way to explore such scientific questions.

Mistral is a cold magnetized plasma experiment with a constant magnetic field [3, 4 ,5, 6]. It is a canonical experiment to study various kind of instabilities of weakly magnetized ExB plasmas,such as centrifugal instabilities [7]. In the presence of a magnetic field B perpendicular to an electric field E, charged particles drift in the ExB direction. Combined with plasma inhomogeneities, this drift is favorable to the apparition of instabilities that considerably increase the transport across the magnetic field B (« anomalous transport »). These cross-field configurations are exploited in numerous applications.

Rotating plasma are easily obtained in the Mistral experiment, but there is a lack of control of the azimuthal differential rotation. This can be done by a fine control of the radial electric field profile in the plasma. Indeed, the prediction of the rotation properties and the control of the flow profiles is still an open problem regarding rotating plasmas in such devices. Previous works in our SoPlasma network (https://gitlab.com/soplasma/soplasma), including the Mistral device, have shown that rotation of plasmas can be challenging to control. Recent progress however provides a path to achieve this important objective by the use of concentric cold or hot cathodes [8, 9] . Another possible way to control the plasma column rotation specific to Mistral is to control the energetic ionizing electrons in Mistral by independent concentric grids.

Investigation of the plasma self-organization and experimental control of its rotation is the main objective of this PhD proposition. First, PhD student will study the stability/turbulent areas in the parameter space (plasma pressure and boundary conditions) by experimental acquisition of plasma parameters with Langmuir probes, fast camera and optical tomography. Second, new innovative experimental configurations will be studied to better control the plasma azimuthal differential rotation: concentric grids controlling ionizing electrons injection and cold/hot cathodes placed at the end of the plasma column.

Finally, a comparison of experimental results with theory will allow a better understanding of ExB plasma rotation physics to ultimately propose an experimental setup with a Keplerian plasma rotation. The importance of this part will be modulated, according to the student’s motivation for theoretical work.

The project, being within the framework of AMIDEX (“Excellence initiative” of Aix-Marseille University) project “Table Top Accretion Disks”, is 100% funded.

 

References

[1] G. R. J. Lesur, J. Plasma Phys. 87 205870101 (2021) [2] Burrows et al, Astrophys. J 473, 437 (1996),

https://jwst.nasa.gov

[3] N. Claire, A. Escarguel, C. Rebont, F. Doveil, Phys.Plasma 25, 061203 (2018)

[4] A. Escarguel, Eur. Phys. J. D, 56, 209-214 (2010).

[5] Th. Pierre, A. Escarguel, D. Guyomarc’h, R. Barni, C. Riccardi, Phys. Rev. Lett., 92, 065004 (2004).

[6] S. Aggarwal, Y. Camenen, A. Escarguel, and A. Poye, Journal Plasma Phys., 89(3), 905890310 (2023).

[7] R. Gueroult et al, Phys. Plasmas 082102 (2017)

[8] B. Trotabas and R. Gueroult, Plasma Sources Sci. Technol. 31, 025001 (2022)

[9] V.Désangles et al, J. Plasma Phys. 87, 905870308 (2021) and Désangles, Ph.D. thesis, Ecole Normale Supérieure de Lyon, France (2018)