5 mois (mars à juillet 2021)

M2 internship – TP team – Absolute measurement of a radiofrequency electric field in a plasma by Electric Field-Induced Lyman-alpha Emission (EFILE)

L’objectif de ce stage est d’obtenir la mesure absolue (=calibrée) d’un champ électrique radio-fréquence dans le vide et dans un plasma, en particulier au niveau d’une gaine à l’interface entre le plasma et une paroi. Une gaine est une région où le potentiel varie fortement, créant un champ électrique ayant une influence sur les mouvements des particules char-gées. Nous utiliserons un diagnostic développé dans l’équipe Turbulence Plasma, (EFILE) permettant de mesurer de façon directe un champ électrique dans le vide ou dans un plasma.

Theoretical description and numerical simulations of radiofrequency (RF) sheaths are very active research areas in the field of fusion (ICRF heating) but there is a lack of specific diagnostics and experimental data. This diagnosis should answer this problem. This internship will pro-vide the candidate with a fundamental training in this field of research.

This diagnosis is based on the emission of the Lyman- line (vacuum UV) by a probe beam of hy-drogen in the 2s state, when the beam passes through a region with an electric field. As a result of the 2s-2p coupling created by the field (Stark mixing), there is a reduction in the lifetime of the 2s (metastable) level with the emission of the Lyman- line, whose intensity is proportional to the square of the field amplitude, which makes it possible to measure it.

Experimentally, in order to control as many parameters as possible to validate the diagnosis, we are working under much simpler conditions than in the field of fusion. An electric field is generated in a cylindrical chamber between two parallel plates between which the probe beam is sent. A filament heated by a current emits primary electrons that are accelerated by a potential difference. By collisions, these electrons ionize argon atoms, allowing the crea-tion of a non-magnetized argon plasma.

This subject can lead to several thesis perspectives. One of the direct applications of this intern-ship would be to develop the measurement of the absolute value of the electric field in front of an ion cyclotron heating antenna (ICRF). This measurement should be carried out in an experimental test bench reproducing the plasma edge conditions of a tokamak in order to calibrate field maps obtained by simulation. This work would be the subject of a collabora-tion with the Institut Jean Lamour in Nancy. Another application, in development since Jan-uary 2019 (financed by the SUD/PACA region), is the study of a plasma in a column in a magnetic field on the MISTRAL machine of the Turbulence Plasma team.

Réf : L. Chérigier-Kovacic et al., Rev. Sci. Inst. 86, 063504 (2015).

Laurence Chérigier-Kovacic - Contacter
Equipe TP

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