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Poster communications

Mechanisms and dynamics of unipolar arcs in magnetized plasmas

Abstract : Post-mortem analyses suggest that arcs in contact with metallic walls are the main cause of impurities and dust formation in tokamaks [1,2]. Such dust and impurities represent important operational and safety issues for next-step fusion devices like ITER and DEMO. If dipolar arcs, which occur in between adjacent Plasma Facing Components (PFCs), are well known and can in most of the cases be avoided by adapting the design of the PFCs, it is not the case for unipolar arcs taking place in between PFCs and the plasma. On addition to dust production, unipolar arcs also damage mirrors required by many diagnostics used for studying the plasmas as well as for ensuring a safe operation of the fusion device. Several models for explaining the triggering of unipolar arcs have been developed, but these models are based on assumptions which are difficult to assess experimentally, mainly due to the spatial and temporal scales involved (a few µm to <1mm and ~1ns for the triggering and few µs for the lifetime, respectively). As a result, defining the right threshold required for triggering the arcs is a tricky operation due to the uncertainties in the relative importance of the numerous input parameters which also influence the arc dynamics: surface roughness, secondary electron emission, thermionic emission, desorption of gas trapped in thick deposits, sheath modifications induced by ELMs… With the aim to provide more reliable input parameters and to assess the validity of the models, an experimental setup making possible highly resolved measurements in the linear low-beta ALINE plasma device has been developed. Stereoscopic fast camera measurements are performed and analyzed with the TRACK software, which enables a statistical investigation of the influence of various parameters on the triggering and dynamics of unipolar arcs.
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https://hal.univ-lorraine.fr/hal-02372071
Contributor : Frédéric Brochard <>
Submitted on : Wednesday, November 20, 2019 - 11:19:15 AM
Last modification on : Wednesday, June 24, 2020 - 3:24:02 PM

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Chabha Djerroud, F. Brochard, Jérôme Moritz, Jordan Ledig, Eric Faudot, et al.. Mechanisms and dynamics of unipolar arcs in magnetized plasmas. 17th International Conference on Plasma-Facing Materials and Components for Fusion Applications, May 2019, Eindhoven, Netherlands. ⟨hal-02372071⟩

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