Plasma experiments on earth all need a container: magnetic confinement fusion devices, electrostatic nuclear fusion devices, experimental bench test, satellite thrusters, magnetrons for industrial thin layer depositions, etching devices, decontamination… For unmagnetized plasmas, the Bohm criterion separates the quasi-neutral presheath from the high potential gradient sheath but it has been theoretically demonstrated by simulations that this sheath criterion disappears in magnetic sheath with a low incidence angle1,2 of magnetic field. Several theoretical analysises3,4 explain our experimental results5 exhibiting a thermalization of the Ion Velocity Distribution Function (IVDF) with two physical contradictory phenomena: collisions and ballistic effects. Langmuir probe6 is the most used diagnostic to measure plasma properties by collecting currents through its own surrounding sheath. The interpretation of this diagnostic is an opened issue since a theoretical sheath model is still needed to analyze measurements. For electron emissive surface sheath, if the rate of electron emission is high enough, inverse sheath is predicted7 but is still not experimentally observed. Because of a very low sheath thickness and a high potential gradient, only the non-intrusive laser-induced fluorescence (LIF) diagnostic can be used. This powerful diagnostic of which the supervisor is an expert, allows the space and time Ionic Velocity distribution Function (IVF)8 measurements even if some artefacts have been identified by authors and must be considered9.
This internship proposes to study electrons emissive sheath by a dedicated plasma experiments measuring IVDF in a non-intrusive way by LIF8 diagnostic in a low pressure DC Argon discharge. This study will start with a classical floating sheath study to validate modelisations (kinetic, fluid and PIC simulations developed in another team of the lab) to an electrons emissive sheath. The emissive cathode will be either a heating conducting metallic surface or a heated insulated ceramic.
1. Chodura, R., Phys. Fluids 25, 1628 (1982).
2. Stangeby, P. C., Nucl. Fusion 52, 083012 (2012).
3. Baalrud, S. D. & Hegna, C. C., Plasma Sources Sci. Technol. 20, 025013 (2011).
4. Coulette, D. & Manfredi, G., Phys. Plasmas 22, 043505 (2015).
5. Claire, N., Bachet, G., Stroth, U. & Doveil, F., Phys. Plasmas 13, 062103 (2006).
6. Chen, F. F. Langmuir Probe Diagnostics. IEEE-ICOPAS Meeting (2003).
7. Campanell, M. D., Phys. Rev. E 88, 033103 (2013).
8. Pigeon, V., Claire, N., Arnas, C., Terasaka, K. & Inagaki, S., Phys. Plasmas 27, 043505 (2020).
9. Pigeon, V., Claire, N., Arnas, C. & Doveil, F., Phys. Plasmas 26, 023508 (2019).