Incoherent Thomson scattering investigations in Hall thruster, planar magnetron and ECR ion source plasmas

Abstract : Incoherent Thomson Scattering (ITS) is one of the most reliable techniques for determining electron properties inside a plasma. Key information (electron density, temperature (more generally, the EEDF), and drift velocity) can be determined from the analysis of the intensity and spectral distribution of light of a laser pulse scattered by free electrons. In contrast to many conventional techniques, it is non-perturbative, gives access to spatiotemporally-resolved information, and can be applied in magnetized plasma regions. Yet ITS implementation in low-temperature plasmas poses a particular challenge due to the low plasma densities involved. This thesis focuses on the development and application of a new ITS diagnostic. Traditional diagnostics have commonly used triple grating spectrometers to filter the stray light signal (from Rayleigh scattering and laser reflections) in order to allow detection of the Thomson-scattered signal from electrons. Instead, this work uses a recently-commercialized notch filter for stray light reduction. This approach enabled the development of a diagnostic with notable sensitivity and compactness. This diagnostic, named THETIS, was used to gain insights into three low-temperature, magnetized plasmas sources operating at low pressure. A planar magnetron was studied in continuous (DCMS) and pulsed (HiPIMS) regimes, with these investigations allowing the extraction of spatial and temporal profiles of electron properties. This study focused on the influence of the gas type, repetition rate, and magnetic field intensity. A ECR ion source was also studied in pulsed and continuous regimes. The influence of the gas type, pressure and power were investigated. In addition to the study of the dynamics of electron properties in pulsed operation, insights into species excitation and ionization were gained from the analysis of the plasma emission. Lastly, electron properties along radial and azimuthal directions in Hall thrusters in ``standard'' and ``magnetic shielding'' configurations were investigated. The influence of the magnetic field intensity, configuration, and direction were investigated, as well as the discharge power and voltage. This work revealed not only the presence of anisotropies in electron properties, but also high electron temperatures in magnetized regions of the thruster plasma and the presence of strongly non-Maxwellian EEDFs in such regions. The information on electron properties, and the accompanying physical insights obtained in this work, contribute to current understanding of these plasmas and provides a new path forward for their modeling and development.
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Benjamin Vincent. Incoherent Thomson scattering investigations in Hall thruster, planar magnetron and ECR ion source plasmas. Plasma Physics [physics.plasm-ph]. Université d'Orléans, 2019. English. ⟨tel-02466976⟩

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