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Experimental study of H- negative ion production by electron cyclotron resonance plasmas

Abstract : The present PhD thesis is devoted to the experimental study of hydrogen negative ion (H-) production in microwave-driven (2.45 GHz) multi-dipolar Electron Cyclotron Resonance (ECR) plasma sources. H- sources are required in high-energy accelerators and more importantly in neutral beam injection systems for fusion plasma heating. Towards this directions, two sources (namely, "Prometheus I" and "ROSAE III") are designed, fabricated and studied. Both sources are driven by 2D networks of dipolar ECR elementary sources. It is proven that, negative ion formation in these ion sources is governed by the volume production mechanism, which mostly refers to the dissociative attachment of low energy electrons to vibrationally excited molecules. Contrary to the so called surface sources, volume production sources have the advantage of cesium-free operation. Extended experimental study on fundamental principles of H- production is realized, and possible ways for potential source optimization are tested by means of: electrostatic probes, laser photodetachment, optical emission spectroscopy, both in the visible and vacuum ultra-violet spectral range and finally, vacuum-ultraviolet absorption and induced fluorescence spectroscopy using synchrotron radiation in a specially designed setup ("SCHEME"). Analytically:The source "Prometheus I" is initially studied in detail (EEDF, H- density, optical emission spectra etc), under a wide range of experimental conditions (e.g., pressure, power, ECR-zone location), proving its efficiency for H- volume production, and unveiling optimum operational window and paths for obtaining higher H- densities. The contribution of the dissociative attachment process and neutral resonant ionization to H- production in this source, is evaluated, and the dominance of the former is finally confirmed by an equilibrium model.Due to the importance of the ro-vibrationally excited molecules to the dissociative attachment process, the study is focused on their formation reactions. Two formation reactions are considered by adequately adapted experiments: the recombinative desorption of hydrogen atoms on the surface of various materials (ROSAE III and SCHEME) and the electron impact excitation through temporary singlet states (Prometheus I). The study of recombinative desorption is approached in two different ways. With the source ROSAE III, the indirect impact of the process to the production of negative ions, through the formation of ro-vibrationally excited molecules, is evaluated in ECR plasmas. In the second approach, the source SCHEME is designed for the independent investigation of the recombinative desorption of unexcited atoms using synchrotron radiation based diagnostics. The formation of vibrational states through singlet excitation in the source "Prometheus I" is studied by vacuum-ultraviolet emission measurements.A study that combined vacuum-ultraviolet emission spectroscopy, photodetachment and the characterization of electron kinetics with electrostatic probes, allowed the identification of the factors that limit negative ion production in the ECR plasma of "Prometheus I". Perspectives for overcoming these limitations are finally proposed.
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Submitted on : Monday, March 20, 2017 - 4:45:28 PM
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Spyridon Aleiferis. Experimental study of H- negative ion production by electron cyclotron resonance plasmas. Plasma Physics [physics.plasm-ph]. Université Grenoble Alpes; Panepistīmio Patrṓn, 2016. English. ⟨NNT : 2016GREAI032⟩. ⟨tel-01492954⟩

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