Experimental study of negative ions current production applied to controlled thermonuclear fusion by use of spectroscopic techniques and comparison of the experimental results with a numerical model

The present Phd thesis is an input within the frame of ITER and DEMO projects, which aim to control nuclear fusion reactions. To maintain fusion reactions in a tokamak (i.e. Fusion machine), the plasma fusion should be constantly heated. One of the heating methods used, is the so-called Neutral Beam Injection (NBI). This technique involves the creation of negative ions in an external cold plasma source; after being extracted, they will be accelerated, neutralized, before being injected into the fusion reactor. Negative hydrogen ions are created on surfaces and / or in volume. The surface mechanisms require cesium injection, which could contaminate the accelerating stage, and could lead to voltage disruptions. This thesis aims to improve the production of negative ions, in cesium-free plasma. For this purpose, the ROSAE-III (ecr hydrogen plasma for neutral beam) reactor has been developed and designed by the PMN team of the LPSC. This multi-dipolar ECR (Electron Cyclotron Resonance) plasma reactor (2.45 GHz) favors the volume mechanisms, which, unlike the surface mechanisms, does not require cesium. A detailed study of the fundamental principles of the production of H^- is carried out, and the different ways of optimization are explored by means of: electrostatic probes, laser photo-detachment, optical emission spectroscopy in the visible range and in the VUV range and absorption spectroscopy and laser induced fluorescence (LIF) in the VUV using synchrotron radiation. Two different positions (close and far from the ECR driving zone) are investigated under various conditions of pressure and power. Negative ions are produced in ROSAE-III, in the plasma volume by dissociative attachment (DA). This reaction requires cold electrons, and hydrogen ro-vibrationally excited molecules H₂ (X1Σg⁺,v''). The advantage of ROSAE-III is the possibility to change the plasma facing materials: borosilicate glass cylinder (Pyrex^TM), with low recombination coefficient (γ= 0.004), or moderate (∼γ = 0.5 for metallic surfaces). In order to increase the hydrogen ro-vibrationally excited molecules density, by re combinative desorption (RD), and consequently to generate a higher density of negative ions H^- via DA, materials (tungsten, stainless steel, tantalum and HOPG) have been investigated. Hence, the absolute density of negative ions has been multiplied by a factor of 3.5, from 5 x10¹³ m^-3 to 17.5 x10¹³ m^-3. The SCHEME-II source, designed to be mounted on the DESIRES beam line of the SOLEIL synchrotron, allowed us to determine the relative densities of atoms and excited molecules. The experimental results were supported by a 1-dimensional model of the hydrogen plasma, developed at the LSPM laboratory (at Villetaneuse) which confirmed our results. (author)