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Study of negative-ion surface production and extraction in low-pressure plasmas : application to the neutral beam injector of ITER

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Thesis advisor : Pr Gilles Cartry
Email and address : gilles.cartry
Tel : 04 91 28 89 59

Subject description :
The present work deals with negative-ions for fusion applications in the context of the international projects ITER and its successor DEMO, which aim to demonstrate controlled nuclear fusion for energy production. In tokamaks (nuclear fusion reactors), a plasma composed of deuterium and tritium is magnetically confined and heated to very high temperatures, around 1.5·108 K, to overcome the repulsion between deuterium and tritium nuclei and achieve fusion. In the ITER and DEMO devices, the heating of the plasma will mainly be produced by Neutral Beam Injection (NBI). The ITER NBIs are required to inject 1 MeV beams of neutral deuterium atoms (D) into the tokamak, providing plasma heating and current drive. The production of such D beams relies on neutralization of high-intensity D- beams. D- negative-ions are produced in a low-pressure plasma source and subsequently extracted, accelerated and neutralized. The only up to date solution to produce the high intensity (40 A) beam required is to inject in the plasma source cesium (Cs). Cesium deposits on all surfaces in contact with the plasma and lower the material work function, thus enhancing the capture of electrons by incoming deuterium ions or atoms. While the solution is efficient, it has many drawbacks that could jeopardize NBI operation. Therefore an important research effort is presently being carried out to understand caesium dynamics, negative-ion surface production, and extraction in plasma sources. The final goal is to limit at maximum the Cs injection and solve the associated issues.
PIIM laboratory has developed many tools, both numerical and experimental, to study negative-ion surface production in low-pressure plasmas. It is proposed here to investigate the fundamental parameters of negative ion production and extraction in Cs-seeded hydrogen/deuterium plasmas. The experiment will allow for a large control of operational parameters, with many dedicated diagnostics to measure relevant parameters : H-/D- density will be measured by Cavity Ring Down Spectroscopy, angle and energy distribution functions of H-/D- emitted from the wall will be measured by mass spectrometry, extracted negative-ion flux by calorimetry… The experiment will allow for a benchmark of numerical models developed at LAPLACE laboratory (Toulouse) describing the whole physics of the negative-ion source. Using scaling laws, the experiments and the benchmarked model will be used as development tools to understand and optimize negative-ion production and extraction in real high plasma density negative-ion sources.
Skills and knowledge : Knowledge in plasma science and/or surface science is required. The experimental aspect of physic research must motivate the candidate.