Avis de Soutenance de thèse de Monsieur Basile DUTEIL POURADIER

Avis de Soutenance

Monsieur Basile DUTEIL POURADIER

Soutiendra publiquement ses travaux de thèse intitulés :

« Study and optimization of the use of caesium in large negative ion sources 

for nuclear fusion applications »

Soutenance prévue le mardi 1^er décembre 2026 à 10h30

Lieu : salle René Gravier et par Skype

Composition du jury :

Résumé :

« Efficient operation of the heating neutral beam (HNB) injectors is essential for achieving ITER’s target thermonuclear conditions. At least two injectors will each deliver 1 MeV deuterium beams at 16.5 MW for up to 3600 s. Their development relies on the Neutral Beam Test Facility (NBTF) in Padua, Italy, comprising MITICA, the full-scale injector prototype, and SPIDER, the full-size radio-frequency negative-ion source. 
Because positive-ion neutralization becomes inefficient above 100 keV, the HNBs use negative ions produced mainly by surface conversion, enhanced by caesium deposition through a reduction in surface work function. Controlling Cs transport and surface interactions is therefore crucial for long-pulse stability, beam uniformity, and accelerator voltage holding. This thesis combines experiments, diagnostic development, and modelling to optimize Cs management.

Successive oven-nozzle designs improved Cs deposition uniformity in vacuum while aiming to limit transport into the accelerator. Experiments in the Caesium Test Stand (CATS) showed that Cs sticking involves compound formation and depends on impurities, temperature, and deposition history, explaining changes in density and deposition at constant injection rate. A work-function diagnostic was developed and installed in SPIDER to relate caesiation to surface properties and beam performance. 
A unified hydrogen–caesium collisional-radiative model enabled joint interpretation of both species’ emission, improving the consistency of inferred plasma parameters. Agreement between neutral Cs densities and laser absorption measurements supports optical emission spectroscopy for Cs monitoring and spatial characterization, particularly valuable for MITICA and ITER, where diagnostics will be limited.

SPIDER campaigns demonstrated the importance of plasma-induced Cs redistribution, notably towards inactive regions during partial-source operation. Estimated Cs consumption for HNB operation is comparable to the nominal 10 g inventory per oven, highlighting the need to optimize its use. 
These findings were integrated into CARODAS, a code describing Cs transport with and without plasma and the evolution of surface coverage. Simulations explain partial-source behaviour and magnetic filter-field effects. They indicate that Cs injected during plasma pulses is unlikely to reach the plasma grid (PG) directly, emphasizing recycling from surfaces. The formation of weakly caesiated regions during long pulses could explain deteriorating beam performance. This work supports more predictive Cs management and operating procedures that maintain favourable PG conditions while limiting consumption and accelerator contamination. 

Key words : Plasma, Neutral beam injector, Caesium, ITER, Negative ion source, Transport 

………………………………………………………………………………………………………………………..