Göttingen 2025 – wissenschaftliches Programm

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P: Fachverband Plasmaphysik

P 17: Poster Session II

P 17.34: Poster

Mittwoch, 2. April 2025, 16:15–18:15, ZHG Foyer 1. OG

Towards a deeper understanding of the pronounced increase in co-extracted current density in H-/D- negative ion sources for fusion — •Joey Rubin, Niek den Harder, and Ursel Fantz — Max Planck Institute for Plasma Physics

Negative ion sources for fusion face stringent operational demands. ITER’s sources must achieve a current density of 329 A/m² for H⁻ ions over 1000 s and 286 A/m² for D⁻ ions over 3600 s. Negative ions are generated on caesiated surfaces through the conversion of precursors formed in RF-drivers. Electrons are inherently co-extracted with the negative ions and must be removed from the beam before full acceleration. Magnets embedded in the extraction grid deflect and collect these electrons. During long pulses, the co-extracted electron current density increases, particularly in deuterium operation, limiting the pulse duration. At the ELISE test facility, which hosts a half-ITER-size source, a comprehensive set of diagnostics has been used to investigate the physics behind this phenomenon, and strategies to counteract it have been developed. This contribution reviews the current understanding of the mechanism driving the growth of co-extracted electron current density, highlighting the varying growth rates seen during beam extraction. Correlations with caesium dynamics and potential variations are discussed, along with open questions.

Keywords: neutral beam injection; negative ion source; co-extracted electron current; isotope effect