Extraction system design and commissioning for TRAP negative ion source

An extraction system for the Two Region Arc Plasma (TRAP) negative ion source has been designed, fabricated, and commissioned at KAERI (Korea Atomic Energy Research Institute). Following previous beam line loss reduction from > 80% to ∼ 10% through cryopump installation (Kim et al. 2023; Kim et al. 2024) this work addresses extraction grid optimization. The extraction system consists of three grids: G1 (plasma grid), G2 (extraction grid, in which the electron suppression magnets are embedded) and G3 (acceleration grid). Electromagnetic simulations of the initial slit-type design revealed electron trapping between the grids, prompting a redesign to a circular-aperture geometry before fabrication, with 21 apertures of 5 mm diameter arranged in a line at 12 mm pitch, to improve co-extracted electron suppression. Arcing occurred with the initial electron suppression magnets embedded in the G2 grid (4 mm × 5 mm cross-section SmCo); iterative optimization guided by electromagnetic simulations removed the arcing with a reduced magnet (1 mm × 5 mm cross-section SmCo with 3 mm mild steel backing). IBSimu beam optics calculations (plasma current density 1–10 mA/cm 2 , extraction voltages G2: 0.5–2.5 kV, G3: 5–30 kV) were used to map the operating parameter space and to define the voltage range explored in the experiments. Initial commissioning (60 V arc, 90 A filament, 2.5 × 10 −3 mbar) demonstrated beam extraction with measured target currents up to 27 mA. About 20% of the transmitted current was collected on G3, combining beam interception, secondary electrons and stripping electrons, so enhanced diagnostics are required for quantitative current characterization. This work establishes an operational baseline for TRAP extraction studies.