Spatial Characterization of Plasma Properties in a Kamaboko Negative Ion Source

High-energy negative ion-based neutral beam injectors (N-NBIs) are commonly equipped with filament-arc or radio frequency (RF) ion sources. Both types are based on the tandem concept, which envisages the presence of a static and oriented magnetic field, called the filter field (FF), for preventing negative ion destruction by energetic electrons. As a consequence, plasma parameters strongly depend on the position inside the ion source. To this end, a water-cooled movable Langmuir probe (LP) was developed at Consorzio RFX (Padua, Italy) and operated in the negative ion test stand (NITS) facility in Quantum Science and Technology (QST) (Naka, Japan), specifically in a filament-arc-based negative ion source, the so-called Kamaboko. This study presents a spatially resolved characterization along the axial direction of plasma parameters such as plasma density, electron temperature, and plasma potential. In particular, both the arc and extraction regions were explored under varying experimental conditions, including source pressure and wall polarization. For the same scans, the peak intensity trends of the principal hydrogen Balmer emission lines (\mathrm {H}_{\alpha }, \mathrm {H}_{\beta }, and \mathrm {H}_{\gamma }) measured along two lines of sight are also provided.