Characterization of negative ion production in two-region arc plasma (TRAP) ion source

The Korea Atomic Energy Research Institute is developing a Two Region Arc Plasma (TRAP) ion source for negative hydrogen NBI system. A distinctive feature of this ion source is its capability to confine primary electrons emitted from the filament exclusively to the driver region, thereby eliminating the need for external filter magnets. In this system, the ratio of primary electrons to low-energy electrons in the driver region and extraction region can be controlled by adjusting parameters such as filament position, filament current, arc voltage, and gas pressure. This manipulation facilitates the creation of favorable conditions for negative ion generation, particularly within the extraction region. This study delves into the characterization of negative ion generation by varying these discharge parameters. To evaluate the negative ion density, a diagnostic system is built to measure laser-induced photo-detachment current using a laser system and a Langmuir probe. The experimental results obtained using the established diagnostic system show that it is possible to produce a high density of negative ions in the extraction region without the need for an external magnetic filter by adjusting the discharge parameters. This paper will describe the developed diagnostic system and discuss detailed experimental results.