Numerical investigations of surface negative ion production in the giant NBI source SPIDER

The SPIDER experiment (Source for Production of Ions of Deuterium Extracted from RF plasma), located at the Neutral Beam Test Facility in Padua, is dedicated to the study of the radiofrequency (RF) negative ion source that will be used in the ITER Negative Ion-based Neutral Beam Injectors. The production and extraction of high negative ion currents, particularly Image 1001, are technically challenging due to their short lifetime inside plasmas, requiring a strong magnetic filter and the use of caesium to enhance surface production and limit the fraction of co-extracted electrons. A detailed understanding of the physical processes occurring inside the source is essential to optimize its performance, motivating the development of the 2D plasma fluid code FSFS2D (Fluid Solver For Spider in 2D). For the first time, the code has been equipped with a detailed model for the conversion of particles into negative ions on a caesiated surface and used to investigate future SPIDER operations in deuterium. The results confirm the expected performance improvement, characterized by an increase in negative ion current and a reduction of the electron co-extraction ratio, at higher input power, gas flux, and magnetic filter field strength, indicating, however, that further optimization is still necessary to reach the ITER target requirements.