Laser photodetachment diagnostics of a 1/3-size negative hydrogen ion source for NBI

Abstract

To investigate the flows of charged particles in front of the plasma grid (PG) in a negative hydrogen ion source, the information of the local densities of electrons and negative hydrogen ions (H - ) are necessary. For this purpose, the laser photodetachment is applied for pure hydrogen plasmas and Cs-seeded plasma in a 1/3-size negative hydrogen ion source in NIFS-NBI test stand. The H - density obtained by photodetachment is calibrated by the results from cavity ring-down (CRD). The pressure dependence and PG bias dependence of the local H - density are presented and discussed. The results show that H - density increases significantly by seeding Cs into the plasma. In Cs-seeded plasma, relativity exists between the H - ion density and plasma potential.