The neutral beam injection system (NBI) has the highest heating efficiency and the clearest physical mechanism, so it has become one of the main auxiliary heating methods used in the world's large magnetic confinement controlled thermonuclear fusion devices. The application of NBI system based on negative ion source is increasingly demanding and urgent. Cavity ring-down spectroscopy (CRDS) is a highly sensitive absorption spectrum measurement technique with relatively simple principle, and the measurement results are not limited by electromagnetic field interference and other plasma parameters.

This study aims to explore the characteristics of negative ion generation in negative ion source based on CRDS, and measure negative ion density produced by NBI.

First of all, a CRDS diagnostic system was developed on a large area negative ion source. Then the related characteristics of negative ion production was investigated by sequentially measuring the decay time of pulsed laser in front of the plasma electrode plate under different experimental parameters. Finally, developed CRDS diagnostic system was employed to study the characteristics of hydrogen anion generation under different experimental parameters, such as radio-frequency (RF) power, source pressure and bias voltage.

Experimental results show that produced negative hydrogen ions increase with the increase of RF power and pressure in the source cavity. Due to the different effects of the two changes on the electron temperature, the growth rate of negative hydrogen ions changes opposite under the influence of different power and pressure. There is an optimal value of bias voltage that is conducive to the generation of negative hydrogen ions because of the plasma potential.

Application of CRDS-based measurement approach in this study provides valuable experience for further research on the generation of negative hydrogen ions and the beam quality.