Hydrogen negative ion density measurement by laser photodetachment and the effect of H recombination on the walls

Abstract

A laser photodetachment (LPD) was applied to measure the absolute hydrogen negative ion (H−) density in an RF negative ion source. The ion source has been developed to study a Cs-free ion source utilizing a magnetic filter by permanent magnets to enhance the volume production of H− in the extraction region. Prior to a beam extraction experiment, the laser photodetachment was applied as an initial performance evaluation. The absolute H− densities were obtained by LPD with the variation of the RF power. The H− density reached 1.8×109 cm−3 at an RF power of 5 kW. The comparison with the calculation of a particle balance model showed the discrepancy between the model and the LPD results. The LPD result showed the rapid increase in H− density with respect to the RF power compared to the model. By assuming the wall temperature increase and adjusting the recombination coefficient (γrec) of hydrogen (H) atoms on the walls, the discrepancy could be resolved. The recombination coefficient (γrec) plays an important role in determining the H− density in view of the fact that it can affect both the generation and the destruction processes of H−.