Preliminary Result of Cavity Ring-Down Spectroscopy System for Radiofrequency Negative Ion Source Test Facility

Negative ion source is a core part of neutral beam injection system for magnetic confinement fusion devices. The density of produced hydrogen negative ions is a critical parameter of negative ion source. Cavity ring-down spectroscopy (CRDS) is an ultrasensitive absorption diagnostic technique for density measurement. Based on photo-detachment process, CRDS can measure integrated line-of-sight hydrogen negative ions density in high power ion source. CRDS diagnostic system has been applied to Hefei Utility Negative ion Test Equipment with RF source, which is now one of the references for China Fusion Engineering Test Reactor (CFETR) neutral beam injection system. Typical ring-down signals are obtained to calculate the density of hydrogen negative ions. The time evolution of hydrogen negative ions density is successfully measured. Preliminary experiments show the accurate relationship between RF power and measured hydrogen negative ions density.