Study on Cavity Ring-Down Spectroscopy in High Power Ion Source

High power ion source is a core component of neutral beam injector for tokamak. Negative ion source is the reference source for ITER neutral beam injection system as negative ion beam has a high neutralization efficiency in high energy. Cavity ring-down spectroscopy (CRDS) is a highly sensitive optical absorption spectroscopic technique. CRDS is able to measure the absolute line-of-sight integrated density of negative hydrogen ions based on photodetachment process. Compared with laser photodetachment method and optical emission spectroscopy, CRDS is insensitive to electromagnetic interference and doesn't need plasma parameters for calculating, thus yielding reliable results. CRDS system is composed of laser, optical cavity, detector and data acquisition system. Equipment selection principle was analyzed. The basic principle of CRDS was presented, and then the analytical form of using CRDS to measure H- density was derived in detail. CRDS has already been applied to high power ion source in laboratories of Germany, Japan and Italy. Results displayed that source parameters, including pressure and power, affected H- density. H- density increased from 1016 m(-3) magnitude to 10(17) m(-3) magnitude in Cs seeded operation. The linearity between H- density and extracted current was proved by NIFS.