Spectroscopic diagnostics for the negative ion RF source SPIDER

The negative ion RF source SPIDER, prototype source of the ITER neutral beam injectors, will be built at Consorzio RFX, and it will include a full set of diagnostics, required for safe operation and beam performance optimization. This paper describes the design of spectroscopic diagnostics of both plasma source and accelerated beam. Emission spectroscopy of the plasma source will measure key parameters involved in negative ion (H− or D−) formation and recombination processes, such as temperature and density of electrons and of the plasma gas in atomic and molecular form, density of cesium and other impurities and, finally, the H−/D− concentration. Absorption spectroscopy, with a cavity ring-down technique, will provide a direct measure of the H−/D− density. The extracted beam is then probed by beam emission spectroscopy, to measure the beam homogeneity, divergence and stripping losses. Observing the emission of a selected spectral line with a sufficient number of lines-of-sight will allow a tomographic reconstruction of the two-dimensional beam emission, which is proportional to the beam density.