Diagnostics tools and methods for negative ion source plasmas, a review

Plasma parameter measurements for negative hydrogen (H ^− ) ion sources have been playing an important role in clarifying fundamental physics related to negative ion production and destruction processes. Measured data of beam properties, such as H ^− ion current density with the co-extracted electron current and the emittance, were correlated to local concentration of charged particles and temperature often characterized by Langmuir probes and optical emission spectrometry. Langmuir probes coupled to pulse lasers quantified local H ^− ion densities from early days of H ^− ion source development, while the cavity ring down photodetachment method removed Langmuir probes from contemporary large-size high power density ion sources. Technological progress has made source plasma diagnostics possible during beam extraction, which has thrown light on the transport of H ^− ions during the application of the extraction electric field. The advancement of plasma diagnostics for high intensity H ^− ion sources are summarized in this report together with recent results from the research and development negative ion source being operated for collaborative research programs at National Institute for Fusion Science.