Observation of Hydrogen and Cesium Spectra in a Negative Ion Source for a Neutral Beam Injector using a Multi-Channel Spectrometer

Effective production of hydrogen negative ions (H−) is one of the important issues in high power neutral beam injection (NBI) for fusion experimental devices. Cesium (Cs) vapor is seeded in the H− sources to enhance the H− production rate, which depends strongly on the amount of Cs inside the source. On the other hand, oxygen and water attenuate the production rate. In order to investigate the states of those species inside ion source, we have installed two multi-channel spectrometer systems with different spectral resolution. Hydrogen emissions, Cs emissions and oxygen emission are clearly observed with the time resolution of several handled milliseconds. Most of evaporated Cs atoms ionize in arc discharge plasma, and the emission increases strongly due to the ion back streaming with beam extraction. Oxygen neutral emission increases at the beginning of the beam conditioning, and its evaporation depends on the condition of the grid system.