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

Abstract

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.

Optical measurement of Cs distribution in the large negative ion source

“…The Cs + spectrum is stronger than the Cs 0 spectrum, which means that more than 99% of the Cs is ionized in arc discharge plasma. 6 We have not observed the W 0 spectrum around 400 nm, which is different from the result in the KAMABOKO-III source at CEA. 8 Figure 2͑a͒ shows time trace of the arc power ͑P arc ͒ and the acceleration drain current ͑I acc ͒; here the horizontal axis is the elapsed time since it turns on the filament power supplies.…”

“…In order to investigate the behavior of Cs distribution in the large negative ion source by using arc discharge, we have installed the optical diagnostic tools on the LHD-NBI source. 6 Experiments have been carried out to demonstrate Cs evaporation by backstreaming positive ions and Cs distributions in uniform and nonuniform discharges. Figure 1 shows a schematic view of the optical configuration on the LHD-NBI source.…”

“…Each LOS is arranged at y = 435 mm, y = 87 mm, y = −87 mm, and y = −435 mm, respectively. 6 We have arranged the low resolution ͑1.5 nm͒ survey spectrometer ͑PLASUS EmiCon system͒ to take many spectra simultaneously from 200 to 870 nm, and the high resolution ͑0.12 nm͒ spectrometer ͑ANDOR Shamrock system͒ with charge coupled device ͑CCD͒ detector with the size of 1024 pixels in width and 256 pixels in height to take close-up spectra into the 35 nm narrow wavelength range.…”

Optical Measurement of Cesium Behavior in a Large H− Ion Source for a Neutral Beam Injector

“…The H γ intensities and arc discharge current were stable during beam extraction. Since no electrostatic probe measurement was applied in the beam extraction experiment, the electron density (n e ) and the electron temperature (T e ) measured in experiments with only arc discharge of the same arc discharge power (80 kW) [10] were used as a reference. From this result, shown in Fig.…”

“…In the Large Helical Device Neutral Beam Injector (LHD-NBI) H − ion source, we have reported increasing Cs + ions in the arc chamber caused by back-streaming positive ions and changes in the uniformity of the Cs using a view port on the back plate [9,10]. The behavior of Cs near the plasma grid (PG) and the effects of long beam extraction are very interesting.…”