Application of passive spectroscopy to the He I line intensity ratiotechnique: a new tool for electron temperature and density measurement infusion boundary plasmas

Abstract

The emission of helium neutrals recycling on a poloidal limiter is analysed,so as to combine passive spectroscopy with the He I line intensity ratiomethod, widely used in combination with atomic helium beams in fusion devices,to measure the plasma electron temperature Te and density ne. The inherentproblem of passive spectroscopy, in contrast to active spectroscopy, is thatthe measured intensities result from the integration of the emission over thewhole line of view. Local information is therefore difficult to obtain. Here,however, it is demonstrated that this problem can be overcome by choosing thecorrect observing geometry that integrates the dominant plasma emission overnearly constant Te and ne chords in front of a poloidal limiter. In this paperonly radial electron temperature profiles are deduced from two He I lines,although ne measurements using a different He I line pair is also possible. Theexperimental results are in good agreement with other diagnostics. A futurepossibility of this method could be two-dimensional imaging of Te and ne inthe plasma boundary with good temporal resolution. The combination of passivespectroscopy and the He I line intensity ratio technique would be especiallyattractive in reactor-relevant conditions, where helium will be naturallypresent in the plasma.