Oxygen ion impurity in theTEXTOR tokamak boundary plasma observed and analysed by Zeemanspectroscopy

Abstract

Oxygen ion impurity radiation is a potential source ofinaccuracy in ion temperature determination with the aid of thecommonly used C VI transition n = 8→n' = 7, produced bycharge-exchange recombination (CXR) of C6+ ions, since thecorresponding transition in O VI cannot be resolved undertypical plasma conditions in the tokamak. In order todemonstrate the possible importance of oxygen ion impurityradiation, we have selected a convenient spectroscopic `window'(about 8 Å wide) containing the major Zeeman components oftwo prominent lines in the visible (multiplet 1), one emittedby C2+ and one by O+. Observations have beenperformed in this wavelength range, both tangentially andperpendicularly to the magnetic flux surfaces, in the secondcase with the aid of a special graphite test limiter.Measurements include the case of special plasma discharges inwhich oxygen gas was introduced from the test limiter. Thetemperatures of both species are evaluated from the Dopplerbroadening of the respective Zeeman components, and comparedwith the results from a model for collisional heating by impactwith hot protons (deuterons) in the plasma edge. The spectra andderived results show that impurity identification in tokamakedge plasmas should not be carried out with the aid of spectrallines from highly excited levels populated by CXR, but usinglines corresponding to much morespecies-specific transitions from lower ionization stages. Theidentification and quantitative analysis should be performedwith the aid of carefully measured and calculatedZeeman-(Paschen-Back-) broadened line profiles, since these havefeatures practically unique to the species under investigation.Some allowance may, however, be required for deviation,from a statistical distribution, of population amongfine-structure sublevels.