Measuring plasma impurities in Alcator C-Mod as a function of time in the extreme ultraviolet

A flat-field grazing-incidence grating spectrometer has been installed on the MIT Alcator C-MOD tokamak, following the installation of such a spectrometer on the Livermore electron beam ion trap facility and on the National Spherical Torus Experiment at Princeton. The spectrometer employs a variable space grating with an average spacing of 2400 lines/mm and covers the 10-70 Å wavelength band. It is being used to monitor the presence of impurity ions such as boron, oxygen, fluorine, argon, calcium, iron, molybdenum, and tungsten, and to calibrate spectral diagnostics of astrophysical plasmas. The Alcator instrument provides spectral emission data in a density regime that is one to three orders of magnitude higher than that covered by the other two instruments and thus produces complementary data. (author)