Highly-ionized atoms in fusion research plasmas

Topical problems involving the interpretation of line emission from impurities or diagnostic test ions in Tokamak fusion devices are discussed briefly. The particular use of Tokamaks for precision (approx. 10 ppm) wavelength spectroscopy of Δn = 0 transitions in highly ionised atoms with between 2 and 12 electrons is described in more detail. Experimental wavelengths are compared with ab initio theory and with semi-empirical isoelectronic extrpolations. The Tokamak data for ions with Z >= 3-electrons lead to improved isoelectronic extrapolations which reveal errors in the previous data and provide transfer standards of wavelength in the XUV and VUV regions. Wavelengths of the 2-electron systems are of sufficient accuracy to be sensitive to the 2-photon exchange term in the Breit interaction as well as the 2-electron contribution to the Lamb shift. Experimental wavelengths are in agreement with recent theoretical calculations taking these contributions into account. (Auth.)