A comparison of theoretical line intensity ratios for Ni XII with extreme ultraviolet observations from the JET tokamak

Recent R-matrix calculations of electron impact excitation rates in Ni XII are used to derive the emission line ratios R-1 = I(154.17 Angstrom)/I(152. 15 Angstrom), R-2 = I(152.95 Angstrom)/I(152.15 Angstrom) and R-3 = 1(160.5 5 Angstrom)/I(152.15 Angstrom). This is the first time (to our knowledge) t hat theoretical emission line ratios have been calculated for this ion. The ratios are found to be insensitive to changes in the adopted electron dens ity (N-e) when N-e greater than or equal to 5 x 10(11) cm(-3), typical of l aboratory plasmas. However, they do vary with electron temperature (T-e), w ith for example R-1 and R-3 changing by factors of 1.3 and 1.8, respectivel y, between T-e = 10(5) and 10(6) K. A comparison of the theoretical line ra tios with measurements from the Joint European Tents (JET) tokamak reveals very good agreement between theory and observation for R-1, with an average discrepancy of only 7%. Agreement between the calculated and experimental ratios for R-2 and R-3 is less satisfactory, with average differences of 30 and 33%, respectively. These probably arise from errors in the JET instrum ent calibration curve. However, the discrepancies are smaller than the unce rtainties in the R-2 and R-3 measurements. Our results, in particular for R -1, provide experimental support for the accuracy of the Ni XIII line ratio calculations, and hence for the atomic data adopted in their derivation.