Sensitivity of calculated neutral helium line intensities and their ratiosto uncertainties in excitation rate coefficients

The neutral helium line intensity ratios 667.8 nm/728.1 nm and 728.1 nm/706 .5 nm are measured at JET to provide essential information on the parallel and radial profiles of election density and temperature both above and at t he divertor targets. The accuracy of the derived electron density and tempe rature values depends on the uncertainties in the atomic data used to calcu late line intensity ratios. Work has been carried out to assess the sensiti vity of the population calculations to typical uncertainties in the electro n impact excitation rate coefficients using ADAS (Atomic Data and Analysis Structure). We find that the uncertainty in the 3s S-1-3p P-1 excitation ra te dominates the error in the line ratios.