Sensitivity of calculated neutral helium line intensities and their ratios to 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 electron density and temperature both above and at the divertor targets. The accuracy of the derived electron density and temperature values depends on the uncertainties in the atomic data used to calculate line intensity ratios. Work has been carried out to assess the sensitivity of the population calculations to typical uncertainties in the electron impact excitation rate coefficients using ADAS (Atomic Data and Analysis Structure). We find that the uncertainty in the 3s 1 S-3p 1P excitation rate dominates the error in the line ratios.