Benchmark of collisional?radiative models for ITER beams at the Alcator C-Mod tokamak

The beam-based plasma diagnostics on ITER require high accuracy and reliability which accordingly put challenging requirements on the collisional-radiative (CR) models used for description of the excited states of beam atoms. These states are known to play an essential role in implementation and interpretation of neutral beam-based diagnostics, such as the motional Stark effect (MSE), charge-exchange recombination spectroscopy (CXRS), beam-emission spectroscopy (BES) and in the dynamics of beam penetration. The latest analyses demonstrate that the widely used assumption of statistical populations among the beam excited states is questionable for high beam energies and high magnetic fields. Here we report an empirical verification of the recently developed non-statistical nkm-resolved CR model (Marchuk et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 011002) and the short extrapolation to the relevant parameter range for ITER. The experiment was performed on the Alcator C-Mod tokamak, which operates in a unique range of parameters well suited for testing CR models for ITER beams. Beam emission spectra are collected for a selected range of plasma parameters. The line ratios sigma(1)/sigma(0), pi(4)/pi(3) and Sigma sigma/Sigma pi are measured and compared to the n-resolved statistical population model and the new non-statistical nkm-resolved CR model. The measured ratios show clear deviations from the statistical model and a good agreement with the non-statistical results. The difference between the experimental values and nkm-resolved simulations for the most part is within 10% for all three line ratios. The largest deviation here is for the sigma(1)/sigma(0) ratio that reaches up to 14% for the lowest electron density of 0.6 x 10(20) m(-3). In contrast, the difference between experiment and the n-resolved (statistical) model is within 13-27% for the 50 keV/u Alcator C-Mod beam and expected to be up to 32% for 100 keV/u and up to 43% for 500 keV/u ITER beams. The discussed effect must be taken into account for the proposed CXRS/BES and MSE diagnostics for ITER.