Comparison of fast ion collective Thomson scattering measurements at ASDEX Upgrade with numerical simulations

Collective Thomson scattering (CTS) experiments were carried out at ASDEX Upgrade to measure the one-dimensional velocity distribution functions of fast ion populations. These measurements are compared with simulations using the codes TRANSP/NUBEAM and ASCOT for two different neutral beam injection (NBI) configurations: two NBI sources and only one NBI source. The measured CTS spectra as well as the inferred one-dimensional fast ion velocity distribution functions are clearly asymmetric as a consequence of the anisotropy of the beam ion populations and the selected geometry of the experiment. As expected, the one-beam configuration can clearly be distinguished from the two-beam configuration. The fast ion population is smaller and the asymmetry is less pronounced for the one-beam configuration. Salient features of the numerical simulation results agree with the CTS measurements while quantitative discrepancies in absolute values and gradients are found.