Deconvolution of electron density from lithium beam emission profiles in high edge density plasmas

Active beam emission spectroscopy with high energy Li beams has been shown to be a successful technique for determining electron density profiles of low and medium density plasma edges of a tokamak. For high density plasma edges, however, the reconstruction algorithm necessary to extract the electron density profile from the Li beam emission profile at 671 nm encounters a singularity point at which numerical instabilities can occur. This behaviour is particularly pronounced for noisy input data. An 'integral method' has been developed at JET to replace the standard calculation method in the singularity region of the profile, and the 'normal' deconvolution method is then used for data in the high density region. Using this approach, routine deconvolution of emission profiles extending from low density regions well into high density regions of the plasma has been made possible. The sensitivity of the algorithm to noisy input data and errors on input plasma parameters has been examined for typical conditions expected at JET. The simulations show that even for unfavourable conditions electron density profiles can be recovered with an overall accuracy better than 25%.