Density fluctuation measurements via beam emission spectroscopy (invited)

Previous studies of plasma microturbulence have indicated that the fluctuation power scales with radial wave number, k⊥ , like k⊥−2→k⊥−3.5 for k⊥ ≥2 cm−1. This implies that low k fluctuations may dominate the spectrum. Beam emission spectroscopy (BES) has been developed to provide spatially localized measurements of density fluctuations in this low k region of the spectrum (k⊥ ≤2 cm−1). A 20-channel system has been installed on TFTR which images one of the heating neutral beams (via fiber optics) onto a set of photoconductive photodiode detectors. Fluctuations in the fluorescent Dα emission from the beam can be related to the local plasma density fluctuations via a model of the atomic excitation processes. The analysis of BES data utilizes many of the standard statistical analysis techniques such as power spectra, coherency and cross phase, and correlation analysis which are also used in the analysis of, for example, Langmuir probe data. In the case of BES however, these techniques require some special modifications to account for systematic effects such as photon statistics and fluctuations in the neutral beam density induced by the strong fluctuations near the plasma edge.