Development of fast helium beam emission spectroscopy on MST

A fast helium beam emission spectroscopy (BES) diagnostic is being developed to measure electron temperature and density fluctuations on MST. The light signals for two HeI triplet wavelengths (587.6nm and 706.5nm) and two HeI singlet wavelengths (501.6nm and 667.8nm) have been observed and analyzed. The observed local emission fluctuations from a fast neutral helium beam injected into the plasma may be inverted to the local plasma parameter fluctuations by making use of a full collisional-radiative model, the Atomic Data and Analysis Structure (ADAS) code package. The existence of metastable fractions (2$^{1}$S and 2$^{3}$S) in the fast neutral helium beam affects the beam stopping and emission coefficients. The local metastable fractions must be known to calculate the plasma parameter fluctuations from the observed local emission fluctuations. A system of statistical balance equations is solved to estimate the local metastable fractions and thus the total line emission intensities at a given wavelength. This requires $T_{e}$, $n_{e}$, and $Z_{eff}$ profiles as an input and therefore implies that the calculated emission fluctuation reflects both the local plasma fluctuation and the global plasma parameters. This work was supported by the US Department of Energy.