Performance prediction of upgrading lithium beam emission spectroscopy to sodium beam emission spectroscopy diagnostic on EAST

The alkali beam emission spectroscopy diagnostic is an active spectroscopic diagnostic method that can measure edge electron density profile and density fluctuation. Due to the limitations of the previous lithium beam emission spectroscopy (Li-BES) (Zoletnik et al 2018 Rev. Sci. Instrum. 89 063503) installed on the Experimental Advanced Superconducting Tokamak (EAST), providing edge electron density profile and density fluctuation, its upgrade to a sodium beam emission spectroscopy (Na-BES) is now in process. Here, we report a performance prediction of upgrading the existing Li-BES diagnostic detecting the Li-I(2p–2s) line emission to the ongoing Na-BES diagnostic detecting the Na-I(3p–3s) line emission on EAST. This is done by calculating the occupation numbers of different atomic states of sodium and lithium resulting from collisions with the bulk plasma particles using a collisional-radiative model. During the calculations, the beam energy and the electron density profile (starting from an experimental measurement in an EAST H-mode discharge) are respectively varied. Calculation results demonstrate that, in contrast to Li(2p) state, the radial extension of Na(3p) occupation number distribution is shallower (still appropriate for pedestal measurements) and less influenced by the change in either beam energy or electron density profile. Moreover, although the occupation numbers of Na(3p) are approximately half those of Li(2p), under identical beam current conditions, the lower velocity of the sodium particles with a much higher density results in a higher total population of Na(3p) than that of Li(2p). Considering the significantly higher spontaneous transition coefficient of Na(3p) compared to Li(2p), along with the improved optical system, the signal intensity of Na-BES with a 50% pixel size is predicted to be 2–6 times higher than that of Li-BES in the edge region of EAST. When we further consider a much lower background impurity emission close to the Na-I line wavelength, our calculation suggests that a significantly improved system performance of such a Li-BES to Na-BES upgrade may be accomplished on EAST.