Measurement of edge electron density profile with lithium beam emission spectroscopy (Li-BES) diagnostic on the experimental advanced superconducting tokamak (EAST)

Highlights: • Electron density profile is measured with Li-BES diagnostic on EAST. • The whole optical system of Li-BES diagnostic has been shifted outwards. • It's confirmed that transmissivities of APD channels in the edge region are improved. • Electron density profile behaviors during fast transient events on EAST are observed. -- Abstract: Lithium Beam Emission Spectroscopy (Li-BES) diagnostic has been developed on the Experimental Advanced Superconducting Tokamak (EAST), aiming at providing the edge electron density profile and density fluctuation. This system has been upgraded to 32 $\times$ 4 (radial $\times$ poloidal) channels since the 2016 campaign with a radial observation range of about 30 cm at the midplane on the low field side. The spatial resolution of Li-BES diagnostic is about 1 cm. The whole optical system of Li-BES diagnostic has been shifted 9 cm away from the tokamak axis perpendicular to the flange at horizontal D-port on EAST since the 2017 campaign. It's confirmed that transmissivities of Avalanche PhotoDiode (APD) channels in the edge region are improved by comparing the relative light intensity calibration factors of the same row of APD channels before and after the shift. Spatial calibration and relative light intensity calibration of this system have been performed. Based on Schweinzer's algorithm [J. Schweinzer et al., Plasma Phys. Control. Fusion 34, 1173 (1992)], the edge electron density profile is reconstructed and shown to be consistent with those provided by other diagnostics on EAST. The electron density profile evolution during the plasma current ramp-down process is observed with Li-BES diagnostic in both SOL region and pedestal region. With an injected Li-beam under the high frequency chopping control, the temporal resolution of the reconstructed electron density profile can be up to 10 $\mu s$, which reveals temporal behaviors of fast transient events on EAST, such as the L-H transition and ELM crashes.