Upgrades of the W-band Doppler reflectometry for the core region plasma measurement on EAST

The five-channel tunable W-band Doppler reflectometry installed in the Experimental Advanced Superconducting Tokamak (Feng et al 2019 Rev. Sci. Instrum. 90 024704) has been upgraded. The signal launching and receiving circuits have been separated in order to circumvent the reflection of the W-band filter on the launching microwave, thereby reducing the background noise in a wide frequency band. The frequency interval between adjacent channels has been increased from 400 MHz to 1 GHz to broaden the diagnostic range in the core. Following upgrades and the careful optimization of the power distribution between different channels, the performance in the high frequency range has been significantly enhanced. Current available bandwidth is 75 ∼ 100.4 GHz, and the region within ρ ∼ 0.2 can be covered in low density discharges (n_e < 2 × 10¹⁹ m⁻³) with a toroidal field B_t = 2.5 T, which meets the fundamental requirements for measuring the plasma in the core region. During the internal transport barrier experiments investigating the fishbone instability, turbulence suppression could be distinctly identified, and the Doppler shift fluctuations related to the fishbone instability have been observed. The fluctuation amplitude calculated by the center of gravity method reaches 1 MHz, which is considerably larger than the Doppler shift arising from the equilibrium electric field (mainly within 300 kHz). In order to elucidate the underlying causes of the observed Doppler shift fluctuations, a 2D full-wave simulation was employed. The simulation results indicate that the Doppler fluctuation amplitude caused solely by the deformation and movement of the cutoff layer due to fishbones is only 0.1 of the experimental value. This suggests that these fluctuations are not dominated by the deformation and movement effects of the cutoff layer.