Wenxiang Shi

A novel multichannel X-mode reflectometer has been developed for density measurements near the ion cyclotron resonance heating antennas on the Experimental Advanced Superconducting Tokamak. This system enables direct measurement of the edge density profile in the immediate vicinity of the antennas. Key challenges associated with long fundamental rectangular waveguides—significant attenuation and dispersion—are addressed through a combination of watt-level amplifiers and a digital mixing method calibrated via a closed-loop waveguide test. Furthermore, a dedicated peak-seeking algorithm is implemented to robustly determine the zero-density starting position (Rstart) under conditions of harmonic interference. Finally, a corrected reconstruction formula accounting for the dual-horn geometry confirms its negligible influence (≲0.5mm) on profile accuracy.

Doppler reflectometry (DR) can measure the turbulence in magnetic confinement fusion plasma by detecting backscattered signals, and has been widely used in fusion devices. On the experimental advanced superconducting tokamak (EAST), a Backscattering and Forward-scattering integrated DR system has been deployed during the 2024 experimental campaign, which can measure the turbulence across multiple scales at the same location, simultaneously. In the scattering process, the wave vector values satisfy the Bragg condition: m k ⊥ = k s − k i . For Doppler backscattering (DBS), it detects the m =−1 scattered signal, whereas for the forward scattering, it detects the scattered signal with m > 0 .   To investigate the scattering process around the cutoff layer for DR, a two-dimensional (2D) plane-shaped O-mode full-wave simulation based on the finite-difference time-domain (FDTD) method has been done. It was found that the signal received by the DBS is mainly the m = −1 scattered signal, while the turbulence measured by the Doppler forward-scattering (DFS) corresponds mainly to low-k fluctuations ( k ⊥ < 3c m − 1 ). Besides, the results indicate that for both backscattering and forward scattering, the intensity of the scattered signals increases with turbulence level, while the Doppler shift frequency remains unchanged. Therefore, based on these simulation results, the integrated Backscattering and Forward-scattering DR system can simultaneously measure the large scale (low- k ) and small scale (high- k ) turbulence at the same position in the plasma.

In the Experimental Advanced Superconducting Tokamak (EAST), a transition from electromagnetic to electrostatic turbulence is observed in the pedestal region as plasma density ramps up. This transition is manifested by the suppression of magnetic fluctuations and the presence of broadband electrostatic turbulence. The frequency domain of the electrostatic turbulence is typically beyond 300 kHz. It leads to a rapid build-up of density gradient and a sharp degradation of energy confinement. By reducing the gas puffing rate, a prolonged intermediate transition phase is observed, and the confinement improved with increasing density in this phase. The emergence of broad electrostatic turbulence is associated with the enhanced turbulence control parameter α _t, together with a weaken edge radial electric field. Furthermore, the impact of the turbulence transition to scrape-off layer (SOL) transport is evaluated. Measurements suggested that radial particle flux and intermittent structures are strengthened after the transition. Moreover, the profiles in the far SOL are broaden with increase of α_t. The properties of the magnetic fluctuations are consistent with the nature of the magnetic coherent mode previously observed in EAST, while the broad electrostatic turbulence is proposed to be an ion temperature gradient mode by the Gyrokinetic Electromagnetic Numerical Experiment simulation. It is clarified that the energy confinement degradation in high-density regimes is primarily driven by the broadband turbulence rather than divertor detachment. These findings advance our understanding of high-density H-mode plasmas and provides additional insights into the interplay between edge turbulence and global confinement properties.

An E-band (60–90 GHz) tunable ten-channel backscattering and one-channel forward-scattering integrated Doppler reflectometer with X-mode polarization has been installed on the O port of the Experimental Advanced Superconducting Tokamak. The ten-channel backscattering system can launch 16 fixed microwave beams in the range of 60–90 GHz with an interval of 2 GHz into the plasma, and 10 probing signals are selected by employing a reference signal and multiple filters. It can measure the turbulence at 10 different radial locations simultaneously. In addition, by the backscattered and forward-scattered signals, this integrated system can measure multi-scale turbulence at the same location. During experiments, the frequency of the reference signals is tunable in the E-band, and the selected probing signals can be changed as needed in 100 μ s . Furthermore, the incident angle can be adjusted from −10° to 20°, and the wavenumber range is 4–25 cm⁻¹ with a wavenumber resolution of Δk/k ⩽ 0.28. The scattering locations and the perpendicular wavenumber are calculated by ray tracing in the experiments.

An E-band (60–90 GHz) multi-channel Doppler backscattering (DBS) system with X-mode polarization has been installed on the Experimental Advanced Superconducting Tokamak (EAST), which can measure the turbulence at five different radial locations simultaneously. This system can launch 31 fixed microwave frequencies in the range of 60–90 GHz with a 1 GHz interval into the plasma, and five probing signals are selected by employing a reference signal and multiple filters. During experiments, the frequency of the reference signal is tunable in the E-band, and the selected probing signals can be changed as needed without any other adjustments, which can be performed in one shot or between shots. Furthermore, the incident angle can be adjusted from −10° to 20°, and the wavenumber range is 4–25 cm−1 with a wavenumber resolution of Δk/k ≤ 0.35. Ray tracing simulations are employed to calculate the scattering locations and the perpendicular wavenumber. In this article, the hardware design, ray tracing, and initial results obtained from the EAST plasma will be presented.