Beam-Tracing-Based Quantitative Reconstruction of Density Fluctuations in QUEST Using Doppler Backscattering
A three-channel X-/Ku-band Doppler backscattering (DBS) system has been developed and installed on QUEST for turbulence and electric-field measurements. In spherical tokamaks, the large magnetic-field pitch angle increases the geometric mismatch between the probing beam wave vector and the local magnetic-field vector, reducing the effective perpendicular projection and resulting in a systematic underestimation of the measured scattering intensity. In addition, in QUEST, where low plasma density requires a low-frequency probe beam, beam propagation effects become increasingly significant, further complicating the interpretation of the measured DBS power in terms of local density fluctuation amplitude. To address these issues, a quantitative correction methodology based on the synthetic DBS code SCOTTY was established. All relevant diagnostic response effects were evaluated using SCOTTY along ray trajectories, yielding a correction factor for reconstructing the local turbulence amplitude from the measured scattering signal. The correction factor exhibits strong spatial and frequency dependence, varying by up to an order of magnitude between the plasma core and edge regions, highlighting the necessity of frequency-dependent corrections. By applying the derived correction factor to experimental measurements, quantitative density fluctuation amplitudes were reconstructed from the detected scattering signals. Evaluation of the fluctuation amplitude indicates enhanced turbulence activity in the plasma edge region, where a finite negative radial electric field is inferred. This work demonstrates the first quantitative turbulence evaluation using low-frequency X-/Ku-band DBS measurements in QUEST and establishes a framework for quantitative DBS analysis in spherical tokamaks.