A novel, multichannel, comb-frequency Doppler backscatter system

TLDR

Doppler backscattering is a powerful diagnostic in high‑temperature fusion plasmas, sensitive to turbulence flow and used to determine radial electric fields and study geodesic acoustic modes, zonal flows, and intermediate‑scale density turbulence. The authors present a novel, stable multichannel V‑band (50–75 GHz) Doppler backscatter system that simultaneously monitors turbulent flows and fluctuation levels at eight distinct spatial locations. The system employs a high‑frequency, low‑phase‑noise comb‑frequency generator combined with a filter bank and quadrature detection to achieve this capability. When deployed on DIII‑D, the system has successfully monitored flow and turbulence across the plasma radius during events such as the L‑H transition.

Abstract

Doppler backscattering has emerged in recent years as a powerful diagnostic tool in high temperature fusion plasmas. The technique is sensitive to plasma turbulence flow and has been utilized to determine radial electric field and to study geodesic acoustic modes, zonal flows, and intermediate scale density turbulence. The current manuscript describes a novel technique for creating a stable, multichannel system covering the V-band frequency range (50-75 GHz) which enables simultaneous monitoring of turbulent flows and fluctuation levels at eight distinct spatial locations. The system is based on a high-frequency, low phase noise comb-frequency generator combined with a filter bank and quadrature detection system. The system is now in operation on DIII-D and has allowed monitoring of the flow and turbulence levels across the plasma radius during events such as the L-H transition.