A new Q-band comb-based multi-channel microwave Doppler backward scattering diagnostic developed on the HL-3 tokamak

The Doppler backscattering (DBS) diagnostic is widely used to measure the localized density fluctuations and the propagation velocity of turbulent structures. Microwave is launched at a frequency that approaches a cutoff layer in the plasma and at an angle that is oblique to the cutoff layer. A new Q-band multichannel DBS system based on a comb generator has been designed and tested for application on the HL-3 tokamak. With the comb generator and heterodyne scheme, the stability and flexibility of the new DBS system are improved. The new DBS diagnostic has a high output power (~ 10 dBm), good power flatness (< 5 dB in Q-band) and frequency stability, and the inter-frequency separation is tunable remotely. This article introduces the system design, laboratory test results, and initial experimental results from the HL-3 tokamak. With the help of the newly developed multichannel DBS, the turbulence information can be studied with high temporal and spatial resolution.