W-band tunable, multi-channel, frequency comb Doppler backscattering diagnostic in the ASDEX-Upgrade tokamak

This article presents the design, implementation, and first data of a uniquely flexible, multi-channel, frequency comb Doppler backscattering diagnostic recently made operational in the ASDEX-Upgrade tokamak [A. Gruber and O. Gruber, Fusion Sci. Technol. 44, 569 (2003)]. It uses a double side-band signal fed into a ×6 frequency multiplier to produce a multiple-frequency output spectrum. Seven of these frequencies are simultaneously measured in the receiver via a two-step frequency down-conversion and traditional I/Q demodulation. The frequency comb spectrum is fully tunable to sit anywhere in the W-band. The inter-frequency separation is also uniquely tunable remotely between 0.1 and 6 GHz without any hardware changes. The diagnostic can be operated in both O and X-mode polarizations and at both oblique and normal incidence to the cutoff layer. The time evolution of backscattered signals, in excess of 30 dB, from seven distinct frequencies sampled simultaneously is presented across an L-to-H-mode confinement regime transition.