Speaker
Description
Frequency-comb microwave reflectometer enables observation of dynamic behaviors of reflection layers corresponding to different cutoff densities by injecting microwave composed of multiple discrete frequency components into plasma. This is thus a diagnostic tool for simultaneously observing the spatiotemporal structure of the plasma density, including microscopic density fluctuations, at multiple spatial locations.
Here, we report the development of a frequency-comb microwave reflectometer for mid-sized helical device, Heliotron J. A Ka-band (26.5-40 GHz) frequency-comb oscillator was developed to cover the typical edge electron density range of the Heliotron J (0.8-2.0x10^19 m^(-3)). The frequency spacing between adjacent comb components is 0.5 GHz, corresponding to a spatial separation of approximately 5 mm (~5*rho_i) between neighboring cutoff layers (about 28 spatial measurement channels).
The frequency-comb microwaves are guided to the Heliotron J vacuum vessel through coaxial cables and rectangular waveguides and were radiated into free space by a corrugated horn antenna. A single antenna is used for both transmission and reception via a directional coupler. An ellipsoidal mirror installed in front of the diagnostic window focuses the beam near the cutoff layer (beam waist: 2 cm) to enhance the reflected signal. The incident and reflected signals are directly recorded by a digital storage oscilloscope with a bandwidth of 51 GHz and a sampling rate of 160 GSa/s.
This system was applied to Heliotron J as an O-mode Doppler backscattering diagnostic. The Fourier spectra of the scattered signals exhibit significant broadening, indicating the presence of turbulence-like fluctuations near the cutoff layer. Moreover, the spectral broadening is asymmetric with respect to the incident frequency, showing a clear Doppler shift. This Doppler shift is attributed to poloidal flow at the cutoff layer. Owing to its simultaneous multi-point measurement capability, this diagnostic enables evaluation of the radial structures (autocorrelation lengths) of flow velocity and density fluctuations, as well as correlations between the flow shear and fluctuations. In addition, it is expected to facilitate the identification of ballistic radial propagation of fluctuations and density gradients (particle avalanches).