Influence of the probing wave phase modulation on the X-mode radial correlation Doppler reflectometry performance in the FT-2 tokamak

Abstract

The cross-correlation function of high field side radial correlation X-mode Doppler reflectometry (DR) measured in the FT-2 tokamak experiment is shown to be a factor of three narrower than that computed using the fast linear (Born approximation) version of the X-mode DR synthetic diagnostics developed in the framework of the ELMFIRE global gyrokinetic modeling of the FT-2 ohmic discharge. This difference is observed in spite of the fact that the computed DR signal frequency spectra are shown to be similar to those measured. A modest phase modulation of the probing and backscattering waves by the long-scale turbulent density fluctuations is shown, both experimentally and in computation, to be responsible for the observed effect.

Validation of full-f global gyrokinetic modeling results against the FT-2 tokamak Doppler reflectometry data using synthetic diagnostics

“…The role of the probing wave phase modulation in the FT-2 X-mode RCDR experiment was studied in [31], where the random phase perturbation of the backscattered wave was computed using the geometrical optics approximation and measured. The standard deviation of the computed DR signal phase was shown to be 0.93, whereas for the measured phase it is equal to 1.23.…”

Studies of Spectral Broadening of the Lower Hybrid Wave Line in the Current-Drive Regimes and Ion Heating at the FT-2 Tokamak

“…, where N ||0 is the initial slowing factor of the pump wave, and is the level of density fluctuations. According to the gyrokinetic calculations, confirmed experimentally in [19], in the peripheral plasma of the FT-2 tokamak, the level of density fluctuations is ∆n e /n e ≈ (1-2) × 10 -1 , and in the core plasma, it is ∆n e /n e ≈ 2 ×10 -2 .…”

Investigation of nonlinear effects in Doppler reflectometry using full-wave synthetic diagnostics

“…Unfortunately these values lie in the range where direct computation from ELMFIRE is impossible due to the strong numerical noise, as mentioned previously. [13] and impact of parallel velocity [15] is presented in previous works [16] and we refer the readers to these works for more information.…”

“…While the results obtained have demonstrated a qualitative agreement with the analytical predictions, the subject of the future work would be to perform quantitative analysis on the nonlinear scattering regime offset. Analytical criteria provided by [4]- [7] would need to be adapted for the parameters domain of FT-2 experiments, possibly including modifications due to the cylindrical geometry effects and compared against the modeling results as well as other nonlinearity interpretations, such as those presented in [15] and [16]. Nonlinear effects are shown to be weaker for O-mode probing in the studied cases, while a faster transition to the nonlinear regime is demonstrated for RCDR compared to DR in both probing scenarios.…”

“…instability cannot be correctly resolved by ELMFIRE, which is not a problem as the wavenumbers probed by DR are limited by k θ ρ s = 0.7, placing ETG mode outside of the accessible wavenumber range. Further details on the FT-2 tokamak turbulence and its modeling with ELMFIRE can be found in previous papers [13], [15], [16].…”