Radial correlation reflectometry at oblique probing wave incidence (linear scattering theory predictions)

Abstract

Turbulence radial correlation length measurements by making use of Doppler reflectometry are analysed in this paper assuming the linear (in density perturbation amplitude) regime of the probing wave scattering. Based on the Born approximation applicable in this case, we get a general integral representation of the scattering signal amplitude, which is then analysed in detail both analytically and numerically. Asymptotic analytical evaluation of the scattering signal amplitude gives a criterion on the incidence angle, exceeding which leads to suppression of the poor localized forward scattering contribution to the Doppler reflectometry signal. In the latter case, fast decay of correlation in the two frequency channels appears to be similar to that expected from the turbulence correlation function. The analytical estimations and predictions are confirmed by numerical evaluation of the scattering signal amplitude. The analysis performed proves the feasibility of the radial correlation Doppler reflectometry technique for measurements of the correlation length of a modest level turbulence at oblique launching of the probing wave beam.

Enhanced Doppler reflectometry power response: physical optics and 2D full wave modelling

“…The dominance of the cutoff contribution justifies the application of PO for this case. It is important to remark that although the weaker contributions away from the cutoff are not relevant for the power response, they may be important for radial correlation length studies, where signal contributions of very low amplitude can still give strong correlations [41].…”

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

“…[13][14][15] In the former case, the small-angle scattering contribution could be suppressed by choosing a large enough incidence angle, satisfying the condition proposed in Ref. 8. However, in the latter (non-linear) case, the turbulence wavenumber spectrum measurements are questionable, and only the long-scale plasma turbulence mean velocity could be determined (but with poor spatial resolution).…”

“…In the computation, we used the analytical relationship between the measured signal CCF and the density fluctuation two-point CCF derived in Ref. 8 and assumed v ¼ 0. As is seen in Fig.…”

“…6 Recently, the radial correlation modification of this technique, 7 providing information on the turbulence radial structure, was justified. [8][9][10] Unfortunately, the interpretation of the radial correlation reflectometry data is complicated by the contribution from the poorly localized small-angle scattering along the wave trajectory, which can lead to the overestimation of the turbulence radial correlation length in the linear scattering regime 11,12 and to its underestimation in the nonlinear regime (multiple scattering or strong phase modulation). [13][14][15] In the former case, the small-angle scattering contribution could be suppressed by choosing a large enough incidence angle, satisfying the condition proposed in Ref.…”

Development and application of radar reflectometer using micro to infrared waves

“…In fluctuation diagnostics of plasmas, the important issues are to obtain wavenumber resolved information as well as temporally and spatially resolved ones. Due to this point of view, radial and poloidal correlation reflectometry [21,22] and Doppler reflectometry which uses an oblique microwave beam and picks up the backscattered signal [23,24] have been applied to the measurements. In order to perform localized fluctuation measurements over a wide range of radial positions, an ultrafast sweeping source [25] or a comb generator source [26] are often utilized as well as one or two fixed frequency sources, since the fluctuations are 'frozen' during the measurement time.…”