Small-angle scattering and spatial resolution of fluctuation reflectometry: comparison of 2D analytical theory with numerical calculations

Abstract

Three methods to study the effect of density fluctuations on the reflected signal, based on a 2D full-wave analysis and Born approximation numerical and analytical calculations, are developed. The explicit analytical expression for the signal produced by an island-like perturbation running in poloidal direction is obtained and shown to be in nice agreement with results of numerical techniques for small level density perturbations. It is shown, both numerically and analytically, that due to small-angle scattering effects, fluctuations far from the cut-off layer may produce a significant and, in some realistic cases, dominant response. The conditions under which the fluctuation reflectometry signal is generated only in the cut-off vicinity are given.

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

“…It is possible to estimate N using the already developed non-linear conventional reflectometry theory from [20,42]. For normal incidence, it predicts the critical turbulence level for transition to the non-linear regime as,…”

“…In the experiment θ can be extracted from k ⊥ = 2k 0 sin θ where k ⊥ is typically obtained using ray tracing, and the refractive index gradient length L ref can be computed from the density and magnetic field profiles. The linearity criterion for normal incidence [42] can be generalized to oblique incidence using the physical optics linearity criterion Eq. 17, obtaining…”

“…This effect is investigated and a simple formula to describe it has been proposed as a first approximation. Finally applications to Doppler reflectometry data analysis based on these results are discussed, in particular the linearity criterion from [42] has been extended to include measurements at oblique incidence and a formula to assess the linearity of the Doppler reflectometry measurements has been provided. The Kirchhoff approximation limits the applicability of the model to small fluctuation levels.…”

Microwave reflectometry for fusion plasma diagnosis

“…[31,17,32]. However, numerical simulations are computationally intensive, therefore simpler models or analytic approaches are also important [22,33]. As noted previously Wang [13] has used extensive 2D full-wave simulation scans to map the level of coherent reflection vs turbulence amplitude and wavenumber which is then employed to interprete experimental signals.…”

A numerical study of fixed frequency reflectometry measurements of plasma filaments with radial and poloidal velocity components

“…From the whole set of amplitude signals we observe evidence of constructive and destructive interference effects as amplitude dips and peaks emerge depending on the poloidal size of the filaments. A complete analysis of Gaussian perturbations running in poloidal direction has already been made previously 10 .…”