Effects of reflectometer asymmetries on fluctuation measurements

Effects on reflectometer phase φ-tilde and power P-tilde fluctuation signals due to (a) asymmetries in the transmit-plasma-receive antenna geometry (misalignments), and (b) asymmetries in the plasma cut-off layer perturbations (distortions from sinusoidal or Gaussian) are studied using a two-dimensional (2D) physical optics model. Results show the onset of phase runaway with antenna misalignment and/or sawtooth type perturbations when the perturbation amplitude exceeds some critical value. For broadband (Gaussian) turbulence antenna misalignment leads to Doppler shifts in the φ-tilde spectrum - provided the reflectometer beam width w and spectral width kw of the turbulence are sufficiently large. Misalignment also generates coherence between φ-tilde and P-tilde at Bragg backscatter frequencies with quadrature (±π/2) phase difference. Sawtooth (asymmetric gradient) perturbations also generate phase-power coherence in quadrature, but at frequencies determined by w and kw, i.e. not Bragg. Cusped or spiky (asymmetric amplitude) perturbations generate asymmetric phase distributions (nonlinear phase offsets) and low-frequency phase-power coherence with 0 or π phase difference. The simulations indicate that a combination of antenna misalignment (or plasma tilt) and cusped reflection layer perturbations can account for a wide range of experimentally reported features in reflectometer signals. (author)