Comparison of reflectometer fluctuation measurements from experiment and two-dimensional numerical simulation

A comparison is made between the statistical properties of phase and power signals from a homodyne reflectometer on the STOR-M tokamak and simulated signals from a two-dimensional distorted surface model. Experimental results from edge density fluctuations show phase fluctuations of less than half a fringe with a Gaussian distribution and no phase ramping. Reflected power fluctuations are substantial (up to 45%) and are non-Gaussian distributed. Both phase and power signals display broadband turbulent frequency spectra with spectral indexes of −3.5. Fluctuations in the scattered electric field are calculated in the model using physical optics principles with a Gaussian reflectometer incident beam profile and structured surface distortions to replicate variations in the plasma cutoff layer. Simulation results display a wide range of features depending on three parameters, surface fluctuation amplitude, transverse wavenumber spectrum, and incident beam width. With the beam width fixed by the experiment (50 mm) and (realistic) fitted values of 1–2 mm for the root mean square fluctuation depth and 2 cm−1 for the k-spectral width—good agreement is found with the experiment.