SCATTERING OF REFLECTOMETER SIGNALS FROM RIPPLED SURFACES

Transverse coherent density fluctuations at the plasma cutoff layer are modeled by a rippled conducting surface. Two simulations are presented which give the reflectometer back scattered power as functions of ripple wavelength, amplitude, and phase. Results from an experimental simulation of the STOR-M tokamak geometry and a numerical simulation using the Helmholtz integral and a paraxial Gaussian probe beam, agree in showing three ripple wavelength (LAMBDA) sensitivity regions defined by beam radius (w): Short wavelengths LAMBDA/lambda much less than 2w/lambda experience large Bragg scattering. Long wavelengths LAMBDA/lambda much greater than 10w/lambda experience no scattering. Intermediate wavelengths experience complex scattering with strong modulation of reflected power at twice ripple frequency.