Signal amplitude effects on reflectometer studies of density turbulence in tokamaks

The effect of amplitude fluctuations on reflectometer measurements of density turbulence has been investigated through comparison of reflectometry and Langmuir probes on the CCT and DIII-D tokamaks. Power spectra, turbulent radial correlation lengths, and root-mean square magnitude variations (at the I-I-mode transition) of the homodyne reflectometer signal (given by S-Re = E cos(phi), which depends strongly on the amplitude E and nonlinearly on the phase phi), show good agreement with Langmuir probes. The homodyne signal is found to be dominated by the amplitude fluctuations and not by the phase for high density fluctuation levels. Correspondingly, power spectra and correlation lengths deduced from the phase data alone show agreement with the homodyne signal only at low density fluctuation levels. It is concluded that for these plasma parameters the homodyne signal (S-Re) is closely representative of the density fluctuation behaviour and that this response is related to the reflectometer amplitude E. This correspondence of the homodyne signal and density fluctuations is in contrast to most theoretical/modelling work which has typically concentrated on the phase. A one-dimensional simulation of resonance absorption effects upon the amplitude and phase of a reflectometer is presented as an example of how amplitude fluctuations might arise due to processes internal to the plasma. The implications of these results and the connection to theory are discussed.