Analysis of fluctuating signals from a multichord interferometer

Abstract

Fluctuating signals from multichord interferometers contain information about the spatial structure of the corresponding plasma perturbations. We present results from the interferometer used with the reversed-field pinch device ZT-40M, and discuss methods of systematically surveying the spectrum of fluctuations so as to search for patterns representing coherent spatial structures in the plasma. Three such structures or ‘‘modes’’ are identified, and it is shown how the amplitude distributions of each can be separated out from the general background due to other modes. Knowledge of the amplitude distributions is then used to construct linear combinations of the eight interferometer signals, which represent the time variation of each of the coherent modes; power spectra of these linear combinations show prominent peaks at frequencies characteristic of the modes even when the spectra of the individual interferometer signals are bland and featureless. From an analysis of the amplitude distributions it is shown that the three modes comprise (a) a broad-band symmetric (m=0) mode concentrated in the outer quarter of the plasma; (b) an antisymmetric (m=1) mode at ∼30 kHz confined to the discharge core; and (c) a second antisymmetric mode at ∼100 kHz confined to the outer region of the plasma.