Cross-power spectrum measurements of plasma turbulence

An analog cross-power spectrum analyzer operating up to 30 MHz has been constructed. The device requires only a fixed delay to measure the quadrature components of the cross-power spectral density. Also, because of its analog character, the device should be capable of being scaled to much higher frequencies than 30 MHz. The device has been applied to the study of ion-acoustic wave turbulence in a positive column. In principle, it should be possible to determine the complete (ω, k) spectrum of such waves by recording the cross-power spectrum of the signals from two probes at different positions in the plasma, and then taking the spatial transform of these spectra considered as a function of the vector displacement between the probes. The phase velocity of such waves in the case of a one-dimensional, weakly-excited spectrum was determined. (author)