Measurement of velocity fluctuations in plasmas by a conventional visible spectrometer

A conventional visible spectrometer system has been applied to velocity fluctuation measurements in plasmas. By measuring the number of photons, the effect of quantum noise is subtracted, and real velocity fluctuations are obtained. The first measurements in a reversed-field-pinch plasma show finite velocity fluctuations of about 1.7×103 m/s when the system views off-center chords. When it views the central chord, finite fluctuations are not found. These results impose an upper limit to spurious Doppler broadening, which could arise from fast and large vibrations of cold plasma. The effect of the spurious Doppler broadening is found to be small.