Interferometric measurements of plasma density fluctuations during the formation phase in the ZT-40 reversed-field pinch

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Abstract
A seven-chord infrared interferometer is used to study plasma density fluctuations during the formation phase in the ZT-40 toroidal pinch device. The method consists of taking experimental ensemble averages of the products of fluctuating interferometric phases for pairs of chords, forming an experimental covariance matrix. The diagonal components provide (with suitable normalization) the equal-time mean correlation between chord-integrated fluctuations along differing paths. These mean quantities are found to depend systematically on the level of magnetic shear in the pinch discharge, in a manner which suggests that the density disturbances are elongated structures oriented parallel to the local magnetic field. Within this picture, the spatial correlation results are compared with earlier work on the Zeta device.