Correlation broadening and the study of turbulence in reversed-field pinch plasmas

Abstract

The use of the spatial correlation function to describe the structure of turbulence in a plasma is well established. The authors consider the extent to which further information about the turbulence can be obtained from the study of the broadening of the correlation function when a time delay is introduced between the correlated signals. They define an experimentally accessible dimensionless measure of the broadening rate and illustrate its use with data obtained from the ZETA and HBTX reversed-field pinch (RFP) devices. They discuss the interpretation of these results in terms of the random motion of turbulent elements or of the broadening due to resistive diffusion, and show that the true value of the fine-scale correlation length in ZETA was about 2.5 cm, half the value given by the magnetic field fluctuations. The results are compatible with an overall picture of the ZETA discharge as dominated by parallel motion along stochastic field lines near the magnetic axis but by radial convective motions near the edge; it remains to be seen whether other RFP devices show a similar pattern.