Progress toward a practical magnetic field diagnostic for low-field fusion plasmas based on dual mode correlation reflectometry

Previously, the proof of principle of measurement of magnetic field strength, \B\ internal to a plasma by cross correlation of ordinary (O) and extraordinary (X) mode fluctuation reflectometer signals has been demonstrated in a linear plasma device. It was found that dual mode (O-X) reflectometry data could be interpreted by a one-dimensional numerical model to determine \B\ in the vicinity of the reflectometer cutoff positions. Radial correlation properties of turbulence are also measured simultaneously with \B\. This technique is potentially well suited to measurement of \B\ in low-field fusion devices (e.g., B < 0.6 T) where standard magnetic field diagnostics are expected to be difficult to implement. However, transfer of this method to toroidal magnetic fusion devices presents a number of potential difficulties such as the effects of magnetic shear, steep density gradients, and limited time for cross correlations. In addition, recent simulations suggest that two-dimensional modeling may be required to interpret experimental data when density fluctuation levels are high. Dual mode correlation reflectometry experiments on the National Spherical Torus Experiment are presented, and progress toward the implementation of this technique as a practical diagnostic is discussed. (C) 2003 American Institute of Physics.