Linear Mode Conversion between Ordinary and Extraordinary Waves in Plasma with Large Magnetic Shear

Abstract

Linear mode conversion between ordinary- and extraordinary-mode microwaves in plasma with large magnetic shear is analyzed numerically. The purpose of the analysis is a basic-design study of a microwave reflectometer using the mode conversion available in the fusion reactor large helical device (LHD) with large magnetic field shear created by a pair of helical windings. We call this a mode conversion reflectometer. Analysis is performed assuming a simple one-dimensional model and practical plasma parameters. The results indicate that mode conversion is sensitively affected by the density gradient near the two cutoff points. A steep gradient results in a large degree of mode conversion. In some cases, the converted amplitude becomes larger than 10% that of the incident signal. It is shown that, for steep density profiles, the interference patterns obtained by two converted (daughter) signals are applicable to the measurement of the density profile without any conventional phase detection.