Investigation of a high spatial resolution method based on polar coordinate maximum entropy method for analyzing electron density fluctuation data measured by laser phase contrast.

Laser phase contrast is a powerful diagnostic method to determine the spatial distribution of electron density fluctuations in magnetically confined plasmas, although its applicability depends on magnetic field configurations. The spatial resolution of fluctuations is linked with the resolution of the propagation direction that is derived from the two-dimensional spectral analysis of the wavenumber for the fluctuations. The method was applied to fluctuation measurements in a compact helical system. In order to improve the resolution of the propagation direction with a relatively small number of data points, the maximum entropy method with polar coordinates was employed. A spatial resolution of the order of 1 cm was obtained, which is satisfactory in a plasma with a 20 cm minor radius.