Application of laser phase contrast (LPC) method for density fluctuation spectra in plasmas

A laser phase contrast (LPG) method is a useful diagnostic technique especially for the wave-number spectra of the turbulent density fluctuations, because it applies imaging optics. Recently this line-of sight measurement has been improved to yield spatially resolved information by making use of the direction of fluctuation wave-vectors with respect to the sheared magnetic field lines in toroidal devices. This paper describes the overview of the principle of this method, the verification of the orientation-resolving system, and results applied to the plasma discharges in Heliotron E helical device. This method have made it possible to analyze poloidally counter-propagating pair of waves, electron and ion branches, separately, which are expected to be important in identifying the fluctuations. (author)