Measurement of density fluctuation propagation direction via the far-forward collective scattering diagnostic based on polarimeter-interferometer.

ABSTRACT

Previously, the 17-channel three-wave polarimeter-interferometer system (POLARIS) on the J-TEXT tokamak has been implemented to measure far-forward collective scattering (FFCS) from electron density fluctuations. Recently, this system has been exploited to measure the propagation direction of density fluctuation. After considering the refraction of the laser probe beam passing through plasma, the ray tracing result shows that the detector of POLARIS may receive asymmetric far-forward scattering beams. Thus, the heterodyne detection of FFCS is available to identify the propagation direction of density fluctuation by resolving the asymmetric scattering spectrum. Experimentally, the transform of the heterodyne scattering spectrum from symmetry to asymmetry has been observed, while the refraction effect becomes strong demonstrating the capacity of measuring the propagation direction of fluctuation. Furthermore, by changing the plasma potential through the use of an applied positive electrode biasing, the reverse of frequency shift for the heterodyne scattering spectrum is identified, confirming the validity of direction discrimination of density fluctuation.