Optical layout and mechanical structure of polarimeter-interferometer system for Experimental Advanced Superconducting Tokamak

Bench test of phase measurement on dispersion interferometer for EAST

In recent years, different traditional interferometers have been the necessary diagnostic of electronic density measurement on fusion devices. Until now, two main problems always influence the density measurement: the mechanical vibration and fringe jump in the calculation. The dispersion interferometer (DI) with a long-wavelength infrared wavelength is a good choice because mechanical vibrations can be canceled and the fringe jump can be inhibited. This paper describes the bench test of phase measurement using a wedge instead of plasma on the DI. The results show good agreement with the theoretical calculations. In the background measurement, this DI without a vibration isolation system has good performance, and the drift of the baseline is less than 2 × 10¹⁷ m^-2 in 3 s and less than 5 × 10¹⁷ m^-2 in 400 s. Plasma data will be obtained during the next campaign on EAST (Experimental and Advanced Superconducting Tokamak).

Optical configuration optimization and calibration for the POINT system on EAST

Calibration of the polarimeter system is one of the key elements to determine the overall measurement accuracy. The anisotropic reflection and transmission properties of the mesh beam splitters can easily distort the polarization state of the circularly polarized beams. Using a rotating crystal quartz λ/2-waveplate to replace the plasma can effectively allow us to obtain the ratio of the measured Faraday rotation angle to the known rotation angle of the waveplate. This ratio is used to estimate the calibration factor for each chord to be accurately determined and help to minimize distortions introduced by the wire-mesh beam splitters. With the novel configuration optimization, the distortion of polarization state is effectively eliminated.

Faraday-effect polarimeter-interferometer system for current density measurement on EAST

A multichannel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique is under development for current density and electron density profile measurements in the EAST tokamak. Novel molybdenum retro-reflectors are mounted in the inside wall for the double-pass optical arrangement. A Digital Phase Detector with 250 kHz bandwidth, which will provide real-time Faraday rotation angle and density phase shift output, have been developed for use on the POINT system. Initial calibration indicates the electron line-integrated density resolution is less than 5 × 10(16) m(-2) (∼2°), and the Faraday rotation angle rms phase noise is <0.1°.