Demonstration of two-laserPolarimeter-Interferometer(PIer) scheme for simultaneous measurements of Faraday rotation angle and electron density on HL-2A

Abstract

A novel two-laser Polarimeter-Interferometer (PIer) diagnostic scheme, in which Faraday rotation angle (αF) and electron density (ne) can be simultaneously measured by taking advantage of two lasers and two detectors for each channel, has been successfully demonstrated on HL-2A tokamak through upgrading one channel of existing monofunctional Faraday-effect polarimeter. In comparison with the conventional three-laser PIer diagnostic, two-laser PIer generates only one intermediate frequency (IF), avoiding the overlap of IF frequency bands, so as to increase the time resolution and decrease the phase noise of system. The single channel two-laser PIer was firstly put into operation in 2016 HL-2A experimental campaign, and both Faraday rotation angle and electron density phase have been measured with a fast time resolution of 1.0 μs and a phase resolution of 0.1̂ and 1.0̂, respectively. This work is valuable for next step far-infrared (FIR) laser PIer construction on HL-2M tokamak, as well as the future International Thermonuclear Experimental Reactor (ITER).

An FFT overlap method for high bandwidth far-infrared laser Interferometer in HL-2A

“…Some density fluctuation frequency, such as reverse shear Alfven eigenmodes (RSAE), can reach above 80 kHz [8,9], while some Low-Frequency Zonal Flows (LFZF) frequency is lower than 4 kHz [11]. Driven by the demand for high-bandwidth electron density diagnostics, the modulation 2021 JINST 16 P09010 signal frequency of the FIR interferometer is upgraded to 1-5 MHz with two methanoic acid lasers [12][13][14]. However, the phase difference measurement system is optimized for high accuracy, which limits high bandwidth measurement [15][16][17].…”