Development of an HCN dual laser for the interferometer on EAST

A two-color continuous wave（CW） discharge-pumped far-infrared（FIR） hydrogen cyanide（HCN） laser was developed as the source of an interferometer for measuring the line-averaged electron density in the Experimental Advanced Superconducting Tokamak（EAST）. The output power of the dual laser system was about 120 m W from each laser on the 337 μm（0.89 THz）line. The polarization of each output beam was fixed using thin tungsten filaments and oscillated in the EH11 mode. Different megahertz intermediate frequencies（IF） and a slight frequency offset（～1 MHz） were generated in this system to replace the traditional rotating grating with～10 k Hz IF, and this can improve the time resolution of the interferometer significantly. The experimental result showed that different IF signals were obtained by successfully adjusting the cavity length. In particular, the beat frequency was captured at ～1.3 MHz by a Schottky mixer when the length of the resonant cavities was changed by 5 μm by an automatic adjustment system. In order to study the character of IF, a long time record of the IF signal was carried out,and the IF signal could be stabilized for a few minutes in the range of 2 MHz to 3 MHz. A realtime IF stability control system was initially designed for long pulse discharge experiments on the EAST. The ～MHz frequency response and good phase sensitivity of the dual laser HCN interferometer will allow the system to track fast density profiles and resolve fast MHD events,such as tearing/neoclassical tearing, disruptions, etc.

Real-Time Dynamic Spectrum Analysis for Plasma Electron Density and Faraday Rotation Angle Measurement on HL-2A

Electron density and Faraday rotation angle are important physical parameters in nuclear fusion research.To measure them simultaneously,the three-wave polarimeter/interferometer diagnostic system is applied.Both the final probe output signal and the reference signal contain three frequency components.The time-varying phase difference curve of each frequency component can be measured by the Real-time Dynamic Spectrum Analysis（RDSA）method based on Field-Programmable Gate Array（FPGA）.The phase difference precision is better than 0.1° and the real-time feedback delay is less than 1 ms,which satisfy the requirements of HL-2A.

Simultaneous measurement of vibration amplitude and rotation angle based on a single-channel laser self-mixing interferometer

Based on a single-channel laser self-mixing interferometcr, we present a new silnultaneous measurement of the vibration amplitude and tile rotation angle of objects that both affect the power spectrum containing two peaks of the interferometer signals. The fitted results indicate that the curve of the peak frequency versus the vibration amplitude follows a linear distribution, and the curve of the difference of the two-peak power values versus the angle follows a Gaussian distribution. A vibration amplitude with an error less than 3.0% and a rotation angle with an error less than 11.7% are calculated from the fitted results.