Performance evaluation of short-wavelength FIR laser polarimeter with dual silicon photoelastic modulators

Abstract

Measurement of an internal magnetic field distribution in magnetically confined fusion devices is indispensable for both understanding the plasma physics and controlling plasmas. A polari-interferometer based on the Faraday effect has been used for such a purpose. This paper describes performance evaluations of a part of the polarimeter with photoelastic modulators (PEMs) of the short-wavelength far-infrared (FIR) laser polari-interferometer. The wavelengths of the light source are 57.2 and 47.7 μ m, which are suitable for a high-density operation and large fusion devices. The PEM for the FIR region is newly developed with high-resistive silicon in which absorption of the FIR laser is small. The polarization angle is successfully measured and an angle resolution of 0.01° with a time resolution of 1 ms is achieved. A drift of the baseline of about 0.1° for 1000 s is observed and is found to be caused by changes in the room temperature.

Keywords:

• polarimeter
• interferometer
• far-infrared laser
• photoelastic modulator

Acknowledgements

This work is supported by a Grant-in-Aid for Scientific Research on Priority Areas “Advanced Diagnostics for Burning Plasmas” (16082208). The authors thank Dr Linda Hirschy and Dr Theodore C. Oakberg of HINDS Instruments for the development of the PEMs and fruitful discussions.