The 7-channel FIR HCN Interferometer on J-TEXT Tokamak

TL;DR

This paper presents a seven-channel FIR HCN interferometer designed for plasma density measurement on the J-TEXT tokamak, featuring a 3.4 m cavity laser, phase modulation, and improved vibration control for enhanced accuracy.

Contribution

It introduces a novel seven-channel FIR HCN interferometer with phase modulation and vibration reduction techniques for precise plasma density diagnostics on J-TEXT.

Findings

Achieved phase sensitivity of 0.06 fringe

Demonstrated stable laser power under different silicone oil viscosities

Improved vibration reduction enhances measurement accuracy

Abstract

A seven-channel far-infrared hydrogen cyanide (HCN) laser interferometer has been established aiming to provide the line integrated plasma density for the J-TEXT experimental scenarios. A continuous wave glow discharge HCN laser designed with a cavity length 3.4 m is used as the laser source with a wavelength of 337 {\mu}m and an output power up to 100 mW. The system is configured as a Mach-Zehnder type interferometer. Phase modulation is achieved by a rotating grating, with a modulation frequency of 10 kHz which corresponds to the temporal resolution of 0.1 ms. The beat signal is detected by TGS detector. The phase shift induced by the plasma is derived by the comparator with a phase sensitivity of 0.06 fringe. The experimental results measured by the J-TEXT interferometer are presented in details. In addition, the inversed electron density profile done by a conventional approach is also given. The kinematic viscosity of dimethyl silicone and vibration control is key issues for the system performance. The laser power stability under different kinematic viscosity of silicone oil is presented. A visible improvement of measured result on vibration reduction is shown in the paper.