C O 2 (λm=10.6 μm) He–Ne (λc=633 nm) two-color laser interferometry for low and medium electron density measurements in the TJ-II Stellarator

Abstract

In this article we describe the setup, installation, calibration, and operation of the two-color (CO2–HeNe) laser heterodyne interferometer installed in the TJ-II Stellarator. This diagnostic has been routinely working during the last TJ-II experimental campaign (Autumn 2003) in low-density [electron cyclotron resonance (ECRH) heated, ne≅1019 m−3] and medium density [neutral beam injection (NBI) heated, ne≅5×1019 m−3] plasmas with line integral error better than 2×1017 m−2. This work demonstrates the possibility of using low-cost two wavelength laser interferometric diagnostics to measure low and medium-density plasmas instead of the complicated far infrared (FIR) interferometric systems that also suffer from fringe-jumps errors. The issues of low sensitivity to plasma density and mechanical vibration subtraction typical of these laser interferometers are solved through proper calibration of the interferometric system in the machine. First results on NBI heated plasmas measured by the laser interferometric system in the Stellarator TJ-II will be shown.