Application of Interferometry and Faraday Rotation Techniques for Density Measurements on ITER

Abstract

There is a need for real time, reliable density measurement for tokamak plasma density control, compatible with the restricted access and radiation environment on ITER. Line average density measurements using microwave or laser interferometry techniques have proven to be robust and reliable for density control on contemporary tokamaks. In ITER, the large path length, high density and high density gradients, limit the wavelength of a probing beam to shorter then about 50 µm due to refraction effects. In this paper we consider the design of short wavelength vibration compensated interferometers and Faraday rotation techniques for density measurements on ITER. These techniques allow operation of the diagnostics without a prohibitively large vibration isolated structure and permit the optics to be mounted directly on the radial port plugs on ITER. A beam path designed for 10.6 µm (CO₂ laser) with a tangential path through the plasma allows both an interferometer and a Faraday rotation measurement of the line average density with good density resolution while avoiding refraction problems. Plasma effects on the probing beams and design tradeoffs will be discussed along with radiation and long pulse issues. A proposed layout of the diagnostic for ITER will be presented.