Coherence imaging for plasma spectro-polarimetry

Optical coherence imaging (CI) systems developed at ANU are compact polarization-interferometers that facilitate 2D time-resolved imaging of simple plasma spectroscopic scenes. These systems simultaneously analyse the spectrum and polarization properties of relatively narrow-band spectral scenes (transmitted by an optical interference pre-filter) such as Doppler broadened emission lines or polarized multiplets. The physical parameters determining the emission spectral and polarization properties are recovered from the interferometric phase and amplitude images at one or more appropriately chosen optical path length delays.
CI has been used successfully for Doppler imaging studies of boundary and divertor flows and temperatures in a number of tokamaks around the world, and a system is now being prepared for installation on ITER. By combining with a suitable front-end polarimeter, CI systems have also been used for imaging the motional Stark effect polarization orientation (IMSE) in KSTAR, DIII-D and ASDEX-U. Other applications include synchronous Doppler imaging of plasma instabilities and RMP islands, imaging of spectral line ratios for isotope relative abundances and Thomson scattering.
In this presentation I will describe the optical principles underpinning these interferometric systems and will review Doppler and IMSE results obtained on a number of devices. In particular I will discuss the particular challenges faced by the ITER CI system, including large Zeeman effect and high stray background radiation. I will then consider possible applications for CI systems on the TCV tokamak.