THz multi line-of-sight polarimeter for fusion reactors

In this paper we present the first preliminary study of a new polarimetry diagnostic system. The device foresees multiple lines of sight, so that the measure of the plasma parameters can be performed at different chords along the poloidal plane, parallel to the equatorial direction, in a single acquisition cycle. Considering the typical plasma conditions (i.e. ASDEX Upgrade) of the actual magnetic confinement machines, we need to employ sources in the range of the low (<3) THz to have appreciable rotation angles. As source, the diagnostic foresees the use of Quantum Cascade Lasers (QCL) which represents a very promising solution, given their ability to operate at the expected frequency of 1.6 THz at 4.2 K. Since the power of the probe beam is in the order of tenths microwatts, a cryo-detector, such as kinetic inductance detector (KID), is required. This opens the field for a very compact modular machine, composed by a single cryogenic cooler encasing source, detector and the optical section. The assessment study has been performed taking into account the performances, reliability and adaptability to multiple machines, and enriched with estimates of the Faraday rotation at different conditions, using as base data coming from the ASDEX Upgrade (AUG) tokamak located at IPP in Garching.