Scanning beam medium infra-red interferometry for plasma density measurements

In magnetically confined fusion experiments interferometry is commonly used to measure plasma electron density. This because interferometry is a very reliable technique, not affected by calibration problem. The main drawbacks of interferometers are the integral characteristic of the measurement and the fringe loosing problem. Scanning beam interferometry can always overcome the former problem and in some cases the second as well. The advantages of the scanning beam technique can better exploited in two colours vibration compensated medium infra-red (MIR) interferometers. Indeed the short wavelength of the probing beams provide small beam sizes allowing plasma density measurement along many non-overlapping paths, furthermore the vibration compensation system relaxes the requirement of massive interferometer structures embracing the fusion device, actually allowing to install back-reflecting mirror directly attached to the experimental machine. As an example of application of this technique the two-color medium-infra-red-compensated scanning beam interferometer installed on the Frascati Tokamak Upgrade (FTU) experiment is presented. We present also a preliminary design of a scanning interferometer for the new proposed Fusion Advanced Studies Torus (FAST) experiment. The interferometer uses retroreflectors installed inside the vessel to back reflect the scanning beams; this will allow realizing scanning beams measuring from the plasma edge. This feature in principle can solve the fundamental problem of interferometers: the fringe loosing problem.