Dispersion scanning beam medium infra-red interferometry for divertor plasma density measurements in DTT

Dispersion Interferometers (DI) present the fundamental advantage against conventional ones to be insensitive to mechanical vibration without requiring a second wavelength interferometer to measure path length variations. On the other side their optical setup requires a duplication of nearly all optical components for any additional channel making in this way quite complicate the realization of a multi-channel interferometer which is often required to translate the line integral measurements to local (or nearly local) measurements. To get the advantage of the dispersion interferometer without its drawback in this work we propose to join the dispersion technique to the bean scanning one, which has been already successfully implemented in the convention mid-infrared two-color interferometer [1]. To this aim we present a preliminary design of a DI scanning interferometer for the new Divertor Test Tokamak (DTT) presently in the design phase. The role of the DTT facility [2] is to help the development of a reliable solution for the power and particle exhaust in a reactor. To this aim, DTT has been designed to study a large suite of alternative divertor magnetic configurations in order to ensure acceptable conditions at the walls while maintaining sufficient core performance. In this contest measurements of plasma parameters in the divertor region is import though not often easy. To contribute to improve divertor measurements the proposed interferometer will measure the density along the divertor legs from the strike points up toward the X-point. The interferometer will use a CO2 laser (l=10.6 ?m) and a double pass optical scheme. Phase modulation method [3] is used to improve the resolution of the measurement and to extent measurement range above the 10-19 m-2 line integral limitation of the standard homodyne implementation. Both improvements are important in this application considering the expected high extension of density range in the DTT divertor region. Comparing to shorter wavelength, easily used in the DI interferometer, CO2 wavelength improves density resolution while providing good immunity to diffraction effect due to the expected high density gradient.