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

Dispersion Interferometers (DI) present the fundamental advantage over conventionalones to be insensitive to mechanical vibrations without requiring a second wavelength interferometerto measure path length variations. On the other hand, their optical setup requires duplication ofnearly all optical components for any measuring chord. This makes the realization of a multi-channel interferometer that is needed to obtain density profiles via Abel inversion of line integralmeasurements more complicated. To overcome such drawback of the DI, in this work we proposeto join the dispersion technique to the beam scanning one, which has been already successfullyimplemented in the conventional mid-infrared two-colour interferometer. In particular, we presenta preliminary design of a DI scanning interferometer for the new Divertor Test Tokamak (DTT)facility, presently in construction. DTT is designed to study a large suite of alternative divertormagnetic configurations in order to ensure acceptable conditions at the walls while maintainingsufficient core performance. In this contest, measuring plasma parameters in the divertor regionis very important though it often presents various difficulties. To improve divertor measurementsthe proposed interferometer will measure the density along the divertor legs from the strike pointsup the X-point. The interferometer will use a CO2laser (?=10.6?m) and a double pass opticalscheme. Phase modulation method will be used to improve the resolution of the measurement andto extend the measuring range above the 1020m-2line integral limitation of the standard homodyneimplementation. Both improvements are important in this application, considering the wide densityrange expected in the DTT divertor region. Comparing to shorter wavelengths, more commonlyused in the DI interferometers, the CO2wavelength improves density resolution while providinggood immunity to the diffraction effect due to the expected high density gradient.