Study for a tangential dispersion interferometer/polarimeter for DTT

The Divertor Tokamak Test (DTT) facility [1], whose construction is starting in Frascati, will require robust and reliable diagnostics for the correct operation of the machine and the characterization of the plasma discharge. For this purpose, we are studying a common-path dispersion interferometer/polarimeter for the detection of plasma electron density and magnetic field in two different tangential chords in the equatorial plane. The physical principle is based on the generation of a second harmonic which crosses the plasma collinearly with the beam at its fundamental. Being the plasma a dispersive medium, the two beams are subject to different phase shifts from which it is possible to retrieve the plasma free electron density. Moreover, the unconverted part of the fundamental can be used for polarimetric measurement. Two different implementations of the interferometer have been considered, one with a CO2 laser (lambda = 10.6/5.3 mu m) and another one with Nd:YAG (lambda = 1.064/0.536 mu m). The former is more sensitive to lower plasma densities and to Faraday rotation, while the latter is more robust to fringe jumps. We have studied the main aspects of these two possible solutions. In particular, we analysed the optics to be used and the expected maximum signals for the interferometry and the polarimetry in a possible