ITER Density Interferometer Polarimeter: theoretical estimation of temperature, pressure and humidity variation effect on phase measurement and correction strategy

Abstract A phase-modulated dispersion interferometer combined with a polarimeter is currently being designed in ITER. This diagnostic, called DIP (Density Interferometer Polarimeter), aims at performing reliable line-averaged electron density ( n e ) measurements with a time resolution of 1 ms and an accuracy of 10% during ramp up/down phases, and 2% during flat-top, serving as a complementary system for ITER's main n e diagnostic, the TIP (Toroidal Interferometer/Polarimeter). ITER's DIP is based on a CO 2 laser ( λ = 9.6 μm). It is inherently insensitive to mechanical vibrations and, thanks to the combined polarimeter, can correct fringe jump errors. Phase-modulation is performed using a photo-elastic modulator (PEM). This work presents recent updates on the system design. In particular, a model that takes into account the variations of air and ZnSe optical windows refractive index due to environmental parameters (temperature, pressure and humidity), eventually affecting phase measurements, is here described.