Thermo-optical effect in zinc selenide windows for two-color interferometer for fusion plasma diagnostics

After some improvements in the two-color interferometer for plasma density measurements in the TJII fusion device, an effect, which we attribute to the thermo-optical characteristics of zinc selenide windows, has been put in evidence. Part of the 53.2 GHz mm radiation, used to heat the plasma, heats the windows and produces an optical path length difference between the two laser beams of 10.6 and 0.633μm wavelength. The effect induces a deformation of the line integral density measured by the interferometer. It is important in shots with high plasma reflectivity at the electron cyclotron resonance heating (ECRH) frequency. Some experiments and calculations have been done. The effect is explained and its order of magnitude is justified on the basis of some approximations. Several ways to avoid the effect including a correction procedure are discussed. Other possible heating sources are also discussed. The observed effect could be very relevant for experiments where ECRH is the main heating mechanism.