Upgrade of the JET far infrared interferometer diagnostic

In recent years there has been a major upgrade of the JET far infrared diagnostic system consisting of a new laser system with the wavelength at 118.8 μm at and more advanced processing electronics for phase counting. This provides a second colour measurement of the electron plasma density on the vertical system. Due to the shorter wavelength, the plasma induced laser beam refraction is reduced by a factor of three alleviating density errors caused by loss of signal (so-called "fringe jumps" [A. Murari et al., Rev. Sci. Instrum. 77, 073505 (2006)]), in particular during high performance plasmas experiments in JET.