JET Far Infrared (FIR) interferometer/polarimeter diagnostic system – 40 years of lessons-learned

Originally designed for five years of plasma operations, the JET Far Infrared (FIR) interferometer/polarimeter diagnostic system was still operating at its full capabilities nearly forty years later in ITER relevant conditions (eg. metal-wall, tungsten divertor) for multiple D-T campaigns, albeit with significantly lower neutron fluences. The original design had to adapt substantially over the years due to machine changes, leading to reduced signal and access to mirrors etc, and the diagnostic still worked due to the excellent dynamic range of the detectors. This paper will discuss invaluable lessons learned designing, operating, optimising, and enhancing such a complex system and how these can be used for developing the new class of laser-based diagnostics for the next generation reactor grade machines.