JET far-infrared interferometer/polarimeter diagnostic system—40 years of lessons learned

Originally designed for 5 years of plasma operations, the JET far infrared interferometer/polarimeter diagnostic system was still operating at full capability nearly 40 years later in ITER-relevant conditions (e.g. metal wall, tungsten divertor) for multiple D–T campaigns, albeit with significantly lower neutron fluences. The original design had to be adapted substantially over the years due to machine changes, leading to reduced signal and access to mirrors etc, and the diagnostics still worked due to the excellent dynamic range of the detectors. This paper will discuss invaluable lessons learned from designing, operating, optimising and enhancing such a complex system and how these can be used for developing a new class of laser-based diagnostics for next-generation reactor-grade machines.