40 years of ICRF operation on JET: challenges and achievements

After 40 years of successful operation and multiple Deuterium-Tritium (D-T) campaigns, the Joint European Torus (JET) concluded its operations in December 2023. From the outset, Ion Cyclotron Range of Frequency (ICRF) heating was identified as a key auxiliary heating system, with the first ICRF system becoming operational in 1985. This system underwent significant development, culminating in a high-power (32 MW installed), wideband (23-57 MHz range), and highly versatile system. This contribution provides an overview of the JET RF system's evolution over its lifetime, highlighting selected operational challenges and technological achievements. It addresses the rationale behind the development and operation of the ICRF system, including the implementation and operation of load-resilient systems (3 dB hybrid coupler, External Conjugate-T, ILA), as well as the operational limits and system protection measures, such as the specific mode of operation for Ion Cyclotron Wall Conditioning (ICWC). The impact of transitioning from a carbon wall to the JET-ILW metal wall (tungsten divertor and beryllium first wall) on the system's operation and the challenges posed by D-T operations are also emphasized. Finally, key lessons learned from the technical challenges, constraints, and achievements of the JET experience are highlighted, with an eye towards the design and operation of ICRF systems in future machines.