Latest Achievements of the JET ICRF Systems in View of ITER

The capability of Ion Cyclotron Resonance Frequency (ICRF) systems to reliably inject high power levels into ELMy plasmas is essential both for the JET research program and for future ITER operations. In parallel with the installation and commissioning of the ITER-like antenna (ILA), the conventional ICRF A2 antenna system has been significantly modified to improve its performance. The ILA consists of a closely packed array of 4 Resonant Double Loops with in-vessel matching capacitors and low impedance Conjugate-T points. It has demonstrated efficient trip-free ELM tolerant operation on ELMy plasmas with high power densities (up to 6.2 MW/m2 on L-mode and 4.1 MW/m2 on H-mode) and voltages up to 42 kV without significant impurity production. The ILA further provided validation of the ICRF coupling modeling code Topica. This is relevant for antenna design for ITER as it demonstrates that with ITER's presently estimated density proles its proposed ICRF system is able to couple 20 MW with a system voltage below 45 kV. The conjugate-T junction principle was also used by pairing straps from A2 antennas C and D thus creating 4 conjugate-T points outside the vacuum vessel. Additionally, the load tolerant circuit installed in 2005, based on 3 dB hybrid couplers feeding straps from A2 antenna arrays A and B in pairs, was updated for more extensive use. Both load tolerant approaches have demonstrated robust and reliable performance during operations in a variety of plasma loading conditions including on large Type I ELMs. Depending on the plasma scenario, the trip-free time-average power levels coupled to ELMy H-mode plasma during long pulse operations have reached 3.3 MW for the 3 dB hybrid and 4 MW for the External Conjugate-T (ECT) system. Up to 7 MW total ICRF power was coupled to ELMy plasmas simultaneously by all four conventional JET ICRF antennas.