Tests of load-tolerant external conjugate-T matching system for A2 ICRF antenna at JET

A prototype of ICRF antenna matching system tolerant to fast plasma loading perturbations has been successfully tested at JET. The technique is based on the use of an external (outside the tokamak vacuum vessel) conjugate-T circuit tuned to low resistive impedance by coaxial line-stretchers (trombones). Further impedance transformation, required to complete the entire circuit matching is achieved by a conventional variable stub and trombone tuner. The experimental set-up involved one pair of adjacent straps of the JET A2 antenna array powered by a single RF amplifier. The tests fully confirmed the feasibility of the matching scheme both for vacuum and plasma loading. Clear evidence of high load-tolerance during plasma sawtooth oscillations and Edge Localised Modes (ELMs) was observed in agreement with the circuit simulations. Reliable trip-free performance was demonstrated in the 32–51 MHz frequency band at ≤1 MW power levels. The proposed approach appears as a viable alternative to the in-vessel conjugate-T matching schemes and offers such additional advantages as full compatibility with coaxial transmission line technology, manageable tuning accuracy, separation of launching and matching sub-systems and capability to conjugate remote antenna straps.