Development of the JET Ion Cyclotron Resonance Frequency Heating System in Suppor of ITER

Three new improvements have recently been implemented to the existing JET Ion Cyclotron Resonance Frequency antennae to (a) increase power density and (b) develop matching systems that are tolerant to rapid coupling variations during Edge Localised Modes (ELMs); both of which are key developments for the future design of the ITER ICRF antenna. Firstly, 3dB couplers have been fitted to two antennae in 2004/5. Initial results from these antennae have highlighted the need for parallel development of arc detection and ELM-tolerant systems. Secondly, a new ITER-like antenna (ILA) was installed during 2007 to couple an ITER-relevant power density at the required plasma/antenna spacing using a close-packed array of straps. ELM tolerance is incorporated using an internal (in-vacuum) conjugate-T junction with each strap fed through in-vessel matching capacitors from a common vacuum transmission line. The mechanical engineering design challenges posed by the ILA were significant, given the need for the antenna to: withstand high disruption forces; operate for long pulse length; and achieve a demanding positional accuracy on the in-vessel capacitor tuning system. In addition, it has proved necessary to develop both existing and novel arc detection systems. Prior to installation, the antenna has been high power tested on a test-bed. First results, as well as related simulations, have highlighted the challenges inherent in developing a matching system for an ICRF antenna with closely spaced straps. Thirdly, an externally-mounted conjugate-T (ECT) system has been installed on antennae C and D during the 2006/07 shutdown. The detailed engineering design features of all three developments; the results achieved to date; and the implications for the ITER antenna design will all be reported.