G. Rey, P. Bibet, P. Froissard, F. Kazarian, M. Goniche, G. Tonon, J.-G. Wegrowe
Association Euratom-CEA , departement de Recherche sur la Fusion Controlee, CEA/Cadarache 13108 SAINT PAUL LEZ DURANCE CEDX FRANCE
Ph. Schild, J. Dobbing, A. Ekedahl, C. Gormezano, A. Kaye, M. Lennholm, F. Sšldner
JET Joint Undertaking, Abingdon, Oxon OX14 3EA UK
The efficient capability of the Lower Hybrid Waves (LHW) to drive and control a large fraction of the plasma current, particularly at moderate density and medium temperature, makes this system strongly attractive for the ramp-up phase, off-axis current drive and for the 'advanced tokamak' scenarios which are projected to achieve steady state operations in ITER.
A conceptual design of a LHW system for ITER EDA was elaborated ( ITER task IVA-LH 1993), based on the novel concept of the passive active multijunction (PAM) which associates a good coupling of the slow wave with an efficient cooling of the grill mouth. The PAM is fed by an oversized section (hyperguide) excited by several mode converters which ensure the poloidal sharing of the RF power. Specific components, such as the mode converter and hyperguide have now been successfully tested at high power and long pulse duration (up to 1000 s at Tore Supra). The present design takes into account the new dimensioning of the ports and is more compact to meet remote handling requirements. The 50 MW required to cover the envisioned scenarios, can be coupled by two launchers located in two ports of the machine. The working incident power density is 23 MW/ m2 which is routinely achieved in present experiments at 3.7 GHz. The RF losses are below 4% and for density at the grill mouth between 2 & 4 times the cut-off density, the reflection coefficient remains below 4 %. The launched N// spectrum of the wave centered at N// = 2, is produced by a PAM made of 29 passive waveguides (WG) and 28 active WG with a 270 degrees phasing between adjacent WG. The passive WG facing the plasma are Be coated (or alternatively CFC brazed on) DS copper and actively cooled to sustain the 0.5 MW/m2 of radiated power and 1 MW/m2 of neutron flux. The 4 PAM which compose the antenna are inserted in a mechanical frame attached to the backplate of the first wall. The total heat load on the antenna is 11 MW ( 6 due to the neutrons) and the total weight of the RF plug is 35 tons which can be introduced (or disassembled) by a telemanipulator. The main RF transmission line is composed of 8 circular waveguides working in the circular mode TE01. The klystron plant is composed of 64, 5 GHz ,1 MW klystrons which requires some industrial development.