On October 28, the new ITER prototype Lower Hybrid Current Drive (LHCD) antenna (Passive Active Multi- junction (PAM) antenna, see FN October 2009) started its commissioning phase on the Tore Supra tokamak. The entire system operated successfully as soon as the second day, coupling 450 kW into the plasma for 4.5 seconds, which is an outstanding result after such a short commissioning period. Besides the need for long pulses, ITER poses a second challenge for the Radio Frequency (RF) power systems. This challenge takes the form of a large gap between plasma and antenna which causes higher reflections (see the JET ICRH article in this issue about the effect of reflections). Besides the active waveguides that transmit the power, the PAM antenna design incorporates passive waveguides which receive part of the reflected power, preventing it from affecting the RF generator. The first experimental results clearly meet the main design specifications: Even with a large plasma-antenna gap, less than two percent of the supplied power was reflected towards the transmission line and RF generator, compared to up to ten percent for conventional antennas. Thus the new system may enable a more continuous and reliable power injection into the plasma.

Thanks to Annika Ekedahl, CEA, for her input