LHCD and Coupling Experiments with an ITER-like PAM launcher on the FTU tokamak

Succesful experimental tests on the PAM (passive active multijunction) antenna for the Lower Hybrid (LH) waves similar to that foreseen for ITER have been carried out for the first time on the FTU tokamak. They validate the main features indicated by the simulation codes, concerning the power handling and the coupling. More than 1.5 times the power requested for ITER and scaled for the different LH frequencies (8 GHz in ITER, 5 GHz in FTU) can be routinely managed for the maximum time allowed by the LH power plant, namely 0.9 s, without any fault in the transmission lines. The power level achieved in FTU is 250 kW against a design value of 170 kW, which correspond respectively to 50 and 33 MW/m2 through the ITER active area of the antenna. The power reflection coefficient Rc is always £ 2% , once the PAM launcher has been properly conditioned, even with the grill mouth retracted 2 mm inside the port shadow, with density in front of the launcher very close to the cut-off value. The current drive efficiency is comparable to a conventional grill in similar conditions, provided the lower directivity is taken into account. The flexibility in the N|| spectrum is confirmed by the HXR spectrum, detected with a fast electron bremsstrahlung camera. Conditioning the PAM to operate at the ITER equivalent power level has required only one day of RF opeartion, without a previous baking at 200 °C of the waveguides.