Performance of toroidally wide-separation loop antennae for JT-60U ICRF experiments

The performance of two array antennae of phased 2 × 2 loops for JT-60U is presented in the initial operation for higher harmonic ion cyclotron resonant frequency (ICRF) experiments. The array antenna is designed so as to have a sharp coupled power spectrum of npeak ≈ 3 with Δn ≈ 2.5 when out of phase with toroidally adjacent loop currents ((π, 0) phasing) at 116 MHz by separating the loops widely by 0.44 m. The maximum coupled power up to the present is 6.4 MW with (π, 0) phasing at Bt = 4 T, which corresponds to the power density of about 6.1 MW m−2. Anomalous antenna breakdown is observed during combined neutral beam injection (NBI) and ICRF heating for the particular magnetic field strength Bt = 3.0 T at which an ion cyclotron resonance layer of 7ωcD is located near the antennae. A fairly large antenna loading resistance is obtained, i.e. 4 Ω for (π, 0) phasing at 0.13 m of the distance δSF between the separatrix and the Faraday shield, and 5 Ω for in-phase ((0, 0) phasing) at very large distances of more than 0.33 m. results show the decrease in the loading resistance at the L to H transition is about 30%, from 5 to 3.7 Ω with (0, 0) phasing at δSF = 0.32 m. The ratio of the incremental radiation loss to ICRF power decreases with increasing ICRF power and is saturated at small values less than 10% at power levels more than 4 MW. The increase in the radiation loss seems to be mainly caused by the light impurities from the first wall.