Achievement of the H-mode with a screenless ICRF antenna in ASDEX Upgrade

The removal of the Faraday screen from one of the four ICRF antennas in ASDEX Upgrade and the achievement of the H‐mode with that antenna alone allowed for the first time H‐mode specific effects of screenless antenna operation to be analyzed in a divertor tokamak. Only in particular cases could small but consistent differences be seen as compared to an antenna with screen. The threshold power is marginally (<10%) higher for a screenless antenna. In the power range that has been reached with only one ICRF antenna (up to 1.3 MW, ELMy and ELM‐free conditions, but still near the threshold), there is a slight difference in quality of the H‐mode, reflected in a lower edge electron temperature and a lower total plasma energy for the antenna without screen (−10%). The screenless antenna was also operated in combination with neutral beam injection (type I ELMs) and under CDH conditions (small type III ELMs, detached divertor) with no significant differences to an antenna with screen. Only modulation experiments allowed to detect a minimal difference (<5%) in plasma stored energy under reversed field conditions, i.e. when the Faraday screen is misaligned to the magnetic field by about 30°, in favour of the antenna with screen.