Status and development of the ICRF antennas on ASDEX Upgrade

The 3 dB couplers on the ICRF system of ASDEX Upgrade allow operation of the ICRF system, even in the presence of strong ELMs. The refurbishing of the power tetrodes has restored the nominal power of the generators. Under those conditions, the limiting factor on the power to the plasma can become the voltage stand-off of the antennas. The paper reports the status, progress and plans in this area. By doubling the radius of the sides of the antenna straps (from 5 mm to 10 mm) the edge electric field of the antenna straps was reduced. Experiments in a vacuum test set-up confirmed an increase of the voltage stand-off of the antenna, from typically 27 kV to 41 kV for 10 s pulses and from 42 kV to 58 kV for 1 s pulses. The effect of plasma scrape-off layer variations on the voltage stand-off was investigated with a high voltage RF probe of a simple geometry inserted in the plasma edge. The voltage stand-off of the probe is reduced during the period of the edge density increase correlated with an ELM. The reduction in voltage stand-off can be understood as an effect due to plasma getting into the antenna. This implies that in addition to the need to make a system ELM resilient from the point of view of matching, and optimised from the point of view of vacuum electric fields, one still needs to consider the possible influence of the edge plasma on the voltage stand-off. To prevent plasma from penetrating into the antenna during the ELM phase an optically closed Faraday screen could be used