Further upgrades of the ASDEX Upgrade ICRF system

The original plans were to operate the tokamak ASDEX Upgrade (AUG) for ten years. The flexibility of the experiment lead to the fact, that AUG is now operating for over three decades. As the Ion Cyclotron range of Frequencies (ICRF) heating was AUG’s first auxiliary heating system, which went into operation in 1992, regular upgrades of different ICRF components are mandatory for different reasons. For example, due to a lack of supply, it became necessary to replace the pre-driver stage tetrode by a widely available triode in the ICRF generators. This change reduced the frequency range of the rf generators from 30 – 120 MHz to 30 – 60 MHz, still covering the experiment requirements. A long-time limitation was the behaviour of the high voltage supply of the driver stage of the rf generator. A new power converter was extensively tested on a test bed with promising results. Further upgrades include a redesign of the antenna vacuum pumping system to replace the maintenance intense cold heads. A redesign of the antenna vacuum feed through has also been made to increase the voltage standoff. For the implementation of a new rf generator in the near future, which will extend the frequency range from 30 MHz down to 24 MHz, changes in the antenna matching systems were made on two of the four ICRF antennas by adding a third stub tuner. For the integration of this rf generator into the AUG ICRF network, additional coaxial switches, which were recycled from a shortwave broadcasting station, are integrated in the ICRF system.