The prospects of ICRF generators at ASDEX upgrade

The ICRF system at ASDEX Upgrade (AUG) currently uses five RF generators with a total nominal output power of 10 MW. The contract for delivering the initial 4 generators was signed in 1987 and operation started in 1992. In 2018, we added a fifth generator, one that had been put into operation in 1983 for the AUG predecessor, the ASDEX tokamak and later also used for the Wendelstein 7-AS advanced stellarator. While this is a testimony to the longevity of the generators in this frequency range - a legacy of their extended development as radio transmitters - it does rely on spare parts remaining available, in particular tetrodes. Since the initial operation starting in 1983 and 1992, the original tetrodes for the pre-driver stage (Siemens RS 1054) and final stage (ABB CQK 650-2) became unavailable. The pre- driver tetrode can be either replaced by a widely used triode or by a semiconductor amplifier. As a replacement for the ABB final stage tetrode, we successfully tested an EIMAC 4CM2.500KG tetrode in the ASDEX/W7-AS generator mentioned above. Recently however, it was announced that the EIMAC tetrode might no longer be available. Consequently only a single electron tube manufacturer remains, able to produce a tetrode of the type needed for the final stage. Security of supply and lack of competitive pricing could become a cause of concern. We investigate to what extend it is already possible now to replace tube based ICRF generators with semiconductor based RF amplifiers and what future developments can be expected.