Development of an active overvoltage protection for the new ASDEX upgrade divertor coils

There is proposed a new upper divertor for the ASDEX Upgrade (AUG) tokamak experiment [1]. It is planned to be equipped with internal coils for investigation of advanced magnetic configurations like e.g. "snowflake" [2]. Due to the close vicinity of the coils to the plasma, high induced and very stiff voltages are expected during disruption events, as shown in [3,4],. This voltage is a high risk for either damaging the connected power supplies and/or for turning into high currents causing damage to the tokamak by Lorentz forces. The proposed device named "ripping crowbar" detects and overtakes the full high voltage of up to 5 kV during a plasma disruption at full load current of up to 3 kA to protect the connected power supply which can deal with voltage not higher than 340 V absolute value. The concept will be given and discussed. Experiments were started at much lower current and voltage by use of commercially available hardware. This is a starting point to enhance the working regime in a time-saving way, because there exist no commercially available solution for the target values. The only known field with comparable power stage solutions can be found in the field of High Voltage Direct Current (HVDC) circuit breakers [5]. The final solution should be scalable in voltage, current and power to adapt it to other applications in and outside the field of fusion technology. Part of the development are the Insulated Gate Bipolar Transistor (IGBT) based powerstage, the drivers, the measurement unit to detect disruptions and the triggering unit. At the end, most important will be reliability of this safety relevant device to protect multi-million investments into power supply and new divertor.