Electromagnetic FEM studies of disruptions and engineering consequences for the power supply and coils design of planned upper divertor at ASDEX Upgrade

A new upper divertor is proposed for the ASDEX Upgrade (AUG) tokamak experiment. It is planned to be equipped with internal coils for investigation of alternative magnetic configurations like e.g. "snowflake" (Herrmann et al., 2017 [1]). Since the coils are close to the plasma, high induced and very stiff voltages are expected during disruption events. Because only very vague analytical estimates of voltages, forces and coupling factors were available, an improvement by the help of Finite Element Method (FEM) was envisaged. Therefore, recorded measurements of currents, plasma position, plasma profile and the geometry were integrated in an electromagnetic simulation as boundary conditions to calculate resulting field distributions during selected AUG disruptions. The time resolution can be better than 100 mu s and the required computing resources are comparably small due to the assumption of 2D axis-symmetry. The results were compared with magnetic probe measurements integrated into the tokamak ultimately observing good agreement. After this, the FE model was modified including the new divertor to calculate all relevant parameters. The output of these calculations has strong implications for the coil and power supply design: i) The power supply will be protected by a new kind of crowbar to avoid damage of the power supply due to overvoltage and uncontrolled current and force rise of the coils. The concept of this so called "ripping crowbar" is introduced, which is now under development. ii) The coil cable should be coaxial shaped to monitor isolation faults and to become inherently safe against single-turn shortcuts, identified as destructive fault scenario.