Shunt analysis in the isolated-target divertor model for plasma detachment measurement in DEMO

A recently proposed isolated-target design solution for the DEMO divertor introduces a shunt resistor between the target plate and the cassette body, which are otherwise electrically isolated from each other. Such a design serves as a diagnostic tool for measuring thermo-currents (voltages) through a shunt to control plasma detachment. Ideally, large shunt resistances would be desired in order to provide measurable voltage signals. However, during global plasma instabilities, very large electrical currents may be induced between the target plates which need to be routed through a shunt towards the massive cassette body to avoid damage of the cooling pipes. An optimal shunt resistance is therefore required to provide measurable thermo voltage signals and to protect the piping from the effects of the most extreme plasma instabilities. In this study, a steady-state finite-element analysis is performed for the latest divertor geometry design assuming realistic extreme loading conditions of VDE-IV slow down with 30/89 kA applied on the inner/outer plasma facing components and channel-like pipe fixations that prevent pipe bending. In particular, the number of shunts, their maximum resistances and optimal locations are proposed from the requirement that the temperature of the water-cooling pipes does not exceed the water boiling temperature and that all the pipes remain their structural integrity during the VDE-IV disruption. The results of the analysis suggest that four shunts of-600 mu Omega will protect the pipes during the VDE-IV event and provide a-64 mV signal during the assumed 150/150 A thermo-current measurement.