Study of electromagnetic disruption forces for plasma detachment measurements in DEMO

Abstract

Global plasma instabilities such as plasma disruptions and vertical displacement events (VDEs) can generate huge transient electrical currents in the conductive parts of the divertor. The interaction of the currents with the magnetic field induces Lorentz force loads. Particularly, the force induced by a VDE can impose a critical impact on the divertor water-cooling pipes due to high current density. A recently proposed isolated-target design solution for the DEMO divertor introduces a shunt element between the target plate and the cassette body to divert electrical current towards more massive cassette body. Such a design solution is also considered as a diagnostic tool for measuring thermo currents (voltages) through a shunt resistor to control plasma detachment.In this study, a finite-element analysis is performed to provide a conservative upper value for the shunt resistance by investigating the impact of the electromagnetic disruption forces on the integrity of water-cooling pipes in the isolated-target design solution of the DEMO divertor. In the analysis, a maximum allowed pipe current is estimated assuming realistic material properties and magnetic fields, and true geometries of the divertor cassette and water-cooling pipes fixed to the cassette. The results show that local pipe yielding (representing damage initiation) develops for electrical currents larger than ∼3.5 kA, which sets a conservative (assuming extreme VDE halo currents) upper value for the shunt resistance to ∼40 μΩ.